AlGaN p-i-n Photodiode Arrays for Solar-Blind Applications
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AlGaN p-i-n Photodiode Arrays for Solar-Blind Applications P. Lamarre, A. Hairston, S. Tobin, K. K. Wong, M. F. Taylor, A. K. Sood and M. B. Reine BAE SYSTEMS, Lexington, Massachusetts and Nashua, New Hampshire, U.S.A. M. J. Schurman and I. T. Ferguson Emcore Corporation, Somerset, New Jersey, U.S.A. R. Singh and C. R. Eddy, Jr. ECE Dept. and Photonics Center, Boston University, Boston, Massachusetts, U.S.A. ABSTRACT This paper presents UV imaging results for a 256×256 AlGaN Focal Plane Array that uses a back-illuminated AlGaN heterostructure p-i-n photodiode array, with 30×30 µm² unit cells, operating at zero bias voltage, with a narrow-band UV response between 310 and 325 nm. The 256×256 array was fabricated from a multilayer AlGaN film grown by MOCVD on a sapphire substrate. The UV response operability (>0.4×average) was 94.8%, and the UV response uniformity (σ/µ) was 16.8%. Data are also presented for back-illuminated AlGaN p-i-n photodiodes from other films with cutoff wavelengths ranging between 301 and 364 nm. Data for variable-area diagnostic arrays of p-i-n AlGaN photodiodes with a GaN absorber (cutoff=364 nm) show: (1) high external quantum efficiency (50% at V=0 and 62% at V=-9 V); (2) the dark current is proportional to junction area, not perimeter; (3) the forward and reverse currents are uniform (σ/µ=50% for forty 30×30 µm² diodes at V=-40 V); (4) the reverse-bias dark current data versus temperature and bias voltage can be fit very well by a hopping conduction model; and (5) capacitance versus voltage data are consistent with nearly full depletion of the unintentionally-doped 0.4 µm thick GaN absorber layer and imply a donor concentration of 3-4×1016 cm-3. INTRODUCTION We are developing 256×256 solar-blind UV Focal Plane Arrays (UV-FPAs). The solar-blind spectral region is generally defined as wavelengths less than 280 nm. At these wavelengths, the atmospheric ozone strongly absorbs solar radiation, preventing it from reaching the earth’s surface [1]. The hybrid UV-FPA, illustrated in Fig. 1, consists of a 256×256 back-illuminated AlGaN p-i-n photodiode array that is bump-mounted to a matching 256×256 silicon CMOS readout integrated circuit (ROIC) chip. This hybrid FPA architecture has been extensively developed and manufactured for the infrared spectral region, incorporating back-illuminated photodiode arrays of narrow-gap semiconductors such as InSb and HgCdTe.
Figure 1. Hybrid UV Focal Plane Array (FPA), consisting of a back-illuminated AlGaN photodiode array on a UV-transparent substrate, hybridized with indium bump interconnects to a silicon CMOS ReadOut Integrated Circuit (ROIC) chip.
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The first back-illuminated GaN UV FPA was reported in 1998 by NASA Goddard Space Flight Center [2,3]. This FPA used a 256×256 array of 30×30 µm² GaN photoconductors, formed in a semiinsulating GaN MOCVD film on a sapphire substrate. We are using back-illuminated AlGaN p-i-n junction photodiodes, which are inherently more linear than photoconductors and which have negligible 1/f noise when operated at ze
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