Si Nanowires Light Absorption

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Nano Energy (2012) 1, 714–722

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

RAPID COMMUNICATION

Geometrical control of photocurrent in active Si nanowire devices Amit Solankia,n, Pascal Gentilea, Vincent Calvoa, Guillaume Rosaza,b, Bassem Salemb, Vincent Aimezc, Dominique Drouinc, Nicolas Pauca SiNaPS/SP2M, UMR-E CEA/UJF-Grenoble 1, INAC, Grenoble F-38054, France  Laboratoire des Technologies de la Microelectronique, CNRS, Grenoble, France c    Centre de recherche en nanofabrication et nanocaracterisation, Universite de Sherbrooke, Quebec, Canada b a

Received 1 March 2012; received in revised form 23 May 2012; accepted 23 May 2012 Available online 2 June 2012

KEYWORDS
Nanowires; Absorption; Photocurrent; Photovoltaic devices; Mie resonance; EBIC

Summary
The growing research in the field of photovoltaics has led to various strategies for increasing the light interaction in absorbers, for instance the use of nanostructures like nanowires where leaky mode resonances enhanced absorption efficiency. Towards this goal, we present a study of the light absorption in single Si nanowires, by means of microphotocurrent spectroscopy combined with transport measurements of carrier diffusion length using the electron beam induced current technique. The study is performed on different diameter nanowires with Schottky junctions created by doping modulation during Chemical Vapor Deposition–Vapor Liquid Solid growth. We show that the photocurrent spectra of single Si nanowires do not follow monotonous profiles as bulk silicon, but rather have steep valleys and peaks whose position and intensity are diameter dependent. These sharp modulations result from a resonant coupling between incident photons and cavity modes of the nanowires. A good agreement between the experiment and the theoretical fit using Mie theory is observed with a red shift in the absorption spectrum with increasing diameters. & 2012 Elsevier Ltd. All rights reserved.

Introduction
Abbreviations: NW, nanowire; EBIC, Electron Beam Induced Current; I–V, current–voltage; CVD, Chemical Vapor Deposition; VLS, Vapor Liquid Solid n Corresponding author. Tel.: +33 4 38 78 18 06; fax: +33 4 38 78 58 17. E-mail address: amit.solanki@cea.fr (A. Solanki). 2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2012.05.010

NWs have been extensively studied for their novel properties which encompass their application in future electronics [1–5], photonics [6], thermoelectric [7,8], sensors [9,10], etc. One field of immense potential is energy sector where new routes are being unraveled to fuel the future of the world energy consumption [11,12]. The tremendous growth in the photovoltaics in last few years has led to great deal of

Geometrical control of photocurrent in active Si NW devices research in forming and characterizing junction in NWs to achieve a NW array based solar cell. With current progress the NW arrays with both axial and radial junction have been studied with efficiencies comparable to thin film and bulk Si solar cells [13,14]. Studying the single NW electro-optical properties is thus of critical importance to get an optimum design in the future NW array based solar cells. In our work, we modulate the doping along the NWs during single VLS growth to obtain p+-intrinsic or n +intrinsic (p +i and n +i respectively) dopant distribution in NWs. In this way, by using same contacts on either side of the nanowire, we fabricate ohmic contact on one side and rectifying contact on the other side to obtain active devices where no external bias is required to generate photocurrent. This approach uses conventional doping methods to create built in electric fields, which can be viewed as an alternative approach to the metal–semiconductor–metal photodetectors made on intrinsic Ge nanowires [15]. In this case, the nature of the electrical contact can be tuned by...
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