Strained epitaxial films are often unstable to morphological transformations. The classic example of this phenomenon is the SiGe system. A nonplanar morphology results in non-uniform strains in the system (Figure 1). These strain fields can modify the local chemical potential of a uniform alloy driving Ge enrichment in the regions of low compression.
Recent work has shown that annealed films of SiGe on Si{001} exhibit both vertical and lateral segregation during the transition from a metastable planar film to an array of ﹛100﹜ directed island. Vertical segregation is evidenced by an enrichment of Ge in the last few monolayers of the film while lateral segregation is seen as an enhancement of this surface layer in the region near the island peaks. This segregation is due to surface diffusion as the bulk diffusion lengths are too short. The case of annealed, metastable films can be compared to growing films.