HCl-gas etching behavior of (001) β-Ga₂O₃ under oxygen supply

The planar and lateral HCl-gas etching behavior of (001) β-Ga₂O₃ under oxygen supply were investigated at partial pressures of P ⁰(O₂) = 0-2.5 kPa and 645-1038°C, while maintaining a constant HCl supply partial pressure of P ⁰(HCl) at 63 Pa. At 747°C, the planar etch rate (PER) exhibited a slight decrease with increasing P ⁰(O₂). Notably, at P ⁰(O₂) = 1.25 kPa, the PER increased with temperature, demonstrating a plateau between 747 and 848°C, whereas the thermodynamically calculated etching driving force did not. Even minimal O₂ supply effectively suppressed root mean square (RMS) roughness to <1 nm at 747°C. At P ⁰(O₂) = 1.25 kPa, RMS roughness remained at  <2 nm at up to 847°C, but sharply increased to  >7 nm above 947°C, indicating that lower temperatures realize smoother surfaces. Lateral etch rate (LER) analysis, employing a spoke-wheel pattern mask at 747°C revealed significant anisotropy, demonstrating a kidney-like polar plot pattern, with minimum values in the <100 > direction and maximum values in the  <010> direction. Although P ⁰(O₂) had a limited effect on anisotropy, temperature increase significantly enhanced the LER, particularly along the ± 20°-rotated directions from  <100> . Above 947°C, etched sidewalls exhibited a multi-faceted morphology owing to the formation of {310} and {3̅10} facets depending on the spoke direction, whereas the sidewalls were relatively smooth below 848°C. These findings underscore the potential of controlled HCl-gas etching for the plasma-free processing of β-Ga₂O₃, enabling the fabrication of high-performance devices.