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.

We have investigated the electron spin relaxation in undoped InGaAsP grown on an InP substrate. Time-resolved spin-dependent pump and probe reflection measurements revealed a spin relaxation behavior between 10 and 300K. We have observed the electron spin relaxation times of 980ps at 10K and 95ps at room temperature. The observed presence of the carrier density dependence and the weak temperature dependence of spin relaxation time imply that the Bir-Aronov-Pikus (BAP) process is effective at 10-30K. At 100-300K, the observed presence of the temperature dependence and the absence of the carrier density dependence of spin relaxation time indicate that the D'yakonov-Perel' (DP) and Elliott-Yafet (EY) processes are effective. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on the study of InGaAsP solar cells grown by solid-state molecular beam epitaxy (MBE) on InP. The effect of growth temperature on device performance is investigated. Under standard one-sun air-mass 1.5 global (AM1.5) illumination, an efficiency of 18.8% has been obtained for In0.78Ga0.22As0.45P0.52 single-junction solar cells grown at high temperature. Time-resolved photoluminescence (PL) results demonstrate that the In0.78Ga0.22As0.48P0.52 optical quality is greatly improved in the case of a high growth temperature. A longer PL decay time of In0.78Ga0.22As0.48P0.52/InP in contrast to In0.78Ga0.22As0.48P0.52/In0.52Al0.42As indicates that InP is more promising as the back surface field for future solar cell performance improvements. (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).

The carrier recombination dynamics of InGaAsP material with a bandgap energy of 1 eV for quadruple-junction solar cells grown by solid-source molecular beam epitaxy have been investigated by the employment of time-resolved photoluminescence (PL) measurement. For the nominally undoped material, the PL decay time increases with increasing temperature, which indicates that radiative recombination dominates the recombination process. The radiative and the nonradiative recombination time constants were calculated on the basis of the temperature-dependent PL decay time and the integrated PL intensity. With the incorporation of Be (as the p-type dopant) into the material, the PL decay time decreases with increasing temperature, and a double-exponential PL decay curve is observed in the case of the material with a higher doping density. An InGaAsP-based single-junction photovoltaic device with a bandgap of 1 eV was fabricated, and an efficiency of 16.4% was obtained under the AM1.5G solar spectra. (C) 2014 Elsevier B.V. All rights reserved.

We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100K. Two PL peaks were observed at 1.511 eV (peak 1) and 1.497 eV (peak 2) at 10K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.3-3.1 ns at 10-100K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 10-77K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism. (C) 2014 AIP Publishing LLC.

The spin relaxation process of low-temperature-grown GaAs is investigated by spin-dependent pump and probe reflectance measurements with a sub-picosecond time resolution. Two very short carrier lifetimes of 2.0 ps and 28 ps, which can be attributed to nonradiative recombinations related to defects, are observed at 10 K. The observed spin polarization shows double exponential decay with spin relaxation times of 46.2 ps (8.0 ps) and 509 ps (60 ps) at 10K (200 K). The observed picosecond spin relaxation, which is considerably shorter than that of conventional GaAs, indicates the strong relevance of the Elliott-Yafet process as the spin relaxation mechanism. For the first (second) spin relaxation component, the temperature and carrier density dependences of the spin relaxation time indicate that the Bir-Aronov-Pikus process is also effective at temperatures between 10K and 77 K, and that the D'yakonov-Perel' process is effective between 125K (77 K) and 200 K. (C) 2014 AIP Publishing LLC.

The ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layer grown by the thermal oxidation of ZnS with gallium was investigated by the time-resolved photoluminescence as a function of measuring temperature and excitation power. With increase of excitation power, the (DX)-X-0 emission is easily saturated than the DAP emission from ZnO/ZnGa2O4 composite layer, and which is dramatically enhanced as compared with that from pure ZnO layer grown without gallium. The radiative recombination process with ultra-long lifetime controlled the carrier recombination of ZnO/ZnGa2O4 composite layer. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

We have investigated the optical anisotropy of single-layer highly uniform InAs quantum dots (QDs) by photoluminescence (PL) measurements. The high uniformity of the QDs, whose PL spectrum was clearly separated into each energy level, enabled us to observe the optical anisotropy of the ground state, the first excited state and the second excited state separately. The degree of linear polarization (DLP) was obtained to be 15.8%, 14.3% and 8.3% for the ground state, the first excited state and the second excited state, respectively. This result shows that the DLP is smaller for higher-energy states. We also measured the time evolutions of the PL polarization components using a streak camera. The DLP was found to remain constant during the measured time range. (c) 2013 Elsevier B.V. All rights reserved.

Photoluminescence and Raman scattering measurements were performed on Si-doped GaInP grown on Ge. The deep broadband photoluminescence (PL) emission centered around 1.4 eV exhibits a strong dependence of strength on Si dopants, which is suggested to be due to [Ge-(Ga,Ge-In)-V-(Ga,V-In)] complexes. A strong evidence to support the existence of [Ge-(Ga,Ge-In)-Si-(Ga,Si-In)] complexes in Si-doped GaInP is shown by Raman spectra. The blue shift of broad PL emission and the increased recombination life time with increased temperatures were explained by the competition between [Ge-(Ga,Ge-In)-V-(Ga,V-In)] and [Ge-(Ga,Ge-In)-Si-(Ga,Si-In)] complexes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737611]

We have investigated the exciton and carrier spin relaxations in InGaAs lattice-matched to Ge substrates. Time-resolved spin-dependent pump and probe reflectance measurements revealed a spin relaxation behavior between 10 and 300 K. The presence of the carrier density dependence of spin relaxation time at 10-200K implies that the Bir-Aronov-Pikus process is effective. At 250-300 K, the strong temperature and weak carrier density dependences of spin relaxation time show that the D'yakonov-Perel' process is dominant. The longest observed spin relaxation time of 2.6 ns at 77K is explained by the decrease in the spatial overlap of electrons and holes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730386]

