Charged muonium diffusion in indium oxide and other transparent conducting oxides

Abstract

Transparent conducting oxides (TCO) are a class of semiconducting materials that are transparent to visible light, commonly used as transparent electrodes in semiconductor optical devices, such as LEDs and solar cells. Hydrogen is commonly used to passivate defects at the semiconductor – TCO interface to increase the transparency of the interface. Over time, H can diffuse away from the interface, causing interface defects to reactivate and reducing the transparency. The manner in which H diffuses in the TCO and semiconductor layers on either side of the interface is of interest. Muon Spin Relaxation (MuSR) is a useful experimental technique that uses positive muons as a light proton analog to determine how muonium (Mu = μ^++e^-) defects act in a material. The Mu defect is treated as a light isotope of H and the results for Mu can be extrapolated to H, with zero point energy differences properly accounted for. This PhD dissertation investigates Mu^+ motion in indium oxide (In2O3) and a series of other TCO materials. Zero external magnetic field (ZF) MuSR experiments were performed on the TCO samples to track the Mu^+ hop rates that are directly related to diffusion. The In2O3 ZF data show three Mu^+ states are present over the temperature range tested. Mu^+ motion that is interrupted by trapping at an unknown defect is treated with a two-state, trap and release model. The barrier for site-to-site hopping motion for the mobile Mu^+ state is found to be 0.75 eV, which compares well with the calculated H^+ migration barrier of 0.76 eV. In progress fits, initial analysis of results, and plans moving forward are discussed for ZF-MuSR measurements on CdO, SnO2, TiO2, Ga2O3, and ZnO.