Annealing Effects on the Structural, Optical, and UV Photoresponse Properties of ZnO Nanostructures Prepared by RF-Magnetron Sputtering at Different Deposition Temperatures

Undoped ZnO nanostructures were deposited on SiO2/Si substrates via radio frequency magnetron sputtering at different deposition temperatures (room temperature, 200, 300, and 400 degrees C). The prepared samples were annealed at 500 degrees C for 2 h under an N-2 flow. The structural, surface morphological, optical, and photoresponse characteristics of ZnO nanostructures as deposited and after annealing were then investigated. The energy bandgaps of all samples after annealing (3.22-3.28 eV) decreased compared with those of the as-deposited specimens. The barrier height increased when the deposition temperature increased and reached 0.77 eV at 400 degrees C after annealing with a leakage current of 0.17 mu A at a 5 V bias. The UV photodetector device which was deposited at the optimal temperature of 300 degrees C, has 12.51 x 10(3)% photosensitivity, 2.259 mu A dark current, 0.508 s response time, and 0.466 s recovery time. The dark current significantly decreased for all samples after annealing. The proposed UV photodetectors exhibit high performance, high photosensitivity, shorter response and recovery times, and excellent stability at lower bias voltages of 5 and 2 V.