Cubic Ti1−xAlxN (x = 0.5, 0.6) films were deposited using RF (radio frequency) magnetron sputtering and cathodic arc ion plating methods on a thermally oxidized Si substrate to evaluate the temperature dependence of their electrical characteristics by Hall measurements from 10 K to 295 K. The temperature dependence of the hole concentration and the electrical conductivity above about 200 K shows that cubic TiAlN in this study had semiconductor characteristics, indicating that the hole conduction in the valence band is dominant in a high temperature range above about 200 K. In contrast, the variable range hopping conduction is dominant in a low temperature range below about 100 K. In addition, the hole mobility of Ti0.4Al0.6N deposited by RF sputtering increased with increasing temperature above 220 K, even though the phonon scattering increases. This result shows that a disordered grain boundary between polycrystalline TiAlN acts as a barrier layer for the hole conduction.

In this paper, we report the effects of surface barrier layers on the characteristics of AlGaAs/GaAs solar cells. The external quantum efficiency (EQE) spectra for AlGaAs barrier samples with different barrier layer AlAs fractions and thickness of the surface barrier layer were measured to increase the solar cell efficiency. The results show that the surface barrier layer is effective to block diffusing photoexcited electrons to the surface while the thicker barrier layer absorbs higher energy photons to generate carriers which recombine at the surface. The optimal surface barrier structure is a 50 nm thick Al0.7Ga0.3As. (C) 2015 Elsevier B.V. All rights reserved,

The effect of excitons in AlxGa1-xAs/GaAs superlattice solar cells has been investigated. We have shown that the superlattice active layers are effective to improve the solar cell performances because of the exciton enhanced photo-absorption. External quantum efficiency spectra show sharp and intense increase at the absorption edge due to excitonic absorption. This result indicates that excitonic photo-absorption can be stabilized at room temperature by using a superlattice structure. Optical properties of superlattice solar cells depend on the superlattice parameters because they determine the excitonic confinement effect, the tunneling effect and the sub-band structure. In this study, we compare external quantum efficiency for solar cells with different superlattice parameters to optimize the structure. The optimal barrier layer thickness is determined to be 1 nm for the Al0.5Ga0.5As/GaAs superlattice solar cell with 2-mu m-thick active layer. (C) 2015 Elsevier B.V. All rights reserved.

High quality CuGaSe2 and CuInSe2 single crystalline layers are grown on GaAs (001) by employing the deposition sequence of migration enhanced epitaxy using a solid source molecular beam epitaxy system. When CuGaSe2 is grown on CuInSe2 at moderate temperatures, severe interdiffusion takes place at the heterojunction of CuGaSe2/CuInSe2. This problem has been solved by optimizing the growth temperature and deposition rates of the constituent elements. Thus, we have successfully grown CuGaSe2/CuInSe2 single quantum well with sharp interfaces on GaAs (001) for the first time. Intense photoluminescence from the single quantum well with 10 nm well width is demonstrated. (C) 2015 Elsevier B.V. All rights reserved.

A 20 nm-thick AlON buffer layer consisting of Al2O3, graded AlON, AlN, and thin Al2O3 amorphous films was used to grow AlN on a sapphire substrate by molecular beam epitaxy with radio frequency plasma for nitrogen source (RF-MBE). Mirror-smooth AlN layers were successfully obtained using this new AlON buffer layer. The total threading dislocation densities of AlN layers are comparable to those reported for the high-quality AlN layers grown by RF-MBE using the conventional low-temperature (LT) buffer layer. (C) 2015 Elsevier B.V. All rights reserved.

In-situ reflection high-energy electron diffraction (RHEED) observation and Xray diffraction measurements were performed on heterojunction interfaces of CuGaSe2/CnInSe(2)/CuGaSe2 grown on GaAs (001) using migration-enhanced epitaxy. The streaky RHEED pattern and persistent RHEED intensity oscillations caused by the alternate deposition of migration-enhanced epitaxy sequence are observed and the growths of smooth surfaces are confirmed. RHEED observation results also confirmed constituent material interdiffusion at the heterointerface. Cross-sectional transmission electron microscopy showed a flat and abrupt heterointerface when the substrate temperature is as low as 400 degrees C. These have been confirmed even by X-ray diffraction and photoluminescence measurements. (C) 2015 Author(s).

We fabricated AlGaN/GaN high electron mobility transistors (HEMTs) on h-BN/sapphire substrates and transferred them from the host substrates to copper plates using h-BN as a release layer. In current-voltage characteristics, the saturation drain current decreased by about 30% under a high-bias condition before release by self-heating effect. In contrast, after transfer, the current decrement was as small as 8% owing to improved heat dissipation: the device temperature increased to 50 degrees C in the as-prepared HEMT, but only by several degrees in the transferred HEMT. An effective way to improve AlGaN/GaN HEMT performance by a suppression of self-heating effect has been demonstrated. (C) 2014 AIP Publishing LLC.

We demonstrated the transfer of GaN-devices from sapphire substrates to foreign materials using an h-BN release layer. In LEDs transferred onto indium sheets, no discernible degradation of the light emission performance occurred. Remarkably, the self-heating effect was suppressed in HEMTs transferred to copper plates because of improved heat dissipation.

We demonstrate that hexagonal boron nitride (h-BN) can work as a release layer that enables the mechanical transfer of gallium nitride (GaN)-based device structures onto foreign substrates. We illustrate the potential versatility of this approach by growing an AlGaN/GaN heterostructure, an InGaN/GaN multiple-quantum-well (MQW) structure, and a multiple-quantum-well light-emitting diode on h-BN-buffered sapphire substrates. These device structures, ranging in area from five millimeters square to two centimeters square, are then mechanically released from the sapphire substrates and successfully transferred onto other substrates.

We studied the structure of graphene layers grown by chemical vapor deposition on polycrystalline nickel. The conditions of the polycrystalline nickel catalyst (size of fine crystals and surface roughness) were controlled by cyclic heating and cooling, and its effect on the graphene layer formation was evaluated. By increasing the average size of the nickel fine crystals and thereby increasing of the surface roughness, nonuniformity of the graphene sheet numbers tends to increase. A marked change in graphene sheet number tends to occur at discontinuities in the polycrystalline nickel surfaces. From the structural analysis, the graphene layer is found to be made up of single or multiple crystal graphene thin films with different crystallographic directions. The size of each thin film is independent of and not restricted by the size of the nickel fine crystals, and a certain thin film passes over the discontinuities. (c) 2013 The Japan Society of Applied Physics

We have successfully released an InGaN/GaN light-emitting diode (LED) from a sapphire growth substrate and transferred it to a piece of commercially available adhesive tape using a mechanical transfer method called "MeTRe'' (Mechanical Transfer using a Release layer). By this method, a 3-nm-thick hexagonal BN (h-BN) layer inserted between the sapphire substrate and the GaN-based layer acts as both a buffer layer for the growth of a high-quality GaN-based layer and a release layer in the transfer process. A very thin (<0.1 mm) vertical LED prototype wrapped with two pieces of adhesive tape emitted violet-blue light. (c) 2012 The Japan Society of Applied Physics

Nitride semiconductors are the materials of choice for a variety of device applications, notably optoelectronics(1,2) and high-frequency/high-power electronics(3). One important practical goal is to realize such devices on large, flexible and affordable substrates, on which direct growth of nitride semiconductors of sufficient quality is problematic. Several techniques-such as laser lift-off(4,5)-have been investigated to enable the transfer of nitride devices from one substrate to another, but existing methods still have some important disadvantages. Here we demonstrate that hexagonal boron nitride (h-BN) can form a release layer that enables the mechanical transfer of gallium nitride (GaN)-based device structures onto foreign substrates. The h-BN layer serves two purposes: it acts as a buffer layer for the growth of high-quality GaN-based semiconductors, and provides a shear plane that makes it straightforward to release the resulting devices. We illustrate the potential versatility of this approach by using h-BN-buffered sapphire substrates to grow an AlGaN/GaN heterostructure with electron mobility of 1,100 cm(2) V-1 s(-1), an InGaN/GaN multiple-quantum-well structure, and a multiple-quantum-well light-emitting diode. These device structures, ranging in area from five millimetres square to two centimetres square, are then mechanically released from the sapphire substrates and successfully transferred onto other substrates.

We investigated the temperature dependence of the current gain of npn-type GaN/InGaN double-heterojunction bipolar transistors (DHBTs) in the low-temperature region. The current gain increased with decrease in device temperature due to the reduction of the recombination current in the p-type base layer. The current gain reached as high as 5000 at 40 K, which is the highest among nitride-based HBTs. For conventional HBTs made of InP or GaAs, the current gain decreased with decreasing device temperature. However, no reduction of the current gain was observed in this study, suggesting that the minority carrier mobility in the p-type InGaN base layer has negative temperature dependence, presumably because the ionized impurity scattering is relatively unaffected owing to the carrier freezeout and the high activation energy of Mg in the p-InGaN base layer. (C) 2009 Elsevier B.V. All rights reserved.

We investigated the temperature dependence of the common-emitter I-V characteristics of a pnp AlGaN/GaN heterojunction bipolar transistor (HBT) at temperatures ranging from RT to 590 degrees C. The HBT operated at 590 degrees C in air with a current gain of 3. Even at 590 degrees C, the collector-emitter leakage current was as low as 9 mu A at the collector-emitter voltage of 40 V. Although there is no significant degradation of the HBT characteristics only by annealing in air at 400 degrees C for 2 h, the current gain reduced to 30% of the initial one after the common-emitter operation at 400 degrees C for 2 h.

To obtain a guideline for improving the device performance of GaN-based Metal-Insulator-Semiconductor (MIS) heterostructure field-effect transistors (HFETs), both HFET and MIS HFETs were fabricated and compared, whose gate capacitances are designed to be equal. With an increased insulator thickness and decreased AlGaN thickness, a high trans-conductance was successfully obtained that was equivalent to that of HFET, in addition to the reduced gate leakage current in MIS HFETs. Moreover, Al2O3-based MIS HFETs were revealed to exhibit superior DC characteristics over Si3N4 MIS HFETs. Since deposition of insulators changes the electrical properties of heterostructures, considering the insulator/AlGaN structures as the total barrier layers is effective and indispensable in designing the MIS AlGaN/GaN HFETs. On the basis of the device design, MIS HFETs can be fabricated that exhibit superior device characteristics over those in conventional HFETs. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Hexagonal boron nitride (h-BN) has a potential for optical device applications in the deep ultraviolet spectral region. For several decades, only amorphous and turbostratic boron nitride (BN) films had been grown by chemical vapor deposition and metalorganic vapor phase epitaxy. By introducing flow-rate modulation epitaxy (FME), which enables us to reduce parasitic reactions and lower the optimal growth temperature, we have succeeded in growing single-phase h-BN epitaxial films on nearly lattice-matched (1 1 1) Ni substrates. The h-BN epitaxial films exhibit near-band-gap ultraviolet luminescence at a wavelength of 227 nm in cathodoluminescence at room temperature. The combination of FME and the lattice-matched substrate paves the way for the epitaxial growth of high-quality h-BN. (C) 2008 Elsevier B.V. All rights reserved.

We investigated the high-temperature characteristics for AlxGa1-xN-based (x = 0-0.57) vertical conducting diodes. In the forward current-voltage (I-V) characteristics, the offset voltage decreases with temperature because of the reduction of the built-in potential due to the decrease in the bandgap energy with temperature. In spite of an increase in SiC substrate resistance with temperature because of a decrease in the electron mobility, the on-state resistance of the diode with Al0.22Ga0.78N is as low as 1.45 m Omega cm(2) even at 250 degrees C because of the reduced resistance of the p-InGaN layer due to an increase in the hole concentration. In the reverse I-V characteristics, the breakdown voltage increases with increasing Al composition, x, of AlxGa1-xN layer because the higher the Al composition of the AlxGa1-xN layer is, the higher the critical electric field becomes. Although the reverse leakage current of AlxGa1-xN-based diodes increases with increasing temperature, the breakdown voltage at elevated temperatures is similar to that at room temperature. These features indicate the feasibility of AlxGa1-xN-based diodes for high-temperature operation.

We fabricated pnp AlGaN/GaN heterojunction bipolar transistors (HBTs) with various emitter areas on GaN substrates and investigated their common-emitter current-voltage and high-power characteristics at room temperature. The HBTs with an emitter area of 30x50 mu m(2) exhibited a high performance: a maximum current gain of 85 at a collector current of 30 mA and a maximum collector current density of 7.3 kA/cm(2) at a collector-emitter voltage of 30 V, which corresponds to the maximum power dissipation density of 219 kW/cm(2). The current gain and the collector current density increased compared to those on sapphire substrates. For the HBT with the emitter area of 270x450 mu m(2), the current gain was still as high as 47 and the maximum collector current reached as high as 1 A, and this single HBT showed a high-power dissipation of 30 W. This high performance of the HBTs is ascribed to the low dislocation density and relatively high thermal conductivity of the GaN substrate. Although the emitter crowding and self-heating effects remain, the obtained values are highest among pnp nitride-based HBTs. (C) 2008 American Institute of Physics.

We fabricated Pnp AlGaN/GaN heterojunction bipolar transistors (HBTs) with various base widths W(B) and investigated their common-emitter current-voltage characteristics at room temperature to clarify their carrier transport mechanisms. The current gain beta increased as W(B) decreased. The maximum current gain beta(max) was 40 in a HBT with a W(B) of 30 nm. HBTs with different base widths exhibited almost the same tendency for beta to increase with increasing the collector current I(C), indicating that the carrier transport mechanism is the same in all the n-GaN base layers. With a low I(C), recombination in the emitter-base depletion region is the dominant carrier transport mechanism. beta was less affected when I(C) was high, and the carrier transport was dominated by the minority hole diffusion in the neutral base layer. The minority hole diffusion length obtained from the HBT characteristics agrees well with previous results obtained with electron beam induced current measurements, also indicating that beta(max) was determined by the minority hole diffusion length in the n-GaN base layer. (c) 2008 American Institute of Physics.

Three types of growth defects commonly found epitaxial diamond films grown by chemical vapour deposition (CVD), namely unepitaxial crystals (UCs), hillocks with flat top (FHs) and pyramidal hillocks (PHs), were etched using hydrogen/oxygcn plasma to discuss their origin. UCs formed at random locations on the grown layer without any apparent relation with the substrate. Their nucleation might be due to contaminants and their development controlled by the growth conditions in the plasma. In contrast, dislocations formed from impurities segregated at the interface between the substrate and the CVD layer, were found to be the origin of the FHs and the PHs. A simple crystal model that involves micro-faceting or twinning at an intrinsic stacking fault originating from the dislocation core is proposed to explain the formation and the evolution of the growth defects. (C) 2007 Elsevier B.V. All rights reserved.

We investigated high-temperature (300 degrees C) characteristics of npn-type GaN/In(0.07)Ga(0.93)N double heterojunction bipolar transistors (DHBTs). The current gain obtained at 300 degrees C is as high as 308, although it decreases with increasing temperature. In Gummel plots, both base and collector current increases with increasing base-emitter voltage. While the maximum collector current at 300 degrees C is similar to that at room temperature (RT), the maximum base current monotonically increases with increasing temperature. Consequently, the reduction of the Current gain with temperature is attributed to the increase in the base current. Since the activation energy of the reduction of the current gain is larger than the expected value assuming the increase in the hole back-injection current from the base into the emitter, ail increase in the carrier concentration of the P-In(0.07)Ga(0.93)N base with temperature is considered to be attributed to the reduction of the current gain. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report the growth of (Cu,C)Ba2CuOy thin films by molecular-beam epitaxy (MBE). The c-axis oriented film is obtained with a = 3.959 A and c = 8.140 A. The T-c values of the film on an NdGaO3 substrate are T-c(on) similar to 35 K and Tc(zero) similar to 18 K. The T-c(on) is increased to 70 K for a 12 nm-thick film deposited on an infinite-layer CaCuO2 buffer layer. This suggests that the layer-by-layer growth method using (Cu,C)Ba2CuOy and [CaCuO2](n-1) is a promising way to obtain higher member (Cu,C)Ba2Can-1CunOy (n >= 2) thin films with a higher T,. We also find that the Ba-Cu-O phase with c similar to 4.0 A, which is known as the'infinite-layer BaCUO2' phase, can be obtained only in a slightly CO2 atmosphere. This means that the phase contains carbon in its structure. (C) 2007 Elsevier B.V. All rights reserved.

We report the deposition of NdBa2Cu3O7-theta(NBCO) thin films with a large diameter of over 5 in. by molecular-beam epitaxy (MBE). We examined the uniformity of the film composition and T-c for areas with a diameter of 140 mm. The film composition was Ba/Nd:Cu/ Nd = 1.79 +/- E 0.02:2.97 +/- 0.04, which indicates a uniform deposition over a large area. The Tc values only varied from 90.4 K to 91.6 K, which also confirms the uniform deposition. Thus our deposition method appears to be promising for producing thin film on a commercial scale. (C) 2007 Elsevier B.V. All rights reserved.

We investigated the temperature dependence of the common-emitter current-voltage (I-V) characteristics of npn-type GaN/InGaN double heterojunction bipolar transistors. Although the current gain decreases with increasing measurement temperature, the current gain measured at 300 degrees C is still as high as 308. The reduction of the current gain with temperature is attributed not only to the hole back-injection current from the base into the emitter but also to the shorter minority carrier diffusion length due to the increase in the carrier concentration of the p-InGaN base. (c) 2007 American Institute of Physics.

Nonpolar AlBN (1120) and (1100) films were grown using flow-rate modulation epitaxy. The transmission electron diffraction and lattice image reflect the wurtzite crystal structure of an AlBN (1120) film. The boron compositions in AlBN (1120) and (1100) films (B similar to 2%), estimated by x-ray diffraction assuming the wurtzite structure, agree well with the compositions measured by secondary ion mass spectroscopy, indicating that boron atoms are incorporated exactly into the wurtzite lattice sites. The (1120) face is more promising than the (1100) one for the growth of nonpolar AlBN because it has fewer dangling bonds of nitrogen on the surface.

The effects of sputtered and room temperature plasma enhanced chemical vapour deposition (RT-PECVD) SiNx passivation on the dc and microwave performance of AlGaN/GaN high electron mobility transistors (HEMTs) are studied. The pulsed I - V characteristics from a class B quiescent bias point and transient measurements indicate that the sputtered SiNx passivation is more efficient in suppressing lag effects in AlGaN/GaN HEMTs. Dispersion-free sputtered SiNx passivated AlGaN/GaN HEMTs were obtained using this technique. Continuous-wave (CW) measurements without active cooling give a maximum output power density of 6.6 W mm(-1) at V-gs=- 4 V, V-ds = 50 V and a maximum power added efficiency of 51.3% at V-gs=- 4 V, V-ds = 30 V at 3 GHz on 2 x 50 mu m AlGaN/GaN HEMTs on the sapphire substrate, with a gate length of 2 mu m and without field-plated gates. To the best of our knowledge, this is the highest level power density reported on the sapphire substrate without field-plate design. The extrinsic cut-off frequency (f(t)) and maximum oscillation frequency (f(max)) are 51 GHz and 100 GHz, respectively, on 2 x 50 x 0.15 mu m HEMTs. To our knowledge, the sputtered SiNx passivation for AlGaN/GaN HEMTs is a unique technique, which has never been published before.

We report room-temperature (RT) observation of near-band-gap ultraviolet luminescence at a wavelength of 227 nm in cathodoluminescence from h-BN heteroepitaxial layers. The h-BN layers were grown on single crystal Ni(111) substrates by flow-rate modulation epitaxy (FME), which is based on the alternate supply of triethylboron and ammonia. The h-BN layers grown with longer NH3 supply time exhibit stronger intensity and narrower full width of half maximum (FWHM) of the near-band-gap luminescence. X-ray diffraction measurements reveal that the FWHM of (0002) h-BN X-ray rocking curves decreases with increasing NH3 supply time. The reduction of lattice defects in h-BN grown by FME with longer NH3 supply time could be the reason for the improved near-band-gap ultraviolet luminescence at RT. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Free-exciton emission from AlN (0001) surface was characterized by angle-dependent photoluminescence (PL) measurement. As the radiation direction was inclined from the surface normal (c-axis direction), the emission intensity increased. This is because the optical transition between the conduction band and the top valence band is mainly allowed for light with the electric field parallel to the c-axis direction of AlN (E parallel to c) and consequently the free-exciton emission is strongly polarized for E parallel to c. By analyzing the angle-dependent PL intensities, the polarization ratio was estimated to be 0.995. This high polarization ratio results from the large negative crystal-field splitting energy. (c) 2007 American Institute of Physics.

A multienergy oxygen ion implantation process was demonstrated to be compatible With the processing of high-power microwave AlGaN/GaN high electron mobility transistors (HEMTs). HEMTs that are isolated by this process exhibited gate-lag- and drain-lag-free operation; A maximum output power density of 5.3 W/mm at V-gs = -4 V and V-ds = 50 V and a maximum power added efficiency of 51.5% at V-gs = -4 V and V-ds = 30 V at 3 GHz were demonstrated on HEMTs without field plates on sapphire substrate. This isolation process results in planar HEMTs, circumventing potential problems with enhanced gate leakage due to the gate contacting the 2-D electron gas at the mesa sidewall.

We have investigated the characteristics of pup GaN/InGaN/(Al)GaN double heterojunction bipolar transistors (DHBTs) grown by metalorganic vapour phase epitaxy. In GaN-collector DHBTs, the collector-base breakdown voltage under the open-emitter condition increased with increasing collector thickness. By fitting the experimental results to the theoretical model of the punch-through diode, we obtained the breakdown field of 3.1 MV/crn and the residual carrier concentration of 8.5 x 10(16) cm(-3) for the GaN collector layer. On the other hand, in a 500 nm thick AlGaN collector, the breakdown voltage was 190 V. Assuming that the bias was uniformly applied in the AlGaN collector, the corresponding breakdown field was calculated to be 3.8 MV/cm. The results indicate that the use of an AlGaN collector in HBTs is a promising way to increase the breakdown voltage without increasing the collector thickness. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report the growth of (Cu C)Ba2Ca(n-1)CUnOY (n = 1-4) superconducting thin films by the molecular beam epitaxy method. The film was c-axis oriented for n = 1, and was changed to a-axis oriented with 90 degrees domains for n = 2-4. All the films exhibited superconductivity and their T-c's were T-c(on) = 35 K and T-c(zero) = 18 K for n = 1, T-c(on) = 80 K and T-c(zero) = 52 K for n = 2, T-c(on) = 85 K and T-c(zero) = 42 K for n = 3, and T-c(on) = 105 K and T-c(zero) = 53 K for n = 4. (c) 2006 Elsevier B.V. All rights reserved.

We investigated the current-voltage characteristics of pup AlGaN/InGaN/GaN double heterojunction bipolar transistors grown by metalorganic vapor phase epitaxy. The InGaN base layer was heavily doped with Si in the range from 4 x 10(19) to 2 x 10(20) cm(-3). Base sheet resistance was less than 100 Omega/square for the base width of 30 nm and Si-doping concentration of 2 x 10(20) cm(-3). This is the lowest resistance ever reported for nitride-based heterojunction bipolar transistors. In this device, the current gain was 2.7 at the collector current of 23 mA. We observed no significant degradation in the crystal quality of InGaN in separate Hall-effect measurements of Si-doped InGaN and in the device operation, even though the Si-doping concentration was above 10(20) cm(-3). The capability to decrease the base resistance will be advantageous for high-frequency operation.

We investigated the interface microstructure of sandwich-type MgB2/Al-A1O(x)/MgB2 Josephson tunnel junctions by cross-sectional transmission electron microscopy (TEM) in order to clarify the non-idealities in the junction characteristics. The results indicate that there are poor-crystal line MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between the AlOx barrier and upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/Al-AIO(x)/MgB2 junctions.

We have succeeded in obtaining the high critical electric field exceeding 8 MV/cm measured using an AlGaN p-i-n vertical conducting diode on n-SiC substrate grown by low-pressure metal organic vapor phase epitaxy. The critical electric field of AlGaN with Al composition of 57% is as high as 8.1 MV/cm, the highest among semiconductors with a doping concentration of less than 10(17) cm(-3), at which the avalanche multiplication process takes place. The critical electric field is proportional to the bandgap energy to a power of 2.7. In the forward current-voltage characteristics, the on-state resistance of the diode increases with increasing Al composition. Since there is a tradeoff between the breakdown voltage (VB) and the on-state resistance (R-on) the figure of merit V-B(2)/R-on has its maximum when the Al composition is about 30% and is twice as high as that for GaN-based diodes. This indicates that AlGaN-based electronic devices are more promising for high-power operation than GaN-based ones.

The present status of diamond-based transistors for high-frequency and high-power applications is reviewed. We have achieved the drain current density of 550 mA/mm, cut-off frequencies for current gain (f(T)) and power gain (f(MAX)) of 45 GHz and 120 GHz, respectively, and output-power density of 2.1 W/mm at 1 GHz in class-A operation of a field-effect transistor (FET) with hydrogen (H)-surface-terminated diamond. We have found that gate capacitance can be separated into depletion-layer capacitance and insulator capacitance. Concerning a stability of H-surface termination, no apparent decrease in the current for an FET without a gate contact was observed, but gate bias stress results in a slight decrease in the drain current and simultaneously an increase in the gate leakage current. (c) 2007 Elsevier B.V. All rights reserved.

The growth of thin boron nitride (BN) films on graphitized 6H-SiC substrates was investigated in an attempt to reduce the large lattice mismatch between 6H-SiC and BN, which would improve the three-dimensional ordering in BN thin films grown by metalorganic vapor phase epitaxy (MOVPE). BN thin films were grown by low-pressure (300 Torr) MOVPE using triethylboron and ammonia on graphitized 6H-SiC substrates with surfaces displaying (1 x 1) reconstruction as determined by low energy electron diffraction (LEED). The (1 x 1) surfaces were formed by annealing at 1500 degrees C in ultrahigh vacuum with a base pressure of 10(-10) Torr. The LEED patterns showed that the. surfaces were covered with single-crystal graphite several monolayers thick. X-ray diffraction revealed that the c-axis lattice constant of the BN was 6.72 angstrom, which is close to the 6.66 angstrom of bulk hexagonal BN. In contrast, BN films grown on non-graphitized 6H-SiC substrates by MOVPE under the same conditions were mostly amorphous. Use of a graphitized 6H-SiC substrate covered with graphite several monolayers thick improves the degree of three-dimensional ordering in BN thin films grown by MOVPE.

The authors show that high-pressure and high-temperature (HPHT) annealing is very effective for the activation of ion-implanted dopants in diamond. The HPHT annealing condition is located in the thermodynamically stable region for diamond in the phase diagram and is, therefore, much more efficient for the recovery of implantation-induced damage and for the activation of ion-implanted dopants than thermal annealing in vacuum. The B-implanted film after HPHT annealing showed a high mobility of 632 cm(2)/V s with a sheet hole concentration of 4.8x10(10) cm(-2) at 300 K and the doping efficiency of similar to 7%. The mobility is the highest so far for ion-implanted diamond. In the entire annealing temperature range, the HPHT annealing is more efficient than the thermal annealing in vacuum. (c) 2007 American Institute of Physics.

Nonpolar AlN (11(2)over bar0) and (1(1)over bar00) films were grown on SiC substrates by flow-rate modulation epitaxy (FME), wherein trimethylaluminum and NH3 were alternately supplied. FME provides both AlN (11(2)over bar0) and (1(1)over bar00) films with good crystallinity and smooth surfaces, whereas AlN (1(1)over bar00) films obtained by conventional metal-organic chemical vapor deposition exhibit poor crystallinity and rough surfaces with deep trenches consisting of (000(1)over bar) and (1(1)over bar01) N-face microfacets. FME effectively eliminates these trenches, because the microfacets are unstable and have faster growth rates because of the enhanced migration of Al atoms in the absence of excess N surface coverage under the Al-rich condition. (c) 2007 American Institute of Physics.

To systematically examine the effect Of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si3N4. SiO2, AlN, and Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (N-s) was observed for all the insulators with the order of N-s(Al2O3) > N-2 (AlN) similar to N-2(SiO2) > N-s(Si3N4) > N-0 (N-0: N, without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R-0 similar to R(SiO2) (R-0: R Without insulators). This order is the same as that of N-s except for SiO2. where the 2DEG mobility largely degraded due to the diffusion Of Si atOITIS into nitride layers. The increase in N, was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) file surface potential barrier (phi(B)), and (ii) the interface charge (N-lnt) between in insulator and AlGaN. phi(B) (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between phi(B) and the bandgap of the insulator. N-lnt (10(13) cm(-2)) was estimated to be similar to 0 (Si3N4), 0.1 (SiO2), 0.3 (AlN), and 0.5 (Al2O3); thus, the interface Was found to be positively charged for AlN and Al2O3, whereas it was found to be almost neutral for Si3N4 and SiO2. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for Understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited its surface passivation and/or gate insulators.

To clarify the mechanisms governing the formation and reduction of threading dislocations (TDs) in aluminum nitride (AlN) layers grown on SiC (0 0 0 1) substrates by metalorganic vapor phase epitaxy (MOVPE), we characterized the mosaicity and the growth mode. High-density (similar to 10(11) cm(-2)) three-dimensional (3D) AlN islands nucleate on the substrate. Because the islands are slightly misoriented with respect to each other, dislocations are generated with a high density of 10(10)-10(11) cm(-2) as the islands coalesce. However, most of the dislocations are annihilated because their propagation direction changes horizontally during the island growth. Thus, at the initial growth stage, the dislocation density is drastically decreased to 10(8)-10(9) cm(-2). Consequently, as the layer thickness increases, the defect-free region becomes larger and the misorientation becomes smaller. On the other hand, we found that the TDs induce a large tensile strain and that the residual strain decreases with decreasing dislocation density. From the relationship between a- and c-lattice strains, the Poisson ratio of AlN was determined to be 0.19. (c) 2006 Elsevier B.V. All rights reserved.

We fabricated InGaN p-n junction diode structures on SiC substrates by metalorganic vapor phase epitaxy, and investigated the minority carrier diffusion length in n- and p-InGaN layers by electron beam induced current measurements. The minority electron diffusion length in p-InGaN was little affected by the In content in InGaN. The diffusion length decreased with increasing Mg-doping concentration. The minority hole diffusion length in n-InGaN was little affected by Si-doping concentration but slightly decreased with increasing In content in InGaN. We also fabricated pup AlGaN/InGaN/GaN double heterojunction bipolar transistors and investigated their common-emitter current-voltage characteristics. The Si-doping concentration in the base was 4 x 10(19) cm(-3). The maximum current gain was 21 at a collector current of -10 mA for an emitter size of 30 mu m x 50 mu m. This good performance is ascribed to the large conduction band discontinuity between the AlGaN emitter and InGaN base. (c) 2006 Elsevier B.V. All rights reserved.

We investigated the Al composition and thickness dependence of the resistance of graded AlxGa1-xN buffer layers for vertical conducting devices on n-type 6H-SiC substrates. To measure current-voltage characteristics, a Ti/Au ohmic contact was formed on the backside of the SiC substrate. The Al composition of the buffer layer was varied from 2% to 10%. We found that the optimal buffer layer thickness for the lowest resistance decreases with increasing Al composition because the nucleation seeds on SiC substrate coalesce faster. Since the optimal layer thickness depends on the Al composition, the sheet Al concentration, which is calculated from the Al composition and the thickness, should be considered as a critical parameter for low buffer resistance of vertical conducting devices on n-SiC substrates. (c) 2006 Elsevier B.V. All rights reserved.

We investigated the resistance of conductive AlGaN buffer layers and the current-voltage characteristics of GaNp-i-n vertical conducting diodes on n-type 4H-SiC substrates grown by low-pressure metalorganic vapor-phase epitaxy. High Si doping of the AlGaN buffer layer at the AlGaN/SiC interface produces ohmic current-voltage characteristics in spite of the large band offset between AlGaN and 4H-SiC. Owing to the optimization of the AlGaN buffer layer, a low on-resistance (R-on) of 1.12 m Omega cm(2) with high breakdown voltage (V-B) of 300 V is obtained for a GaNp-i-n vertical conducting diode on a 4H-SiC substrate, leading to the figure of merit (V-B(2)/R-on) of 80 MW/cm(2), which is larger than that for the diode with the same structure on a 6H-SiC substrate (62 MW/cm(2)). This result indicates that 4H-SiC is preferable for fabricating GaN-based electronic devices with a low on-resistance and high breakdown voltage. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs), i.e., GaN-based metal-insulator-semiconductor (MIS) HFETs, have been fabricated that exhibited excellent DC and RF characteristics together with the reduced gate leakage current (Ig). The Al2O3/Si3N4 bilayer gate insulator was used to simultaneously utilize (i) the high-quality interface between Si3N4 and AlGaN, and (ii) the high resistivity and high dielectric constant of Al2O3. The Ig was less than 10(-4) A/min even at a gate voltage of +3 V. In a device with a gate length of 0.1 mu m, the drain current was 1.30 A/mm, and the cut-off frequency (f(T)) and maximum oscillation frequency (f(max)) were 70 and 90 GHz, respectively. Moreover, the deposition effect of insulators on the electrical properties in AlGaN/GaN heterostructures has been examined and theoretically analyzed for Si- and Al-based insulators (Si3N4, SiO2, AIN, and Al2O3), because insulators are commonly used for surface passivation as well as the gate insulator, and hence, clarifying the insulator deposition effect is a fundamental issue in GaN-based HFETs. The increase in the two-dimensional electron gas (2DEG) was observed for all the insulators, and the effect was found to vary among insulators. One result is that Al2O3 was most effective to increase N-s. The results were explained in terms of the change in the potential profile. The band engineering including insulators is proposed to be indispensable for interpreting and designing the device performance, because, through the potential profile change, the essential device parameters are altered such as the source resistance, the channel resistance under the insulated-gate, and its threshold voltage.

Single-phase (0 0 0 1) hexagonal boron nitride (h-BN) heteroepitaxial layers were successfully grown on lattice-matched Ni (I 1 1) substrate by flow-rate modulation epitaxy (FME) using triethylboron (TEB) and ammonia (NH3) at 1020 degrees C for the first time. The structural properties of the h-BN layers were investigated by high-resolution X-ray diffraction (XRD). The first-ever X-ray rocking curve (XRC) measurement of an h-BN layers was performed and the minimum full-width of half-maximum of 0.7 degrees was obtained. For the h-BN FME growth, larger NH3 flow rates and longer NH3 exposure time are preferable for improving the crystal quality. Our approach enables us to drastically improve the crystal quality of h-BN at relatively low growth temperature. (c) 2006 Elsevier B.V. All rights reserved.

The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (C-GS)-voltage (V-GS) results extracted from measured s parameters, the authors found a plateau in C-GS within a certain V-GS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative V-GS, as negative V-GS is increased, C-GS increases steeply. This results from holes penetrating the energy barrier. (c) 2007 American Institute of Physics.

We report on narrow photoluminescence (PL) spectra obtained from spatially localized excitons in InGaN quantum wells (QW). These PL lines (less than 1 meV wide) are clearly detected in QWs on several buffer structures and substrates with the micro-PL technique in low temperature regions. A narrow PL spectrum is one of the characteristics of an exciton confined in a quantum dot (QD). Our results directly confirm that QD-like nano-objects exist in InGaN QWs. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Aluminum nitride (AlN) has an ultrawide direct band gap of approximately 6.1 eV at low temperature and is fully miscible with gallium nitride. This makes AlN a promising material for ultraviolet optoelectronic applications. Here, we apply cathodoluminescence, photoluminescence, and reflectance spectroscopies to the same AlN layer grown by metalorganic vapor phase epitaxy on silicon carbide. In cathodoluminescence and photoluminescence, we observe strong near band edge emission at approximate to 6 eV. The contribution appearing at an energetic position of 5.983 eV could be identified as A free exciton recombination, strongly redshifted due to strain effects. The spectra obtained by reflectance measurements show features at 5.985 eV and approximate to 6.2 eV which we assign to the A exciton-in accordance to our luminescence measurements-and a combination of the B and C free excitons, respectively. (c) 2007 American Institute of Physics.

We discuss the growth mechanism of GaN films and report very high two-dimensional electron gas (2DEG) mobility in AlGaN/AlN/GaN heterostructures fabricated on sapphire using BGaN micro-islands as novel buffers by metalorganic vapor phase epitaxy. The three-dimensional growth of BGaN (formation of BGaN micro-islands) occurs due to the phase separation of BGaN. However, the surface of the overgrown GaN on the BGaN micro-islands becomes smooth and continuous through the epitaxial lateral overgrowth process. The threading dislocations (TDs) in GaN consist mainly of pure edge-type ones and are effectively annihilated using single and double layers of BGaN micro-islands from 2 x 10(10) to 2 x 10(9) and 2 x 10(8) cm(-2), respectively. An n-type GaN film shallowly doped with Si exhibits an electron concentration and high Hall mobility of 3.0 x 10(16) cm(-3) and 669 cm(2)/Vs at room temperature (RT). Very high Hall 2DEG mobility in an Al0.10Ga0.90N/AlN/GaN heterostructure is obtained: 1910 and 20,600 cm(2)/Vs at RT and 77 K, respectively. The sheet carrier density had almost constant values of 6.9-5.7 x 10(12) cm(-2) in the temperature range from 77 to 500K, indicating that the parallel conduction due to the residual electrons in the GaN underlying layer was negligible. (c) 2006 Elsevier B.V. All rights reserved.

The electrical properties in AlGaN/GaN heterostructures with Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been examined and analyzed. By insulators deposition, significant increase in the two-dimensional electron gas (2DEG) density (N-s) was observed with the order of N-s(Al2O3) > N-s(AlN) N-s(SiO2) > N-s(Si3N4) > N-0 (N-0: N-s without insulators). As the result, the decrease in the sheet resistance (R) was observed; the smallest order of R was R(Al2O3) < R(AlN) < R(Si3N4) < R-0 similar to R(SiO2) (R-0: R without insulator). The insulators deposition effect has thus been shown to be significant and different among insulators. The increase in N-s was analyzed in terms of the change in the potential profile, and the observed differences in N-s among insulators have been interpreted. The band engineering including insulators is indispensable in understanding and designing AlGaN/GaN HFETs, since insulators are commonly used for the surface passivation as well as for the gate insulators, and the insulators deposition is to alter the essential device parameters such as the source resistance. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by - 2 times and the luminescence bands from 270 to 320 min, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm(2)/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films. (c) 2006 Elsevier B.V. All rights reserved.

We characterized high-quality polycrystalline diamond with large grains and fabricated polycrystalline diamond field effect transistors (FETs). The polycrystalline diamond had (110) preferred orientation, and its typical grain size was similar to 100 mu m. Well-resolved free exciton related emissions were observed at room temperature in cathodoluminescence. The FETs showed extremely high DC and RF performance. The cut-off frequency for current gain (f(T)) and power gain (f(max)) were 45 and 120 GHz, respectively. The maximum drain current (I-DS) was 550 mA/mm. These values are the highest among diamond FETs, including those fabricated from single-crystal diamond. These results suggest that high-quality polycrystalline diamond, whose maximum size is 4 in., is very promising for diamond electronic devices. (c) 2006 Elsevier B.V. All rights reserved.

For n-type Si-doped AlN with a low Si doping concentration of 3x10(17) cm(-2), a high room-temperature electron mobility of 426 cm(2) V-1 s(-1) was achieved, and at 220 K the mobility reached 730 cm(2) V-1 s(-1), the highest value ever reported for AlN. At Si doping concentrations lower than 10(18) cm(-3), dislocation scattering is the most dominant scattering mechanism, and the mobility can therefore be increased significantly by reducing the dislocation density. (c) 2006 American Institute of Physics.

p-InGaN/n-GaN vertical conducting diodes were grown on freestanding n-GaN substrates by metal organic vapor phase epitaxy. The homoepitaxial growth produced a high-quality GaN layer, as evidenced by the full width at half maximum of the (0002) x-ray rocking curve is as low as 34 arc sec. For a diode with a 3.6-mu m-thick n-GaN layer, a high breakdown voltage (V-B) of 571 V is obtained with a low on-state resistance (R-on) of 1.23 m Omega cm(2), leading to the figure of merit, (V-B)(2)/R-on, of 265 MW/cm(2). This is the highest value among those previously reported for GaN-based vertical conducting Schottky and p-n junction diodes. (c) 2006 American Institute of Physics.

We report the growth of (Cu,C)Ba(2)Ca((n-1))Cu(n)Oy (n = 2,4) thin films on NdGaO3(100) and LaSrGaO4(001) substrates by molecular-beam epitaxy (MBE). For n = 2, we synthesize superconducting films with T-c(on) similar to 85 K and T-c(zero) similar to 50 K on NdGaO3. The film is a-axis oriented with 90 degrees domains, and the lattice parameters are a = 3.894 angstrom and c = 11.634 angstrom. We also obtained a-axis oriented superconducting films for n = 4 with T-c(on) similar to 105 K and T-c(zero) similar to 53 K. (c) 2006 Elsevier B.V. All rights reserved.

We have succeeded in obtaining high critical electric fields from AlGaN layers using the p-InGaN/i-AlxGa1-xN/n-AlxGa1-xN (x = 0-0.22) vertical conducting diodes grown on n-SiC substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE). The breakdown voltage (V-B) increases with increasing Al composition of the AlGaN layer. The corresponding critical electric fields are calculated to be 2.4 MV/cm for GaN and 3.5 MV/cm for Al0.22Ga0.78N. The critical electric field is proportional to the bandgap energy to a power of 2.5. This bandgap energy dependence is much stronger than that in the empirical expression proposed by Sze and Gibbons. The figure of merit, (V-B)2/R-on, increases with increasing Al composition, indicating the AlGaN-based p-i-n diodes are promising for high-power and high-temperature electronic device applications. (c) 2006 Elsevier Ltd. All rights reserved.

A series of InGaN quantum wells (QWs) emitting blue-green, blue, violet, or ultraviolet light was grown on InGaN underlying layers (ULs). The potential fluctuation in these InGaN QWs was carefully measured using time-resolved photoluminescence, taking several steps to reduce the quantum confinement Stark effect. The potential fluctuation of InGaN QWs on InGaN ULs was smaller than that on conventional GaN ULs with the identical emission wavelength. A violet-light-emitting diode using an InGaN UL had the electroluminescence intensity approximately five times higher than the one using a conventional GaN UL under the low injection-current conditions, indicating that an InGaN UL effectively eliminates the nonradiative recombination centers in the InGaN QWs. (c) 2006 American Institute of Physics.

Using high-quality polycrystalline chemical-vapor-deposited diamond films with large grains (similar to 100 mu m), field effect transistors (FETs) with gate lengths of 0.1 mu m were fabricated. From the RF characteristics, the maximum transition frequency f(T) and the maximum frequency of oscillation f(max) were similar to 45 and similar to 120 GHz, respectively. The fT and f(max) values are much higher than the highest values for single-crystalline diamond FETs. The dc characteristics of the FET showed a drain-current density-I-DS of 550 mA/mm at gate-source voltage V-GS of -3.5 V and a maximum transconductance g(m) of 143 mS/mm at drain voltage V-DS of -8 V. These results indicate that the high-quality polycrystalline diamond film, whose maximum size is 4 in at present, is a most promising substrate for diamond electronic devices.

We proposed and demonstrated GaN growth on sapphire substrates by metalorganic vapor phase epitaxy using novel buffer layers of Al2O3/graded-AlON/AlN/Al2O3. The buffer layers were deposited by electron cyclotron resonance plasma sputtering at room temperature. The total thickness of the buffer layers was around 20 nm. The cross-sectional bright-field transmission electron microscope observation showed that the dislocations were bent at the initial stage of the growth, indicating the enhancement of lateral growth. Typical dislocation density in the GaN layer was 6.5 x 10(8) cm(-2). We obtained high electron mobility of 640 cm(2)/Vs at the electron concentration of 2.0 x 10(17) cm(-3) for Si-doped GaN. We also obtained high electron mobility of 1760 cm(2)/Vs at a sheet electron concentration of 8.3 x 10(12) cm(-2) for an AlGaN/GaN heterostructure. These results indicate that the quality of the epitaxial layer grown on sapphire substrates using the electron cyclotron resonance plasma sputtered buffer is comparable or superior to that of GaN layers using the conventional low-temperature buffer layer. (c) 2006 Elsevier B.V. All rights reserved.

Al2O3/Si3N4 insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) have been fabricated, where the regrown ohmic structure was incorporated to reduce the contact resistance. Excellent DC and RF characteristics have been obtained together with the low gate leakage current as the result of employing the metal-insulator-semiconductor (MIS) structure. An HFET with a gate length (L-g) of 0.1 mu m has exhibited a drain current density (I-d) and a transconductance (g(m)) of 1.30 A/mm and 293 mS/mm, respectively, with a reduced contact resistance of 0.3 Omega mm. The gate leakage current (I-g) was as low as 1 x 10(-8) A/mm in the reverse vias region, and only 4 x 10(-5) A/mm even at a forward bias voltage of +3 V. In this device, the cutoff frequency (f(T)) and maximum oscillation frequency (f(max)) were estimated to be 70 and 90 GHz, respectively. In the HFETs with longer L-g of 0.7 and 1.0 mu m, f(T) and f(max) were 20 and 48 GHz (L-g = 0.7 mu m), respectively; and 14 and 35 GHz (L-g = 1.0 mu m), respectively. Thus, the Al2O3/Si3N4 MIS HFETs have proved to also exhibit excellent RF characteristics. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Compact high-efficiency ultraviolet solid-state light sources(1) such as light-emitting diodes (LEDs) and laser diodes - are of considerable technological interest as alternatives to large, toxic, low-efficiency gas lasers and mercury lamps. Microelectronic fabrication technologies and the environmental sciences both require light sources with shorter emission wavelengths: the former for improved resolution in photolithography and the latter for sensors that can detect minute hazardous particles. In addition, ultraviolet solid-state light sources are also attracting attention for potential applications in high-density optical data storage, biomedical research, water and air purification, and sterilization. Wide-bandgap materials, such as diamond(2) and III - V nitride semiconductors (GaN, AlGaN and AlN; refs 3 - 10), are potential materials for ultraviolet LEDs and laser diodes, but suffer from difficulties in controlling electrical conduction. Here we report the successful control of both n-type and p-type doping in aluminium nitride ( AlN), which has a very wide direct bandgap(11) of 6 eV. This doping strategy allows us to develop an AlN PIN (p-type/intrinsic/n-type) homojunction LED with an emission wavelength of 210 nm, which is the shortest reported to date for any kind of LED. The emission is attributed to an exciton transition, and represents an important step towards achieving exciton-related light-emitting devices as well as replacing gas light sources with solid-state light sources.

We have investigated the current-voltage (I-V) characteristics of p-InGaN/n-GaN vertical conducting diodes grown on n(+)- SiC substrates by low-pressure metal organic vapor phase epitaxy (MOVPE). The breakdown voltage (V-B) of 250 V was obtained with a low on-state resistance (R-on) of 1.28 m Omega cm(2) when the n-GaN layer thickness was increased to 1800 nm, leading to the high figure-of-merit, (V-B)(2)/R-on, of 49 MW/cm(2). With increasing measurement temperature from room temperature (RT) to 520 K, the on-state resistance decreased due to the reduced contact resistance of the p-InGaN layer, while the breakdown voltage remained almost constant because of fewer defects in the n-GaN layer. These I-V characteristics are preferable for high-power and high-temperature electronic device applications.

This paper reports the first Studies on temperature dependent DC and RF characteristics of diamond metal-semi conductor field-effect transistors along with circular-transmission-like-method measurements on hydrogen-terminated diamond surface. In general, the device under study is thermally stable up to 100 degrees C as it does not deteriorate at higher temperatures with the cut-off frequency for current gain maintained at 8 similar to 9 GHz. It is found that the sheet resistance is almost totally independent of temperature, contact resistance is negligible, and channel conductance underneath the gate decreases with increasing temperature. The threshold voltage for the device is found to shift to the negative side with increasing temperature. A small-signal equivalent circuit analysis reveals that both transconductance and gate-source capacitance decrease with increasing temperature,. which results in the almost constant cut-off frequency for current gain. The experimental results call be explained by the fact that with increasing temperature, the band near the Al/H-terminated diamond surface bends upward more weakly, which leads to a decrease of buffer capacitance. At the same time the mobility decreases and the transconductance therefore decreases. (c) 2006 Elsevier B.V. All rights reserved.

Temperature dependent DC and RF characteristics of p-type diamond metal-semiconductor field-effect transistors (MESFETs) on hydrogen-terminated surfaces are investigated. The device is thermally stable up to 100 degrees C. because it does not deteriorate at all at higher temperatures. Temperature coefficients of transconductance (g(m)), drain conductance (g(ds)), gate-source capacitance (C-gs), gate-drain capacitance (C-gd), cut-off frequency (f(T)), and maximum drain current (I-ds) were obtained from small-signal equivalent circuit analysis. The cut-off frequency (fT) is almost totally independent of temperature. Intrinsic g(m), g(ds), and C-gs decrease with increasing temperature. C-gd is almost totally independent of temperature. The threshold voltage shifts to the negative side with increasing temperature. We propose a band model of an Al-gate contact/H-terminated diamond to explain the temperature dependence of these components.

We investigated the a- and c-axis lattice constants of GaN buffer and 180-nm-thick p-InGaN layers grown on SiC and sapphire substrates using reciprocal space mapping of the X-ray diffraction intensity. It was found that the a-axis lattice constant of the GaN buffer layer on a SiC substrate is larger than those of unstrained GaN and a GaN buffer layer on a sapphire substrate. As a result, the p-InGaN layer oil GaN/SiC is fully strained even at the In mole fraction of 9.0% where that on GaN/sapphire is relaxed. This result means that fewer defects are generated in p-InGaN on GaN/SiC at higher In mole fractions. This is another advantage of SiC Substrate for npn-type GaN/InGaN heterojunction bipolar transistors, in addition to its high thermal conductivity. The collector current density dependence of current gain shows the ideality factor of 2 for GaN/ InGaN HBTs on both SiC and sapphire substrates. This is ascribed to the recombination current at the emitter-base interface, which arises from the threading dislocations generated at the interface between the substrate and nitride buffer layer.

We prepared various MgO substrates cut from various ingots with different impurity-Ca concentrations and deposited NdBa2Cu3O7-delta (NBCO) thin-films on them by using molecular beam epitaxy (MBE). As a result, we found that the room temperature resistivity (rho(RT)) of NBCO thin-films and the Ca concentration in an MgO substrate are quantitatively correlated. Moreover we clarified the permissible impurity-Ca concentration in MgO substrates in terms of practical microwave applications.

We report the current-voltage characteristics of AlxGa1-xN (x=0-0.22) p-i-n vertical conducting diodes grown on n-SiC substrates by low-pressure metal organic vapor phase epitaxy. An increase in the breakdown voltage was experimentally demonstrated with increasing Al composition. The corresponding critical electric fields were calculated to be 2.4 MV/cm for GaN and 3.5 MV/cm for Al0.22Ga0.78N. The critical electric field is proportional to the band gap energy to a power of 2.5. The forward voltage drop also increases with increasing Al composition but it is still as low as 5.2 V even in the case of the Al0.22Ga0.78N p-i-n diode. (c) 2006 American Institute of Physics.

We report great improvement of RF output power for H-terminated diamond field-effect transistors (FETs). For the FET device with a gate width of 1 mm and a gate length of 0.4 mu m, the maximum output power (P-out) is 1.26 W, the maximum power gain is 23.2 dB, and the power added efficiency (PAE) is 56.3%. The increase in the device temperature when output power is 0.84 W is only similar to 0.6 degrees C. This is due to diamond having the highest thermal conductivity. (c) 2006 Elsevier B.V. All rights reserved.

We report great improvement of RF output power for H-terminated diamond field-effect transistors (FETs). For the FET device with a gate width of 1 mm and a gate length of 0.4 mu m, the maximum output power (P-out) is 1.26 W, the maximum power gain is 23.2 dB, and the power added efficiency (PAE) is 56.3%. The increase in the device temperature when output power is 0.84 W is only similar to 0.6 degrees C. This is due to diamond having the highest thermal conductivity. (c) 2006 Elsevier B.V. All rights reserved.

Boron nitride (BN) layers on 6H-SiC substrate were grown by metalorganic vapor phase epitaxy (MOVPE) using triethylboron (TEB) and ammonia (NH3). The growth rate of the BN decreased as the NH3 flow rate increased, indicating that a strong parasitic reaction occurred between TEB and NH3. Flow-rate modulation epitaxy (FME), which is based on alternating the gas Supply, was applied to the BN growth for the first time and it was found that the parasitic reactions could be effectively reduced. The Structural properties of BN grown by FME were also investigated by X-ray diffraction (XRD) and transmission electron microscopy. In contrast with amorphous BN layers grown by MOVPE. the BN structure crown by FME was turbostratic with a weakly preferred orientation to the c-axis.

The authors report the effects of nonradiative recombination on the properties of spatially localized excitons in InGaN quantum well structures studied using a microphotoluminescence (PL) technique. Sharp PL lines (linewidth of less than 1 meV) are clearly obtained by combining the PL and nanolithographic techniques. The PL originates from localized excitons induced by quantum-dot-like local potential minima where indium is accumulated. A systematic study with various kinds of samples reveals that suppressing the density of the nonradiative centers is crucially important in terms of observing the exciton localization effects rather than increasing the effects of indium accumulation. © 2006 American Institute of Physics.

Piezoelectric effects on PL properties are clarified in InGaN quantum wells (QWs) of 1-10 nm wide at 17 K. An extremely large change in PL peak energy (up to 200 meV) and PL decay time (two orders of magnitude) are found in a 10 nm-wide QW with increasing excitation intensity. We compare the distinctive characteristics of the field effects with QCSE in GaAs QWs and discuss possible optical device functions. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

On the basis of the thin barrier surface (TSB) model, the mechanism of gate current leakage under reverse gate-source bias in nitride-based heterostructure field effect transistors (HFETs) and metal-insulator-semiconductor (MIS) HFETs with an ultrathin (1 nm/0.5 nm) Al2O3/Si3N4 bilayer has been investigated. The simulations show that the electron tunneling through the Schottky barrier is the dominant mechanism for gate current in conventional HFETs due to the high density of donor like defects on the surface. An Al2O3/Si3N4 bilayer insulator can substantially reduce the donor like surface defect density and then significantly suppress the gate current leakage in nitrides-base MIS-HFET devices.

The device performance of AlGaN/GaN-based metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with a very thin AlGaN barrier and a heavily doped GaN channel for high 2DEG density has been investigated at elevated temperatures up to 250 degrees C. The devices exhibited ultra-low gate current leakages under the reverse gate bias with very reasonable transconductance characteristics at both RT and high temperatures. MIS-HFETs with doped channel showed much higher saturation drain current and weaker current collapse than the conventional devices at RT and high temperatures. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nitride heterojunction bipolar transistors (HBTs) are promising for high-power and high-temperature applications. However, two major issues for nitride HBTs have been low current gains and large offset voltages. Recently, to resolve these two issues, we developed a technique for extrinsic base regrowth of p-InGaN which led to the successful fabrication of Npn-type GaN/InGaN HBTs with high current gains and low offset voltages. These GaN/InGaN HBTs have a low-resistivity p-InGaN base and wide-bandgap n-GaN collector. We demonstrated their high breakdown voltage characteristics as well as high current density characteristics. These characteristics indicate that GaN/InGaN HBTs are favorable for high-power electronic devices. (c) 2006 Wiley Periodicals, Inc.

Nitride heterojunction bipolar transistors (HBTs) are promising for high-power and high-temperature applications. However, two major issues for nitride HBTs have been low current gains and large offset voltages. Recently, to resolve these two issues, we developed a technique for extrinsic base regrowth of p-InGaN which led to the successful fabrication of Npn-type GaN/InGaN HBTs with high current gains and low offset voltages. These GaN/InGaN HBTs have a low-resistivity p-InGaN base and wide-bandgap n-GaN collector. We demonstrated their high breakdown voltage characteristics as well as high current density characteristics. These characteristics indicate that GaN/InGaN HBTs are favorable for high-power electronic devices. (c) 2006 Wiley Periodicals, Inc.

Wurtzite AlBN thin films were fabricated using flow-rate modulation epitaxy (FME), wherein group-III and group-V sources are alternatively supplied to the growing surface. A 350-nm-thick wurtzite Al1-xBxN (x similar to 0.015) film grown by FME exhibited a single sharp peak in the omega-2 theta scan of x-ray diffraction, a smooth surface with the root-mean-square roughness of 3.4 nm, and a sharp epitaxial interface with SiC(0001) substrate, whereas that prepared by conventional metalorganic chemical vapor deposition contained faceted columnar AlBN(11_01) crystallites several ten nanometers in width in an AlBN(0001) matrix. FME is useful for fabricating high-quality wurtzite AlBN thin films, because it suppresses polycrystallization through enhancement of the surface migration of boron atoms. (c) 2006 American Institute of Physics.

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photo luminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 mu m spatial resolution. A sharp PL line (linewidth of < 0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions. (c) 2006 Elsevier Ltd. All rights reserved.

p-InGaN/n-GaN vertical conducting diodes have been grown on n(+)-SiC substrates by low-pressure metalorganic vapor phase epitaxy and their current-voltage characteristics have been investigated. The typical forward voltage drop was 3.8-4.0 V at a forward current density of 100 A/cm(2) with an on-state resistance of similar to 1.3 m Omega cm(2). The ideality factor of the samples was similar to 2, meaning that the tunneling current through defects is small enough in these devices. The breakdown voltage (V-B) increased with increasing n-GaN layer thickness, while it increased with decreasing carrier concentration of the layer by substituting undoped GaN for n-GaN. When the undoped GaN layer thickness was increased to 1800 nm, the highest breakdown voltage of 305 V was obtained with a low on-state resistance (R-on) of 1.51 m Omega cm(2), leading to the figure-of-merit, (V-B)(2)/R-on, of 62 MW/cm(2). (c) 2005 Americian Institute of Physics.

Great improvement in the output power density of diamond FETs on a diamond homoepitaxial layer grown using a high-purity source gas is reported. For a device with a gate length of 0.1 mu m and gate width of 100 pro, at 1 GHz, maximum output power density of 2.1 W/mm, maximum power gain of 10.9 dB, and power added efficiency of 31.8% were obtained.

Channel-doped AlGaN/GaN heterostructure field-effect transistors (HFETs) with metal-insulator-semiconductor (MIS) structures have been fabricated to obtain the high drain current density and reduced gate leakage current. A thin bilayer dielectric of Al2O3(4 nm)/Si3N4(1 nm) was used as the gate insulator, to simultaneously take advantage of the high-quality interface between Si3N4 and AlGaN, and high resistivity and a high dielectric constant of Al2O3. A MIS HFET with a gate length of 1.5 mu m has exhibited a record high drain current density of 1.87 A/mm at a gate voltage (V-g) of +3 V, which is ascribed to a high applicable V-g and a very high two-dimensional electron gas (2DEG) density of 2.6x10(13) cm(-2) in the doped channel. The gate leakage current was reduced by two or three orders of magnitude, compared with that in normal HFETs without a gate insulator. The transconductance (g(m)) was 168 mS/mm, which is high in the category of the MIS structure. Channel-doped MIS HFETs fabricated have thus been proved to exhibit the high current density, reduced gate leakage current, and relatively high transconductance, hence, promising for high-power applications. (C) 2005 American Institute of Physics.

We have grown NdBa2Cu3O7-delta films under silver atomic flux by molecular-beam epitaxy, which show a drastic improvement in microstructure and also crystallinity leading to a 30% enhancement in critical current density. The most remarkable point is that the final film is free from silver. The key to our process in achieving a silver-free film was the use of rf-activated oxygen that oxidizes silver, nonvolatile, to silver oxide, volatile at the deposition temperature. This process enables one to utilize the beneficial effects of silver in the growth of oxide films and at the same time ensures that the final film be free from silver, which is important for high-frequency applications. This method can be used in the growth of thin films of other complex oxide materials. (c) 2005 American Institute of Physics.

Temperature-dependent time-resolved PL measurements were performed for blue-purplish InGaN multiple quantum wells grown on various kinds of underlying layers (ULs). By using an InGaN UL, excitons recombined radiatively at low temperatures, being confined in the shallow potential minima (7.1 meV), while they radiatively recombined two-dimensionally with high luminescent efficiency at around room temperature, being delocalized thermally from the potential minima. Therefore, the exciton localization is not necessary in order to obtain high luminescent efficiency, but it is important to annihilate the nonradiative recombination centers by incorporation of indium atoms into ULs. (c) 2005 American Institute of Physics.

AlGaN/GaN-based metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayer as insulator have been investigated in detail, and compared with the conventional HFET and Si3N4- based MIS-HFET devices. Al2O3/Si3N4 bilayer-based MIS-HFETs exhibited much lower gate current leakage than conventional HFET and Si3N4-based MIS devices under reverse gate bias, and leakage as low as 1 X 10(-11) A/mm at -15 V has been achieved in Al2O3/Si3N4-based MIS devices. By using ultrathin Al2O3/Si3N4 bilayer, very high maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been obtained in the MIS-HFET device with gate length of 1.5 mu m, a reduction less than 5% in maximum transconductance compared with the conventional HFET device. This value was much smaller than the more than 30% reduction in the Si3N4-based MIS device, due to the employment of ultra-thin bilayer with large dielectric constant and the large conduction band offset between Al2O3 and nitrides. This work demonstrates that Al2O3/Si3N4 bilayer insulator is a superior candidate for nitrides-based MIS-HFET devices.

