Trap-assisted photomultiplication in a-IGZO/p-Si heterojunction ultraviolet photodiodes

Many applications require a photodetector (PD) with a large gain to detect very low levels of ultraviolet (UV) light. However, a large gain is generally produced using an impact ionization process, which requires a very large applied voltage. This study demonstrates a photomultiplication (PM)-type UV PD with a simple structure that uses an a-IGZO/p-Si heterojunction. The device exhibits an external quantum efficiency (EQE) of 2565% under 325 nm illumination at a reverse bias of −5 V. The oxygen vacancy (Vo) state in a-IGZO is relaxed to a deep bandgap but is fully occupied by two electrons. Vo with doubly charged Vo 2+ or singly charged Vo + can be excited by UV light. This produces free electrons. There is a high EQE at low reverse-bias because trapped electrons are emitted from the Vo. The IGZO/Si heterojunction also has a high response speed (∼1 ms) in the self-powered mode because the built-in potential separates electron–hole pairs immediately. This study shows that an a-IGZO/p-Si heterojunction not only acts as a PM-type PD with a low driving voltage but also as a high-speed self-powered PD to reduce power consumption.