Dendritic growth of amorphous gallium oxide in mixed GaOx/WOx thin films

The structure and morphology of mixed GaOx/WOx thin film systems prepared by either sequential or simultaneous oxide deposition has been studied by Analytical (High-Resolution) Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). GaOx and WOx were prepared by thermal evaporation of the corresponding oxides (β-Ga2O3 and WO3, respectively) in 10-2 Pa oxygen onto a NaCl substrate, which was held at temperatures between 298 and 580 K. The film morphology of differently prepared mixed GaOx/WOx films not only differs significantly from those of the pure oxides, but also strongly depends on the preparation conditions and the order of deposition. Upon co-deposition of GaOx/WOx films onto NaCl(0 0 1) at temperatures above 580 K a gallia morphology was observed, which is strikingly different from hitherto reported pure GaOx films, i.e., growth of small-diameter (20-25 nm) sub-stoichiometric gallia dendrites. The dendritic growth is apparently mediated by the GaOx/NaCl(0 0 1) contact area rather than being an intrinsic feature of the three-dimensional mixed-oxide bulk. In particular, neither sequentially deposited GaOx/WOx films nor co-deposited films on top of a pre-deposited WOx interlayer exhibited this peculiar morphology.