Applied Surface Science 268 (2013) 16–21 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Initial stages of the adsorption of Sc and ScN thin films on GaN(0 0 0 1): First principles calculations J. Guerrero-Sánchez a , Gregorio H. Cocoletzi a,∗ , J.F. Rivas-Silva a , Noboru Takeuchi b a Benemérita Universidad Autónoma de Puebla, Instituto de Física “Ing Luis Rivera Terrazas”, Apartado Postal J-48, Puebla 72570, Mexico b Universidad Nacional Autónoma de México, Centro de Nanociencias y Nanotecnología, Apartado Postal 2681, Ensenada, Baja California, 22800, Mexico a r t i c l e i n f o Article history: Received 11 October 2012 Received in revised form 15 November 2012 Accepted 17 November 2012 Available online 6 December 2012 Keywords: Gallium nitride Scandium nitride Growth Adsorption Surfaces a b s t r a c t Using first principles total energy calculations we have investigated the initial stages of the adsorption of Sc and ScN thin films on GaN(0 0 0 1) surfaces under both N and Ga rich conditions. In an ideally GaN(0 0 0 1) bulk terminated surface, and when the Sc atom is constrained to remain on top of the surface, the T4 site configuration is the most favorable. However a structure in which the Sc atom replaces a Ga atom of the first monolayer and the displaced Ga atom occupies a T4 site (forming bonds with Ga atoms only) has the lowest energy. Results are similar for Ga rich conditions: if the Sc atom is constrained on top of the surface, it occupies the T4 site. However, if it is allowed, it will occupy sites in the third (from top) Ga layer and it will form ScN. For a full monolayer of Sc atoms, three different configurations are possible, in all of them there is formation of scandium nitride: a ScN bilayer terminated configuration for N rich conditions, a ScN bilayer underneath a Ga bilayer for Ga rich conditions, and a ScN bilayer under a Ga layer for intermediate configurations. In all three geometries, the ScN are in wurtzite like configurations with distorted bond angles and the surfaces are metallic. © 2012 Elsevier B.V. All rights reserved. 1. Introduction GaN, a semiconductor that crystallizes in the wurtzite struc- ture, is the focus of extensive experimental and theoretical research because of its excellent physical properties that include a wide band gap of 3.4 eV, large bulk modulus and a high thermal con- ductivity. It is used in a large variety of technological applications, such as high-power and high temperature electronic devices [1–6]. The ability of GaN to form alloys and heterostructures with other group III semiconductors, such as InN, (resulting in materials with continuously varying band gaps) is an additional reason for the interest in GaN. Band gap engineering can lead to the fabrica- tion of optoelectronic devices with very specific properties. For example it is used in the fabrication of blue laser diodes that are fundamental in the search for a more efficient white light produc- tion [7–9]. However, for some applications it is better to replace the InN: due to their great strength and durability (they have been used at extreme conditions of temperature and pressure, for example in rocket nozzles and drill bits), it has been suggested that transition metal nitrides can be used instead. Indeed, tran- sition metal-GaN (TM-GaN) alloys have a high thermal stability and are considered promising candidates for applications in high ∗ Corresponding author. E-mail address: cocoletz@ifuap.buap.mx (G.H. Cocoletzi). radiation microelectronics and for the manufacture of electronic transistors at high temperature and high power [10]. In particular, it has been shown that it is possible to fabricate Sc x Ga 1-x N thin films with the band gap varying linearly from 2 to 3 eV [11]. Scan- dium nitride (ScN) is a semiconductor with a smaller energy band gap of 2–2.4 eV, and although it crystallizes in the rock salt phase [12,13], in the ScGaN thin films ScN forms a hexagonal structure [14–17]. The GaN/ScN system is also important, because in the device applications it is essential to have good quality GaN films. However, since the majority of GaN devices are grown on substrates such as silicon, silicon carbide, or more often sapphire, this type of growth results in a high density of threading dislocations and other defects. It has been demonstrated that the use of ScN interlayers reduce the number of those defects in GaN [18,19]. To further improve the quality of the material, a fuller under- standing and control of the growth mechanism is needed. Although there are previous experimental and theoretical studies of the adsorption of other metals on GaN surfaces [20–25], very little is known about the initial stages of the adsorption of Sc and ScN on GaN surfaces. In this paper we examine the initial adsorption and diffusion of Sc ad-atoms on GaN(0 0 0 1) surfaces. We have also studied the formation of full monolayers of Sc and ScN on those sur- faces. The paper is organized as follows: In Section 2 we describe the method. In Section 3 we present the results and in Section 4 we make conclusions. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.094