Mechanisms controlling growth quality of Si–Ge superlattices in different growth directions

Abstract

We apply an empirical potential-based approach to the study of growth quality of Si–Ge/Si superlattices in various directions. The particular emphasis is on understanding quantitatively the strain energetics at the interface. Recent experimental work by Kuan and Iyer show that the best growth morphology is obtained in the [100] directions, while twin formation substantially affects quality on [110] and [111] surfaces. We show that twins do not relax misfit strain, as originally thought, but that twin surfaces allow growth to switch over to energetically more favorable twin directions. We obtain the relative stability of strained Si–Ge superlattices for different orientations which are consistent with the predictions of harmonic elasticity theory. The results should also be applicable to III–V semiconductor superlattices. KI also showed that long-range ordering ocurred only on thick relaxed SiGe layers grown on [100] surfaces and not on thin strained-layer superlattices. We present some numerical results and discuss the mechanism for this phenomenon.