Atomistic models of dislocation formation at crystal surface ledges in Si1-xGex/Si(100) heteroepitaxial thin films

Mechanisms of defect formation near surface ledges of a diamond cubic cryst al subjected to compressive strain parallel to the surface are investigated as precursory processes to dislocation nucleation in Si1-xGex/Si(100) hete roepitaxial thin films under surface diffusion conditions. This study is mo tivated by our preliminary calculations of dislocation formation at surface ledges in a model crystal characterized by the 6-12 Lennard-Jones interato mic potential, and by our controlled annealing experiments on evolution of a Si1-xGex/Si(100) film from an atomically flat, defect-free, surface morph ology to an undulating surface morphology with cusp-like surface features a nd dislocation formation at the cusp valley. When subjecting such films to high temperature anneals, we observed nucleation and growth of three types of dislocations: the 60 degrees glide dislocations, the 90 degrees Lomer-Co ttrell dislocations with stair rod Shockley partials and twinned wedge disc linations with twofold Sigma 9 coincidence boundaries between the wedge and matrix. The objective of this paper is to examine the sequence of atomisti c processes which lead to the formation of each of these three types of def ects, and in doing so we hope to foster a link between the continuum pictur e of dislocation nucleation in thin films and the quantum mechanical pictur e of the unstable collapse of surface ledges due to compressive strain. Whi le we do not actually perform quantum mechanical calculations in this paper , attempts will be made to identify the critical problems that need to be a ddressed at the quantum mechanics level. Although the present study should be of general interest in the study of dislocation formation near crystal s urface ledges, we will confine our discussion to the problem of dislocation formation during stress driven mass transport via surface diffusion in het eroepitaxial thin films which are morphologically unstable under cycloid-li ke variations in surface shape. The following process is envisioned as undu lations form in the film surface. As the curvature at the root of a surface 'valley' increases, the local elastic strain magnitude rises as a result o f the local geometrical magnification of stress. The strain continues to in crease in magnitude as the root sharpens, until atomic ledges collapse to f orm dislocations. Due to the resulting stress relief, subsequent mass trans port reverses its direction of flow and causes such dislocations to be trap ped as bulk defects as the film surface moves away from the nucleation site s. Through this mechanism, dislocations can be nucleated without glide by s urface trapping in films with even a modest level of nominal compressive mi smatch strain.