Morphology effect of 1D ZnO nanostructures designed by hydrothermal and thermal annealing for fast ultraviolet photodetector applications

ZnO nanorods are grown on a ZnO seed layer using the hydrothermal method and subsequently annealed at temperatures of 400-800 degrees C. The ZnO nanorods annealed at 400 degrees C exhibit a morphology similar to that of the unannealed ZnO nanorods. However, the tips of the ZnO nanorods gradually become rounded and their density and diameter increase with an increase in the annealing temperature. The intensity of the near-band-edge emission increases gradually with an increase in the annealing temperature from 400 to 600 degrees C but decreases sharply in the case of nanorods annealed at 800 degrees C. With respect to the deep-level emissions, broad yellow, orange, and green emissions are observed. Further, the low-temperature photoluminescence spectrum measured at 12 K of the ZnO nanorods annealed at 600 degrees C contains a donor-bound exciton emission, as well as emissions related to the donor-acceptor pair (DAP) transition and the first-order and second-order longitudinal optical phonon replicas of the DAP transition. Finally, with respect to the photoresponse, the dark current and photocurrent of the nanorods decrease and their photosensitivity increases with the increase in the annealing temperature.