Stabilizing reconstruction induced by O protrusions of the ZnO (0001) polar surface

The stability of the ZnO polar surfaces which is crucial for their applications is still being debated intensely. Here, we demonstrate that O extrusion outside the outermost bilayer is a universal reconstruction behavior of the Zn-terminated (0001) surface (with or without terraces) to compensate the well-known instability of such polar surfaces. The study is based on first-principles energetics of several atomic reconstruction configurations that emerge spontaneously in ab initio molecular dynamics simulations, including on-site O extrusions, O interstitials and O extrusions as adatoms at hollow sites, compared with previously proposed O-rich compensation models with O adatoms and Zn vacancies. The O extrusion process is specifically favored for Zn-rich environments, which provides a theoretical support for investigating the defect-process of a ZnO polar surface.