Focused gold ions beam for localized epitaxy of semiconductor nanowires J. Gierak * , A. Madouri, E. Bourhis, L. Travers, D. Lucot, J.C. Harmand Laboratoire de Photonique et de Nanostructures CNRS, Route de Nozay, F-91460 Marcoussis, France article info Article history: Received 14 September 2009 Received in revised form 25 November 2009 Accepted 27 November 2009 Available online 3 December 2009 Keywords: Focused ion beam Gold ions Deposition Nanowires abstract We report on the optimization of a focused gold ion beam source which is used for local and shallow implantation of gold in GaAs wafers. The ion source uses pure liquid gold and the ion optics is designed for high resolution patterning. Imaging and etching performances are evaluated. Then, arrays of implanted gold dots are fabricated at 20 keV in GaAs substrates, which are subsequently used for epitax- ial growth. Formation of organized GaAs nanowires is observed. Their diameter can be lower than 10 nm. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Epitaxial growth techniques like Molecular Beam Epitaxy (MBE) and Metalorganic Vapor Phase Epitaxy (MOVPE) are known for their excellent control of one crystal dimension (the growth direc- tion), but these techniques can also be investigated to grow crys- tals of limited lateral size. The Vapor–Liquid–Solid (VLS) [1] epitaxy regime is very attractive to produce semiconductor nano- wires (NWs) with diameter of few tens of nanometres. Since the middle of the 1990’s the growth of Au-catalyzed GaAs, InP or InAs NWs arrays has attracted a considerable attention [2,3]. Their for- mation is promoted by liquid metal droplets deposited at the sub- strate surface. These NWs could become the building blocks of future 1D nanodevices. To this aim, gold patterning techniques are necessary in order to control the exact substrate location where the nanowire growth takes place. This control has been already demonstrated by patterning the catalyst material on the semicon- ductor substrate prior to the growth. Regular nanowires arrays have been obtained by using e-beam lithography [4] or nanoim- print technologies [5]. In this article, we demonstrate that focused ion beam (FIB) is another technique which can predetermine accu- rately the position of catalyst seeds for nanowire growth. 2. Experimental set-up 2.1. The gold LMIS emitter Since the pioneering works of Sudraud [6] and Wagner [7] there have been very few developments on pure gold liquid ion sources. Initially liquid gold ion sources were found promising be- cause of their excellent emission stability and reproducibility. These good characteristics were mainly explained by the high dif- fusivity of gold layers over tungsten surfaces and by the absence of oxidation of the supply material. On the other hand, limitations have been rapidly evidenced such as a relatively large spreading of ion energy (10 eV for 1 lA emission current) [8] and mostly, the existence of various emission modes from the gold surface leading to the emission of single charged Au + , or doubly charges Au ++ ions [6]. Finally, short source lifetimes were observed, repre- senting a major limitation. Indeed parasitic thermal evaporation along the heating filament was often noted as a source of impor- tant mass losses. Practically a liquid gold ions source consists of a tungsten needle wetted with a liquid gold film permanently flowing from a reservoir towards the tip emitting apex. Heating of the gold reservoir is usu- ally obtained via Joule effect by circulating a current in a hairpin spring-rolled filament. Ideally, the tip and the reservoir must be maintained at the same temperature, slightly above the melting temperature of the gold metal (1063 °C). But in practice, due to the limited space available, large temperature gradients often ap- pear on the heating filament. These thermal gradients have a delete- rious effect in causing supply metal migration to the ‘‘hot spots”. Consequently, unwanted evaporation occurs and the source lifespan is limited to a couple of hours of continuous operation. Another dif- ficulty related to the high temperature of the tip, is to preserve a good alignment between the source and the extractor. This align- ment is realized at room temperature beforehand. Indeed mechan- ical stress, metal diffusion over the filament and electrical load variations of the temperature control electronics can produce lateral displacements of the emitting apex. To reduce these effects, eutectic alloy sources of low melting point, such as AuSi [9], were developed. 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.11.164 * Corresponding author. Tel.: +33 01 69 63 60 75; fax: +33 01 69 63 60 06. E-mail address: jacques.gierak@lpn.cnrs.fr (J. Gierak). Microelectronic Engineering 87 (2010) 1386–1390 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee