κ/β-GaO Type-II Phase Heterojunction.

Ultrawide-bandgap gallium oxide (GaO) holds immense potential for crucial applications such as solar-blind photonics and high-power electronics. Although several GaO polymorphs, i.e., α, β, γ, δ, ε, and κ phases, have been identified, the band alignments between these phases have been largely overlooked due to epitaxy challenges and inadvertent neglect. Despite having similar stoichiometry, heterojunctions involving different phases may exhibit band offsets. Here, β-GaO/κ-GaO-stacked "phase heterojunction" is demonstrated experimentally. This phase heterojunction has a sharp and well-defined interface, and subsequent measurements reveal an unbeknown type-II band alignment with significant valence/conduction band offsets of ≈0.65 eV/0.71 eV. This alignment is promising for self-powered deep ultraviolet (DUV) signal detection, necessitating an internal electric field near the junction and matching the absorption properties for effective electron-hole separation. The fabricated phase heterojunction photodetector displays a responsivity of three orders of magnitude higher at 17.8 mA W, with improved response times (rise time ≈0.21 s, decay time ≈0.53 s) under DUV illumination and without external bias in comparison to the bare β-GaO and κ-GaO photodetectors, confirming the strong interfacial electrical field. This study provides profound insight into GaO/GaO heterojunction interfaces with different polymorphs, allowing the use of phase heterojunctions to advance electronic device applications.