Solar-Blind AlGaN Heterostructure Photodiodes
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Internet Journal Nitride Semiconductor Research
Solar-Blind AlGaN Heterostructure Photodiodes J.D. Brown1, Jizhong Li1, P. Srinivasan1, J. Matthews1 and J.F. Schetzina1 1Department
of Physics, North Carolina State University,
(Received Friday, August 11, 2000; accepted Wednesday, September 13, 2000)
A backside-illuminated solar-blind UV detector based on an AlGaN p-i-n heterostructure has been successfully synthesized, fabricated and tested. The p-i-n photodiode structure consists of a 1.0 µm n-type Al0.64Ga0.36N:Si layer grown by MOVPE onto a low temperature AlN buffer layer on a polished sapphire substrate. On top of this base layer is a 0.2 µm undoped Al0.47Ga0.53N active layer and a 0.5 µm p-type Al0.47Ga0.53N:Mg top layer. Square mesas of area A = 4 x 10-4 cm2 were obtained by reactive ion etching using BCl3. The solar-blind photodiode exhibits a very narrow UV spectral responsivity band peaked at 273 nm with a FWHM = 21 nm. Maximum responsivity R = 0.051 A/W at 273 nm, corresponding to an internal quantum efficiency of 27%. R0A values up to 8 x 107Ω-cm2 were obtained, corresponding to D* = 3.5 x 1012 cm Hz1/2 W-1 at 273 nm.
1
Introduction
Atmospheric absorption of sunlight by ozone and oxygen gives rise to a narrow UV absorption band at the earth's surface from about 240-285 nm that is termed the "solar-blind" UV region [1] [2] . UV detectors that respond exclusively to radiation in this wavelength region are termed "solar-blind" detectors since they can detect objects that emit radiation in this narrow wavelength window without interference from the sun — provided that both the detector and the object of interest are at or near the surface of the earth. Solar-blind detectors are to be distinguished from "visible-blind" UV detectors and detector arrays that have UV responsivity bands outside of the solar-blind window but remain blind to visible radiation. Visible-blind UV detectors and detector arrays based on III-V nitride structures have been reported in a number of recent publications [2] [3] [4] [5] [6] [7] [8] [9] [10] This paper reports the successful synthesis, fabrication, and testing of a backside-illuminated solar-blind UV detector based on an AlGaN p-i-n heterostructure. The structure of the device is first described. This is followed by a description of the experimental procedures employed to synthesize and characterize the various layers of the heterostructure. Processing procedures employed to prepare discrete 200 µm x 200 µm mesa photodiodes are next described . This is followed by a discussion of the device characteristics obtained from
spectral responsivity and R0A measurements from which the photodiode detectivity D* was estimated. 2 2.1
Experimental Details Photodiode Structure
Photodiode structures were prepared by MOVPE at North Carolina State University using a low-pressure, vertical-flow reactor that employs high speed substrate rotation during film growth [6] [7] [8]. The photodiode structures were deposited onto 2 inch diameter c-plane sapphire substrates. A thin (~30 nm) Al
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