Growth and shape transition of small silicon self-interstitial clusters

The growth behavior of small self-interstitial clusters in crystalline Si is presented based on extensive combined Metropolis Monte Carlo, tight-binding molecular dynamics, and density-functional theory calculations. New stable structures for small interstitial clusters (I_n,5≤n≤16) are determined, showing that the compact geometry appears favored when the cluster size is smaller than 10 atoms (n<10). The fourfold-coordinated dodecainterstitial (I₁₂) structure with C_2h symmetry is identified to serve as an effective nucleation center for larger extended defects. This work provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around n=10.