Variation of dislocation morphology with strain in Ge_xSi_1−x epilayers on (100)Si

Abstract

A change in microstructure, including dislocation Burgers vector, length, and behavior, has been observed to occur when the epilayer mismatch is varied in Ge_xSi_1−x layers grown on (100) Si. At low mismatches (<1.5%), there is an orthogonal array of very long 60° misfit dislocations. At higher mismatches (>2.3%) there is an orthogonal array of short edge dislocations. At intermediate mismatches (1.5 to 2.3%) there is a mixture of 60° and edge dislocations. The nature of the microstructure has a pronounced effect on the density of threading dislocations in the epilayer, which increase by a factor of ∼60× through a relatively small range of mismatch (1.7 to 2.1%, corresponding to x ranging from 0.4 to 0.5). These morphologies are discussed in the light of recent work on the sources of misfit dislocations. While mechanisms for the introduction and propagation of dislocations at low mismatch have recently been observed and explained, the high misfit case is clearly very different; i.e., surface nucleation seems to be likely in the latter case as opposed to operation of an internal source in the former. A mechanism for edge dislocation formation is proposed.