The samples under study consist of strained InAs/In0.52A10.48As single quantum wells, grown on InP substrates by molecular beam epitaxy. All samples were capped by a 15 nm-thick lattice matched InGaAs layer. By photoreflectance investigations, transitions which do not arise from the strained InAs quantum well have been observed. They have been interpreted as transitions from a surface quantum well formed by the InGaAs cap layer confined between the InAlAs barrier layer on one side and the sample surface on the other side. Photoluminescence excitation on the photoluminescence peak attributed to the first level of this well has been performed at 5 K. The obtained spectra are typical of a two-dimensional system. A modelisation of this structure is proposed and theoretical calculations of the expected quantum levels have been performed and fit well with experimental results. Finally, etching of the InGaAs cap layer has been done to confirm the origin of the observed transitions.

The stability of GaAs/Si superlattices grown on GaAs substrates using molecular beam epitaxy is described. Typical superlattice structures consisted of ten periods of thin (less than 6.5Å thick) layers of pseudomorphic silicon alternating with thick GaAs layers. We have examined the As2/Ga flux conditions required for the growth of high quality superlattices and have found that the structural perfection is extremely sensitive to the V/III flux ratio. The best superlattices were grown under condition that were just barely enough arsenic to produce a stable (2×4) surface reconstruction in the GaAs layers; increases in the arsenic overpressure resulted in a progressive trend towards 3-D growth of the GaAs on the pseudomorphic Si. In addition, we have examined the stability of GaAs/Si superlattices towards post-growth annealing. Double crystal x-ray diffraction scans showed little change in superlattice structure following rapid thermal anneals at 800°C; at 900°C, however, all but the first order satellite reflections disappeared. We attribute this behavior to the relaxation of pseudomorphic strin and the generation of misfit dislocations at the higher anneal temperature.

II-VI semiconductor alloys have recently received considerable attention for their possible use in double heterostructure (DH) blue laser diodes (LDs).1-4 The purpose of this paper is to present the empirical pseudopotential method within virtual crystal approximation for calculating the band structure of MgZnSSe quaternary alloy. The dependence of band gap energies on alloy composition has shown that MgZnSSe can be a direct or an indirect semiconductor. Electron and hole effective masses are calculated for different composition. Camel's back structure for the X valley conduction band has been found for certain composition range.

The perturbations on the optical properties introduced by increased interface roughness in 3×3 short-period AlAs/GaAs superlattices (SL) were investigated through an examination of the position and line shape of the E0 (direct gap) feature in photoreflectance (PR) and spectroscopic ellipsometry (SE). The degree of interface roughness in the SLs was controlled by a choice of the growth conditions and the buffer layer thickness. The structural behavior was characterized by X-ray diffraction. The measured spectra from PR and SE were compared to those from an AlxGa,1-xAs (x≈0.5) alloy reference specimen grown under nearly identical conditions. The Гand E0values from the Al0 5Gao 5As alloy sample are valuable in this comparative analysis since they represent the lowest and highest bounds for the SL Гand E0, respectively. The results show that increasing interface roughness enlarges the linewidth, Г, from 42 meV to 64 meV, and the corresponding transition energy, E0, shifts from 2.060 eV to 2.112 eV; the reference values from the AlxGa,1-xAs (x≈0.5) sample are 26 meV and 2.137 eV, respectively. The upshift of E0 from the SLs with increasing interface roughness may be understood as a consequence of the roughening-induced intermixing and the eventual reduction in the modulation depth of the SL potential. The increase of Гalso with roughening suggests that additional mechanisms, such as roughening-induced scattering, are important.

Recent studies have shown that highly strained InAs/GaAs quantum wells of high crystalline quality can be grown by conventional solid source molecular beam epitaxy. These wells have been optically characterized by photoluminescence and structurally characterized by electron microscopy. We present the first report of the electrical investigation of InAs/GaAs quantum well confined electronic states using capacitance transient spectroscopy and steady state capacitance-voltage profiling techniques. These measurements are combined with high resolution electron microscopy and high-angle annular dark-field (HAADF) imaging to characterize the highly strained ultrathin quantum wells and heterointerfaces. HAADF, in conjunction with nanometer resolution X-ray analysis, gives the compositional distribution of the InAs monolayers. Correlation with photoluminescence is also performed.

Reference dielectric function spectra,ε-ε1+ iε2, for strained Si1-x Gex have been generated for 0<x<0.255. Used in an interpolation procedure they allow multilayer analysis of pseudo-dielectric function spectra, <ε <ε1 + i<ε2, measured by spectroscopic ellipsometry (SE) on strained Si1-xGex/Si structures of arbitrary composition, x≤0.255. These include multi-quantum well (MQW) structures important for application as infrared detectors.

The Gas Source MBE growth of high quality GaInAsP/InP Bragg reflector stacks for operation at 1.5μm followed by growth of an MOVPE AlInGaAs/InGaAs MQW structure is demonstrated. An asymmetric Fabry-Perot non-linear etalon structure was completed by dielectric mirror deposition. A clear bistable response was observed for the device as a function of incident light intensity, with low critical switching power (<lmW) and excellent thermal stability.

