IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 18 (2007) 105307 (5pp) doi:10.1088/0957-4484/18/10/105307 Sulfur passivation for ohmic contact formation to InAs nanowires D B Suyatin 1,2 , C Thelander 1 , M T Bj ¨ ork 3 , I Maximov 1 and L Samuelson 1 1 Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, S-221 00, Lund, Sweden 2 Division of Microelectronics, Institute of Nuclear Physics, Moscow State University, Leninskie Gory, GSP-2, 119992, Moscow, Russia 3 IBM Research GmbH, Z¨ urich Research Laboratory, S¨ aumerstrasse 4, 8803, R¨ uschlikon, Switzerland E-mail: Lars.Samuelson@ftf.lth.se Received 1 December 2006 Published 6 February 2007 Online at stacks.iop.org/Nano/18/105307 Abstract We have studied the formation of ohmic contacts to InAs nanowires by chemical etching and passivation of the contact areas in an ammonium polysulfide, (NH 4 ) 2 S x , water solution. The nanowires were exposed to different dilution levels of the (NH 4 ) 2 S x solution before contact metal evaporation. A process based on a highly diluted (NH 4 ) 2 S x solution was found to be self-terminating, with minimal etching of the InAs. The stability of the contacts was investigated with electrical measurements as a function of storage time in vacuum and air. (Some figures in this article are in colour only in the electronic version) 1. Introduction In the past few years, considerable progress in the control of semiconductor nanowire formation has been achieved, and already, a number of different nanowire-based applications have been demonstrated [1–4]. Nanowires of different materials are not only of interest as potential building blocks for future electronics and photonics, but they also let researchers access low-dimensional systems with interesting properties [5, 6]. A key to electrical transport studies on such nanometre-scale objects is the ability to form reliable contacts to individual nanowires. The surface oxides must be removed, and the semiconductor contact areas should ideally be passivated before contact metal deposition to prevent re- oxidation. Semiconductor surface passivation was actively pursued in the 1960s in the development of III–V based high speed transistors. Since the mid 1980s, research on chalcogenide passivation for bulk III–V semiconductors has attracted a lot of attention. In particular, ammonium polysulfide, (NH 4 ) 2 S x , solutions have been found to remove native oxides and contaminants from III–V semiconductor surfaces, and to provide passivation with covalently bonded sulfur atoms. The passivation allows good short-term surface stability without considerable reoxidation in air and aqueous solutions [12, 13]. It also provides both chemical and electrical passivation of a surface. Sulfur-adsorbed III–V compound semiconductor surfaces have been studied extensively by various surface- sensitive techniques and noticeable improvement of various device-related properties was observed [12, 13]. Surface passivation becomes progressively more impor- tant for a device as the surface-to-volume ratio of the struc- ture increases. In spite of this, only a limited number of studies on passivation of semiconductor nanowire structures have been reported so far [8–11]. The key difference in chalcogenide passivation technology for nanostructures, compared to bulk materials, is the importance of etching rates [7]. In bulk, a certain amount of surface material can be sacrificed without the loss of device functionality. However, semiconductor nanostructures in general, and nanowires in particular, are extremely sensitive to removal of surface material due to their large surface-to- volume ratio. The aim of this study was to find optimum process conditions which provide ohmic contact formation to InAs nanowires together with minimal semiconductor material removal. We describe a method for passivation of InAs nanowires using both highly diluted and regular (NH 4 ) 2 S x 0957-4484/07/105307+05$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK