A Molecular Dynamics Study Of The Surface Structure And Thermal Stability Of Semiconductor Nanowires

Semiconductor nanowires have been the research focus in recent years in the research fields of nanotechnology and condensed matter physics. Due to their unique and fascinating properties, semiconductor nanowires are expected to have a variety of applications for nanoscale devices, such as lasers, detectors, sensors, transistors, emitted-light diodes, logic-gate electrocircuits, spintronic devices, and quantum computers. Semiconductor nanowires can also be used as effective wires to connect different nanoscale devices.Due to their large surface-to-volume rations, the physical properties of semiconductor nanowires are largely determined by its surface structures. However, the knowledge of the surface structure of a semiconductor nanowires the knowledge about the surface structure of a semiconductor nanowire has been very limitted. Thin semiconductor nanowires have been studied by first-principles calculations. Although first-principles calculations can give accurate results, the system size studied are limited by the computational cost and thus are not comparable with practical nanowires found in experiments. Another problem is that it is almost impossible to use first-principles methods to study the thermal stability of semiconductor nanowires. Therefore, we have studied in this thesis the surface structure and thermal stability of semiconductor nanowires using the molecular dynamics simulation based on many-body empirical potentials. Using many-body empirical potentials significantly reduces computational cost, which allows us to use the standard simulated annealing global optimization technique to search for the most stable surface structure of nanowires with hundreds and thousands of atoms in a long simulation time.The thesis consists of six chapters. Chapter one is the introduction. In chapter two, we introduce the theory and applications of molecular dynamics simulations . In chapter three, typical many-body empirical potentials are discussed. In chapter four, we report results of molecular dynamic simulations for the surface structure of pristine silicon nanowires and germanium nanowires with bulk cores oriented along the [110] direction and bounded by the (100) and (110) surfaces in lateral directions. Our results show that no reconstruction exists on the (110) surfaces and the 2x1 symmetrical dimers form on the (100) surfaces. The dimer rows are perpendicular to or parallel to the axis wire following the direction of the surface dangling bands. The dimer length is 0.236nm and 0.249nm for the silicon dimers and germanium dimers on the (100) surfaces, respectively. In chapter five, we show how axial tensile/compressive stress affects the (100) surface structure for the silicon nanowire. We find that different stress results in different surface struture on the (100) surface of a silicon nanowire. In chapter six, we report results of melting of silicon nanowires. We show that the melting temperature of a small silicon nanowire is significantly lower than the melting temperature of bulk silicon. Furthermore, we show that the melting temperature of a silicon nanowires decreases as its diameter decreases. In the last part of this thesis, we summarize our results and discuss some future research directions related to our work.