The technology of heteroepitaxially growing compound semiconductors, particularly GaAs, on Si has attracted an increasing volume of attention in the past 3–5 years. In the past two years there have been few major advances in the growth recipes, which all include a high temperature Si preheat, a two-step growth temperature profile, and the use of misoriented substrates. The need for mechanistic understanding of the effect of these parameters is crucial to advancing the state of the art beyond this current practice. This work focusses on the effect of the misorientation in inducing asymmetry in early stages of the molecular beam epitaxy of GaAs on Si substrates. The strain in the films is found to have greatly different rates of relaxation in the plane of the film when measured in the two orthogonal <220> directions. This asymmetry persists to greater than 30 nm film thickness at 400°C. The nucleation morphology was also examined as a function of substrate misorientation. At low substrate tilts, nucleation density was sparse and there was not substantial ordering of the nuclei. As the tilt was increased, a distinct habit emerged where collections of nuclei were quite coherent along the steps for several 100 nm, with dimensions of the order of 10's of nm perpendicular to the steps. The density of steps with nuclei was also substantially less than that predicted by the widely accepted “array of double-height steps” used to explain the curious lack of anti-phase disorder in these films.

Dislocations are produced at the interface between epilayers and the substrate when there is a lattice mismatch. When GaAs is grown on Ge substrates, these dislocations can propagate into the epilayers. They can then interact with one another or with antiphase boundaries which are generated when the polar-material is grown on a non-polar materials.The interactions between these defects have been investigated using the weak-beam imaging technique of transmission electron microscopy. Possible interactions between the misfit dislocations and heterojunctions were examined in a specially prepared layer structure model of GaAs-AlxGal−xAs.

Molecular beam epitaxy (MBE) growth of GaAs on Si was investigated for three Si substrate orientations: exact (100), 4° off (100) towards (011), and 4° off towards (010). Cross-sectional transmission electron microscopy (X-TEM) analysis indicated a high dislocation density at the GaAs-Si interface that decreased away from the interface. Changing the orientation significantly affected the dislocation arrangement in the films.

In the exact (100) case, dislocations from different glide systems formed pyramids, and dislocation annihilation resulted in linear defects propagating to the bottom of pits on the GaAs surface. On Si substrates oriented 4° off of (100), dislocation pyramids were not observed which we attribute to the different stresses acting on different glide systems. Planar TEM sections indicated that the dislocation densities at th surfaces of the 2-μm-thick films were 8 × 108 /cm2 for exact (100), 3.4 × 108/cm2 for 4° off (100) towards (010), and 1.6 × 108/cm2 for 4° off towards (011) orientations. When etching was used to evaluate anti-phase domain (APD) density, the exact (100) and off (100) orientations toward (010) showed APD's in some areas; off (100) toward (011) orientations were apparently APD-free. Results of photoluminescence (PL) spectroscopy of each of the wafers showed marked differences in peak intensities for the different orientations. Secondary ion mass spectrometry (SIMS) showed that roughly 1/4 of a monolayer of Si was incorporated in the GaAs, mostly concentrated in the first 250 nm near the GaAs-Si interface.

A UHV MBE apparatus in which the deposition of both group IV and group III-V components is possible without breaking vacuum has been utilized to compare the growth of GaAs epilayers on non-polar Si(100) and Ge coated Si(100) substrates. In addition, a comparison of GaAs epilayers grown on substrates cleaned by ex-situ techniques and on substrates given all UHV in-situ surface preparation was made. Defect reduction by the incorporation of strained-layer superlattice dislocation filters and by post-growth rapid thermal anneal (RTA) thermal cycles was also investigated. Optical and material properties comparable to MBE grown GaAs/GaAs were obtained for GaAs grown on Ge coated Si(100) substrates.

Antiphase boundaries are observed in epilayers of GaAs grown by organometallic vapor phase epitaxy on Ge substrates and are then invariably found to show a tendency to facet. Stacking-fault-like fringes caused by the translation of adjacent grains give the information on the relative displacement of the two grains at these interfaces and show that this translation does not have a fixed magnitude for a particular interface but varies with the orientation of the interface. Preferred orientations of the antiphase boundaries and the rigid-body translations have been studied using transmission electron microscopy. Interactions between antiphase boundaries and interfaces have been examined here in heterolayer structures consisting of alternating layers of GaAs and AlxGal−xAs grown on an (001) Ge substrate. The possibility of using atomic-resolution imaging to investigate the atomic structure of APBs is illustrated and the images are compared with those predicted by image simulation.

