Heteroepitaxial Layer Morphology and Misfit Defect Formation
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INTRODUCTION The demands of modern electronic device fabrication are placing increasing importance upon the growth of semiconductor heteroepitaxial layers. Although low strain hetero-systems such as AIGaAs/GaAs remain the basis of many device structures, there is a marked trend also to use more highly strained combinations such as InGaAs/GaAs and SiGe/Si. However, the growth of the latter epitaxial systems must be approached with great care in order to achieve the optimum layer structural quality. Of course, for any given alloy layer composition, interfacial misfit defects in general will be introduced when the layer thickness exceeds a critical value, as originally described by Frank and Van der MerweI and Jesser and Matthews 2 . In addition, the morphology of such strained layers must be given very careful attention. It is the purpose of the present article to examine our current understanding in this latter area. When any particular epitaxial layer is deposited, initially it might be thought that a flat surface would result under ideal growth conditions, when internal defects have been eliminated. This would be expected to minimise the surface step density and, hence, the surface energy. However, whilst for homoepitaxial layers in the absence of kinetic effects this expectation virtually can be realised, the presence of strain in an heteroepitaxial system can severely affect the observed layer morphology.
THEORETICAL BACKGROUND The first theoretical treatment of morphological instability driven by stress in solids was presented by Asaro and Tiller 3 and related to stress corrosion cracking. Description of the stressdriven instability relevant to a range of other circumstances has been set out subsequently4- 9 and, in particular, formulation in the context of epitaxially-strained solid films has been provided 1°' 7 . At a phenomenological level, the driving force for the formation of non-planar surfaces may be understood as follows. Figure 1 shows in diagrammatic form a film compressively 303 Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
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Fig. I Diagram showing elastic distortion of vertical lattice planes in a morphologically undulating heteroepitaxial layer under compressive stress upon its substrate. strained upon a substrate, where surface distortions are present as undulations with relatively rounded peaks and groove- or cusp-like troughs: as will be seen below, this type of morphology can be formed in many experimental situations. The vertically-aligned lattice planes are also shown and it is immediately clear that the unconstrained lateral edges of the surface mounds allow these planes to dilate by lateral expansion. There is then partial elastic stress-relief of the epitaxial material within each mound and this can be quantitatively modelled using finite element analysis18 . However, a consequence of this relaxation is that there is a complementary additional compression of the lattice planes at the locations of the surface grooves. Nevertheless, this latter compress
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