Low Thermal Budget Heteroepitaxial Gallium Oxide Thin Films Enabled by Atomic Layer Deposition

This work explores the applicability of atomic layer deposition (ALD) in producing highly oriented crystalline gallium oxide films on foreign substrates at low thermal budgets. The effects of substrate, deposition temperature, and annealing process on formation of crystalline gallium oxide are discussed. The Ga2O3 films exhibited a strong preferred orientation on the c-plane sapphire substrate. The onset of formation of crystalline gallium oxide is determined, at which only two sets of planes, i.e., alpha-Ga2O3 (006) and beta-Ga2O3 ((4) over bar 02), are present parallel to the surface. More specifically, this work reports, for the first time, that epitaxial gallium oxide films on sapphire start to form at deposition temperatures >= 190 degrees C by using an optimized plasma-enhanced ALD process such that alpha-Ga2O3 (006)parallel to alpha-Al2O3 (006) and beta-Ga2O3 ((2) over bar 01)parallel to alpha-Al2O3 (006). Both alpha-Ga2O3 (006) and beta-Ga2O3 ((2) over bar 01) planes are polar planes (i.e., consisting of only one type of atom, either Ga or O) and, therefore, favorable to form by ALD at such low deposition temperatures. Ellipsometry and van der Pauw measurements confirmed that the crystalline films have optical and electrical properties close to bulk gallium oxide. The film grown at 277 degrees C was determined to have superior properties among as-deposited films. Using TEM to locate alpha-Ga2O3 and beta-Ga2O3 domains in the as-deposited crystalline films, we proposed a short annealing scheme to limit the development of alpha-Ga2O3 domains in the film and produce pure beta-Ga2O3 films via an energy-efficient process. A pure beta-Ga2O3 phase on sapphire with beta-Ga2O3 ((2) over bar 01)parallel to alpha-Al2O3 (006) was successfully achieved by using the proposed process at the low annealing temperature of 550 degrees C preceded by the low deposition temperature of 190 degrees C. The results of this work enable epitaxial growth of gallium oxide thin films, with superior material properties offered by ALD, not only with potential applications as a high-performance material in reducing global energy consumption but also with an energy-efficient fabrication process.