We have investigated carrier spin relaxation in InAs columnar quantum dots (CQDs) using time-resolved photoluminescence measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxations of the 3- and 35-period SL CQDs show double exponential decay up to 50 and 130 K, respectively. The spin relaxation times of the fast component, whose amplitudes are 4-11 times larger than that of the slow component, are around 100 ps for the two samples. For the 3-period SL CQDs, the fast spin relaxation time shows no temperature dependence up to around 50 K, indicating the relevance of the Bir-Aronov-Pikus process. The slow spin relaxation time of the 35-period SL CQDs was found to decrease from 3.42 ns at 10 K to 0.849 ns at 130 K. This large change may be explained by the Elliott-Yafet process considering acoustic phonon scattering. (C) 2012 The Japan Society of Applied Physics

The spin relaxation process of In0.53Ga0.47As/AlAs0.56Sb0.44 quantum wells is investigated by spin-dependent pump and probe reflectance measurements with a high time resolution of 200 fs. The observed spin relaxation time of 17.5 ps at 20K indicates high potential for applications to high-speed optical devices. A positive temperature dependence of the spin relaxation time due to the unique band structure is observed at 30-100 K. The spin relaxation is found to be mainly governed by the Bir-Aronov-Pikus process [Sov. Phys. JETP 42, 705 (1976)] below 100K and by the D'yakonov-Perel' process above 100 K. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690833]

We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells by time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of 1.38 mu m-electron-heavy-hole excitons at 150 K is obtained to be 34 43 ps at an excitation power of 50 80 mW. The observed carrier density dependence and temperature independence of the spin relaxation time indicate that the spin relaxation mechanism is dominated by the Bir-Aronov-Pikus process.

We have investigated the carrier spin relaxation in GaInNAsSb/GaNAsSb/GaAs quantum well (QW) by time-resolved photoluminescence (PL) measurement. The sample consists of an 8-nm-thick GaIn0.36N0.006AsSb0.015 well, 5-nm-thick GaN0.01AsSb0.11 intermediate barriers and 100-nm-thick GaAs barriers grown by molecular beam epitaxy on a GaAs(100) substrate. The spin relaxation time and recombination lifetime at 10 K are measured to be 228 ps and 151 ps, respectively. As a reference, we have also obtained a spin relaxation time of 125 ps and a recombination lifetime of 63 ps for GaInNAs/GaNAs/GaAs QW. This result shows that crystal quality is slightly improved by adding Sb, although these short carrier lifetimes mainly originate from a nonradiative recombination. These spin relaxation times are longer than the 36 ps spin relaxation time of InGaAs/InP QWs and shorter than the 2 ns spin relaxation time of GaInNAs/GaAs QW.

ZnO/ZnGa2O4 composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa2O4 composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa2O4. (C) 2010 Elsevier B. V. All rights reserved.

We have investigated the exciton spin relaxation in InGaAs/AlAsSb quantum wells using time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of 1.46 mu m electron-heavy hole excitons at room temperature is obtained to be 27-54 ps for an excitation power of 20-100 mW. The carrier density dependence of the exciton spin relaxation time was clearly observed, suggesting that the spin relaxation mechanism is strongly related to the Bir-Aronov-Pikus process at room temperature. (C) 2010 The Japan Society of Applied Physics

Time-resolved photoluminescence measurement was performed on one-dimensional GaN nanorods with c-axis-oriented single-crystalline wurtzite structures. The GaN nanorods were grown on Si(111) substrates by an improved hydride vapor phase epitaxy without a catalyst. Fast carrier recombinations of less than 10 ps were observed. The short recombination times of the GaN nanorods with few defects show the presence of nonradiative fast recombinations at the surface and interface. (C) 2010 The Japan Society of Applied Physics

We have investigated spin polarization-related localized exciton photoluminescence (PL) dynamics in GaInNAs quantum wells by time-resolved PL spectroscopy. The emission energy dependence of PL polarization decay time as well as polarization-independent PL decay time suggests that the acoustic phonon scattering in the process of localized exciton transfer from the high-energy localized states to the low-energy ones dominates the PL polarization relaxation. By increasing the excitation power from 1 to 10 mW, the PL polarization decay time is changed from 0.17 to more than 1 ns, which indicates the significant effect of the trapping of localized electrons by nonradiative recombination centers. These experimental findings indicate that the spin-related PL polarization in diluted nitride semiconductors can be manipulated through carrier scattering and recombination process. (C) 2009 The Japan Society of Applied Physics

Time-resolved studies using ultra-short laser pulses unveil interesting aspects of carrier and spin dynamics in compound semiconductors. Here, thermally-activated carrier transfers between CdTe/ZnTe quantum clots (QDs) and ultrafast spin-relaxations in bulk GaN are reported. Carrier transfer among CdTe/ZnTe self-organized QDs was studied using time-resolved photoluminescence measurements. The carriers in the high-energy ground states of small QDs are confirmed to transfer to the lower-energy ground states of larger QDs, even at 10 K. The energy dependence of the PL decay time changes uniquely With increasing temperature. The change in the energy dependence of the PL decay time call be explained by thermally-activated carrier transfer. Excitonic spin-relaxations in hexagonal GaN and cubic GaN are observed. The A-band free exciton in hexagonal GaN shows a sub-picosecond spin-relaxation of 0.47 ps at 1.50 K. The acceptor-bound exciton in hexagonal GaN shows spin-relaxation times of 1.40 - 1.14 ps at 15 - 50 K. Meanwhile, the spin-relaxation times in cubic GaN at 1.5 - 75 K are found to be longer than 5 ns. The long nanosecond spin-relaxation time in cubic GaN is consistent with the observation that spin-relaxation time becomes longer for wider-band-gap zincblende semiconductors.

We have investigated carrier spin dynamics in InAs columnar quantum dots (CQDs) by time-resolved photoluminescence (PL) measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxation time was found to be prolonged from 1.6 to 5.3 ns by increasing the number of SL periods from 3 to 35. The PL decay time also increased from 0.93 to 1.9 ns, indicating a decrease in the spatial overlap of electron and hole wave functions. The changes in both the spin relaxation time and the PL decay time suggest that the Bir-Aronov-Pikus process is the main spin relaxation mechanism. (C) 2009 The Japan Society of Applied Physics

We have investigated the exciton spin relaxation in a GaInNAs/GaAs quantum well. The recombination from free and localized excitons is resolved on the basis of an analysis of the photoluminescence characteristics. The free exciton spin relaxation time is measured to be 192 ps at 10 K, while the localized exciton spin relaxation time is one order of magnitude longer than that of the free exciton. The dependence of the free exciton spin relaxation time on the temperature above 50 K suggests that both the D'yakonov-Perel' and the Elliot-Yafet effects dominate the spin relaxation process. The temperature independence below 50 K is considered to be due to the spin exchange interaction. The ultralong spin relaxation time of the localized excitons is explained to be due to the influence of nonradiative deep centers. (c) 2008 American Institute of Physics.

The exciton radiative lifetime in submonolayer (SML) InGaAs/GaAs quantum dots (QDs) grown at 500 degrees C was measured by using time-resolved photoluminescence from 10 to 260 K. The radiative lifetime is around 90 ps and is independent of temperature below 50 K. The observed short radiative lifetime is a key reason for the high performance of SML QD devices and can be explained by the theory of Andreani [Phys. Rev. B 60, 13276 (1999)] calculating the radiative lifetime of QDs formed at the interface fluctuations of a quantum well, as the SML QDs are 20-30 nm in diameter and embedded within the lateral InGaAs QW. (C) 2008 American Institute of Physics.

Nonresonant carrier tunneling dynamics in GaAs/AlGaAs double quantum wells as a function of carrier density was investigated by using time-resolved pump and probe measurements. At low temperatures, tunneling time increases with increasing carrier density. At high temperatures, no dependence of tunneling time on carrier density was shown. This unique behavior is well explained by the thermodynamic effect between excitons and free carriers as a function of carrier density on the tunneling process. At low temperatures, a larger fraction of excitons at a higher carrier density, which have a relatively slow tunneling time in contrast to free electrons, leads to a slow tunneling time. The independence of the tunneling time at high temperatures is attributed to the insensitivity of the exciton population to the increased carrier density. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The energy relaxation time of hot carriers photoexcited in bulk InGaN is measured. The time-resolved pump and probe transmission measurements with subpicosecond time resolution show that the hot-carrier relaxation time is 0.92 ps at 15 K. The hot-carrier relaxation time becomes significantly shorter at higher temperatures. At temperatures higher than 150 K, there are no meaningful differences between rise times. This strong temperature dependence indicates that electron-phonon scattering dominates the carrier relaxation process. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The competition between the quantum-confined Stark effect (QCSE) and the free-carrier screening effect in AlGaN/GaN multiple quantum wells (MQWs) has been investigated by time-resolved photoluminescence (PL) measurement. AlGaN/GaN MQWs is a promising material for the next-generation ultraviolet light-emitting diodes and laser devices. The large charges in the PL energy and the decay time are observed with changing carrier density. We show that the energy shift and the change in decay time are explained well by the free-carrier screening effect that compensates for the internal electric field. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Carrier spin relaxation in undoped GaAs/AlGaAs asymmetric double quantum wells (QWs) at room temperature was studied by spin-dependent pump and probe reflection measurements. The electron tunneling, which occurs between the two lowest conduction bands in the narrow and wide QWs, has a negligible effect on the electron spin relaxation. The most striking feature is that a long hole spin relaxation time was directly observed in the case of fast electron tunneling while it was not observed in the case of relatively slow electron tunneling. The hole that remains in the QWs due to the escape of the electron tunneling out makes the direct observation of hole spin relaxation possible. The measured hole spin relaxation time varies from 107 ps to 80 ps when the excitation power changes from 10 mW to 40 mW. This study provides direct experimental evidence of the important effect of the interaction between electrons and holes on the hole spin relaxation. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Highly circularly polarized light is generated from highly uniform InAs/GaAs quantum dots. These quantum dots allowed us to observe the spin dynamics at each energy state selectively without disturbing the inhomogeneous broadening. Time-resolved spin-dependent pbotoluminescence measurements show that the spin polarization at the second excited state reaches a maximum of 45% at 10 K, and that the spin polarization is greater at higher-energy states, This large spin polarization is explained by the spin Pauli blocking effect. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nonresonant carrier tunneling has been studied as a function of temperature in GaAs/AlGaAs double quantum wells (DQWs). Time-resolved pump and probe reflectance measurements allow the direct observation of tunneling at any temperature between 15 K and room temperature. We found that for two DQWs with different barrier thicknesses, the tunneling time abruptly decreases above a critical temperature while it remains almost constant below the critical temperature. This critical temperature is shown to correspond to the exciton binding energy. Rate equation analysis explains this behavior as the thermalization of excitons into free electrons that have a faster tunneling time than excitons.

The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. The spin relaxation time is found to be proportional to T-0.175, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process [Sov. Phys. JETP 42, 705 (1976)]. (c) 2006 American Institute of Physics.

Carrier transfer among CdTe/ZnTe self-organized quantum dots (QDs) was studied using time-resolved photoluminescence (PL) measurements. The authors have confirmed that carriers in the high energy ground states of small QDs transfer to the lower-energy ground states of larger QDs even at 10 K. The energy dependence of PL decay time changes uniquely with increasing temperature. They have found that the change in the energy dependence of PL decay time can be explained by thermally activated carrier transfer. (c) 2006 American Institute of Physics.

We report carrier spin dynamics in highly uniform self-assembled InAs quantum dots and the observation of antiferromagnetic coupling between semiconductor quantum dots. The spin relaxation times in the ground state and the first excited state were measured to be 1.0 and 0.6 ns, respectively, without the disturbance of inhomogeneous broadening. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200ps at 130 K. This large change in the spin relaxation time is well-explained in terms of the mechanism of acoustic phonon emission. In coupled quantum dots, the formation of anti ferromagnetic coupling is directly observed. Electron spins are found to flip at 80 ps after photoexcitation via the interdot exchange interaction. The antiferromagnetic coupling exists at temperatures lower than 50-80K. A model calculation based on the Heitler-London approximation supports the finding that the antiferromagnetic coupling is observable at low temperature. These carrier spin features in quantum dots are suitable for the future quantum computation. (c) 2006 Elsevier B.V. All rights reserved.

The excitonic spin relaxation process in cubic GaN is observed by spin-dependent pump and probe reflectance measurements with subpicosecond time resolution. The spin polarization presents at temperatures lower than 100 K. The spin relaxation times at 15-75 K are found to be longer than 5 ns and short spin relaxation times on the picosecond order are not present. Although these long spin relaxation times are in striking contrast to the subpicosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band gap zinc blende semiconductors. (c) 2006 American Institute of Physics.

