The authors demonstrate improved device performance of GaN metal-semiconductor-metal ultraviolet (UV) photodetectors (PDs) by ultrathin HfO2 (UT-HfO2) layer on GaN. The UT-HfO2 interfacial layer is grown by atomic layer deposition. The dark current of the PDs with UT-HfO2 is significantly reduced by more than two orders of magnitude compared to those without HfO2 insertion. The photoresponsivity at 360 nm is as high as 1.42 A/W biased at 5 V. An excellent improvement in the performance of the devices is ascribed to allowed electron injection through UT-HfO2 on GaN interface under UV illumination, resulting in the photocurrent gain with fast response time. © 2015 American Vacuum Society.