The use of Tetraethyltin (TESn) as a source of donors in GaAs and its A1xGal-xAs alloys with x varied from 0 to 1 has been investigated for the growth of heterostructures by low pressure metalorganic chemical vapor deposition. (LP-MOCVD). The donor activity increases in the binary and ternary alloys as a function of tetraethyltin molar flow fraction and with increasing temperature. High quality one quarter wave Distributed Bragg reflectors (DBRs) were grown with TESn as an n type dopant source with no degradation in reflectivity observed. No surface accumulation of tin was observed and the morphology of all epitaxial samples was excellent. Sn incorporation in AlAs produced n type material as determined by Van der Pauw Hall measurements. The use of TESn as a convenient and controllable dopant source in structures such as vertical cavity surface emitting lasers which incorporate DBRs as well as other heterostructure devices is therefore practical..

The dependence of carbon concentration on growth temperature and V/HII ratio for high composition AlxGal-xAs (x>0.40) grown by metalorganic chemical vapor deposition using trimethyl sources has been investigated. The carbon concentration exhibits at least two temperature regimes having different trends with temperature. In the region of 600-675°C, the carbon concentration decreases with temperature, while in the range of 700-800°C, the carbon concentration increases with temperature. This dependence was observed in samples grown in two separate reactors. High values of V/III ratio have been found to suppress the low temperature carbon incorporation in AlAs. The results are qualitatively explained in terms of the chemical reactions and surface kinetics that may occur during the growth of GaAs or AlxGal-xAs.

Double crystal x-ray diffraction and Hall effect measurements have been performed on GaAs layers grown by molecular beam epitaxy at low substrate temperature (200-250°C). The concentration of excess As incorporated in undoped material is found to be a strong function of growth temperature but not of As4/Ga beam equivalent pressure ratio at a given temperature. Doping with Si or Be at concentrations of 1019cm−3 or greater has resulted in significant reduction of excess As concentration in layers grown at 250'C. This effect is seen to diminish with reducing substrate temperature. The effect is not seen when Se is used as the doping source. Reducing As overpressure in the presence of such a high doping concentrations has led to highly electrically active n- and p-type layers grown at 250'C.

It is shown that the low electrical activity of Zn in MOCVDgrown InGaAlP is related to hydrogen passivation during growth and that the effect is sensitive to growth conditions. We also discuss the results of hydrogen plasma treatment of p- and n-InGaAlP layers.

Semi-insulating Al0.3Ga0.7AS epitaxial films grown by molecular beam epitaxy at 400°C have been characterised by a recently proposed technique known as zero quiescent bias voltage transient current spectroscopy (ZBTCS). Separated electron trap spectra and hole trap spectra are obtained by ZBTCS. From these spectra, a prominent hole trap with an activation energy of 1.01eV, a prominent electron trap with an activation energy of 0.86eV and a continuum of states are detected. The trap concentrations of the detected hole trap and electron trap are calculated to be about 3.6x1015cm−3 and 9.7x1015cm−3 respectively. The continuum of states is believed to be due to the surface states of the Al0.3Ga0.7AS film. By passivating the surface with (NH4)2S solution, the magnitudes of the continuums in the spectra reduce significantly. With the reduction of the surface states, hole injections that interfere the electron trap spectra are suppressed, and consequently electron trap spectra with more pronounced peaks are produced.

We have performed molecular dynamics simulations of thin layers of InGaAs on GaAs substrates for different In concentrations to determine the critical thickness before strain relaxation occurs. We have considered both dislocation formation and islanding as possible mechanisms for strain relief. The results for the critical thickness for strain relief by misfit dislocations is slightly lower than that found using elasticity theory. For high In concentrations, facetted islands are found to be stable and are energetically favoured.

We study the optical properties of metalorganic vapour phase epitaxy (MOVPE) grown (Ga,In)As-GaAs single quantum wells as a function of the growth parameters. Our objective is to study the properties of the type II light-hole excitons with light-hole wave function delocalized in the thick GaAs layers and electrons confined in the ternary alloy. However, marked structures (like for type I excitons) are observed in the reflectivity spectra. This we interpret in the context of a novel approach of the exciton problem via a self-consistent calculation of the electron-hole interaction.

The performance of InGaAs/GaAs pseudomorphic high electron mobility transistors is anticipated to improve with increased channel thickness due to reduced effects of quantum confinement. However, greater channel thicknesses increase the probability of forming misfit dislocations which have been reported to impair device properties. We characterized the composition and thickness of the active layer in Al0.25Ga0.75As / In0.21Ga0.79As structures with different channel thicknesses (75 Å - 300 Å) to within ± 0.005 and ± 8 Å using high resolution x-ray techniques. We determined, using Hall and rf measurements, that the device properties of these structures improved with increasing thickness up to about 185-205 Å; degraded properties were observed for thicker channel layers. Cathodoluminescence results indicate that the mosaic spread observed in x-ray triple axis rocking curves of these device structures is due to the presence of misfit dislocations. Thus, even though misfit dislocations are present, the device structure performs best with a channel thickness of ∼185 Å. These results demonstrate that one can fabricate functional devices in excess of critical thickness considerations, and that these x-ray techniques provide an effective means to evaluate structural properties prior to device processing.

In-plane anisotropic strain can be employed in the design of a new class of optoelectronic devices, such as high contrast, polarization sensitive spatial light modulators. One of the key issues involved in realizing these devices is obtaining a controllable and uniform in-plane strain. We have studied the uniformity of thermally induced in-plane strain in MOCVD grown GaAs lift-off thin films mounted on LiTaO3 or CaCO3 substrates. The experiment exploits the straininduced splitting of the excitonic interband transition at low temperature through absorption measurements using a Ti-Sapphire laser focused to a spot size less than 100 μm. The polarization vector of the incident light was oriented along an axis which enhances both features. From the energy positions of these transitions, the magnitude as well as the type of the in-plane strain was determined. Topographic scans performed over a 1.4mm X 1.4mm area for the sample bonded to CaCO3, and along a 2 mm line for that bonded to LiTaO3 revealed variations in strain of less than 5%.

Similar phenomena associated with the lateral thickness modulation have been observed in two different types of GaxIn1-x strained layer multiple quantum well structures grown on InP by MOVPE. The thickness modulations were always quasi-periodic and developed anisotropically in the [110] direction. It has been found that lattice distortions were caused by such thickness modulations, resulting probably in overall surface elastic relaxation. Once the thickness modulation had started at some stage of the growth, it would continue in the subsequent layer with approximately the same modulation period and an amplified amplitude. This is attributed to the effect of the lattice deformation at the growth surface on the deposition rate of the subsequent layer. The effect of such factors as the magnitude of strain, the growth temperature and the V/III gas ratio on the onset of the thickness modulation is discussed.

Heterostructural properties of pseudomorphic (AlGaAs/GaAs), partially strained (GaInAs/GaAs), and highly strained (GaAs/Si) semiconductor systems have been studied using High Resolution Double-Crystal X-ray Diffraction (DXRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM). Using the high resolution DXRD with CuKα1 and two-reflection Si (220) monochromator, we obtained (004) symmetric and (115) or (224) asymmetric reflection rocking curves for samples grown by molecular beam epitaxy. With 0.5 μm thick samples, perpendicular and in-plane lattice mismatches were calculated using elastic theory and compared with each other. The different degree of relaxation for these samples was observed and correlated with the lattice mismatch, X-ray layer peak broadening (i.e., full width at half maximum), and SEM surface morphology. For a GaInAs/GaAs sample, the strain relaxation along one of the <110> directions was more than the other direction, that is, the strain relaxation is not isotrophic. Also we observed that the lines were mainly parallel in one direction, i.e., they did not form a cross-hatch pattern. TEM images from both cross-sectional and planar views of the samples will be presented.

We have investigated the structural and electronic properties of partially strain-relaxed InxGal-xAs/GaAs heterojunctions, grown by molecular beam epitaxy (MBE) on both misoriented and nominally flat (001) GaAs substrates. Mobility measurements using Hall bars aligned along the [110] and [110] in-plane directions reveal an asymmetry in bulk InGaAs electron mobility. This asymmetry is correlated with an anisotropic bulk strain relaxation and interfacial misfit dislocation density, determined from high-resolution x-ray rocking curves (XRC), as well as a polarization anisotropy in cathodoluminescence (CL).

InxGa1-xAs epilayers (x < 0.2) have been grown on GaAs substrates by atmospheric pressure OMVPE. The effects of varying the substrate temperature and the gas composition on the properties of the epilayers were investigated. The investigations have shown that higher mobilities were obtained at low growth temperatures (610°C), while optimum optical properties were obtained at higher growth temperatures (690°C). Variation of the AsH3 overpressure yielded optimum electrical and optical properties at a V/III ratio of 50. 77 K mobilities higher than 42 000 cm2/V.s and photoluminescence linewidths as low as 4.4 meV were obtained for x = 0.087 and x = 0.137 epilayers, respectively.

Ga0.65In0.35As layers of a varying nominal epilayer thickness (10 – 1000 nm) have been grown by the MBE technique on GaAs (100) substrates and characterized by the combined use of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). The surface roughness and morphology have been investigated. The GIXD and AFM results show that the thinnest films are characterized by an asymmetric strain relaxation along the two <110> directions with no surface crosshatched pattern but with a misfit dislocation network. AFM images on the thickest films show also well-oriented protrusions along the [110] direction, which increase in size and become more elongated as the nominal film thickness increases.

