High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition

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High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition U. Chowdhury; M. M. Wong; C. J. Collins; B. Yang; T. G. Zhu; A. L. Beck; J. C. Campbell; and R. D. Dupuis The University of Texas at Austin, Microelectronics Research Center, 10100 Burnet Road, Building 160, Austin, TX 78758 USA Phone: +1-512-471-0537, Fax: +1-512-471-0957, e-mail: [email protected] ABSTRACT We report the growth, fabrication and characterization of back-illuminated AlGaN p-i-n photodetectors operating in the solar-blind wavelength range. A peak external quantum efficiency (EQE) of 45.5% (0.1A/W) was obtained at a wavelength of 270 nm. This EQE was obtained at zero volt bias and without the use of any anti-reflection coating on the back of the substrate. To our knowledge, this is the highest EQE reported to date for p-i-n photodetectors operating in the solar-blind wavelength range. INTRODUCTION Solid-state optoelectronic devices such as light emitters and photodetectors operating in the ultraviolet (UV) wavelength range enjoy widespread research attention for both commercial and defense applications. Particularly, photodetectors insensitive to radiation of wavelengths longer than ~290 nm are of increasing interest for various UV detection applications. The atmospheric ozone layer absorbs light at wavelengths shorter than ~290 nm from the solar spectrum, making it possible for such a photodetector to operate at or near the Earth’s surface in the absence of a high background of solar radiation noise. Because of this feature, photodetectors satisfying the above criteria are called “solar-blind”. Also, because of the convenience of flip-chip mounting of the photodetectors on electronic driving circuitry, it is desirable to fabricate a back-side (or substrate-side) illuminated photodetector for incorporation into electronic circuits, e.g., UV photodiode imaging arrays. The common approach to the design of a back-side-illuminated solar-blind photodetector is the fabrication of a p-i-n diode structure of AlxGa1-xN with x~0.45 grown on top of a thick “window” layer composed of AlxGa1-xN with x~0.60. Since the bandgap equivalent wavelength of such a p-i-n diode is around 275 nm, such a device produces a photocurrent response under illumination at wavelengths around 280 nm and shorter. In this work, the AlGaN p-i-n epitaxial layers are grown by metalorganic chemical vapor deposition (MOCVD) on double polished sapphire substrates and a thick Al0.60Ga0.40N template layer serves to reduce the dislocation density in the device active region. The composition for the template is chosen so that it is transparent to the wavelength of detection interest and also does not have too large of a lattice mismatch with the p-i-n region, which might lead to defect formation from lattice relaxation. Commonly, the n-side of the diode is grown first (on the substrate) to reduce the device impact of the spreading resistance in the relatively high resistivity p-material and to reduce the memory effect from Mg, which is