One monolayer of In on a Si(111) surface forms 4×1 structure at room temperature (RT). This In/Si(111)-4×1 surface has a quasi- one dimensional (1D) metallic electronic structure and undergoes a symmetry-lowering phase transition into an insulating 8×2 phase at low temperature (LT) upon cooling the sample below approximately 130 K [1]. Adsorption of Na atoms on the surface was reported to convert the 4x1-RT phase into the 8×2-LT phase even at RT [2]. We investigated using low-energy electron diffraction (LEED) the influence of alkali-metals (Na, K, Li) adsorption on the structural phase transition of this quasi-1D In/Si(111) surface. Introducing the alkali metals with low coverage on the surface at room temperature was found to affect the 4×1-to-8×2 structural phase transition by lowering the transition temperature (Tc). The Tc decreased almost linearly and the transition becomes more rounded when the amount of deposited alkali-metals atoms was increased. The decrease of Tc with the increase of the amount of adsorbed Na atoms is suggested to be due to the doping of electrons from adsorbates to the substrate [3]. In the high-coverage regime, new superstructures appear in LEED with the adsorption of alkali metals. Comparison of the adsorbate-induced superstructures in low-coverage and high-coverage regimes will be made and discussed.

[1] H. W. Yeom, et al., Phys. Rev. Lett. 82, 4898 (1999)

[2] S. S. Lee, et al., Phys. Rev. Lett. 88, 196401 (2002)

[3] H. Shim, et al., Appl. Rev. Lett. in press (2009)