Strained-layer Single Quantum Wells Research Articles

Effects of temperature on transition energies of GaAsSbN/GaAs single quantumwells

The energy transitions of GaAsSbN/GaAs strained-layer single quantum wells (QWs),grown by molecular-beam epitaxy, are studied in detail, using photoluminescence (PL) andphotoreflectance (PR) spectroscopies. The optical transitions energy observed in the PLand PR spectra of GaAsSbN/GaAs QWs show a strong decrease with a small increase inthe N composition. These effects are explained through the interaction between theconduction band and a narrow resonant band formed by nitrogen states in the GaAsSbNalloy. The temperature dependence of ground-state energy of strained-layer QWs isanalyzed using the Bose–Einstein relation in the temperature range from 9 to 295 K. Theparameters that describe the temperature variations of the ground-state energies areevaluated and discussed.

Temperature-dependent photoluminescence spectra of GaAsSb/AlGaAs and GaAsSbN/GaAs single quantum wells under different excitation intensities

The mechanism for low-temperature photoluminescence (PL) emissions in GaAsSb/AlGaAs and GaAsSbN/GaAs strained-layer single quantum wells (SQWs), grown by molecular-beam epitaxy, is studied in detail, using PL spectroscopy as a function of temperature and excitation intensity. In all samples, the PL peak energy as well as the full width at half maximum (FWHM), as a function of temperature, present anomalous behaviors, i.e., the PL peak energy shows a successive red/blue/redshift (S-shaped behavior) and the FWHM shows a successive blue/red/blueshift ("inverted S-shaped curve") with increasing temperature. At sufficiently low excitation intensity and in a narrow temperature interval (50 - 80 K), the nitrogen-containing samples present two clear competitive PL peaks. The low-energy PL mechanism (8 - 80 K) is dominated by localized PL transitions, while the high-energy PL mechanism is dominated by the ground state (e1-hh1) PL transition. Additionally, these PL peaks show different temperature dependence with the low-energy PL peak, showing a stronger redshift than the high-energy PL peak. A competition process between localized and delocalized excitons is used to discuss these PL properties.

Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2–100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity.

Optical properties of ZnS/ZnMgS strained-layer quantum wells

We report optical properties of ZnS/ZnMgS strained-layer single quantum wells. The main photoluminescence (PL) peak from ZnS is attributed to light-hole (LH) free excitons. The emission due to heavy-hole (HH) excitons was also observed. With decreasing well width, these emission lines shift to higher energy owing to quantum confinement. Time-resolved PL spectra suggest rapid relaxation from HH band to LH band in the ZnS well. Localization of excitons in the ZnMgS barrier, which is due to fluctuation of alloy composition, is also suggested. On the other hand, localization of excitons in the ZnS well, caused by the fluctuation of the well width, was hardly observed for the sample with good quality. The LH exciton emission showed fast initial decay time of 16–36 ps, which seems to also depend on the sample quality.

Photoluminescence study of ZnS/ZnMgS single quantum wells

We report a photoluminescence (PL) study of ZnS/ZnMgS strained-layer single quantum wells. The main PL peak from ZnS is attributed to light-hole free excitons. Quantum confinement causes it to shift from 3.76 eV to higher energy, 3.84 eV, with decreasing well width. Hydrostatic and shear deformation potentials are determined from energies of light- and heavy-hole exciton emission, to be a=−6.4 eV and b=−1.0 eV, respectively.

Localized excitonic emissions of ZnCdSe/ZnSe quantum wells grown on a GaAs(110) cleaved surface

Anomalous optical properties of Zn1−xCdxSe/ZnSe strained-layer single quantum wells (SQWs) fabricated on cleaved GaAs(110) surfaces in ultrahigh vacuum by molecular beam epitaxy have been investigated. From the temperature-dependent photoluminescence measurement, the origins of localized excitonic emissions of two kinds of SQW structures with different Cd compositions are suggested. Localization centers of SQWs with low Cd composition (14%) are mainly originated by the well thickness fluctuation. However, SQWs with high Cd composition (27%) showed localized excitonic emissions at low temperature due to the Cd composition fluctuations rather than the lack of thickness uniformity of the well layer. Estimated averaged depths of localization of excitons by model calculations are 9 and 14 meV for the wells with 14% and 27% Cd compositions, respectively.

The determination of the band offsets in strained-layer InxGa1-xAs/GaAs quantum wells by low-temperature modulation spectroscopy

We report a determination of the conduction-band offset ratio Oc of the InxGa1-xAs/GaAs strained-layer single quantum wells by low-temperature modulation spectroscopy. Various modulation reflectance measurements include contactless electroreflectance (CER), photoreflectance (PR) and piezoreflectance (PZR) are carried out on four different samples at room temperature and 20 K. The samples have In composition x=0.05 (sample A), 0.11 (sample B), 0.14 (sample C) and 0.21 (sample D) respectively. A number of allowed and forbidden excitonic transitions have been observed. The light-hole transitions were unambiguously identified by comparing the CER spectra with different modulation strengths. The origins of the various spectral features have been assigned by comparison with an envelope-function calculation taking into account the effects of strain. Good agreement between experiments and theory is achieved by taking Qc=0.46+or-0.05, 0.50+or-0.10, 0.54+or-0.07 and 0.63+or-0.05 for samples A, B, C and D respectively. It has been shown that Oc depends linearly with the In composition x between 0.05 and 0.21.

Growth of CdTe/CdZnTe strained-layer single quantum wells by modified hot-wall epitaxy method and their properties

Several CdTe/CdZnTe strained-layer single quantum well (SLSQW) structures, with CdTe well width ranging from 15 to 240 Å, were grown on GaAs(100) substrates, by modified hot-wall epitaxy (HWE) method for the first time. Our HWE system is equiped with a gold tube radiation shield which is more effective in heat confinement and temperature stability than conventional metal tubes. Photoluminescence (PL) measurements of SLSQW showed that the sharp e 1h 1 excitonic transition peaks in the range of 1.597 to 1.624 eV shifted to the higher energy with decreasing well width. From the PL excitation (PLE) measurements of SLSQW, we obtained the n = 1, 2, 3 and n = 1, 2 excitonic transition peaks for the 240 and 120 Å CdTe well width SLSQW structures, respectively. The excitonic transition energies were calculated considering strain effects and this results were agreed well with experimental ones considering the exciton binding energy. Moreover, the FWHMs of excitonic transition peaks obtained from PL and PLE measurements were about 2 meV and Stokes shifts between PL and PLE spectra were about 1 meV. It indicates that the SLSQWs grown by HWE in this study have a high quality comparable to MBE-grown ones.

Metalorganic molecular beam epitaxial growth and characterization of CdSe/ZnSe strained-layer single quantum wells and superlattices on GaAs substrates

Reflection high-energy electron diffraction (RHEED) oscillations that continued up to three cycles were observed during metalorganic molecular-beam epitaxial growth of CdSe on a ZnSe buffer grown on a GaAs substrate, showing two-dimensional layer-by-layer growth despite the large lattice mismatch (6.85%). Based on this result, we could definitely control the growth of CdSe/ZnSe strained-layer single quantum wells with well widths of 1–3 monolayers and of short-period superlattices under in situ RHEED monitoring. Optical properties strongly demonstrated the successful formation of well-defined structures consisting of these highly strained II-VI semiconductor multilayers.

Optical properties of CdTe/Cd1-xZnxTe strained-layer single quantum wells.

The electronic states in a series of molecular-beam-epitaxy-grown CdTe/${\mathrm{Cd}}_{0.79}$${\mathrm{Zn}}_{0.21}$Te strained-layer single quantum wells have been studied both theoretically and experimentally. Calculations based on model solid theory and deformation-potential theory have shown that, at Zn concentration of 0.21 in the barriers, the band lineup is type I for heavy holes and type II for light holes. The confined states were calculated using the three-band envelope function model. Photoluminescence measurement at temperatures between 9 and 100 K indicates that the broadening mechanism is dominated by the polar optical phonon (longitudinal optical) interactions. The circularly polarized excitation spectra have shown predominant heavy-hole feature confirming that the band lineup is type I for heavy holes and type II for light holes. With well width between 25 and 75 \AA{}, our study has shown that exciton binding energy decreases with decreasing well width.

Growth of II–VI semiconductor quantum well structures under in situ RHEED observations

Growth of ZnSe/ZnSSe and ZnCdSe/ZnSSe strained-layer single quantum wells (SQWs) has been carried out by metalorganic molecular beam epitaxy (MOMBE) under in situ RHEED observations. ZnSe/ZnSSe SQWs exhibited intense and sharp photoluminescence (PL) at 4.2 K. The linewidth of a ZnSe(28Å)/ZnS 0.1Se 0.9 SQW is 1.2 meV, which is the narrowest value ever reported for any ZnSe-based quantum wells with similar structures. In contrast to ZnSe/ZnSSe SQWs, ZnCdSe/ZnSSe SQWs showed intense excitonic emission with a relatively large linewidth at 4.2 K. No evidence for the presence of monolayer fluctuations has been observed in both kinds of SQWs despite the high quality of the excitonic features. The interface qualities of the SQWs are discussed in terms of the PL linewidth.

MBE growth of InAs/AlInAs strained-layer multi quantum wells for optical device applications

InAs/ Al 0.48 In 0.52 As strained-layer single quantum wells and strained-layer multi quantum wells have been grown by molecular beam epitaxy (MBE) on (100)-InP substrates. Although there is highly biaxial compressive strain in this material, high-crystalline quality without misfit dislocations is demonstrated by transmission electron microscopy (TEM) and low temperature photoluminescence (PL) measurements. PL peaks have been successfully observed from the MQWs with well width L z of 9–40 Å. As for the sample with L z = 9 Å, a large energy shift of 750 meV from the band-gap of bulk InAs was confirmed. A laser diode with InAs/AlInAs MQW active layer was also fabricated and the stimulated emission at 1.15 μm wavelength was confirmed at 77 K for the first time.

Energy Loss Rates for Light Holes in Ingaas/GaAs Strained-Layer Single Quantum Wells

ABSTRACTEnergy relaxation rates for light holes in InGaAs/GaAs strained layer quantum wells are measured. Two techniques were used to measure light hole temperatures as a function of power dissipated in the hole gas. For Th < 20K, Shubnikov-de Haas oscillations were used and for Th> 20K, a photoluminescence technique was employed.