GaAs layers were grown directly on misoriented (2° off (100)→[011]) Si substrates by Metalorganic Chemical Vapor Deposition. The threading dislocation density at the surface of 4 μm thick layers was typically 108cm−2, as determined by both preferential etching and transmission electron microscopy. Rapid thermal annealing (900°C, 10s) improved the crystalline quality of the GaAs near the heterointerface while allowing no detectable Si diffusion into this layer. Two deep electron traps were observed in the undoped GaAs, but were present at a low concentration (∼ 1013 cm−3 ). The (400) x-ray diffraction peak width from the GaAs was significantly reduced with increasing GaAs layer thickness, indicating improved material quality. This is supported by Si implant activation data, which shows higher net donor activity in thicker layers.

In an effort to evaluate the near surface crystal quality of GaAs on Si wafers, {224} plane diffraction were investigated using a conventional double crystal x-ray diffractometer without any high intensity radiation source. The x-ray incident angle to wafer surface varied from 3.6 to 9.6 degrees for different {224} planes due to the substrate tilt angle of 3 degrees. The GaAs to Si rocking curve intensity ratio increased significantly as the incident angle decreased. For the diffraction with 3.6 degree incident angle, only the GaAs peak was detected from the 3.5 um thick GaAs on Si wafer and the GaAs peak became narrower. These indicates that this conventional x-ray diffraction technique is applicable for the near surface quality evaluation of GaAs on Si wafers.

This paper will review some of the recent progress in GaAs-on-Si devices and circuits, and discuss the issues involved in realizing large scale ICs in GaAs-on-Si wafers. With a recent success in fabricating 1 Kbit static RAMs in GaAs-on-Si wafers, it has become apparent that this material is indeed acceptable for LSI complexity circuits. GaAs MESFETs fabricated using a standard process show an excellent threshold voltage uniformity which is comparable to that for bulk GaAs devices. GaAs bipolar devices on GaAs-on-Si have realized a gain of 25 which is the highest reported for bipolar devices in GaAs-on-Si material. Bipolar ring oscillators and 256-bit ROMs consisting of more than 100 gates have also been realized. In spite of these successes, however, there are numerous issues that need to be solved before this material becomes practical.

The status of monolithic GaAs/Si (MGS) integration is reviewed and its future prospects are discussed. The MGS material now available is satisfactory for majority-carrier devices, but improvement in quality is needed for minority-carrier devices. By using a suitable sequence of processing steps, fabrication of GaAs MESFETs and Si MOSFETs on the same wafer and monolithic integration of GaAs/AlGaAs double-heterostructure LEDs and Si MOSFETs have been achieved. The GaAs and Si FETs show characteristics comparable to those for devices fabricated on separate GaAs and Si substrates. The LEDs have been modulated up to 27 Mb/s by applying a pattern of voltage pulses to the MOSFET gate. Two promising applications of MGS integration are optical interconnects between Si VLSI chips and monolithic microwave integrated circuits. Much better performance should be obtained by operating MGS ICs at liquid nitrogen temperature.

Deep electron traps were investigated in MBE GaAs grown directly on Si substrates with orientations a few degrees off the <100> axis. Capacitance Deep Level Transient Spectroscopy (CDLTS) revealed the presence of eleven electron traps in the GaAs epilayer. Their activation energies ranged from 0.21 eV to 0.83 eV below the conduction band. Most of these traps were previously observed in homoepitaxial GaAs films grown under As-rich conditions (VPE, MBE), or in electron irradiated bulk GaAs. Trap concentrations tracked Si dopant density and MBE growth conditions. Observed deep levels are not introduced by metals or other contaminants present at the GaAs-Si interfaces. Rather, they are caused by defect complexes. These complexes involve native point defects, whose formation is favoured by As-rich environments, by lattice mismatch, and by different thermal expansion coefficients. Si dopant atoms may also participate in the formation of these defects. A similar deep level generation mechanism is proposed for the electron traps in homoepitaxial MBE GaAs layers.

We show that MBE-grown GaAs on Si exhibits only a modest increase in the concentrations of the well-known electron traps typical of MBE-GaAs with no evidence for any new electron deep levels in the upper half of the bandgap in spite of the dislocations and other defects in the material. As shown by Au-GaAs Schottky contacts, the defects are unnoticeable when the device is forward biased but become very active in reverse biased condition, causing large leakage current and low breakdown voltage (although the device is still acceptable for many applications, especially FET's). The defects become more active after hydrogenation and more inactive after a post-growth rapid thermal annealing (RTA). Performance of devices made on thermally annealed GaAs on Si is comparable to those of GaAs on GaAs. Also, following the application of a large- current the device behavior improves, indicating a self-annealing action as a result of internal heating. The reverse current in the as-grown material shows a very weak temperature dependence, indicating its origin is not thermionic emission or carrier generation. It is speculated that a large part of the leakage current in the as-grown GaAs on Si is due to the defect assisted tunneling. After RTA, the average spacing between defect clusters increases, thus reducing the tunneling probability and tremendously improving the device characteristics.

