Ab initio study of Yb on the Ge(111)–(3 × 2) and Si(111)–(3 × 2) surfaces S. Özkaya a,b, ⁎, M. Çakmak a , B. Alkan c a Department of Physics, Gazi University, 06500 Ankara, Turkey b Department of Physics, Aksaray University, 68100 Aksaray, Turkey c Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Ankara, Turkey abstract article info Article history: Received 21 January 2010 Accepted 26 July 2010 Available online 4 August 2010 Keywords: Density functional theory Low index surface Ge Si Adsorption Pseudopotential method The surface reconstruction, 3 × 2, induced by Yb adsorption on a Ge (Si)(111) surface has been studied using first principles density-functional calculation within the generalized gradient approximation. The two different possible adsorption sites have been considered: (i) H 3 (this site is directly above a fourth-layer Ge (Si) atom) and (ii) T 4 (directly above a second-layer Ge (Si) atom). We have found that the total energies corresponding to these binding sites are nearly the same, indeed for the Yb/Ge (Si)(111)–(3 × 2) structure the T 4 model is slightly energetic by about 0.01 (0.08) eV/unitcell compared with the H 3 model. In particular for the Ge sublayer, the energy difference is small, and therefore it is possible that the T 4 , H 3 , or T 4 H 3 (half of the adatoms occupy the T 4 adsorption site and the rest of the adatoms are located at the H 3 site) binding sites can coexist with REM/Ge(111)–(3 × 2). In contrast to the proposed model, we have not determined any buckling in the Ge = Ge double bond. The electronic band structures of the surfaces and the corresponding natures of their orbitals have also been calculated. Our results for both substrates are seen to be in agreement with the recent experimental data, especially that of the Yb/Si(111)–(3 × 2) surface. © 2010 Elsevier B.V. All rights reserved. 1. Introduction There has been a considerable interest in one and two dimensional (1D and 2D) superstructures because of their fundamental and technological importance. A large amount of both theoretical and experimental work has been performed on this issue in parallel with the aim of elucidating the nature of such systems [1–5]. The various metals, such as alkali metals (AM) [6,7], alkaline-earth metals (AEM) [8,9], and rare-earth metals (REM) [10,11], depending on the metal coverage and the substrate temperature, induce 1D reconstructions, when adsorbed on the Si(111) and Ge(111) surfaces. Among these systems, the (3 × 1) reconstruction is an interesting and well known 1D surface. The surface sensitive techniques, like scanning tunneling microscopy (STM) [12–14] and low-energy electron diffraction (LEED) [15–17], show that the Si(111) and Ge(111) surfaces undergo the (3 × 1) reconstruction on the adsorption of the AM and Ag [18] at the 1/3 monolayer (ML) coverage. The experimental works appear to support that these phases have similar structures. Since the AM/Si(111)–(3 × 1) surfaces have an even number of valence electrons in the unit cell, it is expected that the electronic band structure of this system must have a semiconductive character. This semiconductive behavior has also been supported by the angle- resolved photoelectron spectroscopy (ARPES) measurements [19,20]. On the other hand, the (3 × 1) phase induced by the adsorption of the AEM with an odd number of the valence electrons must be metallic. However, ARPES results [21,22] show that this structure exhibits a semiconductive property similar to the AM/Si(111)–(3 × 1) surface. An et al. [9] interpreted this inconsistency as the evidence for a Mott– Hubbard insulator phase. The other well-known example for the 1D systems, induced by the adsorption of AEM and REM, is the (3 × 2) reconstruction. This phase has been reported to have the honeycomb chain-channel (HCC) reconstruction as observed on the AM induced (3 × 1) surfaces at 1/3 ML of metal atom in the channel. Recently, the REM/Ge(Si)(111)– (3 × 2) systems have gained a renewed interest because of their potential use in opto-electronic and micro-electronic devices. ARPES studies by Takada et al. [23] showed that Yb/Si(111)–(3×2) reconstruction had five surface states in the fundamental band gap region. The dispersions of three of them matched those of the surface states of the AM/Si(111)–(3 × 1) structure, while the dispersion of two other surface states matched those of the Ca/Si(111)–(3 × 2) recon- struction. Similar works have also been carried out for the Sm and Eu atoms [11,23–26]. In a recent work, Kuzmin et al. [27] have investigated the atomic and electronic structure of the Yb/Ge(111)–(3 × 2) surface with 1/6 ML coverage, using high-resolution synchrotron-radiation photoelec- tron spectroscopy and low-energy electron diffraction (LEED). In order to explain the substrate structure of the Yb/Ge(111)–(3×2) phase, they have proposed a new model based on the classical HCC structure. The stability of this model has been attributed to the buckling of the Ge = Ge double bond in the top layer. In addition, it Surface Science 604 (2010) 1899–1905 ⁎ Corresponding author. Department of Physics, Gazi University, 06500 Ankara, Turkey. E-mail address: sozkaya@gazi.edu.tr (S. Özkaya). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.07.025 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc