? 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Since its bandgap is close to optimum value, AgGaTe2is regarded as a promising material for solar cells. This paper reports the result of photoluminescence (PL) experiments on AgGaTe2thin films prepared by the two-step closed space sublimation method. In particular, the effect of by-products including Ag2Te or AgGa5Te8on PL properties of AgGaTe2is investigated. The radiative recombination process is also explored. PL measurements of thin films are performed by changing the excitation intensity and measured temperature. It is confirmed that the emission band probably derived from defects and impurities exists near 1.02 eV and the emission band derived from free exciton (FE) exists around 1.32 eV, and the variation of FE emission intensity is affected by the Ag/Ga ratio in AgGaTe2. It is also investigated that the activation energy (EA) of the FE emission is about 7.8 meV.

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim CuGaTe2 layers are successfully grown on Si (001) and Mo/quartz substrates using two-step closed space sublimation (CSS). The surface morphology of the Cu2Te first layer is affected by the surface free energy of the different substrates. Cu2Te grown on Si (001) comprises hexagonal islands which are highly oriented to the (0001). Conversely, Cu2Te layers formed on Mo/quartz substrates forms a thin film. The surface morphology and the quality of the crystals in the CuGaTe2 layers are investigated. Overgrowth by CuGaTe2 of the Cu2Te islands on Si substrates is confined to the islands only and there is no nucleation on the bare substrate. On the contrary, overgrowth by CuGaTe2 on the continuous Cu2Te films on Mo/quartz substrates produces continuous thin films. Therefore, the surface morphology of the CuGaTe2 layers can be controlled by changing the substrate material because they have different surface free energies.

AgGaTe2, AgAlTe2, CuGaTe2, and Ag(Ga,Al)Te2 layers were deposited by the close spaced sublimation method. The surface morphology and crystal quality of these Te-based chalcopyrite layers were systematically evaluated. Controlling the stoichiometry of these layers grown by the closed space sublimation was very difficult because Te preferentially detached from the source materials during the sublimation process and then Te vapor leaked out from the reactor. To solve this problem, the gap between the lid and reactor boat was minimized, and the vapor was encapsulated. As a result, the crystal quality of the AgGaTe2 and AgAlTe2 was improved. However, the controlling stoichiometry of CuGaTe2 remained difficult even after the Te vapor leakage was minimized. This behavior was attributed to the large vapor pressure difference between Cu and Te. The surface morphology of the grown AgGaTe2 and CuGaTe2 layers exhibited scattered grain structure, while that of AgAlTe2 possessed a continuous film structure. These different surface structures contributed to differing wettability between the chalcopyrite materials and substrates. It was found that AgAlTe2 exhibited a high wettability against a sapphire substrate, which promoted continuous film formation.

The AgGaTe2 layer was formed on Ag2Te/Si structure with two different procedures by eliminating the melt-back etching. Diffusion of the Ga2Te3 source material into the Ag2Te layer and formation of the AgGaTe2 layer were both occurring during the growth when Ag2Te and Ga2Te3 source mixture was used to form AgGaTe2. It was also clear the AgGaTe2 could be formed by deposition and annealing of Ga2Te3 layer on top of the Ag2Te/Si structure. Solar cells were fabricated using the p-AgGaTe2/n-Si heterojunction, and showed conversion efficiency of approximately 3%.

Molecular beam epitaxy was used to fabricate Ag(In,Ga)Se-2 (AIGS) thin films. To improve the diffusion of Ag, high-temperature deposition and high-temperature annealing methods were applied to fabricate AIGS films. The as-grown AIGS thin films were then used to make AIGS solar cells. We found that grain size and crystallinity of AIGS films were considerably improved by increasing the deposition and annealing temperature. For high-temperature deposition, temperatures over 600 degrees C led to decomposition of the AIGS film, desorption of In, and deterioration of its crystallinity. The most appropriate deposition temperature was 590 degrees C and a solar cell with a power conversion efficiency of 4.1% was obtained. High-temperature annealing of the AIGS thin films showed improved crystallinity as annealing temperature was increased and film decomposition and In desorption were prevented. A solar cell based on this film showed the highest conversion efficiency of 6.4% when annealed at 600 degrees C. When the annealing temperature was further increased to 610 degrees C, the performance of the cell deteriorated due to loss of the out-of-plane Ga gradient.

Ag(In, Ga)Se-2 (AIGS) thin films were deposited by a modified three-stage method, using a molecular beam epitaxy apparatus. The influence of sodium on the properties of AIGS films was investigated, using Mo-coated soda-lime-glass (SLG) and quartz as substrates. A sodium concentration gradient was observed along the thickness direction of the AIGS film. This resulted from sodium diffusion from the SLG substrate. No sodium was observed in the AIGS film fabricated on the quartz substrate. The grain size and crystallinity of AIGS increased upon sodium doping. Photoluminescence (PL) experiments on the AIGS film showed that exponentially tailed donor states introduced by indium atoms were passivated by sodium. This led to free excitons being observed in the PL spectrum. Potential fluctuation was used to explain the emission spectra of both sodium-containing and sodium-free samples. The donor and acceptor levels included in the quasi-donor-acceptor pair recombination were observed at approximately 35 meV interstitial defect (GaAg) and 100 meV (V-Se), respectively. Solar cell performance was significantly improved by sodium doping, probably because of defect passivation.

ZnTe electro-optic waveguide device has a great potential for the practical applications. The introduction of low Mg% cladding layer to the ZnMgTe/ZnTe/ZnMgTe waveguide structure was performed to circumvent the effect of large lattice mismatch between ZnMgTe and ZnTe, and to improve the crystallographic properties. Various two-step index ZnTe waveguide structures were fabricated by molecular beam epitaxy. The structure with 0.1 μm (Mg 9%) interlayer and 0.6 μm (Mg 26%) cladding layer showed significant defect density reduction compared to the single-step index ZnTe waveguide (Mg 20%) due the much smaller lattice mismatch between the ZnTe core layer and the ZnMgTe interlayer. However, the average Mg content of ZnMgTe layers needs to be carefully controlled to prevent the drop of the optical confinement.

Nucleation of Cu-Te layers was performed by the closed space sublimation method using various source materials, source temperatures, and Si substrates with different surface orientations. The objective was to produce nuclei layers with high quality for use as nucleation centers for CuGaTe2. The grown samples were evaluated by x-ray diffraction and scanning electron microscopy. Cu2Te deposits were obtained using either a CuTe or Cu2Te source, but the latter gave a higher area coverage of Cu2Te nuclei. Highly oriented nuclei were obtained when they were grown on Si (001) at a source temperature of 640 °C and substrate temperature of 590 °C. When the source temperature was raised to 750 °C and the corresponding substrate temperature was 700 °C, nonuniform but highly oriented nuclei were obtained. Both nuclei layers exhibited a strong preference for (0001) orientation. The crystallographic features of the Cu2Te nuclei formed on Si (111) were similar to those of the Cu2Te nuclei formed on Si (001).

ZnTe electro-optic waveguide device has a great potential for the practical applications. The introduction of low Mg% cladding layer to the ZnMgTe/ZnTe/ZnMgTe waveguide structure was performed to circumvent the effect of large lattice mismatch between ZnMgTe and ZnTe, and to improve the crystallographic properties. Various two-step index ZnTe waveguide structures were fabricated by molecular beam epitaxy. The structure with 0.1 mu m ( Mg 9%) interlayer and 0.6 mu m ( Mg 26%) cladding layer showed significant defect density reduction compared to the single-step index ZnTe waveguide ( Mg 20%) due the much smaller lattice mismatch between the ZnTe core layer and the ZnMgTe interlayer. However, the average Mg content of ZnMgTe layers needs to be carefully controlled to prevent the drop of the optical confinement. (C) 2017 American Vacuum Society.

Nucleation of Cu-Te layers was performed by the closed space sublimation method using various source materials, source temperatures, and Si substrates with different surface orientations. The objective was to produce nuclei layers with high quality for use as nucleation centers for CuGaTe2. The grown samples were evaluated by x-ray diffraction and scanning electron microscopy. Cu2Te deposits were obtained using either a CuTe or Cu2Te source, but the latter gave a higher area coverage of Cu2Te nuclei. Highly oriented nuclei were obtained when they were grown on Si (001) at a source temperature of 640 degrees C and substrate temperature of 590 degrees C. When the source temperature was raised to 750 degrees C and the corresponding substrate temperature was 700 degrees C, nonuniform but highly oriented nuclei were obtained. Both nuclei layers exhibited a strong preference for (0001) orientation. The crystallographic features of the Cu2Te nuclei formed on Si (111) were similar to those of the Cu2Te nuclei formed on Si (001). (C) 2017 American Vacuum Society.

Zinc telluride layers were grown on highly-lattice-mismatched sapphire substrates by molecular beam epitaxy, and their crystallographic properties were studied by means of X-ray diffraction pole figures. The crystal quality of the ZnTe thin film was further studied by scanning electron microscopy, X-ray rocking curves and low-temperature photoluminescence measurements. These methods show that high-crystallinity (111)-oriented single domain ZnTe layers with the flat surface and good optical properties are realized when the beam intensity ratio of Zn and Te beams is adjusted. The migration of Zn and Te was inhibited by excess surface material and cracks were appeared. In particular, excess Te inhibited the formation of a high-crystallinity ZnTe film. The optical properties of the ZnTe layer revealed that the exciton-related features were dominant, and therefore the film quality was reasonably high even though the lattice constants and the crystal structures were severely mismatched.

The structural and electrical properties of the junction at Ag(In,Ga)Se(2)AIGS/Mo, and Cu(In,Ga)Se-2 CIGS/Mo layers were characterized. The region between the CIGS and Mo featured a MoSe2 layer with a layered hexagonal structure and thickness of 10-15 nm. The c-axis of the MoSe2 was oriented perpendicular to the Mo layer, and the c-value was 12.6 A. However, no such layer was observed at the interface between AIGS and Mo. This result was also confirmed by energy-dispersive X-ray spectrometry and X-ray diffraction measurements of the MoSe2 layer. The GIGS/Mo with a MoSe2 layer formed an ohmic contact, while the AIGS/Mo without the MoSe2 layer formed a Schottky contact. This Schottky contact showed a barrier height of 0.8 +/- 0.02 eV, a nonideality factor of 1.5 +/- 0.1, and a series resistance of 370 +/- 8 Omega. A schematic band diagram of the AIGS/Mo junction was constructed on the basis of the above results.

ZnTe thin films on sapphire substrate with nanofaceted structure have been studied. The nanofaceted structure of the m-plane (10-10) sapphire was obtained by heating the substrate at above 1100A degrees C in air, and the r-plane (10-12) and S-plane (1-101) were confirmed. ZnTe layers were prepared on the nanofaceted m-plane sapphire substrates by molecular beam epitaxy (MBE). The effect of the nanofaceted structure on the orientation of the thin films was examined based on x-ray diffraction (XRD) pole figures. Transmission electron microscopy (TEM) was also employed to characterize the interface structures. The ZnTe layer on the nanofaceted m-plane sapphire substrate exhibited (331)-plane orientation, compared with (211)-plane without the nanofaceted structure. After thermal treatment, the m-plane surface vanished and (211) layer could not be formed because of the lack of surface lattice matching. On the other hand, (331)-plane thin film was formed on the nanofaceted m-plane sapphire substrate, since the (111) ZnTe domains were oriented on the S-facet. The orientation of the ZnTe epilayer depended on the atomic ordering on the surface and the influence of the S-plane.

Ag(In, Ga)Se-2 (AIGS) has been considered as a promising candidate material for the top cell of chalcopyrite-based tandem solar cells. In this work, the process of (AIGS) film growth by a three-stage molecular beam epitaxy method is studied. The diffusion of silver and grain growth of AIGS films from the first stage-deposited (In, Ga)(2)Se-3 is investigated. Energy dispersive spectroscopy mapping is used to reveal the distribution of silver in the film during each stage of the deposition process. A sharp silver accumulation at the surface of the film at the early stage of the deposition process is observed. This has led to the formation of a silver-rich phase, which gradually diffused into the film to produce a homogeneous distribution. X-ray diffraction results illustrate the phase changes of AIGS films. Based on the result, a growth model for AIGS films from the first stage to the second stage is suggested as (In, Ga)(2)Se-3 -> Ag-9 (In, Ga)Se-6 at the surface and (In, Ga)(2)Se-3 at the bottom of the film -> silver-rich AIGS film. During the third stage, the silver-rich film was converted to silver-poor film. The performance of AIGS solar cells fabricated from various samples was compared, and a highest efficiency of 7.1% was obtained.

Domain structures of ZnTe layers grown on a-plane sapphire substrates were investigated by changing the crystallographic properties of the surface and interface. Pole figure images were obtained and we investigated the domain structure in the grown film and the orientation relationships between films and substrates. It was confirmed that two kinds of {111} domains were oriented by annealing the buffer layer at 350 degrees C, while the (100) domain was obtained by annealing the buffer layer at 300 degrees C. From the results of the rocking curve measurement, the introduction of a step-terrace surface through the high-temperature treatment of the substrate resulted in an improved crystallographic quality. However, it did not affect the domain structure in the layer. The introduction of an off-angle on the substrate surface resulted in the formation of a single (111) domain layer. These crystallographic features were mainly affected by the surface atom arrangement of the sapphire substrate and its chemical nature. (C) 2017 The Japan Society of Applied Physics

AgGaTe2 layers were prepared on Si substrates by a closed space sublimation method using a mixed powder source of Ag2Te and Ga2Te3. Ag2Te buffer layer deposition was introduced to eliminate melt-back etching. The effect of the molar ratio of Ag2Te and Ga2Te3 in the mixed source on the crystallinity of the AgGaTe2 layer was investigated. The composition and the phase of the layer was found to change depending on the molar ratio in the deposits, which could be controlled by the source molar ratio along with the Ag2Te buffer layer thickness. It was confirmed that (112) oriented uniform AgGaTe2 layer with an abrupt interface between AgGaTe2 and Si was formed after those parameters were tuned. The obtained layer exhibited the acceptor concentration of around 2.5x10(16)cm(-3). A solar cell was fabricated using the p-AgGaTe2/n-Si heterojunction, and exhibited a conversion efficiency of 1.15%.

ZnTe/sapphire heterostructures have attracted attention for their suitable properties as a terahertz wave detector material. In this study, we focused on the selective growth of ZnTe on SiO2-masked sapphire substrates by molecular beam epitaxy (MBE). When ZnTe was grown at a high temperature (=350 degrees C), low growth rate (0.3mh(-1)), and low J(Te)/J(Zn) flux ratio (0.83) compared with the conventional film growth conditions, the formation of ZnTe nuclei on the SiO2 mask was avoided. The Te flux intensity significantly affected the selectivity of ZnTe growth. The selective growth of ZnTe on sapphire was revealed to be limited by the desorption of Te adatoms from the SiO2.

Zinc telluride (ZnTe) epilayers were grown on S-plane (10 (1) over bar1) sapphire substrates by molecular beam epitaxy, and the epitaxial relationships between the two were compared with data previously obtained for layers grown on c-plane (0001), m-plane (10 (1) over bar 10) substrates, and r-plane (1 (1) over bar 02). The crystallographic relationship between the (111) plane of the ZnTe layer and (0001) plane of the substrate was studied using x-ray diffraction pole figure measurements. It was confirmed that two kinds of {111} oriented domains were formed on the S-plane substrate, and the dominant domain was (111)-oriented. Layers grown on S-plane substrate and on m-plane substrate exhibited the same epitaxial relationship, while the epitaxial relationship of the layer grown on the c-plane substrate exhibited a 60 degrees rotation. These findings would be applicable to control the orientation of ZnTe epilayer surface for various device applications and for various physical property characterizations.

AgGaTe2 layers were successfully grown on Si substrates by the close-spaced sublimation method. The Si substrates were confirmed to be etched during AgGaTe2 layer growth when the layer was grown directly on the substrate. To eliminate melt-back etching, a buffer layer of Ag2Te was introduced. It was found that the Ag2Te buffer layer changed into the AgGaTe2 layer during the growth process, and a uniform AgGaTe2 layer with an abrupt interface was formed. Both the diffusion of Ga into Ag2Te and the growth of AgGaTe2 occurred simultaneously. It was confirmed that uniform AgGaTe2 layers could be formed without any traces of the Ag2Te layer or melt-back etching by tuning the growth parameters. A solar cell was also fabricated using the p-AgGaTe2/n-Si heterojunction. This solar cell showed conversion efficiency of approximately 3%.

A ZnMgTe/ZnTe electro-optic (EO) waveguide has great potential to be utilized for practical applications. A low-dislocation ZnMgTe/ZnTe waveguide can be fabricated when the cladding layer thickness is below 20-fold the calculated critical layer thickness (CCLT x 20). To improve optical confinement, a waveguide with a thicker cladding layer or a higher Mg% should be considered. However, the device performance might be affected because of crystal quality deterioration since the lattice mismatch between MgTe and ZnTe was around 4.1%. In this study, optical confinement and propagation loss were examined by changing the dimensions of the ZnMgTe/ZnTe waveguide structure. The propagation loss, EO characteristics, and crystal quality of the fabricated waveguides were mainly studied. A waveguide with a cladding layer thickness of around 1.5-fold the 1/e penetration depth of the evanescent wave (d(p1/e) x 1.5, corresponding to CCLT x 100) showed better optical properties than other waveguides, although its interface defect density was reasonably high. (C) 2016 The Japan Society of Applied Physics

ZnTe/sapphire heterostructures were focused, and ZnTe thin films were prepared on highly mismatched sapphire substrates by molecular beam epitaxy. A sapphire substrate possessing an atomically-smooth step-terrace structure was used to improve the crystallinity and morphology of the produced ZnTe film. The growth mode of the ZnTe thin film on a sapphire substrate with an atomically-smooth step-terrace structure was found to shift to a two-dimensional growth mode, and a ZnTe thin film possessing a flat surface was obtained. The crystallographic properties of the ZnTe film suggested that the resulting layer consisted of a single (111)-oriented domain. The photoluminescence property was also improved, and the interface lattice alignment between the ZnTe and sapphire was also affected by the atomically-smooth step-terrace structure.

ZnMgTe/ZnTe waveguide is a high potential electro-optical device. Thick and high Mg composition cladding layers are required for high optical performance waveguides. However, adding Mg would increase the lattice mismatch between ZnMgTe and ZnTe which would cause dislocation defects and generate asperities at interfaces. In this article, influence of the lattice mismatch strain to the waveguide surface morphology was studied. Waveguide structures were prepared by molecular beam epitaxy. The surface morphologies were observed using atomic force microscope and propagation loss of the waveguides were studied. Total of 0.5-0.7m asperities were observed on the surface of waveguides with 20% of Mg composition and 1.2m of the total cladding layer thickness. The asperities on the surface became larger (1.0-1.2m) when the Mg composition increased to 40%. The propagation loss of the waveguide was suppressed to 7dB by tuning the growth parameters and the resulting interface roughness.

ZnTe thin films were grown on m-plane sapphire substrates with nano-faceted structure by MBE. The growth process of the ZnTe thin film was analyzed by AFM. The influence of the nano-facet on the crystal quality of the epilayer was studied by means of XRD pole figure and PL measurements. It was confirmed that (331)-plane ZnTe layer was formed on the m-plane sapphire substrate with nano-faceted structure and the ZnTe epilayer grown of the nano-facet structure exhibited the single domain structure with high optical properties.

ZnMgTe(Cladding)/ZnTe(Core)/ZnMgTe(Cladding) thin film waveguide had been grown by molecular beam epitaxy (MBE) and presented a great potential to be a high performance Electro-optical (EO) modulator. For a low propagation loss ZnMgTe/ZnTe waveguide, thick cladding layer with high Mg composition (Mg %) is needed. However, the in-plane lattice mismatch of the fabricated device with high Mg % and thick cladding layer was large. It might cause the cladding/core interface roughness and asperities due to the misfit dislocation, and degrade the device performance. Because EO property of waveguide device is primarily influence by the structure thickness, the device efficiency improvement in this study was only considered to reduce the defects asperities that would cause propagation loss without decreasing the Mg % or the total thickness of the cladding layers. Therefore, we introduced a low Mg % layer between the cladding and the core layer to circumvent the effect of large lattice mismatch. The in-plane lattice mismatch of the devices was monitored using reciprocal space mapping, and surface morphologies were also observed using atom force microscope. The two-step index ZnMgTe/ZnTe waveguide had shown to have lower degree of relaxation compared to that of single-step index waveguide device with close Mg % and cladding layer thickness. Therefore, the crystal degradation of the two-step-index waveguide caused by lattice mismatch was successfully suppressed by introduction extra low Mg % layers. The morphologies of the two kinds of waveguide structures have similar surface asperities, which indicated the extra inserted layers did not produce additional large scale asperities at the interfaces that would increase the propagation loss.

Ag(Ga,Al)Te-2 layers were grown by the closed space sublimation method on c-plane sapphire substrates. The source used was AgAlTe2/AgGaTe2 mixture or AgAlTe2/Ga2Te3 mixture. The crystallographic property of Ag(Ga,Al)Te-2 layers was analyzed by X-ray diffraction (XRD). XRD spectra of layers exhibited very strong 112 diffraction peaks regardless of the variation of the source material mixture. In addition to crystallographic characterizations, optical properties of the Ag(Ga,Al)Te-2 layer were evaluated through transmittance measurements. The bandgap energy was decreased when the source mole ratio of Al to Ga was decreased. It was revealed that control regulation of x composition of Ag(Ga1-x,Al-x)Te-2 was feasible by varying the source mole ratio Al/(Ga+Al). (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnTe epilayers were grown on R-plane (10 (1) over bar4) and S-plane (10 (1) over bar1) sapphire substrates by molecular beam epitaxy, and the crystal orientation and the optical property were studied. The crystal orientation of ZnTe layers on sapphire substrates was studied using X-ray diffraction pole figure measurements. It was confirmed that (111)-oriented domains were formed on the R-plane as well as on the S-plane substrate. Layers grown on R-plane exhibited higher film quality. From the lowtemperature photoluminescence, emissions caused by ZnTe exciton were observed. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cu2ZnSnS4 (CZTS) precursor was fabricated from a CZTS nano crystal ink, which was obtained by a ball-milling method. The precursor was then annealed in the sulfur atmosphere. The growth mechanism of CZTS was studied in this work. It was found that with the extension of annealing time, the grain size as well as the crystallinity was improved significantly. And growth of grains from domain area of CZTS nano particles was observed. The samples experienced a decomposition process during the annealing. Secondary phases, such as ZnS, SnS, CuS and CuSnS3, were detected at the beginning of annealing process. However, with the increase of annealing time, the secondary phases recombined to form uniform CZTS film, which lead to the growth of grain size and improvement of crystallinity. The growth mechanism of CZTS films during the annealing process was concluded as: CZTS(Nanoparticle)->(Cu2S, ZnS, SnxS)-> CZTS (large grains).

The AgGaTe2 layer was formed on Ag2Te/Si structure with two different procedures by eliminating the melt-back etching. Diffusion of the Ga2Te3 source material into the Ag2Te layer and formation of the AgGaTe2 layer were both occurring during the growth when Ag2Te and Ga2Te3 source mixture was used to form AgGaTe2. It was also clear the AgGaTe2 could be formed by deposition and annealing of Ga2Te3 layer on top of the Ag2Te/Si structure. Solar cells were fabricated using the p-AgGaTe2/n-Si heterojunction, and showed conversion efficiency of approximately 3%.

Cu2ZnSn(S,Se)(4) (CZTSSe) is a compound semiconductor which replaces a part of S in the CZTS crystal by Se. The bandgap varies from 1.05 eV to 1.51 eV depending on the mole ratio between S and Se. In this paper, CZTSSe thin films were prepared by the selenidation of CZTS film, and the Se mole ratio was tuned by changing annealing conditions. The film quality of the obtained CZTSSe was characterized by X-ray diffraction (XRD) and Hall measurements. It was revealed that both the supply of Se vapor pressure and the annealing temperature of CZTS were controlling parameters for the selenidation of the film. The crystal quality of CZTSSe was also influenced by the supply of Se vapor pressure.

ZnTe epilayers were grown on transparent a-plane (11-20) sapphire substrates by molecular beam epitaxy (MBE). The insertion of a low temperature nucleated buffer layer was carried out, and the influence of the buffer layer annealing prior to the film growth on crystallographic properties of the epilayer was investigated. Pole figure imaging and wide range reciprocal space mapping (RSM) measurements were used to study the domain distribution within the layer, and epitaxial relationships between the ZnTe thin films and the sapphire substrates. The orientation of ZnTe domains formed on a-plane sapphire substrates was controlled by the annealing condition of the buffer layer. Two different {111} domains were formed from the sample the buffer layer was annealed at 350 degrees C. while (100) layers were obtained from the sample the buffer layer was annealed at 300 degrees C. These crystallographic features were probably originated from the atom arrangements of ZnTe and sapphire at the interface. (C) 2015 Elsevier B.V. All rights reserved.

AgGaTe2, AgAlTe2, and Ag(Ga,Al)Te-2 layers were grown by the closed-space sublimation method on c-plane sapphire substrates. The crystallographic properties of the AgGaTe2 and AgAlTe2 layers were then analyzed by x-ray diffraction (XRD). Very strong 112 diffraction peaks were observed in the XRD spectra of both layers, and it was clear the (112) orientation was predominant in both layers. The orientation and domain structure of the AgAlTe2 layer were carefully analyzed by XRD pole-figure measurement, and the effect of substrate surface atomic arrangement on the crystallinity of the AgAlTe2 layer was investigated. On the basis of the pole-figure image, six types of {112} domain were confirmed in the layer, and [ 10] in one domain was regarded as aligned with the sapphire's a-axis when the layer was formed on the c-plane sapphire substrate surface. In addition to crystallographic characterization, the optical properties of the Ag(Ga,Al)Te-2 layer were evaluated on the basis of transmittance measurements. The optical bandgap, derived from the transmittance spectrum, was approximately 2.0 eV. This value was between the bandgap values of AgGaTe2 and AgAlTe2. These results showed that an alloy of AgGaTe2 and AgAlTe2 had been grown successfully.

ZnTe epilayers were grown on r-plane (1102) and n-plane (1123) sapphire substrates by molecular beam epitaxy. The ZnTe domain distribution in the layer and the influence of the substrates' c-plane location on the orientation of the epilayer were studied by means of X-ray diffraction pole figure measurements. Computer simulation was used to analyze the diffraction patterns. The (100)-plane ZnTe was formed on the r-plane substrate whereas two different {111}-plane mixed with the (511) domain were formed on n-plane substrate, respectively. The orientation of ZnTe layers on r-plane sapphire substrates could not change even by varying the buffer layer growth condition. From these results, it was revealed that the relationship of the sapphire c-plane and the (111) ZnTe strongly affected the orientation of the film. ZnTe layers grown on n-plane sapphire substrates, on the other hand, exhibited different relationships and (111) of ZnTe was not aligned to the sapphire c-plane. (C) 2015 The Japan Society of Applied Physics

The AgGaTe2 layer growth was performed by the closed space sublimation method on the Mo/glass substrate. The Ag2Te buffer layer was inserted between AgGaTe2 and Mo layers, to improve the quality of grown layers. Crystallographic properties were analyzed by x-ray diffraction (XRD), and the surface morphologies were analyzed by scanning electron microscopy (SEM). The Ag2Te layer grown on the Mo/glass exhibited a membrane filter structure from the SEM observation. XRD spectra of layers grown with and without the buffer layer were compared. The AgGaTe2 layer with the Ag2Te buffer layer exhibited peaks originating from AgGaTe2, and a very strong diffraction peak of 112 was observed. On the other hand, it was cleared that the layer grown without the buffer layer exhibited no strong peaks associated with AgGaTe2, but Ga-Te compounds. From this, crystallographic properties of the AgGaTe2 layer were drastically improved by the insertion of the Ag2Te buffer layer. Moreover, the surface morphology exhibited a smooth surface when the Ag2Te buffer layer was inserted. The nucleation site density of AgGaTe2 was probably increased since the membrane filter structure exhibited numbers of kinks at the edge. Chemical reaction between Ga and Mo was also eliminated. It was cleared that the insertion of the buffer layer and its surface morphology were an important parameter to grow high quality AgGaTe2 layers.

AgAlTe2 layers were grown on a-and c-plane sapphire substrates using a closed space sublimation method. Grown layers were confirmed to be single phase layers of AgAlTe2 by X-ray diffraction. AgAlTe2 layers were grown to have a strong preference for the (112) orientation on both kinds of substrates. The variation in the orientation of grown layers was analyzed in detail using the X-ray diffraction pole figure measurement, which revealed that the AgAlTe2 had a preferential epitaxial relationship with the c-plane sapphire substrate. The atomic arrangement between the (112) AgAlTe2 layer and sapphire substrates was compared. It was considered that the high order of the lattice arrangement symmetry probably effectively accommodated the lattice mismatch. The optical properties of the grown layer were also evaluated by transmittance measurements. The bandgap energy was found to be around 2.3 eV, which was in agreement with the theoretical bandgap energy of AgAlTe2. (C) 2014 AIP Publishing LLC.

The electrooptic effect in ZnTe has recently attracted research attention, and various device structures using ZnTe have been explored. For application to practical terahertz wave detector devices based on ZnTe thin films, sapphire substrates are preferred because they enable the optical path alignment to be simplified. ZnTe/sapphire heterostructures were focused upon, and ZnTe epilayers were prepared on highly mismatched sapphire substrates by molecular beam epitaxy. Epitaxial relationships between the ZnTe thin films and the sapphire substrates with their various orientations were investigated using an X-ray diffraction pole figure method. (0001) c-plane, (1-102) r-plane, (1-100) m-plane, and (11-20) a-plane oriented sapphire substrates were used in this study. The epitaxial relationship between ZnTe and c-plane sapphire was found to be (111) ZnTe//(0001) sapphire with an in-plane orientation relationship of [-211] ZnTe//[1-100] sapphire. It was found that the (211)-plane ZnTe layer was grown on the m-plane of the sapphire substrates, and the (100)-plane ZnTe layer was grown on the r-plane sapphire. When the sapphire substrates were inclined from the c-plane towards the m-axis direction, the orientation of the ZnTe thin films was then tilted from the (111)-plane to the (211)-plane. The c-plane of the sapphire substrates governs the formation of the (111) ZnTe domain and the ZnTe epilayer orientation. These crystallographic features were also related to the atom arrangements of ZnTe and sapphire. (C) 2014 AIP Publishing LLC.

AgGaTe2 and AgAlTe2 layers were grown on a-plane sapphire substrates by closed-space sublimation. These compounds replace Cd in CdTe with group I and III elements, and are, hence, expected to be ideal novel candidate materials for solar cells. The grown layers were confirmed to be stoichiometric AgGaTe2 and AgAlTe2 by x-ray diffraction (XRD). The AgAlTe2 layers had strong preference for the (112) orientation. The XRD spectrum of the AgGaTe2 layer was different from that of the AgAlTe2 layer, and strong peaks were observed for (103) and (110) diffraction. The variation in orientations of the grown layers was analyzed in detail by use of XRD pole figures, which revealed that the AgGaTe2 layers had an epitaxial relationship with the a-plane sapphire substrates.

ZnTe epilayers have been grown on 2A degrees-tilted m-plane sapphire substrates by molecular beam epitaxy. Pole figure imaging was used to study the domain distribution within the layer, and the pole figures of 111, 220, 004, and 422 ZnTe and sapphire were measured. Computer simulation was used to analyze the symmetry of the diffraction patterns seen in the pole figure images. Stereographic projections were also compared with the pole figures of 422 and 211 ZnTe, confirming that single-domain (211)-oriented ZnTe epilayers had been grown on the 2A degrees-tilted m-plane sapphire substrates. Although differences in crystal structure and lattice mismatch were severe in these heterostructures, precise control of the substrate surface's lattice arrangement would result in the formation of high-quality epitaxial layers.

ZnMgTe/ZnTe/ZnMgTe thin film waveguide with high crystal quality were grown by molecular beam epitaxy (MBE). The in-plane mismatch between the ZnMgTe cladding layers and ZnTe core layer was about 0.02% which was measured by X-ray reciprocal space mapping (RSM). It indicated that films were grown coherently with high crystal quality. The Electro-Optical characterization of waveguide was evaluated using 1.55 mu m polarized lights and bias applied on the waveguide device from -15 V to + 15 V. The dependence of light phase shift passed though the waveguide on the applied voltage bias was studied. The electro-optical characterization of the waveguide device was about 7% of the theoretical calculation. It could be improved by increasing the resistance ratio between the ZnMgTe/ZnTe/ZnMgTe waveguide structure and substrate so that the electric field applied on the waveguide structure could be improved. It was indicated that the ZnMgTe/ZnTe/ZnMgTe thin film waveguide has the potential to become a high efficiency electro-optical device. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnTe epilayers were grown on transparent (10-10) oriented (m-plane) sapphire substrates by molecular beam epitaxy (MBE). Pole figure imaging was used to study the domain distribution within the layer. (211)-oriented ZnTe domains were formed on m-plane sapphire. The presence of only one kind of (211) ZnTe domain formed on the 2 degrees-tilted m-plane sapphire substrates was con-firmed. Thus, single domain (211) ZnTe epilayers can be grown on the m-plane sapphire using MBE. Although differences in the crystal structure and lattice mismatch are large, precise control of the substrate surface lattice arrangement result in the formation of high-quality epitaxial layers. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

AgGaTe2 layers were grown on a-and c-plane sapphire substrates by a closed space sublimation method with varying the source temperature. Grown films were evaluated by theta-2 theta and pole figure measurements of X-ray diffraction. AgGaTe2 layers were grown to have strong preference for the (103) orientation. However, it was cleared the Ag5Te3 was formed along with the AgGaTe2 when the layer was grown on c-plane sapphire. The orientation of the film was analyzed by using the pole figure, and resulted in AgGaTe2 without Ag5Te3 layers could be grown on a-plane sapphire. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnTe epilayers were grown on transparent (10 (1) over bar0) oriented (m-plane) sapphire substrates by molecular beam epitaxy (MBE). The insertion of a low-temperature buffer layer was carried out, and the influence of the buffer layer annealing on crystallographic properties was investigated. Pole figure imaging was used to study the domain distribution in the layer. It was shown that strongest (211)- and (100)-oriented ZnTe epilayers were formed on m-sapphire when a ZnTe buffer layer annealed at 340 degrees C for 5-min was inserted. Also, it was confirmed that only (211) ZnTe epilayers were formed on the 2 degrees tilted m-plane sapphire substrate. Thus, the single domain (211) ZnTe epilayer can be grown on the m-plane sapphire using MBE. (C) 2014 The Japan Society of Applied Physics

Chalcopyrite I-III-VI2 compounds are considered as novel potential materials for solar cells. Among the variety of those compounds, CuGaTe2 and AgAlTe2 films were grown by a closed space sublimation method. Crystallinity and stoichiometry of grown films were evaluated by X-ray diffraction measurements. When CuGaTe2 powder source was used to grow CuGaTe2 films at around 840 degrees C of the source temperature, stoichiometric CuGaTe2 could not be formed. X-ray diffraction measurements revealed that the deposited material was Cu-Ga compound. The desorption of Te was compensated by adding elemental Te in the source material. These stoichiometric CuGaTe2 films were achieved when the volume of additional Te was around twice weight of CuGaTe2. On the other hand, growth of AgAlTe2 films could be achieved when the source temperature was 800 degrees C, and no additional Te was introduced in the source. From X-ray diffraction measurements, it was found that these two compounds films had strong preference for the (112) orientation on the quartz substrate. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

AgGaTe2 layers were grown on a- and c-plane sapphire substrates by a closed-space sublimation method. Various samples have been prepared with various source temperatures, holding times, and temperature differentials. In this study,. the variation of source temperature was primarily aimed at improving the stoichiometry of the film. The grown films were evaluated by X-ray diffraction (XRD) measurements. When the sample was grown at a high temperature, namely, above 800 degrees C, the formation of Ag-Te compounds was observed. The Ag-Te compounds exhibited a high degree of crystallinity when the layer was grown on c-plane sapphire substrates. By using a pole figure, it was possible to study the orientation of the film, and AgGaTe2 layers were shown to have a preferential orientation in the (103) on a-sapphire. (C) 2014 The Japan Society of Applied Physics

AgGaTe2 layers were deposited on Si substrates by the closed-space sublimation method. Multiple samples were deposited with various source temperatures and holding times, and constant temperature differential. Variation of the source temperature was used primarily to improve the stoichiometry of the film. Deposited films were evaluated by the theta-2 theta method of x-ray diffraction (XRD) and transmission electron microscopy. These results confirmed that the deposited films were stoichiometric (after optimizing the above parameters). From XRD, it was also clear that films deposited on Si (111) have strong preference for (112) orientation.

AgGaTe2 layers were grown on a-plane sapphire substrates by a closed space sublimation method. Various samples were prepared with varied source temperature, holding time and temperature differential. The variation of source temperature was used primarily to improve the stoichiometry of the film. Grown films were evaluated by XRD measurements. AgGaTe2 layers were grown only at low source temperature (about 780 degrees C), and to have strong preference for the (103) orientation. By using the pole figure, it is possible to study the orientation of the film, and the substrate surface arrangement has resulted in this unique relationship. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Single-crystalline and single domain ZnTe thin films are sought for high-performance terahertz wave detectors, and ZnTe/sapphire heterostructures were considered since the Electro-Optical (EO) effect could be obtained only from epilayers. ZnTe epilayers were grown on m-plane sapphire substrates by molecular beam epitaxy, and the potential of single domain epilayers was explored. Through the X-ray diffraction pole figure measurement it was confirmed that one (100) oriented ZnTe domain along with two kinds of (211) oriented domains were formed on the m-plane sapphire when the layer was grown at 340 degrees C. When the layer was grown at 350 degrees C, the (211) oriented domain dominated the film. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnTe epilayers were grown on transparent substrates by molecular beam epitaxy. The insertion of a low-temperature buffer layer was carried out, and the influence of the buffer layer thickness and its annealing on the crystallographic property were investigated. Pole figure imaging was used to study the domain distribution in the layer. It was shown that the (111) ZnTe epilayer with the decreased number of domains could be formed on c-sapphire when a 3.5-nm-thick annealed ZnTe buffer layer was inserted. It was shown that the XRD pole figure imaging was a useful means of analyzing domain distributions in the film. (C) 2012 The Japan Society of Applied Physics

The electro-optical (EO) effect of the ZnMgTe/ZnTe waveguide structure grown on (001) ZnTe single crystal substrates using molecular beam epitaxy (MBE) was studied. The EO properties of ZnTe were investigated by optical confinement with an electric field. The effect of the applied electric field and the rotation of polarization wave effect are discussed. (C) 2012 The Japan Society of Applied Physics

The electro-optical effect of ZnTe is recently highlighted, and various device structures utilizing ZnTe are explored. ZnTe substrates are recently commercially available, and high quality homoepitaxial layers can be grown. On the other hand, the cost of the substrate could be a concern for the practical device application. In this study, Si and Al2O3 substrates were used since they are widely used for many kinds of device applications. The lattice mismatch between those substrate materials and ZnTe is above 10% and those mismatch strain should be carefully considered. ZnTe layers were nucleated on chemically treated substrate surfaces. (111), (110), (211) oriented Si substrates and (0001), (11-20), and (10-10) planes of Al2O3 were used. By taking the pole figure data and studying the crystal symmetry of the domain, detail information associated with the formation of domains in the layer was discussed. The pole figure analysis revealed that several kinds of (110) oriented ZnTe domains rotated 60 degrees each other were formed on (110) oriented Si surface. On the other hand, preferentially oriented single domain of (111) was confirmed for the layer grown on the (0001) plane of Al2O3. ZnTe layers grown on (10-10) plane Al2O3 exhibited a unique domain formation. Pole figure data combined with the conventional theta-2-theta measurement indicated that (211) oriented plane of ZnTe was a dominant domain formed on the (10-10) plane Al2O3. These crystallographic features would be related to the interface valency as well as the interfacial bond structure. X-ray pole figure method was a useful and powerful method to study the domain formation of the severely lattice mismatched structures. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ba2ZnS3:Mn (BZS), SrGa2S4:Eu, and BaAI(2)S(4):Eu nanoparticles phosphor materials were prepared by a break down method, namely the ball-milling method. Several-nanometer-size stoichiometric and dispersed nanoparticIes were achieved. Red color phosphor material, namely BZS, was sprayed on the glass substrate. Mn doped BZS absorbs ultra violet light and emits red light peaking at around 640nm. The transparent nanoparticle layer absorbed short wavelength light and converted to long wavelength light for the Si solar cell application.

ZnMgTe/ZnTe/ZnMgTe layered structures were grown on (0 0 1) ZnTe substrates by molecular beam epitaxy. This structure was designed to apply to waveguides in various optoelectronic devices to reduce light loss. Since the lattice mismatch between ZnTe and ZnMgTe was not negligible, the critical layer thickness (CLT) was theoretically derived. Structures with varying Mg composition and layer thickness of ZnMgTe cladding layer were grown and examined for crystal quality with respect to theoretical data. The crystal quality was investigated by means of cross sectional transmission electron microscopy (TEM) and reciprocal space mapping (RSM). Optical confinements were observed by irradiating a laser beam from one end of the sample and monitoring the transmitted light from the other end. (C) 2011 Elsevier B.V. All rights reserved.

Conventional phosphor materials are doped ternary or quaternary compounds
hence it would be difficult to prepare nanoparticles of those materials by build up methods. Ba2ZnS3:Mn (BZS), SrGa2S 4:Eu, and BaAl2S4:Eu nanoparticles were prepared by a break down method, namely the ball-milling method. Transmission electron microscopy (TEM) and TEM- energy-dispersive X-ray spectroscopy (EDX) measurements showed several-nanometer-size stoichiometric and dispersed nanoparticles were achieved. ZnO-coating was performed and the uniform coating layers were formed on the phosphor nanoparticles. The ZnO-coated nanoparticles exhibited an improved stability in Photoluminescence. Red color phosphor material, namely BZS, was ball-milled and sprayed on the glass substrate. Mn doped BZS absorbs ultra violet light and emits red light peaking at around 640nm. When the single crystal Si solar cell was placed under the transparent nanoparticle layer, short wavelength light was absorbed and converted to long wavelength light. © 2012 Materials Research Society.

ZnMgTe/ZnTe layered structures were grown on ZnTe substrates by molecular beam epitaxy, and the crystal structures were characterized using X-ray diffraction methods. This structure would be the waveguide for various optoelectronic devices. Therefore, the crystal quality of this layered structure would be very crucial for the realization of high performance devices. ZnMgTe is lattice mismatched to ZnTe, and the increase of the ZnMgTe layer thickness or Mg mole fraction ratio would result in the crystal quality deterioration of the layered structure. The critical layer thickness (CLT) was theoretically derived, and various structures with various ZnMgTe layer thickness and Mg mole fraction were grown. The lattice mismatch strain relief and crystal quality of those samples were investigated by means of X-ray reciprocal space mapping (RSM) and cross sectional transmission electron microscopy (TEM). The dislocation formation and the lattice mismatch relaxation were confirmed for various samples and it was revealed that the calculated CLT values could be used as an appropriate guideline to design the dislocation free and high performance device structures. © 2010 Wiley-VCH Verlag GmbH &amp
Co. KGaA.

ZnMgTe/ZnTe layered structures were grown on ZnTe substrates by molecular beam epitaxy, and the crystal structures were characterized using X-ray diffraction methods. This structure would be the waveguide for various optoelectronic devices. Therefore, the crystal quality of this layered structure would be very crucial for the realization of high performance devices. ZnMgTe is lattice mismatched to ZnTe, and the increase of the ZnMgTe layer thickness or Mg mole fraction ratio would result in the crystal quality deterioration of the layered structure. The critical layer thickness (CLT) was theoretically derived, and various structures with various ZnMgTe layer thickness and Mg mole fraction were grown. The lattice mismatch strain relief and crystal quality of those samples were investigated by means of X-ray reciprocal space mapping (RSM) and cross sectional transmission electron microscopy (TEM). The dislocation formation and the lattice mismatch relaxation were confirmed for various samples and it was revealed that the calculated CLT values could be used as an appropriate guideline to design the dislocation free and high performance device structures. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

SrGa2S4:Eu has been recently widely studied. Annealing treatment in a vacuum and a H2S atmosphere is used for the improvement of the crystallinity and optical properties. H2S annealed nanoparticles showed better characterizations than vacuum annealed ones. Good samples in optical property indicate also good data in crystallinity.

Conventional phosphor materials are doped ternary or quaternary compounds; hence it would be difficult to prepare those nanoparticles by build up methods. Ba(2)ZnS(3):Mn, SrGa(2)S(4):Eu, and BaAl(2)S(4):Eu nanoparticles were prepared by a break down method, namely ball-milling method. Transmission electron microscopy (TEM), and TEM-energy-dispersive X-ray spectroscopy (EDX) measurements showed several-nanometer-size stoichiometric and dispersed nanoparticles were achieved. The annealing was performed after the milling for those particles to cure the mechanical damages. The annealing in a vacuum at around the calcination temperature of starting materials showed the cured photoluminescence (PL). ZnO-coating was performed and the uniform coating layer of the thickness about 10 nm was formed on the phosphor nanoparticles. The ZnO-coated nanoparticles exhibited an improved stability in PL. (C) 2009 The Japan Society of Applied Physics

ZnS:Mn2+ nanoparticles and other sulpher-based ternary compound nanoparticles were achieved using a ball-milling method. Several-nanometer-sized ZnS:Mn2+ nanoparticles were achieved by comminuting materials in a solvent. Both X-ray diffraction (XRD) and transmission electron microscopy (TEM) suggested that the particles had been milled down to the nanometer size. TEM-energy-dispersive X-ray spectroscopy (TEM-EDX) suggested that the stoichiometry of the starting powder was not affected by the ball-milling process. The orange-colored luminescence originating from Mn2+ was improved by reducing the particle size to several nanometers. Red, green and blue (RGB) colored phosphors were also studied using this technique. Ba2ZnS3:Mn (Red), SrGa2S4:Eu (Green), BaAl2S4:Eu (Blue) and phosphor materials with sizes of about several micrometers were comminuted to nanoparticles without affecting the crystal structure and the stoichiometry. Bright red, green and blue luminescence were confirmed from those nanoparticles.

The co-doping technique known as an effective method to circumvent the dopant compensation problem was applied for the molecular beam epitaxy (MBE) growth of homoepitaxial ZnTe layers. The co-doping concept is to introduce two oppositely polar atoms at the same time in a 2:1 ratio, forming metastable three-atom complexes located at adjacent crystal sites. Al donor and N acceptor were sandwiched between undoped ZnTe spacing layers. In order to reduce the beam intensity of the RF plasma excited nitrogen species, N-2 gas was diluted by Ar gas. From the low temperature PL spectra, the increase of the Al doping efficiency was confirmed for co-doped layers, especially co-doped using the diluted N-2 gas. (c) 2007 Elsevier B.V. All rights reserved.

3-D branching GaN nanowires have been grown using the intermediate and Ga-rich growth regimes of plasma assisted molecular beam epitaxy. Evidence that the growth is due to an auto-catalytic VLS process is obtained through SEM images showing droplet termination heads, the composition of which is essentially pure Ga. TEM analysis revealed a defect free crystal structure, even in the trunk to branch junction. Cathodoluminescence from the trunk of the branching nanowires produced a strong luminescence feature at 3.44 eV, while a slight decrease in energy to 3.1 eV was observed at the interface between the nanowire and epilayer or kink site. No yellow luminescence was detected, further suggesting a defect free growth. Preliminary I-V measurements give mixed results, suggesting intrinsic n-type nanowires. © 2007 Materials Research Society.

In order to get around the high and activated n-doping levels for ZnTe, the co-doping technique known as effective method to overcome the dopant compensation was explored. The co-doping concept is to introduce two oppositely polar atoms at the same time in a 2:1 ratio, forming metastable three-atom complexes located at adjacent crystal sites. In this study, two types of deposition process were considered to place the co-dopants at the intended site, and the co-doped ZnTe:(Al, N) layers were grown on (001) oriented ZnTe substrates by molecular beam epitaxy. From the low temperature PL spectra, the increase of Al activation was confirmed by using co-doping sequences, and the activation of N dopant was also confirmed for one doping sequence even thought the doping level was maintained the same. © 2006 WILEY-VCH Verlag GmbH &amp
Co. KGaA,.

ZnSe/MgCdS and ZnCdS/MgCdS superlattices were grown on semi insulating (001) oriented GaAs substrates by molecular beam epitaxy (MBE). The crystal quality and optical properties were examined for both superlattices. The observed photoluminescence (PL) peak intensity of ZnCdS/MgCdS was much stronger than that of ZnSe/MgCdS while ZnSe/MgCdS showed sharper luminescence line width. The PL peak energy position was compared with the theoretical value derived from the Kronig-Penny model. The degradation of crystal quality was observed by increasing Mg content in MgCdS layer and drastic PL property degradation was observed when the Mg content in the layer exceeded a certain value. Two superlattices were applied to the UV-A sensor of the metal-semiconductor-metal (MSM) configuration. Both photodetectors exhibited a high sensitivity in the UV-A region with a sharp cut-off in the visible wave-length region
the ON-OFF ratio of sensor was about 103 for both structures. © 2006 WILEY-VCH Verlag GmbH &amp
Co. KGaA,.

In order to get around the high and activated n-doping levels for ZnTe, the co-doping technique known as effective method to overcome the dopant compensation was explored. The co-doping concept is to introduce two oppositely polar atoms at the same time in a 2:1 ratio, forming metastable three-atom complexes located at adjacent crystal sites. In this study, two types of deposition process were considered to place the co-dopants at the intended site, and the co-doped ZnTe:(Al, N) layers were grown on (00 1) oriented ZnTe substrates by molecular beam epitaxy. From the low temperature PL spectra, the increase of Al activation was confirmed by using co-doping sequences, and the activation of N dopant was also confirmed for one doping sequence even thought the doping level was maintained the same. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

ZnSe/MgCdS and ZnCdS/MgCdS superlattices were grown on semi insulating (001) oriented GaAs substrates by molecular beam epitaxy (MBE). The crystal quality and optical properties were examined for both superlattices. The observed photoluminescence (PL) peak intensity of ZnCdS/MgCdS was much stronger than that of ZnSc1MgCdS while ZnSe/MgCdS showed sharper luminescence line width. The PL peak energy position was compared with the theoretical value derived from the Kronig-Penny model. The degradation of crystal quality was observed by increasing Mg content in MgCdS layer and drastic PL property degradation was observed when the Mg content in the layer exceeded a certain value. Two superlattices were applied to the UV-A sensor of the metal-semiconductor-metal (MSM) configuration. Both photodetectors exhibited a high sensitivity in the UV-A region with a sharp cut-off in the visible wavelength region; the ON-OFF ratio of sensor was about 10(3) for both structures. (c) 2006 WILEY-VCH Veriag GmbH & Co. KGaA, Weinheim.

The application of visible blind ultraviolet (UV)-A sensors was extensively studied for wide-bandgap II-VI compound epitaxial layers. The growth of ZnMgCdS quaternary alloys and MgCdS/ZnCdS short period superlattices were performed. and UV-A sensors were fabricated. The mole-fraction control of the tertiary alloy would be much easier than that of the quaternary alloy, and various structures with certain bandgap energy can be designed. The cross-sectional TEM observation has revealed that well-defined superlattice structures could be achieved. Low-temperature photoluminescence (PL) exhibited that the intense and sharp luminescence peak was observed from superlattice samples compared with quaternary alloy layers having the similar bandgap energy. The PL peak position could be controlled by changing the layer thickness as well as the mole-fraction ratio of the alloy. The PL peak energy positions agreed well with theoretically predicted values. After the film growth, Au electrodes were evaporated and photo-sensitivity was characterized for metal-semiconductor-metal configurations. Sharp cut-off profiles were observed for samples, and its position was consistent with the PL peak position. The ON-OFF ratio of the device was similar to that of the device using the quaternary alloy layers. (c) 2005 Elsevier B.V. All rights reserved.

The influence of lactic acid or porphyrins on the optical properties of tissue fluorophores is investigated by autofluorescence (AF) spectroscopy measurement with a GaN-based ultraviolet laser diode along with Fourier-transform IR (FTIR) spectroscopy measurement. As the lactic-acid concentration becomes dense, the AF peak intensity from elastin and desmosine solutions become wholly weak. A similar reduction in the AF intensity is observed for nicotinamide adenine dinucleotide (NADH) solutions. FTIR analysis indicates that the lactic acid causes the conformational change in elastin and the oxidation of NADH, which can be related to changes in the AF properties. The peak intensity of the tissue fluorophores also becomes weak when porphyrins are added, although the conformational change in each tissue fluorophore is not confirmed from FTIR analysis. Judging from the change in the scattering-light intensity of the excitation source, the observed change mainly originates from the absorption of the excitation source by porphyrins. (C) 2004 Optical Society of America.

Zincblende ZnMgCdS quaternary alloy layers were grown by molecular beam epitaxy. The ZnMgCdS alloy could lattice match to GaAs and could exhibit the band-gap energy of approximately 3 eV. Zn, Mg, and CdS were used as source materials. Incorporation of Mg was more effective compared with Zn in the alloy. Stabilizing the substrate temperature especially at the initial growth stage was crucial to obtain high quality layers. The epilayer was applied to the metal-semiconductor-metal photodetector, and the rejection rate above 10(3) have bean achieved. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Autofluorescence (AF) measurement of human bronchus tissues are practically used for the detection of early stage lung cancers. Problems associated with the interference between the excitation light and the AF signal and the lack of the precise analysis of the AF characteristics limit the capability of the technique. In this study, we used a ultraviolet laser diode (UV-LD) and a ultraviolet light emitting diode (UV-LED) as the alternative light source for the AF measurement. An AF analysis system that can analyze the spectroscopic data of about 5mm × 5mm with a spatial resolution of about 10 micro meters were established. The AF spectrum was measured for 512 × 512 pixels and the intensity mapping as a function of a certain emission wavelength was obtained. The numerical calculation of the data (including differential imaging and the chromaticity diagram projection) was performed and the precise feature of the AF was revealed. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

Zincblende ZnMgCdS quaternary alloy layers were grown by molecular-beam epitaxy. The ZnMgCdS alloy lattice matched with GaAs and exhibited the band-gap energy of approximately 3 eV. Zn, Mg, and CDs were used as source materials. Cd was replaced by Zn or Mg, and the alloy composition was controlled when the growth temperature was set to approximately 210degreesC. Low-temperature photoluminescence showed dominant band-edge features indicating the high quality of the epilayer. The epilayer was applied to the metal-semiconductor-metal photodetector, and visible-blind UV sensing characteristics were observed.

Recent success in the fabrication of the bulk ZnTe has enabled the realization of large area single crystal substrates. The growth and application of ZnTe homoepitaxial layers by molecular beam epitaxy should involve the pretreatment of the substrate surface prior to the nucleation. An atomic hydrogen treatment was performed to remove the surface oxide of ZnTe. The oxide could be removed at around 100degreesC with the assist of the atomic hydrogen beam. The reconstructed surface could be achieved by annealing at around 230degreesC. The two dimensional nucleation was also confirmed. The homoepitaxial layer has shown such a high quality that interface, or defects could not be confirmed by the transmission electron microscopy. (C) 2003 American Vacuum Society.

Ultraviolet laser diodes (UV-LD) were used for the excitation source of the autofluorescence (AF) measurements and spectroscopic imaging of the AF originated from the human bronchus was obtained. The AF spectra from the normal bronchus tissues were measured and the clear AF spectrum was obtained by using a short wavelength (400 nm) laser diode; the overlap of the AF signal and excitation source could be substantially eliminated. In order to study the origin of the AF intensity deterioration from the bronchus tissue due to the formation of the tumor tissues, the fluorescence spectrum was measured for various AF substances under various conditions. The blue AF signal of the elastin and NADH solutions which could not be easily studied by the conventional excitation light sources as well as the green AF became weak by adding the lactic acid. The AF spectrum was measured for 512 x 512 pixels and the intensity mapping as a function of the emission wavelength was obtained. Two-dimensional information of the AF signal intensity distribution for a certain wavelength component was measured. The feature originated from the region as small as about 100 mum could be recognized. The numerical calculation of the data was performed and the precise feature of the AF was revealed.

A GaN based ultraviolet (UV) laser diode (LD) was used to study the autofluorescence (AF) spectrum of the normal and tumor human bronchial tissues under ex vivo conditions. The UV LD generates a coherent short wavelength (around 400 nm) light beam with an intensity of about a few watts. AF spectrum data can be obtained without interference by excitation light. A clear blue peak located at around 483 nm was observed along with a green peak at around 560 nm in the normal tissue. The peak intensities observed were very weak for the tumor tissues. The AF imaging and spectrum analysis were performed along with a histopathological study. The spatial distribution of the elastin in the bronchial tissue affected the intensity of the AF whereas the spectrum shape was not affected. Strong AF was observed from regions that include a high density of the elastin. Biopsy measurements were performed for ex vivo samples, and depth profiling of the elastin was studied along with variations of the AF spectrum. AF spectra excited by the UV LD for fluorescence materials including FAD, NADH, and elastin were measured. The spectrum shape of the elastin as well as of NADH was similar to that of normal bronchial tissues. (C) 2002 Society of Photo-Optical Instrumentation Engineers.

Thermal diffusion of Se was performed in ZnTe substrates and the resulting material was characterized by TEM-EDS and X-ray reciprocal space mapping methods. ZnSe layers were formed on the surface of ZnTc substrates, and ZnSeTe ternary alloy layers with a certain alloy fraction region as well as a graded alloy fraction region were formed. The crystal quality of ZnSe and ZnSeTe region were affected by the diffusion conditions. High temperature diffusion and the use of H-2 carrier gas have resulted in the degradation of the bulk ZnTe region.

Purpose. The GaN-based ultraviolet laser diode (UV-LD), which has recently became commercially available, was used to study the autofluorescence (AF) of human bronchial tissue. The conventional AF measurement system uses a large excitation light source, and the measured data have problems associated with the intermixing of the excitation source and the AF signal. The wavelength of the UV-LD is much shorter than that of the He-Cd laser which is employed for the LIFE system, and the AF measurement is performed without interference of the excitation light using the UV-LD. AF data were compared with those obtained using the He-Cd laser. Method. The spectroscopic analysis of the AF was performed for 13 resected specimens of the human bronchus using the UV-LD. The relationship between the origin of the AF signal and the histopathological structure was also studied. Result. The AF characteristics were similar to those obtained using the He-Cd laser or the xenon lamp. Blue AF, the peak wavelength of which was at around 483 nm was additionally observed from normal tissues. The spatial distribution of the AF signal intensity revealed that a strong AF signal obtained from the region where the high density of the elastin was confirmed. Conclusion. The AF measurement in a wide wavelength region can be easily performed by using the UV-LD as the excitation light source.

Self-assembled nanocrystalline dots of CdS grown within a ZnSe host have been studied by steady state and transient optical spectroscopies. This material system features an unusually low density of the dots, into which the excitation transfer of excitons from the host has been identified. (C) 2000 American Institute of Physics. [S0021-8979(00)01921-6].

Cubic (zinc-blende) InGaN/GaN double-heterostructure LEDs were fabricated on GaAs (001) substrates. The device performance and crystal quality were investigated. The emission wavelength was controlled by the In content in the cubic InGaN active layer. The violet-blue electroluminesence was observed around 435 nm with a FWHM of 55 nm from a cubic In0.07Ga0.93N/GaN DH LED. The forward voltage was 4.9 V at 20 mA and the reverse leakage current was 5 mA at -10 V. X-ray reciprocal space mapping measurement was performed to investigate the phase purity and strain in InGaN/GaN heterostructure. The mixing of the stable hexagonal phase in the cubic GaN was observed and the hexagonal phase content was about 10%. In-situ spectroscopic ellipsometry measurement showed that most of the mixed hexagonal domains were likely to be formed in the Mg-doped GaN layer. In addition, the anisotropic lattice relaxation occurred in the InGaN active layer. The elimination of the hexagonal phase inclusions plays an important role for the realization of high performance devices.

Atomic scale characterization of cleaved surfaces of cubic GaN (c-GaN) epilayers was established in real space. Using cross-sectional scanning tunneling microscopy (XSTM), c-GaN epilayers grown on GaAs (001) by low pressure MOVPE were investigated. The analysis of STM observations revealed that the atomic arrangement of the c-GaN (1 (1) over bar 0) surface depended on the crystal quality of the c-GaN epilayer. For high quality c-GaN regions, the STM image confirmed that the c-GaN (1 (1) over bar 0) surface was a 1 x 1 reconstruction in analogy to GaAs. On the other hand, for poor quality c-GaN regions, a structure different from the 1 x 1 reconstruction was observed, which was the disordered structure composed of atomic rows along the growth direction. Tn addition, stacking faults parallel to the c-GaN (111) plane, which gave rise to hexagonal GaN inclusions in the c-GaN epilayer, were characterized in real space.

Cubic (zinc-blende) InGaN/GaN double-heterostructure LEDs were fabricated on GaAs (001) substrates. The device performance and crystal quality were investigated. The emission wavelength was controlled by the In content in the cubic InGaN active layer. The violet-blue electroluminescence was observed around 435 nm with a FWHM of 55 nm from a cubic In0.07Ga0.93N/GaN DH LED. The forward voltage was 4.9 V at 20 mA and the reverse leakage current was 5 mA at -10 V. X-ray reciprocal space mapping measurement was performed to investigate the phase purity and strain in InGaN/GaN heterostructure. The mixing of the stable hexagonal phase in the cubic GaN was observed and the hexagonal phase content was about 10%. In-situ spectroscopic ellipsometry measurement showed that most of the mixed hexagonal domains were likely to be formed in the Mg-doped GaN layer. In addition, the anisotropic lattice relaxation occurred in the InGaN active layer. The elimination of the hexagonal phase inclusions plays an important role for the realization of high performance devices.

Self-assembled quantum dots (QDs) of CdS were grown on ZnSxSe1-x. Low-temperature photoluminescence (PL) properties of CdS QDs were studied and a blue shift was observed when the S composition in ZnSxSe1-x was increased. This shift is related to the band line-up between ZnSxSe1-x and CdS; the band line-up is type-II. Blue-light-emitting diode (LED) structures embedded in CdS QD layers in the pn junction of ZnSe or ZnSxSe1-x were fabricated. I-V characteristics were compared for two device structures. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

The realization of wurtzite structure II-VI material would overcome the problem associated with the lifetime of the devices. ZnCdS epilayers were grown on GaAs(0 0 1) and(1 1 1)B substrates, and crystal structure was controlled, ZnCdS eyilayers grown on GaAs(0 0 1) substrates showed slight signs of the wurtzite phase. Phi (phi) scan of the X-ray diffraction was performed, and ZnCdS epilayers grown on GaAs(1 1 1)B substrates showed mixed crystal structures. The substrate temperature and the Cl doping would affect the volume ratio of the wurtzite structure. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

CdS quantum dots (QDs) were formed on ZnSe and ZnSxSe1-x. The nucleation process was monitored by reflection high energy electron diffraction, and it was suggested that QDs were formed without the wetting layer, The low temperature (13 K) photoluminescence (PL) measurement showed that the peak energy could be controlled by the amount of the CdS deposit. The PL peak energy covered most: of visible range, indicating that the band lineup between CdS QD and ZnSe (or ZnSxSe1-x) host would be type II. Bright green and blue light emitting diode (LED) structures were fabricated in which CdS QDs were embedded in pn junctions of ZnSe and ZnSxSe1-x. The control of QD size as well as the band gap of the host material enabled the luminescence peak wavelength of the LED to be varied. (C) 2000 American Vacuum Society.

A new candidate for long lifetime short-wavelength light emitters made of hexagonal II-VI compounds has been proposed. In this paper, the results of theoretical design of ZnMgCdOSSe-based light emitters fabricated on (111) plane of GaAs and InP substrates is presented. The band lineups of CdZnS/ZnSSe and CdZnS/ZnMgS structures were estimated with Harrison's LCAO theory. Further, as for the first step of an experimental investigation on the growth of high phase-quality hexagonal II-VI compounds, growth of CdZnS epilayers were examined on GaAs(111) substrate. The epilayers were characterized by high-resolution X-ray diffraction reciprocal space and pole figure measurements. In this study, the optimum growth temperature was 300°C for hexagonal phase CdZnS, and the inclusion of cubic phase CdZnS drastically decreased with increasing Cd content at 300°C.

There is a great deal of interest in thin firm deposition techniques which can achieve good crystal quality at low substrate temperatures. Pulsed laser deposition (PLD), well-known as a reliable technique for fabrication of high critical temperature superconductor thin films, has a number of characteristics which may make it. suitable for such applications. In particular, PLD is characterized by a relatively large average species energy, which can be controlled by the laser fluence at the target. This paper describes the growth of silicon on silicon films using PLD over substrate temperatures between 500 and 700 degreesC, and in-situ characterization using reflection high-energy electron diffraction (RHEED). Transmission electron microscopy confirms the growth of single crystal oriented films, and atomic force microscopy indicates smooth films with an rms surface roughness of less than 2 Angstrom.

Inclusion of hexagonal GaN (h-GaN) phase domain associated with the mosaicity of the cubic GaN (c-GaN) phase have been investigated for c-GaN epilayer grown on (001) GaAs by rf-MBE. In the c-GaN epilayer with 10 % h-GaN phase, two diffraction peaks near (002) c-GaN were observed in the X-ray reciprocal space Mapping. One is the strong and narrow peak arising from the pure c-GaN phase and the other is the weak and broad peak arising from the secondary c-GaN phase with mosaicity. The broad peak showed its peak shift from the calculated diffraction angle, i.e., the tilt of c-GaN [001] axis is attributed to the secondary phase c-GaN domain. The diffraction peak for (10-11) h-GaN also showed its peak shift from the calculated diffraction angle. Therefore, it is considered that the h-GaN phase domain was preferentially included in the secondary phase c-GaN domain.

The luminescence properties of self-assembled CdS quantum dots (QDs) were studied. CdS QD structures formed on ZnSe buffer layers without capping layers showed intense photoluminescence (PL). The PL peak position could be controlled by changing the amount of CdS deposited. Introduction of the capping layer enhanced the PL intensity. The buffer layer material selected also affected the luminescence properties of the CdS QDs, and brighter PL with the higher transition energy was observed by replacing the ZnSe buffer layer with ZnSSe. A light emitting diode structure based on CdS QDs sandwiched in the pn junction of ZnSe was fabricated and bright blue or green luminescence from current injection was observed at 77 K as well as at room temperature. © 1999 American Vacuum Society.

Cubic InGaN/GaN heterostructures were grown on GaAs(001) substrates by MOVPE. X-ray reciprocal space mapping (RSM) measurements were used to investigate the structural defects. It was revealed that the cubic InGaN layers contained high density stacking faults parallel to the cubic InGaN {111} planes, and hexagonal domains with their c-axes parallel to the cubic InGaN [111] directions. The hexagonal domains were preferentially oriented to the-cubic InGaN [111]A directions rather than the [111] B directions. It was clarified that the cubic InGaN layer was anisotropically strained; and this would be originated from the anisotropic mixing of the hexagonal phase in the cubic InGaN epilayers.

ZnSe substrate surfaces were treated by the citric acid-based etchant. Homoepitaxial layers of ZnSe grown by MBE using this etching treatment showed excellent film qualities based on X-ray rocking curve and transmission electron microscopy observations. The electrical properties of the n-ZnSe/n-ZnSe interface prepared using this etchant was studied, and the C-V measurement showed that a good electrical interface was formed. (C) 1999 Elsevier Science B.V. All rights reserved.

We found that the direction of the [001] axis of the c-GaN epilayer grown on (001) GaAs was slightly tilted towards the direction of the N-beam during the epitaxial growth by rf-MBE, where the N-beam was incident obliquely to the surface normal. When the tilt of the c-GaN [001] axis was observed, the typical X-ray rocking FWHM value for (002) c-GaN was about 60 arcmin. This indicates that the large mosaicity is related to the tilt of the c-GaN [001] axis. Because the tilt is easily observed for the c-GaN epilayers grown at high growth rate, this implies that the large amount of N species on the growing surface is the origin of the tilt of c-GaN [001] axis. As for the inclusion of the h-GaN phase domain, we found that the peak shifts for the (1011) h-GaN planes occurred in accordance with the peak shift of (002) c-GaN in reciprocal space mappings.

Quantum dots are spectroscopically investigated based on CdS and ZnSe heteroepitaxy by molecular beam methods. The lattice mismatch between CdSe and ZnSe is about 3%, a relative modest value. Another unusual characteristic is that MBE grown CdS layers show a metastable crystallographic feature.

X-ray diffraction reciprocal space maps and pole figures were used to analyze the cubic GaN epitaxial layers grown on (0 0 1) GaAs by r.f. plasma source MBE
the presence of hexagonal phase in cubic GaN layers was detected by high resolution x-ray analysis and the relationships among various crystal axes of cubic and hexagonal phase GaN were analyzed with respect to V/III source-supply ratio. As for the growth conditions of the epitaxial layers, the V/III ratio was found to drastically affect the quality of the layers. High-temperature growth under near-stoichiometric conditions was necessary to obtain high quality epitaxial layers. It was found that inclusion of the hexagonal phase in the cubic GaN layers could be less than 0.4%, though previously reported typical c-GaN epitaxial layers included as much as 10-20% hexagonal phase GaN. On the basis of the measurements and analyses of reciprocal space maps and pole figures, it was revealed that the orientation of crystal axes of the hexagonal phase was unique in the present GaN epitaxial layers and they were different from those of previously reported c-GaN epitaxial layers.

Self-assembled quantum dots of CdS were introduced into the pn junction of ZnSe. Bright luminescence was observed at 77 K and room temperature. The peak wavelength can be tuned by altering the dimensions of the quantum dot structure.

A two-step atomic-hydrogen (atomic-H) treatment of GaAs substrate was investigated; this includes low-temperature cleaning and high-temperature smoothening of GaAs (001) substrate surface. It was found that atomically flat GaAs surface with one monolayer-height steps and ragged step edges could be obtained by the atomic-H treatment at high temperatures. Further, growth of high quality cubic GaN (c-GaN) on atomic-H treated GaAs (001) was examined by rf plasma-assisted MBE. The c-GaN epilayers were characterized by high resolution X-ray diffraction analysis. The results show that the FWHM of the X-ray rocking curve for the (002) c-GaN was as small as 70-90 arcsec and the inclusion of hexagonal GaN phase was about 0.4% (or below). This indicates that the atomic-H irradiation treatment of the GaAs substrate was an efficient method obtaining High quality c-GaN.

