Understanding ZnO surface defects from first-principles simulation

The single native defects on ZnO surfaces have been experimentally found to play an essential role in different applications of ZnO nanostructures. In this work, by means of first-principles density functional theory combined with atomistic thermodynamics and the nudged-elastic band method, we investigated the electronic properties, stability under the temperatures and pressures, and migration mechanism of the six different point defects on the non-polar (1010) and (1120) ZnO surfaces. Our results elucidate the defective energy states formed within the band gap of the ZnO surfaces, their orbital properties, and the defect-induced local surface magnetic moments. The former quantity is shown to be in good agreement with the available experimental photoluminescence measurements, reflecting this work's highly accurate theoretical method. The thermodynamic phase diagrams of the pristine surfaces against the formation of the defects demonstrate that under the thermodynamic equilibrium condition with the oxygen reservoir, the most stable surface structures are the free-defect structures, consistent with the scanning tunneling microscopy measurement, while the oxygen-vacancy defect surfaces are only stable at the lower oxygen chemical potentials. Furthermore, we simulate the energy barriers of the different defect migration mechanisms, which describe how the defects are suppressed to form pristine surfaces.