Mechanisms of Epitaxial Growth of Polar Semiconductors on (001) Silicon
During the last few years, the use of a two-step process in the growth of compound semiconductors on (001) silicon substrates, by chemical vapor deposition or molecular beam epitaxy, has been found to produce films which are uniformly single crystalline with good epitaxial relationship to the substrate. In the first step, a so-called buffer layer is grown at a relatively low temperature to form a continuous film on the substrate and, subsequently, the substrate temperature is increased and growth continued at a higher deposition rate. This is in contrast to the one-step process where the substrate is heated to the growth temperature and deposition carried out in one go. Using the latter technique of growth, the morphology of the resulting film is often poor and it is sometimes a polycrystalline aggregate. The epilayers grown on an (001) Si substrate contain a variety of lattice defects. The formation of these defects is often attributed to stresses resulting from mismatch in lattice parameters (“coherency strains”), or mismatch in coefficients of thermal expansion giving rise to thermal stresses in the cooling stage following the growth. However, the literature shows a number of unexpected similarities in the defect configurations and densities of most epitaxial systems with varying degrees of lattice or thermal mismatch. In this paper, the one-step and two-step growth techniques are discussed in terms of homogeneous and heterogeneous surface nucleation. The reason for the different microstructures obtained by the two techniques are attributed to these two types of nucleation. Subsequently, the formation of various line defects is briefly reviewed. The characteristics of planar faults are then summarized, and a recently proposed mechanism for their formation which is based on deposition errors in the early stages of heteroepitaxial growth is described.
KeywordsBurger Vector Heterogeneous Nucleation Homogeneous Nucleation Critical Thickness Misfit Dislocation
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