SCANNING-TUNNELING-MICROS COPY AND PHOTOEMISSION-STUDY OF AN ALKALI-METAL-INDUCED STRUCTURAL PHASE-TRANSITION - SI(111)-(7X7) INTO SI(111)-NA(3X1)

The structural phase transition from the clean Si(111)-(7 x 7) reconstruction to the Na-stabilized (3 x 1) reconstruction of this surface is studied using scanning tunneling microscopy as well as core- and valence-level photoemission. By combining the laterally integrating technique of photoemission with the highly spatially resolving technique of scanning tunneling microscopy, the kinetics of the phase transition is explained on an atomic scale. In addition to the information on the kinetic pathway of the phase transition, we present a simple model of the dynamics, which explains the time dependence of the transition rate from the (7 x 7) to the Na(3 x 1) reconstruction of the Si(111) surface. It is demonstrated that samples can be prepared with both surface reconstructions coexisting on the surface in any desired ratio. These coexisting islands show the properties of the pure reconstructions, which demonstrates a strong lateral modulation of the position of the Fermi level in the semiconductor band gap at the surface.