A two-step atomic-hydrogen (atomic-H) treatment of GaAs substrate was investigated; this includes low-temperature cleaning and high-temperature smoothening of GaAs(001) substrate surface. It was found that atomically flat GaAs surface with one monolayer-height steps and ragged step edges could be obtained by the atomic-H treatment at high temperatures. Further, growth of high quality cubic GaN (c-GaN) on atomic-H treated GaAs(001) was examined by rf plasma-assisted MBE. The c-GaN epilayers were characterized by high resolution X-ray diffraction analysis. The results show that the FWHM of the X-ray rocking curve for the (002) c-GaN was as small as 70-90 arcsec and the inclusion of hexagonal GaN phase was about 0.4% (or below). This indicates that the atomic-H irradiation treatment of the GaAs substrate was an efficient method obtaining High quality c-GaN.

A citric acid based etchant was applied to ZnSe. The etching rate could be controlled by adjusting the mixture ratio as well as the etchant temperature. By choosing an appropriate mixture ratio and temperature, a surface smoothness of ZnSe after citric acid etching was as good as those of molecular beam epitaxy (MBE) grown ZnSe epilayers. Homoepitaxial layers of ZnSe grown by MBE using this etching treatment showed excellent film qualities based on X-ray rocking curve, cross-sectional transmission electron microscopy, and low-temperature photoluminescence measurements.

Substrate surface treatment techniques and growth conditions are the key of growing high-quality cubic GaN (c-GaN) epitaxial layers on (0 0 1) GaAs substrates. An atomically flat (0 0 1) GaAs substrate surface can be obtained by a two-step atomic hydrogen (atomic-H) irradiation technique. The typical surface of the substrate treated by the atomic-H consists of narrow terraces of one monolayer step height with acute-angle polygonal structures. The X-ray rocking curve FWHM of GaN (0 0 2) grown on the atomic-H treated substrate is less than 90 arcsec. It is shown that high-density monolayer steps and kinks at the surface terrace edge would be preferable for the uniform nucleation of the c-GaN buffer layer. (C) 1998 Elsevier Science B.V. All rights reserved.

Initial growth stages of cubic GaN (c-GaN) films on GaAs(0 0 1) in low-pressure MOVPE were studied by spectroscopic ellipsometry. The GaN buffer layer was deposited at 500 degrees C and was then annealed at 700 degrees C with H-2/nonomethylhydrazine (MMHy) ambient. Spectroscopic ellipsometry revealed that the buffer layer thickness was increased and the surface was roughened during the annealing process. The change of surface morphology was also confirmed by the atomic force microscopy measurement, These observations would be associated with the nitridation of the GaAs substrate by the ambient MMHy and re-crystallization of the GaN buffer layer. Spectroscopic ellipsometry is a helpful technique to study the initial growth stages of GaN films on GaAs substrates. (C) 1998 Elsevier Science B.V. All rights reserved.

Cubic GaN epilayers were grown on atomic hydrogen treated (0 0 1)GaAs substrates by RF-radical source molecular beam epitaxy. The crystalline quality was characterized by a high-resolution X-ray diffractometer. It was found that high-quality c-GaN can be grown at temperatures above 680 degrees C; the FWHM of (0 0 2)GaN rocking curve was as small as 80-90 arcsec. In order to further precisely investigate how and to what extent the h-GaN phase is included in the c-GaN layer, the X-ray reciprocal space maps were measured for the X-ray beams incident along both [1 1 0] and [1 (1) over bar 0] azimuths. It was found that the stacking faults exist predominantly on {1 1 1}A planes. The inclusion of h-GaN was estimated by comparing the integrated XRD intensities for h-GaN {1 0 (1) over bar 1} and c-GaN(0 0 2) planes. It was found that the inclusion of h-GaN phase drastically decreased with increasing growth temperature above 680 degrees C to reach about 4 x 10(-3) (or below) at temperatures of 710-740 degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.

CdS quantum dot (QD) structures were grown by molecular beam epitaxy on (001) ZnSe. Circular QDs were observed from the sample, grown at 220 degrees C, whereas rectangular QDs were observed from the sample grown at 280 degrees C. The difference of the dot shape may be related to the metastable nature of CdS since CdS thin films grown at 220 degrees C show zincblende structures and thin films grown at 280 degrees C show wurtzite structures. The PL peak position and linewidth were strongly affected by the growth condition of QDs; QD samples with a thicker CdS deposition showed redshifts of the peak position along with the narrowing of the linewidth. (C) 1998 American Vacuum Society.

Cubic GaN (c-GaN) layers were grown by rf-plasma source MBE on (001) GaAs prepared by atomic-hydrogen treatment at "high temperatures", and the structural properties of the epilayers were investigated by the high-resolution X-ray rocking curve and the reciprocal space mapping measurements. The growth temperature was varied from 620 to 740 degrees C. It was found that single domain "device-quality" c-GaN layers could be grown for the first time; the FWHM of the X-ray rocking curves for the (002) c-GaN could be as small as 70-90 arcsec and the inclusion of h-GaN phase in the c-GaN epilayers grown at temperatures above 680 degrees C could be less than 4x10(3).

Growth of high quality c-GaN on atomic-II treated (001) GaAs was examined by rf plasma-assisted MBE. First the initial growth stages (atomic-H treatment of GaAs, nitridation/deposition of a thin GaN buffer layer, the post deposition annealing of the buffer layer, and epitaxy of c-GaN) were studied by RHEED and AFM observations. It was found that atomically flat GaAs surface with one monolayer-height steps and ragged step edges could be obtained by the atomic-II treatment at high temperatures. It was found that the atomic-II treated GaAs surface was preferable to grow high quality c-GaN; the FWHM of the X-ray rocking curve for the (002) c-GaN was as small as 70 - 90 arcsec.

ZnS1-xTex and Zn1-yMgyS1-xTex epilayers were grown on (1 0 0)GaAs by MBE. Ternary and quaternary alloys can be lattice matched to GaAs by adjusting the composition of the group VI and group II elements. Zincblende structure layers were grown and p-doping was achieved. A fairly good ohmic contact profile was obtained between gold and the alloy. By using the Au/p-ZnS1-xTex contacting layer, ohmic contact was obtained to p-ZnSe. p-Zn1-yMgyS1-xTex layers were applied to cladding layers of the LED, and bright luminescence was observed. © 1998 Elsevier Science B. V. All rights reserved.

Growth of high-quality and/or "purely cubic" GaN layers on (001) GaAs has been investigated by an rf-radical source MBE:paying particular attention to the effect of surface cleaning: and smoothing on the structural properties of the GaN epilayers. Device-quality c-GaN layers can be grown on the (001) GaAs treated by atomic hydrogen at high temperatures; the FWHM of X-ray rocking curve for the c-GaN (002) plane is as small as 80-90 arcsec and also the inclusion of the hexagonal phase GaN in the epilayer can be less than 4x10(-3).

Growth of high-quality cubic GaN (c-GaN) layers on GaAs has been investigated by an rf-radical source MBE in terms of nitridation of the substrate and buffer layer deposition at initial growth stage. The nitridation of the (001) GaAs surface at 620 degrees C resulted in the formation of c-GaN with rough surface. In order to obtain flat surface of c-GaN, we have examined the nitridation process at low temperature. It was found that the nitridation of GaAs surface at 500 degrees C was suitable, because the reaction of Ga with N was slow enough to form c-GaN uniformly and the surface was kept flat even after the nitridation.

Detailed investigation of Al2O3 pre-treatment and initial nucleation process were performed in order to grow a high quality GaN thin film on Al2O3(0001) by molecular beam epitaxy. The atomic step of an Al2O3 surface was successfully obtained by chemical etching using H3PO4:H2SO4. It was found that a very thin (26 Angstrom) amorphous buffer layer of atomic steps could form a uniform nucleation which consists of minute nuclei (RMS = 0.17 nm). The epilayer, which grew on this nucleation layer showed a significant improvement in crystal quality. Band edge emission (357 nm/FWHM = 22meV) without any deep emission band was observed with low temperature photoluminescence and the FWHM of the X-ray rocking curve was 20 arcmin.

Spectroscopic ellipsometry (SE) was used to study the surface stoichiometry of the cubic GaN layers during MOVPE growth. The relationship between the surface conditions and the crystal quality of the cubic GaN layers was clarified. The signal change corresponding to the variation of the surface stoichiometry was observed; the N coverage of the cubic GaN surface was drastically decreased when the V/III molar ratio was below 20 at 800 degrees C. The crystal quality of the cubic GaN layers was characterized by using X-ray reciprocal space mapping. The growth under the Ca-rich condition was suitable to improve the crystal quality of the cubic GaN. The surface stoichiometry of the cubic GaN layers during MOVPE growth strongly affected the crystal quality of GaN epilayers and the precise control of the surface stoichiometry is important to minimize the inclusion of the hexagonal phase.

The cubic GaN epilayers were grown on (001) GaAs under different V/III source-supply ratio by rf-radical source MBE. The inclusion of hexagonal phase in cubic GaN layers was detected by high resolution X-ray analysis and the relationships among various crystal axes of cubic and hexagonal phase GaN were analyzed by X-ray diffraction reciprocal space mappings and pole figures. The V/III ratio was found to drastically affect the quality of the layer. High-temperature growth under near-stoichiometric condition was necessary to obtain high quality epilayers. It was found that the inclusion of hexagonal phase in the cubic GaN layers could be less than 0.4 %. The measurements and analyses of reciprocal space maps and pole figures revealed that the orientation of crystal axes of the hexagonal phase was quite unique in the present GaN epilayers and they were different from those of previously reported c-GaN epilayers.

An atomically flat GaAs(001) surface can be obtained by a two-step atomic hydrogen (atomic-H) irradiation technique. This method includes low-temperature cleaning and high-temperature smoothening of the GaAs substrate surface. The reflection high energy electron diffraction (RHEED) and AFM study showed that a wide terrace with a 1 monolayer step height was observed when a GaAs(001) surface was cleaned at 400 degrees C and smoothened at 540 degrees C with atomic-H irradiation. The irradiation of atomic-H during the high temperature process maintained a certain surface stoichiometry, and resulted in an atomically flat substrate surface. This technique is useful for heterovalent epitaxy systems involving a single chamber growth system.

Growth of high-quality cubic GaN layers on the (001)GaAs substrate was investigated by an rf plasma-assisted molecular beam epitaxy. We paid a particular attention to the effect of substrate-surface cleaning/smoothing on the crystallinity and/or structural properties of GaN epitaxial layers. First, we have found that atomically flat (001)GaAs surface can be obtained by an atomic hydrogen irradiation technique. The surface of the typical atomic-hydrogen treated (001)GaAs consists of a narrow terrace with an acute-angled polygonal terrace with one monolayer step height. Further, it was found that quite high-quality cubic GaN layers can be grown at substrate-temperatures above 680℃. Typical value of FWHM of X-ray rocking curve of (002) plane for 0.3 mm-thick GaN epilayer was as small as 90 arcsec. The reason why such high quality GaN epilayers can be grown has been attributed to the quite large number of monolayer-height steps and kinks on the atomic hydrogen treated (001)GaAs surface. Because such surface will be preferable for uniform and high-density GaN nucleation on the surface at the very initial stage of epitaxial growth of GaN.

The metastable nature of molecular-beam epitaxy grown CdS layers was studied. Undoped CdS layers were grown on (100) GaAs substrates/(100) ZnSe buffer layers, and crystal structures were characterized by x-ray diffraction and transmission electron microscopy. Undoped films showed both zincblende and wurtzite structures depending on the growth condition; higher substrate temperatures and smaller VI/VII beam intensity ratios preferentially form wurtzite structures. The Ga doping significantly affected the crystal structure and Ga doped CdS layers preferentially formed wurtzite structures. Thermal instability of the CdS him was observed through various ex situ annealing experiments. The wurtzite structure phase was developed from the zincblende structure CdS layers by thermal annealing where the annealing temperature was lower than the growth temperature. (C) 1996 American Vacuum Society.

The defect zincblende structure Ga2Se3 was inserted at the interface of ZnSe and ZnTe so that the valence band discontinuity (ΔEv) could be modified. The valence band discontinuity was determined from the capacitance-voltage (C-V) measurement technique. The measured ΔEv depended on the thickness of the Ga2Se3 interfacial layer. The discontinuity became negligible when the thickness of the interfacial layer was about 4 Å. The current-voltage (I-V) characteristics for the Au/p-ZnTe/Ga2Se3/p-ZnSe/p-GaAs structure exhibited a perfectly linear plot when the thickness of the interfacial layer was about 8 Å. These results suggest that the valence band discontinuity between ZnTe and ZnSe can be reduced by introducing the Ga2Se3 interfacial layer.

Growth and p-type doping processes in MOVPE of ZnSe have been studied by optical probing methods called RD (reflectance difference) and SPI (surface photo-interference). As for the dopant source, tertiarybutyl-amine (tBNH2) was extensively used in this work, because it is one of the most useful dopant sources in photo-assisted MOVPE of widegap II-VI compounds. Oscillations with monolayer periodicity in RD signal traces have been successfully detected for the first time using a He-Ne laser beam in MOVPE of ZnSe. One of the most important findings is that tBNH2 molecules are selectively adsorbed on the "Zn-terminated" surface and not on the "Se-terminated" surface
this suggests that selective doping during Zn-source (DMZn) supply and/or on Zn-terminated surfaces will be effective for improving the doping efficiency.

Interfacial layers were inserted at the interface of ZnSe and ZnTe in order to reduce both (1) the effect of strain and (2) the valence band discontinuity. The interfacial layer adapted in this study is the III-VI compound (Ga,Se). The layered structure GaSe is favorable for the present work, because it can be a buffer layer to relax the lattice mismatch at the interface. All layers including ZnTe, (Ga,Se) and ZnSe were grown on (100) GaAs substrate by conventional molecular beam epitaxy. The crystal structure of the (Ga,Se) on ZnSe was investigated. The growth of the layered structure GaSe layer on (100) ZnSe was very difficult, though the defect zinc-blende structure Ga2Se3 layer could be easily grown. The defect zinc-blende structure Ga2Se3 was inserted at the interface of ZnSe and ZnTe so that the valence band discontinuity could be modified. The discontinuity was decreased to about 0.1 eV when the thickness of the interfacial layer was about 8Å. The current-voltage characteristics were measured for the sample with Ga2Se3 interfacial layer. The structure with Ga2Se3 exhibited the ohmic property. These results suggest that the valence band discontinuity between ZnTe and ZnSe can be reduced by introducing the Ga2Se3 interfacial layer.

A new in-situ optical probing method with an atomic-scale resolution in thin film deposition and/or heteroepitaxial growth is proposed in this paper. The new method is named surface photo-interference (SPI) because it is essentially concerned with a photo-interference in the deposited layer. The principle of the SPI is as follows; the complex refractive indices of the atomic or molecular layers, which alternately appear on top of the surface during film deposition and/or heteroepitaxy, greatly affect the total phase-shift of probing light during propagation in the epilayer, resulting in the change in the photo-interference signal intensity. The experimental setup of the method is very similar to that for another optical probing method called surface photo-absorption (SPA), but the principle is quite different between those two. One of the features of the SPI is that the experimental setup is quite simple and inexpensive though it is quite useful in real time monitoring of the thin film deposition.

A study of biaxial strain as a function of temperature in a ZnSe epilayer grown on a GaAs substrate is presented. The strains are determined by measuring the heavy- and light-hole related excitonic transitions via photomodulated spectroscopy. The strain is found to increase with increasing temperature. The data are compared with a calculation using a previously determined elastic constant and thermal expansion coefficients. The temperature dependence determined here allows a comparison of various other optical measurements performed at different temperatures. (C) 1995 American Institute of Physics.

Theoretical design of pseudo-ternary and quaternary alloys by superlattice structures consisting of(Zn,Cd)(S,Se) binary II-VI compounds has been studied. For pseudo-ternary ZnCdS and ZnCdSe alloys, the superlattices with two layers in a cycle, i.e., ZnS/CdS and ZnSe/CdSe are considered, and for pseudo-quaternary ZnCdSSe alloy, the two superlattice structures with more than two layers in a cycle are considered. In order to design and evaluate these superlattices, the expression for the equilibrium in-plane lattice constant of these superlattices has been derived by minimizing the total elastic strain energy in the cycle. The combinations of layer thicknesses in a cycle and the effective bandgap of these superlattices have been calculated while the elastic strain effect was included. The usefulness of these superlattice structures has been evaluated.

In this paper, recent advances in the temperature epitaxy of the ZnSe-based widegap II-VI compounds on GaAs and their application to blue/green laser and light emitting diodes are discussed. It is shown that the maximum net acceptor concentration in the these widegap II-VI compounds is saturated at about 2X10(18) cm(-3), although several attempts to improve the maximum concentration have been done. As for the life time of blue/green laser diodes, the degradation of devices has been correlated to the propagation of crystal defects generated during growth. To reduce these defects such as stacking faults, it is believed now that incorporation of GaAs buffer layer is a useful and necessary technique. This indicates that double-chamber MBE system, in which GaAs buffer and laser structure are grown in different chambers connected through high- vacuum tunnel. The results when using ''epi-ready'' GaAs substrate which was grown in a separate MBE system are also briefly discussed. Further, recent advances in p-type doping in MOVPE-based growth are briefly shown.

The surface reaction mechanism in MOMBE‐ALE of ZnSe and CdSe using DMZn, DMCd, and H2Se as reactants is studied with a newly developed optical in‐situ monitoring method. The new method is named surface photo‐interference (SPI), because the concept of the SPI is essentially concerned with a photo‐interference in the heteroepilayer. Fairly low‐energy photons for which the epilayer is transparent can be used as probing light in SPI. Reflecting this feature, SPI is especially useful for studying heteroepitaxial growth kinetics of wide‐gap II‐VI compounds, because the effect of photocatalytic growth rate enhancement can be avoided when using such low energy photons as probing light. It is shown that there is a quite significant difference in the surface reaction mechanism between the MOMBE‐ALE of ZnSe and CdSe. That is, in the ALE of ZnSe when none of the source gases are per‐cracked, the reaction to form ZnSe can take place when the group‐II source (i.e., DMZn) is supplied, resulting in an alternate appearance of Zn terminated and H2Se terminated surfaces during growth. On the other hand, the reaction to form CdSe can do when the group‐VI source (i.e., H2Se) is supplied, resulting in an alternate appearance of Se terminated and DMCd terminated surfaces during growth. Copyright © 1995 WILEY‐VCH Verlag GmbH &amp
Co. KGaA

We report on the molecular beam epitaxial (MBE) growth of sulfur-based binary and ternary widegap II-VI alloys using elemental sulfur source. The growth temperatures used arranged from 180 to 310°C. The valved-cracker cell with three independently controllable heaters is used for the S source. The epitaxial layers were grown using thermally cracked S source at 200, 500 and 900°C, and the film quality was characterized by X-ray diffraction and photoluminescence.

Theoretical design of ZnSSe/ZnCdSSe laser diodes emitting 460 nm at room temperature (RT) was studied by means of threshold current analysis. The threshold current density calculation was based on the laser theoryestablished for the III-V laser diode (LD) system. The result of the threshold current density calculation indicatedthat the reduction of carrier overflow is an essential issue in realizing the device with a reasonable threshold currentlevel at RT. Another indication is that 30% S content in the ZnSSe cladding layer would be a suitable targetvalue for a practical 460 nm LD at RT, along with a multiple quantum barrier structure. The predicted thresholdcurrent density of such a LD could be as low as 450 A/cm2 at RT. © 1995 The Japan Society of Applied Physics.

A new in-situ optical probing method for studying surface reactions during heteroepitaxial growth is proposed. The new method is named surface photo-interference (SPI), because the concept of the SPI is essentially concerned with a photo-interference in the heteroepilayer. The experimental setup of the method is very similar to another optical probing method called surface photo-absorption (SPA), but the principle is quite different between the two. Unlike SPA, fairly low energy photons that are transparent for the epilayer can also be used in SPI as a probing light. Reflecting this feature, SPI is especially useful for studying growth kinetics in heteroepitaxial growth of widegap II-VI compounds, because the effect of photocatalytic growth-rate enhancement can be avoided when using such low energy photons as a probing light. A brief theoretical description of SPI signal is given, and experimental results detected in metalorganic molecular beam energy (MOMBE) of ZnSe and CdSe are mainly discussed. Further, how SPI differs from SPA will be shown.

The surface reaction mechanism in MOMBE-ALE of ZnSe and CdSe using DMZn, DMCd and H2Se as reactants has been studied with a newly developed optical in-situ monitoring method named surface photo-interference (SPI). It is shown that there is a quite significant difference in the surface reaction mechanism between the MOMBE-ALE of ZnSe and CdSe. That is, in the ALE of ZnSe when none of the source gases are pre-cracked, the reaction to form ZnSe can take place when the group-II source (i.e., DMZn) is supplied, resulting in an alternate appearance of Zn-terminated and H2Se-terminated surfaces during growth. On the other hand, the reaction to form CdSe can take place when a group-VI source (i.e., H2Se) is supplied, resulting in an alternate appearance of Se-terminated and DMCd-terminated surfaces during growth.

A new in-situ optical probing method with an atomic-scale resolution in heteroepitaxial growth is proposed. The experimental set-up of the method is very similar to another optical probing method called surface photo-absorption (SPA), but the principle is quite different. The new method is named surface photo-interference (SPI) because it is essentially concerned with photo-interference in the heteroepilayer. The principle of the SPI is as follows: the complex refractive indices of the atomic or molecular layers, which alternately appear on top of the surface during epitaxial growth of compound semiconductors, greatly affect the total phase shift of probing light during propagation in the epilayer, resulting in a change in the photo-interference signal intensity. One of the features of the SPI is that, unlike in the case of SPA, fairly low energy photons which are transparent for the epilayer can also be used as probing light. This is very preferable for studying the growth kinetics and/or surface reaction in photo-excited epitaxial growth of widegap II-VI compounds, because the effect of photocatalytic growth-rate enhancement can be avoided when using such low energy photons as probing light.

Nitrogen and helium mixed gas plasma was used to control the acceptor concentration of p-ZnSe and p-ZnSSe. Using the mixed gas, the acceptor concentration of p-ZnSe can be controlled from 6 X 10(16) to 7 X 10(17) cm-3, whereas the acceptor concentration of p-ZnSSe can be controlled from 4 X 10(16) to 4 X 10(17) cm-3 . Doping characteristics such as the acceptor concentration and the PL properties depend on the gas mixing ratio and the RF power. p-Layers grown with this technique were used for blue-green laser diode structures. Lasing was observed at 77 K under pulsed operation.

The metalorganic molecular beam epitaxy (MOMBE) growth process of ZnSe on GaAs was characterized by the surface photo-interference (SPI) method. The SPI signal traces were monitored in various experiments where the source gas cracking conditions were varied. The signal features, such as intensity and polarity, were drastically modified by the Se source gas cracking, whereas the signal was hardly affected by the Zn source gas cracking. Zn-terminated surfaces formed by uncracked dimethylzinc (DMZn) gas supply as well as the cracked gas supply are likely to be covered with Zn atoms. On the other hand, the Se-terminated surface formed by the uncracked H2Se gas supply is probably covered with H2Se molecules.

In this paper, we propose two superlattice structures as pseudo-quaternary ZnCdSSe layers. They consist of three layers or four layers in a cycle with three kinds of binary compound materials, i.e., ZnS, ZnSe and CdS. In order to estimate the equilibrium in-plane lattice constant of these layers, a general expression for superlattices consisting of n (n=2, 3, 4, …) layers in a cycle has been derived by minimizing the total elastic strain energy. The effective bandgap for the superlattices with different combinations of the layer thickness in a cycle were calculated using Kronig-Penney model, where the strain effect was taken into account in calculating the bandgap and the band offset. The calculated results of the effective bandgaps were compared with the bandgap of ZnCdSSe quaternary alloy.

The piezomodulated-, electromodulated-, and photomodulated-reflectivity spectra of a pseudomorphic ZnTe epilayer, grown on an InAs epilayer by molecular-beam epitaxy, exhibit heavy- and light-hole excitonic signatures split by the lattice mismatch induced biaxial compressive strain. This splitting in the pseudomorphic epilayer is studied as a function of applied hydrostatic pressure using photomodulated reflectance spectroscopy at 80 K. With increasing hydrostatic compression, the compressive strain is progressively compensated by the pressure-induced tensile strain. At similar to 55 kbars the epilayer becomes strain-free, and is under a biaxial tension at higher pressures. The separation between the heavy- and light-hole signatures is superlinear in pressure, suggestive of a strain or volume deformation-dependent shear deformation-potential constant. We also compare the pressure dependence of she Raman LO phonon of the ZnTe epilayer on InAs with that of a bulk ZnTe sample at 13 K. The pressure-dependent strain is found to be linear. Accurate values of the first-order strain derivatives of the LO phonons and mode Gruneisen constants are obtained.

Nitrogen and helium mixed gas plasma was used to grow p-ZnSe. Using the mixed gas, the acceptor concentration could be controlled from 6×1016 to 7×1017 cm-3 while films doped using the nitrogen plasma exhibited the acceptor concentration of 3×1017 cm-3. Doping characteristics such as the acceptor concentration and the PL properties depend on the gas mixing ratio and the rf power. Plasma spectroscopy was used to characterize the variety of the species in the plasma. Although the variety of the nitrogen related peaks in the spectrum were not significantly affected by the gas mixing, several peaks (for example 745nm and 825nm) showed intensity variation that was similar to the acceptor concentration variation with respect to the N2 and He gas mixing ratio.

The successful p-doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy has been an important factor leading to the recent realization of blue-green diode lasers and light-emitting diodes. This letter reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels approaching the 1019cm-3range are reported along with electrical, optical, and microstructural characterization. Highly doped ZnTe provides an opportunity for forming an ohmic contact to p-ZnSe.

The heavy-hole and light-hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch-induced splitting between the light-hole and heavy-hole related transitions increases in a continuous and reversible manner because of the additional pressure-induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer undergoing a structural phase transition at approximately 30 kbar. For pressures between 20 and 30 kbar, the behavior of the CdTe epilayer is similar to that of the bulk sample; the strain appears to have been relaxed due to the structural phase transition which has taken place in InSb. Values of the first- and second-order pressure coefficients for bulk CdTe and for the CdTe epilayer as well as values of the hydrostatic and shear deformation potentials are obtained at 14 and 80 K and compared with previously quoted values.

The heavy-hole and light-hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch-induced splitting between the light-hole and heavy-hole related transitions increases in a continuous and reversible manner because of the additional pressure-induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer undergoing a structural phase transition at approximately 30 kbar. For pressures between 20 and 30 kbar, the behavior of the CdTe epilayer is similar to that of the bulk sample; the strain appears to have been relaxed due to the structural phase transition which has taken place in InSb. Values of the first- and second-order pressure coefficients for bulk CdTe and for the CdTe epilayer as well as values of the hydrostatic and shear deformation potentials are obtained at 14 and 80 K and compared with previously quoted values.

The use of a nitrogen radio frequency plasma source together with an appropriate quantum well configuration have recently resulted in the successful realization of p-type ZnSe by molecular beam epitaxy. This has enabled a variety of pn heterojunction based devices to be built including the first semiconductor injection lasers operating in the blue/green portion of the spectrum first reported by 3M and the Brown/Purdue group. In this paper, we discuss two lattice matched multiple quantum well structures that produce laser emission in the blue and blue/green portion of the spectrum.

In recent months the successful p-doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy was one factor leading to the realization of diode lasers and light emitting diodes. This paper reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels exceeding the 10(19) CM-3 range are reported along with electrical, optical, and microstructural characterization. The nitrogen-doped ZnTe is used to implement p-ZnTe/n-AlSb diodes; the growth and characterization of these heterojunction diodes are described.

We describe the performance of pn junction MOW diode lasers and LEDs which are grown on both p-type and n-type GaAs substrates. Efforts to minimize dislocations by lattice matching the II-VI region to the GaAs substrate in some cases resulted in obtaining dislocation densities below 10(5) cm-2. We have obtained pulsed high power, high quantum efficiency laser emission up to room temperature conditions, and continuous operation at liquid nitrogen temperatures. Efficient LED and display devices operate in the blue (490-494 nm) at room temperature.

Nitrogen and helium mixed gas plasma was used to grow p-ZnSe. Using the mixed gas, the acceptor concentration can be controlled from 6 x 1016to 7 x 1017cm-3while films doped using the nitrogen plasma exhibited the acceptor concentration of 3 x 1017cm-3. Doping characteristics such as the acceptor concentration and the PL properties depend on the gas mixing ratio and the rf power. p-ZnSe layers grown with this technique were used for blue-green laser structures. Lasing was observed at 77 K under pulse operation. © 1993 The Japan Society of Applied Physics.

Planar doping of nitrogen into ZnSe is examined. The dependence of doping efficiency on the particular surface termination (Zn- or Se-stabilised) on (100) ZnSe is investigated through capacitance-voltage measurement and low-temperature photoluminescence (PL) spectroscopy. Using planar doping on the Zn surface, we achieved a hole concentration of 4.5 x 1017 cm-3, and the PL spectrum is dominated by strong donor-acceptor pair (DAP) emissions. By contrast, the film planar-doped on the Se plane shows rather low hole concentration, and the spectrum is dominated by excitonic features along with very weak DAP emissions. © 1993 The Japan Society of Applied Physics.

Two methods were examined for doping nitrogen into ZnSe and ZnCdSe. N2gas was thermally excited in the first method. The photoluminescence (PL) spectra showed that acceptor levels were formed in the film
this feature was similar to that observed in the samples doped with nonexcited N2gas. The second method utilized pho-toexcited N2O and N2gases using vacuum ultraviolet (VUV) light. A deuterium lamp was used for the VUV-light source. Deep-emission-dominant PL spectra were observed when films were doped using photoexcited N2O gas. N2O gas was then mixed with N2gas to control the photodecomposition process and improve the film quality. Samples doped using the mixed gas exhibited well-resolved near-band-edge features. The PL spectrum of the ZnO.9 Cdo.iSe sample doped using the photoexcited mixed gas showed donor-acceptor pair emission. © 1993 The Japan Society of Applied Physics.

Low-threshold current density operation has been obtained from (Zn, Cd)Se/Zn(S, Se) quantum-well diode lasers in the blue-green spectrum. The devices, with reflective facet coatings, have been operated at room temperature under pulsed conditions with threshold current densities of I(th) almost-equal-to 1 kA/cm2.

pn junction characteristics and LED action in ZnSe-based multilayers grown by molecular beam epitaxy is demonstrated. In particular, we show that (Zn,Cd)Se/ZnSe/Zn(S,Se) structures containing (Zn,Cd)Se quantum wells, grown on p-type GaAs epilayers, and designed with a heavily doped n+-ZnSe top contact layer may be appropriate for display device applications in the blue-green portion of the spectrum.

Laser diode operation has been obtained from (Zn,Cd)Se/ZnSe and (Zn,Cd)Se/Zn(S,Se) quantum well structures in the blue and the green. The devices, prepared on p- and n-type (In,Ga)As or GaAs buffer layers for lattice matching purposes to control the defect density, have been operated at near-room-temperature conditions and briefly at room temperature with uncoated end facets. Quasi-continuous wave operation has been obtained at T=77 K.

The successful p doping of ZnSe by substitutional nitrogen using a plasma cell incorporated into the molecular beam epitaxy chamber has led to the development of electroluminescent devices based on carrier injection at a pn junction. The light emitting diode structures described here are grown on a GaAs substrate using a tetragonally distorted (In,Ga)As buffer layer to provide lattice matching between the substrate and the active II-VI region. The result of the incorporation of the buffer layer is an essentially dislocation-free active region. The letter discusses optical properties as well as the x-ray and transmission electron microscopy characterization of the quantum well device structures.

Recent developments were reviewed in ZnSe-based quantum wells and heterostructures with emphasis on physics and engineering towards light emitter applications at blue-green wavelengths. To illustrate current progress, examples are given of both diode laser and light emitting diode device research.

The successful p-doping of ZnSe by substitutional nitrogen using a plasma cell incorporated into a molecular beam epitaxy chamber has led to the development of electroluminescent devices employing carrier injection at a p-n junction and in which the active region is based on (Zn,Cd) Se/Zn(S,Se) multiple quantum well structures. We report on the performance of p-n junction MQW diode lasers and LEDs which are grown on both p-type and n-type GaAs substrates, and where sulfur is, or is not incorporated. For all structures, efforts were made to minimize dislocations by lattice matching the active II-VI region to the GaAs substrate. Some designs have dislocation densities below 105 cm-2. In this work we have obtained pulsed high power, high quantum efficiency laser emission up to room temperature conditions, and continuous operation at liquid nitrogen temperatures. Efficient LED devices are described which operate in the blue (494 nm) at room temperature.

Sputter deposited indium tin oxide layers have been used as the top contact for blue LEDs and diode lasers in (Zn,Cd)Se/Zn(S,Se) quantum well heterostructures. The contact resistance to n-Zn(S,Se) is comparable to that with indium or gold. The optically transparent contacts have been utilized, as an example, in the fabrication of a numeric display device and to show that LED emission is of excitonic origin in these type I quantum wells.

Blue (494 nm) light emitting quantum well diodes based on Zn(S,Se) p-n junctions are demonstrated at room temperature. P-type Zn(S,Se) is realized by using a nitrogen rf plasma cell. The light emitting diode is formed on homoepitaxial GaAs buffer layers by molecular beam epitaxy. The S fraction of the alloy is selected to provide a lattice match between the II-VI active region and the GaAs buffer; the result is an active region having a dislocation density below 10(5)/cm-2. The letter discusses emission characteristics as well as the x-ray rocking curve and transmission electron microscopy characterization of the structures.

The p doping of molecular-beam epitaxially grown ZnSe by substitutional nitrogen using a plasma cell has enabled the fabrication of pn junction injection electroluminescent devices. The light emitting diode structures, including pulsed laser diodes described here, are grown on GaAs substrates and incorporate multiple quantum well structures as the active gain region. This article discusses optical properties as well as the x-ray and transmission electron microscopy (TEM) characterization of the quantum well device structures. The TEM imaging reveals low dislocation densities in both device configurations.

The optical properties of (Zn,Cd)Se/ZnSe quantum wells are reviewed with emphasis on recent results on quasi-two-dimensional excitons, light emitting diodes, as well as on optically pumped lasers and diode lasers in this type I system.

Atomic structures of three Ga2Se3epilayers grown on GaAs(100) or ZnSe(100) epilayers by molecular beam epitaxy were studied by transmission electron microscopy. A new vacancy ordered structure was found in the heteroepitaxial interface regions of Ga2Se3epilayers. The ordered structure is described as an alternate stacking of two types of Ga planes in the growth direction. One half of the sites are vacancies in one type of Ga plane, while the other type of Ga plane is fully occupied by Ga atoms. Comparison of three Ga2Se3epilayers indicates a strong influence of surface chemistry of substrate epilayers on the formation of the vacancy ordering. A highly developed two-dimensional ordering has occured in the epilayer grown on a GaAs(100) surface with (4 × 3) rereconstructions. © 1992.

