Characteristics of back-illuminated visible-blind UV photodetector based on Al_xGa_1-xN p-i-n photodiodes

In this work, we reported the growth, fabrication and characterization of an Al_xGa_1-xN heteroepitaxial back-illuminated visible-blind UV photodetector designed for flip-chip mounting. This device was grown on one side of a polished sapphire substrate using a low-temperature AlN buffer layer created by six-pocket multi-wafer system metalorganic chemical vapor deposition (MOCVD) with a vertical reactor. In order to obtain the wavelength of the visible-blind region, the Al_xGa_{1- x}N layer was grown under various conditions of growth time and gas flow rate, after optimizing the AlN buffer layer. This device consisted of a 1.3 μm thick Al_0.15Ga_0.85N "window layer", a 0.16 μm thick Al_0.08Ga_0.92N i-layer, a 0.46 μm thick Al_0.08Ga_0.92N p-layer, a 0.1 μm thick GaN p-layer, followed by a 30 nm GaN:Mg p⁺-contact layer. All of the device processing was completed using standard semiconductor processing techniques that included photolithography, metallization and etching. In this device, the zero-bias peak responsivity was found around 0.052 A/W at 340 nm, corresponding to an external quantum efficiency of 19%. The rise and fall time of the photoresponse was 20.8 ns. Moreover, this device exhibits a low dark current density of 17 pA/cm² at zero-bias.