We report on experiments and simulations of charge transport through the surface state of Si(111)(√3 x √3)-Ag in UHV. This surface exhibits a parabolic and isotropic surface state S1. Adjusting the amount of additional Ag atoms on the surface shifts the onset of the S1 state below the Fermi Energy EF and forms a two dimensional electron gas (2DEG) [2,4]. We present Scanning Tunneling Spectroscopy (STS) data that evaluate the onset of the S1 state ~ 250mV below EF. The in-plane electrochemical potential µec(x,y) of this current-carrying 2DEG has been imaged by means of Scanning Tunneling Potentiometry (STP). At every scan point, first the tip-sample distance is controlled in the usual constant current mode. The tip z-position is subsequently fixed for a 40-ms sampling interval and the tip-sample voltage is adjusted to zero net tunneling current, thus providing µec(x,y) with an accuracy of 1µV [1]. Variations of µec(x,y) in the vicinity of scattering centers were imaged on the nanometer scale. The experimental data allow to distinguish between scattering at defects and nonlocal scattering. The data proves that the surface state S1 contributes to the charge transport. Simulations of spreading resistance networks were made. Effective transmission probabilities for scattering at defects as well as resistances for nonlocal scattering are derived. Strongest scattering, i. e. low transmission probability, is observed at island edges that naturally form during the deposition process. The spatial resolution demonstrated here gives complementary information to conductance measurements by micro-four-point probes [2]. This work was supported by the SFB 602 TP A7.
[1] M. A. Schneider, et. al., Appl. Phys. Lett. 69, 1327 (1996)
[2] I. Matsuda et. al., Phys. Rev. Lett. 93, 236801 (2004)
[3] M. Ono et. al., Phys. Rev. Lett. 96, 0168011 (2006)
[4] S. Hasegawa, Surf. Sci. 386, 322 (1997)
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