| Currently, micro gas sensors have become one of the major MEMS fields, and semiconductor gas sensors are the mainstream. Tin oxide (SnO2) is widely applied as the gas sensing material. The measurement of its gas sensing characteristics and the relationship between the characteristics and the micro structure of thin films fabricated with different processes have been significantly studied and progressing rapidly, while the fundamental research studies are progressing slowly.Thin films are widely used in MEMS with different thicknesses from micron, sub-micron to nanometer scale. Thermal problems become significant in such micro devices. However, the characteristics of materials at micro-scale are different with which at macro-scale. To study the micro-scale world, the molecular dynamics simulation and the Ab initio method in quantum mechanics have been proved very efficient methods. The Material Studio?software, which is developed for the purpose of molecular simulation by Accelrys Ltd., is chosen as the simulation platform.In this thesis, the SnO2 gas sensing thin film (grains) is chosen as the research subject. The heat capacity of SnO2 thin film (grains) at micro-scale is calculated with molecular dynamics simulation method. Firstly the heat capacity of thin film at several hundreds nm is calculated. Then the size of the thin film shrinks and the heat capacity of grains at nanometer scale is studied. Several conclusions can be reached from the simulation results, including that heat capacity is not a constant at micro-scale, but the function of scale; the value of it at micro-scale is bigger than its value of bulk material and the heat capacity of nano material depends on the distance between the grains in some degree. In addition, the energy band structure of SnO2 is calculated based on the Density Functional Theory (DFT) of solid state energy band theory. The electron distribution of SnO2 and Sb doped SnO2 are calculated respectively in Chapter 4. |
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