Layered phase composition and microstructure of kappa-Ga2O3-dominant heteroepitaxial films grown via MOCVD

Phase and microstructural evolution of gallium oxide (Ga2O3) films grown on vicinal ( 0001 ) sapphire substrates was investigated using a suite of analytical tools. High-resolution transmission electron microscopy and scanning transmission electron microscopy of a film grown at 530 & DEG;C revealed the initial pseudomorphic growth of three to four monolayers of alpha-Ga2O3, a 20-60 nm transition layer that contained both beta- and gamma-Ga2O3, and a top & SIM;700 nm-thick layer of phase-pure kappa-Ga2O3. Explanations for the occurrence of these phases and their sequence of formation are presented. Additional growths of Ga2O3 films in tandem with x-ray diffraction and scanning electron microscopy investigations revealed that the top layer varied in phase composition between & SIM;100% kappa-Ga2O3 and & SIM;100% beta-Ga2O3; the surface microstructure ranged from poorly coalesced to completely coalesced grains as a function of growth temperature, growth rate, or diluent gas flow rate. In general, it was found that the kappa-phase is favored at lower growth temperatures and higher triethylgallium flow rates (low VI/III ratios). The growth of nominally single-phase kappa-Ga2O3 within the top layer was observed in a temperature range between 500 and 530 & DEG;C. Below 470 & DEG;C, only amorphous Ga2O3 was obtained; above 570 & DEG;C, only the beta-phase was deposited.