InxGa1-xAs crystals, 0 < x < 0.2, have been successfully grown by LPEE on the GaAs substrates which, prior to growth, were coated with SiO2 layer and patterned by photolithography. The ternary seeds originated in the oxide free seeding windows, 2 – 20 μm wide and 100 μm apart, extended over SiO2 coating and merged to result in a continuous, monocrystalline layer with no apparent trace of boundaries of the merging islands. A considerable improvement in structural perfection of the ternary crystals has been achieved: the FWHMs of the rocking curves and dislocation density was in the 30 –40 arc seconds and low 105cm−2 range, respectively. Structural perfection of InxGa1-xAs crystals grown on patterned GaAs substrates is far superior to that of the ternary buffer layers grown by MBE or MOCVD.

There is a broad spectrum of electronic devices, e.g., high electron mobility, heterojunction bipolar (HBTs) and modulation doped field effect (MODFETs) transistors, and optoelectronic devices, e.g., low-threshold quantum-well (QW) lasers, which employ strained InyGa1-yAs layers to improve device performance due to the enhanced flexibility in the electronic structure. These devices have very promising applications. For example, the ultra-low threshold lasers are going to play a major role in computer interconnect circuits due to their low-power consumption. Strainlayer InGaAs/GaAs QW lasers are of considerable interest as a possible candidate for pumping the upper states of Er-doped silica fiber amplifiers of optical networks.

In the design of GaAs devices, material selection for heteroepitaxy is limited due to the large amount of strain produced by lattice mismatch. To overcome this problem, a new growth method has been proposed called van der Waals Epitaxy (VDWE). This process attempts to decouple the interface through the growth of a van der Waals bonded crystal layer in-between the two materials with dissimilar lattice constants, thereby accommodating the lattice mismatch. Typical van der Waals materials such as GaSe, have hexagonal crystal structures and are grown with the c-axis perpendicular to the surface due to the nature of the bonding. These materials have a low surface energy and are not very reactive. In addition, they are only stable at relatively low temperatures. This poses two constraints to the growth of GaAs on GaSe. Initial experiments were done to investigate the possibility of growing GaAs (111) layers homoepitaxially at temperatures compatible with van der Waals crystals, with high crystallinity and a smooth surface. Twinned growth layers 1.7μm thick, were obtained at 350° C on a GaAs (111)B substrate using MBE. Further experiments looked at homoepitaxial growth on (111)A substrates and at slightly higher temperatures. These results were then compared to GaAs layers grown on GaSe layers in which nucleation occurs on a van der Waals surface as opposed to the covalently bonded GaAs substrate. The relative quality of the layers was analyzed using RHEED and TEM.

A single crystal GaAs/Si heterostructure has been fabricated using the “two step” version of the traditional halogenide VPE technique. The X-ray diffraction spectroscopy, the high resolution electron microscopy observation and the Raman scattering spectroscopy have been used to analyze the structure of the GaAs epitaxial film on Si, while the X-ray photoelectron energy spectroscopy has been used to determine the chemical structure of the GaAs epilayer. The results indicate that the epilayer is a stoichiometric GaAs single crystal film.

The incorporation of low-temperature (LT) GaAs intermediate layers grown at 230°C had been shown to have the effects of improving the crystalline quality of GaAs epilayers on Si. The use of this LT-GaAs intermediate layer between the GaAs nucleation layer and the GaAs overlayer has improved the photoluminescence (PL) peak intensity by about five times, and reduced the GaAs (004) X-ray diffraction full width at half maximum (FWHM) by 59 arcsecs. The PL results were subsequently confirmed by cathodoluminescence images. The dominant deep level electron trap in the LT-GaAs epilayer grown on Si substrate was the same as that found in LT-GaAs epilayer grown on GaAs substrate.

Optimum growth conditions were investigated to obtain high quality heteroepitaxial GaAs layers on InP substrates by liquid phase epitaxy (LPE). Addition of about 0.005wt% of Se to the Ga growth melt effectively suppressed dissolution of the InP substrates into the melt during the initial stage of the growth, resulting in a significantly improved surface morphology. The crystallinity and the surface morphology could be further improved by growing undoped GaAs layers on thin Se-doped buffer GaAs as well as using InP substrates patterned with grating structure. The transmission electron microscopy observation indicated that the misfit dislocations interact with each other at the grating region, resulting in a lower dislocation density in the upper GaAs layer. The (400) double crystal X-ray diffraction peaks of the undoped GaAs showed fullwidth- at-half-maximum of about 380 arcsec, which is comparable with the previously reported values using more sophisticated growth techniques.