Abstract
Si atoms adsorption behavior, Si atoms dynamic properties, and the manipulation of Si atoms on the Pb/Si(111) surface have been studied with variable-temperature scanning tunneling microscopy (VT-STM). This thesis has been divided into three parts. In the first part, the adsorption of single Si atoms and the adsorption sites on the surface at low temperature (LT) have been reported. The dynamic properties and stability of the Si atoms have been discussed in the second part of this thesis. In the third part, Si adatoms were manipulated either along or across the trimer row by using STM tip. Additionally, the nature and dynamics of Si dimer have been explored. In the first part, the adsorption of single Si atoms on the Pb/Si(111) surface at low temperature (~125 K) has been studied. It is well known that at room temperature a monolayer Pb on Si(111) surface exhibits (1×1) structure. This surface structure transformed reversibly into a low- symmetry row-like (√7×√3)-Pb structure at a transition temperature ~270 K. At 125 K, the adsorbed Si atoms are found to be immobile and their adsorption sites were examined in details. Single Si atoms tend to appear near the on-top site (T1 site) of the Si(111) substrate. The adsorbed Si atoms have different appearance at empty- and filled-state STM images in high tunneling conditions; however, they have the same appearance at low tunneling conditions. The atomic-resolved STM images revealed that adsorbed Si atoms prefer to occupy either at T1A or T1B sites. The adsorption behavior of single Si atoms on the surface provided a strong evidence of breaking of mirror symmetry in the (√7×√3)-Pb structure. In the second part, the stability and dynamics of Si atoms have been studied with increasing sample temperature. We have found that Si atoms started to switch between T1A and T1B sites inside a Pb trimer at the temperature higher than 150 K. When the temperature raised above ~160 K, the adsorbed Si atoms could hop to other trimers along the same trimer row. Below i ~170 K, short hops to adjacent trimers dominated, but long hops dominated at temperature above ~170 K. The activation energy and prefactor for the Si atoms diffusion were derived through analysis of continuous-time imaging at temperature from 160 to 174 K. In addition, irreversible aggregation of single Si atoms into Si clusters started to occur at the phase boundaries or defective sites at temperature above ~170 K. At temperature above ~180 K, nearly all Si atoms disappeared on the surface and aggregated into clusters, which may have important implications on the atomic mechanism of epitaxial growth of Si on the Pb/Si(111) surface. The lateral manipulation of single Si atoms was demonstrated at ~125 K using the tips of an STM and this study has been included in the last part of this thesis. At this sample temperature, Si adatoms are not mobile. STM tips were used to move Si adatoms either along or across the surface trimer rows by employing single-point I-Z spectroscopy. One Si adatom was brought closer to another Si adatom on the same trimer row. When a Si dimer was formed, it diffused rapidly on the Pb-covered surface until it was trapped by the domain boundaries or defect sites. This work demonstrates the prospective to use atomic manipulation methods to reveal the surface dynamic processes that hardly be observed with STM alone. In addition, the manipulation revealed the true atomic positions of Si adatoms on the sample surface, which solves a common problem that STM images may not reflect the real positions of adsorbed adatoms.