Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Optical properties of GaAs1−x N x alloys grown by molecular beam epitaxy

Optical properties of GaAs_1?x N _x alloys grown by molecular beam epitaxy using GaAs (001) as the substrate have been studied. These include photoluminescence (PL), cathodoluminescence (CL), photocurrent and photomemory effects. The low-temperature (77?K) PL characteristics were measured on samples with 0–0.105% N content. The wide emission band indicates the defective nature of the materials. The widening of the band for materials with increasing nitrogen concentration also suggested that the concentration of defect states in these materials dramatically increased with increasing nitrogen content. The PL and CL spectra for GaAs_1?x N _x layer 1854 did not show identical characteristics. Some layers showed a very sharp fall in photocurrent at low temperatures, indicating a very sharp photoquenching and an interaction between antisite, interstitial and vacancy defects. The photomemory effect, which causes photoquenching and the transition from the EL2 to the EL2? metastable state, was strongly influenced by the optical exposure and thermal history of the sample.     

Photoluminescence characteristics of 1.5-μm Ga1-xInxNyAs1-y/GaAs structures grown by molecular beam epitaxy

We report the photoluminescence characteristics of molecular beam epitaxy grown GaInNAs/GaAs single quantum wells that emit near 1.5-m wavelength at room temperature. The photoluminescence properties were investigated by varying the excitation wavelength, excitation intensity and sample temperature. It is found that the spectroscopic properties in the 1.5-m material are quite different from those of similar GaInNAs/GaAs structures emitting at 1.2 m. The related radiative mechanisms in the 1.5-m samples are interpreted in terms of the optical transitions of phase-separated quantum-dot states rather than quantum-well states. PACS 42.62.Fi; 78.55.Cr; 78.66.Fd; 78.67.De     

Strained In x Ga1−x As/GaAs multiple quantum wells grown by MOVPE

Luminescence properties of strained In _x Ga_1−x As/GaAs multiple quantum wells of different thickness and In content, prepared by metal organic vapour phase epitaxy were studied. The influence of the quantum well material composition on the shape of luminescence spectra was investigated. The experimental results were fitted by the Model Solid Theory. This fit was improved by the use of adjustedQ parameter. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by Grant Agency of Czech Republic under grants numbers 202/98/0074, 102/99/0414 and Grant Agency of Academy of Sciences No. A 10110807/1998.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

Hydrogen sulphide doping of GaAs and AlxGa1−xAs grown by molecular beam epitaxy (MBE)

H₂S gas has been used during molecular beam epitaxy (MBE) growth of GaAs and Al _x Ga_1–x As as sulphur vector forn-type doping. Doping efficiencies are less than 10^–3 at usual growth temperatures, and are limited by an incorporation competitive surface process, probably 2Ga+H₂SGa₂S+H₂. In Al_xGa_1–x As forx0.2 the doping efficiency is further reduced by carrier freeze-out at deep levels. Measured thermal activation energies depend on growth conditions and remain relatively low even up to the direct-indirect bandgap crossover for substrate temperatures in the 585–645 C range.     

Magnetic properties of GaAs/δ〈Mn〉/GaAs/In x Ga1 − x As/GaAs quantum wells

The field and temperature dependences of the magnetization of GaAs/δ〈Mn〉/GaAs/In _x Ga_{1 ? x} As/GaAs quantum wells with the δ〈Mn〉 layer separated from the well by a 3-nm GaAs spacer have been studied in the temperature range of 3–300 K in a magnetic field up to 6 T. An external magnetic-field-induced phase transition to a ferromagnetic state with a magnetization hysteresis loop shifted from a zero magnetic field has been found to occur at a temperature below 40 K. A theoretical model is proposed that implies the coexistence of ferromagnetically and antiferromagnetically ordered regions within the GaAs layers.     

Evolution of surface morphology and photoluminescence characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots grown by molecular beam epitaxy

Evolution of surface morphology and optical characteristics of 1.3-μm In0.5Gao.5As/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are investigated by atomic force microscopy (AFM) and photoluminescence (PL). After deposition of 16 monolayers (ML) of In0.5Ga0.5As, QDs are formed and elongated along the [110] direction when using sub-ML depositions, while large size InGaAs QDs with better uniformity are formed when using ML or super-ML depositions. It is also found that the larger size QDs show enhanced PL efficiency without optical nonlinearity, which is in contrast to the elongated QDs.     