Laser diode action in the blue-green has been observed from (Zn,Cd)Se quantum wells within ZnSe/Zn(S,Se) p-n heterojunctions up to 250 K. Operation is reported for two different configurations for which the GaAs substrate serves either as the n- or p-type injecting contact. In pulsed operation, output powers exceeding 0.6 W have been measured in devices prepared on both n-type and p-type GaAs epitaxial buffer layers and substrates.

The paper addresses two issues related to II-VI/III-V heterostructures. The first topic is a discussion of the nature of the bonding at CdTe/InSb and ZnSe/GaAs interfaces grown by molecular beam epitaxy. The bonding is studied by means of in situ X-ray photoelectron spectroscopy (XPS). A comparison of the In 3d core level features from an InSb epilayer surface, a Te-reacted InSb surface, very thin (a few monolayers thick) CdTe/InSb epilayer/epilayer heterostructures and from deliberately grown (In,Te) epilayers, indicate that the Te-reacted layer, the CdTe/InSb heterostructures and the (In,Te) epilayers exhibit similar In bonding characteristics. A parallel XPS study of ZnSe/GaAs interfaces reinforces the conclusion that there is a general tendency for the formation of a III-VI interfacial compound when forming II-VI/III-V junctions. The second topic concerns a description of pn junction light emitting devices based on ZnSe. In the structures described, carriers are injected from nitrogen doped p-ZnSe and chlorine doped n-ZnSe into a multiple quantum well structure having (Zn,Cd)Se wells and ZnSe barriers. The bright CW photon emission originates from quantum well transitions.

The heavy-hole and light-hole excitons of a pseudomorphic ZnSe film grown on a GaAs epilayer by molecular-beam epitaxy, are studied as a function of applied hydrostatic pressure using photomodulated reflectance spectroscopy. At ambient pressure, the signature in the spectrum due to the heavy-hole exciton occurs at an energy lower than that of the light-hole exciton, a consequence of the compressive biaxial strain in ZnSe due to its lattice mismatch with GaAs. As the pressure is increased, the two signatures approach each other in energy and coalesce at 36.2 kbar. The difference in the compressibility of ZnSe from that of GaAs generates a tensile strain that progressively compensates the lattice-mismatch-induced compressive strain and finally, at 36.2 kbar, the heterostructure is strain free. Beyond this pressure, the strain in ZnSe transforms from biaxial compression to biaxial tension, the light-hole signature now occurring at the lower energy. The transformation of strains via pressure tuning is continuous and reversible. The separation between the heavy-hole and light-hole signatures is superlinear in pressure, suggestive of a pressure-dependent shear-deformation-potential constant.

In this paper in situ x-ray photoelectron spectroscopy (XPS) is used to study the nature of the bonding at ZnSe/GaAs and CdTe/InSb interfaces grown by molecular-beam epitaxy. X-ray photoelectron spectra support the transmission electron microscopy evidence that ZnSe/GaAs MIS capacitors exhibiting low interface state densities are associated with the formation of an interfacial layer of zinc blende Ga2Se3. An interfacial layer can be deliberately introduced by reacting an As deficient GaAs surface with an incident Se flux. A comparison of the Se 3d core level features from the ZnSe epilayer surface, a Se-reacted GaAs surface, and from separately grown Ga2Se3 epilayers, clearly indicate the same Se bonding characteristic for the Se-reacted layer, and the Ga2Se3, epilayer. A parallel XPS study of CdTe/InSb interfaces leads to the conclusion that there is a general tendency for the formation of a III-VI interfacial compound when forming a II-VI-on-III-V junction.

A highly developed two dimensional superstructure was found at the Ga2Se3/GaAs epitaxial interface by transmission electron microscope observations. The atomic arrangement of the superstructure was determined by the analysis of electron diffraction patterns and high resolution transmission electron microscope images. The structure is described as a c(2 x 2) ordered arrangement of vacancies on the interfacial Ga plane. A possible role of the mismatch of electronic configurations at the Ga2Se3/GaAs interface in the formation of the vacancy ordering is discussed.

Despite the valence difference across the junction, it has been shown that ZnSe/GaAs epilayer/epilayer interface state densities can be reduced to values comparable to the (Al,Ga)As/GaAs interface. We have previously reported a transmission electron microscopy study indicating that ZnSe/GaAs structures exhibiting low interface state densities are associated with the formation of an interfacial layer of zinc-blende Ga2Se3. In this letter we describe a procedure whereby an interfacial layer can be deliberately introduced prior to nucleation of ZnSe. In situ x-ray photoelectron spectroscopy is used to study the nature of the bonding at the interfacial layer. A comparison of the Se 3d core level features from the ZnSe epilayer surface, a Se-reacted GaAs surface, and from a separately grown Ga2Se3 epilayer, clearly indicates the same Se bonding characteristic for the Se-reacted interfacial layer and the Ga2Se3 epilayer.

Despite the valence difference across the junction, it has been shown that ZnSe/GaAs epilayer/epilayer interface state densities can be reduced to values comparable to the (Al,Ga)As/GaAs interface. We have previously reported a transmission electron microscopy study indicating that ZnSe/GaAs structures exhibiting low interface state densities are associated with the formation of an interfacial layer of zinc-blende Ga2Se3. In this letter we describe a procedure whereby an interfacial layer can be deliberately introduced prior to nucleation of ZnSe. In situ x-ray photoelectron spectroscopy is used to study the nature of the bonding at the interfacial layer. A comparison of the Se 3d core level features from the ZnSe epilayer surface, a Se-reacted GaAs surface, and from a separately grown Ga2Se3 epilayer, clearly indicates the same Se bonding characteristic for the Se-reacted interfacial layer and the Ga2Se3 epilayer.

A highly developed reconstruction structure was found at the Ga2Se3/GaAs epitaxial interface by transmission electron microscope observations. The atomic structure of the reconstruction was derived by the analysis of electron diffraction patterns and high resolution transmission electron microscope images. The structure, which is described as an ordered arrangement of structural vacancies on the Ga sublattice, suggests that the reconstruction results from the valence mismatch at the Ga2Se3/GaAs interface.

In this paper we describe a series of MnTe/CdTe/MnTe and MnTe/InSb/MnTe single quantum well structures. For the CdTe quantum wells we report the observation of luminescence covering the entire visible range from red to blue; a quantized state in the InSb well is used to implement resonant tunneling. X-ray diffraction and transmission electron microscopy (TEM) were used to evaluate the microstructural quality of the structures. Dark-field TEM showed that, in spite of the 2.3% lattice mismatch, the MnTe layers remained pseudomorphic and dislocation-free. High resolution images (also used to determine dimensional details) indicated that the interfaces were atomically abrupt, and that the CdTe and InSb wells were essentially unstrained in each of the structures; most of the strain was contained in the MnTe barrier layers. Optical properties of the single quantum well structures have been studied using photoluminescence and photoluminescence excitation spectroscopy. Blue luminescence at 2.59 eV (n = 1 transition) has been observed from a structure with a 10 angstrom CdTe well. The negative differential resistance observed from MnTe/InSb resonant tunneling structures represents, to our knowledge, the first report of a dimensionally quantized state in InSb.

Epitaxial ZnSe/epitaxial GaAs interfaces are formed by molecular beam epitaxy and evaluated by several techniques including capacitance-voltage (C-V) measurements. The GaAs surface stoichiometry is systematically varied prior to the nucleation of ZnSe. A dramatic reduction of interface state density occurred when the GaAs epilayer is made As deficient. The ZnSe/GaAs interfaces exhibiting low interface state densities are associated with the presence of an interfacial layer of zincblende Ga2Se3. In situ X-ray photoelectron spectroscopy (XPS) is used to study the nature of the bonding at the interfacial layer. The character of Se 3d core level features from the interfacial region and from separately grown Ga2Se3 epilayers support the identification of the interfacial layer as Ga2Se3.

Comparative photoluminescence and excitation spectra of ZnSe/GaAs epilayers grown by molecular beam epitaxy (MBE) and hot-wall epitaxy (HWE) show likewise features in the exciton energy regime. Two strain-split components of the free exciton are observed, as well as characteristic sets of transitions from or into ground and excited states of acceptor- and donor-bound excitons. However, all respective lines are shifted to lower energies in the HWE samples, due to the increased thermally induced strain as a consequence of the substrate temperatures being enhanced compared to the MBE growth. Whereas the dominant donors are of the same nature in both kinds of samples, specific acceptor centers are incorporated in the HWE films. Although the MBE-grown layers are of superior quality, it is shown that HWE under optimized growth conditions is a cheap and useful alternative to obtain ZnSe epilayers of reliable properties.

A series of strained single-quantum-well (SQW) CdTe/MnTe structures with well thicknesses ranging from approximately 50 to 10 angstrom were studied. Raman experiments indicate that the lattice mismatch of approximately 3.2% is coherently accommodated if the MnTe barrier layers remain sufficiently thin (typically approximately 40 angstrom). As an example of the effects of the strong quantum confinement in these heterostructures, photoluminescence spectra (normalized amplitudes) obtained from three CdTe quantum-well samples with well thickness of approximately 22, 15, and 10 angstrom, respectively, are shown. The emissions occur in the red, yellow, and blue for the n = 1 exciton recombination in the structures. The blue emission (at approximately 478 nm) shows how confinement effects are responsible for increasing the optical gap of CdTe by approximately 1 eV. Steady-state and transient spectroscopy were applied to the quantum wells to detail the confined electronic states. An approximate conduction-to-valence band offset ratio of approximately 10:1 and excitonic binding energies in the range of 20-25 meV were inferred. Temperature-dependent linewidth studies gave a measure of the exciton-optical phonon coupling strengths that was a factor of approximately 5-8 larger than values reported for the GaAs quantum wells.

We have employed short-wavelength holographic laser lithography and reactive ion etching to define wire and dot-like patterns in ZnSe thin epitaxial films and heterostructures with spatial feature size to better than 100 nm. Photoluminescence measurements suggest that surface damage from etching may be much less severe than in III-V semiconductors.<hedend>.

For the first time, ZnSe epitaxial layers grown by molecular-beam epitaxy are shown to exhibit large contrast, low power, and extremely long-term stable thermally induced absorptive optical bistability. It is observed over a wide temperature range between 169 K and RT, with a strongly variable loop width and a switch-back adjustable to be less than 50% of the respective switch-down value. Critical slowing down as well as switch-down times are studied in dependence of the excess over the switch-down intensity values.

Interfaces of pseudomorphic (100) ZnSe/GaAs heterostructures grown by molecular beam epitaxy have been studied by transmission electron microscopy. The observation of three different heterostructures suggests the existence of a transition structure at the ZnSe/GaAs interfaces which have formed on As-deficient GaAs surfaces. The transition structure appears as a bright line in dark field images of the 200 reflection, while it becomes a dark line in dark field images of the 400 reflection. These observations are explained by assuming the existence of an interface layer which has a zinc blende type structure having vacancies in one of the face centered cubic sublattices.

Epitaxial ZnSe/epitaxial GaAs interfaces have been formed by molecular beam expitaxy and evaluated by several techniques including capacitance-voltage measurements. In the study reported here, the GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe. A dramatic reduction of interface state density occurred when the GaAs epilayer was made As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.

A range of optical studies has been carried out on a series of single quantum wells of CdTe/MnTe. The structures appear to be nearly pseudomorphic and show evidence for very effective electron-hole confinement. For thin quantum-well layers, efficient low-temperature photoluminescence up to yellow-green photon energies has been obtained. Coupling of excitons to longitudinal-optical phonons has also been measured. © 1990 The American Physical Society.

MBE-grown ZnSe epilayers on (100) GaAs substrates are optically investigated under moderate to high excitation densities. Below the light-hole and heavy-hole exciton lines Xlhand Xhh, two lines I2and I'2are dominant in the luminescence spectra which are related to transitions from a neutral-donor-exciton complex (D0, X). For the first time, comparative excitation spectroscopy and magnetooptics of bound excitons in ZnSe epilayers allowed to determine their term structure, g-values and diamagnetic shifts. In the excitation spectra of the I2line, a set of resonances located 1.2 meV to 2.9 meV on its high-energy side can be interpreted as specific transitions into excited states of the neutral-donor-exciton complex. For excitation densities until 50 kW/cm2, high-excitation bands appear, superimposing the exciton luminescence. They are discussed to be due to exciton-exciton collision processes and can be compared with high-density transitions known from bulk ZnSe crystals. © 1989.

Optical characteristics of a new metastable wide-gap II-VI semiconductor heterostructure ZnTe/MnTe are reported. Single ZnTe/MnTe quantum wells show strong n = 1 exciton resonance manifesting, for example, in pronounced enhancement of the Raman cross-section. Pseudomorphic nature of the structures is inferred from strain induced shifts in the optical phonon spectrum. © 1990.

Structures of ZnSe/GaAs epitaxial interfaces have been studied by cross-sectional transmission electron microscopy. Narrow dark bands are observed along these interfaces in their bright field and high resolution electron microscope images, suggesting a certain form of modifications of the structure. Despite the appearance of the dark hands, perfect coherent lattice structures are observed in high resolution images of the interfaces. Weak beam dark field images indicate the presence of lattice distortions in the neighborhood of the interfaces. These observations are explained by assuming the existence of a thin interface layer which maintain the coherent lattice relation with matrix crystals. As-grown and annealed ZnSe/GaAs epitaxial interfaces are compared in order to investigate the structural origin of the change of the electrical property of the interface by the post-growth annealing. The observation suggests that the interfacial layer remains after annealing but has changed into a slightly broader one. © 1990.

The difficulty in achieving the amphoteric doping of wide gap II-VI semiconductors for use in light emitting devices suggests the merits of heterojunction injection. This paper discusses the first epilayer/epilayer ZnTe/AlSb/GaSb heterojunctions grown by MBE. Pseudomorphic ZnTe layers were nucleated on pseudomorphic AlSb layers; the AlSb was grown on homoepitaxial GaSb buffer layers. Free exciton related features were observed in the photoluminescence of ZnTe; dominant features of the PL corresponded to near band-edge transitions. Microstructural quality was examined using TEM and X-ray rocking curves. Cross-sectional TEM images reveal atomically flat interfaces. X-ray rocking curve full width at half maximum values were close to the theoretically expected broadening due to the finite layer thickness. © 1990.

New advances in epitaxial growth techniques have contributed to an improved understanding of the properties of II-VI materials and their alloys. The creation of novel heterojunctions, quantum well structures, and strained-layer superlattices, composed of II-VI/III-V and II-VI/II-VI multilayers, can be attributed to their successful epitaxial growth by molecular beam epitaxy (MBE). The objective of this paper is to discuss some recent advances in the growth of II-VI/III-V and II-VI/II-VI heterostructures, with emphasis on two particular topics. In the first, pseudomorphic epilayer/epilayer heterojunctions consisting of ZnTe on AlSb, a configuration with potential for the development of injection light emitting devices, have been grown by MBE. Microstructural and optical evaluation has indicated a high structural quality. The second topic describes the use of MBE for the growth of novel structures incorporating a previously hypothetical magnetic semiconductor, the zincblende phase of MnTe. Characterization using TEM and X-ray diffraction reveals only the zincblende phases of MnTe. Whereas bulk crystals of MnTe having a NiAs structure with a bandgap of 1.3 eV, the zincblende phase exhibits a bandgap of 3.2 eV. Single quantum well structures incorporating zincblende MnTe exhibit 2D electron and hole confinement in both CdTe and ZnTe quantum wells, and serve to confirm the zincblende MnTe bandgap at 3.2 eV. © 1989.

With the incorporation of cubic zincblende MnTe, a range of optical studies have been carried out on single quantum wells of ZnTe/MnTe and CdTe/MnTe. By using thin MnTe barrier layers the structures appear to be nearly pseudomorphic and show evidence for good electron-hole confinement. © 1989.

A range of optical studies have been carried out on a series of single quantum wells of CdTe/MnTe. The structures appear to be nearly pseudomorphic and show evidence for robust electron-hole confinement. Exciton states have been characterized in terms of lifetime and coupling to optical phonons. © 1990.

MBE-grown ZnSe epilayers released from their GaAs substrates exhibit thermally induced absorptive optical bistability of high contrast ratios and low switching powers in a wide temperature range up to room temperature. For increasing sample thickness or for enhanced reflection realized by inserting the samples into a resonator, counteracting dispersive switching becomes visible. The interplay of both the absorptive and dispersive effects is studied for different samples and varied experimental parameters. Long-term stability of the samples with regard to all switching effects makes them promising candidates for application studies. © 1990.

A series of pseudomorphic (100) ZnSe/GaAs heterostructures were grown by molecular beam epitaxy on GaAs epilayers which had different As coverages of surfaces. A large variation of the interface state density was observed among the heterostructures by capacitance-voltage measurements. Transmission electron microscope observations of cross-sectional samples have revealed existence of an interface layer in the heterostructures grown on As-deficient GaAs surfaces. Analyses of dark field images and high resolution microscope images have shown that the interface layer has a structure similar to that of Ga2Se3which crystallizes in a zincblende type structure with vacancies on the Ga sublattice. © 1990, The Japan Institute of Metals. All rights reserved.

ZnSe epilayers grown by MBE on (100) GaAs substrates are optically investigated in comparison with bulk ZnSe crystals using photoluminescence and excitation spectroscopy under high excitation densities. High‐density luminescence bands appear in both materials for relatively low excitation densities (about 30 kW/cm2), but their energetic positions do not correspond. The intensity‐dependent shape, and the behaviour of the dominant band in bulk ZnSe in excitation spectroscopy are similar to that of the M band in other II VI semiconductors. The main bands growing up in the ZnSe/GaAs films are much more corresponding to typical P‐band emission of exciton‐exciton collision. Further, the transmission of ZnSe epilayers released from the GaAs substrate is studied in dependence on the laser intensity. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA

A pseudomorphic epilayer/epilayer heterojunction consisting of ZnTe on AlSb, having potential for the development of novel visible light emitting injection devices, has been grown by MBE. A variety of microstructural and optical evaluation techniques have provided evidence of high structural quality. The nonequilibrium growth capability of the MBE technique has enabled the growth of heterostructures incorporating a previously hypothetical widegap magnetic semiconductor, the zincblende phase of MnTe. Electron diffraction measurements of cross-sectional samples reveal only zincblende phases. Double barrier structures incorporating zincblende MnTe are found to exhibit 2D electron and hole confinement in CdTe and ZnTe quantum well layers, and serve to confirm the zincblende MnTe bandgap at 3.2 eV. © 1990.

