Highly efficient ultraviolet photodetection in nanocolumnar RF sputtered ZnO films: a comparison between sputtered, sol-gel and aqueous chemically grown nanostructures.

Highly efficient ultraviolet (UV) photodetection has been realized in nanocolumnar radio frequency (RF) sputtered ZnO thin films as compared to sol-gel nanocrystalline films and aqueous chemically grown nanowire (NW) arrays. The photo-to-dark current ratio in the columnar films reaches a value as high as 10(6) which is at least 2-4 orders of magnitude higher than that of the sol-gel films and NWs. Such a high value is attributed to a dense compact columnar morphology of the sputtered films with higher surface-related trap states. Within 2 s of UV illumination, five orders of change in the photocurrent have occurred in the sputtered film in contrast to three orders of change in the NWs. The photocurrent decay under steady UV illumination in the sputtered films is only 23%, compared to 50% in the NWs. However, the fall in current under the UV 'off' condition in the sputtered film takes a longer time than in the NWs. These changes are correlated to the deep defect levels of the nanocolumnar films.