Surface structure of Si(1 1 1)-(8  2)–In determined by reflection high-energy positron diffraction Y. Fukaya a, * , M. Hashimoto a , A. Kawasuso a , A. Ichimiya a,b a Advanced Science Research Center, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan b Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Bunkyo-ku, Tokyo 112-8681, Japan article info Article history: Received 1 February 2008 Accepted for publication 17 May 2008 Available online 24 May 2008 Keywords: Surface structure Phase transition Reflection high-energy positron diffraction Total reflection Silicon Indium abstract By using reflection high-energy positron diffraction (RHEPD) and first-principles calculations, we inves- tigated an In/Si(1 1 1) surface on which the quasi-one-dimensional In atomic chains that exhibit the metal–insulator transition were formed. From the analyses of rocking curves, we found the transforma- tion of the zigzag chain structure of In atomic chains to hexagon structures below 130 K along with the phase transition from the 4  1 to the 8  2 periodicities. The band structure calculated with the opti- mum hexagon structure displays the gap opening of 60 meV, which indicates the semiconducting char- acter. This confirms the recent theoretical prediction that the hexagon structure is energetically favored at low temperatures [C. González, F. Flores, J. Ortega, Phys. Rev. Lett. 96 (2006) 136101]. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction One-dimensional structures frequently exhibit a metal–insula- tor transition originating from the distortion due to the Peierls instability that results in lower symmetries [1]. The In-adsorbed Si(1 1 1) (In/Si(1 1 1)) surface is one of such one-dimensional struc- tures [2,3]. On this surface, In atomic chains with a zigzag chain structure having a periodicity of 4  1 are formed at room temper- ature, as shown in Fig. 1a [4–6]. One-dimensional metallic conduc- tion occurs along the In atomic chains [7]. In 1999, Yeom et al. found that the periodicity changed from 4  1 to 8  2 below 130 K [8]. Based on the angle-resolved photoemission spectros- copy (ARPES) observations, they reported that below 130 K, the metallic conduction along the In atomic chains vanishes [8]. From the electrical conductivity measurements using the microscopic four-point probe method, Tanikawa et al. confirmed the occurrence of metal–insulator transition below 130 K [9]. To determine the structure of In atomic chains at low tempera- tures, many extensive studies have been carried out using scanning tunneling microscopy (STM) [10–14], photoemission spectroscopy (PES) [15,16], high-resolution electron-energy-loss spectroscopy (HREELS) [17], low-energy electron diffraction (LEED) [18], surface X-ray diffraction (SXRD) [19], and first-principles calculation [20– 22]. Most of the results support the fact that at low temperatures, In atomic chains consist of trimers, as shown in Fig. 1b [19,22]. However, it is proposed that more drastic structural changes, such as the hexagon formation as shown in Fig. 1c, should be considered to explain the appearance of the band gap [23,24]. Thus, more de- tailed structural analyses are required to reveal the structural changes in the In atomic chains at low temperatures. Reflection high-energy positron diffraction (RHEPD) is a power- ful surface method owing to total reflection [25,26]. When the glancing angle of a positron beam is sufficiently small, the posi- trons are totally reflected at the first surface layer due to the pres- ence of positive potential barriers. The critical angle is given by h c = arcsin(V/E) 1/2 , where V and E denote the mean inner potential of crystals and the accelerating voltage of the incident positron beam, respectively [26]. For example, at E = 10 kV, h c is estimated to be 2.0° when V = 12 V for an Si crystal. Under the total reflection condition, the diffraction intensities are found to be very sensitive to the structures and thermal vibration of the first surface layers. In this study, by using RHEPD, we examined the structure of In atomic chains on the In/Si(1 1 1) surface at low temperatures. We also per- formed the first-principles calculations using the optimized struc- ture. Finally, we show the electronic surface band structures calculated using the optimized structure. 2. Experimental procedure A specimen was cut from an mirror-polished n-type Si(1 1 1) wafer with a resistivity of 1–10 X cm. To obtain a clean 7  7- reconstructed surface, the specimen was flashed several times at 1473 K in an ultra-high vacuum chamber evacuated with a base 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.05.021 * Corresponding author. Tel.: +81 27 346 9330; fax: +81 27 346 9432. E-mail address: fukaya.yuki99@jaea.go.jp (Y. Fukaya). Surface Science 602 (2008) 2448–2452 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc