Photo-Enhanced Inverse Metal-Assisted Chemical Etching of α-Ga2O3 grown on Al2O3
Authors: Choi Woong , Moon Sanghyun , Kim Jihyun 
Published: September 2023

Si-based semiconductor devices are not suitable for stable operations in the aerospace environments. Among the ultra-wide bandgap semiconductor materials, gallium oxide (Ga2O3) is attracting attention as the next-generation material for high-power semiconductor devices in extreme condition beyond Si, owing to its large band gap of 4.5-5.3 eV, high breakdown electric fields of 7-10 MV/cm, excellent chemical and thermal stability and radiation hardness.

Alpha gallium oxide (α-Ga2O3) has the largest bandgap (4.8-5.3 eV) and the highest breakdown electric fields (~10 MV/cm) among the five Ga2O3 polymorphs, which facilitates the application of α-Ga2O3 as a high power device. However, the research on etching technology for α-Ga2O3 is rare. Etching technology is a crucial step of device fabrication. Chemical etching is free from plasma-induced damage which leads to superior device performance compared with dry etching, and high throughput with low cost is beneficial to industrial implementations.

Chemical stability of Ga2O3 hinders the effective reaction with most chemical etchants. However, the photo-enhanced inverse metal assisted chemical (I-MAC) etching, where a noble metal with a high work function is utilized as a catalyst under UV irradiation, has emerged as a candidate for the chemical etching of Ga2O3. Deep-UV irradiation generates electron-hole pairs (EHPs) in the uncovered region of Ga2O3, and the metal electrode withdraws carriers from the photo-generated EHPs. The holes that are accumulated in the uncovered region of Ga2O3 react with gallium ions which produce gallium fluoride (GaF3) in a reaction with Hydrofluoric acid (HF). The etching process continues as the oxidant re-oxidizes the metal. The etch rate can be controlled by various parameter, including concentration and temperature of the etchant.

In this work, for the I-MAC etch of α-Ga2O3 on sapphire substrate grown by halide vapor phase epitaxy, Pt was deposited on α-Ga2O3. Etch rate and surface roughness were characterized by atomic force microscopy after each step of the I-MAC etch.

The I-MAC etch using HF and potassium persulfate (K2S2O8) solution with 185-nm UV irradiation was performed at constant durations under different etchant temperature conditions. The etch rates obtained at each temperature condition were fitted with the Arrhenius plot to estimate the activation energy of 0.898 eV. The etch depth showed a linear increase with time and the etch rate exhibits a direct dependency on the temperature of the etchant, indicating that the I-MAC etching of α-Ga2O3 is an activation-controlled reaction. As the I-MAC etching proceeded, surface roughness of α-Ga2O3 showed a tendency to increase. The rate at which the surface roughness increased increased with raising the etchant temperature. In the case of Pt there was no significant difference in the surface roughness after etching. After the completion of the I-MAC etch, the remaining Pt can be removed using aqua regia. The I-MAC etch process of α-Ga2O3 can allow us to fabricate α-Ga2O3 without plasma-damage.

Figure 1