Dislocation nucleation near the critical thickness in GeSi/Si strained layers

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Abstract
A theoretical and experimental study is presented of dislocation nucleation processes in the initial stages of coherency breakdown in GeSi/Si(100) strained layers. It is shown that the use of dislocation core parameters appropriate to semiconductors leads to far higher predictions than in previous studies for the activation energy for half-loop nucleation. Homogeneous nucleation is thus unlikely at low misfit in the absence of stress concentrations at large surface steps. Experimentally the actual nucleation processes in GeSi epilayers at low (less than 1%) misfit are deduced from the microstructure at the earliest stages of dislocation introduction. The first dislocations are shown to be 60[ddot] (a/2) half-loops on the inclined [111] planes. Detailed investigation shows that these can be attributed to the operation of a completely new type of (heterogeneous) regenerative nucleation source. This involves the dissociation of a pre-existing (a/6) stacking fault (the ‘diamond defect’) to emit one of several possible (a/2) dislocations on the appropriate glide plane. This heterogeneous mechanism may prove central to the breakdown of coherency in a number of low-misfit systems.