The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. These spin relaxation times are slightly longer than those of the A-band free excitons of 0.47-0.25 ps in GaN at 150-225 K. The spin relaxation time is found to be proportional to T-0.175, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The dependence of radiative recombination rate and efficiency on GaN quantum-dot (QD) size and temperature is studied by time-resolved photoluminescence (PL) spectroscopy. The emission is dominated. by radiative recombination at low temperatures (< 125 K) and exhibits high PL efficiency at room temperature. The radiative lifetime and the relative quantum efficiency decrease with the decreasing QD size.

Near-field and time-resolved photoluminescence measurements show evidence of exciton localization in vertically and laterally coupled GaN quantum dots (QDs). The binding energies in multiple period QDs (MQDs) are observed to be stronger by more than six times compared to single period QDs (SQDs). Excitons in MQDs have a short (450 ps) lifetime and persist at room temperature, while SQDs exhibit extraordinarily long (> 5 ns) lifetime at 10 K due to reduced spatial overlap of electron and hole wave functions in strained QDs.

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver. (C) 2005 Optical Society of America.

The spin-relaxation process of A-band exciton in GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The spin-relaxation times at 150-225 K are 0.47-0.25 ps. These are at least one order of magnitude, shorter than those of the other Ill-V compound semiconductors. The spin-relaxation time tau(s) is found to be proportional to T-1.4, where T is the temperature. (C) 2004 American Institute of Physics.

We have investigated the exciton spin relaxation mechanism between 13 and 300 K in InGaAs/InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, tau(s) above 40 K was found to depend on temperature, T, according to tau(s)proportional toT(-1.1), although the spin relaxation time is constant below 40 K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40 K, although the carrier density dependence is weak above 40 K. These results imply that the main spin relaxation mechanism above and below 40 K are the D'yakonov-Perel' process and the Bir-Aronov-Pikus process, respectively. (C) 2004 American Institute of Physics.

We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission. (C) 2004 American Institute of Physics.

We have investigated antiferromagnetic coupling between semiconductor quantum dots. Electron spin is observed to flip at 80 ps after photoexcitation via the interdot-exchange interaction. The spin relaxation time under the antiferromagnetic order is extended to 10-12 ns, one order of magnitude longer than that in isolated quantum dots. The antiferromagnetic order exists at temperatures lower than 50-80 K. The photoluminescence experiments for various carrier densities show that antiferromagnetic coupling disappears when the electron pairing probability is low. A model calculation based on the Heitler-London approximation supports the finding that the antiferromagnetic order is observable at low temperature.

Self-assembled GaN quantum dots (QDs), grown on AIN by molecular beam epitaxy, were investigated by time-resolved photoluminescence spectroscopy. We investigate the emission mechanism in GaN QDs by comparing the carrier recombination dynamics in single and multiple period QDs. At 100 K, the PL decay time in single period QD structures is considerably shorter than in stacked QDs: Compared to single period QDs, the room temperature PL efficiency is considerably enhanced in 20 period QDs due to the reduction in nonradiative recombination processes.

The intersubband carrier relaxation dynamics in InAs quantum dots of high-uniformity has been investigated using time-resolved photoluminescence measurement. The clearly separated photoluminescence spectra of the ground state and the second state were observed. The rate equation analysis gives the intersubband relaxation time from the second state to the ground state of 360 ps at 10 K. The slow intersubband relaxation can be attributed to the phonon relaxation bottleneck. The intersubband relaxation time at room temperature is evaluated to be 320 ps by extrapolation of 10-200 K data.

We have investigated carrier spin dynamics in self-assembled InAs/GaAs high-uniform quantum dots. The high-unifonn quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of the inhomogeneous broadening. Spin Pauli blocking due to which the spin polarization in the second state is greater than that in the ground state has been clearly observed by time-resolved spin-dependent photoluminescence measurements. The spin relaxation times in the ground state and the second state were measured to be 1 ns and 0.8 ns, respectively.

Using time-resolved photoluminescence measurements, the recombination rate in an In0.18Ga0.82N/GaN quantum well (QW) is shown to be greatly enhanced when spontaneous emission is resonantly coupled to a silver surface plasmon. The rate of enhanced spontaneous emission into the surface plasmon was as much as 92 times faster than QW spontaneous emission into free space. A calculation, based on Fermi's golden rule, reveals that the enhancement is very sensitive to silver thickness and indicates even greater enhancements are possible for QW's placed closer to the surface metal coating.

We studied the influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN multiple quantum wells (MQWs) mainly in relation to a quantum-confined Stark effect. We performed a systematic time-resolved photoluminescence measurement of MQWs for various carrier densities and three different well widths (2.5, 4.0, and 5.5 nm). We show that the energy shift and the change in carrier lifetime are explained well by the free carrier screening effect which compensates for the internal electric field. (C) 2002 American Institute of Physics.

InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm. was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit, However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E-0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E-0, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In0.1Ga0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDS. (C) 2002 Elsevier Science B.V. All rights reserved.

We have investigated magnetic coupling between semiconductor quantum dots. The antiferromagnetization process based on the interdot exchange interaction has been directly observed using time-resolved spin-dependent photoluminescence measurements. The antiferromagnetic order that proves the interdot exchange interaction energy to be negative is found to exist at temperatures lower than 50-80K. The spin relaxation time under the antiferromagnetic order is extended to 10-12ns, an order of magnitude longer than that in isolated quantum dots. (C) 2002 Elsevier Science B.V. All rights reserved.

Optical properties of fully-strained wurtzite and zincblende InxGa1-xN/GaN multiple quantum well (MQW) structures were compared to discuss the origin of exciton localization. In contrast to the hexagonal InGaN MQWs, the photoluminescence (PL) peak energy of cubic InGaN MQWs showed a moderate blueshift with decreasing well thickness, L, and low-temperature PL decay time of the cubic MQWs did not depend strongly on L. The results imply that the wavefunction overlap in cubic InGaN MQWs was not reduced compared to the hexagonal ones, since they do not suffer from the electric field normal to the QW plane due either to spontaneous or piezoelectric polarization. Both MQWs exhibited a large and composition-dependent bandgap bowing, and time-resolved PL (TR-PL) signals showed a stretched-exponential decay even at room temperature. The exciton localization is considered to be an intrinsic property of InGaN.

We have performed time-resolved photoluminescence and electroluminescence measurements for an InGaN quantum well at 300 K to investigate the carrier dynamics. Electroluminescence measurement yields more direct information of the carrier dynamics of device operation, because carriers are generated by current injection in optoelectronic devices. The time evolution of EL spectra indicates that spatial indium fluctuations become significant with increasing In content in nitride-based optoelectronic applications.

Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.(C) 2001 American Institute of Physics.

To clarify the influence of diffused Mg impurities on the carrier recombination in InGaN multiple quantum wells (MQWs), we have performed a systematic study using time-resolved photoluminescence (PL) measurements. It was found that the MQWs with a p-contact layer and the MQWs with a nondoped GaN layer had almost the same carrier lifetime and PL intensity below 200 K. However, the MQWs with the p-contact layer had shorter carrier lifetime and lower PL intensity than the MQWs with the nondoped GaN laver above 200 K. This degradation of the PL for the MQWs with the p-contact laver can be attributed to nonradiative recombination caused by the diffusion of Mg impurities from the p-contact laver into MQWs.

Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1-xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1-xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1-xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material's inhomogeneity is much enhanced compared to that of the hexagonal polytype. (C) 2001 American Institute of Physics.

We demonstrate the ultrafast modulation of interband (IB)-resonant light (1.0-1.1 mum) by near-infrared intersubband (ISB)-resonant light (1.55-1.9 mum) in n-doped InGaAs/AlAsSb multiple quantum wells (QWs) using non-degenerate pump-probe spectroscopy. A modulation with an absorption recovery time of 1.0-2.0 ps has been observed in a planer-type modulation device due to ultrafast ISB relaxation of the carriers. The IB carrier relaxation process in the absence of an ISB-resonant light has also been investigated by time-resolved photoluminescence (PL) measurement. The modulation speed is determined by the inter- and intra-subband relaxation of the carriers in the conduction subband. The modulation speed at 1.1 mum due to an ISB-resonant pump light at 1.95 mum has been observed to be 1.4 ps at excitation energy of 500 fJ/mum(2).

We have directly observed the spin-polarized electron tunneling process between double quantum dots (QDs) using spin-dependent time-resolved photoluminescence measurements. The spin relaxation times in isolated In0.9Al0.1As Stranski-Krastanow (SK)-mode QDs and InAs SK-mode QDs were measured as 1.3 and 1.2 ns, respectively. The fact that these times are longer than those of quantum wells indicates that the spin relaxation rate is suppressed in QDs by the increased zero dimensionality. The measurement of tunneling between double QDs which consist of In0.9Al0.1As QDs and InAs QDs indicates that the spin relaxation time is not affected by the tunneling process. The spin-polarized carriers were found to tunnel from one dot to another dot while retaining spin polarization. (C) 2001 Elsevier Science B.V. All rights reserved.

We have directly measured carrier tunneling times between vertically aligned double quantum dots (QD's) using time-resolved photoluminescence measurement. The vertically aligned double QD structure consists of In0.9Al0.1As QD's, a GaAs barrier layer, and InAs QD's. The tunneling times were measured for the three different barrier thicknesses. The dependence of the tunneling time on the barrier thickness is in agreement with the Wentzel-Kramers-Brillouin approximation. The nonresonant tunneling rate between QD's is found to be suppressed to one-tenth of the tunneling rate between quantum wells.

We report the spin polarization of carriers photoexcited in bulk InGaN by circularly polarized femtosecond optical pulses. No spin polarization is observed in the picosecond time region using spin-dependent pump and probe absorption measurements with a time resolution of 0.35 ps. This is in contrast to the existence of spin polarization in GaAs quantum wells or in InGaAs quantum wells which have a spin relaxation time in the picosecond time region. The unique band structure of InGaN, which has weak spin-orbit interaction, and an in-plane potential fluctuation due to the compositional inhomogeneity of In explains the lack of spill polarization. (C) 2000 Elsevier Science B.V. All rights reserved.

Photogenerated carrier dynamics in an AlGaN/GaN single quantum well has been studied using a conventional degenerate pump and probe technique at room temperature. Photoinduced absorption at the exciton resonance has been observed. It is explained by the absorption coefficient change, through the quantum-confined Stark effect and the quantum-confined Franz-Keldish effect, caused by the photoinduced internal electric-field screening. In comparison with biased GaAs multiple quantum wells, a slower time evolution of differential transmission signals has been also found. Its origin is attributed to the longer carrier sweep-out time due to the potential profile of the sample in conjunction with the longer carrier recombination time. (C) 2000 American Institute of Physics. [S0003-6951(00)02404-9].

The spin-relaxation process of electrons at room temperature is investigated for GaAs/AlGaAs multiple-quantum wells (MQWs) and InGaAs/InP MQWs. The spin-relaxation times are measured for various well thicknesses using time-resolved spin-dependent pump and probe absorption measurements. The spin-relaxation time, tau(s), for GaAs MQWs was found to depend on the electron confinement energy, E-1e, according to tau(s) proportional to E-le(-2.2), demonstrating that the main spin-relaxation mechanism at room temperature is the D'yakonov-Perel' process. The measured tau(s) of InGaAs MQWs vary depending on the quantum confinement energy, E-le, according to tau(s) proportional to E-le(-1.0). tau(s) for QWs by the Elliott-Yafet process is calculated and shown to vary according to tau(s) proportional to E-le(-1). The spin-relaxation mechanism and possible applications using this fast spin-relaxation process are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

We report on the spin relaxation in GaAs-based quantum wires and wells studied at 50 K < T < 180 K by analyzing the circularly polarized photoluminescence transients. A 3.5-times shortening of the electron spill relaxation time is observed in GaAs/AlGaAs quantum wires compared with that in quantum wells at 180 K. The observed results suggest that a spin Rip in the quantum wires is caused predominantly by the D'yakonov-Perel' effect due to spin-orbit interaction. The strong quantum confinement of electrons in the reduced dimensionality of the quantum structure leads to a larger spin splitting energy and a stronger temperature dependence of spin relaxation.

We have investigated the spin-relaxation process of electrons at room temperature in undoped GaAs/AlGaAs multiple-quantum wells (MQWs) and InGaAs/lnP MQWs. The spin-relaxation times are measured for different well thicknesses using time-resolved spin-dependent pump and probe absorption measurement. The spin-relaxation time, tau(s), for GaAs MQWs was found to depend on the electron confinement energy, E-1e, according to tau(s) proportional to E-ie(-2.2), demonstrating that the main spin-relaxation mechanism at room temperature is the D'yakonov-Perel' process. The measured spin-relaxation times of InGaAs MQWs whose band-gap is about half that of GaAs MQWs are about 5 ps and vary depending on the quantum confinement energy EI,, according to tau(s) proportional to E-ie(-1.0). The spin-relaxation time by Elliott-Yafet process, which becomes stronger for narrower band-gap materials, is calculated for quantum wells and shown to vary according to tau(s) proportional to E-ie(-1). The spin-relaxation mechanism and possible applications using this fast spin-relaxation process are discussed.

We have investigated the electron mobility (mu) dependence and the electron quantized energy dependence of the electron spin relaxation time (tau(s)) in n-type and undoped GaAs/AlGaAs multiple quantum wells at room temperature, tau(s) proportional to mu(-1) obtained from the experimental results is consistent with the theoretical prediction based on the D'yakonov-Perel' theory.

This article reviews aspects of carrier dynamics concerning semiconductor quantum wells in picosecond and femtosecond regime with emphasis on tunneling and spin relaxation which we were experimentally involved in. We refer to recent significant developments in the field. We also pick up intriguing aspects of quantum dots on possible applications to both electronic and photonic devices related to the carrier dynamics. (C) 1998 Elsevier Science S.A. All rights reserved.

The electron spin relaxation of InGaAs/InP multiple-quantum wells is investigated at mom temperature using time-resolved polarization-dependent absorption measurements. The spin relaxation time is dependent on the quantum confined energy, E-1e, according to tau(s) proportional to E-1e(-1). The dependence differs from that of the D'yakonov-Perel' interaction, which governs the spin relaxation of GaAs quantum wells at room temperature, and suggests the possibility of the existence of an additional spin relaxation mechanism.

We demonstrate femtosecond bleached absorption recovery using a type-Il tunneling biquantum well (TBQ) structure tailored for long-wavelength operation. The type-IT TBQ structure consisted of strained-layer InGaAlAs narrow wells and InGaAsP wide wells with staggered band lineup, separated by thin InAlAs barriers. In the structures, the photocreated electrons in InGaAlAs layers energetically relax to InGaAsP layers via LO-phonon assisted tunneling through InAlAs barrier! resulting in the bleaching recovery due to the ultrafast spatial separation of electrons and holes. The bleached absorption recovery time was as fast as 350 fs in the excitation wavelength range of 50 nm near the InGaAlAs bandgap.

We have investigated the temperature dependence of the exciton spin relaxation between 10 and 300 K in undoped GaAs quantum wells by using the nonlinear transmission pump-probe and the four-wave-mixing techniques. The electron spin relaxation rate above 40 K is found to be proportional to the product of the temperature and the momentum relaxation time, which indicates that the D'yakonov-Perel' interaction governs the spin-flip dynamics. The temperature independence below 30 K is considered to be due to the band-mixing effect that determines the hole spin relaxation, and the spin exchange is effective to the exciton spin relaxation in these temperatures.

The electron spin relaxation of InGaAs/InP multiple-quantum wells (MQW) is investigated using time-resolved polarization-dependent absorption measurement. The MQW has an excitonic absorption at 1.54 mu m which is suitable for application in optical communications. A theoretical prediction assuming the D'yakonov-Perel' interaction as the main relaxation mechanism gives a spin relaxation rate for the InGaAs quantum well over twice as high as that for the GaAs quantum well. The spin relaxation time measured at room temperature is 5.2 ps and found to be an order of magnitude faster than that of a GaAs quantum well. (C) 1997 American Institute of Physics.

We study the induced absorption due to biexciton formation in absorption bleaching pump-probe signals and the quantum beats between exciton and biexciton in the four-wave-mixing signals. The origin is confirmed by the polarization dependence of the signals in both experiments. The definite discrimination between exciton-biexciton quantum beats and the others by the polarization choices is demonstrated clearly. We also show the well-width dependence of the biexciton binding energy that is determined by the period of the quantum beats in the range of well width 45-150 Å of GaAs/(Formula presented)(Formula presented)As (x=0.3-1.0) multiple quantum wells. © 1997 The American Physical Society.

We study the induced absorption due to biexciton formation in absorption bleaching pump-probe signals and the quantum beats between exciton and biexciton in the four-wave-mixing signals. The origin is confirmed by the polarization dependence of the signals in both experiments. The definite discrimination between exciton-biexciton quantum beats and the others by the polarization choices is demonstrated clearly. We also show the well-width dependence of the biexciton binding energy that is determined by the period of the quantum beats in the range of well width 45-150 Angstrom of GaAs/AlxGa1-xAs (x=0.3-1.0) multiple quantum wells.

Ultrafast all-optical switch is proposed and demonstrated using picoseconds spin-polarization relaxation in a multiple-quantum-well (MQW) etalon structure, The decay time of the conventional all-optical switching in MQW etalon is restricted by the carrier lifetime, typically in nanoseconds, Using carrier spin relaxation, the polarization change of the probe beam has been demonstrated to be switched with a pulse width of 4 ps and a contrast of 4:1 at a pump pulse energy of 50 fJ/mu m(2). In the present device, the contrast is determined by the polarization rotation angle of the probe beam, and the polarization-rotation angle has been shown to be proportional to the total well thickness, It is predicted that a contrast can be improved over 13 dB by optimizing the MQW etalon structure, indicating potential applicability to ultrafast optical communication systems, The optimization would improve the transmission from a present value of about 1%.

We have investigated the spin relaxation process of electron at room temperature in nominally undoped GaAs/A1GaAs multiple-quantum wells. The spin relaxation times are measured for different well thicknesses using time-resolved polarization absorption measurement. The spin relaxation time tau(s), is found to depend on the electron confined energy E(le), according to alpha E(le)(-2.2), showing that the main spin relaxation mechanism at room temperature is the D'yakonov-Perel' Perel' interaction. (C) 1996 American Institute of Physics.

