Initial Study of Positively Charged Muonium Motion in ZnO, CdO, TiO2, and SnO2

Transparent conducting oxide materials ZnO, CdO, TiO2 and SnO2 are being studied to determine the motional characteristics of diamagnetic muonium. These materials are being studied with an eye for future applications in solar cells, LEDs, and other transparent electrode applications. Theoretical studies show that H+ is the only stable form of hydrogen seen in these materials; therefore we expect to only observe a Mu+ state except at the very lowest temperatures. Zero field muon spin depolarization measurements have been performed to determine Mu+ motional characteristics in these samples; with the aim of addressing a controversy regarding the diffusion barrier for hydrogen in ZnO. The results of initial data fits are discussed and interpreted in light of the data on H diffusion. We observe ionization of shallow donor Mu states at low temperatures, confirming earlier observations that are consistent with hydrogen as an n-type dopant in these compounds.