Modification of near-surface layers of alpha-gallium oxide under irradiation with ultra-high ion doses

A detailed study of $\alpha$-Ga$_2$O$_3$ epitaxial layers grown by halide vapor phase epitaxy has been carried out both before and after P and Ta and molecular PF$_4$ ion bombardment to doses as high as 45 dpa. A wide range of complementary analytical techniques were used: X-ray diffraction, atomic force microscopy, Rutherford backscattering spectrometry in channeling mode and X-ray photoelectron spectroscopy. Completely amorphous layer is formed from the surface to the crystal bulk as a result of irradiation. Monatomic P and Ta ions form thicker amorphous layer than molecular PF$_4$ ions. Small step appears at the border between virgin and irradiated areas of the sample revealing sample swelling ($\sim$7 nm after irradiation to 45 dpa). The root mean square roughness of the $\alpha$-Ga$_2$O$_3$ surface remains approximately unchanged (0.7 to 0.5 nm) after irradiation to a dose up to 45 dpa, regardless of the ion kind. The smoothing of small-scale topography (reflecting atomic steps) due to irradiation-induced amorphization of the sub-surface layer is found, whereas the large-scale topography remains virtually unchanged. Ion bombardment leads to surface layer decomposition with oxygen loss and partial reduction of gallium to Ga$^0$ and Ga$^+$ states.