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Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free-energy minimum surface of the epilayer is not atomically flat, but three-dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high-energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.
The correlation between the surface crosshatched morphology and the 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 those with small lattice-mismatched (f<2%) systems. A poorly defined crosshatched morphology was found on layers grown at relatively low temperature (400 °C). As the lattice mismatch of the strained layer becomes larger than 2%, a roughly textured surface morphology is commonly observed in place of actual cross-hatching. Few threading dislocations are observed in the strained layer when the crosshatched pattern develops. It is also noted that the surface crosshatched pattern develops after the majority of the interfacial misfit dislocations are generated. The result suggests that the surface crosshatch pattern is directly related to the generation of interfacial misfit dislocations through glide processes.
The formation, interaction, and propagation of misfit dislocations in molecular-beam epitaxial InGaAs/GaAs heterointerfaces have been studied by transmission electron microscopy. With the lattice mismatch less than 2%, most of the interfacial dislocations are found to be 60° mixed dislocations introduced by glide processes. Sessile edge-type dislocations can also originate from the combination of two 60° mixed dislocations. The ratio of densities of edge dislocations to 60° dislocations was increased during the later part of the elastic strain relaxation. These sessile edge dislocations may be generated in appreciable numbers through a climb process. For large lattice-mismatched systems, the majority of the misfit dislocations are pure edge dislocations and high threading dislocation density is generally found. The interfacial dislocation network is found to contain regions of dislocations with the same Burgers vector that extend over several micrometers. The results support a mechanism that involves misfit dislocation multiplication during the molecular-beam epitaxial growth process.
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