Ultra-violet photo-response characteristics of p-Si/i-SiO₂/n-ZnO heterojunctions based on hydrothermal ZnO nanorods

Hydrothermal zinc oxide (ZnO) nanorod (NR)-based p-Si/n-ZnO and p-Si/i-SiO₂/n-ZnO heterojunctions were fabricated, and the effects of interfacial native SiO₂ (~4 nm) on the I-V characteristics of heterojunctions under dark and ultra-violet illumination conditions were investigated. First, the structural and optical properties of ZnO seed crystals grown by sol-gel method and hydrothermal ZnO NRs on two different substrates of p-Si and p-Si/i-SiO₂ were examined, and more improved optical and crystalline quality was obtained as revealed by photoluminescence and X-ray diffraction. The p-i-n heterojunctions showed ~3 times greater forward-bias currents and enhanced rectifying property than those of p-n junctions, which is attributed to the role of native SiO₂ in carrier confinement by promoting the electron-hole recombination current through the deep level states of ZnO crystal. The measured ratios of photocurrent to dark current of the p-i-n structure were also greater under reverse bias (92–260) and forward bias (2.3–7.1) conditions than those (28–225 for reverse bias, 1.6–6.8 for forward bias) of p-n structure, and the improved photosensitivity of the p-i-n structure under reverse bias is due to lower density of recombination centers in the ZnO NR crystals. Fabricated ZnO NR heterojunction showed repeatable and fast photo-response transients under forward bias condition of which response and recovery times were 7.2 and 3.5 s for p-i-n and 4.3 and 1.7 s for p-n structures, respectively.