Effects of Stress on Step Energies and Surface Roughness

Strain relaxation in lattice-mismatched, heteroepitaxial systems is one of the classic problems in materials physics, which has gained new urgency with the increased applications of strained layers in microelectronic systems. In general both the structure and the integrity of the thin films are strongly influenced by strain. For instance it has long been known that under strain, the growth changes from an initial layer-by-layer growth mode to one with three-dimensional islanding. In the seminal works of van der Merwe, and Matthews and Blakeslee, this change in growth mode is explained in terms of the introduction of strain-relieving misfit dislocations, which appear when the film has reached some critical thickness. Recently it has become clear that this change in growth mode can take place even without the introduction of misfit dislocations. Such dislocation-free coherent islanding, or “roughening,” has been observed experimentally both in Ge/Si and in InGaAs/GaAs systems. Furthermore recent experiments show that in Ge/Si(100) systems, the thin films display a curious asymmetry with respect to the sign of the strain: Films under compression roughen by forming coherent islands while those under tension remain relatively smooth. A possible mechanism behind this strain-induced type of roughening is the subject of this article.