We have used MBE to grow graded InxGa1−xAs/GaAs superlattices on GaAs substrates as buffers for larger period, constant composition InxGa1−xAs/GaAs superlattices. TEM analysis shows that there are numerous crystal defects associated with the presence of strained layers, but, in many cases, the buffer layers confine these defects to areas outside of the strained-layer superlattices. PIN detectors fabricated from these structures show external quantum efficiencies up to 2.7 %.

We have examined spectrally resolved photoconductivity and photoluminescence from InAs/Ga1–xInxSb strained-layer superlattices, which have been proposed as infrared detectors in the 8-14 μm region. Our measurements indicate that the energy gaps of the strained–layer superlattices are substantially smaller than those of InAs/GaSb superlattices with similar layer thicknesses, in agreement with previous theoretical predictions. Measurements on InAs/Ga1–xInxSb superlattices with x=0 and 0.25 and layer thicknesses of 25 – 45 A indicate superlattice band gaps of 3 – 15 μm, in excellent agreement with gaps calculated by a two band k · p model. Our results demonstrate that far-infrared energy gaps are compatible with the thin layers necessary for strong optical absorption in type-IT superlattices, and suggest that InAs/Ga1–xInxSb superlattices are promising candidates for far-infrared detection.

A discussion of resonant tunneling physics in both diode and transistor heterojunction structures is presented. It is evident the In(GaAI)As/InP system is significantly superior for this application. We also present results on resonant tunneling in lower dimensional systems.

We have examined the diffusion of Zn from the base of GaAs-AIGaAs heterojunction bipolar transistor (HBT) structures during growth by organometallic vapor phase epitaxy. The role of Si doping in the emitter-contact, emitter, and collector/subcollector in enhancing the Zn diffusion has been determined by separately doping each layer. For a growth temperature of 675°C Zn shows no observable redistribution up to concentrations of 3x1019 cm−3 without Si doping. The addition of Si to the adjacent AIGaAs emitter and GaAs collector/subcollector layers causes significant diffusion from the base, while Si doping of the GaAs emitter-contact results in even greater Zn redistribution. Silicon counter-doping in the base region retards the Zn diffusion. These results are consistent with a recent model which shows that the n-type surface layer enhances the formation of gallium interstitials which diffuse into the structure and displace the Zn in the base via a kick-out mechanism.

We have studied the thermoelectric power in quantum dots (QDs) of non-parabolic semiconductors in the presence of a classically large magnetic field and we have taken A3N B2V, ternary chalcopyrite, II-VI and III-V semiconductors. It is found that the thermopower increases with increasing film thickness and decreasing electron concentration respectively in all the cases. The numerical values are greatest for Cd3As2 and least for InAs and the theoretical results are in ageement with the experimental observation as reported elsewhere.

We have studied the Einstein relation for the diffusivity. mobility ratio (7PT) on the basis of a newly derived electron energy spectrum in QW f tetragonal semiconductors, within the framework of K. P method by considering all types of anisotropies of the energy band parameters. It is found, taking n-Cd3 As2 as an example that the DUTZ increases with electron concentration and decreases with film thickness in an oscillatory manner respectively. The theoretical results are in good aoreement with the suggested experimental method of determining the DMR in degenerate semiconductors having arbitrary dispersion law.

A variety of optical methods are now available for studying surface processes and for monitoring layer thicknesses and compositions during semiconductor crystal growth by molecular beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD), and related techniques. Spectroellipsometry (SE) and spectroreflectometry (SR), the older, primarily bulk-sensitive probes, are now augmented by new, primarily surface-sensitive probes such as reflectance-difference spectroscopy (RDS), second-harmonic generation (SHG), and laser light scattering (LLS). Examples of real-time growth studies now include SE determinations of thicknesses and compositions of AlxGa1–xAs layers on GaAs by organometallic molecular beam epitaxy (OMMBE) to 10 Å thickness scales, RDS determinations of surface dielectric anisotropy spectra of various (001) GaAs surfaces relevant to crystal growth by MBE, and LLS determinations of the evolution of surface roughness during chemical vapor deposition (CVD) growth on Si. Proven capabilities suggest new applications, particularly to growth-interrupted and metastable systems.

The application of some characterization methods to the study of heteroepitaxy is illustrated by recent work in the authors laboratory on the epitaxy of Cu on Mo and W(110). In particular, the use of PEEM and LEEM in the study of the epitaxy of Cu, Au and Co on Si(111) is discussed.

