Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a molecular oxygen flux

The authors report a study of molecular beam deposition of MgO films on amorphous SiO₂ and (0001) GaN surfaces over a large range of temperatures (25–400 °C) and molecular oxygen growth pressures (10⁻⁷–10⁻⁴ Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO₃), all components of MgO become volatile at normal epitaxial growth temperatures (≥250 °C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 10⁻⁵ Torr O₂ is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.