The Department of Energy’s Council on Materials Science convened a Panel charged with assessing the present scientific understanding of epitaxial growth and identifying fruitful research opportunities in this area. The Panel, chaired by Paul S. Peercy, a member of the Department of Energy’s Council on Materials Science and of the Solid State Sciences Committee, was composed of scientists in materials science, physics, and chemistry from academia, government labs and industry. Panel members were: Ernst G. Bauer, Brian W. Dodson, Daniel J. Ehrlich, Leonard C. Feldman, C. Peter Flynn, Michael W. Geis, James P. Harbison, Richard J. Matyi, Pierre M. Petroff, Paul S. Peercy, Julia M. Phillips, Gerald B. Stringfellow and Andrew Zangwill. The Panel met in January, 1989; its activities were supported by the Materials Sciences Division of Basic Energy Sciences. Deliberations emphasized artificially structured materials and resulted in a Panel Report which has been submitted to the Journal of Materials Research. With permission from the Department of Energy, this article excerpts from the report.

We present a simple example of the time evolution of temperature, excess stress and strain relaxation during growth and subsequent annealing of strained heterostructures.

We discuss the kinetic barriers to misfit dislocation nucleation, propagation and interaction in lattice-mismatched GexSi1-x/Si epitaxy. Experimental real-time observations of the strain relaxation process via in-situ annealing experiments in a transmission electron microscope enable each of these processes to be separately studied. Quantitative parameters defining misfit dislocation processes may be derived; these are found to be highly dependent upon the structure geometry. The approximations involved in extending these measurements to a description of the relaxation process during growth are described in detail.

The molecular beam epitaxial growth of strained InGaAs films grown on GaAs(100) substrates has been studied using in situ reflection high-energy electron diffraction (RHEED). Both the intensity, shape and position of the diffracted beams were monitored during growth. Growth was found to be layer-by-layer up to a strain dependent thickness, at which point three-dimensional clusters were formed. These clusters exhibited (114) facets and were elongated in the [011] direction. The onset of 3D cluster formation was simultaneous with measurable lattice relaxation. The relaxation was determined using electromagnetic deflection of the RHEED pattern across two detectors. With this arrangement, the lattice constant could be measured to within 0.003Å. The onset could be delayed by lowering the growth temperature. For misfit strain greater than about 2%, the onset occurs at thicknesses less than the Matthews-Blakeslee critical thickness. For smaller strains, the onset occurs at larger thicknesses.

The glide of a threading dislocation in a strained layer may be impeded as it encounters interface misfit dislocations on intersecting glide planes. An estimate of the change in driving force on the threading dislocation during this interaction is discussed within the framework of elastic dislocation theory.

A thermodynamic model is presented which predicts a significant sample size effect on the elastic moduli of superlattice thin films possessing small bilayer repeat lengths. Owing to the presence of incoherent (incommensurate) interfacial stresses, biaxial in-plane strains are created which are approximately inversely proportional to the bilayer repeat length Λ. When Λ is of the order of 1 nm, these strains can be of the order of 1%, inducing nonlinear elastic behavior. This model is able to explain in a quantitative way the existence of a “supermodulus” effect in certain metallic superlattices.

We have investigated the molecular beam epitaxial growth of GexSi1-x on small growth areas patterned in Si substrates. Electron beam induced current, etch-pit density measurements, transmission electron microscopy, and photoluminescence were used to compare dislocation densities in GexSi1-x on patterned and unpattemed substrates. We find a dramatic reduction in both misfit and threading dislocation densities for the patterned substrate growth. Our results also show that dislocation introduction is dominated by heterogeneous nucleation.

In the study of elastic strain relaxation in semiconductor heterostructures, a number of misfit dislocation generation mechanisms have been suggested to account for the high interfacial dislocation density observed in these almost defect-free crystals. Several MBE-grown GexSi1-x/Si heterostructures, both in the as-grown and annealed condition have been studied using transmission electron microscopy. The results indicate that some of the popular theories of dislocation generation are less important or not applicable based on both theoretical and experimental considerations. Specifically, it will be shown that: (i) heterogeneous sources play a dominant role in the nucleation mechanisms, (ii) the strain relaxation behaviour during MBE growth may be different from that observed in metastable structures annealed after growth and (iii) the Hagen-S trunk multiplication mechanism is inoperative under most conditions in this system.

We have observed that the nature of misfit dislocations introduced near the critical thickness in GexSi1-x alloys on (001)Si changes markedly in the region 0.4 ≤ x ≤ 0.5. At or below the lower end of this compositional range, the observed microstructure is comprised almost entirely of 60° type dislocations, while at the high end, the dislocation structure is almost entirely Lomer edge type. Concurrent with this change, the dislocation density at the top of the epilayer varies by a factor of about 60X. Similarly, several other observables (e.g. dislocation length and spacing) also change appreciably.

Part of the reason for the morphological variation seems to be a change in the source for dislocation introduction, in conjunction with a change in glide behaviour of dislocations as a function of film thickness. Evidence will be presented that indicates strain, as well as thickness, has a critical value for some dislocation introduction mechanisms, and that these together determine the resulting microstructure.

Furthermore, it appears unlikely that the edge-type Lomer dislocations which appear at about x = 0.5 are either introduced directly, by climb, or grown in, as in the three-dimensional island growth and coalescence which occurs when x approaches unity. Instead, a two-step mechanism involving glissile dislocations is proposed and discussed.

Annealing experiments in the 450-1000 °C temperature range on various MBE grown Si1-xGex/(100)Si heterostructures have revealed an abrupt thermally activated transition from coherently strained to partially relaxed microstructures. Misfit dislocations at densities 1 to 104 cm-2 have been monitored using Nomarski interference microscopy of defect etched surfaces and charge collection microscopy (EBIC) of Si1-xGex/Si heterojunctions in the scanning electron microscope. The kinetics of misfit relaxation by dislocation glide can be characterized by a relaxation temperature which for 30 minute furnace anneals was ~500 °C for an uncapped Si0.83Ge0.17 stramed layer, and ~600 °C for a similar buried strained layer typical of heterojunction bipolar transistor geometries. Rapid thermal anneals (5s) delayed the onset of relaxation to higher temperatures (~ 200 °C increase) and allowed misfit dislocation velocities to be determined for the temperature range 500-900 °C. The activation energy for dislocation glide was found to be 2.2±0.1eV for a Si0.9Ge0.1/Si strained layer superlattice, 1.6±0.2eV for a Si capped Si0.88Ge0.12 layer and 1.6±0.2eV for a Si0.83Ge0.17 uncapped strained layer.

Plastic flow is driven by excess stress, which can be calculated from lattice strain and dislocation line tension. In order to better understand and model plastic flow, we have derived an expression for the dislocation line tension in thin film structures for double kink (dislocation dipole) extension of a threading dislocation. From these calculations, we conclude that the presence of a free surface (i.e. the vacuum/solid interface) significantly reduces the line tension for an extended double kink. For the specific case of a 50nm thick Si0.7 Ge0.3 layer capped with lum of silicon, we find that the line tension is approximately 30% less than the estimate of an analogous model that neglects the influence of a free surface. Therefore, in order to obtain an accurate estimate of the excess stress for a double kink, one must allow for the influence of a free surface.

The thermal stability of Si/Si0.85Ge0.15/Si p-type modulation doped double heterostructures grown by the Ultra High Vacuum/ Chemical Vapor Deposition technique has been examined by Hall measurement, transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy. As deposited heterostructures showed two-dimensional hole gas formation at the abrupt Si/SiGe and SiGe/Si interfaces. Annealing at 800 °C. for 1 hr. caused the diffusion of boron acceptors to the heterointerfaces, degrading the hole mobilities observed in the two dimensional hole gas. Rapid redistribution of boron, causing a loss of the 2 dimensional carrier behavior, was observed after a 900 °C, 0.5 hr. anneal. Neither Ge interdiffusion nor the generation of misfit dislocations were observed in the annealed heterostructures, evincing the defect-free crystal quality of these as-grown strained heteroepitaxial layers. The superior stability of these heterostructures have strong positive implications for Si:Ge heterojunction devices.

We report the MBE growth of various (SimGen)p atomic layer superlattices (ALS) and their characterization by Raman scattering spectroscopy, x-ray diffraction and photoluminescence. The structural properties of ALS prepared on (100) Si, (100) Ge and on various Si1-xGex (0.5<×<1) buffers were compared. Phonon peaks due to folding of acoustic modes were seen by Raman scattering spectroscopy in the frequency range 15-250 cm-1. The observed Raman spectra from the ALS were interpreted on the basis of a theoretical analysis of these systems. The study provided an estimation of the interfacial sharpness of the ALS. The photoluminescence investigation on annealed specimens revealed features between 800 and 900 meV that were ascribed to known dislocation lines in Si. No strong luminescent signal that could be unambiguously related to a direct bandgap behavior was detected.

