GaAs epilayers on (100)Si substrates have been grown by ionized cluster beam(ICB) method under conventional high vacuum conditions. The developed deposition method has two characteristics: first is cleaning the Si surface by accelerated As clusters and second is adopting double layer structure in order to confine dislocations within a primary layer. As a result of these features, the etch pit density came to be 2×1O6cm−2 and the crystal quality was comparable to that of molecular beam epitaxy(MBE) or metal organic chemical vapor deposition(MOCVD) method. In the present paper, we discuss those characteristic phenomena of ICB method and demonstrate MESFET's fabricated on our GaAs grown on Si substrates.

UCGAS (undercut GaAs on Si) structures in which a part of the GaAs layer is separated from the Si substrate are newly developed to reduce both the thermal stress and the defect density. The stress distribution in the fabricated structures is evaluated by photoluminescence. Quite a good agreement between the measured and the analyzed distributions is obtained, and the stress in UCGAS is less than 1/10of that in planar GaAs on Si. The EPD (etch pit density) in UCGAS after thermal cycle annealing is also less than 1/10of that in the planar layer. UCGAS-LED is fabricated for the first time and tested. The UCGAS-LED emits in 7 meV shorter wavelength and has longer lifetime compared to the mesa-type LED fabricated from the same wafer.

The role of interface strain and dipole layers in detenrining the electronic properties of two lattice matched HI-V semiconductor superlattices, InAs/AIGaAsSb and GaAs/AlAs, has been investigated with the self-consistent pseudopotential method. Values for the total energies, interface structures, valence band offsets, and charge density distributions are reported. In the InAs/AlGaAsSb system, since both the group III and V sublattices are different, large intrinsic strains are present at the interface. If the interface strain is neglected in the calculation, the valence band offset is found to be 0.05 eV, in poor agreement with the experimental value of 0.21 eV obtained from infrared photoluminescence spectroscopy. The valence band offset for the relaxed structure (obtained by minimizing the total energy with respect to atomic relaxations at the interface) is 0.24 eV, in excellent agreement with the experimental value. The interface relaxations modify the interface dipole through charge transfer between interface cation and anion. In addition to the intrinsic interface strain, the effect of extrinsic interface dipole layers, both III-V (InAs, GaSb) and group IV (Si and Ge) interlayers, has been investigated in the GaAs/AlAs lattice-matched system. The III-V interlayers are not effective in changing the band offset due to small charge asymmetry and large screening of the induced dipole. On the other hand, the group IV interlayers are found to have a dramatic effect, changing the GaAs/AlAs band offset by ∼1 eV. The group IV interlayers produce a step potential at the interface, resulting in the large offset change.

The introduction of dislocations into initially dislocation free strained epitaxial layers is examined. Examples are given of several different sources of dislocations, and the influence of these upon the final microstructure is shown. The sources most prevalent appear to be growth defects in epilayers with strains less than about 0.01, and topography-induced surface nucleated half-loops at strains greater than about 0.02.

The effect of repetitive rapid thermal annealing (RTA) at 800°C was investigated in the strained-layer superlattices (SLS's) In.2Ga.8As/GaAs (80Å/80Å) and In.3Ga.7As/GaAs (80Å/80Å) using photoluminescence (PL) and the highly sensitive absorption technique photothermal deflection spectroscopy (PDS) [1]. The samples were repetitively annealed at 800 °C for 15 seconds intervals up to a total time of 45 seconds. Between each anneal, PL spectra were taken at 1.4 K. PL measurements show initial red shifts and subsequent blue shifts with increased annealing times. These shifts were accompanied by a decrease in the intensity of the PL peaks. PDS absorption data were measured at room temperature for the as-grown materials and for the 45 seconds RTA. The PDS data measured at room temperature for 0 and 45 seconds RTA were consistent with the PL data. The spectral shifts may indicate a competition between the effects of thermally driven strain relaxation and chemical interdiffusion.

