A Study Of High Frequency Characteristics Of<111>and<112> Silicon Nanowires Based On Keating Potential Model

In recent years, with the rapid development of silicon micro nano manufacturing technology, the silicon micro beam, which is the basic unit of Micro-Electro-Mechanical System (MEMS), has become increasingly unable to meet the necessities of our life. Nano-Electro-Mechanical System (NEMS) device with silicon nanobeams as the basic unit has much more excellent performance than MEMS does, such as a high vibration frequency(GHz), a high quality factor(105), a low power consumption (10-17 watts) and so on. In addition, the silicon nanobeams have unusual thermal conductivity, field emission and other physical properties. Therefore, the silicon nanobeam has been developed in optoelectronic devices, the utilization of new energy and many other fields that the resonator, mixer, filter and biological chemical sensors are widely used. However, there are still a lot of problems in current research of the silicon nanobeams. When the size of silicon beams steps into nano scale, the size effect, surface effect and quantum effect of silicon material are becoming particularly significant, and the atomic behavior of silicon nanobeams will be more complex. Although so far the silicon nanobeams have been simulated by molecular dynamics software and fabricated in some laboratories, the precise theory model are still necessary. On the other hand, the inherent continuum theory model can not accurately explain the high frequency characteristics of the silicon nanobeams. Consequently, a new theory model is urgently to be established to accurately describe the high frequency properties of silicon nanobeams.The main work of this dissertation is described as following:(1) The theory calculation models for the high frequency of ideal silicon nanobeams with orientation <111> and<112> are established based on Keating potential model and semi continuum model. The theoretical computing model which can describe the behavior of silicon atoms is a further development for semi continuum model of silicon nanobeams. This model not only has simplified the computational process of calculation, but also has ensured the high precision of the frequency of the silicon nanowires.(2) For the silicon nanobeams in<111> orientation with different cross sections such as triangle, rhombus and hexagonal, the theoretical models of high frequency characteristics are built. The results based on these models are given in chapter 3 and the impact of cross section on the high frequency characteristics of the silicon nanobeams is discussed.(3) The impacts of crystal orientation, length, width, thickness, surface relaxation and surface reconstruction on the high frequency characteristics of silicon nanobeams are comprehensively studied in theory.(4) The results of theoretical calculation and molecular dynamics simulation are compared. The trend of simulation results is in accord with that of the calculation data.From the calculation result of the theoretical model, the frequency is inversely proportional to the length of silicon nanobeams with<111> and<112> orientation. With the increasing of width, thickness and the length of side, the frequency increases in varying degrees. For the silicon nanobeams in<111> orientation with three cross sections such as triangle, rhombus and hexagonal, the surface to volume ratio (SVR) is different from one another, and the frequency is also different. The frequency characteristics of silicon nanobeams are also greatly influenced by the surface relaxation coefficient and the surface reconstruction.In this dissertation, the frequency characteristics of silicon nanobeams with different size can be acquired from the semi continuum model based on the Keating potential model. Molecular dynamics simulation results and the experimental results can also verify this theoretical calculation model. Because the silicon nanobeams can be used as the basic unit of the NEMS devices, this high frequency characteristic theory model of the silicon nanobeams has certain reference value for the design and optimization of NEMS devices based on silicon.