WSe₂/β-Ga₂O₃ p–n Heterojunction-BasedNormally Off Phototransistors forSelf-Powered UV‑C Detection

A WSe₂/β-Ga₂O₃ heterojunction (HJ)-based enhancement-mode (E-mode) phototransistor with self-powered operation was developed for ultraviolet-C (UV-C) photodetector applications, featuring a top-gate p-type WSe₂ (p-WSe₂) and an n-type ultrawide-bandgap β-Ga₂O₃ that serves as both the conductive channel and UV-C absorption layer. To increase the hole concentration in WSe₂ dry-transferred onto β-Ga₂O₃, the top few layers of WSe₂ were oxidized to tungsten oxide (WO_<i>x</i>) (2 < <i>x</i> < 3) with a high work-function value via UV–ozone treatment. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed excellent interface quality with atomic-scale layer uniformity in the HJ of WO_<i>x</i>/WSe₂/β-Ga₂O₃. The WSe₂/β-Ga₂O₃ p–n HJ diode exhibited a high rectification ratio (∼10⁹) and an extremely low reverse current (14 fA), allowing the demonstration of a normally off n-channel β-Ga₂O₃ phototransistor integrated with an ultrathin p-WSe₂ stack. The type-II band alignment of this HJ promoted efficient separation of photogenerated electron–hole pairs under UV-C illumination, allowing us to achieve excellent optoelectronic performance under standalone operation. Without an external power source, the WSe₂/β-Ga₂O₃ E-mode phototransistor exhibited excellent optoelectronic performance, including responsivity of 2.1 A W^–1, a photo-to-dark current ratio of 1.5 × 10³, external quantum efficiency of 10.2%, specific detectivity of 6.4 × 10⁷ Jones, UV-A selectivity with a rejection ratio (<i>R</i>_254nm/<i>R</i>_365nm) of 4, and a fast response without persistent photoconductivity. These findings highlight the potential of p-WSe₂/β-Ga₂O₃ heterostructure UV-C phototransistors with high sensitivity and energy efficiency because of their compact and standalone deep-UV optoelectronic architecture.