We present a systematic study of the photoluminescence of undoped GaAs layers deposited by MOCVD on Si substrates. The study includes an examination of substrate and layer thickness effects in thin GaAs layers, and a detailed investigation of the stress effects on the intrinsic band-edge transitions in thicker samples. For sample thickness,t ≤ 0.5(μm), we observe strong midgap emission bands associated with defects close to the interface. These bands depend strongly on the nature of the Si substrate. The crystal quality improves with sample thickness, and fort ≥ 0.5 μm the emission is dominated by lines in the band edge region which are relatively independent of substrate preparation. Photoluminescence excitation spectra reveal that the highest energy line is due to an intrinsic exciton transition, and that a splitting of this line observed fort ≥ 2 μm reflects the presence of two different regions of strain in the material. The magnitude of the strain is estimated from the shift of the exciton lines relative to unstrained GaAs, and is found to be consistent with an upper limit provided by the thermal expansion mismatch between GaAs and Si.

During molecular-beam epitaxy, growth interruption at each heterointerface of single quantum wells results in very narrow multiple peaks in photoluminescence (PL) spectra. These peaks correspond to extended monolayer-flat regions of the wells, differing in well thickness by one monolayer. PL and PL-excitation spectroscopies show that excitons tend to diffuse from narrower-well regions to thicker-well regions before radiative recombination takes place. This is also observed in time-resolved PL. The intensity of the higher-energy peak decreases with time, while that of the lower-energy peak increases with time. The obtained exciton diffusion time of ≂250 ps is in good agreement with the estimated size (≤1 μm)of the monolayer-flat islands from spectrally resolved cathodoluminescence.

Several GaAs/AlxGa1−xAs single-quantum-well samples are examined via photoluminescence and excitation spectra to emphasize that in many cases there is a poor correlation between the perceived qualities of quantum wells as deduced from each of the two different kinds of spectra.

We report growth of ultrapure GaAs by molecular beam epitaxy (MBE) and show that the arsenic source has a dramatic effect on the purity. With every thing else the same, by changing the arsenic source from 6N grade double refined chunks to a 7N grade slug which closely fits the 35 cm3 crucible, the deep electron trap and the residual acceptor densities were reduced by nearly two orders of magnitude. When intentionally doped with Si to an electron density of 3×1013 cm−3, the measured mobility at 77 K was 205 600 cm2 V−1 s−1, which increased to 294 700 cm2 V−1 s−1 at 42 K. These mobilities are the highest ever observed in intentionally n-doped MBE GaAs. Low-temperature photoluminescence studies of both undoped and Si-doped GaAs layers grown with the As slug show a marked decrease in acceptor- and defect-related luminescence over that observed when the As chunks are used. Our study conclusively proves that the major source of residual acceptor impurities and typical MBE GaAs traps observed by deep level transient spectroscopy is the chemical impurities present in the arsenic source, although the exact nature of these impurities is yet to be identified. These impurities or their complexes with the native crystal defects, and not the native defects alone, are responsible for the deep traps.

A new method has been developed for the growth of graded band-gap AlxGa1-x As alloys by molecular beam epitaxy which is based upon electron beam evaporation of the Group III elements. The metal evaporation rates are measured real-time and feedback controlled using beam flux sensors. The system is computer controlled which allows precise programming of the Ga and AJ evaporation rates. The large dynamic response of the metal sources enables for the first time the synthesis of variable band-gap AlxGa1-xAs with arbitrary composition profiles. This new technique has been demonstrated in the growth of unipolar hot electron transistors, graded base bipolar transistors, and M-shaped barrier superlattices.

We report the results of studies which have been made on heteroepitaxial layers of GaAs and AlGaAs grown by metalorganic chemical vapor deposition on composite substrates that consist of four different types of heteroepitaxial layered structures of Ge and Ge-Si grown by molecular beam epitaxy on (100)-oriented Si substrates. It is found that of the four structures studied, the preferred composite substrate is a single layer of Ge ∼1 µm thick grown directly on a Si buffer layer. The double-crystal X-ray rocking curves of 2 µm thick GaAs films grown on such substrates have FWHM values as small as 168 arc sec. Transmission electron micrographs of these Ge/Si composite substrates has shown that the number of dislocations in the Ge heteroepitaxial layer can be greatly reduced by an anneal at about 750° C for 30 min which is simultaneously carried out during the growth of the GaAs layer. The quality of the GaAs layers grown on these composite substrates can be greatly improved by the use of a five-period GaAs-GaAsP strained-layer superlattice (SLS). Using the results of these studies, low-threshold optically pumped AlGaAs-GaAs DH laser structures have been grown by MOCVD on MBE Ge/Si composite 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.

Very high-quality GaAs/Al0.37Ga0.63As single quantum wells with extended monolayer-flat regions have been grown by molecular beam epitaxy with growth interruption at hetero-interfaces. Low-temperature photoluminescence and excitation spectroscopy have been used to study these samples. The spectra exhibit well-resolved multiple sharp peaks which are due to changes in well thickness of one monolayer (ML). These peaks are the sharpest and best resolved to date for narrow wells. For example, the full widths at half maximum for single wells of width 57 Å (20 ML), 28 Å (10 ML), and 17 Å (5 ML) are 1.0, 1.7, and 6.0 meV, respectively. Although growth interruption introduces neutral acceptors, presumably carbon, they do not affect the 77 K mobilities of selectively doped heterostructures with thin spacers.

The MBE growth and energy levels of GaAs-(Al, Ga)As quantum well structures with half-parabola and two-stepped square composition profiles are described. Pulsed beam growth is used for the half-parabolic wells. Energy level spacings are determined independently for electrons and holes from photoluminescence excitation measurements.

High quality GaAs/AlxGa1−xAs single and multiquantum well heterostructures were grown for the first time by chemical beam epitaxy. Studies using low-temperature photoluminescence and excitation spectroscopy techniques show that on the average these samples are similar in quality to similar structures from this laboratory grown by molecular beam epitaxy (MBE) and in certain specific characteristics superior to the MBE ones. Furthermore, in some important respects, they are also superior to those grown by organometallic chemical vapor deposition (OMCVD). The very small red shifts observed between emission and n=1 exciton transition E1h with continuous growth show that the emission is dominated by E1h excitons. An interface roughness of δL≲±a/2, where a is the lattice constant, and very square wells (undistorted) even with continuous growth, in contrast to OMCVD are inferred from the excitation spectra. Unusually sharp exciton transition peaks up to E3h including forbidden transitions were obtained in single quantum wells. Such high quality line shape has not been obtained in MBE or OMCVD grown wafers so far. The excitation spectra also show no evidence of band filling due to holes or electrons from the AlxGa1−xAs layers which is a common problem with the OMCVD technique. From this study, it is also shown that the GaAs and AlxGa1−xAs materials are of very high purity.

The growth system and growth kinetics of chemical beam epitaxy (CBE) are briefly described and its differences from molecular beam epitaxy (MBE) and organometallic chemical vapor deposition (OMCVD) are discussed. We characterize the capability of this technique in preparing high quality multilayer heterostructures by growing GaAsAlxGa1−xAs single and multiquantum wells and double-heterostructure lasers. Studies using low-temperature photoluminescence and excitation spectroscopy techniques show that on the average the quantum wells are similar in quality to similar structures from the laboratory grown by MBE and in some important respects, superior to those grown by OMCVD. An interface roughness of δL ≲±a/2, where a is the lattice constant, and very square wells (undistorted) even with continuous growth, in contrast to OMCVD, are inferred from the excitation spectra. Unusually sharp exciton transition peaks up to E3h including forbidden transitions were obtained in single quantum wells. Such high quality lineshape has not been obtained in MBE- or OMCVD-grown wafers so far. The excitation spectra also show no evidence of band filling due to holes or electrons from the AlxGa1−xAs layers which is a common problem with the OMCVD technique. From this study, it is also shown that the GaAs and AlxGa1−xAs materials are of very high purity. The first GaAs/AlxGa1−xAs double-heterostructure lasers grown by CBE is also achieved. Very low averaged current threshold densities of ≌ 500 A/cm2 were obtained for wafers with active layer thicknesses of ∼ 500–1000 Åand confinement layers of Al0.5Ga0.5As. Such current threshold densities were similar to those obtained from the best wafers grown by other techniques. The present results unequivocally established that CBE is capable of producing high optical quality multi-layer heterostructures for state-of-the-art device applications.

Photoluminescence spectra from Al0.37Ga0.63As/AlAs multiple quantum well structures with staggered band alignments are presented which provide the first direct optical measure of the valence-band offset at a semiconductor heterojunction. The experiment takes advantage of the crossover occurring at a critical aluminum concentration above which the indirect X minima in the AlAs become the lowest energy conduction bands in the system, and recombination occurs across the interface. The resulting emission fixes the valence-band offset at ΔEv =342±4 meV for this structure.

Low-temperature excitation and photoluminescence spectra are described for single GaAs/Al0.37Ga0.63As quantum wells grown by molecular beam epitaxy with and without a 2-min interruption of growth at the heterointerfaces. The spectra from samples grown with interruption include well-resolved multiple sharp peaks which are due to changes in well thickness of one monolayer and to bound excitons. These peaks are as narrow as 1.0, 1.7, and 6.0 meV for single wells of width 57, 28, and 17 Å, respectively.