Determination of effective Ga/N ratio to control GaN growth behavior in liquid-target reactive magnetron sputter epitaxy

The optimization of magnetron sputter epitaxy (MSE) for the high-volume production of high-quality GaN films is increasingly important. This study concerns the influence of key MSE process parameters - including the partial pressure of process gas, target-to-substrate distance (TSD), and growth temperature (TG) - for the synthesis of GaN thin films using a liquid Ga target. It is observed that the effective Ga/N ratio on the substrate surface determines the film's growth behavior and affects material's composition and luminescence properties. A lower Ar/N2 partial pressure ratio substantially enhances the crystalline quality, evidenced by the reduction in peak width of x-ray rocking curves from approximately 1.25° (N-rich regime) to 0.35° (Ga-rich regime) and improved GaN bandgap emission. While target sputtered in a highly Ga-rich condition significantly reduces the GaN growth rate (R), primarily due to Ga desorption in nitrogen-limited condition at elevated TG. Ion mass spectrometry and rate monitor measurements demonstrate that the Ga/N ratio can be controlled by adjusting Ar/N2 pressure ratio in MSE process. A reduction in TSD from 9.3 cm to 7 cm resulted in an increased R from 541 nm/h to 731 nm/h, corroborated by Simulation of Metal Transport (SIMTRA) analysis. Temperature-dependent studies revealed that films grown above 900 °C exhibited flat surface with high crystalline quality.