In this work, we reported the fabrication and characterization of an Al<sub>x</sub>Ga<sub>1-x</sub>N /GaN hetero-epitaxial front-illuminated visible-blind UV photodetector with very high external quantum efficiency. This device was grown on one side of polished sapphire substrate using a low-temperature AlN buffer layer created by three-pocket multi-wafer system metalorganic chemical vapor deposition (MOCVD) with a vertical reactor. This device consisted of a 2.5μm thick GaN n-layer, a 0.4μm thick GaN i-layer and Al<sub>0.1</sub>Ga<sub>0.9</sub>N "window layer", followed by a 10 nm GaN:Mg p+ contact layer. In order to investigate the effect of p- Al<sub>0.1</sub>Ga<sub>0.9</sub>N thickness on the characteristics of the photodetector, three samples only with different p-AlGaN thicknesses of 0.1μm and 0.15μm were fabricated. All of the device processing was completed using standard semiconductor processing techniques that included photolithography, metallization and etching. Compared the results of these three samples, the sample with 0.15μm thick p-AlGaN possesses the highest quantum efficiency and its zero-bias peak responsivity was found around 0.20A/W at 365 nm, corresponding to an external quantum efficiency of 85.6%. Moreover, this device exhibits a low dark current density of 3.16nA/cm<sup>2</sup> at zero-bias.
The persistent photoconductivity (PPC) effect was generally observed in many III-V compound semiconductors and it was always related to yellow luminescence. In this paper, the PPC effect in unintentionally doped GaN was investigated. The GaN photoconductive detector response time measured by changing the chopper frequency of modulator was studied under various ultraviolet-radiation intensities. The maximum value of UV intensities used in test was approximately 0.2W/m<sup>2</sup>. Experimental results show that the response time of unintentionally doped GaN PC detector is independent of the wavelength of the ultraviolet radiation in the range of 300~365nm and it decreases with the increasing of UV radiation intensity. The longest response time getting in experiments was 7.64ms and the shortest 2.89ms. Fourier transformation and lock-in amplifier was used to reduce the noise at AC frequency of 50Hz and the results show that Fourier transformation was more effective to eliminate the low frequency noise. The experimental data fit the theoretical curve very well, better than the results reported previously. Finally, these phenomena were tried to be explained using a mechanism that minor carriers were captured by deep acceptors. The deep acceptors were deduced to be V<sub>Ga</sub> related complexes. In strong UV radiation, the photo-generated holes (minor carriers) were no longer captured by deep acceptors and the recombination opportunities with majority carriers were increased. Consequently the response time was reduced. The other possible reason was that there were metastable states which were related to Ga vacancy.
In recent years, Al<sub>x</sub>Ga<sub>1-x</sub>N semiconductor alloys, with a direct bandgap tunable between 3.4eV and 6.2eV, become the most suitable materials for the fabrication of UV detectors. In this paper we describe the fabrication and characteristics of an UV 64×1 focal plane array (FPA) based on front illuminated GaN p-i-n photodiodes. The diode structure consists of a base n-type layer of GaN followed by unintentionally doped and p-type layers deposited by metal organic chemical vapor deposition on GaN buffered sapphire substrate. Standard photolithographic, Ar<sup>+</sup> ion beam etching, SiO<sub>2</sub> passivation and metallization procedures were employed to fabricate the devices. I-V, responsivity and spectral response were tested. The linear photodiode array was indirectly hybridized to a silicon readout integrated circuit (ROIC) chip. The ROIC chip consists of capacitor feedback transimpedance amplifier (CTIA) input circuits, correlated double sampling (CDS) circuits, shift registers etc. The 64×1 UV linear FPA was packaged into a 28-pin chip carrier. The response ununiformity is 1.86%. The mean detectivity is about 2.0×10<sup>9</sup>cmHz<sup>1/2</sup>W<sup>-1</sup>.
The contact resistivity of Ni/Au contact on p-type GaN was drastically decreased through the surface treatments in sequence using alcohol-based HCl and KOH solution. The surface oxide on p-type GaN formed during epitaxial growth was removed in the alcohol-based HCl and KOH solution, The O 1s and C 1s core-level peaks in the x-ray photoemission spectra showed that the alcohol-based HCl treatment was more effective in removing of the surface oxide layer. Compared to the KOH solution treated sample, the alcohol-based HCl-treated sample showed a Ga 2p core-level peak which was shifted toward the valence-band edge by 0.3 eV, indicating that the surface Fermi level was shifted toward the valence-band edge. These results suggest that the surface barrier height for hole injection from Ni/Au metal to p-type GaN be lowered by the surface treatment, which results in a drastic reduction in specific contact resistance.