We discuss the benefits and limitations of scanning tunneling microscopy (STM) for use in studies of the early stages of heteroepitaxy. We describe a system merging in vacuo an MBE, STM, XPS, LEED, etc. We present results for the initial stages of GaAs growth on vicinal Si(100), including As termination, Ga capping and subsequent 2D and 3D competition.

The structural quality of GaAs layers grown at 200°C by molecular beam epitaxy (MBE) was investigated by transmission electron microscopy (TEM). We found that a high crystalline perfection can be achieved in the layers grown at this low temperature for thickness up to 3 μm. In some samples we observed pyramid-shaped defects with polycrystalline cores surrounded by microtwins, stacking faults and dislocations. The size of these cores increased as the growth temperature was decreased and as the layer thickness was increased. The upper surface of layers with pyramidal defects became polycrystalline at a critical thickness of the order of 3μm. We suggested that the low-temperature GaAs becomes polycrystalline at a critical thickness either because of a decrease in substrate temperature during growth or because strain induced by excess As incorporated in these layers leads to the formation of misoriented GaAs nuclei, thereby initiating polycrystalline growth. The pyramidal shape of the defects results from a growth-rate hierarchy of the low index planes in GaAs.

Magnetic-field-dependent microwave absorption Is shown to be a useful non-destructive and contact-free technique to study transport behavior in GaAs/AlGaAs devices. It allows quick measurement of resistance, mobility and carrier concentration in the two-dimensional electron gas of heterostructure quantum wells. The two and three dimensional conductivities may be separately evaluated allowing detailed study of conduction in the active layer of high electron mobility (HENT) devices. Applications of the approach to examination of device structural dependencies, carrier-density conduction behavior and the effects of etch processing on quantum well integrity are presented.

Lattice parameter, RBS channeling, and T-dependent Hall effect and resistivity are measured in 5 μm GaAs layers MBE-grown at 200°C, 250°C and 305°C, and subsequently annealed. Perpendicular lattice parameter was increased by 0.15% for 200 °C, 0.10% for 250°C, and 0.05% for 305°C sample. No parallel mismatch was observed. In annealing up to 800°C, most recovery of lattice parameter occured at 350°C-450°C. For samples annealed at 500° C − 600°C, the conductivity activation energy was higher in the furnace annealed sample than in the RTA-annealed sample, indicating possible depth nonuniformity in furnace annealed sample. The 600°C RTA annealed sample indicated a donor activation energy of 0.61 eV. A 600°C furnace annealed sample indicated the deep donor concentration of 1.5×1018 cm−3. Maximum mobility in the 500–600°C annealed sample was 3000–3500 cm2/Vs, limited by the neutral impurity scattering and polar optical phonon scattering.

We demonstrate the novel use of CV simulation to determine the level of charge at each interface in multilayer GaAs/AlGaAs heterostructures. The gettering of charged centres at the earliest grown interfaces is quantitatively appraised and the parallel application of PL decay lifetime measurements determines their effectiveness as recombination centres. A direct relationship is found between the magnitude of the charged states and the recombination velocity. Estimates of a 1eV activation energy and a hole capture cross section of 10−15 cm2 were also obtained. This suggests that the dominant recombination centre could be observed as a hole trap.

HgCdTe heterostructures consisting of a thin n-type widegap (250 meV or 5 μm cutoff) layer deposited on an n-type narrowgap (100-125 meV or 10-13 μm cutoff) layer offer the promise of very high performance metal-insulator-semiconductor (MIS) photocapacitors for long wavelength infrared (LWIR) detection. Molecular Beam Epitaxy (MBE) is a candidate growth technology for these two layer films due to its fine control in composition, thickness and doping concentration. The critical materials issues are reducing the defect content associated with twins in the grown layers, achieving low net donor concentrations in the widegap layer, and avoiding the formation of misfit dislocations at the HgCdTe heterointerface. This paper will report on our recent progress in these directions.

The initial stages of germanium heteroepitaxy on vicinal Si(100) have been studied using in-situ deposition in a UHV STEM. Germanium was deposited using molecular beam techniques onto substrates misoriented 1° and 5* toward <110> held at room temperature, 375°C and 525°C. Film thicknesses were in the range 4-6 ML, just greater than the stable intermediate layer of 3-4ML (1ML = 0.14nm). The Ge clusters were observed using biassed secondary electron (b-SE) imaging with nanometer resolution. Comparisons were made between deposition at the elevated temperatures, and room temperature deposition followed by anneals at the same temperatures.

