High Responsivity UV Photodetector Based on ZnO Nanowire Arrays

ZnO has demonstrated considerable promise for use in UV photodetection, owing to its advantageous characteristics, including wide band gap (3.37 eV), high exciton binding energy (60 meV), high thermochemical stability and low processing cost [1]. In addition, ZnO nanostructures (nanoparticles, nanowires) with a high surface-to-volume ratio show improved light absorption and charge collection. Different solutions have been employed to improve the performance of ZnO photodetectors, such as light trapping [2], NWs passivation with a SnO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> shell [3], decoration with nanoparticles [4], [5], etc. In this work we focused on the development of high-sensitivity UV photodetectors based on ZnO nanowires (NWs) oblique grown on interdigitated gold electrodes (line/gap 2 or 3 μm). The device is composed of a Schottky diode Au/ZnO reverse biased, a ZnO photoconductor (oblique ZnO NWs interconnecting the electrodes) and a Schottky diode Au/ZnO forward biased. ZnO NWs were grown by hydrothermal method on SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>/Si substrates with pre-patterned gold electrodes from a solution of zinc nitrate hexahydrate and hexamethylenetetramine The morphology and properties of ZnO NWs were controlled by varying the growth conditions, including the concentration and pH of the solution, the temperature and duration of the hydrothermal process, as well as the substrate pre-treatment. The aim was to obtain dense, thick, sufficiently long and obliquely oriented NWs to provide good electrical paths between the neighbouring electrodes for ensuring a good collection of the photocarriers. We were able to obtain oblique NWs selectively grown on Au electrodes (fig. 1a), as well structures with oblique NWs between the electrodes and vertical NWs on the Au electrodes and pads (fig. 1b) that improves the light absorption in the Schottky diode Au/ZnO by light trapping.