Graded (Ca,Ba)F2 layers consisting of near lattice matched CaF2 at the Si interface and of BaF2 with 14% increased lattice constant at the top surface were grown by molecular beam epitaxy (MBE) on Si(111). Smooth and crackfree layers exhibiting Rutherford backscattering (RBS) channeling minima below 5% were obtained. Device quality epitaxial layers of PbTe, PbSe and (Pb,Sn)Se were grown on top of these structures. Mechanical stress at 300K was relaxed by athermal mechanisms in the fluoride- as well as in the Pb-salt films. - In preliminary runs, epitaxial CdTe-layers were obtained on Si(111) using the same fluoride-buffer film technique and which showed clear SEM electron channeling patterns.

Epitaxial single crystal growth of zirconia (ZrO2) and yttria stabilized zirconia (ZrO2 ·Y2O3) films on sapphire substrates was achieved for the first time by using reactive sputtering. And the relations of crystallographic orientations between the epitaxial films and sapphire substrates was determined. Yttria stabilized zirconia films seem to offer hiah quality SOI substrates, since the crystal structure of ZrO2·Y2O3 is cubic fluorite and its lattice constant is closely matched to those of semiconductors such as Si and GaAs.

Activities in semiconductor heterostructures are reviewed, with focus on the fundamental physics aspects in the quantum regime and with emphasis on recent experimental observations. The GaAs-GaAlAs is used to illustrate the general electronic properties of interest, which are obtained through various measurement techniques. Other heterostructures, including III-V, II(IV)-VI, and IV materials, are covered to the extent that they exhibit new and specific features.

A variety of theoretical models have been used to calculate the electronic properties of semiconductor superlattices. The methods vary in their ease of implementation, number of empirical parameters, and ability to incorporate physical effects. There is no “best” method; the choice of model is made on the basis of the desired property under investigation, such as subband energy levels, energy band dispersion and effective mass, strain effects, or optical spectra. The strength and limitations of the Kronig- Penney, envelope function, and tight-binding models will be reviewed, including one-, two-, and multi-band versions. The relationship of superlattice to bulk band structure, and the issue of dispersion in the growth and in-plane directions will be illustrated with the examples of the CaAs-GaAlAs and HgTe-CdTe superlattices.

Employing temperature dependent photoconductivity, photoluminescence and photoreflectivity measurements, we have analyzed a GaAs-AlAs multiple quantum well. The above optical techniques clearly resolve the fundamental inter-subband transitions, including heavy hole-light hole splittings. At T < 60K an anomalously high photoconductivity was discovered below the direct inter-subband transitions and is attributed tentatively to the presence of extrinsic interface states within the bandgap. For T > l00K the fundamental indirect transition was discovered and associated with LO (L) - phonon absorption.

By using a DLTS technique, we measured the drain current transient from a gate bias pulse for a HEMT. Two negative peaks and one positive peak were observed. From the analysis of the spectra, we found that the positive peak was due to the interface trap. The density of the interface trap was determined from a DLTS fitting procedure. The effect of the interface traps on the electrical properties of the heterostructure are discussed.

The use of an AlGaAs/n-GaAs superlattice in place of the n-AlGaAs layer in MODFET devices reduces the light and temperature sensitivity of the threshold voltage. This paper considers the stability of Si doped superlattices under annealing conditions required for activation of the implant in the self-aligned gate MODFET fabrication process. Rapid optical annealing does not significantly degrade the superlattice structure. The DX center concentration in the superlattice structures is a factor of 30 less than measured in conventional MODFET structures. High performance MOOFET devices have been fabricated using the self-aligned gate process with rapid optical annealing.

A novel method has been used to obtain a direct and accurate measure of the valence-band discontinuity AlyGa1−yAs/AlAs heterojunctions in quantum-well structures. The technique takes advantage of the crossover occurring at a critical Al concentration above which the indirect X minimum in the AlAs becomes the lowest-energy conduction band in the system. Within these “staggered” band alignment structures, photoexcited electrons and holes are spatially separated, and recombination occurs across the interface. The resulting emission fixes the valence-band offset to within 1% without accurate knowledge of other system parameters, such as effective masses and exciton or dopant binding energies. These measurements represent the first direct optical confirmation of staggered band alignments in this technologically important material system.

Superlattices of HgTe and CdTe, grown on (100) and (111) CdTe, have been characterized by He ion backscattering spectrometry and x-ray double crystal diffractometry. Backscattering spectrometry gave basic information about the depth-distribution of layers, i.e. the number of layers in the superlattice, the thickness of CdTe overlayer and the location of damage in the underlying CdTe. Symmetric and asymmetric rocking curves gave information about crystal quality and layer thicknesses. Average strains in the directions perpendicular and parallel to the layer interfaces were obtained directly from the rocking curves. Strains and thicknesses of the HgTe and CdTe layers, as well as variations from layer to layer, were found by fitting the symmetric rocking curves with theoretical calculations based on a kinematic model for the diffraction. A tilt of the atomic planes in the epitaxial layers, relative to those in the substrate, was observed by means of rocking curves recorded from the same diffracting planes, but with reversed incident and diffracted beam directions.

The effects of implantation and annealing on an AlAs-GaAs superlattice grown by OMCVD is examined with SIMS (secondary ion mass spectrometry). Several 180 keV 28Si+ implants, with doses ranging from 3 × 1013 to 3 × 1015 cm−2, are examined before and after a three hour 850 C anneal. While the implantation by itself causes some intermixing in the vicinity of the projected range, the 850 C thermal anneal induces significant mixing at depths well beyond the implant range. In the region of maximum implant damage, however, the post-thermal mixing effect is inhibited. Depth dependent diffusion lengths of Al and Si are derived from the SIMS data. The diffusion coefficient of Si is markedly enhanced in the mixed regions.

