Surface roughening of heteroepitaxial thin films

Heteroepitaxial structures with strained semiconductor thin films are widely used in electronic and optoelectronic devices. One of the more important defect creation processes in these films is related to a stress-induced morphological instability that tends to roughen the film surface by mass diffusion during film growth or annealing. Interestingly, the same mechanism of surface roughening can be utilized for fabrication of quantum dot devices. This article gives an overview of a series of theoretical and experimental studies on surface roughening in heteroepitaxial films. It is shown that the strain caused by lattice mismatch drives the diffusional atomic flux along the film surface in such a way that an initially flat film evolves into an undulating profile with cusp-like surface valleys with singular stress concentration near the cusp tip. The essential features of this evolution process are described by a family of mathematical curves called cycloids. The fundamental length and time scales associated with surface roughening can be obtained from thermodynamic and kinetic considerations. The stress concentration at cycloid-like surface valleys caused by roughening is found to create dislocations of various characters that participate in the overall strain relaxation of a heterostructure.