The competing oxide and sub-oxide formation in metal-oxide molecular beam epitaxy

摘要

The hetero-epitaxial growth of the n-type semiconducting oxides β-Ga_2O_3, In_2O_3, and SnO_2 on c-and r-plane sapphire was performed by plasma-assisted molecular beam epitaxy. The growth-rate and desorbing flux from the substrate were measured in-situ under various oxygen to metal ratios by laser reflectometry and quadrupole mass spectrometry, respectively. These measurements clarified the role of volatile sub-oxide formation (Ga_2O, In_2O, and SnO) during growth, the sub-oxide stoichiometry, and the efficiency of oxide formation for the three oxides. As a result, the formation of the sub-oxides decreased the growth-rate under metal-rich growth conditions and resulted in etching of the oxide film by supplying only metal flux. The flux ratio for the exclusive formation of the sub-oxide (e.g., the p-type semiconductor SnO) was determined, and the efficiency of oxide formation was found to be the highest for SnO_2, somewhat lower for In_2O_3, and the lowest for Ga_2O_3. Our findings can be generalized to further oxides that possess related sub-oxides.