Thick p-InGaN layers were grown on GaN by metalorganic vapor phase epitaxy to investigate the strain inside p-InGaN using a reciprocal space map of X-ray diffraction intensity. It was found that a large part of p-InGaN grows coherently on the GaN buffer layer, even though it is much thicker than the calculated critical thickness. This result means that few dislocations are generated at the InGaN/GaN interface. Using this strained thick p-InGaN as a base, a GaN/InGaN heterojunction bipolar transistor was fabricated on a sapphire substrate. Its maximum current gain was as high as 1000 and its offset voltage as low as 0.2 V, which matches that calculated from the conduction-band discontinuity between the n-GaN emitter and the p-InGaN base.

Device performances have been compared between two types of AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors (MIS-HFETs) with Al2O3/Si3N4 bilayers and a Si3N4 single layer. Al2O3/Si3N4 bilayer-based MIS-HFETs have much lower gate current leakage than Si3N4-based MIS devices by more than 3 orders of magnitude under reverse gate biases. An ultralow gate leakage of 1 x 10(-11) A/mm at -15 V has been achieved in the Al2O3/Si3N4 bilayer-based MIS devices though higher maximum drain-source current has been obtained in the Si3N4-based MIS devices. A maximum transconductance of more than 180 mS/mm with ultra-low gate leakage has been achieved in the ultrathin Al2O3/Si3N4 bilayer-based MIS-HFET device with a gate length of 1.5 mu m, which is much higher than that of less than 130 mS/mm in the Si3N4-based MIS devices. The reduction in the transconductance of Al2O3/Si3N4 bilayer-based devices was much smaller than that in the Si3N4-based MIS devices due to the employment of ultrathin bilayers with a large dielectric constant. This work demonstrates that an Al2O3/Si3N4 bilayer insulator is a superior candidate for nitride-based MIS-HFET devices.

Sandwich-type all-MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated with as-grown MgB2 films formed by molecular-beam epitaxy. The junctions exhibit substantial superconducting current (IcRN product similar to 0.8 mV at 4.2 K), a well-defined superconducting gap (Delta = 2.2-2.3 mV), and clear Fraunhofer patterns. The superconducting gap voltage of Delta agrees well with the smaller gap in the multigap scenario. The results demonstrate that MgB2 has great promise for superconducting electronics that can be operated at T similar to 20 K. (c) 2005 American Institute of Physics.

An advanced structure of AlGaN/GaN heterostructure field-effect transistors (HFETs) has been proposed and fabricated, which is characterized by the following structural features: (i) a metal-insulator-semiconductor (MIS) structure using an Al2O3/Si3N4 bilayer gate insulator to reduce the gate leakage current, (ii) a thin AlGaN barrier with a doped channel to simultaneously obtain the high transconductance and high drain current, and (iii) a regrown ohmic structure to reduce the contact resistance. The fabricated devices have been proved to exhibit attractive characteristics such as low gate leakage current, low contact resistance, high drain current, and high transconductance. An HFET with a gate length of 0.1 mu m has exhibited a gate leakage Current density of below 10(-4) A/mm even at a gate voltage of +3 V. It has also exhibited a low contact resistance of 0.3 Omega mm, a high maximum drain current density of 1.23 A/mm, and a high transconductance of 280 mS/mm, which is the highest transconductance ever reported in the category of MIS-HFETs. The cutoff frequency and maximum oscillation frequency, measured with the pad capacitances included, were 52 and 75 GHz, respectively. The proposed structure has thus been proved to be effective in further improving the device performance in GaN-based HFETs.

We observe cavity polaritons in InGaN quantum well (QW) microcavities at room temperature. The crack-free microcavities with a high quality factor (Q) of 400 are fabricated by the wafer bonding of InGaN QW layers and dielectric distributed Bragg reflectors (DBRs). The anti-crossing behavior of cavity polaritons is confirmed with a vacuum-field Rabi splitting in the reflection measurements. We also observe the splitting in photoluminescence (PL) spectra. The oscillator strength of the InGaN QW excitons is found to be one order of magnitude larger than that of GaAs QW excitons. © 2005 WILEY-VCH Verlag GmbH &amp
Co. KGaA, Weinheim.

We observe cavity polaritons in InGaN quantum well (QW) microcavities at room temperature. The crack-free microcavities with a high quality factor (Q) of 400 are fabricated by the wafer bonding of InGaN QW layers and dielectric distributed Bragg reflectors (DBRs). The anti-crossing behaviour of cavity polaritons is confirmed with a vacuum-field Rabi splitting in the reflection measurements. We also observe the splitting in photoluminescence (PL) spectra. The oscillator strength of the InGaN QW excitons is found to be one order of magnitude larger than that of GaAs QW excitons. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p-n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108 cm(-2). It increased from 220 to 950 nm with decreasing Mg doping concentration from 3 X 10(19) to 4 X 10(18) cm(-3). For relatively high dislocation density above 109 cm(-2), it was less than 300 nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250 nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length. (C) 2005 American Institute of Physics.

A pnp AlGaN/GaN heterojunction bipolar transistor (HBT) with a thin n-GaN base shows high-voltage operation with high current gain in the common-emitter configuration at room temperature. The device structure was grown by metalorganic vapor phase epitaxy on a sapphire substrate. The emitter area is 30 mumx50 mum. The HBT can operate at high voltage of 70 V with the maximum current gain of 40 at the collector current of 10 mA. The maximum output power density is 172 kW/cm(2). Transport characteristics in the HBT were also investigated. At small collector current, the current gain is dominated by the recombination current at the emitter-base heterojunction. At moderate collector current, the calculated minority hole diffusion length well agreed with that determined from electron beam induced current measurements, indicating the current gain is dominated by the minority carrier diffusion. At large collector current, a high injection effect was observed in the current gain characteristics. (C) 2005 American Institute of Physics.

We fabricated a Pnp AlGaN/GaN heterojunction bipolar transistor and investigated its common - emitter current - voltage characteristics at room temperature. ne device structures were grown by metalorganic vapor phase epitaxy on the sapphire substrates. The buffer layer was a newly developed Al2O3/AlN/AlON/ Al2O3, resulting in the dislocation density of 6 x 108 cm 2 in MOVPE-grown GaN layer. This relatively low dislocation density led to the high voltage operation in the devices, corresponding to the breakdown field of 2.4 MV/cm. AlGaN/(Al)GaN superlattices were applied to the emitter and subcollector to increase the hole concentrations in these layers. An n-type GaN base width was 80 run. The sheet resistivity and the specific contact resistance were 900 Omega/square and 2.6 x 10(-5) Omega-cm(2) for a 80 nm base, respectively. The base sleet resistivity of Pup AlGaN/GaN HBT was two orders of magnitude smaller than that of Npn AlGaN/GaN HBTs. The maximum current gain was 8 at the collector current of 11.5 mA for the 30 [mu m x 50 mu n device. It operated at the collector current of 20 mA at the collector - emitter voltage of 65 V with a current gain of 5. The corresponding current density and power density were 1.3 kA/cm(2) and 84.5 kW/cm(2). High power operation was achieved by using the relatively low dislocation density GaN and low resistance superlattices. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report discrete photoluminescence (PL) spectra with narrow linewidths from spatially localized excitons obtained in InGaN quantum wells (QWs) that are of a similar structure to those used in conventional optical devices. A micro-PL measurement combined with submicrometer-scale fabrication techniques allows us to create a small number of excitons in a laser spot and to detect the PL from single spatially localized excitons. A sharp PL line (linewidth: 0.34 meV limited by the resolution) is clearly obtained in a 0.2 mu m mesa-shaped QW, which originates from a sin le localized exciton. We show that the technique is a more powerful method of examining excitonic effects-than previously reported methods.

Al2O3/Si3N4 insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) have been fabricated, where excellent RF characteristics have been obtained in addition to the low gate leakage current as the result of employing the metal-insulator-semiconductor (MIS) structure. In an HFET with a gate length (L-g) of 0.1 mu m, the cutoff frequency (f(T)) and maximum oscillation frequency (f(max)) were estimated to be 70 and 90 GHz, respectively. The drain current density (I-d) and transconductance (g(m)) were 1.30 A/mm and 293 mS/mm, respectively. The gate leakage current (I-g) was as low as 4 x 10(-5) A/mm even at a forward bias voltage of +3 V.

BxGa1-xN (x similar to 0.02) micro-islands provide novel buffers for growing GaN films and AlGaN/GaN heterostructures on sapphire substrates. These films and heterostructures show low threading dislocation density (TDD), low residual carrier concentration, and high two-dimensional electron gas (2DEG) mobility: Non-doped GaN films had the TDD of 2 x 10(8) cm(-2) and the residual electron concentration of 9.4 x 10(9) cm(-3) at 433 K. AlGaN/GaN heterostructures exhibited 2DEG Hall mobility of 1720 and 13100 cm(2) V-1 s(-1) at 300 and 77K, respectively. An almost constant sheet electron density (2.9-4.2 x 10(12) cm(-2)) was obtained in the wide temperature range from 500 to 77 K, indicating the absence of parallel conduction in the GaN buffer layers.

For n-type Si-doped AlN, we have obtained an electron mobility and concentration of 125 cm(2) V-1 s(-1) and 1.75x10(15) cm(-3) at 300 K, respectively. At 250 K, the mobility reached the maximum of 141 cm(2) V(-1)s(-1). To explain the temperature dependence of the mobility, we calculated mobilities limited by specific scattering mechanisms. We found that the mobility is limited by neutral impurity scattering rather than ionized impurity scattering or lattice scattering because of a large donor ionization energy (similar to250 meV). (C) 2004 American Institute of Physics.

InGaN multiple quantum wells were grown on InGaN underlying layers 50 nm thick by metalorganic vapor phase epitaxy. Photoluminescence (PL) measurements were performed by selective excitation of the quantum wells under a weak excitation condition. The PL intensity was almost constant at temperatures ranging from 17 to 150 K. Assuming that the internal quantum efficiency (eta(int)) equals unity at 17 K, we obtained eta(int) as high as 0.71 even at room temperature. The reason for the high eta(int) is the reduction of nonradiative recombination centers by the incorporation of indium atoms into the underlying layer. (C) 2004 American Institute of Physics.

The crystallographic relationship between InN and its interfaces with GaN and sapphire was investigated using selected area diffraction patterns obtained with a transmission electron microscope, and was confirmed using X-ray diffraction pole figures. The lattice of InN grown on a GaN template corresponded to that of the GaN in the c-plane, while that of InN grown directly on sapphire was rotated with respect to the sapphire by 30degrees. The polarity of InN was investigated using convergent-beam electron diffraction. The InN had the same N-polarity along the growth direction as that reported for Molecular beam epitaxy growth. The growth conditions for high-quality InN with photoluminescence (PL) at room temperature, and no metal indium inclusions are described. The dependence of PL characteristics on the growth conditions was investigated. The PL peak was observed to shift to higher energies as the growth temperature rose. However, it is noteworthy that hardly any PL peak shift was observed with the measurement temperature. From the data reported up to now, uncooled and high-power operation laser diodes for optical communications systems can be expected. (C) 2004 Elsevier B.V. All rights reserved.

The (111)-oriented chemical-vapor-deposited diamond homoepitaxial layers with low defect density exhibited well-resolved free-exciton transitions in cathodoluminescence at 13K and a sharp peak at 1332 cm(-1) (linewidth: 1.9 cm(-1)) in Raman scattering. Furthermore, using these (111) layers, we fabricated metal-semi conductor field-effect transistors (FETs). FETs with an 11-mum-long gate exhibited a maximum drain current of 24 mA/mm and maximum transconductance of 14 mS/mm. These values are of the same order as those for the (001) orientation.

The Magnetic and electric field effects of photoluminescence of excitons bound to nitrogen atom pairs in GaAs have been investigated for nitrogen delta-doped samples grown on (001) GaAs substrates. The excitons bound to nitrogen atom pairs produce a number of photoluminescence lines. However, these lines are much fewer than those observed in uniformly nitrogen-doped samples because of the limited spacing between two-dimensionally distributed nitrogen atoms. Among these lines, those appearing at 1.488, 1.476, and 1.428 eV are the most dominant. In this study, the characteristics of these dominant lines are investigated by an applying external field. The observed phenomena are explained by assuming that there is a continuous flow of excitons from a lower to a higher binding energy state under continuous excitation. Each photoluminescence line is found to split into two or more lines without applying an external field. The lines show a further split under a magnetic field and are finally quenched when the magnetic field is increased. The photoluminescence intensity of each line is modulated by the localization of excitons by a magnetic field and by the delocalization by an electric field.

The electronic properties of Si-doped InGaN thin films with different fit compositions were investigated. The samples were grown by metalorganic vapor phase epitaxy (MOVPE), and then evaluated using photoluminescence, X-ray diffraction and variable temperature Hall effect measurements. The Si donor activation energy was found to decrease with the increase in In composition of InGaN as a result of shrinking bandbap energy, and determined to be 17 meV, 10 meV and 6 meV for 4%, 9% and 13% of In compositions through the least square fitting of experimental carrier concentrations versus temperatures. InGaN alloy with 9% of In composition exhibited the best electronic properties with the lowest compensation ratio and the highest mobilities among those three samples over the whole range of measurement temperature. The relatively high mobility of 227 cm(2)/Vs at room temperature was achieved in this sample. Scattering mechanism in InGaN alloy was also studied using a simple model.

An Al2O3 insulated-gate (IG) structure was utilized for controlling the surface potential and suppressing the gate leakage in Al0.2Ga0.8N/GaN heterostructure field effect transistors (HFETs) having thin AlGaN barrier layers (less than 10 nm). In comparison with the Schottky-gate devices, the Al2O3 IG device showed successful gate control of drain current up to V-GS = +4V without leakage problems. The threshold voltage in the Al2O3 IG HFET was about -0.3V, resulting in the quasi-normally-off mode operation.

An InGaN-based horizontal-cavity surface-emitting laser diode (HCSELD) was fabricated by dry-etching of an InGaN-based multilayer on a SiC substrate and selective-area regrowth of a Mg-doped GaN layer. The InGaN-based HCSELD is a Fabry-Perot laser diode equipped with outer micromirrors that reflect the laser beams upward. The cavity mirrors and outer micromirrors are vertical {11 (2) over bar0} and inclined {11 (2) over bar2} facets of the regrown Mg-doped GaN layers, respectively. These grown facets are very smooth and had little angle misalignment. The InGaN-based HCSELD lased by current injection at room temperature. Current-injection lasing for group-III-nitride-based surface-emitting lasers is reported. (C) 2004 American Institute of Physics.

GaN/Alumina/GaN structures were fabricated to reduce dislocation density in GaN. Alumina films were deposited on GaN templates by electron cyclotron resonance plasma Sputtering. GaN was regrown on the alumina films by metalorganic vapor phase epitaxy. It was found that GaN on alumina selectively regrown from pinholes in alumina by annealing prior to GaN regrowth. Hardly any of the dislocations in the GaN template propagated through the interlayer. Most of dislocations were terminated or bent at the interface between the alumina and regrown GaN. On the other hand, new vertical dark lines and horizontal dislocations were generated in regrown layer. The vertical lines were found to correspond to an inversion domain from convergent beam electron diffraction analysis.

The regrowth of p-InGaN as the extrinsic base layer of Npn-type GaN/InGaN heterojunction bipolar transistors (HBTs) significantly improves the ohmic characteristics of the base layer. The specific contact resistance is 7.8 x 10(-4) Omega(.)cm(2) even after the dry etching. This value for a non-alloyed ohmic contact is much better than that for as-grown p-GaN (1.7 x 10(-3) Omega(.)cm(2)) and decreased turn-on voltage in the emitter-base diodes, which results in high current gains of up to 2000 and reduced offset voltages in the GaN/InGaN HBTs. These results show that the p-InGaN extrinsic base regrowth is an effective way to improve nitride HBT characteristics.

High-power characteristics have been investigated for GaN/InGaN double heterojunction bipolar transistors (HBTs) on SiC substrates. A base-collector diode showed a high breakdown voltage exceeding 50 V, which is ascribed to a wide band gap of a GaN collector. The maximum collector current is proportional to the emitter size in the emitter-size ranging from 1.5x10(-5) to 1.4x10(-4) cm(2). The corresponding maximum collector current density is as high as 6.7 kA/cm(2), indicating the high current density characteristics of bipolar transistors. A 50 mumx30 mum device operated up to a collector-emitter voltage of 50 V and a collector current of 80 mA in its common-emitter current-voltage characteristics at room temperature. The corresponding power density is as high as 270 kW/cm(2), showing that nitride HBTs are promising for high-power electronic devices in terms of both the material and the device structure. (C) 2004 American Institute of Physics.

Using heavily Si-doped AlN, a triode-type field emission display is demonstrated. The device consists of the heavily Si-doped AlN field emitter, mesh grid, and phosphor-coated anode screen. The device exhibits a low turn-on electric field of 11 V/mum, and the field emission current exponentially increases as the grid voltage increases. The field emission current reaches 9.5 muA at an electric field strength of 23 V/mum. Luminescence from the phosphor excited by the field-emitted electrons is uniform over the anode screen and is intense enough for the display application. The field emission current is stable over time. (C) 2004 American Institute of Physics.

Electron transport properties in AlGaN/InGaN/GaN double heterostructures have been investigated. Samples were grown by metalorganic vapor phase epitaxy and evaluated using x-ray diffraction and variable temperature Hall effect measurements. Much higher two-dimensional electron gas density of up to 50% has been obtained in AlGaN/InGaN/GaN structure than in a typical AlGaN/GaN structure due to the larger polarization effect while the mobilities are comparable at room temperature and above in these structures, which demonstrates the suitability of an AlGaN/InGaN/ GaN structure for high-power device applications. Theoretical-simulations were done to investigate the carrier transport mechanism, and they suggest that alloy disorder and interface roughness scattering have a very strong impact on the electron transport properties in AlGaN/InGaN/GaN structures. (C) 2004 American Institute of Physics.

The influence of the crystalline and surface properties of diamond homoepitaxial layers on device properties in H-terminated surface-channel diamond field effect transistors (FETs) is investigated. Crystalline defects inside the layers result in gate current leakage in FET DC operation. We confirmed incorporation of boron acceptors inside the layers. Their residual acceptors result in buffer leakage in DC operation, but do not affect RF characteristics much. Adsorbates from the environment change the concentration of the surface holes generated by the H-termination. (C) 2003 Elsevier B.V. All rights reserved.

We have confirmed the potential of a bulk AlN substrate for high current operation of nitride ultraviolet-light-emitting diodes (UV-LEDs). For the high flux UV extraction from nitride UV-LEDs, transparency and high thermal conductivity of the substrates are important issues. The bulk AlN is one of the best candidates, because it satisfies requirements above, and has the same crytallographic symmetry with those of AlGa(In)N families, which is beneficial to the high-quality crystal growth of the nitride device structures. We formed AlGaN-based UV-LEDs on a bulk AlN substrate and compared its performance with that of a reference device grown on an AlN-template grown on a sapphire substrate. The output power linearly increases with a saturation injection current of 300 mA, which is two times higher than that of the reference device. The emission spectrum under high current injection is much more stable than that of conventional substrate. (C) 2004 American Institute of Physics.

The potential of high power extraction from AlGaN-based ultraviolet light emitting diodes (UV-LEDs) is described. Improvements of UV-LEDs are shortly introduced from the viewpoints of nitride epitaxial growth, heterostructure optical characteristics based on the internal polarization field, and p-n junction design. The UV light extraction enhancement by utilizing the GaN-free transparent UV-LED structure and highly efficient UV-LEDs fabricated by introducing a high-quality AlN template on sapphire substrate are described. The maximum output powers are 8.6 mW and 5.5 mW at an injection current of less than 150 mA, at the emission wavelength of 350 rim and 340 rim, respectively. The highest external quantum efficiencies are 2.2 and 1.7 %, respectively. The application to white lighting and the potential of the high-flux UV-extraction utilizing bulk AlN substrate are also investigated.

We fabricated a basic triode-type field emission display (FED) structure by using heavily Si-doped AlN and demonstrated its action. The FED consisted of the heavily Si-doped AlN field emitter, mesh grid, and phosphor-coated anode screen. We used the grid voltage to control the emission current. This device exhibited an emission current of 2 muA for an electric field of 23 V/mum, and luminescence from the phosphor excited by the field-emitted electrons was observed. The brightness of the luminescence increased as the grid voltage was increased, and it was uniform over the entire field-emission area. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Crack-free AlGaN thin films were directly grown on SiC substrates by metalorganic vapor phase epitaxy, and their threading dislocation density was reduced by one order of magnitude using 1-2 nm thick, heavily Si-doped AlN multiple interlayers. The interlayers form SixAl1-xN ternary alloys, where the Si molar fraction ranges typically from 0.07 to 0.17. This technique enables us to grow crack-free AlGaN films, since the film thickness of about 1 mum is much smaller than that required in conventional epitaxial lateral overgrowth techniques. Both termination and looping of threading dislocations were observed near the interlayers using cross-sectional transmission electron microscopy. Light emitting devices with the SixAl1-xN multiple interlayers showed a remarkable improvement in the intensity and spectral width of electroluminescence and the series resistance. (C) 2003 American Institute of Physics.

Uniformly Si-doped AlN/Al0.5Ga0.5N superlattices were shown to have a high electron concentration. Even with high average At content of approximately 80%, the high electron concentration reached 3.2 x 10(18) cm(-3), which is about eight times higher than that of a bulk Si-doped AlGaN layer with the same Al content. In the AlN/AlGaN system, the conduction band offset is larger than the ionization energy of the Si donor in AlN. Therefore, the Si donors in the AlN barriers are fully activated and the corresponding electrons are transferred to the AlGaN wells. In addtion, the large band bending caused by the strong strain-induced piezoelectric and spontaneous polarization increases the activation of the donors in the AlGaN wells. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Stacking faults containing microtwins in (111)-oriented diamond layers grown on a high-pressure high-temperature (HPHT)-synthesized diamond substrate by chemical vapor deposition start to form just on the substrate. The microtwins in the stacking faults form on the {(1) over bar 11} plane, not on the (111) substrate plane. To explain these results, we propose an atomic-scale model in which a foreign atom remains on the HPHT substrate surface and a C atom on the foreign atom cannot form a covalent bond with it. Therefore, twinning of the C atom occurs on the {(1) over bar 11} plane. The next C atoms bond with the twinned C atom in an untwinned (normal crystalline) relation. Consequently, the formation of stacking faults that contain microtwins occurs. (C) 2003 American Institute of Physics.

GaN/InGaN double heterojunction bipolar transistors with a regrown p-InGaN extrinsic base have been fabricated on SiC substrates. The maximum common-emitter current gain exceeds 2000 at the collector current of 15 mA for a 50 mumx30 mum device. In addition, the offset voltage in the common-emitter current-voltage characteristics was reduced from 5 to 1 V. This indicates that the large offset voltage reported previously was mainly due to the degraded base ohmic characteristics. The regrowth of p-InGaN is effective for improving the characteristics of nitride heterojunction bipolar transistors. (C) 2003 American Institute of Physics.

We have formed ohmic contacts to p-GaN using a strained p-InGaN contact layer, and achieved the lowest contact resistance of 1.1 x 10(-6) Omega-cm(2) at room temperature by optimizing the contact layer thickness and its In mole-fraction. We have also evaluated thermal stability of ohmic contacts to p-GaN using. the strained p-InGaN contact layer. The contact resistance decreased to 2 x 10(-7) Omega-cm(2) at 100degreesC, and increased with elevating temperature above 100degreesC. In the temperature range up to 400degreesC, the contact resistances of the samples with the p-InGaN contact layer were smaller than those of the samples without the contact layer. Furthennore, the ohmic characteristics of the strained p-InGaN contact layer were less degraded even after the thermal process, compared with those of the sample without a content layer. These results indicate that the strained p-InGaN contact layer is favorable for practical application.

The Mg-acceptor activation mechanism and transport characteristics in a Mg-doped InGaN layer grown by metalorganic vapor phase epitaxy are systematically investigated through their structural, optical, and electrical properties. The In mole fraction was from 0 to 0.13, and the Mg concentration varied from 1x10(19) to 1x10(20) cm(-3). X-ray rocking curves for Mg-doped InGaN layers indicate that the structural quality is comparable to that of undoped and Si-doped InGaN layers. Their photoluminescence spectra show emissions related to deep donors emerged at lower energy when Mg doping concentrations are above 2-3x10(19) cm(-3). The electrical properties also support the existence of these deep donors in the same Mg concentration range because the hole concentration starts to decrease at around the Mg concentration of 2-3x10(19) cm(-3). These results indicate that self-compensation occurs in Mg-doped InGaN at higher-doping levels. The temperature dependence of the hole concentration in Mg-doped InGaN indicates that the acceptor activation energy decreases with increasing In mole fraction. This is the reason the hole concentration in Mg-doped InGaN is higher than that in Mg-doped GaN at room temperature. In addition, the compensation ratio increases with doping concentration, which is consistent with the deep donor observed in PL spectra. For Mg-doped InGaN, impurity band conduction is dominant in carrier transport up to a relatively higher temperature than that for Mg-doped GaN, since the acceptor concentration for Mg-doped InGaN is higher than that of Mg-doped GaN. (C) 2003 American Institute of Physics.

We investigated the Schottky characteristics of Au, Pd and Ni on n-GaN using capacitance-voltage and current-voltage measurements for mesa-structure diodes. Even though these metals have a similar metal work function of around 5.1 eV, their Schottky barrier heights are considerably different each other. The obtained Schottky barrier heights are 0.96, 1.50 and 1.34 eV for Au, Pd and Ni, respectively. The high Schottky barrier height for Pd is preferable for the gate metal in a field effect transistor and the ohmic metal for p-type nitride semiconductor. Although the work function difference between Au and I'd is just 0.02 eV, the barrier height difference is as high as 0.54 eV, meaning that the interfacial reaction between these metals and GaN or the Fermi-level pinning mechanism is quite different. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Npn-type AlGaN/InGaN/GaN heterojunction bipolar transistors have been fabricated for high power application. Their common-emitter current-voltage characteristics showed a high breakdown voltage of 120 V, corresponding to the breakdown electric field of 2.3 MV/cm in the collector. The cross-section transmission electron microscopy image showed that the V-shape defects in InGaN were filled with wider bandgap AlGaN layers during the emitter growth. This is considered to prevent the leakage current from the base to the collector, resulting the high breakdown electric field.

We have investigated high-power characteristics of GaN/InGaN double heterojunction bipolar transistors on SiC substrates grown by metalorganic vapor phase epitaxy. The p-InGaN extrinsic base layers were regrown to improve ohmic characteristics of the base. Base-collector diodes showed low leakage current at their reverse bias voltages due to a wide bandgap of a GaN collector, resulting in a high-voltage transistor operation. A 90 mum x 50 mum device operated up to a collector-emitter voltage of 28 V and a collector current of 0.37 A in its common-emitter current-voltage characteristics at room temperature, which corresponds to a DC power of 10.4 W. A collector current density and a power density are as high as 8.2 kA/cm(2) and 230 kW/cm(2), respectively. These results show that nitride HBTs are promising for high-power electronic devices.

We applied a bulk AlN substrate to an AlGaN-based ultraviolet light emitting diode (UV-LED) and found that this combination enables high injection current, which shows the LED's potential for large ultraviolet flux extraction. Heat dissipation is an important issue for LEDs. Bulk AlN substrate has high thermal conductivity, a wurtzite crystal symmetry the same as that of nitride emitters, and transparency in the ultraviolet wavelength range. An UV-LED grown on a bulk AlN substrate shows output power linearity up to high injection current up to 300 mA, whereas a similar device grown on an AlN-template formed on a sapphire substrate only shows linearity up to an injection current of about 150 mA. It also showed very stable emission peak wavelength. For example, the emission peak shift is less than 2 nm in spite of the large injection cur-rent of 200 mA. Both findings are attributed to the heat dissipation afforded by the high thermal conductivity of the bulk AlN. This LED still suffers from internal absorption loss caused by the residual color centers in the AlN at present. However, further improvement of bulk AlN substrates will lead to high flux and highly efficient ultraviolet sources.

We have fabricated Pnp AlGaN/GaN heterojunction bipolar transistors and investigated their common-emitter current-voltage (I-V) characteristics at the temperatures from room temperature to 300 degreesC. Good saturation properties were observed in their common-emitter I-V characteristics. The current gain of 20 at room temperature increased with elevating the temperature. The maximum current gain was as high as 71 at 300 degreesC with the collector current from 20 to 200 muA. This increase in current gain is ascribed to the enhancement in the hole injection efficiency from the p-AlGaN emitter to the n-GaN base as well as the reduction of the device resistance and the improved Ohmic characteristics of the subcollector. These results indicate that Pup HBTs have the potential for high temperature operation.

Field effect of photoluminescence due to excitons bound to nitrogen atom pairs in GaAs has been investigated for uniformly doped and atomic-layer-doped samples grown on (00 1) GaAs substrates. The intensities of excitonic photoluminescence lines due to distant nitrogen atom pairs decrease much more rapidly than those from closer pairs when the electric field is increased. In addition, photoluminescence clue to tile nearest neighbor pairs in atomic-layer-doped samples exhibits much more stable characteristics than that Of Uniformly doped samples against an applied electric field, This stability is observed only when the electric field is applied in either the [110] or [(1) over bar 10] direction. This anomalous field effect can be explained by considering the electron trapping process to the isoelectric N traps modulated by the electric field.

We fabricated a Pnp AlGaN/GaN heterojunction bipolar transistor and investigated its common-emitter current-voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The Al mole fraction in an AlGaN emitter layer was 0.18. The thickness of the GaN base layer was 0.12 mum and its Si doping concentration was as high as 1x10(19) cm(-3), so its base resistance decreased two orders of magnitude compared with the reported Npn nitride heterojunction bipolar transistors. The transistor showed good saturation current-voltage characteristics and the maximum common-emitter current gain of 28 was obtained for collector current of -2x10(-5) A at room temperature. (C) 2002 American Institute of Physics.

Valence-band discontinuities between InGaN and GaN were evaluated using the capacitance-voltage characteristics of p-InGaN/n-GaN heterojunction diodes with high hole concentrations in p-InGaN. This capacitance-voltage method is effective to evaluate valence-band discontinuities because the influence of the piezoelectric charges at the heterojunction is ignored due to high acceptor concentrations. The built-in potential obtained from the capacitance-voltage measurements decreased with the In mole fraction of p-InGaN. This result indicates that the valence-band discontinuity (DeltaE(V)) increases with the In mole fraction (x) and is expressed as DeltaE(V) (eV) = 0.85x for xless than or equal to0.28. The DeltaE(V) value obtained in this work is about 50% lower than that reported previously using the photoluminescence (PL) method.

We fabricated a pnp GaN bipolar junction transistor and investigated its common-emitter current-voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The base thickness was 0.12 mum and its doping concentration was estimated to be lower than mid-10(17) cm(-3). We have obtained a maximum common-emitter current gain of 50 at room temperature for collector current ranging from -10(-5) to -10(-4) A. (C) 2002 American Institute of Physics.

We propose a new method to evaluate the polarization charge density induced at hetero-interfaces using capacitance-voltage characteristics of p-n heterojunctions and have applied this method to p-GaN/n-AlGaN heterojunction diodes. This method uses p-n heterojunction structures instead of undoped ones, so the obtained result is less influenced by background impurities or deep levels. Furthermore, it is possible to evaluate the negative polarization charge densities for GaN on top of AlGaN heterostructures. The experimental results indicate that the negative polarization charges with their densities more than 5 x 10(12) cm(-2) are induced at the GaN/AlGaN hetero-interfaces for the Al mole fractions above 11%.

Npn InGaN/GaN double heterojunction bipolar transistors have been fabricated using p-type InGaN and n-type GaN as base and collector layers, respectively. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1 x 10(19) cm(-3) corresponding to a hole concentration of 5x10(18) cm(-3) at room temperature. The common-emitter I-V characteristics showed good saturation characteristics and the maximum current gain of 20 was obtained at room temperature. Using this current gain, a minority carrier diffusion length for electrons was calculated to be 0.32 mum in the p-InGaN base layer. The corresponding minority carrier lifetime was 400 ps. These results mean the good crystal quality of p-InGaN.

We have fabricated N-AlGaN/p-InGaN/n-GaN heterojunction bipolar transistors to evaluate their common-emitter current-voltage (I-V) characteristics at room temperature. The device has a Si-doped N-AlGaN emitter, a Mg-doped p-InGaN base, and a Si-doped n-GaN collector. The common-emitter I-V characteristics were observed up to a collector-emitter voltage of 70 V and a collector current of 14.5 mA. The breakdown voltage is as high as 100 V The corresponding breakdown electric field is 2 x 10(6) V/cm, which is comparable to the expected one for GaN. This high breakdown electric field is ascribed to a less damaged p-InGaN and a wide bandgap of an n-GaN collector. Furthermore:, the leakage path through the defects in InGaN layers is considered to be eliminated by filling them with wide bandgap AlGaN layers during the emitter growth.

Radiative recombination rate of excitons bound to nitrogen atom pairs in the AlGa-As/GaAs single quantum wells is investigated by comparing the recombination process between the quantized levels by using nitrogen atomic layer doped samples. When the nitrogen-doped layer is located at the center of the well, photoluminescence due to the quantized levels almost vanishes. The quantum well photoluminescence appears when the nitrogen-doped layer is shifted from the center of the well toward the heterojunction. It is found that the recombination process can be relatively modulated by changing the spatial distance of the nitrogen-doped layer from the heterojunction or, by applying an electric field in the direction perpendicular to the well.

We fabricated pnp GaN bipolar junction transistors and investigated their common-emitter and common-base current-voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The base thickness was 0.12 mum and its electron concentration was estimated to be 3 x 10(17) cm(-3) from the common-emitter current-voltage characteristics and the base conductivity. The common-emitter current-voltage characteristics showed very low leak current. The maximum current gains at room temperature were 50 and 69 from the common-emitter and the common-base current-voltage characteristics, respectively.

We investigated non-alloy Ohmic contact to p-type GaN using Mg-doped InGaN contact layer. The thickness and In mole fraction of the p-type InGaN were varied from 2 to 15 urn and from 0.14 to 0.23, respectively. Strained InGaN contact layers are effective in reducing the contact resistance. The lowest specific contact resistance of 1.1 x 10(-6) Omega cm(2) was obtained using a contact layer of 2 nm thick strained In0.19Ga0.81N. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to the large polarization-induced band bending at the surface as well as the high hole concentration in p-type InGaN.

InGaN/GaN double heterojunction bipolar Npn transistors have been fabricated using p-type InGaN and n-type GaN as base and collector layers, respectively. The structures were grown on SiC substrates by low-pressure metalorganic vapor phase epitaxy. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1 x 10(19) cm(-3) corresponding to a hole concentration of 5 x 10(18) cm(-3) at room temperature. The common-emitter I-V characteristics showed good saturation characteristics and a maximum current gain of 20 was obtained at room temperature.

A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1x10(-6) Omega cm(2). The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN. (C) 2001 American Institute of Physics.

InGaN/GaN double heterojunction bipolar transistors have been fabricated using p-type InGaN as a base layer. The structures were grown on SiC substrates by metalorganic vapor phase expitaxy and defined by electron cyclotron resonance plasma etching. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1x10(19) cm(-3) corresponding to a hole concentration of 5x10(18) cm(-3) at room temperature. From their common-emitter current-voltage characteristics, the maximum current gain of 20 was obtained at room temperature. (C) 2001 American Institute of Physics.

We investigated the effect of In atoms in p-GaN on the damage induced by electron cyclotron resonance etching. After etching the surface of p-GaN without In atoms, the I-V characteristics between two Ni/Au electrodes on the surface showed non-Ohmic behavior. This is ascribed to the damage induced by this etching process, as previously reported. The Ohmic characteristics were much improved for p-GaN doped with In atoms compared with those for p-GaN without In atoms. The Ohmic characteristics were improved as the In mole fraction in the p-(In)GaN was increased. The non-Ohmic behavior arises from the damaged layer between the Ni/Au electrode and the p-(In)GaN layer. This damaged layer is considered to become thinner as the In mole fraction is increased, resulting in improved Ohmic characteristics. (C) 2000 Elsevier Science B.V. All rights reserved.

We investigated the electrical properties of Me-doped InxGa1-xN (0 less than or equal to x < 0.75) grown by metalorganic vapor-phase epitaxy with various growth conditions, such as Mg-doping concentration, growth-rate and growth temperature. The hole concentration depends on the growth-rate, the In mole fraction and the crystal quality of the InGaN layers. The hole concentration of Mg-doped InxGa1-xN layers below x = 0.15 increased with the In mole fraction, while those above x = 0.15 decreased. We realized the p-type InGaN with the room-temperature hole concentration above 10(18)cm(-3) and obtained the maximum hole concentration of 7.8 x 10(18) cm (-3) for x = 0.2 by optimizing the growth conditions. (C) 2000 Elsevier Science B.V. All rights reserved.

We investigated the electrical properties of uniformly Mg-doped AlGaN/GaN superlattices (SLs) with various Al mole fractions and SL period thicknesses. We found that their sheet hole concentration depended strongly on the Al mole fraction and SL period thickness. We found the maximum spatial averaged hole concentrations for SLs with period thicknesses of 360 Angstrom and 100 Angstrom to be 3 x 10(18) cm(-3) at room temperature. The resistivities of the SLs were less temperature dependent than those of GaN and AlGaN, indicating that the enhanced hole generation in these SLs could be ascribed to the large energy shift of the valence band edge due to the piezoelectric field.

We investigated the electrical properties of Mg-doped InGaN with an In mole fraction of less than 0.2 grown by metaloganic vapor phase epitaxy. We obtained p-type InGaN with a hole concentration above 10(18) cm(-3) at room temperature. The hole concentrations of Mg-doped In0.04Ga0.96N and In0.14Ga0.86N were 1.2 x 10(18) and 6.7 x 10(18) cm-3, respectively, while that of Mg-doped GaN was 3.0 x 10(17) cm(-3) with the same Mg doping concentration. The activation energy of Mg in InGaN, calculated from the temperature dependence of the hole concentration, decreases with the increase in the In mole fraction. Furthermore, the electrical activity of Mg in InGaN increases with the In mole fraction. As a result, higher hole concentrations were obtained at room temperature for Mg-doped InxGa1-xN (x < 0.2) with higher In mole fractions.

We achieved spatially averaged hole concentrations above 10(19) cm(-3) at room temperature in Mg-doped InxGa1-xN/GaN (4nm/4nm) superlattices grown by metalorganic vapor phase epitaxy. The hole concentrations for the InxGa1-xN /GaN superlattices increased with the In mole fraction, and the maximum hole concentration reached 2.8 x 10(19) cm-3 for the In0.22Ga0.78N/GaN superlattice. The hole concentrations for the superlattices are larger than those for the InGaN bulk layers with the same average In mole fraction. The weak temperature dependence of the resistivities for InGaN/GaN superlattices with higher In mole fractions indicates highly efficient hole generation in the superlattice.

Short period alloy superlattice of Al0.14Ga0.86N/Al0.18Ga0.82N has been successfully applied to ultra-violet light emitting diode as transparent conductive layer. Hole concentration achieved was 1 x 10(18) cm(-3) with 3-run-period. Small difference of Al molar fraction and short period of this superlattice still keep the bandgap as wide as 3.7 eV, which shows the possibility of the conductive layer of much higher Al content materials. By utilizing this short period alloy superlattice, we demonstrate 343 nm operation of AlGaN-based light emitting diode.

Magnesium doping of AlGaN/GaN strained superlattice (SLS) is studied. Hole generation is enhanced by introducing SLS structure. Secondary ion mass spectroscopic characterization revealed that Mg concentrates in the AlGaN barrier layers of the SLS structure with the thickness precision of 2.5 nm. By utilizing highly conductive p-type SLS cladding layer, pulse operation of InGaN MQW LD with cleaved facet mirrors has been achieved at the threshold current density of 11.3 kA/cm(2).

Lower p-type contact resistances were realized using Mg-doped InGaN and InGaN/GaN superlattices. The contact resistances of the Mg-doped InGaN system were smaller than that of Mg-doped GaN using both Ni/Au and Pd/Au metallization schemes. In addition, ohmic characteristics to surfaces damaged by electron cyclotron resonance plasma etching were greatly improved to show low p-type contact resistances using the Mg-doped InGaN system because of high hole concentrations in the Mg-doped InGaN system and reduction of the surface damage by In atoms.

An InGaN/GaN double heterojunction bipolar transistor has been fabricated using p-type InGaN as a base layer for the first time. The structure was grown on a sapphire substrate by metalorganic chemical vapor deposition and defined by electron cyclotron resonance plasma etching. The In mole fraction of the p-type InGaN base layer was 0. 14 and its hole concentration was 4x10(18) cm(-3) at room temperature. The device showed a current gain of 1.2 at room temperature for the 80-nm base layer.

The strained In0.3Ga0.7As1-xNx (x = 0.001)/GaAs quantum wells (QWs) with 10 nm well thicknesses were grown by MOVPE at 500 degrees C using dimethylhydrazine (DMHy) as the nitrogen source. The nitrogen was nonlinearly incorporated into the solid by increasing the partial pressure of DMHy in the vapor phase. The peak energy of photoluminescence (PL) was red-shifted by increasing the composition of nitrogen up to x = 0.002 and showed a large band-gap bowing of - 82 eV. The as-grown In0.3Ga0.7As0.99N0.01 QW had a weak PL intensity of more than two orders of magnitude lower than that of In0.3Ga0.7As QW, but by annealing in a N-2 atmosphere at 650 degrees C, the PL intensity recovered and peaked at 1.26 mu m at 10 K. The PL recovery seems to have been related to the depassivation of hydrogen. From a SIMS analysis, the as-grown In0.3Ga0.7As0.99N0.01 QW showed a hydrogen concentration as high as 6 x 10(19) cm(-3), but decreased to 2.5 x 10(19) cm(-3) after annealing in N-2 for 1 h. (C) 1998 Elsevier Science B.V. All rights reserved.

We describe a selective area growth based on ultra-high-vacuum (UHV)-scanning-tunneling-microscopy (STM) lithography. After nitridation of GaAs surfaces and selective depassivation by UHV-STM, an array of uniform 6.4 +/- 0.8-nm high GaAs dots was successfully grown on the depassivated areas (50 nm x 50 nm) using trimethylgallium (TMG) and tertiarybutylarsine. On the side walls of dots, (114) or (117) facets appeared. It was found that unintentional growth on the nitrided mask surface is due to TMG decomposition on the nitrided surface. The unintentional growth can be suppressed by using an amorphous-like nitrided surface and by increasing the thickness of the nitrided surface layer, and consequently the selectivity is improved. (C) 1998 Elsevier Science B.V. All rights reserved.

After amorphous-like N-passivated GaAs surfaces with a low defect density are obtained as a mask layer for selective growth, nanometer scale patterning of the surfaces is achieved using ultra-high vacuum scanning tunneling microscopy to selectively depassivate surface N atoms. After patterning, GaAs dots with well-controlled size (typically 6 nm high and 50 x 50 nm(2)) can be successfully grown using trimethylgallium and tertiarybutylarsine in the specific area where the underlying GaAs layer appeared.

Dilute GaAs1-xNx, alloys (x<0.3%) were grown by metalorganic vapor phase epitaxy to investigate their photoluminescence and photoluminescence excitation characteristics. Photoluminescence excitation spectra show clear excitonic absorption peaks at low temperatures and their peak energy drastically decreases with increasing nitrogen concentration due to the band-gap bowing in the GaAsN system. This result indicates that the band-gap bowing starts at a nitrogen concentration as low as 10(18) cm(-3), and its bowing parameter is -22 eV. According to this band-gap bowing, the GaAsN alloys show two photoluminescence lines whose peak energy decreases with increasing nitrogen concentration. Their dependence on the nitrogen concentration suggests that these lines correspond to excitonic and carbon-related transitions in the GaAsN alloy. (C) 1997 American Institute of Physics.

A selective-area GaAs growth technique in a nanometer scale has been demonstrated by a combination of nitrogen (N)-passivation mask formation, ultra-high-vacuum scanning-tunneling-microscopy (STM) pattern modification, and metalorganic molecular beam epitaxy (MOMBE). GaAs(0 0 I) surfaces were passivated with N radicals dissociated from Na molecules, and were modified by STM in a nanometer scale, and on the surface GaAs nanostructures were grown using trimethylgallium and tertiarybutylarsine. Uniform 6 nm-high, 50 x 50 nm(2) dots were formed on the STM-modified areas, where the underlying GaAs layer appeared.

A technique for the selective-area growth of GaAs on a nanometer scale is described. The technique comprises nitrogen (N)-passivation mask formation, scanning-tunneling-microscopy (STM) pattern modification, and metalorganic molecular-beam epitaxy. GaAs (001) surfaces are passivated with N radicals dissociated from N-2 molecules and are modified by STM on a nanometer scale. GaAs nanostructures are then grown on the modified areas using trimethylgallium and tertiarybutylarsine. Uniform 6-nm-high and 50 x 50-nm(2) dots were formed on 50 x 50-nm(2) STM-modified areas. The advantage of the technique is that size-controlled nanostructures can be fabricated in specific positions and these nanostructures are free from contamination because all processes are performed in a vacuum. (C) 1997 American Institute of Physics.

Nitrogen atomic-layer-doping and nitrogen uniform doping were performed on (001) GaAs by metalorganic vapor phase epitaxy using dimethylhydrazine as a nitrogen doping source. Nitrogen doped GaAs layers showed several sharp photoluminescence lines relating to excitons bound to nitrogen pairs at 8 K. We investigated the origins of these sharp photoluminescence lines, that is, the distance between the nitrogen atoms of a pair and their pairing direction. The photoluminescence spectra for nitrogen atomic-layer-doped GaAs are different from those for uniformly nitrogen-doped GaAs, as expected from the different dimensions of the nitrogen atom distribution. To estimate the distance between the nitrogen atoms of a pair and their pairing direction, we performed double atomic-layer doping and varied the distance between the two atomic-layer-doped planes. When the two planes are brought to 1 nm of each other, lines observed only in uniformly doped GaAs appear. From this result, it was determined that the pairing directions corresponding to these lines are not included in the (001) plane and the corresponding distance between the nitrogen atoms of a pair is around 1 nm.

Scanning tunneling microscopy was used to study surface morphologies for various GaAs surfaces after nitridation using atomic nitrogen. For the (100) surface, 3-nm-wide stripes were observed along the [011] azimuth. Smooth surfaces densely covered with small GaN grains were observed for the (311)A and (111)A surfaces, while large GaN grains with some voids were observed for the (111)B surface. These results suggest that the (n11)A surfaces are promising for the growth of GaN on GaAs. Among the (100), (111)B, and (111)A GaAs surfaces, the (111)A surface after nitridation showed the largest selectivity of GaAs growth in metalorganic molecular beam epitaxy using trimethylgallium and tertiarybutylarsine. The surface roughness of GaAs after nitridation was also found to have a major effect on the selectivity of GaAs growth.

This paper reports on the nitridation of (001) GaAs using atomic nitrogen and the etching characteristics of the nitride layers formed using atomic hydrogen. Nitrogen and hydrogen molecules were cracked by a hot W filament to produce atomic nitrogen and hydrogen. After nitridation and etching, GaAs cap layers were grown by molecular beam epitaxy to form GaAs/GaN/GaAs structures. We determine the sheet nitrogen atom concentration for these structures by secondary ion mass spectrometry analysis. As the nitridation temperature decreases, the etched nitrogen atom concentration increases. The N-As bonds might be responsible for this increased concentration. The etching rate also depends on the substrate temperature. (C) 1997 Elsevier Science Ltd.

Nitrogen atomic-layer-doped and uniformly doped GaAs were grown by MOVPE using dimethylhydrazine on a (001) plane. They showed several sharp photoluminescence lines with a full width at half maximum less than 1 meV at 8 K. Compared with uniformly doped GaAs, the photoluminescence intensity of the nitrogen-related line at the longest wavelength is enhanced in nitrogen atomic-layer-doped GaAs, suggesting that it is easier to form nitrogen pairs during atomic layer doping. To investigate the sharp nitrogen-related lines, we also grew GaAs with double atomic-layer-doped planes and varied the distance between the two planes. When the two planes are brought close to 1 nm, two new lines, NNC, and NND, appear between the two nitrogen-related lines, NNA and NNB, observed in a single nitrogen atomic-layer-doped GaAs. The NNC and NND lines are also observed in uniformly doped GaAs. Therefore, NNA and NNB originate from excitons bound to pairs of nitrogen atoms, both of which are in the (001) plane, while NNC and NND originate from those bound to pairs of nitrogen atoms, of which pairing directions are not included in the (001) plane. From the photoluminescence characteristics, distances between nitrogen atoms of a pair are estimated for each line.

Nitrogen atomic-layer-doping on Ga-terminated and misoriented GaAs surfaces was performed by metalorganic Vapor Phase Epitaxy (MOVPE) using dimethylhydrazine to investigate the role of the surface for nitrogen atom incorporation. Compared with the As-terminated surface, dimethylhydrazine molecules are preferentially decomposed on the Ga-terminated surface due to the catalytic effect, resulting in a higher doping efficiency. We also investigated the crystal orientation dependence of the nitrogen doping concentration. Nitrogen atoms are preferentially incorporated on the (n11)A surfaces (n greater than or equal to 3) than the (100) surface. For the (n11)A surface, the nitrogen doping efficiency increases with decreasing n value. This indicated the doping efficiency increases with the step density. In contrast, nitrogen atoms are incorporated less on the (n11)B surfaces (n greater than or equal to 3) than the (100) surface and their doping efficiency decreases with increasing step density. These results are ascribed to the difference of atomic bonding geometries for adsorption sites between (n11)A and (n11)B surfaces. The doping efficiency for A-type steps is twice as high as that for the (100) terraces while that for B-type steps is negligibly small.

We have performed nitrogen atomic-layer doping into GaAs, AlGaAs, and AlGaAs/GaAs single quantum wells using atomic nitrogen cracked by a hot tungsten filament. While the atomic-layer-doped GaAs layers show a series of sharp and strong photoluminescence lines relating to excitons bound to nitrogen atoms at 8K, atomic-layer-doped AlGaAs layers show several broad nitrogen-related lines. For the atomic-layer-doped single quantum well at the center of the GaAs layer, the quantum well luminescence itself disappears and a dominant and sharp luminescence is observed at a longer wavelength. It is found that the As pressure during the atomic-layer doping greatly affects the luminescence characteristics.

We performed the nitridation of (001) GaAs surfaces using nitrogen molecules (N-2) cracked by a hot tungsten filament, After nitridation of a GaAs surface at 620 degrees C for 90 min, the [110] RHEED pattern shows the mixed spotty pattern of GaAs and GaN, while the <[(1)over bar 10]> RHEED pattern shows a streak pattern with the spacing of a GaAs lattice. The nitridation rate is independent of the substrate temperature below 530 degrees C. Above 590 degrees C, N-2 desorbs from the surface during nitridation. This desorption rate of N-2 from the nitrided GaAs surface is the smallest among those of column V molecules in InAs, InP, GaAs and GaP, due to the large standard heat of formation for GaN.

Using scanning tunneling microscopy (STM), we perform nanometer-scale modifications on nitrogen (N)-passivated GaAs (001) surfaces. After the surface is passivated with nitrogen gas through a heated tungsten filament in an ultrahigh-vacuum chamber, STM modification is performed by increasing the tunnel current. A 200X200 nm(2) square groove was successfully fabricated. The smallest grooves are 0.5 nm deep and 5 nm wide when sample bias is -3 V and tunnel current is 5 nA. The threshold current for modification is 5 nA for surfaces with N passivation, but more than 50 nA for surfaces without N passivation. (C) 1996 American Institute of Physics.

We have applied nitrogen atomic-layer-doping to AlGaAs/GaAs single quantum wells. The atomic-layer-doping was performed at the center of the GaAs quantum well. The resulting structures show sharp photoluminescence lines with a full width at half maximum of 0.3 meV at 8 K. These lines are observed at longer wavelength than those obtained for undoped single quantum wells, indicating that they correspond to the excitons bound to nitrogen atoms in the quantum wells. The full width at half maximum of these nitrogen-related lines depends on the quantum well width. The lines remain sharp above a well width of 10 nm, while they become broad at 5 nm. This suggests that the diameter of the exciton bound to N atoms is about 10 nm. Furthermore, the binding energy of the excitons increases with increasing substrate temperature during the atomic-layer-doping.

We have studied GaAs (001) surfaces passivated with nitrogen (N) radicals at submonolayer N coverage mainly using scanning tunneling microscopy and transmission electron microscopy. We determined that GaN-rich regions are elongated in the [<(1)over bar 10>] direction, suggesting that N passivation proceeds in the [<(1)over bar 10>] direction. This can be explained in terms of minimization of the tensile strains in the [<(1)over bar 10>] direction induced when the supplied N atoms replace first-layer As atoms on the (2x4)surface. (C) 1996 American Institute of Physics.

We performed nitrogen atomic-layer doping into GaAs grown by molecular beam epitaxy using nitrogen molecules (N-2) cracked by a hot tungsten filament. While uniformly nitrogen-doped GaAs layers show relatively weak nitrogen-related photoluminescence lines, nitrogen atomic-layer-doped GaAs layers show a series of sharp and strong photoluminescence lines. The dominant photoluminescence line was observed at 1.4437 eV, where an exciton bound to the nitrogen isotropic traps has the highest binding energy. (C) 1995 American Institute of Physics.

This letter reports the nitridation of GaAs surfaces using N-2 through a hot tungsten filament. After nitridation, GaAs cap layers were grown by molecular beam epitaxy to form GaAs/GaN/GaAs structures. For these structures, we determine the sheet nitrogen atom concentration by secondary ion mass spectrometry analysis. The sheet nitrogen atom concentration is proportional to the square root of the N-2 pressure, indicating that N-2 molecules are decomposed into nitrogen atoms to adsorb on the GaAs surfaces. The activation energy of this decomposition process is 3.6+/-0.4 eV. (C) 1995 American Institute of Physics.

We demonstrate hole accumulation in unintentionally doped (In)GaSb/AlSb quantum wells, using an InAs surface as a cap layer. From the hole concentrations in quantum wells, the energy difference between the conduction band edge of InAs and the valence band edge of GaSb is estimated. Relatively high two-dimensional hole mobilities of 650 and 7000 cm(2)/V . s are obtained from Hall measurements at 300 and 77 K. For a strained InGaSb/AlSb quantum well, mobility enhancement is observed.

By an in-situ surface photo-absorption study of the ALE process and carbon atomic layer doping, an other than the so far proposed carbon incorporation mechanism via formation of a CH2 bridge bond has been found, i.e. carbon incorporation via an exchange reaction between dissociated CH3 radicals from TMG and As atoms of the surface onto which TMG is deposited. We also propose a method for reducing the carbon incorporation by thermal desorption of the CH3 groups.

Carbon (C) atomic layer doping in AlGaAs is demonstrated. During undoped AlGaAs growth by conventional low-pressure metalorganic chemical vapor deposition, C atomic layer doping is performed by supplying trimethyl-gallium onto AlGaAs surfaces under arsine-free conditions. C incorporation increases with increasing Al composition of the surface layer onto which trimethylgallium is supplied, suggesting that the C incorporation mechanism is that As atoms of the AlGaAs surface are partially replaced by C atoms of methyl groups to become acceptors. The capacitance-voltage measurement of the C atomic layer doped AlGaAs shows a carrier profile with a full width at half-maximum as narrow as 8.0 nm at a peak carrier concentration of 2.1 x 10(18) cm-3, indicating that the diffusion of C atoms in AlGaAs is not serious during growth at 610-degrees-C. A p-type modulation doped AlGaAs/GaAs structure was fabricated using C atomic layer doping, and a mobility of 1. 3 x 10(5) cm2/V - s was obtained at 1.5 K for a sheet hole density of 3.9 x 10(11) cm-2. Because of the high quality two-dimensional hole gas, a region limited by acoustic phonon scattering can be seen in the temperature dependence of hole mobility.

Self-aligned high-frequency InP/InGaAs double heterojunction bipolar transistors (DHBT's) have been fabricated on a Si substrate for the first time. A current gain of 40 was obtained for a DHBT with an emitter dimension of 1.6 x 19 mum2. The S parameters were measured for various bias points. In the case of I(C) = 15 mA, f(T) was 59 GHz at V(CE) = 1.8 V, and f(max) was 69 GHz at V(CE) = 2.3 V. Due to the InP collector, breakdown voltage was so high that a high V(CE) of 3.8 V was applied for I(C) = 7.5 mA in the S-parameter measurements to give an f(T) of 39 GHz and an f(max) of 52 GHz.

Si atomic-layer doped n-AlGaAs/GaAs structures have been grown by metalorganic chemical vapor deposition using triethyl sources, arsine and silane. Two dimensional electron mobility of 1.3 x 10(6) cm2/V.s is obtained at 4.2 K for a sheet electron concentration of 4.8 x 10(11) cm-2. Because of the high quality of the two-dimensional electron gas, acoustic phonon scattering can be seen in the temperature dependence.

Carbon atoms are used as a p-type dopant for modulation-doped p-AlGaAs/GaAs heterostructures grown by metalorganic chemical vapor deposition. Carbon impurities are effectively doped into AlGaAs layers by flow-rate modulation epitaxy, which is a modified method of metalorganic chemical vapor deposition. A mobility of 6.0 x 10(4) cm2/V.s is obtained at 13 K for a sheet carrier concentration of 4.1 x 10(11) cm-2, indicating that a high quality two-dimensional hole gas is realized by abrupt heterointerfaces and sharp carbon doping profiles.

Longitudinal and transverse magnetoresistance experiments were carried out on GaAs with two impurity-doped planes in steady high magnetic fields up to 30 T. A large oscillation was observed in the field range higher than 10 T. The double Si-doped planes behave as a single well even for 15 nm separation of the atomic planes.

The first InP/InGaAs double heterojunction bipolar transistors on Si substrates were grown by metalorganic chemical vapor deposition. The transistor with an InP buffer layer thickness of 4-mu-m exhibited high current gains of more than 250 and an ideality factor of 1.3. (The base doping concentration and its thickness were 1.5 x 10(19) cm-3 and 700 angstrom, respectively.) The characteristics of the DHBT on Si with the 4-mu-m buffer layer are comparable to those of transistors on InP substrates.

We report, for the first time, the successful fabrication of InP/InGaAs double-heterojunction bipolar transistors (DHBT's), grown by metalogranic chemical vapor deposition (MOCVD) on Si substrates. The transistors exhibit high current gains over 200, which is comparable to those in transistors grown on InP substrates. The dislocations are found to increase the recombination current very little in the neutral base region, but increase the generation-recombination current at the emitter-base interface.

This letter investigates carbon doping in AlGaAs using flow-rate modulation epitaxy (FME). The tendency of hole concentration increasing with Al composition is explained by the thermal decomposition of trimethylaluminum molecules on the substrate surface. The highest hole concentration obtained is 3 X 10(20) cm-3 in Al0.4Ga0.6As layers. An AlGaAs/GaAs heterojunction bipolar transistor with an FME-grown compositionally graded carbon-doped base layer was fabricated for the first time. It exhibits a current gain of 25 with 2 X 10(19) cm-3 base doping.

We describe the performance of InP/InGaAs heterostructure bipolar transistors (HBTs) grown at low temperature by metalorganic chemical vapor deposition (MOCVD). HBTs with a base thickness of 70 nm and doping level of 1 x 10(19) cm-3 show excellent current gain characteristics (current gains h(FE) > 300, ideality factors n(B) < 1.25) at growth temperatures ranging from 500 to 575-degrees-C. In addition, Zn out-diffusion from the base layers into the undoped spacer layers is well suppressed to as low as 5 x 10(17) cm-3.

Surface photo-absorption (SPA) is a newly developed in-situ optical monitoring technique for the epitaxial growth process. This method is based on the reflectivity measurement of p-polarized light incident at the Brewster angle. This configuration minimizes the bulk GaAs contribution to the total light reflection. The small change in reflected light intensity between Ga and As atomic surfaces during GaAs growth is thus detected with a high signal-to-noise ratio. By using this characteristic, GaAs growth rate can be monitored in-situ on an atomic scale. In addition to the in-situ monitoring of growth rate, the decomposition processes of Ga and As precursors in MOVPE can be studied by SPA. We demonstrate an investigation of the decomposition process of Ga organometallic, and discuss the growth mechanism of atomic layer epitaxy.

This paper discusses the p+-n+ GaAs tunnel junction diodes grown by flow-rate modulation epitaxy using carbon and silicon as p-type and n-type dopants, respectively. It also investigates the effect of the conditions of p+-n+ junction interfaces on the excess current. While the excess current drastically increases when p+- and n+ -layers are overlapped on an atomic scale, it gradually decreases and tends to saturate when the undoped layer thickness inserted at the interface is increased. These results indicate that the p+-n+ junction interfaces grown by flow-rate modulation epitaxy are quite abrupt.