Faceting control by the stoichiometry influence on the surface free energy of low-index bcc-In2O3 surfaces

The faceting of In2O3(0 0 1), (0 1 1), and (1 1 1) grown by plasma-assisted molecular beam epitaxy on yttria-stabilised zirconia was investigated under different growth conditions-conventionally used oxygen-rich growth conditions with and without heavy Sn-doping, and indium-rich growth conditions-by in situ reflection high-energy electron diffraction, scanning electron microscopy, x-ray diffraction, and atomic force microscopy. In a simple thermodynamic model that considers surface free energy only, the observed faceting is compared to recent theoretical predictions of the surface tension (also termed surface free energy) anisotropy and the related equilibrium crystal shape derived from a Wulff construction. These predictions and our comparison include the variation with growth-condition-dependent oxygen chemical potential. Our results demonstrate how the experimentally changed oxygen chemical potential controls the faceting or island shape of In2O3 by changing the surface tension anisotropy. While the experimental results largely agree with the theoretically derived surface tension anisotropy, they strongly suggest a lower relative surface tension of the (0 0 1) surface at lower oxygen chemical potential (In-rich growth conditions) than theoretically predicted or a significant surface entropy contribution.