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 E-A=0.17 and 0.37 eV. The change of E-A is accompanied by a change of the diffusion prefactor by eight orders of magnitude. This indicates that E-A 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, mu(H), was estimated. With increasing temperature, mu(H) decreases with a rate of approximate to 0.0013 eV/K. At H concentrations of less than 10(17) cm(-3) mu(H) is pinned. The hydrogen density of states was derived from H effusion data, which is consistent with a diffusion activation of about 1.0 eV 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.
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