We report the epitaxial growth of GaAs films on bulk Si, Ge and Ge coated Si substrates by MOCVD. This work is directed at comparing the structural, electrical, optical and optoelectronic properties of the GaAs films grown on these substrates. The quality of the GaAs films was evaluated by a number of techniques including X-ray, TEM, RBS, I-V and C-V. We have also obtained good photoresponse for Au-GaAs/Si, Au-GaAs/Ge and Au-GaAs/Ge/Si Schottky diodes at 0.87μm. We have observed significant differences in defect density, strain, crystalline quality, structural order, doping density depth-profile, dark current, diode ideality factor, photoresponse, inter-diffusion and interface structure among these three different structures. The GaAs/Ge structure exhibited the best structural and electrical characteristics of all. The GaAs/Ge/Si structure showed material properties and Schottky diode performance that are better than those of the GaAs/Si stucture. Although Ge and Si both out-diffuse into GaAs, that the out-diffusion of Si into GaAs appears to cause more degradation in the electrical and optoelectronic properties of GaAs.

Photoluminescence and photoluminescence excitation spectroscopies are utilized to study excitons in GaAs/AlGaAs quantum wells (QW's) fabricated using MBE on MOCVD grown GaAs/Si. The experimental results are understood in terms of the biaxial tension of approximately 3 kbar present in the plane of growth for both the QW's and the GaAs buffer. An important consequence of the biaxial tension is that for QW's with well widths larger than ≈15 nm the light- and heavy- hole sub-bands cross each other in energy. This results in the light-hole exciton energy being lower than that of the heavy-hole exciton, opposite to the case of QW's grown on GaAs substrates.

We present a systematic study of the low-temperature photoluminescence from undoped GaAs layers grown directly on Si substrates by MOCVD. GaAs layers from 100Å to 4 μm in thickness were deposited on Si substrates prepared with a variety of doping levels and orientations. The emission from thicker samples is dominated by pairs of lines in the band-edge region. Photoluminescence excitation measurements show that this multiplicity results from two regions in the material with different levels of strain. The stress-induced splitting of the valence band is also studied using excitation spectroscopy. In thinner samples we observe strong emission in the midgap range due to stoichiometric defects. The nature of the defects near the interface depends strongly on the character of the substrate.

Using 4.2 K selective photoluminescence (PL) excitation and PL excitation spectroscopy, the n = 2 excited state of the light-hole exciton is observed for the first time in GaAs/Si. The excited state is about 3 meV above the ground state, similar to exciton results for GaAs/GaAs. A spectral width of 1.8 meV is observed for the ground state transition, the narrowest yet reported.

Work on GexSi1−x strained layer epitaxy is reviewed including: The limits of single layer and superlattice growth, and comparison with theory; Physical properties including bandgap and heterostructure band alignment; Work on non-random alloys; Device applications including MODFET's, PIN and APD photodetectors.

An analysis has been made of a Si/SiGe HEMT structure, grown on a (001) oriented substrate, by adapting an existing model for III–V HEMT's. Strain plays a critical role in the behaviour of these SiGe structures by separating the degenerate conduction band minimum into 2- and 4- fold symmetric minima. This effect has been incorporated into a ID solution of Poisson's equation in the device structure. The effects of strain, Schottky barrier height, layer width and doping density on both enhancement and depletion mode HEMT properties are all considered. In addition initial estimates of the effect of strain on electron mobility are presented.

A systematic study of the band-to-band optical transitions in commensurate strainedlayer superlattices of Ge and Si grown on (001) Si substrates show the presence of new electronic energy bands. Our measurements suggest that superlattice structure on the same scale as the unit cell results in a band structure significantly different from that of a random alloy.

First principles total energy and electronic structure calculations for SinGen superlattices grown epitaxially on a Si (001) substrate reveal a nearly direct band gap despite the pronounced indirectness of the Si0.5Ge0.5alloy. While the new direct superlattice transitions have their origin in folded indirect transitions of the random alloy, their transition matrix elements are considerably enhanced by superlattice atomic relaxation and the superlattice ordering potential. For a superlattice grown on a substrate with an lager lattice contant than we predict a nearly direct band gap.

The optical and electronic properties of pseudomorphic SixGel1−x/Ge heterostructures are reviewed in brief. Potential applications of conduction and valence band alignments for modulation doped field effect transistors are outlined. Optical transition energies are calculated for ultra-thin, alternating four monolayer, Si/Ge heterostructures on (001) Ge.