First-principles calculations were made of the properties of atomic and molecular H in pure bulk material. The results indicated that the H penetrated in atomic form. Within the sample, atomic H tended to form H2 molecules on tetrahedral sites. These were deep energy wells for H2. The H2* defect, which consisted of a H atom in a bond-center site and a H atom in an adjacent tetrahedral position, had a higher energy than H2 but was a lower-energy barrier to diffusion. Isolated H could be present as a metastable species. The stable charge state of isolated H was calculated as a function of the Fermi energy. The results suggested that H behaved as a negative-U defect. Consequently, isolated H was expected to be present only as a charged species (positively charged in p-doped samples, negatively charged in undoped and n-doped samples). The conclusions were compared with experimental results and with the results of calculations for H in other semiconductors. The main features of H behavior in GaAs were quite similar to those for Si.

L.Pavesi, P.Giannozzi: Physical Review B, 1992, 46[8], 4621-9