Orientation dependence of growth quality in strained-layer superlattices: A model-potential study of the Si-Ge system

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Abstract
Growth quality of Si-Ge/Si superlattices in various directions is studied using an empirical-potential-based approach. Kuan and Iyer found, experimentally, that growth morphology is best in the [100] direction while twin formation has substantial effects on growth in other directions. We show that twin formation does not relax misfit strain but instead allows growth to switch over to energetically favorable directions. We discuss the relative stability of strained Si-Ge superlattices for different orientations and the applicability of the results to III-V semiconductor superlattices. We also discuss experimental observations of long-range ordering which occur only in thick relaxed Si-Ge layers grown in the [100] direction and not in thin strained-layer superlattices grown in the same direction.