Theoretical study of adsorption of gallium and gallium nitrides on Si(1 1 1) Demeter Tzeli * , Ioannis D. Petsalakis, Giannoula Theodorakopoulos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, Athens 116 35, Greece Received 11 July 2007; in final form 19 September 2007 Available online 25 September 2007 Abstract Adsorption of gallium (Ga, Ga + ) and gallium nitrides (GaN, GaN + , GaN 2 , GaN þ 2 ) on a model Si(1 1 1) surface was studied by density functional theory calculations. Significant binding is found for all the species considered, with binding energies up to 6.13 eV. For GaN, GaN 2 , and GaN þ 2 the lowest energy structures are those with the adsorbate forming a bridge between the Si adatom and rest atom, while for GaN + it is a vertical structure connecting to Si via N. Furthermore, it is found that adsorbed GaN and GaN + are essentially the same species, resembling free GaN  . Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction There is great interest in the study of chemisorption of group III metals and their nitrides at Si surfaces. Subjects of particular interest are the geometric and electronic struc- ture of chemisorbed group III metals on Si surfaces [1,2], the surface changes associated with metal diffusion on the surface, the formation of metal-adsorbate atomic wires as well as the self-assembly of perfectly ordered nanocluster arrays [3–5]. Of particular interest is the growth of GaN films on Si surfaces [6,7], which have great potential for application in optoelectronic devices and high-power, high-temperature electronic devices [6,7]. Even though there are many experimental studies of gallium nitrides at Si(1 1 1) [4–7], to our knowledge, there have not been as many theoretical studies. For Ga deposited on Si(1 1 1), there is a theoretical study of a Ga decamer on a unit-cell model of Si(1 1 1) [8] as well as an earlier theoretical work on Ga at small Si (3–9 Si-atoms) clusters [1]. In the present study the electronic and geometric chem- isorbed structures of gallium (Ga, Ga + ) and different gal- lium nitrides (GaN, GaN + , GaN 2 , GaN þ 2 ) on Si(111) are determined using density functional theory (DFT) calcula- tions and a Si 26 H 22 model of the Si(1 1 1) surface. An ade- quate basis set is employed and the validity of the results is examined by comparison with previous work on the free molecular systems. 2. Computational procedure Chemisorbed structures of Ga, Ga + , GaN, GaN + , GaN 2 , and GaN þ 2 , at Si (111) are calculated at a five-layer one-rest one-adatom (1R–1A) cluster model of Si(1 1 1), (structure 1), constructed as previously [9] using the dimer-adatom-stacking fault (DAS) structure [10] and the LEED data of Tong et al. [11] for the Si(1 1 1) recon- structed surface. Hydrogen atoms (white spheres) have been added to terminate the 26-Si atom cluster (grey spher- es ” Si) at the sides as well as below the lowest Si level, cf. 1, while the adatom and rest atom are left with one dangling bond (i.e., one unpaired electron) each. Preliminary DFT calculations were carried out on dia- tomic and triatomic structures (GaN, GaN + , SiGa, SiN, GaN 2 , GaN þ 2 ) and on the 1R–1A Si(111) cluster so as to choose an adequate combination of functional and basis set with respect to the available experimental and theoret- ical data. On the basis of the above calculations, the 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.060 * Corresponding author. Fax: +30 210 7273 794. E-mail address: dtzeli@eie.gr (D. Tzeli). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 448 (2007) 88–92