The effect of valence subband-mixing on the optical properties of strained and unstrained quantum wells (QWs) is analyzed theoretically. In the model developed, the orientation of the confined carrier in-plane wave vector and the optical polarization vector, which are taken into consideration to obtain the optical matrix elements, are believed to have a negligible effect and have been ignored in most previous calculations. Results presented here show that there are large variations, due to subband mixing in the of compressive strained QW, such as InGaAs/GaAs, and which addresses the controversial issue of whether a non-mixing calculation, such as the parabolic band structure approximation, should be used for strained material systems. The effect of orientation also shows an over-estimation and under-estimation of the TE absorption coefficient when non-parabolic and parabolic band structures are used, respectively, under isotropic orientation. These results indicate the importance of an accurate model for the determination of optical properties in the QW structures.

Band-gap luminescence from Si1-xGex/Si heterostructures grown by Low Pressure Chemical Vapour Deposition (LPCVD) is studied for a composition range of x=.04 to x=.22 and thickness from l00nm to 1300nm. Special attention is paid to the influence of the growth conditions. The growth temperature was varied between 820°C and 600°C. We compare results obtained from our samples with results in the literature obtained from similar samples grown by other techniques, namely by molecular beam epitaxy (MBE). We show that with a growth temperature of 650°C we have achieved samples exhibiting a luminescence spectrum dominated by the excitonic recombination at the alloy, free from dislocation lines and defect related recombination. The dependence of the luminescence with Ge concentration and thickness of the alloy layer is well described by theory. Results for boron doped layers are also presented.

Theoretical results are presented showing how quantum well disordering modifies the quantum-confined Stark effect in strained InGaAs/GaAs single quantum wells. An error function distribution is used to model the constituent atom composition after interdiffusion. It is shown that for a sufficiently long interdiffusion the exciton Stark shift in the disordered quantum well is greater than in the as-grown quantum well and that the change in electroabsorption near the fundamental absorption edge is larger in the disordered well than in the as-grown well for the same applied electric field. These results demonstrate the potential of using disordering to achieve improved performance in strained InGaAs/GaAs quantum well electroabsorption modulators.

Silicon and Si1-x Gex layers have been epitaxially grown on silicon < 100 substrates in a radiantly heated rapid thermal LPCVD reactor using SiH4/GeH4/H2 gas chemistry with the addition of B2H6 and PH3 for in-situ doping. Oxygen-free Si1-xGex layers (0.05<x<0.2) have been grown at temperatures down to 530°C, whereas oxygen-free silicon growth has only been achieved at temperatures above 690°C. The difference is attributed to the formation of volatile GeO during Si1-x Gex growth.

The target application for these Si1-x Gex layers has been as the base of heterojunction bipolar transistors. In-situ boron-doped Si1-x Gex layers with a doping level of 5xl018cm-3 and thicknesses down to 30nm are flanked by undoped Si,.xGex spacers, typically of 15nm in width, to accommodate boron out-diffusion. These have been grown sequentially at 600°C, preceded by silicon buffer growth at 710°C and followed by silicon cap growth also at 710°C.

The process and system development necessary for the production of ultra-thin multiple layer structures are described, addressing the issues of compositional and dopant profile control.

DX centers in compositionally graded GaAs/AlxGa1-x As: Si heterojunctions have been studied using Deep Level Transient Spectroscopy, and Capacitance-Voltage measurements as a function of temperature. We have found that the DLTS peak position moves as the filling pulse amlplitude is increased and that the capture cross section also changes. Three distinct regions characterized by a constant rate of peak movement are seen. The barrier height of the applied Schottky diodes and the carrier concentration profiles were tracked from 300 K to 100 K. Results from graded junctions were compared to results from abrupt junctions.

We have studied the effect of interface cationic intermixing on the Raman scattering by optical phonons in {113}-oriented superlattices. The theoretical Raman spectra have been calculated using the Bond Charge Model (for lattice dynamics) and the Bond Polarizability Model (for the Raman intensities). We have compared the theoretical results with our experimental data in the spectral range of GaAs-like optical modes. Finally, we have studied the differences between the effect of interface disorder and the effect of the periodic interface corrugation proposed in the literature.

By using an empirical tight-binding method we have studied the effects of superlattice period and alloy composition on the ordering of the lowest conduction-band and valence-band states in (GaAs)n/AlxGa1-xAs)n SLs grown in the [001] and [111] directions. The calculated results of band structure are compared and discussed with the existing photoluminescence (PL) data of Hayakawa et al.

Heterostructures of InGaAs/InP and InGaAs/InGaAsP were grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) in an EMCORE GS3200 system. Highly abrupt interfaces were attained with PL line widths for the InGaAs/InP system comparable to the best values reported in the literature for any crystal growth technique, MOCVD, MBE or CBE. These structures were characterized with low temperature (10K) photoluminescence (PL), transmission electron microscopy (TEM) and high resolution X-ray diffraction (HRXD).

A semiconductor device modeling program based on hydrodynamic balance equations is developed. It is capable of treating multiple carriers in compositionally nonuniform (such as heterostructures) and spatially inhomogeneous device structures, as well as high electric field and associated nonlinear effects. Unlike other balance equation based approaches to device modeling, where the various relaxation rates are imported from Monte Carlo calculations or simply assumed to be constant, our approach is based on the premise of computing these rates as part of the simulation, in the form of nonlinear frictional force and energy transfer functions, with full account of dynamical screening and other quantum mechanical effects embodied in the random phase approximation dielectric function of the system. The simplicity of our technique permit fast and economical modeling of device performance characteristics, requiring only a fraction of the CPU time needed for Monte Carlo simulations.

X-ray scattering has been used to study the interfacial structure of a dry oxide grown at room temperature on a system consisting of four layers of Si epitaxially grown on Ge(001). A 2x1 structure is seen at the interface, whose shape parallels the underlying terrace structure. The model producing the best fit to the specular reflectivity data consists of a Ge single crystal substrate, five intermediate ordered layers and an amorphous oxide layer. X-ray Photoelectron Spectroscopy (XPS) data is also presented, consistent with the specular model, which together indicate that not all of Si is oxidized and that there exists Ge in the SiOx amorphous layer.

Epitaxial-SiGe (epi-SiGe) and polycrystalline-SiGe (poly-SiGe) films with Ge concentrations ranging from 5 to 30% were wet-oxidized with TCA at temperatures ranging from 700 to 1,000°C. The oxidation rate of epi-SiGe decreases with increasing Ge concentration for temperatures ≥900°C, but increases for temperatures ≤800°C. The oxidation of poly-SiGe, on the other hand, is found to depend only weakly on the Ge concentration for oxidation temperatures ≥ 800°C. At 700°C, the rate increases with Ge concentration and can exceed that at 800°C. RBS spectra show that in epi samples oxidized at ≥900°C, Ge is completely rejected from the oxide and Ge rejection results in a pile-up at the oxide/epi-SiGe interface. At 800°C, Ge is partially incorporated in the oxide for high Ge containing films and rejected for lower Ge containing films. At 700°C, Ge is fully incorporated in the oxide. On the other hand, in poly samples oxidized at ≥ 800°C, Ge is completely rejected from the oxide, resulting in a pile-up at the interface and diffusion into the poly-SiGe. At 700°C, however, Ge is partially incorporated into the growing oxide when layers with high Ge concentration (≥20%) are oxidized. Most of the Ge is still rejected from the oxide and diffuses into the poly-SiGe layer. Our results on both epi and poly samples can be explained by assuming that at low temperatures the diffusion of the oxidant through the growing oxide is rate limiting whereas at high temperatures the rate is determined by the concentration of Si at the oxide/alloy interface. In the low temperature limit, the Si to Ge ratio in the oxide is identical to that of the alloy. At high temperatures, SiO2 is formed. In the intermediate regime, the composition of the oxide depends on the degree to which Ge can be rejected from the oxide. The Ge removal rate from the interface of polycrystalline films exceeds that of single crystal films because of the enhanced diffusion of Ge along grain boundaries.

W/Ti refractory metal ohmic contacts to n-type InP and InGaAs have been fabricated by electron-beam evaporation in an ultra-high vacuum metallization system. Excellent surface morphology was observed and good pattern transfer of the metal films was achieved with chemical etching. Ideal ohmic behavior was obtained for samples which were heavily doped either during MBE growth or via ion implantation. The lowest specific contact resistances obtained were: 4.8xl0-7Ωcm2 for ion implanted n+InP and 4.9xl0-6 Ωcm2 for n+InGaAs. The thermal and metallurgical stability of the contacts has been investigated using scanning electron microscopy and Auger electron spectroscopy with sputter depth profiling. The contact samples were subjected to heat treatments using rapid thermal annealing at temperatures of 350 to 800°C. Scanning electron microscopy showed that after annealing at up to 600°C no degradation in surface morphology had occurred. Auger depth profiling indicated that the W-film maintained a sharp interface for all heating treatments, with no degradation up to 600°C. However, some P and In was found at the surface of the contact layer after annealing at 800°C.

Raman and both cw and time-resolved photoluminescence (PL) spectroscopy have been used to characterize 3.5nm/3.5nm Al0.30Ga0.70 As/GaAs multiple quantum well structures that have been patterned by focused ion beam (FIB) implantation followed by rapid thermal annealing (RTA). Raman scattering is used to characterize a FIB-delineated optical waveguide structure by identifying the appropriate RTA conditions that provide for removal of ion implantation induced damage in order to produce compostionally mixed regions that possess crystalline order. Spatially and temporally resolved PL spectra provide information on the degree of compositional mixing, exciton lifetime, and lateral transport. Anisotropic diffusion of excitons is observed in samples patterned with parallel line structures.