Ab initio studies of adsorption and migration surface processes on an a-Al 2 O 3 surface Johanna Rose ´n a, * , Jochen M. Schneider a , Karin Larsson b a Materials Chemistry, RWTH-Aachen, D-52056 Aachen, Germany b Department of Materials Chemistry, A ˚ ngstro ¨m Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden Received 21 December 2004; received in revised form 15 February 2005; accepted 24 March 2005 by H. Eschrig Available online 12 April 2005 Abstract Surface processes on a-Al 2 O 3 (0001) have been investigated theoretically using density functional theory. Ion–surface interactions prior to collision were investigated by means of ab initio molecular dynamics simulations, showing an adsorbate trajectory towards a preferred adsorption site. Furthermore, the adsorption process at different surface sites was investigated, together with calculations of barriers for migration between these sites. Although the equilibrium (bulk) adsorption site was energetically favourable, the presence and magnitude of the migration barriers indicate that activation energy is necessary to form the equilibrium surface structure. These results are of importance for a fundamental understanding of ion–surface interactions. q 2005 Elsevier Ltd. All rights reserved. PACS: 68.43.Bc Keywords: A. Alumina; D. Adsorption; D. Migration 1. Introduction Alumina is a material of large interest for both science and technology, where the applications range from dielec- tric in microelectronics to corrosion and wear protection in mechanical systems. Low temperature vapour phase con- densation (%250 8C) of alumina is known to result in an amorphous structure [1]. Thin films of a-Al 2 O 3 can be achieved for example if energy is supplied through growth at temperatures above 800 8C [2], with a substrate bias of K50 V [3] and K200 V [3] at temperatures around 650 8C, as well as through (local) epitaxy [4]. The supplied energy affects the structure evolution of the material through the ability of the atoms to diffuse, at the surface and in the bulk. To gain fundamental understanding of the evolving structure during film growth, it is essential to theoretically study diffusion/migration. Several investigations concern- ing alumina diffusion can be found in literature, as exemplified in Refs. [5–7]: Belonoshko et al. calculated the temperature dependent diffusivity of hydrogen in bulk alumina [5], and Harding et al. simulated grain boundary diffusion in ceramics, including a-Al 2 O 3 [6]. A theoretical study of Pd migration on the (0001) surface were performed by Lodziana et al., indicating an atom mobility high enough for diffusion at room temperature [7]. In a previous investigation, the adsorption of different Al and O ions on an a-Al 2 O 3 (0001) surface was investigated by means of ab initio calculations [8]. All cases studied resulted in exotherm reactions with clear differences in adsorption energies, depending on both ion charge and adsorption site. The strong site dependence indicates a presence of migration barriers. However, no Solid State Communications 135 (2005) 90–94 www.elsevier.com/locate/ssc 0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.03.048 * Corresponding author. Tel.: C49 241 8025988; fax: C49 241 8022295. E-mail address: rosen@mch.rwth-aachen.de (J. Rose ´n).