Three InAs/GaAs single quantum wells of 2, 3, and 4 monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance and cathodoluminescence are presented.

Using the polarization modulated absorptance technique we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the 3- monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2·10−4 eV/K which is significantly lower than in both constituent materials.

Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. Therefore, photoluminescence results alone do not provide unequivocal information about the fundamental transition energy or the interface quality in quantum wells.

The HRTEM images indicate that while the interfaces of the 2-monolayer sample are smooth and the well thickness is uniform, the 4-monolayer sample has uneven interfaces and contains domains of 2, 3, and 4 monolayers. In agreement with these observations, absorption features broaden with the increased well width. Scanning cathodoluminescence images of the 2- monolayer sample present no evidence of dislocations, which is consistent with the HRTEM observations.

A GaAs/InAs heterostructure was characterized by micro-Raman spectroscopy. A 2 μm thick GaAs layer was grown on an (100) InAs substrate by molecular beam epitaxy. The sample was then angle-lapped so that the interface region is exposed on the bevel. At the close vicinity of the interface, GaAs longitudinal-optic (LO) frequency was lower by about 3 cm−1 than at the asgrown surface. The LO shift decreased with increasing distance from the interface. The spectral width did not strongly depend on the position, and thus the shift would be mainly due to strain. A similar experiment was carried out for an InAs/GaAs structure, and a broad peak which was tentatively assigned to the plasmon-LO coupled mode was observed for the InAs layer.

CdTe-ZnTe strained-layer superlattices (SLSs) were used to reduce the density of threading dislocations in CdTe epilayers on GaAs substrates. Three types of SLSs were characterized by double-crystal X-ray rocking curves and transmission electron microscopy (TEM). The SLS blocked dislocations when the thickness of both the individual layers and the total SLS does not exceed the critical layer thicknesses. TEM images of both cross-section and plan-view show that the twothirds of the maximum number of threading dislocations were blocked by the SLS and, on average, half of these were blocked. While, in the other SLSs, new defects (threading dislocations and/or stacking faults) were generated in layers that exceeded the critical thickness. These defects offset the effect of any dislocation bending.

Strain measurements were performed on multilayer films of Mo/Ni using x-ray diffraction with the scattering vector at varying degrees of inclination to the surface normal. In-plane variations in the lattice parameters confirm that there are coherency stresses between the Mo and Ni layers. Assuming the Nishiama-Wasserman orientation for the planes of Ni and Mo allowed the calculation of the complete stress state in both materials. The in-plane lattice parameter, the perpendicular lattice parameter and the unstrained lattice parameter have also been derived from the data. The substrate interaction stress which would be necessary to provide a force balance between the layers was calculated. This was found to be in good agreement with the global stress measured from wafer curvature. Preliminary results of modelling of the high angle superlattice lines to extract interfacial roughness in the growth direction are also presented.

Heteroepitaxial films of (111)BaF2/CaF2 on (111)silicon, and (111)BaF2 on (100)silicon are used as substrates for the growth of IV-VI semiconductors. X-ray diffraction measurements show that p-type PbSxSe1-x(0.4 < × < 0.5) films grown on BaF2/CaF2/Si(111) substrates have bi-crystalline (111) and (100) characteristics. This, combined with thermal stress may cause the heteroepitaxy to peel off from the silicon. Films grown on BaF2(111)/Si(100) substrates display only the (111) peaks, and are robust with respect to thermal strain.

An algorithm has been discovered that greatly simplifies the exhaustive search for all orientations of all known crystals that have a lattice match with crystal film to be epitaxially deposited. It is based on the observations that (1) a two-dimensional lattice match of necessity entails an integer-ratio match between the areas of the unit parallelograms of the substrate and deposit planes and (2) that the unit area A (hkl) is equal to the unit cell volume divided by d(hkl). Using this algorithm, we are currently verifying the area-match tolerances for known cases of epitaxy and developing database-generating and searchprogram software.