Photoconductive UV detector based on high-resistance ZnO thin film

Abstract

As a wide bandgap semiconductor material, ZnO has huge potential in applications such as light emitting devices and sensors. Compared with GaN and SiC, ZnO has a bandgap of 3.37 eV and exciton binding energy of 60 meV at room temperature, indicating it is a promising candidate of UV detector. ZnO based metal-semiconductor-metal photoconductive ultraviolet detector has the advantages of high optical gain and strong responsivity. However, due to the photoconductive relaxation and surface effect of the ZnO material, a ZnO-based photoconductive UV detector has a slow response which is defective for practical application. The intrinsic defects typically generated during the synthesis of ZnO, e.g. oxygen vacancy, should be responsible for the slow response. Therefore, we have fabricated the high-resistive ZnO thin film based UV detector and studied its UV response characteristic. High resistance ZnO thin film is fabricated on glass by RF magnetron sputtering and followed by lift-off photolithography to form Al interdigital electrodes. SEM and XRD images show that the as-fabricated ZnO thin film grows with preferential orientation along c-axis. A linear I-V curve under UV illumination indicates the ohmic contact between Al and ZnO. From these results, we can calculate the resistivities to be 3.71×109 Ω · cm and 7.20×106 Ω · cm respectively when in the dark and under 365 nm UV light of 303 μW/cm2. The light-to-dark current ratio is up to 516 with bias of 40 V. Besides, the ZnO thin film detector shows a stable, rapid, repeatible and reproducible response with a rise time of 199 ms and a fall time of 217 ms when exposed to periodically switched UV light illumination at a bias voltage of 40 V. Moreover, the detector has a high selectivity for 365 nm UV light and the responsivity is 0.15 mA/W with the intensity of 303 μW/cm2. Furthermore, the transient response process is analyzed using the theory of surface recombination and bulk recombination of ZnO semiconductor. For a high resistance ZnO thin film based UV detector, the surface recombination process is weakened ascribed to the decrease of intrinsic defects and the bulk recombination process plays a leading role, resulting in the fast response. Results show that high resistivity ZnO thin film based UV detectors have outstanding UV photoresponse characteristics for potential applications in UV/radiation detection.