Zinc Oxide as a Model Transparent Conducting Oxide: A Theoretical and Experimental Study of the Impact of Hydroxylation, Vacancies, Interstitials, and Extrinsic Doping on the Electronic Properties of the Polar ZnO (0002) Surface

The technology-relevant zinc-terminated zinc oxide (0002) polar surface has been studied at the density-functional theory level using both Perdew-Burke-Ernzerhof (PBE) and hybrid Heyd-Scuseria-Ernzerhof (HSE06) functionals. We have considered a number of surface conditions to better understand the impact of surface hydroxylation and intrinsic and extrinsic surface defects, including zinc vacancies, oxygen vacancies, zinc interstitials, and aluminum dopants on the surface electronic properties. Our calculations point to large variations in surface work function and energy band gap as a function of the surface model; these variations can be attributed to changes in surface charge carrier density and to additional surface states induced by the defects. The calculated shifts in O(1s) core-level binding energy of the surface oxygens in different bonding configurations are in good agreement with experimental X-ray photoelectron spectroscopy data and point to the presence of two distinct OH-species on the ZnO surface. Our results also show that the electron-compensation centers induced by zinc vacancies can be stabilized by intrinsic and/or extrinsic n-type doping near the surface; such n-type doping can lead to better performance of organic opto-electronic devices in which zinc oxide is used as an selective interlayer.