Formation of Nanocrystal Islands During Heteroepitaxial Growth

Growth of nanocrystal islands by heteroepitaxy techniques has attracted the attention of many researchers recently due to the potential applications of the nano-structures. The islands are generally fabricated under the Stranski-Krastanow (SK) growth mode where a film first grows layer by layer on a substrate and then evolves into three-dimensional islands/structures. It was commonly observed that the morphology of the three-dimensional structures as well as the kinetic pathways of the formation process could vary significantly even for similar material systems and growth conditions. The causes of the differences, however, are still an open question, indicating limited understanding of the formation of islands during the SK growth. In this paper we present our theoretical investigation on this issue, based on a continuum model for the SK heteroepitaxial systems. Of particular interest here are the effects of the energetic forces in the systems on the island formation process. The energetic forces include the mismatch strain between the film and the substrate, the film surface energy, and the film-substrate interaction. The effects are determined by energy analyses for the islands and by three-dimensional simulation for the morphological evolution during the SK growth. The results are compared with the experimental findings, revealing possible explanations for the different island formation processes in the experiments.