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.

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 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.

過渡的磁性半導体のスピン物性解明のために、量子井戸内のスピン緩和機構と量子ドット間の交換相互作用の検討を行った。まず、量子井戸内のスピン緩和機構として、Elliott-Yafet効果のスピン緩和時間を理論的に求め、スピン緩和時間が量子化エネルギーに反比例することを明らかにした。これは、さきに実験的に求めたInGaAs/InP量子井戸のスピン緩和時間の量子化エネルギー依存性と同じ依存性であり、E-Y効果が支配的なスピン緩和機構であることを示唆している。InGaAs/InP量子井戸で量子化エネルギーによってスピン緩和時間が制御できることを明らかにしたことにより、光通信に対応する波長1.55ミクロンで、スピンをデバイス応用しうる可能性に大きく道を開いた。また、量子ドット間の交換相互作用の検討では、ハイトラー・ロンドン近似を用いて交換相互作用を計算した。現在、量子ドットをメモリーやコンピューティングに応用する提案がなされているが、とくにドット中に生成されたキャリアのスピンが近接するドットのキャリアスピンと持つどのような相互作用を持つかは、今後、量子ドットのスピンをデバイス応用する際の最重要ファクターの一つになる。このため、今回は、一対の近接した量子ドット(結合量子ドット)に光励起された電子の交換相互作用エネルギーを計算し、さらにドットを立方晶に配置した場合のネール温度を求めた。その結果、結合量子ドットが反強磁性になること、また準現実的なドット構造ではそのネール温度が150Kに達することを明らかにした

過渡的磁性半導体のスピン物性の解明と応用を目指して、InGaNのキャリアのスピン分極を調べるとともに、半導体量子ドットの電子スピンを用いた量子演算の検討を行った。InGaNは、近年の青色レーザなどで注目されるワイドギャップ半導体であるが、キャリアのスピン分極が生成されないことが明らかになった。これは、InGaN固有のバンド構造や結晶構造に起因していると考えられる、InGaNではエピ上でIn組成のゆらぎがあり、この組成ゆらぎのために重い正孔と軽い正孔の準位がエネルギー的に混じっている可能性が高い。半導体量子ドットの電子スピンを用いた量子演算の検討では、3重結合量子ドットにおける2電子状態を用いたゲートを提案した。系を貫く静磁場と量子ドットのサイズに依存した回転磁場、およびフォトン支援トンネリングを用いて量子コンピューティングに必要な1ビットの回転と制御NOT演算を実現できる。スピンの回転とトンネル現象のエネルギー分解能のみを用いても、少なくとも5つの量子ビットまで拡張できることが分かった。また、量子演算を行う際に重要な量子ドット間のトンネル時間を実測した。トンネル時間のバリア層幅依存性を測定し、トンネル時間がWKB近似にのることを明らかにするとともに、量子井戸間のトンネル時間より一桁遅いことを明らかにした

0次元構造である量子ドットは状態密度がデルタ関数的であり、また、振動子強度が大きいためレーザーの閾値電流を下げると期待されている。しかし、量子ドットレーザーなどの光デバイスにおいてはフォノンボトルネックが閾値電流の低減や高速動作の障害となる可能性がある。そこで今年度は量子ドット内のフォノンボトルネック現象を高時間分解計測を用いて調べた。量子ドット中での高エネルギー側の量子準位から下の準位へのキャリアのエネルギー緩和では、エネルギー保存則と運動量保存則を満たす必要がある。通常は、LOフォノン放出をともなうエネルギー緩和が支配的であるが、離散的なエネルギー準位をもつ量子ドットでは、その準位間のエネルギー差と一致するフォノンがないため、LOフォノンを介したエネルギー緩和が抑制され、準位間の緩和時間が遅くなると予想されている。本研究では、極めて高均一な量子ドットを用いることによって各準位の発光の観測を可能にした。明瞭に分離された各準位のPLスペクトルの時間分解フォトルミネッセンス測定を行い、レート方程式を用いて直接的に準位間緩和時間を導出した。その結果、再結合時間は、高温になるにつれ短くなったが、準位間緩和時間は、温度に依存せずほぼ一定であった。この長い準位間緩和時間は、フォノンボトルネックによることが明らかになった。10Kから200Kの結果から300Kの準位間緩和時間を外挿すると、320psとなる。この遅い準位間緩和時間は量子ドットの基底準位にキャリアを供給するのを妨げ、室温での発光デバイスの動作に影響を及ぼすと考えられる。また、一方では高エネルギー準位でのキャリアの滞在時間を引き延ばすため、高エネルギー準位でのキャリアスピンの振る舞いの観測を可能にすると予測される

