Effect of Mesa Shape on Threading Dislocation Density in Ge Epitaxial Layers on Si after Post-Growth Annealing

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The effect of mesa shape on threading dislocation density in Ge epilayers on Si is studied. It is found from the etch-pit counting on the surfaces that the post-growth annealing reduces the threading dislocation densities in Ge mesa structures on Si and in the blanket Ge layers. The dry-etched mesas with vertical sidewalls show dislocation densities identical to those for the blanket layers. This indicates that the reduction in threading dislocation density in the dry-etched mesas is predominantly due to the annihilation of dislocations with opposite Burgers vectors, while the motion of dislocations to the mesa edges does not contribute. On the other hand, the selective-grown mesas with inclined (311) facet sidewalls show lower dislocation densities than those for the blanket layers, indicating that the dislocations can glide out beyond the mesa edges. Since the shear stress thermally induced in Ge is responsible for the dislocation glide motion, the present result suggests that the selective-grown mesas should be different from the dry-etched mesas in terms of the shear stress accumulated during the annealing. Strain simulations for these mesa structures show that the strength and distribution of the thermally induced shear stress are quite different near the top edges between the mesa structures. We discuss the relationship between the distribution of thermal stress and the reduction in dislocation density.
2010-04-25