Polymorphism and Faceting in Ga2O3 Layers Grown by HVPE at Various Gallium-to-Oxygen Ratios

Herein, the influence of the oxygen flow on the formation of metastable polymorphs of gallium oxide (Ga2O3) grown by halide vapor phase epitaxy (HVPE) on c-plane patterned sapphire substrates (PSS), on gallium nitride (GaN) templates, and on m-plane smooth sapphire substrates is reported. X-ray diffraction, scanning electron microscopy, and cathodoluminescence are used to identify different polymorphic phases. The samples deposited on bare PSS exhibit faceted growth of the alpha-Ga2O3 on the cones of the sapphire substrate and the formation of the kappa-Ga2O3 between the cones. In contrast, growth on GaN templates results in hexagonal columns of kappa-Ga2O3 which produce a continuous smooth layer upon coalescence. The growth of Ga2O3 on m-plane sapphire substrates results in overgrown pyramids of the alpha-phase. For all types of substrates, the variation of the oxygen flow affects only the thickness and coalescence of Ga2O3 layers. Thus, the growth of Ga2O3 metastable polymorphs is mainly influenced by the form, orientation, and symmetry of the substrate and shows a weak dependence on stoichiometry.