Scheme for enhancing ef®ciency in resonant-cavity Schottky photodetectors A. Sellai a, * , P. Dawson b a Department of Physics, Sultan Qaboos University, P.O. Box 36, Al-Khod 123, Oman b Department of Pure and Applied Physics, Queen's University of Belfast, BT7 1NN Nothern Ireland, UK Received 18 February 2001; revised 26 March 2001; accepted 2 April 2001 Abstract It is shown that structuring the top layers of a resonant cavity Schottky photodetector in a way that allows coupling between the wave- vector of incident radiation and that of electron-collective oscillations plasmons) at the surface of the metallic electrode leads to practically zero re¯ectance in the case of front illuminated devices. This is expected to result in a consequential enhancement in the quantum ef®ciency in these photodetectors. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Schottky photodetectors; Surface plasmons; Re¯ectance 1. Introduction In addition to their low capacitance, low contact resis- tance and low diffusion current, the advantages of Schottky photodetectors include the simple fabrication process and the possibility of integration with other devices and circuits [1,2]. Various ways have been proposed to enhance the quantum ef®ciency in these detectors. The most studied scheme is the one that uses a resonant micro-cavity structure in which the active region of the device is sandwiched between two re¯ecting mirrors allowing multiple passes of the incident light resulting in maximum absorption and a consequential high-quantum ef®ciency [3±5]. In the proposed designs, the two end mirrors could be con- ventional quarter-wavelength Bragg re¯ectors consisting of a stack of alternating layers of two suitable materials the refractive indexes of which differ substantially. Alterna- tively, the top Bragg re¯ector could be replaced by a metal- lic layer of a Schottky contact [6]. A 100% net re¯ectivity is practically attainable using the Bragg periodic structure. However, the use of a metal layer to serve as a high re¯ec- tivity top mirror instead of a Bragg re¯ector results in signif- icant simpli®cation in the fabrication process as well as a reduction in the detector dimensions. The re¯ectivity at the air-metal interface is obviously undesirable in front illuminated Schottky detectors. Although it can be signi®cantly reduced using a suitable anti-re¯ection coating Si 3 N 4 is often used), no net-zero re¯ectivity is achievable with the anti-re¯ection coating alone. It is shown here that it is possible to attain practically zero re¯ectance if the detector's top constituent layers are structured in a way that allows the matching of the wave- vectors of the incoming radiation to that of the collective ¯uctuations of electrons, known as plasmons, at the surface of the metallic electrode. This is simply attained by inserting a spacer layer of a lower refractive index between the antire¯ection coating and the metallic layer. The spacer layer acts as an optical tunneling barrier and the SPs are excited at the spacer/metal interface by the evanescent opti- cal ®eld extending from the Si 3 N 4 /spacer interface. The matching condition occurs at a particular angle of incidence and is evidenced as a minimum zero) in the calculated re¯ectance. The zero-front re¯ectance is expected to yield further enhancement in the quantum ef®ciency of Schottky photodetectors. 2. Calculation procedure The re¯ectance in a multi-layer structure is calculated, using a 2 £ 2 matrix-formalism [7], from the complex re¯ection coef®cients obtained in terms of the microscopic optical properties and thickness of the different constituent layers. It is assumed that the propagation of plane waves in an isotropic and non-magnetic medium is described by the complex index of refraction N  n 2 jk, n is the real index of refraction and k is the extinction coef®cient. If E 1 z) and Microelectronics Journal 32 2001) 779±782 Microelectronics Journal 0026-2692/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0026-269201)00048-9 www.elsevier.com/locate/mejo * Corresponding author. Tel.: 1968-515486; fax: 1968-513415. E-mail address: asellai@squ.edu.om A. Sellai).