Post-oxidation effects on MgxZn1-xO/ZnO bi-layer metal-semiconductor-metal photodetectors

Abstract

Post-oxidation was conducted for various times (0–8 min) in MgxZn1−xO/ZnO bi-layers and metal–semiconductor–metal photodetectors (MSM PDs) were fabricated. The effects of post-oxidation on the MgxZn1−xO/ZnO bi-layers and the optoelectronic characteristics of the MSM PDs were studied in detail. The leakage current of the MSM PDs decreased as post-oxidation time increased owing to the improvement of the crystal structure of the MgxZn1−xO/ZnO bi-layers. However, a long post-oxidation time (8 min) destroyed the crystal structure and raised the leakage current. After post-oxidation, the visible response of the MSM PDs was greatly reduced, therefore enhancing the ultraviolet (UV) (320 nm)/visible (500 nm) rejection ratio from 77 to 331 (approximately 4.3 times) for the MSM PDs with 5-min post-oxidation compared to the one without post-oxidation. X-ray photoelectron spectroscopy showed that, after post-oxidation, the introduced oxygen atoms compensated the oxygen-vacancy-related defects and that the thermal energy forced interstitial oxygen atoms (Oi) to fill in oxygen vacancies (OV). As a result, the number of lattice oxygen atoms (OL) increased, whereas the number of OV and Oi decreased. The improved crystal structure reduced the leakage current and enhanced the UV/visible rejection ratio. However, for a long post-oxidation time (8 min), the weakly-bonded oxygen atoms leaves the MgxZn1−xO surface, hence increasing the number of OV and decreasing that of OL. Increasing the number of defect degrades the performance of MSM PDs.