Characteristics of back-illumination UV photodetector fabricated with Al_xGa_1-xN heterostructure

We report the growth, fabrication and characterization of an Al _xGa_1-xN heteroepitaxial back-illuminated UV photodetector used for flip-chip mounting. This device is grown on a one-side-polished sapphire substrate with a low-temperature AlN buffer layer by 6-pocket multi-wafer metalorganic chemical vapor deposition (MOCVD) using a vertical reactor. In order to attain the UV region, we increased the Al mole fraction in the Al_xGa_1-xN epilayer and acquired an Al _xGa_1-xN epilayer that shows a crack-free surface morphology when the Al mole fraction was 30%. This device consists of a 1.2-μm-thick Al_0.3Ga_0.7N "window layer", 0.16-μm-thick Al_0.08Ga_0.92N i-layer, 0.46-μm-thick Al_0.08Ga_0.92N p-layer, 0.1-μm-thick GaN p-layer and a 30-nm-thick GaN:Mg p⁺-contact layer. All device processes were completed by standard semiconductor processing techniques that include photolithography, metallization and etching. In this device, the zero-bias peak responsivity is measured to be about 0.1 A/W at 350 nm, which corresponds to an external quantum efficiency of 36%. The rise-and-fall time of the photoresponse is 4.1 ns. This device exhibits a low dark current density of 31.9 pA/cm ² at zero bias. Therefore, we can successfully obtain the back-illuminated UV photodetector with good responsivity, fast photoresponse time and low dark current.