Plane Selective Metal-Assisted Chemical Etching for Good Ohmic Contact to β-Ga2O3
Authors: Choi Woong , Lee Seungyun , Kim Jihyun 
Published: December 2024

With the advent of the first silicon-based transistor in 1954, integrated circuit technology has advanced substantially. Nonetheless, the progression of high-power devices has been hindered by the inherent limitations of silicon's narrow bandgap (1.1 eV) and its relatively low breakdown voltage (~0.3 MV/cm), necessitating the exploration of alternative high-power semiconductor materials. Leading candidates for next-generation high-power semiconductors include wide bandgap materials such as gallium nitride (GaN) and silicon carbide (SiC), as well as ultra-wide bandgap materials like gallium oxide (Ga2O3, with a bandgap of 4.8-5.3 eV and breakdown field of ~10 MV/cm).
Ga2O3 is known for its five different phases (α, β, γ, κ, ε) and exhibits high thermal, chemical, and radiation hardness, along with physical stability, making it suitable for high-power semiconductor devices in extreme and aerospace environments. Among five phases, the β-Ga2O3 is garnering significant research interest for its stability, monoclinic structure, and the capability of being grown over large areas with high crystallinity via melt growth methods.
The physical and chemical stability of Ga2O3 has rendered conventional etching techniques ineffective, prompting the exploration of alternative methods such as inductively coupled plasma reactive ion etching (ICP-RIE) and metal-assisted chemical (MAC) etching. In contrast to ICP-RIE, which may induce defects via ion bombardment in the dry etching process, MAC etching passivates defects through wet etching process. The characteristics of MAC etching enable selective etching of β-Ga2O3 planes through patterning of the metal catalyst which locally amplifies the etching reactions.
The highly asymmetric monoclinic crystal structure (C2/m) of β-Ga2O3 contributes to the anisotropic electrical properties observed in β-Ga2O3. Forming Ohmic contacts with low on resistance is crucial in many electronic devices, and research has been conducted to identify crystal orientations that demonstrate low contact resistance. Previous studies focused on the comparison of contact resistances in surface contacts on substrates with different surface orientations. Comparing across distinct substrates failed to account for the variations in contact resistance caused by doping and defects.
In this study, MAC etching was performed to form trench contacts on an undoped β-Ga2O3 (Nd-Na ~1017) bulk substrate with a (010) plane as the surface orientation. The trenches had orientations of (001), (100), (101), (102), (201), and (-201) planes, and TLM patterns with channel lengths of 5, 10, 15, 20, 25, and 30 μm were fabricated for each plane. This approach eliminated the effects of doping and defects, allowing for the precise examination of contact resistance in each plane orientations. Analyzing the contact resistance of β-Ga2O3 based on plane orientations will contribute to understanding the anisotropic electrical properties of β-Ga2O3 and advance the industrialization of β-Ga2O3 in the field of high-power electronic devices.

Figure 1