Thermodynamically metastable α-, ε- (or κ-), and γ-Ga2O3: From material growth to device applications

Gallium oxide (Ga2O3) has attracted tremendous attention in power electronics and ultraviolet photodetectors because of the large bandgap of 4.9–5.3 eV available to all polymorphs, as well as its high electric breakdown voltage. Recently, there has been increasing research interest in thermodynamically metastable phases such as α-, ε- (or κ-), and γ-Ga2O3, because they are predicted to exhibit superior properties compared with β-Ga2O3, the most stable phase of Ga2O3. For example, α-Ga2O3 (bandgap, Eg = 5.3 eV; expected breakdown field, Ec = ∼10 MV/cm) is expected to be a better potential candidate in power electronics than β-Ga2O3 (Eg = 4.5–4.8 eV; Ec = 8 MV/cm) because of its larger bandgap and higher breakdown field. Because these thermodynamically metastable phases cannot be grown using melt-growth techniques, they are grown heteroepitaxially on foreign substrates. We extensively illustrate the growth of these metastable phases and their alloys by employing various growth techniques and then discuss their doping and electronic properties. Finally, we emphasize their applications in devices, including power devices and solar-blind ultraviolet photodetectors.