Nanoscale superconductivity of ?-Ga islands grown by molecular beam epitaxy

We report on the preparation and superconductivity of metastable γ-Ga islands on Si(111) substrate. The Ga grows in a typical Volmer-Weber mode at a low temperature of 110 K, resulting in formation of Ga nanoislands at various sizes with the identical γ-phase. In-situ low temperature scanning tunneling spectra reveal quantized electronic states in ultrathin Ga islands. It is found that both the lateral size and thickness of the Ga islands strongly suppress the superconductivity. Due to substantial surface energy contribution, the transition temperature Tc scales inversely with the island thickness and the minimum thickness for the occurrence of superconductivity is calculated to be two monolayers.     

Pressure coefficients of the photoluminescence of the II–VI semiconducting quantum dots grown by molecular beam epitaxy

Luminescence properties of CdTe and CdSe quantum dots have been studied under high hydrostatic pressure. The luminescence pressure coefficients of the II–VI quantum dots appear to be very similar to the pressure coefficients of the band-gap of bulk CdTe and CdSe, respectively. In contrary to that, the luminescence pressure coefficients of the III–V quantum dots are significantly lower than pressure coefficients of energy gaps of the appropriate dot materials. The discrepancy can be explained by the theoretical model, which takes into account effects of strain on pressure coefficients in thin strained layers. The experimentally observed pressure-induced quenching of the QDs luminescence is attributed to the “zinc-blende–cinnabar” phase transition in CdTe QDs and to the “zinc-blende–rock-salt” phase transition in CdSe QDs.     

A method to obtain ground state electroluminescence from 1.3μm emitting InAs/GaAs quantum dots grown by molecular beam epitaxy

1.3μm emitting InAs/GaAs quantum dots(QDs) have been grown by molecular beam epitaxy and QD light emitting diodes(LEDs) have been fabricated.In the electroluminescence spectra of QD LEDs,two clear peaks corresponding to the ground state emission and the excited state emission are observed.It was found that the ground state emission could be achieved by increasing the number of QDs contained in the active region because of the state filling effect.This work demonstrates a way to control and tune the emitting wavelength of QD LEDs and lasers.     

High quality above 3-μm mid-infrared InGaAsSb/AIGaInAsSb multiple-quantum well grown by molecular beam epitaxy

The GaSb-based laser shows its superiority in the 3-4 ~tm wavelength range. However, for a quantum well （QW） laser structure of InGaAsSb/AIGaInAsSb multiple-quantum well （MQW） grown on GaSb, uniform content and high com- pressive strain in InGaAsSb/A1GaInAsSb are not easy to control. In this paper, the influences of the growth tempera- ture and compressive strain on the photoluminescence （PL） property of a 3.0μm lnGaAsSb/A1GaInAsSb MQW sample are analyzed to optimize the growth parameters. Comparisons among the PL spectra of the samples indicate that the Ino.485GaAso.184Sb/Alo.3Gao.45Ino.25Aso.22Sbo.78 MQW with 1.72% compressive strain grown at 460 ~C posseses the op- timum optical property. Moreover, the wavelength range of the MQW structure is extended to 3.83 μm by optimizing the parameters.     

High quality above 3-μm mid-infrared InGaAsSb/AlGaInAsSb multiple-quantum well grown by molecular beam epitaxy

The GaSb-based laser shows its superiority in the 3–4 μm wavelength range. However, for a quantum well(QW) laser structure of InGaAsSb/AlGaInAsSb multiple-quantum well(MQW) grown on GaSb, uniform content and high compressive strain in InGaAsSb/AlGaInAsSb are not easy to control. In this paper, the influences of the growth temperature and compressive strain on the photoluminescence(PL) property of a 3.0-μm InGaAsSb/AlGaInAsSb MQW sample are analyzed to optimize the growth parameters. Comparisons among the PL spectra of the samples indicate that the In0.485GaAs0.184Sb/Al0.3Ga0.45In0.25As0.22Sb0.78MQW with 1.72% compressive strain grown at 460 C posseses the optimum optical property. Moreover, the wavelength range of the MQW structure is extended to 3.83 μm by optimizing the parameters.     

Evolution of surface morphology and photoluminescence characteristics of 1.3-μm In_(0.5)Ga_(0.5)As/GaAs quantum dots grown by molecular beam epitaxy

Evolution of surface morphology and optical characteristics of 1.3-μm In0.5Ga0.5As/GaAs quantum dots （QDs） grown by molecular beam epitaxy （MBE） are investigated by atomic force microscopy （AFM） and photoluminescence （PL）. After deposition of 16 monolayers （ML） of In0.5Ga0.5As, QDs are formed and elongated along the [120] direction when using sub-ML depositions, while large size InGaAs QDs with better uniformity are formed when using ML or super-ML depositions. It is also found that the larger size QDs show enhanced PL efficiency without optical nonlinearity, which is in contrast to the elongated QDs.     

Segregation in In x Ga1− x As/GaAs Stranski–Krastanow layers grown by metal–organic chemical vapour deposition

Using quantitative high-resolution transmission electron microscopy we studied the chemical morphology of wetting layers in In _x Ga_{1? x} As/GaAs quantum dot structures which were optimized for applications to optical devices operating around 1.3?µm. The samples are grown by low-pressure metal–organic chemical vapour deposition on GaAs substrates. The In concentration profiles of the wetting layers are evaluated with the composition evaluation by lattice fringe analysis method. The profiles reveal a clear signature of segregation. A fit of the profiles with the Muraki et al. model for segregation reveals a segregation efficiency R?=?0.65?±?0.05 at the growth temperature of 550°C, which is significantly lower than segregation efficiencies observed in samples grown by molecular beam epitaxy at similar temperatures.     

Factors determining the morphology of Cd x Hg1−x Te films in the course of molecular beam epitaxy

Using atomic force microscopy, the dependence of the micromorphology of CdHgTe(301) films grown by molecular beam epitaxy on growth conditions and micromorphology of the buffer CdTe layer is studied. Transmission and high-resolution electron microscopy were used to reveal the interrelation between the micromorphology and microstructure of CdHgTe(301) films. It is found that the roughness of the surface of buffer CdTe layers is inherited during growth of CdHgTe films and initiates the capture of excess tellurium by macrosteps under non-optimal growth conditions, thereby leading to nucleation of growing-in macrodefects. In CdHgTe(301) films grown at the elevated temperature, a periodic wave-like profile and associated lateral modulation of the composition in the [1?03] direction normal to the direction of profile lines [010] are observed. According to the model suggested in the study, formation of a wave-like profile and lateral modulations of the composition are caused by the character of distribution of stresses in the CdHgTe(301) film at the stage of pseudomorphic growth on the buffer CdTe layer.     

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In _x Ga_{1 ? x} As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.     

Photovoltaic spectra of undoped GaAsAl0.25Ga0.75As multiple quantum well structures: Correlation with photoluminescence

The photovoltaic spectra of molecular beam epitaxy GaAsAl_0.25Ga_0.75As multiple quantum wells in the vicinity of the n=1 sub-bandgap region and their correlation with the corresponding photoluminescence spectra have been studied in the temperature range 2–297K. Photovoltaic and photoluminescence spectra are found to agree for the transition of the n=1 heavy- and light-hole excitons. The photovoltaic spectra in the Schottky-barrier configuration show as the relative maxima and minima at the corresponding exciton photoluminescence lines. A possible model of the observed coincidence of photoluminescence and photovoltage is discussed.     

Comparison of blue–green response between transmission-mode GaAsP-and GaAs-based photocathodes grown by molecular beam epitaxy

In order to develop the photodetector for effective blue–green response, the 18-mm-diameter vacuum image tube combined with the transmission-mode Al_(0.7)Ga_(0.3)As_(0.9)P_(0.1)/GaAs_(0.9)P_(0.1) photocathode grown by molecular beam epitaxy is tentatively fabricated. A comparison of photoelectric property, spectral characteristic and performance parameter between the transmission-mode GaAsP-based and blue-extended GaAs-based photocathodes shows that the GaAsP-based photocathode possesses better absorption and higher quantum efficiency in the blue–green waveband, combined with a larger surface electron escape probability. Especially, the quantum efficiency at 532 nm for the GaAsP-based photocathode achieves as high as 59%, nearly twice that for the blue-extended GaAs-based one, which would be more conducive to the underwater range-gated imaging based on laser illumination. Moreover, the simulation results show that the favorable blue–green response can be achieved by optimizing the emission-layer thickness in a range of 0.4 μm–0.6 μm.