Self-assembled InAlAs/AlGaAs quantum dots are grown by the Stranski-Krastanow mode of Molecular Beam Epitaxy (MBE). We measured the spin relaxation time of the structure under the non-resonant optical excitation by using 80 fs pulses by the titanium sapphire laser at wavelength of 780 nm. We used the right circular polarized light for the excitation of carrires to obtain fixed electron spins. The luminescence from the quantum dots at 10 K was divided into the right circular and the left circular components and was temporary resolved by the streak-scope. While the lifetime of the exciton was about 1 18 ns, the spin relaxation time determined by the decay of the polarization of luminescence was 1.31 us, which is comparable to the lifetime. The result indicates that the major relaxation mechanisms in the quantum well are not effective in the quantum dots and that the quantum dot is a good material for the coherent control of electron spins.As the spin-electronisc material for the future generations, wide-gap semiconductors and a polymer was studied with emphasis on the magnetic or spintronic properties

The exciton spin relaxation mechanism was investigated between 13 and 300 K in InGaAs/InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. 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.Carrier spin dynamics was investigated in highly uniform self-assembled InAs quantum dots. 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.The spin relaxation process of A-band exciton in GaN is observed with sub-picosecond's 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 III-V compound semiconductors. The excitonic spin relaxation process in cubic GaN is observed. The spin relaxation times at 15 K -75 K are found to be longer than 5 ns. Although these long spin relaxation times are in striking contrast to the sub-picosecond 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 zincblende semiconductors

1.強磁性スピン系の光操作と光磁化およびその応用(宗片)(1)(Ga,Mn)Asへの窒素添加効果を実験的に調べ、窒素添加によってホール濃度とキュリー温度が低下すると結論した。(2)光誘起才差運動の起源を実験的に検討し、光照射により磁気異方性が変化した結果発生する、持続時間の長い(1-10ns)有効磁場によるものであることを結論した。(3)開発を進めてきたレーザ走査型磁気光学顕微鏡がとうとう立ち上がり、(Ga,Mn)As中で光誘起磁化の直接観測に世界に先駆けて成功した。さらに、(4)作製法に工夫をこらしたIII-V族p-nヘテロダイオードにおいて、再現性の良い円偏光依存光起電力を得ることに成功するとともに、ヘテロ接合に対するスピン拡散過程の理論的解析を世界に先駆けて開始した。2.スピン生成と緩和の光制御と応用(竹内)ワイドギャップ半導体の代表格であるGaN中のキャリア電子のスピン緩和には、六方晶GaN中の1psの超高速過程と立方晶GaN中の10nsの遅い過程の2通りが存在することを実験的に明らかにした。後者については、スピン軌道相互作用の減少によるものであると説明されるが、前者についてはこの物質固有の新しいスピン緩和現象であり、その解明は今後の課題であると結論した。3.近接場光学技術によるナノ光スピントロニクス(斎木)近接場光学顕微鏡(NSOM)の磁気光学効果の高感度化につながる手法として、ホモダイン検出とロックイン検出とを組み合わせる測定技術を引き続き研究し、原子ステップを持つサファイア基板上のNiO薄膜中に形成された段差2nmのナノチャンネルにおいて、上部と下部で1:4という極めて大きなコントラストを有する表面像を得ることができた。本手法の有用性を示した。また、低温・磁場下で動作するNSOMにおいて、ドリフトや振動などの細かい問題点をほぼ解決し、30nmの分解能による単一量子ドットの発光イメージングに成功した。磁気発光分光も可能となり、量子ドット発光のゼーマン分裂や反磁性シフトなどが観測できるようになった

