Due to their large bandgap, high critical electric field, and availability of high-quality large-size melt-grown bulk substrates, III-oxides including Ga₂O₃, Al₂O₃, In₂O₃, and their alloys have been extensively investigated for a myriad of electronic and optoelectronic applications. Recently, β-Ga₂O₃ based power electronics, RF transistors, and ultraviolet (UV) photodetectors have been demonstrated with promising performance. However, p-type β-Ga₂O₃ is still elusive due to high dopant activation energy (>1 eV), large hole effective mass, and hole trapping. This significantly limits the design freedom for β-Ga₂O₃ devices. Other p-type semiconductors have been proposed to form heterostructures with β-Ga₂O₃ such as p-NiO, p-Cu₂O, and p-type III-nitrides. As popular wide bandgap semiconductors, III-nitrides are promising candidates to form III-oxide/III-nitride heterostructures to enable advanced device structures and new functionalities. Furthermore, III-oxides and III-nitrides can be epitaxially grown on each other with small lattice mismatch (< 5% for GaN and β-Ga₂O₃) by the industrial standard epitaxial method MOCVD. For example, vertical GaN violet LEDs grown on n-type β-Ga₂O₃ substrates have been reported. This talk will present our recent work on III-oxide/III-nitride heterostructures in power electronics and optoelectronics.

For power electronics, β-Ga₂O₃/GaN p-n heterojunctions will first be discussed. The heterojunction via mechanical exfoliation shows decent forward rectifying behaviors and thermal stability up to 200 °C but relatively low breakdown voltages (BV). To improve the breakdown capability, we carried out a comprehensive TCAD simulation study to design mesa edge termination for kV-class β-Ga₂O₃/GaN p-n heterojunctions. It was found that the electric field crowding effect is the main reason for the low BV. Several mesa edge termination structures were investigated such as deeply-etched mesa, step mesa, and p-GaN guard ring. Second, normally-off AlN/β-Ga₂O₃ field-effect transistors using polarization-induced doping will be discussed. A large two-dimensional electron gas is formed at the AlN/β-Ga₂O₃ interface due to polarization effects, and p-GaN gate is used to realize tunable positive threshold voltage. The device transfer and output characteristics with different device structures are also studied. For optoelectronics applications, self-powered spectrally distinctive Ga₂O₃/GaN heterojunction UV photodetectors grown by MOCVD will be discussed. Opposite current polarities are observed under different illumination wavelengths due to different carrier transports, which can be utilized to distinguish different spectra. These results indicate that (ultra)wide bandgap III-oxide/III-nitride heterostructures are a promising platform to enable new device structures and functionalities.