Ga interaction with ZnO surfaces: Diffusion and melt-back etching

[Display omitted] •Post-growth diffusion doping of ZnO whiskers with Ga.•ZnO pre-annealing in high vacuum promotes Ga interdiffusion.•In-situ XPS study of Ga diffusion into ZnO.•Gallium acts as a reactant causing selective ZnO etching at diffusion temperatures.•In-situ SEM observation of Ga melt-back etching of ZnO surfaces. Despite being technologically very attractive, highly-doped zinc oxide whiskers with precisely defined morphology and doping level are difficult to prepare. Here, as an advancing step towards this goal, we show that pre-annealing of ZnO in oxygen-poor conditions (e.g. high vacuum) at low temperature encourages a deeper diffusion of Ga into the ZnO crystal lattice in contrast to ZnO pre-annealed in oxygen-rich conditions. We also demonstrate that gallium acts as a reactant causing ZnO etching at diffusion temperatures, contrary to Al-based doping of ZnO systems. This behaviour, being similar to gallium melt-back etching during GaN epitaxy on silicon, has not been observed for ZnO so far and can represent a significant hurdle for the post-growth diffusion doping of ZnO nanostructures. The paper suggests possible ways how to diminish this effect.