High Aspect Ratio β-Ga₂O₃ Fin Arrays with Low-Interface Charge Density by Inverse Metal-Assisted Chemical Etching

β-Ga₂O₃, with a bandgap of ∼4.6-4.9 eV and readily available bulk substrates, has attracted tremendous interest in the wide bandgap semiconductor community. Producing high aspect ratio β-Ga₂O₃ 3D nanostructures without surface damage is crucial for next-generation power electronics. However, most wet etching methods can only achieve very limited aspect ratios, while dry etch usually damages the surface due to high energy ions. In this work, we demonstrate the formation of β-Ga₂O₃ fin arrays on a (010) β-Ga₂O₃ substrate by metal-assisted chemical etching (MacEtch) with high aspect ratio and sidewall surfaces with excellent quality. The etching was found to be strongly crystal orientation dependent, and three kinds of vertical structures were formed after MacEtch. The Schottky barrier height (SBH) between Pt and various MacEtch-produced β-Ga₂O₃ surfaces and sidewalls was found to decrease as the aspect ratio of the β-Ga₂O₃ vertical structure increased. This could be attributed to the different amount of oxygen lost at the surface after etching, as indicated by the XPS and TEM examination. Very little hysteresis was observed in the capacitance-voltage characteristics for the 3D Pt/Al₂O₃/β-Ga₂O₃ MOS capacitor structures, and the extracted interface trap density was as small as 2.73 × 10¹¹ cm^-2 eV^-1, comparable to or lower than that for unetched planar β-Ga₂O₃ surfaces.