GaN/AlGaN back-illuminated multiple-quantum-well Schottky barrier ultraviolet photodetectors A. Teke a,b, * ,S.Dogan a,c ,F.Yun a , M.A. Reshchikov a ,H.Le a ,X.Q.Liu a , H. Morkoc ß a , S.K. Zhang d , W.B. Wang d , R.R. Alfano d a Department of Electrical Engineering, Virginia Commonwealth University, 601 W. Main Street, P.O. Box 843072, Richmond, VA 23284-3072, USA b Department of Physics, Faculty of Art & Science, Balikesir University, 10100 Balikesir, Turkey c Department of Physics, Faculty of Art & Science, Ataturk University, 25240 Erzurum, Turkey d Institute for Ultrafast Spectroscopy and Lasers and New York State Center for Advanced Technology, The City College of The City University of New York, W. 138th Street and Convent Avenue, New York, NY 10031, USA Received 19 December 2002; accepted 9 January 2003 Abstract We report on characterization and operation principle of a set of GaN/AlGaN multiple-quantum-well (MQW) photovoltaic detectors. The structures were grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates and fabricated in the back-illuminated vertical Schottky geometry. Introduction of MQWs into the active region of devices is expected to enhance the quantum efficiency due to the high absorption coefficient. A nearly flat spectral responsivity between 325 and 350 nm with 0.054 A/W peak responsivity was achieved from the single-side polished backside (rough) illuminated GaN/AlGaN MQW devices. The cutoff wavelength of the MQW photodetector can be tunedbyadjustingthewellwidth,wellcompositionandbarrierheight.Amodelhasbeendevelopedtogaininsightinto theoperationprinciplesofMQWsphotodiodes.Thepeakresponsivityincreasedwithdecreasingbarrierthicknessdue to enhanced tunneling of photogenerated carriers. Ó 2003 Elsevier Science Ltd. All rights reserved. 1. Introduction III–Vnitridebaseddevicesareanidealcandidatefor ultraviolet (UV) detection in a number of applications including early missile plume detection, flame sensing, UVastronomy,space-to-spacecommunicationandbio- logical effects [1]. In the past decade, there have been several reports on GaN–AlGaN based photodetectors, such as photoconductors [2], p–n junction [3], p–i–n [4] and p–p–n [5], Schottky barrier [6–8], metal–semicon- ductor–metal [9], metal–insulator–semiconductor [10], field-effect transistors, bipolar junction transistor [11] and avalanche photodiode [12]. Peak responsivities andcorrespondingquantumefficienciesscatterinawide range depending on the device design and material qualities. Recently, Collins et al. [13] reported a peak responsivity for AlGaN solar-blind p–i–n photodiodes as high as 0.12 A/W at 275 nm without bias, giving an external quantum efficiency of 53%. This achievement was attributed to an improved material quality of Al 0:6 Ga 0:4 N. Design consideration and material quality are the key issues for achieving higher device perfor- mance. To date, most of the work on III-nitride UV photodetectors, inclusive of all types, has relied on bulk-like epilayers. Kipshidze et al. [14] reported GaN/ AlGaN MQW in an effort to extend the range of wavelengthinLED[18].Asforinfrareddetectors,other III–V material systems, such as GaAs or InP based, devices containing MQWs in their active region have * Corresponding author. Address: Department of Electrical Engineering,VirginiaCommonwealthUniversity,601W.Main Street, P.O. Box 843072, Richmond, VA 23284-3072, USA. Tel.: +1-804-827-7000; fax: +1-80-827-7040. E-mail address: ateke@mail.vcu.edu (A. Teke). 0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00068-6 Solid-State Electronics 47 (2003) 1401–1408 www.elsevier.com/locate/sse