Atomic Structure and Diffusion of Hydrogen in ZnO

Using first-principles theoretical calculations, we investigate the atomic structure and the diffusion pathway and barrier of hydrogen in ZnO. We find that an interstitial H is energetically more stable at a bond-centered position than at an anti-bonding site of an O atom. We examine the stable geometry of a defect complex consisting of hydrogen and a Zn vacancy and find that the O-H bond is oriented along the c-axis in the wurtzite structure. The energy barrier for the migration of an interstitial H is around 0.4 - 0.5 eV while that for a substitutional H at an O site is about 1.7 eV, indicating that the substitutional H is thermally more stable. Kinetic Monte Carlo simulations confirm that the substitutional H diffuses Out at annealing temperatures higher than that for the interstitial H.