Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (E _F ) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an E _F of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.