0次元構造である量子ドットは状態密度がデルタ関数的であり、また、振動子強度が大きいためレーザーの閾値電流を下げると期待されている。しかし、量子ドットレーザーなどの光デバイスにおいてはフォノンボトルネックが閾値電流の低減や高速動作の障害となる可能性がある。そこで今年度は量子ドット内のフォノンボトルネック現象を高時間分解計測を用いて調べた。量子ドット中での高エネルギー側の量子準位から下の準位へのキャリアのエネルギー緩和では、エネルギー保存則と運動量保存則を満たす必要がある。通常は、LOフォノン放出をともなうエネルギー緩和が支配的であるが、離散的なエネルギー準位をもつ量子ドットでは、その準位間のエネルギー差と一致するフォノンがないため、LOフォノンを介したエネルギー緩和が抑制され、準位間の緩和時間が遅くなると予想されている。本研究では、極めて高均一な量子ドットを用いることによって各準位の発光の観測を可能にした。明瞭に分離された各準位のPLスペクトルの時間分解フォトルミネッセンス測定を行い、レート方程式を用いて直接的に準位間緩和時間を導出した。その結果、再結合時間は、高温になるにつれ短くなったが、準位間緩和時間は、温度に依存せずほぼ一定であった。この長い準位間緩和時間は、フォノンボトルネックによることが明らかになった。10Kから200Kの結果から300Kの準位間緩和時間を外挿すると、320psとなる。この遅い準位間緩和時間は量子ドットの基底準位にキャリアを供給するのを妨げ、室温での発光デバイスの動作に影響を及ぼすと考えられる。また、一方では高エネルギー準位でのキャリアの滞在時間を引き延ばすため、高エネルギー準位でのキャリアスピンの振る舞いの観測を可能にすると予測される

III-V族化合物半導体を用いるスピントロニクス研究では、新デバイスやスピン偏極電子源への応用が期待されている。しかし、スピン緩和時間が調べられた半導体は限られている。本研究では、様々なIII-V族化合物半導体のスピン緩和時間を調べた。その結果、トンネル双量子井戸では、タイプI型とII型ともにスピン緩和時間がトンネル効果の影響を受けることが分かった。また、InGaAs/AlGaAs/AlAsSb結合量子井戸では、高温領域で２種類のスピン緩和過程が働いていることが明らかになった。スピン偏極電子源への応用を目指すGaAs/GaAsP歪補償超格子では、室温で励起光強度を変えても、スピン緩和時間の変化は観測されないことが明らかになった。

　機能性円偏光光源のための高偏極長寿命In系化合物半導体の研究においては、InP基板上に成長したInGaAsPバルク、低温成長GaAsバルク、Beドープしたp型GaAsバルクのスピン緩和時間を、時間分解ポンプ・プローブ測定や時間分解フォトルミネッセンス測定で調べた。　InP基板上に成長したInGaAsPバルクは、10Kで波長1160nm付近に発光ピークがあるが、円偏光ポンププローブ時間分解計測では、この発光ピーク近傍の波長でスピン緩和過程を観測した。その結果、励起光強度10mWで980psのスピン緩和時間が得られた。また、このスピン緩和時間は励起光強度を変えると変化することが分かった。さらに測定温度を室温まで上げると、スピン緩和時間は95psまで速くなった。これらのスピン緩和時間の温度依存性と励起光強度依存性から、10-30Kでは、Bir-Aronov-Pikus効果が支配的であり、100-300KではD'yakonov-Perel'効果が支配的であることが明らかになった。　In系化合物半導体との比較のために、Inを含まないGaAsバルクのスピン緩和時間もあわせて調べている。Beドープしたp型GaAsバルクでは、10Kで1.511 eVと1.497 eVの二つのフォトルミネッセンスピークが観測され、それぞれ10-100Kで1-3nsの比較的遅いスピン緩和時間が観測された。前者はBeアクセプターに束縛された励起子のスピン緩和であり、後者はバンドからアクセプター準位への遷移に関与するスピン緩和であると考えられる。両者のスピン緩和時間かほとんど同じであることから、ともに伝導電子のスピン緩和を観測しているものと考えられる。また、低温成長GaAsバルクでは、10Kで2ps、28ps、158psの3成分の高速のスピン緩和が観測された。この結果からキャリア散乱によりスピンが反転するElliott-Yafet効果が効いている可能性が高いことがわかった。

円偏光光源のための高偏極長寿命スピン緩和量子ドットの研究においては、科研費申請では、量子ドットのスピン緩和研究における未踏領域である波長400-700 nmのフェムト秒光源を導入する予定であった。特定課題研究においては、金額的制約からこの装置の導入は見送り、代替手段によるこの波長域のスピン緩和研究の遂行を図った。このために、今年度は光パラメトリック発振器による波長1.1ミクロンの光を第二高調波発生器を用いて、波長550 nmに変換する実験に注力した。その結果、波長500nm領域で1 mW程度の強度の光が得られるようになった。安定に時間分解フォトルミネッセンス測定を行うためには、この数倍の光強度を得ることが望ましいため、現在、第二高調波発生器の改良を行っている。まもなく波長500nm領域での量子ドット計測の条件がそろう予定であり、CdTe量子ドットなどの測定にとりかかる予定である。また、それ以外の波長領域で円偏光光源となりうる可能性のある材料系として、GaNナノロッド、ZnOナノ粒子、InGaAs/AlAsSb量子井戸の時間分解フォトルミネッセンス測定を行っている。このうちのいくつかについては、発光再結合時間やスピン緩和時間の測定データが得られたので学術誌に投稿中か投稿予定であり、学会発表としては、2010年の秋期の応用物理学会などで発表を行う予定である。

ワイドギャップ半導体の超高速スピン緩和の解明として、GaInNAs/GaAs量子井戸のスピン緩和時間を測定した。GaInNAs量子井戸は波長1.2-1.3ミクロン付近で発光するため、光通信用の発光デバイスへの応用が期待されている。この応用上魅力的なGaInNAs量子井戸のスピン緩和時間の実測例としては、これまでに77-133psという報告と、2 nsという２件の報告があり、スピン緩和時間に１桁もの大きなひらきがあり混乱を生じていた。 我々は、時間分解フォトルミネッセンス分光によって、GaInNAs/GaAs量子井戸のスピン緩和過程を調べた。その結果、光励起後、200psまでは観測できる寿命の短い1.07eVのPLピークと、それとは独立に1.035eVにある長寿命のPLピークがあることを発見した。この二つのピークのスピン緩和時間を測定したところ、非局在励起子によると思われる1.06eVのピークは192psのスピン緩和時間を持つのに対して、局在励起子によると思われる1.035eVのピークは2nsという遅い緩和時間を持つことが明らかになった。 これまで、GaInNAs量子井戸のスピン緩和時間には二説あり混乱を生じていたが、本研究によって、PLピークが二つあり、それぞれのスピン緩和時間が大きく異なることが初めて明らかになった。この問題が解決したことは、GaInNAs量子井戸の今後のスピン緩和メカニズムの解明と応用において大きな意味があると考えられる。　なお、この研究結果は、早速　Applied Physics Letters　に投稿し、２００８年早々にVol.92, p.051908 に掲載された。

ワイドギャップ半導体超高速スピン緩和の解明のために、AlGaN/GaN MQW発光のシュタルク効果とキャリアスクリーニング効果の関わりと、InGaNに光励起されたホットキャリアの緩和過程を調べた。　前者については、AlGaN/GaN MQWの時間分解フォトルミネッセンス測定の結果、AlGaN/GaN MQWの緩和時間とPLエネルギーがキャリア濃度に強く依存していることが実験的に明らかになった。これは窒化物半導体特有の大きな内部電場とキャリアによるスクリーニング効果を考えることで説明できる。この効果をシュレディンガー方程式と、ポアソン方程式を自己無撞着に解くことによって算出した。その結果、内部電場の大きさをフィッティングパラメーター（190 kV/cm）としてキャリア密度に対してPLエネルギーの変化量を計算すると、実験で得られた各励起光強度におけるPLエネルギーの変化量と比較的良い一致を得られることがわかった。上記のモデルがこの依存性をよく説明することが明らかとなった。また、励起光強度8 mW以上では、緩和時間は330 ps程度に収束しフラットバンドが実現されていると考えられる。　後者については、InGaNバルクのバンド端よりも高エネルギーに光励起されたホットキャリアの緩和を調べた。サンプルはサファイア基板上に成長させた厚さ0.15 ミクロンのバルクIn0.05Ga0.95Nである。計測にはポンプ・プローブ時間分解吸収計測を用い、光源には極短パルスチタン・サファイアレーザの第二高調波または第三高調波を励起光として用い、プローブ光には第二高調波を用いた。測定系の時間分解能は光源に用いる光パルスの時間幅のみで決まりサブピコ秒である。測定の結果、立ち上がり時間は第三高調波励起の場合に0.9 psほど第二高調波励起より遅いことがわかった。これは、ホットキャリアの緩和に要した時間と考えられる。

・量子ドット間の反強磁性結合の観測　半導体中の電子スピンを自由に操ることが可能になれば、我々は新しい自由度を手に入れることになる。この自由度を応用すれば、新しい機能を持ったデバイスを実現する可能性が生まれるだろう。半導体の中に3次元的に電子を閉じ込めるナノ構造を量子ドットと呼ぶが、この量子ドットの中の電子スピンの向きをデジタル情報の0と1に対応させれば、情報処理の可能性が生まれる。ただし、隣接するドット間のスピン同士で交換相互作用を働かせる必要がある。我々は、半導体量子ドット間の交換相互作用によって反強磁性結合が形成され電子スピンが反転する過程を、時間分解フォトルミネセンス測定によって直接的に観測した。電子スピンの反転が70-200psで起こること、また、反強磁性秩序が、50-80 K以下の温度で存在することが明らかになった。・高均一量子ドットのスピン緩和時間とスピンパウリブロッキングの観測　高均一量子ドットのスピン緩和時間を、時間分解フォトルミネッセンスを用いて測定した。その結果、10Kでのスピン緩和時間が1nsであることが明らかになった。これは、不均一な量子ドットのスピン緩和時間の測定値1.3nsとよく対応する。　一つの量子ドットに二個の電子が入った場合、両者のスピンが同じであれば、パウリの排他原理により、同じエネルギー準位には入れない。このため、基底準位に1個、第二準位に1個、電子が入ることになる。この効果をスピンパウリブロッキングと呼ぶが、このため第二準位のスピン偏極率が基底準位より大きくなると予測される。実験では、第二準位が基底準位の約2倍のスピン偏極率であることが明らかになった。

　量子ドット間のキャリアのトンネル過程を時間分解フォトルミネッセンス法を用いて調べた。結合量子ドットのサンプル構造は、In0.9Al0.1As量子ドットとInAs量子ドットがGaAsバリア層を挟んだ構造であり、バリア幅を6nm、8nm、10nmと変化させた３種類からなる。この構造では、In0.9Al0.1As量子ドット内の電子は、よりエネルギーの低いInAsドットの基底準位に非共鳴トンネルする。測定の結果、量子井戸間のトンネルと同様に、ドット間のトンネル時間がＷＫＢ近似で記述できることが明らかになった。ただし、このトンネル時間は量子井戸間のトンネル時間よりも約一桁遅い。これは、非共鳴トンネルの主要な散乱過程であるフォノン散乱が抑制されているためと考えられる。次に、スピン偏極させた電子のドット間トンネルを調べた。その結果、結合量子ドットのInAlAsドット内でのスピン緩和時間は、バリア幅6nm、8nm、10nmのサンプルに対して1.6ns、1.6ns、1.8nsであり、トンネル時間（130ps、360ps、850ps）との間に強い相関はないことが明らかになった。スピン緩和時間がトンネル過程に強くは影響されないことから、トンネル時間が短ければ、スピン偏極を維持したまま電子の移動が可能であると考えられる。

