Muonium states and dynamics as a model for hydrogen in semiconductors

An atomistic picture of interstitial hydrogen in semiconductors involves, as it does for hydrogen in metals, determination of the lattice sites and configurations adopted and the mechanisms of diffusion. An additional dimension is added in semiconductors, namely the possibility of examining different charge states of the defect centre--positive, neutral and negative-respectively the diamagnetic proton, the paramagnetic hydrogen atom and diamagnetic hydride ion. Muon spectroscopy has been successful in modelling all these states, determining their local structure, their different stabilities, mobilities and interactions with charge carriers. In semiconductors doped heavily to metallic conductivities, only diamagnetic states are observed, though still with a rich variety of mobilities and trapping sites. Studies of the interaction and pairing with dopant atoms, simulating the important process of passivation, also appear promising. This short review is illustrated with recent results for the elemental semiconductors Si and Ge and the compounds GaAs and InP.