Triggering avalanche-like ultraviolet photomultiplication phenomena in ultrathin amorphous/crystalline gallium nitride heterostructures

Avalanche phenomena, triggered by carrier multiplication through impact ionization under high electric fields, form the operating principle of avalanche photodiodes. However, this process inevitably leads to amplified dark current, thereby limiting the photo-to-dark-current ratio despite large gain. Here, we demonstrate avalanche-like nonlinear photocurrent amplification while we maintain ultralow dark current in an amorphous-gallium oxynitride (a-GaON)/gallium nitride (GaN) heterostructure formed via a two-step "amorphization-recrystallization" process. Instead of relying on the impact ionization process, the engineered amorphous/crystalline interface enables trap-assisted photo-induced carrier multiplication behavior at low voltages, yielding a large gain (3.9 × 10⁶) and ultrahigh responsivity (4.3 × 10⁷ amperes per watt) at 35 volts, with an ultralow dark current (~0.7 picoamperes), which are strongly competitive relative to state-of-the-art ultraviolet avalanche photodiodes. We further present proof-of-concept ultraviolet hardware systems incorporated with the newly constructed device. This amorphous/crystalline interfacial engineering strategy presents an unexploited, simple, and scalable device paradigm for fabricating advanced photodetectors in next-generation compact and integrated optoelectronic systems.