Theoretical study of the formation of a GaAs bilayer on Si(1 1 1) Alfredo Ramírez Torres a,⇑ , Gregorio H. Cocoletzi a,b,c , R.A. Vázquez-Nava d , M. López-Fuentes e , Noboru Takeuchi c a Instituto de Física, Universidad Autónoma de Puebla, Apartado Postal J-48, Puebla 72570, Mexico b Centro de Investigación Científica y de Educación Superior de Ensenada, Km. 107 Carretera Tijuana-Ensenada, Código Postal 22860, Apartado Postal 2732 Ensenada, Baja California, Mexico c Centro de Nanociencia y Nanotecnología, Universidad Nacional Autónoma de México, Ensenada Baja California, Mexico d Centro de Investigación en Óptica, León, Guanajuato, Mexico e Facultad de Ingeniería Química, Universidad Autónoma de Puebla, Puebla, Mexico article info Article history: Received 6 March 2012 Accepted 16 May 2012 Available online 20 June 2012 Keywords: 2D Nanostructure Density functional theory Silicon Gallium arsenide abstract We have performed first principles total energy calculations, within the density functional theory, to investigate the formation of a GaAs bilayer on the Si(1 1 1) surface. For the exchange–correlation energies we have used the generalized gradient approximation, while the electron–ion interactions were treated using ultrasoft pseudo-potentials. We have first studied the adsorption of As and Ga atoms on the Si(1 1 1) surface. A monolayer of As atoms substitutes the topmost Si layer forming an almost ideally bulk termi- nated configuration. On the other hand, the adsorption of 1/3 ML of Ga results in the formation of a p 3  p 3 reconstruction, with Ga atoms occupying T4 sites. The deposit of 1/3 ML of Ga atoms on the As terminated Si(1 1 1) surface also results in the occupation of T4 sites. However, at full monolayer cov- erage, the Ga atoms occupy near top sites and form one dimensional atomic chain. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction We can find applications of silicon in transistors, solar cells, rec- tifiers, integrated circuits and other solid-state devices, which are used extensively in the electronics industry. On the other hand, GaAs possesses some electronic properties which are better to those of silicon in specific applications: first of all, GaAs has a direct band gap, which means that it emits light more efficiently than sil- icon, which has an indirect gap. It has higher electron mobility, allowing electronic devices made from it to function at higher fre- quencies. They also have higher breakdown voltages, and therefore they can be operated at higher power levels. For many years the growth of GaAs-on-silicon was the focus of some intensive re- search at universities, wafer suppliers, and research laboratories. The idea was to provide a platform where III–V optical devices and silicon digital technology could be combined. However, the mismatch between the GaAs and Si lattice constant produces a large number of dislocation defects at the interface of the two materials, making it difficult to grow large amounts of GaAs on Si substrates. However, using techniques such as molecular beam epitaxy, it is possible to form 2D nanostructures of GaAs on Si sur- faces [1], it is therefore important to understand the atomic pro- cesses involved in the formation of such 2D nanostructures. Surface-sensitive core-level spectroscopy measurements of the Si(1 1 1):As-1  1 surface have revealed a well-ordered surface with threefold coordinated As atoms replacing the Si atoms in the outermost layer [2]. The absence of Si dangling bonds in this structure has led to an unreconstructed surface which is highly resistant to contamination. X-ray standing waves in ultra-high vac- uum experiments [3], medium-energy ion scattering measure- ments [4], angle-resolved photoemission measurements [5], medium-energy ion scattering studies and scanning tunneling microscopy images [6] have also confirmed that adsorption of As on the Si(1 1 1) surface results on the replacement of the top half of the Si surface bilayer by As atoms. On the other hand, the adsorption of Ga on Si(1 1 1) takes place with a different coverage and surface symmetry [7]. Low energy electron diffraction (LEED) experiments, k-resolved inverse-photoemission spectroscopy [8], X-ray standing-wave and tunneling-microscope [9], and first prin- ciples pseudo-potential total energy calculations [8,9] have shown that the adsorption of 1/3 ML of Ga atoms on Si(11 1) form a p 3  p 3 reconstruction, with the Ga atoms occupying T4 sites on top of a second layer Si atom. The formation of GaAs structures on the Si(1 1 1) and Ge(1 1 1) surfaces have also been investigated experimentally [1,10], indicating that the atomic structure displays a surface bilayer with the As atoms occupying the outermost atom- ic layer. Our first principles calculations show that a monolayer of As atoms substitutes the topmost Si layer forming an almost ideally 0927-0256/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2012.05.043 ⇑ Corresponding author. Tel.: +52 222 2295610; fax: +52 222 2295611. E-mail address: aramirez@ifuap.buap.mx (A. Ramírez Torres). Computational Materials Science 62 (2012) 216–220 Contents lists available at SciVerse ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci