The behaviour of Na on 1*1 and 7*7 structures of Si(111) and its effect on the oxidation of these structures

Abstract

Deposition of Na on 1*1 and 7*7 structures of Si(111) at room temperature (RT) forms a uniform layer with a rather weak metallic character. The coverage of Na can be determined at any time of the Na deposition by a flux meter. The binding energy of Na is greater on a reconstructed 7*7 than on the 1*1 structure of a Si(111) surface. The initial dipole moment and the maximum Delta phi are greater for Na on the 1*1 structure. Near saturation Na coverage, the oxygen goes initially under the Na layer forming a Na-O-Si complex and subsequently resides on top of it. The presence of Na on Si(111) causes a drastic increase of the initial sticking coefficient of oxygen by about ten times. When oxygen is adsorbed on Na-covered Si(111) surfaces at RT and the system is heated to 700 degrees C, Na is removed from the surface while oxygen forms SiO4 and SiO2. By repeating this cycle several times (seven) SO2 predominates and its average thickness is about 9 AA. However, this thickness is smaller than when Cs is used instead of Na. This indicates that Cs is more effective as a promoter of the oxidation. However, the oxide formation occurs at a lower temperature than with Cs additives. Finally, the effect of Na on the oxidation of Si was almost the same on the 7*7 and 1*1 structures of Si(111) surfaces.