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 stati c equilibrium position for neutral hydrogen is found around the bond-center ed (BC) site as in crystalline Si. As a negative U center, the hydrogen beh aves 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 surrounde d 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 dens ity 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 lo w 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 experi ments is found. Based on these results, the amphoteric behavior of hydrogen , its passivation effects of dopants, and its diffusion mechanisms can be u nderstood in both p-type and n-type GaAs.