Structure and phase transition of a uniaxially incommensurate In monolayer on Si(111)

Shigemi Terakawa, Shinichiro Hatta, Hiroshi Okuyama, and Tetsuya Aruga

Phys. Rev. B 100, 115428 – Published 16 September 2019

Abstract

We have studied the atomic structure and phase transition of a dense metallic monolayer of In on Si(111). Although the monolayer phase was previously considered to have a $(\sqrt{7} \times \sqrt{3})$ periodicity, low-energy electron diffraction and scanning tunneling microscopy observations have revealed that the phase has an incommensurate structure with the In overlayer uniaxially contracted by 2% from $(\sqrt{7} \times \sqrt{3})$ . The uniaxially contracted structure was found to be more stable than the commensurate $(\sqrt{7} \times \sqrt{3})$ structure by first-principles calculations. We also observed a phase transition to a $(\sqrt{7} \times \sqrt{7})$ phase at 250–210 K upon cooling. Angle-resolved photoelectron spectroscopy and macroscopic four-point-probe conductivity measurements demonstrated that the transition induces the disappearance of metallic surface states and a sharp drop in sheet conductivity, respectively. These results indicate an electronic metal-insulator transition.

Received 29 May 2019
Revised 28 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.115428

Physics Subject Headings (PhySH)

Metal-insulator transition Surface reconstruction Surface states

Monolayer films

Angle-resolved photoemission spectroscopy First-principles calculations Low-energy electron diffraction Resistivity measurements Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shigemi Terakawa, Shinichiro Hatta , Hiroshi Okuyama, and Tetsuya Aruga^*

Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

^*aruga@kuchem.kyoto-u.ac.jp

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Issue

Vol. 100, Iss. 11 — 15 September 2019

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Images

Figure 1
LEED patterns of the surfaces with coexisting (a) $(\sqrt{7} \times \sqrt{3})$ -hex and $(4 \times 1)$ phases at RT and (b) $(\sqrt{7} \times \sqrt{7})$ and $(4 \times 1)$ phases at 130 K. The blue (gray) transparent lines indicate the $(1 \times 1)$ and $(4 \times 1)$ spots. (c) and (d) Enlarged images of the region indicated by the dashed rectangles in (a) and (b), respectively. The dotted black, solid red, and double blue circles represent the reciprocal lattice points of $(4 \times 1)$ , $(\sqrt{7} \times \sqrt{3})$ , and $(\sqrt{7} \times \sqrt{7})$ , respectively. The spot indicated by the arrow in (c) is the hex spot belonging to the minor domain rotated by $120^{\circ}$ . Two weak spots enclosed by the squares in (d) are from the ( $2 \sqrt{7} \times \sqrt{7}$ ) structure. (e) and (f) The reciprocal lattices of $(\sqrt{7} \times \sqrt{3})$ and $(\sqrt{7} \times \sqrt{7})$ and the corresponding real-space unit cells. The $(4 \times 1)$ reciprocal lattice points are also depicted in (e). (g) Schematic of the spot pattern of the incommensurate hex phase. The neighboring $(\sqrt{7} \times \sqrt{3})$ spots are also depicted by the open red circles for easy identification of the positional relationship between the hex and $(1 \times 1)$ spots.
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Figure 2
(a) Large-scale STM image ( $2400 \times 1300 Å^{2}$ ) of the surface covered with the hex and $(4 \times 1)$ phases, acquired at sample bias $V_{s} = - 2.0$ V and tunneling current $I = 0.1$ nA. (b) STM image ( $220 \times 400 Å^{2}$ ) of the adjacent hex and $(4 \times 1)$ domains ( $V_{s} = + 1.5$ V, $I = 0.1$ nA). (c) High-resolution STM image ( $140 \times 140 Å^{2}$ ) of the hex phase ( $V_{s} = + 0.3$ V, $I = 0.1$ nA). (d) Enlarged images of the (left) zigzag and (right) linear-type atomic chains of the hex phase, clipped from the dotted and solid rectangles in (c), respectively. The letters Z and L on the left and bottom sides of (c) indicate the types of the chains. The area enclosed by the dashed box corresponds to the structure model shown in Fig. 3. All the STM images were taken at RT.
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Figure 3
(a) Schematic of the $θ = 1.4$ model with the $(\sqrt{7} \times \sqrt{3})$ periodicity given by Park and Kang [25]. The parallelogram indicates the $(\sqrt{7} \times \sqrt{3})$ unit cell. (b) and (c) The model of the incommensurate hex phase. The darkly colored In adatoms are close to the $H_{3}$ sites and protrude more than the brightly colored ones. The rectangle in (c) marks the region enlarged in (b). The letters Z and L on the right side indicate the types of the atomic chains.
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Figure 4
Top and side views of the optimized structure of the ( $7 \times \sqrt{3}$ ) slab model. The rectangle indicates the ( $7 \times \sqrt{3}$ ) unit cell. The dashed line indicates the unit cell of the $θ = 1.4$ model, which, however, is contracted uniaxially by 2% along $[\bar{1} 10]$ .
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Figure 5
(a) Close-up views of the LEED patterns measured during cooling ( $E_{p} = 56 eV$ ). The solid red circle, blue squares, and dotted green circles indicate the spots of the hex, $(\sqrt{7} \times \sqrt{7})$ , and $(4 \times 1)$ phases, respectively. (b) The intensities of the superstructure spots of the three phases as a function of temperature upon cooling.
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Figure 6
(a) Temperature dependence of $σ$ of the surface with coexisting hex and $(4 \times 1)$ phases measured during cooling [blue (light gray)] and heating [red (dark gray)]. Fitting curves are also shown as dashed lines. (b) Fermi surface maps of the surfaces with coexisting (left) hex and $(4 \times 1)$ phases at RT and (right) $(\sqrt{7} \times \sqrt{7})$ and $(4 \times 1)$ phases at 130 K. The maps were obtained with an energy window of 50 meV. The solid lines represent the $(1 \times 1)$ SBZs. The dashed curves indicate the observed Fermi surface of the hex phase. The dotted curves represent the replica of the Fermi surfaces of the triple-domain ( $4 \times 1$ ) phase [17, 18].
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