AlGaN Ultraviolet A and Ultraviolet C Photodetectors with Very High Specific Detectivity D^*

Abstract

The development of AlGaN pin photodetectors sensitive in the UV range with different narrow band active regions is reported in this paper. Structures were grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates using three-dimensional GaN as well as high temperature AlN nucleation. Very high specific detectivities of 1×1014 cm Hz0.5 W-1 can be achieved based on optimized growth conditions of undoped and doped AlGaN layers with an Al-content ranging from 0% up to 100%. The crack-free AlGaN layers have edge dislocation densities in the range of 5×109 cm-2. Based on the two different nucleation types, pin layer structures were grown and fabricated to UV-A (320 to 365 nm) and UV-C (< 280 nm) photodetectors. The electro-optical performance of these photodetectors measured on-wafer will be presented in this paper, supplemented by the data of a single photodetector chip mounted in a TO 18 package.

Improved AlGaN p-i-n Photodetectors for Monitoring of Ultraviolet Radiation

“…1, are grown on c-plane sapphire substrates: a high temperature AlN nucleation, Al x Ga 1-x N buffer layers, a Si-doped n-Al x Ga 1-x N layer, an unintentially doped i-Al x Ga 1-x N layer, and a Mg-doped p-Al x Ga 1-x N layer including a Mg-doped p-GaN cap. Details of material growth, material characterization, and processing into UV photodetector chips have already been published previously [11]. We deliberately avoid the use of epitaxial lateral overgrowth (ELO), homoepitaxy on AlN substrates, selected area growth, or multilayer growth for defect reduction to keep the growth and fabrication of our photodetectors as simple, cost-efficient, and reliable as possible [12]- [17].…”

“…The slight increase in dark current of the UV-C photodetector starting at -2 V bias is due to point defect induced hopping of charge carriers [1]. The number of point defects increases in Al x Ga 1-x N with an Al content around 50 % due to non-optimal growth conditions [11], i.e. the effect is enhanced at UV-C photodetectors with an Al content of 45 and 62 % in the active region.…”

“…The noticeable high series resistance of the UV-C photodetectors results from a low conductivity of the n-doped Al x Ga 1-x N layer. Consequently, the n-contact resistance shows non-ohmic behavior [11]. We achieve ohmic behavior for each p-contact by growing a p-doped GaN cap layer on top of the p-Al x Ga 1-x N layer, as demonstrated elsewhere [11].…”

“…Consequently, the n-contact resistance shows non-ohmic behavior [11]. We achieve ohmic behavior for each p-contact by growing a p-doped GaN cap layer on top of the p-Al x Ga 1-x N layer, as demonstrated elsewhere [11]. The ideality factor n of the various Al x Ga 1-x N photodetectors, shown in Table I, can differ from that of Si photodiodes, where n ranges from 1 to 2, for two reasons: First, the recombination current dominates in Al x Ga 1-x N photodiodes.…”

“…These small area pixels are not suitable for various applications such as luminescence spectroscopy or flame sensors due to a small radiation signal leading to a very small photocurrent [1]. Based on our high quality AlN buffer layer concept [11], we process epitaxially grown Al x Ga 1-x N layers into visible-and solar-blind p-i-n photodetectors, specifically designed to the UV-A, UV-B, and UV-C spectral region. We present photocurrent and response time measurements, I-V characterization, and degradation studies of our p-i-n photodiodes, which result in a superior overall performance compared to the state-of-the-art.…”