ELSEVIER Surface Science 376 (1997) 69-76 surface science Electrical conduction through the surface-state band of the Si( lll)-X/21 x X/21-(Ag + Au) structure Chun-Sheng Jiang 1, Xiao Tong, Shuji Hasegawa *, Shozo Ino Department of Physics, School of Science, Universityof Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 31 May 1996; accepted for publication 29 November 1996 Abstract The Si(111)-~/21 × ~/21 superstructure, which was induced by 0.14 atomic layer Au adsorption onto the Si(lll)-~/3 x ~/3-Ag surface at room temperature, was found to have a very high surface electrical conductance, higher than that of the Si(ll 1)-7 x 7 clean surface by about 3.1 × 10-4 A/V. Photoemission spectroscopies showed that this X/21 structure had a dispersive surface-state band crossing the Fermi level, while the surface space-charge layer was a depletion layer. It was thus concluded that the observed excess surface conductance was due to the two-dimensional band of the surface electronic state. © 1997 Elsevier Science B.V. All rights reserved. Keywords: Angle-resolved photoemission spectroscopy; Reflection high-energy electron diffraction [RHEED); Silicon; Surface electrical transport; Surface electronic phenomena; Surface structure; X-ray photoelectron spectroscopy 1. Introduction Electrical conduction near semiconductor sur- faces is in general classified into three types, each of which is in principle closely related to the surface structure [ 1 ]: (1) Conduction via a surface space-charge layer. Excess charges trapped in the surface state cause band bending below it, resulting in changes of carrier concentrations in the surface space-charge layer, whose width reaches several microns in a lightly doped semiconductor substrate. The surface electronic states of each superstructure thus can *Corresponding author. Fax: + 81 3 5689 7257; e-mail: shuji@phys.s.u-tokyo.ac.jp, Present address: Surface and Interface Laboratory, The Institute of Physical and Chemical Research, Wako, Saitama 351-01, Japan. decisively govern the electrical conductivity through the layer. (2) Conduction via surface-state bands. Two- dimensional bands are formed due to the surface superstructure. The electrons in the bands should be mobile along the surface just like electrons in the three-dimensional bulk bands, so that they contribute to electrical conduction. Conductivity of this type is directly dependent on the nature of the surface-state band (metallic or semiconducting) and also on the mobility of the carriers therein. (3) Conduction via a grown atomic layer. If, for example, a metal atomic layer grows on a semicon- ductor surface at low temperatures, the grown layer dominates the conduction above a percola- tion-threshold coverage. Diffusivity of carrier scat- tering at surface/interface varies depending on the morphology of the surface, leading to changes of carrier mobility. So the growth modes and kinetics, 0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0039-6028 (96)01569-5