Hydrogen migration in single crystal and polycrystalline zinc oxide

Hydrogen diffusion in single crystal and polycrystalline zinc oxide was investigated by deuterium diffusion and hydrogen effusion experiments. Deuterium concentration depth profiles were measured as a function of the passivation temperature, while in H effusion experiments the molecular hydrogen flux was measured as a function of the heating rate. The diffusion coefficient exhibits thermally activated behavior and varies between EA=0.17 and 0.370.3em0exeV. The change of EA is accompanied by a change of the diffusion prefactor by eight orders of magnitude. This indicates that EA is not related to the energetic position of H transport sites or the barrier height between such sites. Using the microscopic diffusion prefactor, the position of the hydrogen chemical potential, μH, was estimated. With increasing temperature, μH decreases with a rate of ≈0.00130.3em0exeV∕K. At H concentrations of less than 10¹⁷0.3em0excm^-3 μH is pinned. The hydrogen density of states was derived from H effusion data, which is consistent with a diffusion activation of about 1.00.3em0exeV as was originally reported by Mollwo [Z. Phys. 138, 478 (1954)] and Thomas and Lander [J. Chem. Phys. 25, 1136 (1956)]. Clear evidence for hydrogen deep traps was found in single crystal and polycrystalline ZnO.