Structure and electrical conductance of Pb-covered Si(111) surfaces

A dense commensurate root 3x root 3 phase was obtained by additional 0.7 ML Pb adsorption onto the Si(111)-root 3x root 3-Pb surface ( 1/3 ML Pb) at r oom temperature (RT). Its electronic structure was similar to that of the h exagonal and striped incommensurate (HIC and SIG) phases. A high surface el ectrical conductance observed for this phase was attributed partly to an in crease of carrier concentration in the surface space-charge layer, and more importantly to the formation of a metallic surface state. The HIC phase, o n the other hand, was found to convert to the SIC phase with only 0.08 ML a dditional Pb adsorption at RT, which was accompanied with a steep increase in conductance. This was due to an increase of carrier concentration in the metallic surface state, and also possibly due to an enhancement of its car rier mobility. Both enhancements are suggested to come from the reduced dom ain-wall density in the SIC phase compared with the HIC phase. [S0163-1829( 99)09731-3].