ZnSe-ZnTe strained superlines (SLSs) were grown by molecular beam epitaxy 9MBE). Th optical properties of the ZnSe-ZnTe SLS were evaluated by photoluminescence (PL). The PL peak position was shifted by tailoring the structure of the superlattice. The luminescence color in the visible changed from blue-green to red. In order to obtain both p- and n-type conduction in wide-bandgap II-VI compound semiconductors, we have prepared ZnSe-ZnTe superlines with a modulation doping technique. When Sb was selectively doped in ZnTe layers, all the samples exhibited p-type conductivity with hole concentrations of (0.5-1.0)X1014cm-3. On the other hand, Ga-doped SLSs were type with electron concentrations of (2-7)X1013cm-3. Furthermore, the growth of ZnSe-ZnS SLS was also demostrated by metalorganic molecular beam epitaxy (MOMBE). Blue luminescence related. to the quantized levels in the SLS was detected. © 1988.

It has been established experimentally in a number of wide-gap II-IV semiconductors, notably ZnTe, that the recombination spectrum of photon energies at and above the bandgap often consists of well-defined peaks separated by LO phonon energies. The physical issues subject to debate have been (1) the distinction between hot luminescence (HL) and resonance Raman scattering (RRS), and (2) whether the energetically relaxing electron-hole pair maintains an excitonlike character throughout the entire secondary emission process. It is shown how the availability of a ZnTe-based quantum well with advanced experimental methods gives direct insight into this problem. Experiments were carried out on a ZnTe/MnTe single-quantum-well structure with very thin MnTe barrier layers (22 angstrom). Tunneling of electron-hole pairs out of the ZnTe quantum well strongly reduces the normal thermalized PL (photoluminescence) emission, so the entire secondary emission from the quantum well is dominated by hot luminescence. The emission, ranging from the laser excitation to the region of the n = 1 excitonic bandgap, shows how the fifth-order LO-phonon-related emission is strongly resonating at this gap. Its linewidth is considerably broadened compared with the lower orders, and a rapid loss of polarization memory with increasing order is observed. To investigate the question of exciton versus hot-electron and hole character in the secondary emission, the spectra were measured in magnetic fields up to 23 T. The key point is that, apart from small spectral shifts, the main features of the spectrum are preserved, a direct indication of the exciton nature of the process of recombination.

The capacitance-voltage (C-V) and the current-voltage characteristics of metal/ZnSe/p-GaAs capacitors have been investigated; both epitaxial layers were grown by molecular beam epitaxy. In the capacitor structures highly resistive, stoichiometric ZnSe functioned as the pseudoinsulator on the doped GaAs layers. The capacitance-voltage measurements demonstrated that the capacitors could be biased from accumulation through depletion, and into deep depletion, with current in the range of 10-8A cm-2. Very little frequency dispersion was observed in the C-V data when measured from 1 kHz to 1 MHz. From the high-frequency C-V curve, the surface state density as a function of position in the GaAs band gap was determined. Surface state densities were comparable to densities reported for (Al,Ga)As/GaAs heterojunctions.

Epilayers of the previously hypothetical zinc-blende MnTe have been grown by molecular beam epitaxy. Epitaxial layers (0.5 μm thick) of MnTe were characterized using x-ray diffraction and transmission electron microscopy; optical reflectance measurements indicate a band gap of ∼3.2 eV. A series of strained single quantum well structures was fabricated with zinc-blende MnTe forming the barrier to CdTe quantum well regions; photoluminescence spectra indicate optical transitions corresponding to strong electron and hole confinement.

Results of a deep-level transient spectroscopy study of Ga-doped ZnSe thin films grown by molecular-beam epitaxy are presented. Two prominent deep levels were observed in all the samples investigated. The concentration of the trap detected at 0.34 eV below the conduction-band edge was essentially independent of the doping concentration and is attributed to native defects arising from Se vacancies in the ZnSe films. The second level with an activation energy of 0.26 eV shows a very strong doping dependence and is tentatively identified as arising from dopant-site (gallium-on-zinc-site) defects complexed with selenium vacancies. Preliminary results also indicate that planar doping of ZnSe significantly reduces the concentration of the Ga-vacancy complex.

A series of pseudomorphic ZnTe/AlSb/GaSb epilayer/epilayer heterostructures, aimed at the realization of novel wide band-gap light-emitting devices, was grown by molecular beam epitaxy. The structures were evaluated by several techniques including transmission electron microscopy (TEM), x-ray rocking curves, photoluminescence (PL), and Raman spectroscopy. Reflection high-energy electron diffraction intensity oscillations were observed during nucleation of ZnTe. The presence of dislocation-free pseudomorphic structures was confirmed by TEM. The PL spectra of ZnTe epilayers showed dominant near-band-edge features composed of free and shallow impurity bound excitons.

The molecular beam epitaxy (MBE) and the microstructural, optical, and electrical characterization of two technologically important heterojunctions, the CdTe/InSb and the ZnSe/GaAs, are described. The II-VI/III-V heterointerface is formed by the epitaxial growth of each layer in either separate MBE growth chambers, or by each layer growth occurring in a single growth chamber. For the case where separate growth chambers are used, the active interface is preserved by employing passivation techniques, or by transferring the sample between growth chambers in an ultrahigh vacuum transfer module. The aforementioned growth approaches allow for the formation of an ‘epitaxial’ heterojunction, to be utilized as an active part of a heterojunction device. The CdTe/InSb heterointerface is approximately lattice matched (<0.05% mismatch), and is motivated by possible device applications provided by InSb quantum wells. The low temperature growth of InSb quantum wells is achieved by the use of an antimony cracking oven to provide Sb2molecules for the growth. No clear indication of mixed interfacial layers of In2Te3is observed by Raman spectroscopy or transmission electron microscopy. The ZnSe/GaAs heterointerface, having a 0.25% lattice constant mismatch, has potential for use in passivation of GaAs devices. The highly resistive, stoichiometric ZnSe is employed as an insulator in two GaAs device configurations: a field effect transistor structure and a metal-insulator-semiconductor capacitor. Electrical characteristics of the ZnSe/GaAs interface provide evidence of the electrical integrity, with measurements of the interface state density resulting in numbers comparable to those reported for the (A1, Ga)As/GaAs interface. © 1989, SPIE.

The integration of several optoelectronic device functions onto a common substrate material is an area which is currently being actively pursued. In an effort to achieve this objective, experiments are under way to examine the epitaxial growth and material properties of a variety of both II-VI and III-V compounds grown on a substrate where the II-VI/III-V heterostructure can be utilized. This paper describes some recent developments involving the molecular beam epitaxial (MBE) growth and characterization of two important II-VI/III-V heterostructures, CdTe/InSb and ZnSe/GaAs. The structures are formed taking three approaches, by using (i) surface passivation techniques, combined with transfer in air from one chamber to the other, (ii) separate growth chambers connected by an ultrahigh vacuum transfer module, and (iii) both III-V and II-VI growth is performed in the same growth chamber. Multiple quantum wells of InSb have been grown at low temperatures of 300°C, using either Sb4or Sb2(the Sb2originates from an antimony cracker). Electrical characterization of metal/pseudomorphic ZnSe/p-GaAs capacitor structures illustrates accumulation, depletion and deep depletion; high frequency capacitance versus voltage data indicate interface state densities for the ZnSe/GaAs interface to be comparable to those reported for (Al,Ga)As/GaAs interfaces. The following paper represents a combined summary of two papers presented at the 5th International Conference on Molecular Beam Epitaxy. © 1989.

The capacitance-voltage and current-voltage characteristics of metal/ZnSe/p-GaAs capacitors, where the p-GaAs and ZnSe epilayers were grown by molecular beam epitaxy, are reported. A thermal anneal process is described which reduces the ZnSe/p-GaAs interface state density to a level comparable to those reported for AlGaAs/GaAs heterojunctions.

ZnSe and ZnTe semiconducting materials are highly promising candidates for the fabrication of short-wavelength light-emitting devices. We have grown ZnSeZnTe strained-layer superlattices (SLSs) on InP substrates by molecular beam epitaxy. In addition to undoped SLSs, two kinds of modulation doped SLS samples were prepared in this study, the first with Ga-doped ZnSe layers and undoped ZnTe layers, and the second with undoped ZnSe layers and Sb-doped ZnTe layers. Van der Pauw measurements of the SLS samples at room temperature showed that their electrical properties can be controlled by using the modulation doping technique. The undoped sample and the Ga-doped sample exhibited n-type conduction, whereas p-type conduction was observed for the Sb-doped sample. Interdiffusion profiles of dopants were measured by secondary ion mass spectroscopy, and significant Ga redistribution was observed. Finally, we have fabricated pn junctions from ZnSeZnTe SLSs, and measured their current-voltage characteristics. © 1988.

A detailed study has been made of interdiffusion in ZnSe-ZnTe strained-layer superlattices (SLSs) grown by molecular-beam epitaxy (MBE) at a growth temperature of 320°C. In x-ray diffraction measurements, the satellite peak intensities relative to the zero-order peak intensity decreased with annealing time. The interdiffusion coefficient D was calculated assuming a linear diffusion model. The values of D=3.6×10- 21 to 2.2×10-19 cm2/s at an annealing temperature of 500°C were obtained for the ZnSe-ZnTe SLSs. In the high-resolution transmission electron microscopy (HRTEM) image of as-grown SLSs, the presence of fine superlattice structures was seen, but for interdiffused samples stripes due to the periodic superlattice structures were not visible and many dislocation lines were observed. These results suggest that the structure of SLSs is significantly modulated by thermal annealing at a temperature higher than the growth temperature.

In order to obtain both p- and n-type conduction in a wide-bandgap II-VI compound semiconductor, we have prepared ZnSe-ZnTe strained-layer superlattices (SLS) by MBE with a modulation doping technique. Modulation-doped superlattices were analyzed by photoluminescence (PL) and the van der Pauw method. The effect of strain on the film quality induced in the SLS structure by lattice mismatch was investigated. Furthermore, the SLS structure has been directly observed by Transmission Electron Microscopy (TEM). © 1987 SPIE.

ZnSe-ZnTe strained-layer superlattices (SLSs) were grown on InP, GaAs and InAs substrates by molecular beam epitaxy (MBE). The samples were characterized by transmission electron microscopy (TEM) observation, and photoluminescence (PL) measurement techniques. Uniform and fine superlattice structures were confirmed by TEM observation. Moreover, a distinguishable change in the direction of (111) lattice-fringes from ZnSe layer to ZnTe layer was clearly observed in lattice image observation. The PL peak energy depended on the thickness of the alternating layers and the luminescence was related to the size quantization of the SLS structure. The luminescence peak was also affected by the substrate materials. © 1987.

The conduction types of ZnSe-ZnTe strained-layer superlattices (SLS's) have been controlled by using the modulation doping technique. Two kinds of modulation-doped SLS's were prepared. One of them consisted of gallium (Ga) doped ZnSe layers and undoped ZnTe layers. The other consisted of undoped ZnSe layers and antimony (Sb) doped ZnTe layers. The conduction types of the samples modulation doped with Ga and Sb were shown to be n and p type, respectively whereas the undoped samples exhibited n-type conduction. The electrical properties of the undoped and modulation-doped samples were evaluated by the van der Pauw method. The carrier concentrations of all types of samples were about 5×1013/cm3 at room temperature. The temperature dependence of the electrical properties was measured for an undoped sample and a sample modulation doped with Sb. The carrier concentrations increased with temperature and reached about 1×101 7/cm3 at 500 K.

Lattice strain and lattice dynamics of ZnSe-ZnTe strained-layer superlattices (SLSs) were examined by means of transmission electron microscopy and Raman scattering. Superlattice structures were confirmed from both transmission electron diffraction patterns and bright field images of the samples. Lattice distortions which originated from misfit strains were observed by the lattice image mode. The crystallinity of the adjacent layers was degraded when the layer thickness became large and polycrystalline structures were observed when the layer thickness exceeded the critical layer thickness. Raman scattering was used to assess the magnitude of the strain in the SLSs. Optical phonon frequencies of the components in SLSs were different from those frequencies of the bulk materials. The frequency shifts were used to quantify the strains in the SLSs. The observed shifts corresponded to the calculated values.

A ZnSe-ZnTe strained layer superlattice (SLS) was grown by molecular beam epitaxy with a 7% lattice mismatch between the components of SLS. InP was used as a substrate material and there was a mismatch of only 3.5% between the components of the SLS and InP. ZnSe was expanded and ZnTe was compressed. Thus the strain was accommodated by the SLS structure and a high-quality superlattice was prepared. Reflective high-energy electron diffraction and X-ray measurements indicated that a high-quality SLS was successfully grown. The superlattice structure was also confirmed by photoluminescence (PL) measurements. Moreover, an interesting phenomenon was observed from the temperature dependence of PL intensity. Strong luminescence was obtained only at a temperature of around 60 K from several samples. © 1986.

Direct observation of ZnSe-ZnTe strained layer superlattice was performed by transmission electron microscopy. The long periodicity of the superlattice was confirmed by the satellite spots of transmissin electron diffraction. The bright field TEM image indicated the superlattice structure without large scale misfit dislocations.

超格子は新しい機能性材料として大きな注目をあびている.本稿では,ひずみ超格子の概念や歴史的経過,設計指針などについて述べる.また,現在までに研究・発表されている各種ひずみ超格子について,その基礎物性を紹介する.

ZnSe thin films were grown by plasma-assisted metalorganic chemical vapor deposition (MOCVD). Plasma-assisted MOCVD is a technique combining rf glow discharge decomposition (at 13.56 MHz) with conventional metalorganic chemical vapor deposition. This process may offer several advantages such as low-temperature growth, high chemical reactivity, cleaning effect of substrate surface, and good surface morphology. The metalorganic sources used were diethylzinc and diethylselenide. The epitaxial films were obtained at the low substrate temperature of 250 °C by plasma-assisted MOCVD. The grown films showed excellent surface morphology and uniformity over a large area. These films were applied to Al/ZnSe:Mn/ITO (indium tin oxide) dc-operated electroluminescent cells. As a manganese source, di-π-cyclopentadienyl manganese [(C5H5)2Mn] was successfully used.

Optical properties of ZnSe-ZnTe strained-layer superlattice (SLS) grown on InP substrates were evaluated by photoluminescence. The luminescence peak intensity and position were affected by the photoexcitation intensity. The line shape was also affected by the growth temperature. The peak position was shifted by tailoring the structure of the superlattice, but it did not correlate exactly with previously reported theoretical values. We have reconsidered their data, and a better result on theoretical calulation was achieved. The luminescence color changed in the visible region from green to red. This SLS could be a new material for optoelectronic devices.

A ZnSe-ZnTe strained-layer superlattice (SLS) was grown on an InP substrate by molecular beam epitaxy for the first time. The x-ray diffraction measurement technique was used to confirm the existence of the high quality SLS structure. Overall quality may also be inferred from the observed quantum size effects of the photoluminescence data.

Al/ZnSe: Mn/ITO (indium tin oxide) dc-electroluminescent cells were fabricated by plasma-assisted organometallic chemical vapor deposition. As a new manganese source, di-lr-cyclopentadienyl manganese [(C, H), Mn] was successfully used for the first time. © 1985 IOP Publishing Ltd.

Two types of ZnSe:Mn dc thin-film electroluminescent (EL) devices were prepared by molecular beam growth: a Au/ZnSe:Mn/GaAs cell with a single-crystal ZnSe:Mn host layer and an Al/ZnSe:Mn/ITO cell with a polycrystal ZnSe:Mn host layer. Modulation doping was used to investigate the active region in the ZnSe:Mn layer. It was found that threshold voltage characteristics of the Au/ZnSe:Mn/GaAs cell were influenced by the Mn doping position in the ZnSe host layer, but in the Al/ZnSe:Mn/ITO cell, there was no dependence of threshold voltage on the Mn doping position. The dependence of luminescence intensity-injection current characteristics on the Mn doping position showed that the excitation probability of Mn luminescence centers by injected electrons increased in the region close to the negative electrode for both types of EL cells.

ZnSe thin films were grown on (100) and (111) oriented Si substrates by molecular beam epitaxy. The single-crystalline ZnSe films were obtained in the substrate temperature range of 300-450°C. The epitaxial layers were evaluated by reflection high-energy electron diffraction, x-ray diffraction, etch pit density, photoluminescence, and Hall effect measurements. Etch pit density was estimated to be 3×105cm-2. The carrier concentration and electron mobility of the epitaxial ZnSe layers at room temperature are 1.3×1017-2.8×1017cm-3and 170-250 cm2/V s, respectively. These epitaxial films were applied to Au/ZnSe:Mn/Si dc-operated electroluminescent cells. The maximum quantum efficiency of 2.2×10-2(30.4 V, 6.3×10-5A/cm2) was achieved.

ZnSe:Mn dc thin-film electroluminescent (EL) devices were prepared by a molecular beam growth method on glass substrates coated with transparent conductive oxide. We found that the EL properties were affected by the grade of the oxide. The EL properties were found to depend on the range of indium diffusion from the oxide to the ZnSe:Mn host layer. The effective thickness of the host layer was varied by the In diffusion. Based upon these results, we have proposed a new type of dc thin-film EL cell, namely an Al/ZnSe:Mn/ZnSe:In/oxide structure. A ZnSe:In layer was prepared as a buffer layer that would improve the crystallinity of the ZnSe:Mn host layer.