Materials Science and Engineering B50 (1997) 311 – 314 GaN and AlGaN metal – semiconductor – metal photodetectors I. Ferguson a, *, C.A. Tran a , R.F. Karlicek Jr. a , Z.C. Feng a , R. Stall a , S. Liang b , Y. Lu b , C. Joseph c a EMCORE Corporation, 394 Elizabeth Ae, Somerset, UK b ECE Department, Rutgers Uniersity, Piscataway, NJ 08855, USA c Physics Department, Rutgers Uniersity, Piscataway, NJ 08855, USA Abstract GaN based interdigital metal – semiconductor – metal (MSM) photodetectors have been successfully fabricated. The MSM structures were patterned on highly resistive GaN and the ternary compound, AlGaN. For the highly resistive GaN detector, the lowest dark current is 0.1 nA and the UV responsivity of the device was about 460 A W -1 at a DC bias of 5 V. The AlGaN with 24% Al exhibited larger gains of up to 10 6 AW -1 at 20 V, but at a very high dark current, 1 mA, and very long detector responses, greater than 60 s. The high gain in this device is not well understood. The preliminary measurements indicate that tunneling occurs at high electric fields since a negative temperature coefficient for the breakdown voltage was observed. © 1997 Elsevier Science S.A. Keywords: Metal – semiconductor – metal photodetectors; GaN; AlGaN 1. Introduction The current interest in developing ultraviolet (UV) photodetectors is driven by applications in military counter measures, aerospace, automotive, petroleum, engine monitoring, flame detection and solar UV detec- tion. The III-Nitride Al x Ga 1 -x N wide direct band gaps are tunable from 365 nm (at x =0) to 200 nm (at x =1) and are direct over the entire alloy composition. These are ideal materials for developing UV photodetectors due to the relatively high mobility, sharp cut-off wave- length and high quantum efficiency. The technology of GaN UV detectors is still at an early stage of its development and although is now receiving intense investigation [1–6]. A recent review of all existing UV photodetector technology including III-Nitrides was completed by Razeghi and Rogalski [7]. 2. The growth of III-Nitrides for photodetector applications The GaN materials and UV detector structures have been grown in an EMCORE D-180 multi-wafer rotat- ing disc low pressure MOCVD system. For AlGaN growth trimethylaluminum, trimethylgallium and am- monia (NH 3 ) were used as the Ga and N sources, respectively, with H 2 as the carrier gas. The typical growth sequence included the low temperature of a 200 A ˚ thick GaN buffer layer deposited at 500°C before the substrate was raised to a temperature of 1050°C for the growth of the active layer. A detailed descrip- tion of the system and growth process has been re- ported elsewhere [8]. A large number of GaN and AlGaN epitaxial films have been grown on (0001) sapphire substrates. These films have been characterized by various techniques; such as X-ray diffraction, C – V profiling, Hall, room temperature and low temperature photoluminescence, Raman scattering, UV-visible opti- cal transmission and reflectance spectroscopy [8]. The ability to complete non-destructive measurement tech- niques to map material properties over large areas is particularly important to be able to produce large area devices and arrays. The focus of this work has been to reproducibly grow III-Nitrides over large areas in a production scale reac- tor on up to six 2 in. wafers in a single growth run and the results obtained to date have been excellent. A FTIR film thickness mapping system has been devel- oped to determine the thickness distributions of various * Corresponding author. 0921-5107/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 21- 5 1 07(97)00 1 96 - 7