WSe₂/β-Ga₂O₃ p–n Heterojunction-Based Normally Off Phototransistors for Self-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 (WOx) (2 < x < 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 WOx/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 (R254nm/R365nm) 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.