Abstract
The low temperature electronic growth and phase transitions of Pb on Si(111) surfaces, and the growth of 1-D Si nanowires on Pb/Si(111) surfaces have been investigated by variable temperature scanning tunneling microscopy (VT-STM). This thesis is organized as follows: First, the low temperature growth of 2-D Pb islands on Si(111)-7×7 surfaces and their evolution with temperature. The second part is about the electronic structure of Pb islands and its effects for the island growth. The third part is the phase transitions of Pb atoms on the Si(111) surfaces at low temperature. The fourth part is on the 1-D Si nanowires on Pb/Si(111) surfaces. In the first topic, the growth of Pb films on the Si(111)-7´7 surfaces has been investigated at low temperatures (~ 200 K). Flat-top Pb islands are formed and at low coverage the thicknesses of islands are confined in the range of 4~9 atomic layers. Among these islands, those of 7-layer height are the most abundant. In low coverage limit, these multi-layer islands prefer to grow in size instead of in thickness, showing a 2D growth property. This growth behavior, different from the conventional growth modes, arises from the quantum size effect (QSE). At higher coverage, the growth also reveals layer-by-layer behavior. The Arrhenius plot of the island density versus temperature shows a linear relationship, indicating the formation of islands is thermally activated. The growth of Pb films on incommensurate (IC) Pb/Si(111) surface at low temperatures is also studied. Flat Pb islands can be grown as well, but the threshold thickness is reduced to two atomic layers instead of four. In the second topic, the real-space STM and local-probe scanning tunneling spectroscopy (STS) are involved in the studying of the electronic structure of Pb islands. Quantized states are detected in the current-voltage (I-V) spectra on individual Pb islands of various thicknesses. The asymmetrical and oscillatory relaxation in the island thickness reveals that the charge distribution of confined electrons can influence the interlayer spacing. A simple model based on the infinite potential well can explain satisfactorily all experimental results. The third topic presents the results about the phase transition of Pb atoms on Si(111) surfaces at low temperature. The room temperature 1×1 phase undergoes a phase transition to ´ phase at ~ 250 K. The transition is found to extend over a range of ~ 40 K. The atomic model, in which Pb atoms displace laterally from their T1 sites to from trimers centered on H3 sites, and the other two Pb atoms between two neighboring rows shows a good agreement with STM observations. It is also discovered that the SIC phase transforms to ´ at ~ 64 K. It indicates a high correlation between IC and 1´1 phases. The surface strain field induced by Pb atoms absorbed at defects, at step edges, and island edges plays an important role for these transitions. The 1-D Si nanowires on Pb/Si(111) surface always appeared in pair are parallel with one of three < > directions on both 1×1 and IC phases. The atomic model shows Si nanowires were composed of Si atoms, Si atoms on one side of nanowires were located on T1 sites, and located on H3 sites on the other side accordingly. On the other hand, the IC phase would tend to appear after the formation of Si nanowires on 1×1 phase. It seems that there is a high correlation between 1×1 and IC phase.