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°C). As compared to the HPDs with as-deposited ZnO, the rectification ratio of the HPDs with 400-°C-treated ZnO increases greatly from 4.22 - 1.92 × 10
⁴
. However, the rectification ratio drops to 114 for HPDs with 500-°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 asdeposited ZnO and 400-°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 (OL) in ZnO increased with treatment temperature due to the higher thermal energy. Conversely, the amounts of oxygen vacancies (OV ) and interstitial oxygen (Oi) defects decreased because the oxygen atoms compensated OV , and Oi diffused into lattice sites of ZnO. In contrast, a higher treatment temperature (500°C) leaded to an increase of OV 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.