Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 47 (2014) 394017 (8pp) doi:10.1088/0022-3727/47/39/394017 Characterization and analysis of InAs/p–Si heterojunction nanowire-based solar cell Anna Dalmau Mallorqu´ ı, Esther Alarc ´ on-Llad ´ o, Eleonora Russo-Averchi, G¨ ozde T ¨ ut ¨ unc ¨ uoglu, Federico Matteini, Daniel R ¨ uffer and Anna Fontcuberta i Morral Laboratoire des Mat´ eriaux Semiconducteurs, ´ Ecole Polytechnique F´ ed´ erale de Lausanne, 1015 Lausanne, Switzerland E-mail: anna.dalmaumallorqui@epfl.ch and anna.fontcuberta-morral@epfl.ch Received 12 March 2014, revised 26 April 2014 Accepted for publication 2 May 2014 Published 11 September 2014 Abstract The growth of compound semiconductor nanowires on the silicon platform has opened many new perspectives in the area of electronics, optoelectronics and photovoltaics. We have grown a1 × 1 mm 2 array of InAs nanowires on p-type silicon for the fabrication of a solar cell. Even though the nanowires are spaced by a distance of 800 nm with a 3.3% filling volume, they absorb most of the incoming light resulting in an efficiency of 1.4%. Due to the unfavourable band alignment, carrier separation at the junction is poor. Photocurrent increases sharply at the surrounding edge with the silicon, where the nanowires do not absorb anymore. This is further proof of the enhanced absorption of semiconductors in nanowire form. This work brings further elements in the design of nanowire-based solar cells. Keywords: InAs, molecular beam epitaxy, solar cell, light absorption (Some figures may appear in colour only in the online journal) 1. Introduction Compound semiconductors (III–V) are of special interest for electronic and optoelectronic devices thanks to their high electron mobility and direct band gap [1–3]. A key issue to make them attractive for industrial applications is their integration on the widely developed Si platform. There are numerous challenges for a successful integration of III–Vs on silicon, such as lattice mismatch, differences in thermal expansion coefficients and polarity [4]. It has been shown that III–V nanowires can overcome these challenges thanks to their small footprint [5–7]. In the case of nanowires the strain can be relaxed laterally thanks to their small diameter and dislocations are present only at the interface, with little impact on the device performance [8, 9]. Alternative high potential devices consider the heterojunction between Si and a monolayer semiconductor such as graphene [10, 11] or MoS 2 [12, 13]. In numerous applications a control on the position and orientation of the nanowires is required. Vertical orientation of nanowires can be achieved on Si (1 1 1) substrates in very high yields [14–16]. In the case of catalyst-free growth, ordered arrays of nanowires can in principle be attained by employing a patterned SiO 2 mask [15, 17–19]. Among the applications of III–V arrays on silicon are tunnel diodes [23], photoelectrochemical water splitting [24, 25] and solar cells [26, 27]. By combining a GaAs nanowire array on a silicon cell, a dual junction with a theoretical efficiency higher than 30% could, in theory, be achieved [28]. Another consequence of the nanowire small diameter is that light absorption can only be understood from a wave optics perspective. This results in the appearance of interesting phenomena such as the presence of guided and leaky modes [29, 30]. Recently, it has been shown that standing semiconductor nanowires are extremely efficient in absorbing light [31–33]. Their absorption cross-section is much larger than their physical bounds. Thanks to this property, the optimal design of nanowire-based solar cells includes arrays with a relatively large pitch, leading to very large void fractions up to 90%. This discovery has led to the achievement of InP solar cells with an efficiency up to 13.8% [34]. The achievement of such an efficiency by other groups is limited to the ability of growing these high quality arrays and by the ability of producing the devices with high yield. The challenges linked to the growth of GaAs arrays on silicon have hampered so far the achievement of a GaAs nanowire array solar cell on silicon 0022-3727/14/394017+08$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK