Embedded Co islands on Ge(001) Harold J.W. Zandvliet ⁎, Arie van Houselt, Petra E. Hegeman Physics of Interfaces and Nanomaterials, MESA + Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands abstract article info Article history: Received 16 February 2011 Accepted 24 March 2011 Available online 6 April 2011 Keywords: Scanning tunneling microscopy Germanium Cobalt Self-assembly The adsorption of Cobalt (Co) on Germanium (Ge) (001) surfaces has been studied using scanning tunneling microscopy. Upon annealing at temperatures of 500–550 K well-ordered rectangular shaped embedded islands are formed. Based on our scanning tunneling microscopy data we propose that the elementary building block of these embedded islands consist of six Co atoms arranged in a hexagonal pattern. A statistical analysis reveals that the embedded Co islands exhibit an attractive interaction in a direction perpendicular to the substrate dimer rows and a repulsive interaction in a direction along the substrate dimer rows. The embedded Co islands eventually convert to perfectly straight and micrometers long nanowires upon annealing at temperatures that exceed 700–750 K. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The synthesis of artificial low-dimensional structures has been a topic of scientific and technological interest for many years. The formation of two-dimensional quantum wells and zero-dimensional quantum dots using epitaxial growth techniques is well established. However, the preparation of one-dimensional nanowires turns out to be more complex. One attractive route is to grow wire like structures at the pre-existing steps on a vicinal surface. These wires are usually irregular because they meander along with the steps. Tersoff and Tromp [1] showed that there is another route to grow virtually kink free nanowires on surfaces. They showed that for the lattice mismatched epitaxial growth strained islands will initially have a compact shape governed by the ratio of the step free energies. However, beyond a critical size strain energy becomes the dominating factor and the island prefers to maximize its perimeter resulting into extremely elongated epitaxial islands, i.e. nanowires [1–3]. For many metal adsorbates on Si(001), such as Ag [1], Bi [4,5] and Er [6], the spontaneous formation of nanowires has been reported. On the closely related Ge(001) surface the deposition of Pt [7–11] or Au [12–17] followed by annealing at sufficiently high temperatures also lead to highly ordered and densely packed nanowire arrays. The adsorption of the 3d transition metals (such as Co, Ni, Mn and Ti) on the Ge and Si (001) surface has been studied extensively due to their suitability as contact material and their importance in dilute magnetic semiconductors [18–25]. Several 3d transition metals, including Co, prefer a subsurface position and thereby generate missing dimer defects in the top layer of the substrate [26–29]. In a recent study Choi et al. [30] found that the diffusion of Co in a subsurface position on Ge(001) goes along with the creation and annihilation of surface vacancies. The deposition of Co on Ge surfaces leads to the formation of Cobalt germanides and different phases are formed depending on the annealing temperature. Interestingly, already at low temperatures (b 575 K) Co reacts with Ge, however clear evidence for the development of an ordered phase is not available to date. X-ray diffraction (XRD) experiments reveal that at temperatures above 575 K a tetragonal Co 5 Ge 7 phase is formed [31,32]. At temperatures exceeding 725 K the Co 5 Ge 7 phase gradually converts to an ortho- rhombic CoGe 2 phase. This phase transformation is complete at a temperature of 875 K. It is the aim of this paper to study the growth of Co on Ge(001) surfaces at low temperatures. Using scanning tunneling microscopy we will attempt to obtain more information on the Co/Ge phase diagram. Since the sensitivity of scanning tunneling microscopy is limited to the topmost layer of the material we will restrict ourselves to the structural an electronic properties of the surface phases. We will pay particular attention to the formation of self-organizing Co nanowires at annealing temperatures exceeding 700 K. 2. Experimental The experiments were carried out in an Ultra High Vacuum (UHV) system equipped with as Scanning Tunneling Microscope and a base pressure of 5 × 10 −11 Torr. After degassing the Ge(001) samples at 700 K they were further cleaned by cycles of Ar + ion bombardment (800 eV, 2 μA/cm 2 , angle of incidence 45° for 30 min) and annealing at 1100 K for 1 min. During annealing the pressure did not rise above 1×10 −9 Torr. This procedure resulted in atomically clean Ge(001) surfaces exhibiting an ordered (2 × 1)/c(4 × 2) domain pattern with a low concentration of missing dimer defects [33]. After ion bombard- ment, annealing and Co deposition, the sample was transferred to the Surface Science 605 (2011) 1129–1132 ⁎ Corresponding author. E-mail address: h.j.w.zandvliet@tnw.utwente.nl (H.J.W. Zandvliet). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.03.021 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc