Amphoteric charge states and diffusion barriers of hydrogen in GaAs

The states and microscopic diffusion mechanisms of hydrogen in crystalline GaAs are investigated by the ab initio molecular-dynamics method. The static equilibrium position for neutral hydrogen is found around the bond-centered (BC) site as in crystalline Si. As a negative U center, the hydrogen behaves as a stable donor around the bond-centered site in the p-type material, while it acts as a stable acceptor around the interstitial site surrounded by four nearest Ca host atoms in the n-type material. The hydrogen cation diffuses along the BC-C-Ga-BC-C-As-BC path in a high valence electron density region with a barrier of 0.46 eV in p-type materials. On the other hand, the hydrogen anion diffuses along the T-Ga-Hex-T-As-Hex-T-Ga path in a low electron density region with a barrier of 0.55 eV in the n-type material. The diffusion is more effective in p-type GaAs. Good agreement with experiments is found. Based on these results, the amphoteric behavior of hydrogen, its passivation effects of dopants, and its diffusion mechanisms can be understood in both p-type and n-type GaAs.