ELSEVIER Surface Science 386 (1997) 322 327 surface science Electrical conduction via surface-state bands Shuji Hasegawa a,* Xiao Tong b, Chun-Sheng Jiang a,1, Yuji Nakajima a, Tadaaki Nagao a a Department of Physics, School of Science, Universityof Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan b CREST, Japan Science and Technology Corporation (JST), Kawaguchi Center Bldg.. Hon-cho 4-1-8, Kawagucht Saitama 322, Japan Received 5 November 1996; accepted for publication 26 February 1997 Abstract We will discuss a two-dimensional electron system originating from a surface-state band and especially its electronic transport properties. We have found a surface superstructure that has a highly conductive surface-state band, the Si( 111 )-X/2T x ~/~R _+10.89 ° structure, induced by adsorption of a 0.14-atomic-layer of Au or Ag onto a Si( 111 )-~/3 x '~/3-Ag surface. Photoemission spectroscopies showed that this ~ x V~ structure had a highly dispersive surface-state band crossing over the Fermi level, while the surface space-charge layer was a depletion layer. It was then concluded that the observed excess surface conductance was due to the two- dimensional band of the surface electronic state. © 1997 Elsevier Science B.V. Keywords: Angle-resolved photoemission spectroscopy; Silicon; Surface electrical transport; Surface structure; X-ray photoelectron spectroscopy 1. Introduction In general, electronic states at surfaces are more or less different from those in the bulk because of abrupt interruption of the crystal periodicity and dangling bonds at the surfaces. These surface states play main roles in their properties as well as in atomic structural reconstructions. Especially, because the surface states are localized only near the topmost atomic layers, the electrons in these states provide an inherently two-dimensional electron system. In contrast to conventional two- dimensional electron-gas systems formed with * Corresponding author. Tel: (+81) 3 3812.2111 (ex)4167; fax: (+ 81 ) 3 5689.7257; e-mail: shuji@phys.s.u-tokyo.ac.jp 1 Present address: Surface and Interface Laboratory, The Institute of Physical and Chemical Research, Wako, Saitama, 351-01, Japan. 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0039-6028 (97)00331-2 bulk-state electrons confined by band bending at heterojunctions [1 ], the two-dimensional electron system in the surface-state band is expected to have novel properties correlating with surface- structural modifications. For example, a clean Si(lll) surface reconstructs into a 7 × 7 superstructure to reduce the number of the dangling bonds [2]. This dangling-bond state on each "adatom" has an unpaired electron, resulting in a half-filled, there- fore metallic, surface-state band [3]. This band is actually detected by angle-resolved ultra-violet photoelectron spectroscopy (ARUPS), denoted "$1" in Fig. la. This surface state is always detected at the Fermi level (EF) at any emission angle, indicating negligible band-dispersion. This may be because the overlap integral between the neighboring dangling-bond states is so small that