High-Gain Ultraviolet Avalanche Photodiodes Using a ZnSe-Based Organic–Inorganic Hybrid Structure

We have developed high-gain and highly sensitive ZnSe-based organic–inorganic hybrid ultraviolet avalanche photodiodes (UV-APDs). The inorganic ZnSe-based wafers (i-ZnSSe active layer/n-ZnSSe) were grown by molecular beam epitaxy (MBE) on n-type GaAs substrates. The inorganic UV-transparent conducting polymer window layers of poly 3,4-ethylenedioxythiophene:poly-styrenesulfonate (PEDOT:PSS) were formed by spin-coating and a photolithography technique instead of the inkjet printing technique, which we previously reported. We have obtained a thin uniform window layer with a mesa-shaped edge by an optimized photolithography process. The leakage current before the breakdown voltage was suppressed to < 10^‑10 A/mm², which is lower than that of the APD device fabricated by inkjet printing. The maximum external quantum efficiency was improved to η_max = 70% (λ = 340 nm) using the photolithography technique compared with the inkjet printing (η_max = 50%). The maximum responsivity was improved from 3 A/W to 10 A/W. The maximum multiplication factor was improved from M = 90 to M = 3100.