これまでのエレクトロニクスにおいて、スピンという量子状態はデバイスの機能としては利用されてこなかった。しかし、近年スピンを利用することによって、従来にない新しいデバイス機能が実現できるのではないかという期待が高まっている。本研究では、InGaAs量子井戸でのスピン緩和過程の測定とその機構の解明に取り組み、量子井戸の設計パラメータを変えることによってどの程度スピンの緩和過程を制御できるのかを調べた。その結果、InGaAs量子井戸での電子のスピン緩和時間は量子化エネルギーに反比例する（マイナス一乗に比例する）ことが明らかになった。InGaAs量子井戸のバンドギャップはGaAs量子井戸のバンドギャップの半分程度と小さいが、バンドギャップの縮小によって大きくなる効果として、伝導帯と価電子帯とのバンドミキシングによって生じるスピン緩和の機構（Elliot-Yafet効果）がある。そこで、量子井戸内のスピン緩和機構として、Elliott-Yafet効果のスピン緩和時間を理論的に求め、スピン緩和時間が量子化エネルギーに反比例することを明らかにした。これは、実験的に求めたInGaAs/InP量子井戸のスピン緩和時間の量子化エネルギー依存性と同じ依存性であり、Elliott-Yafet効果が支配的なスピン緩和機構であることを示唆している。InGaAs/InP量子井戸で量子化エネルギーによってスピン緩和時間が制御できることを明らかにしたことにより、光通信に対応する波長1.55ミクロンで、スピンをデバイス応用しうる可能性に大きく道を開いた。

これまでのエレクトロニクスにおいて、スピンという量子状態はデバイスの機能としては利用されてこなかった。しかし、近年スピンを利用することによって、従来にない新しいデバイス機能が実現できるのではないかという期待が高まっている。本研究では、InGaAs量子井戸でのスピン緩和過程の測定とその機構の解明に取り組み、量子井戸の設計パラメータを変えることによってどの程度スピンの緩和過程を制御できるのかを調べた。その結果、InGaAs量子井戸での電子のスピン緩和時間は量子化エネルギーに反比例する（マイナス一乗に比例する）ことが明らかになった。GaAs量子井戸では、スピン緩和時間が量子化エネルギーのマイナス二乗に比例し、スピン軌道相互作用に基づくスピン緩和機構（Dyakonov-Perel効果）が支配的であることを既に明らかにしたが、この依存性とは異なる。このことから、InGaAs量子井戸でのスピン緩和の機構としては他の要素を考える必要があることが明らかになった。InGaAs量子井戸のバンドギャップはGaAs量子井戸のバンドギャップの半分程度と小さいが、バンドギャップの縮小によって大きくなる効果として、伝導帯と価電子帯とのバンドミキシングによって生じるスピン緩和の機構（Elliot-Yafet効果）などを今後考慮する必要がある。また、応用面を考えると、InGaAs量子井戸は光通信での中心波長である1.55ミクロンの波長に対応するが、スピン緩和時間は同じ量子化エネルギーのGaAs量子井戸より一桁以上早く、その時定数は5ps程度と極めて高速であることが明らかになった。今後、全光ゲートスイッチなどの超高速デバイスへの応用の可能性も考えられる。研究成果の発表1998年1月　 応用物理学会欧文誌刊行会　A. Tackeuchi and O. Wada, Jpn. J. Appl. Phys. 37 (1998) 98., “Electron Spin Relaxation Dynamics in InGaAs/InP Multiple-quantum Wells”