Role of ZrO2 Passivation Layer Thickness in the Fabrication of High-Responsivity GaN Ultraviolet Photodetectors

The importance of a ZrO2 passivation layer in the fabrication of high-responsivity GaN-based ultraviolet (UV) photodetectors (PDs) is discussed. It is found that an optimum thickness of the ZrO2 layer exists, which plays a critical role in controlling the photoresponse and transient response of the device. Beyond the optimal thickness, the performance of PDs deteriorates, which is limited by the restricted tunneling of photogenerated carriers across the oxide layer. At an optimum ZrO2 thickness of 3 nm, a spectral responsivity of 27 AW(-1) at 361 nm is achieved at 4 V applied bias along with the fast response of the device with a rise (fall) time of 28 ms (178 ms), respectively. Such characteristics are found to be similar or better than the recently reported state-of-the-art values for visible blind metal-semiconductor-metal PDs fabricated on GaN. The results confirm that the surface passivation with an optimal thickness of an oxide layer can be used to develop high-responsivity GaN-based UV PDs irrespective of having a large dark current, which is often inevitable due to the presence of a large density of dislocations in GaN epitaxial layers grown on foreign substrates.