Unstrained GaAs Research Articles

Efficient single-mode oxide-confined GaAs VCSEL's emitting in the 850-nm wavelength regime

Single- and multimode vertical-cavity surface-emitting lasers (VCSELs) with three unstrained GaAs quantum wells (QWs) and emission wavelengths around 850 nm have been fabricated using molecular beam epitaxy (MBE) for crystal growth. Wet chemical etching and subsequent selective oxidation are applied for current confinement. The influence of oxide layer position on lateral index guiding is studied in detail in order to increase maximum single-mode output power. A device of 3-μm active diameter and reduced index guiding shows maximum single-mode output power of 2.25 mW with a side-mode suppression ratio (SMSR) of more than 30 dB for high-efficiency oxidized VCSELs.

Tensile-strained barrier GaAsP/GaAs single quantum-well lasers

Device characteristics of tensile-strained barrier AlGaAs/GaAs1−yPy/GaAs separate-confinement heterostructure single-quantum well (SCH-SQW) broad area lasers are reported. Three phosphorous compositions of 9%, 15%, and 22% were used for the tensile-strained GaAs1−yPy barrier. For a 1000 μm long cavity device with a wellwidth of 113 Å and 15% P in the barrier, the threshold current density was as low as 233 A/cm2. The threshold current densities are slightly higher than those observed on similar device structures with tensile-strained GaAsP wells and unstrained GaAs wells with AlGaAs barriers.

Band structure of strained quantum wells grown on novel index surfaces

We present a theoretical study of the in-plane valence subband structure of unstrained GaAsAlGaAs, compressively strained InGaAsAlGaAs and tensile strained GaAsPAlGaAs quantum wells grown along the (001), (111), (011) and (113) directions. From numerical solution of the 6 × 6 Luttinger k · p hamiltonian we find that confinement energies, warping and effective in-plane masses strongly depend on the direction of confinement and on strain. Piezoelectric effects further affect the dispersion for the (111) and (113) directions. Besides, we give analytic expressions for the heavy and light hole in-plane and perpendicular effective masses for any (hkl) growth direction in the limit of uncoupled subbands.

Valence band offsets in strained GaAs1−xPx/GaAs heterojunctions

Valence band offsets at [100]-oriented heterojunctions between tensile-strained GaASj1−xPx and unstrained GaAs are studied experimentally and theoretically. Light-hole (LH) and heavy-hole (HH) offsets are first extracted from the well-width dependence of valence subband splittings observed in luminescence spectra of tensile-strained GaAs1−xPx/GaAs quantum wells of various compositions (x = 0.06,0.09, and 0.19). This data is then combined with results from two other laboratories, yielding a set of 30 independent experimental offset values for junctions with compositions throughout the range 0.06≤x ≤0.32. The data are found to be highly consistent, with linear fits δELH = −140x (meV) and δEHH= −401x (meV) describing the measured offsets to within less than 5 meV on average. Experimental results are then compared with theoretical predictions for the GaAs1−x Px/GaAs system obtained from a tight-binding model for strained heterojunctions. Predictions from the tight-binding calculations are found to lie within experimental scatter for the LH offsets, which define the valence band edge in these heterostructures, while magnitudes of the tight-binding HH offsets exceed measured values by ~20% on average.

Enhanced electron transfer in real-space transfer devices using strained InxGa1- xAs ( x = 0.15, 0.25) channel layers

Real-space transfer devices using strained In x Ga 1- x As ( x = 0.15, 0.25) channel layers and unstrained GaAs channel layers on GaAs substrates are fabricated with Pd/Ge ohmic contacts. The epitaxial structure is studied by transmission electron microscopy. Enhanced electron transfer and strong negative differential resistance are observed in strained InGaAs channel devices. The InGaAs channel devices also show lower leakage currents, higher electron transfer efficiency, and higher drain current peak-to-valley ratios.

Ultrahigh and controllable drain current peak-to-valley ratio in negative resistance field-effect transistors with a strained InGaAs channel

Negative resistance field-effect transistor (NERFET) devices using either strained InGaAs or unstrained GaAs channel layers have been fabricated. The strained InGaAs channel NERFET's show strong negative differential resistance and large drain current peak-to-valley ratio. The peak-to-valley ratio of the InGaAs channel NERFET is more than 3000 at room temperature and larger than one million (10/sup 6/) at 77 K. The peak-to-valley ratio is controllable by adjusting the collector voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Enhancement of electron transfer and negative differential resistance in GaAs-based real-space transfer devices by using strained InGaAs channel layers

Real-space transfer devices using strained InxGa1−xAs (x=0.15, 0.25) channel and unstrained GaAs channel are fabricated. The electron transfer and negative differential resistance of InGaAs channel devices are enhanced. The InGaAs channel devices with a larger indium mole fraction show lower leakage currents and can be operated at a higher collector voltage. For operation in the negative resistance field-effect transistor mode, the drain current peak-to-valley ratios of strained InGaAs channel devices are larger than that of unstrained GaAs channel devices. The peak-to-valley ratio can be increased more than two orders of magnitude by using strained InGaAs channel layers.

Optical properties of symmetrically strained (GaIn)As/Ga(PAs) superlattices grown by metalorganic vapour phase epitaxy

Symmetrically strained (GaIn)As/Ga(PAs) superlattice structures (symm. SLS) have been realized by metalorganic vapour phase epitaxy (MOVPE). The high structural as well as optical quality of these symm. SLS has been established by detailed high-resolution X-ray diffraction (XRD) and in particular photoluminescence (PL) and excitation spectroscopy (PLE) investigations. The optical recombination at low temperatures is due to excitons localized in statistical potential fluctuations in the (GaIn)As well layer. The exciton binding energy is determined in these symm. SLS to (5.5 ± 1) meV according to a theoretical description of the PLE spectra using a two-dimensional (2D) absorption model. Therefore, the exciton binding energy is significantly reduced as compared to unstrained GaAs quantum wells, caused by a considerably smaller in-plane hole mass for the top most valence subband. An estimate of this mass based on the determined exciton binding energy yields a value of about 0.05 m 0.

Defect and strain redistribution in InxGa1−xAs/GaAs multiple quantum wells studied by resonant Raman scattering

Resonant Raman scattering by longitudinal optical (LO) phonons in the GaAs barriers of InxGa1−xAs/GaAs multiple quantum well structures has been used to study the onset of strain relaxation as the number of quantum wells is increased. The intensity of scattering from one and two GaAs LO phonons for excitation in resonance with the GaAs E0+Δ0 band gap is found to be highly sensitive to the formation and/or redistribution of defects and the buildup of strain in the barriers, and thus to strain relaxation in the InxGa1−xAs quantum wells. This behavior allows us to detect the onset of strain redistribution, even in samples where the frequencies of both the GaAs LO phonon in the barrier layers and the GaAs-like LO phonon in the InxGa1−xAs quantum wells show no measurable shift. Here, the GaAs LO phonon frequency observed in structures with fully strained InxGa1−xAs wells and unstrained GaAs barriers is used as a reference.

Band edge offset in strained InxGa1-xAs/GaAs quantum wells measured by high-excitation photoluminescence

A new method is presented for direct and reliable determination of the band edge offset in quantum well (QW) semiconductor heterostructures. It is based on luminescence measurements at various filling levels of the valence and conduction band quantum wells. Magnetoluminescence measurements were made to independently determine the effective masses. The conduction band offset to energy gap ratio for strained InxGa1-xAs QWS in an unstrained GaAs matrix was determined to be 0.79+or-0.05 for x=0.18 and 0.28.

Evidence of type-I band offsets in strained GaAs1-xSbx/GaAs quantum wells from high-pressure photoluminescence.

We have used high-pressure photoluminescence in a diamond-anvil cell to investigate the band offsets between strained ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ and unstrained GaAs for x=0.12. It has generally been expected that this system should display a strongly type-II band lineup, with holes confined in deep ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ wells and the lowest-energy electron states in GaAs. Our results on a multiple-quantum-well sample show that this is not the case. We measure the photoluminescence transition energies up to and beyond the \ensuremath{\Gamma}-X crossover near 36 kbar, where the luminescence becomes indirect. The character of the \ensuremath{\Gamma}-X crossover dependence on well width requires a type-I offset for the X minimum and suggests a type-I offset for the \ensuremath{\Gamma} minimum of the conduction band. This is in good agreement with theory when the strong band-gap bowing in the ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sb}}_{\mathit{x}}$ alloy system is properly taken into account.

Analytic dispersion relations near the Gamma point in strained zinc-blende crystals.

An eight-band k\ensuremath{\cdot}p model of the electronic states near the \ensuremath{\Gamma} point of a strained zinc-blende semiconductor is renormalized using L\"owdin perturbation theory. The renormalization leads to matrices of reduced dimensionality, 2\ifmmode\times\else\texttimes\fi{}2, 4\ifmmode\times\else\texttimes\fi{}4, and 2\ifmmode\times\else\texttimes\fi{}2, for the conduction, light- and heavy-hole, and spin-orbit split-off bands, respectively. The reduced dimensionality of these matrices allows us to obtain analytic dispersion relations in the presence of small but arbitrary strains. Comparison of the analytic dispersion relations is made to those obtained by numerically diagonalizing the eight-band model for the case of biaxially strained and unstrained GaAs.

Raman scattering measurements on InGaAs/AlAs strained MQWs

We present in this paper Raman scattering measurements on a series of strained layer (In xGa 1−xAs) (AlAs) multiple quantum wells (MQWs) in which the modest (x=0.04) indium concentration in the wells gives rise to a 0.3% strain between the alloy layers and the GaAs substrate. Comparison of the measured confined longitudinal optic (LO) mode frequencies with our model yields an effective interface broadening parameter W=1.2 in these samples. Results on a set of unstrained (GaAs) (AlAs) samples gives a similar interface width indicating that in these samples the incorporation of strain in the growth does not adversely effect the quality of the interfaces between the constituent superlattice materials.

Two-dimensional spin confinement in strained-layer quantum wells.

We show that a two-dimensional spin system can occur for ``heavy,'' ${\mathit{m}}_{\mathit{J}}$=\ifmmode\pm\else\textpm\fi{}3/2 holes resulting from a reduction to tetragonal symmetry produced by biaxial strain or confinement in a quantum well. Evidence is produced from magnetotransport measurements on strained ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb/GaSb quantum wells, in which the spin splitting is determined only by the component of magnetic field in the growth direction, and from published work on unstrained GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunctions. Complementary behavior is predicted for ``light'' holes.

Hydrostatic pressure coefficients of the photoluminescence of InxGa1-xAs/GaAs strained-layer quantum wells.

The photoluminescence of strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells, with x up to 0.225, grown pseudomorphically between unstrained GaAs barriers, has been studied under high hydrostatic pressure in a diamond-anvil cell. The measured direct-band-gap pressure coefficients show a marked deviation from theoretical predictions; they decrease strikingly below the GaAs value of 10.7 meV/kbar as the well width or the indium content is increased; the pressure coefficient of the band gap in wide wells depends on well composition x as 10.7-6.0x meV/kbar in the samples studied, compared with a predicted variation of 10.7-1.7x meV/kbar calculated with use of third-order elasticity theory. The experimental data could correspond to a composition dependence of the band-gap hydrostatic deformation potential, scra, of -7.99(1-0.47x) eV for small x. We speculate that the anomalously low measured pressure coefficients may be due to an interplay of effects related to disorder and strain in the alloy quantum wells.

Photoluminescence and photoconductivity measurements on band-edge offsets in strained molecular-beam-epitaxy-grown InxGa

Structure has been observed in the photoluminescence and photoconductivity spectra of ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs (x\ensuremath{\le}1) strained quantum wells grown by molecular-beam epitaxy onto GaAs(001)-oriented substrates. Features in the spectra at energies larger than the energy gap of ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As are interpreted as the allowed excitonic transitions between electron and hole subbands (including the strain-split-off valence band) in ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As. The spectra were analyzed with the conduction-band offset and the energy gap of ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As as adjustable parameters. No strain relaxation in quantum wells with thickness smaller than the critical one was observed. The strain-split-off valence subband in ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As is found to be below the valence band of unstrained GaAs. The ratio of the conduction-band offset to the energy-gap discontinuity was determined to be 0.83\ifmmode\pm\else\textpm\fi{}0.06.

Photoluminescence studies of heteroepitaxial gaas on si

We present a systematic study of the photoluminescence of undoped GaAs layers deposited by MOCVD on Si substrates. The study includes an examination of substrate and layer thickness effects in thin GaAs layers, and a detailed investigation of the stress effects on the intrinsic band-edge transitions in thicker samples. For sample thickness,t ≤ 0.5(μm), we observe strong midgap emission bands associated with defects close to the interface. These bands depend strongly on the nature of the Si substrate. The crystal quality improves with sample thickness, and fort ≥ 0.5 μm the emission is dominated by lines in the band edge region which are relatively independent of substrate preparation. Photoluminescence excitation spectra reveal that the highest energy line is due to an intrinsic exciton transition, and that a splitting of this line observed fort ≥ 2 μm reflects the presence of two different regions of strain in the material. The magnitude of the strain is estimated from the shift of the exciton lines relative to unstrained GaAs, and is found to be consistent with an upper limit provided by the thermal expansion mismatch between GaAs and Si.

X-Ray Rocking Curve Analysis of S-Implanted GaAs

ABSTRACTThe effect of rapid thermal annealing on strain reduction in 1.15 MeV S-implanted GaAs wafers irradiated to a dose of 5 × 1014/cm2 has been studied by double-crystal x-ray diffraction technique. X-ray rocking curves exhibit characteristic thin film fringes between the peak of unstrained GaAs and the major peak of the strained region. The maximum strain, i.e., the separation between the two peaks, as well as the number of minor fringes decreases with increasing RTA temperature, while the relative spacing between the fringes remains constant. At temperatures above 900°C, the main peaks begin to overlap; however, a residual positive strain can be measured for temperatures as high as 1100°C.

Band edge offsets in strained (InGa)As-(AlGa)As heterostructures

The excitonic transitions between the ground electron and hole quantum well sublevels in strained In x Ga 1- x As-Al y Ga 1- y As multiple quantum well structures ( x = 0.12−0.35 and y = 0.2−0.35) have been investigated by means of photoluminescence and photoconductivity measurements. The molecular beam epitaxy grown structures contained an Al y Ga 1- y As matrix with one unstrained GaAs and three strained In x Ga 1- x As quantum wells one of which was in the GaAs cladding layers. The ratio of the conduction band edhe line up to the band gap offset for the strained In x Ga 1- x As-unstrained Al y Ga 1- y As interface has been found to be 0.67 ± 0.08 for the studied regions of x and y.

Photoluminescence in strained InGaAs/GaAs superlattices

We present measurements and analysis of high-quality photoluminescence (PL) data from strained In0.13Ga0.87As quantum wells confined by unstrained GaAs barriers. Data were obtained from low temperature to room temperature on peak positions, intensities, and linewidth. The latter exhibits an unusual minimum at 31 K consistent with defect binding of excitons and one monolayer fluctuations in well size. The high-temperature linewidth agrees with that expected for phonon scattering. Peak positions show excellent agreement with a Kronig–Penny model augmented by a variational calculation of the n=1 bound-state energy of the heavy-hole exciton.