Spin dynamics in GaAs/AlGaAs multiple quantum wells at room temperature are studied in subpicosecond to nanosecond time region by using the optical sampling nondegenerate pump-probe technique. Polarization-dependent absorption signals resonant to the light-hole (lh) exciton revealed the subpicosecond spin relaxation time which was several tens of times shorter than that of the heavy-hole (hh) exciton. Resonant pumping to lh and probing at hh exhibited no spin dependence within the time resolution, showing the spin memory being washed out completely during the energy relaxation from lh to hh. (C) 1996 American Institute of Physics.

We propose and demonstrate two approaches for controlling the recovery time from excitonic absorption bleaching, Each approach has a new feature that the recovery time of optical nonlinearity is controlled by the tunneling or the spin-relaxation effect in quantum well structures. In the tunneling bi-quantum well structure utilizing the tunneling effect, the recovery time is reduced to the picosecond region with the reduction of tunneling barrier thickness. The recovery time is three orders of magnitude shorter than the nanosecond recovery of conventional multiple quantum wells (MQW). The spin relaxation of MQW has been shown to be as fast as several tens of picoseconds. The all-optical gate operations of a Fabry-Perot etalon are demonstrated with a full signal recovery of 17 ps using tunneling and 7-ps recovery using spin relaxation.

Carrier transfer among InAs/GaAs self-organized multi-coupled quantum dots was studied using time-resolved photoluminescence. In the multi-coupled quantum dots, since quantum dots couple with the other dots laterally, the photoexcited carriers tunnel into the relatively larger quantum dots which have lower energy levels. The photoluminescence decay time of multi-coupled quantum dots strongly depends on the energy in contrast with conventional quantum dots. The energy dependence can be explained with a cascade-like tunneling model assuming a tunneling time between quantum dots of 1300 ps.

The performance of the spin-dependent transient rotation (STR) for a multiple-quantum-well (MQW) etalon is studied. An on/off contrast of 4:1 obtained for a pump pulse energy of 50 fJ/mu m(2) at room temperature is larger than that obtained by other picosecond switching methods using etalons. The experimentally observed polarization-rotation angle of the probe beam has been found to be proportional to the total well thickness, which is consistent with the theoretical prediction. It is predicted that the contrast can be improved to over 20:1 for a pump pulse energy of 50 fJ/mu m(2) by using a MQW etalon with a total well thickness of 1 mu m.

An InAs/GaAs in-plane strained superlattice (IFS SL) grown by molecular beam epitaxy (MBE) is investigated using photoluminescence (PL) measurements. The polarization-dependent PL at 0.77 eV (77 K) with a full width at half maximum of 25 meV indicates that a laterally modulated structure is grown. With the use of the empirical nearest-neighbor sp(3)s(double dagger) tight-binding energy band calculation for the IPSSL, we deduced that an In0.74Ga0.26As/In0.26Ga0.74As IPSSL was actually grown. This reveals that an indium composition modulation Delta x of about 0.48 was achieved, which is more than twice as large as that in in-plane superlattices using other material systems.

Tunneling in asymmetric double quantum wells is studied using time-resolved photoluminescence. The photoluminescence lineshape and peak position of the narrow quantum well are strongly influenced by band-gap renormalization caused by the tunneling carriers. Tunneling is quenched in a field regime of +/- 10 kV/cm around the ground state resonance due to excitonic effects.

InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlattices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3 degrees toward the [00 $$($) over bar 1] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ($($) over bar$$ 110) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick)superlattice in addition to a peak corresponding to the In0.5Al0.25Ga0.25As alloy.

We propose a new quantum dot system called multi-coupled quantum dots. In this system, since quantum dots couple with adjacent dots, the photoexcited carriers tunnel into the larger quantum dots which have lower energy states. This energy relaxation results in narrower and stronger photoluminescence than with conventional quantum dots. InAs/GaAs self-organized multi-coupled quantum dots show strong photoluminescence near 1.3 mu m at room temperature, whose intensity is as large as in the well-known highly efficient InGaAs/GaAs quantum wells.

We investigated resonant tunnelling of electrons and holes between coupled quantum wells using time-resolved luminescence spectroscopy. Exponential dependence of tunnelling times on barrier width is observed for electrons but not for holes. The tunnelling times of electrons are correctly described by a model which takes into account inhomogeneous broadening.

We have studied the energy band structure and the GAMMA-X carrier transfer mechanisms for type II tunneling bi-quantum wells consisting of GaAs wells, barriers of different thicknesses, and AlAs layers by cw and time-resolved photoluminescence measurements. The cw photoluminescence spectra of the direct recombination of X electrons in the 7.1 nm thick AlAs layers with GAMMA holes in the 2.85 nm thick GaAs wells show weak zero-phonon lines indicating that the AlAs confined states at X(xy) are lower than those X(z). Time-resolved photoluminescence reveals that the carrier transfer time depends stronger on temperature for thicker AlGaAs barriers. Two scattering mechanisms, temperature-dependent phonon scattering and the temperature-independent interface scattering, are probably involved in the carrier transfer, the latter becoming smaller with increasing AlGaAs barrier thickness. Our results are compared with those obtained for smaller type II GaAs/AlAs superlattices.

We studied the electron tunneling time in tunneling bi-quantum-well and resonant tunneling barrier structures with a pump-probe photo-luminescence system using optical mixing. The tunneling lime decreased as the temperature increased, which is attributed to electrons in an exciton stale thermalizing into free electrons having a faster tunneling lime. (C) 1994 John Wiley and Sons, Inc.

Fast recovery from the excitonic absorption bleaching in tunneling bi-quantum well (TBQ) structures is proposed and demonstrated. The TBQ has a new feature that conventional superlattices will never have; that is, control of the recovery time of optical nonlinearity using tunneling. By reducing the tunneling barrier thickness, the recovery time can be reduced to the picosecond region, which is three orders of magnitude faster than the nanosecond recovery of conventional MQWs. By using type-II TBQs, we have demonstrated all-optical gate operation of a Fabry-Perot etalon. The type-II TBQ etalon with 1.7-nm barriers exhibits a full signal recovery of 17 ps.

We report on the direct observation of ultrafast spin relaxation of both the heavy-hole and the light-hole excitons in GaAs/AlGaAs multiple quantum wells (MQW) at room temperature using the femtosecond absorption bleaching detection. Also, we report on the time-resolved reflection measurements in CdMnTe MQW. A variety of polarization-dependent and wavelength-dependent time profiles are observed in the picosecond time region.

We studied the effect of process damage on exciton absorption recovery time of multiple quantum well narrow wires. The wires were fabricated to the order of 100 nm by focused ion beam lithography and subsequent reactive ion beam etching. The recovery time decreases with increased damage. The recovery times of the damaged samples slightly increase after thermal annealing. The effect of etching damage on the recovery time was discussed by the numerical calculation assuming nonradiative recombination by defects.

We measured the recovery time from excitonic absorption bleaching in tunneling biquantum wells at 77 K and room temperature. The absorption recovery time corresponds to the tunneling time of electrons from narrow to wide wells. We have found that tunneling at 77 K is about 2.5 times slower than at room temperature.

We demonstrate picosecond signal recovery in all optical gate operation using a type II tunneling bi-quantum well (TBQ) etalon. The type II TBQ consists of a series of GaAs wells, AlGaAs barriers, and AlAs layers. In this structure, photoexcited electrons in the GaAs wells escape by tunneling through the AlGaAs barriers toward X states in the AlAs layers. Therefore, the time for recovery from excitonic absorption bleaching in GaAs wells is controlled directly by the AlGaAs barrier thickness. The type II TBQ etalon with 1.7 nm barriers showed a fast signal recovery of 17 ps by carrier tunneling.

We describe a new lateral patterning technique using focused ion beam (FIB) lithography and also two subsequent pattern transfer processes consisting of reactive ion etching and reactive ion beam etching. In the FIB lithography we used a trilevel resist structure consisting of a CMS resist, an Al film, and an SiO2 film. This technique provided 60-nm-wide wires and 100 nm dots in a GaAs/AlGaAs multiquantum well (MQW). Twenty-five-nm-wide wires are also fabricated in GaAs by narrowing the SiO2 mask laterally by wet etching.

We propose the type-II tunneling biquantum-well (TBQ) structure which consists of a series of GaAs wells, AlGaAs barriers and AlAs layers. The type-II TBQ has no significant optical absorption except for the GaAs wells and shows fast recovery from excitonic absorption bleaching. In this structure, photoexcited electrons in the GaAs wells escape by tunneling through the AlGaAs barriers toward X states in the AlAs layers. The recovery time of excitonic absorption bleaching in the GaAs wells was reduced to 8 ps as the AlGaAs barrier thickness was decreased to 1.1 nm.

We have studied narrow multiple quantum-well (MQW) wires using conventional absorption spectroscopy and time-resolved absorption measurements. Wires down to 130 nm were fabricated from MQW's using focused ion-beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward sidewalls and recombine on the sidewall surface. We confirmed from the absorption spectrum of MQW wires that the exciton peak is preserved even in wires 130 nm wide. We show that the excitonic absorption recovery of MQW wires is reduced to 11 ps, preserving excitonic optical nonlinearity.

We studied the nonresonant electron tunneling time in tunneling bi-quantum-well structures with a pump-probe photoluminescence (PL) system using optical mixing. The PL decay time of narrow wells for an AlxGa1-xAs (x = 0.51) barrier of 4.0 nm, determined by tunneling, decreases from 40 ps to 24 ps as the temperature increases from 6K to 132K. We attribute this to electrons in an exciton state thermalizing into free electrons having a faster tunneling rate.

Excitons in a quantum well of a semiconductor exhibit large optical nonlinearity. Therefore, a quantum well of a semiconductor is regarded as a key to realizing a device which can control light using light. However, the lifetime of excitons, which are generated optically, is several nanoseconds to several tens nanoseconds. Therefore, the nonlinear optical effect including absorption saturation continues for the lifetime of the excitons. To shorten the lifetime of the excitons without disturbing the optical nonlinearity, a new superlattice structure is proposed called the “tunneling bi‐quantum well.” This structure uses the tunneling effect and measures the absorption recovery time. As a result, it was found that in a tunneling bi‐quantum well in a GaAs/AlGaAs system with a barrier width of 1.7 nm, the absorption recovery time was as short as 1 ps. Copyright © 1992 Wiley Periodicals, Inc., A Wiley Company

We report the time evolution of excitonic absorption bleaching in resonant tunneling bi-quantum-well (TBQ) structures, in which the ground electron level in a narrow well has the same energy as the second electron level in a wide well. In resonant TBQ structure, we observed a reduction in the absorption recovery time and an increase in the tail-to-peak ratio of the absorption change. By comparing the absorption change of resonant TBQ with the e2-hh2 excitonic absorption change of conventional multiple quantum wells (MQW), we show that the increase in the tail-to-peak ratio of a resonant TBQ can be attributed to the thermally remaining holes in the first excited subband of heavy holes.

We report the time-resolved absorption measurement of narrow multiple quantum-well (MQW) wires to investigate their fast recoveries from excitonic absorption bleaching. Wires down to 130 nm were fabricated from MQWs using focused ion beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward the sidewalls and recombine on the surface of the sidewalls. We show that the strong optical nonlinearity of excitons is preserved, even in wires of 130 nm width, and having a fast recovery time in the picosecond region. We also briefly discuss the possibility of making quantum wires which have a faster recovery time and larger optical nonlinearity.

We discuss the electron tunneling time observed in a new AlGaAs/GaAs superlattice structure, the tunneling bi-quantum well (TBQ). To calculate the nonresonant tunneling time, we made experiments on resonant tunneling to confirm that the 60% rule of conduction-band discontinuity accurately evaluates the free tunneling probability of electrons. We found that the observed recovery time agrees quite well with the calculated longitudinal optical phonon emission tunneling time for thin (less-than-or-equal-to 10 monolayers) barriers.

This letter demonstrates the fast recovery from excitonic absorption bleaching in tunneling biquantum well structures. A tunneling biquantum well consists of a series of narrow and wide wells. Recovery time was reduced to 1 ps using 1.7-nm-thick tunneling barriers, three orders of magnitude shorter than the few nanoseconds recovery due to radiative recombination. This is the fastest tunneling process observed to date.