Molecular beam epitaxy was used to grow Sim Gen superlattices on relaxed Si1-xGex buffer layers which symmetrize the strains between the heteroepitaxial layers. Samples with different superlattïce periodicities and individual layer thickness ratios were prepared. The compositions and defect structures of the GexSi1-x buffers have significant influence on the homogeneity and quality of the overlying superlattices. In particular, greater disorder was found in superlattice structures grown on Si0.5 Ge0.5 buffers than for those grown on buffer layers with significantly higher or lower Ge contents.

Double epilayers of different compositions of GexSi1-x on (001)Si are observed to have dislocation contents which differ markedly from similar single epilayers. An initial epilayer, grown below its critical thickness, underwent substantial misfit dislocation introduction, while a second epilayer, grown at a composition where edge-type misfit dislocations are normally observed to dominate the morphology, contained mostly 60° type dislocations. It is suggested that dislocation entry into the upper, high mismatch epilayer allows many dislocations to enter the buried, low mismatch epilayer, and that this in turn affects the dislocation morphology in the upper layer through strain relief.

Using the nanometer probe available in the dedicated scanning transmission electron microscope (STEM) local structural information can be obtained from individual layers in [100] grown Si-Si1-xGex multilayer structures. Furthermore the small probe size enables cleaved specimens with their very large wedge angles to be analyzed in cross-section. Diffraction patterns are shown from multilayers of varying periodicity. Analysis of the patterns concentrates on the higher order Laue zone (holz) reflections in the high angle excess ring . The behaviour of the excess holz reflections indicates the transition from a strained layer superiattice to a dislocated structure as the thickness of the layers increases for a given composition.

We have made a study of GaAs/InGaAs/GaAs (001) strained layer heterostructures using Transmission Electron Microscopy (TEM) as a structural tool to determine the misfit dislocation structure and density as a function of Indium concentration. The average misfit dislocation spacing varies from > 10 µm for x < 0.3, to a few microns at x = 0.3, and drops to a few hundred Angstroms at x = 0.5. We did in-situ annealing experiments in order to study the strain relaxation process, measuring the temperature at which the structure begins to relax, and the dislocation velocities. Dislocation velocities are a few microns per second at the growth temperature of 450 ° C, and tens of microns per second at 690 ° C. In addition to interfacial dislocations in the usual <110> directions, in samples where x ≥ 0.4, we observed dislocations running in <100> directions. A study of the electrical characteristics of the material was made in parallel with the structural measurements: the mobility of the InGaAs layer was measured, the material was processed into Heterojunction Bipolar Transistors (HBT’s) and the gain was measured. The electrical characteristics initially improved with the addition of In, peaking at x = 0.1 and dropping sharply for higher x.

In this paper we report on the kinetics of strain relaxation in GaAs/InxGa1-xAs/GaAs/AlAs (0.05<x<0.22) layers grown by MBE on GaAs at 520°C. We have characterized the density of dislocations present due to strain relaxation during both film growth and processing by using a large area thinning technique which enables the observation of approximately 2 mm2 areas by plan-view TEM. The thickness of the InxGa1-xAs layers studied was 36.4 nm and four compositions were chosen so that the critical thickness predicted by strain energy considerations was exceeded. Due, however, to sluggish dislocation nucleation and glide kinetics at the deposition temperature, the as-grown misfit dislocation densities were well below the predicted level for fully relaxed films. We have studied the rate at which these metastable strained films relax as a function of post-growth annealing time and temperature.

The correlation between surface cross-hatched morphology and interfacial misfit dislocations in strained III-V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520°C) and with lattice mismatch f < 2%. A poorly defined cross-hatched morphology is found on layers grown at low temperature (400°C). For f > 2%, a rough textured surface morphology is observed in place of cross hatching. Few threading dislocations are observed in the strained layer when cross hatch develops. Cross hatch occurs after most interfacial misfit dislocations are generated. The results suggest that surface cross hatch is directly related to the generation and glide of interfacial misfit dislocations.

We present a study of the structural stability of InGaAs/GaAs strained single quantum wells (SQW) grown with a variety of indium compositions and with well widths close to critical thickness values. The samples were grown by molecular beam epitaxy and were subjected to furnace annealing as well as ion implantation followed by rapid thermal annealing. Changes in low temperature photoluminescence linewidths were used to evaluate the stability of strained SQWs. We observed both strain relief, in wide SQWs and strain recovery, in higher indium composition narrow quantum wells which were partially relaxed (low dislocation density) as-grown.