Enhancing the Photoresponse of p-NiO/n-ZnO Heterojunction Photodiodes Using Post ZnO Treatment

P-NiO/n-ZnO heterojunction photodiodes (HPDs) were fabricated using radio-frequency magnetron sputtering system. A post ZnO treatment was performed by annealing the ZnO film in an oxygen gas atmosphere at various temperatures (200-500 degrees C). As compared to the HPDs with as-deposited ZnO, the rectification ratio of the HPDs with 400-degrees C-treated ZnO increases greatly from 4.22 - 1.92 x 10(4). However, the rectification ratio drops to 114 for HPDs with 500-degrees C-treated ZnO due to the high temperature increase the defects in ZnO through thermal diffusion. The ideality factor decreases from 5.6 to 2.2 for the HPDs with as-deposited ZnO and 400-degrees C-treated ZnO, respectively, suggesting the defects of ZnO were reduced. This result caused the ultraviolet (360 nm)/visible (560 nm) rejection ratio increased from 3 to 347. With increasing treatment temperature, x-ray diffraction showed that the grain size increased from 8.60 nm to 12.96 nm, demonstrating the thermal energy improved the crystal structure of ZnO. X-ray photoelectron spectroscopy revealed that the amount of lattice oxygen (O-L) in ZnO increased with treatment temperature due to the higher thermal energy. Conversely, the amounts of oxygen vacancies (O-V) and interstitial oxygen (O-i) defects decreased because the oxygen atoms compensated O-V, and O-i diffused into lattice sites of ZnO. In contrast, a higher treatment temperature (500 degrees C) leaded to an increase of O-V owing to the thermal diffusion of oxygen atoms, thus degrading the photoresponse of HPDs. Band diagram elucidated the mechanisms of post-treated ZnO in enhancing the photoresponse of HPDs.