Defect induced high photocurrent in solution grown vertically aligned ZnO nanowire array films

Highly oriented and homogeneously distributed single crystalline zinc oxide nanowires (NWs) are fabricated on amorphous glass substrates using soft solution growth approach. The nanowire films and sol-gel grown ZnO films are devised and tested for UV light detection applying four-probe conductivity measurements. As-grown ZnO NWs film device demonstrates three orders enhancement (sensitivity = 440) in conductivity at room temperature under an illumination of 365 nm UV light, while the sol-gel based thick film reveals two orders of enhancement in device conductance. A clear correlation of conductivity and photoluminescence measurements suggest that surface oxygen vacancies (singly charged/Vo+) which render higher green defect luminescence intensity (IG/IUV = 1.8) in ZnO NWs leads to poor dark conductance and higher photo-conductance. Post growth annealing of nanowire arrays either in air (IG/IUV = 0.85) or oxygen ambience (IG/IUV = 0.38) results in reduction of green defects and corresponding suppression of photocurrent. Higher concentration of surface traps also leads to persistent photocurrent due to ionization of oxygen vacancies and creation of perturb host states under UV light excitation.