We have applied the many-body ab-initio diffusion quantum monte carlo (DMC) method to calculate the band gap of ZnO and to study the oxygen vacancy in this material. DMC calculations clearly rule out the oxygen vacancy as the source of the persistent n-type conductivity in ZnO. The DMC results were compared with Hartree-Fock, the Heyd-Scuseria-Ernzefhof (HSE) hybrid functional and other approximations of density functional theory (DFT). DMC predicts the band gap at 3.43(9) eV. DMC and HSE show that the thermodynamic transition levels of the oxygen vacancy are deep, between 1.8 and 2.5 eV from the valence-band maximum. The oxygen vacancy is unlikely to be the source of the persistent n-type conductivity in ZnO, confirming previous DFT calculations. Despite this agreement between DMC and HSE, we found two major differences: (i) the oxygen vacancy formation energy is about 1 eV higher in DMC than in HSE and other DFT approximations and (ii) DMC predicts a positive U behavior for the oxygen vacancy while HSE and other DFT approximations predict the opposite. These results are discussed in conjunction with recent experiments.
A preprint of this paper is available on the arXiv preprint server as arXiv:14026.3169.
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