Cross-sectional Transmission Electron Microscopy, Sputter-Auger spectroscopy, and Raman spectroscopy have been used to study intermixing and residual damage in annealed ion-implanted Al0.3Ga0.7As/GaAs superlattices. Several implant species were studied Nse, Si, Mg, Be). Three different regions can be distinguished in the annealed ionimplanted superlattice samples. The topmost region contains a dense network of stacking faults and microtwins, residual damage from an implantation-amorphized region which has recrystallized during annealing. In the second region, which is relatively defect-free, either total, or at least appreciable intermixing of the GaAs and Al0.3Ga0.7As layers occurs. For fixed annealing conditions, the degree of intermixing varies with the mass of the implanted species. The third region contains many small dislocation loops which form by the agglomeration of point defects during implantation or the subsequent annealing process. Raman spectroscopy is used to compare the degree of intermixing and residual damage between AlGaAs alloys generated by superlattice disordering and uniform “as-grown” alloys of the same composition which have undergone identical implant and anneal treatments.

Cross-sectional transmission electron microscopy (XTEM) has been used to study the defect structure and intermixing of ion-implanted and annealed AlxGa1−xAs/GaAs superlattices. The results show clearly that the layer intermixing depends on mass and energy of the implanted species and the annealing conditions. The temperature and duration of annealing determines mainly the amount of residual damage. In addition it was observed that in all cases the point-defects agglomeration was influenced by the strain field present at the layer interfaces; extended defects nucleate preferentially in the GaAs layers.

The LO phonon frequency evaluated from Raman spectra identifies two compositional disordering mechanisms in GaAs-AlAs superlattices. For a high Se dose, the LO phonons of the Al0.5Ga0.5As alloy are observed from the asimplanted samples. That means the compositional disordering occurred just by Se implantation. The probable mechanism for this disordering is the implantation of Ga atoms into the AlAs layer and of Al atoms into the GaAs layer. The superlattices implanted at a low dose are disordered by the subsequent annealing. The mechanism is the enhanced interdiffusion of both Ga and Al atoms between the GaAs and AlAs layers by Se thermal diffusion.

Growth of Si/SiGe superlattices on Si substrates by molecular beam epitaxy (MBE) is described. Strain symmetrization in the superlattice is achieved with an incommensurate SiGe buffer layer. The concept of strainsymmetrization is explained and properties of buffer and strained layer superlattices are investigated. A twodimensional electron gas with enhanced room temperature mobility and folded phonon modes within the reduced onedimensional Brillouin zone are observed. An n-channel Si/SiGe MODFET demonstrates the device applications of this material concept.

We have used photocurrent spectroscopy to measure the optical absorption spectra of coherently strained layers of GexSi1−x grown on {001} Si by molecular beam epitaxy. A dramatic lowering of the indirect bandgap, relative to that of unstrained bulk Ge-Si alloys, is observed. Our results for 0 < × < 0.7 are in remarkably good agreement with recent calculations of the effects of misfit strain on the band edges of coherently strained Ge-Si heterostructures.

Using a realistic zone-folding scheme for a Ge(x)Si(1−x)/Si strained layer superlattice having a Si-like conduction band structure (i.e ≲85% Ge) we calculate the transition probability for the zone-folding induced direct optical gap and compare it with the indirect band gap absorption probability. The results suggest that such zone-folding induced direct optical transitions are promising for optical devices made from Ge(x)Si(1−x)/Si strained layer superlattices and like structures, provided they are fabricated in such a way that the appropriate zone-folding occurs.

We have developed a procedure, based on conventional Monte Carlo methods, to investigate the limits of stability of a strained layer superlattice (SLS) system as a function of lattice mismatch and layer thickness. The method is demonstrated by the analysis of two-dimensional Lennard-Jones SLS systems, for which the regime of absolute SLS stability is mapped out. Extension of the technique to three-dimensional silicon-like model systems is discussed, and appropriate model potentials for stability analysis of the Si/SiGe system are introduced.

We describe the observation of a strain-induced order-disorder transition in the alloy layers of a GeSi/Si superlattice and discuss the possible implications of the transition for the opto-electronic properties of this important system.

Compositionally modulated amorphous Si/Ge thin films with repeat lengths (wavelengths) between 4.8 nm and 5.83 nm have been prepared using ion beam sputtering. The interdiffusion coefficient was determined from the decrease in the (000) x-ray satellite intensities with annealing, and was found to be relatively large, so that it could easily be measured without crystallization occurring. The effect of copper and oxygen impurities was found to be negligible. The dependence of the interdiffusivity on the modulation wavelength is similar to that of an ordering system. The temperature and wavelength dependence in the range T = 550-630 K is described by Dλ = 1.47×10−10 m.s−1 exp(-l.6 eV/kT)(l-21.8/λ2(nm)). It is suggested that diffusion is governed by the breaking of one bond near a pre-existing dangling bond.

In the present study, Raman spectroscopy (RS) and x-ray diffraction have been used to characterize semiconductor multilayer interfaces. A model for Raman spectra of multilayers is developed and applied to the specific case of the interfaces of a-Si/a-Ge multilayers. Quantification of the ‘blurring’ of interfaces is possible because RS is capable of directly ‘counting’ the total number of chemical bonds of a given type in the film. Multilayers, prepared by various deposition techniques, are compared. Several a-Si/a-Ge multilayers deposited by UHV evaporation (MBD) exhibit exceptionally sharp interfaces (intermixing width <l.0Å) and regular periodicities.