Annealing the low temperature deposits produces coarsening of the islands which is similar on the 1° and 5° samples. Island size distributions and other film growth parameters obtained from the 375°C and 525°C anneals indicate that the coarsening is different at these temperatures and is possibly affected by instabilities in the intermediate layer. Results of the high temperature depositions indicate that neither surface steps nor the edges of islands act as perfect sinks, and that diffusion distances are of the order of several microns. The nucleation density and size distributions are markedly different for deposition at 375°C and 525°C possibly due to competitive capture at strong sinks.

In a parallel set of experiments in a standard UHV chamber, macroscopic wafer samples were analyzed with RHEED, Auger and secondary electron spectroscopy. These correlate well with the intermediate layer thicknesses previously reported in the literature, and the large contrast observed in the b-SE images. Ex situ TEM studies of samples grown in this chamber show islands with various contrast features including those of coherent strain.

Together with the advanced growth technologies of zinc selenide epitaxial films by MOVPE or MBE, severals workers have reported the observation of p-type conductivity in this material. However, there are some inconsistencies in most of the papers reporting such results.

We report here the MOVPE crystal growth of nominaly undoped ZnSe/GaAs S.I. layers using alkyls as precursors, and their optical and electrical characterizations. We show the possibility to observe p-type conductivity in ZnSe due to a hole accumulation at the interface in the GaAs side. This is consistent with a simple model taking Into account the conduction and valence bands discontinuities at the ZnSe/GaAs interface.

The optimization of chemical vapor deposition processes requires an understanding of the influence of various process parameters on the deposition of thin films. A recently developed computer simulation tool provides a powerful means to develop this understanding. This paper describes the use of the computer program, FLUENT, to study the gas flow, temperature, and chemical species distributions during the deposition of CdTe. Numerical results are reported for two operating conditions for an EMCORE vertical high-speed rotating disk growth reactor and are compared to experimental data. The influence of process parameters is discussed. The effects of the addition of significant amounts of Hg (several percent) to the process gas is evaluated.

We have analyzed photoluminescence spectra from CdxZnl−xTe /ZnTe and ZnSexTel−x/ZnTe strained layer superlattices grown by MBE, and obtained the band offsets by fitting to theory. We find that the valence band offset of the CdTe/ZnTe system is quite small (-50± 160 meV). In CdxZnl−xTe /ZnTe superlattices, the electrons and heavy holes are confined in the CdxZn1−xTe layers (type I), while the light holes are confined in the ZnTe layers (type II). On the other hand, the photoluminescence data from the ZnSexTe1−x /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset (−907 ± 120 meV). We also investigated the band bowing in the ZnSexTel−x alloys by optical spectroscopy, and found that there is only a small component of bowing in the valence band, while most of the bowing occurs in the conduction band. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

Molecular beam epitaxy (MBE) grown ZnSe/GaAs and ZnSe/Ge heterointerfaces have been studied by transmission electron microscopy (TEM). Defect characterization of cross-sectional and planar specimens showed that ZnSe epitaxial films contain numerous twins that predominantly arise at the interface. Planar specimens of ZnSe/Ge were in-situ TEM annealed, for 5.5 hours at 873K. The twins are thermally very stable which would indicate that they arise during the growth process. The occurrence of these twins in the ZnSe film is explained by nucleation and growth of normal and twinned nuclei. Some of the ZnSe films grown on (10O)Ge substrates have low-angle boundaries indicating that the initial growth of the film is by the formation of islands.

Little has been published about Te-rich ZnSexTel−x grown at low temperatures, in spite of some successes in the fabrication of wide band gap light emitting devices from ZnSeTe alloys grown at higher temperatures. We present x-ray diffraction and photoluminescence (PL) spectra for ZnSeTe epilayers and ZnSeTe/ZnTe superlattices grown by molecular beam epitaxy (MBE). These we compare with measurements on ZnTe, ZnSe and CdZnTe epilayers and on CdZnTe/ZnTe superlattices grown under similar conditions and also with data published for ZnSeTe alloys grown at high temperatures. Equilibrium phase diagrams for the ZnSeTe alloy system suggest a large miscibility gap at MBE growth temperatures; this may account for some unusual features in the (PL) spectra and for large line widths in the x-ray data. In spite of these possible miscibility problems, we find that ZnSeTe alloys luminesce brightly.