Molecular Dynamics Simulation On In-Plane Thermal Conductivity Of Single-Crystal Si Film At Nanoscale

As functional component and structural unit, thin films with thickness of nanoscale have been widely used in such new areas as nanotechnology, micro-electronic and photoelectronic technologies. The mechanism and characteristics of heat transfer in films are of great significance to the thermal design and thermal management of these micro devices and structures. Thermal conductivities of these films are much different from those of their corresponding bulk materials; yet direct experimental measuring tools face drastic challenges due to the difficulties in preparing the samples and test apparatus. This thesis adopted MD approach, in combination to theoretical analysis, to study the thermal conductivity of single-crystal silicon [001] film with diamond structure, in the direction parallel to the film plane (in-plane direction).The Stillinger-Weber Potential Model is employed to describe the interaction between silicon atoms in the simulation film, and taking the structural characteristics and heat transfer mechanism of solid films into consideration, a steady heat transfer simulation adapted to MD simulation is framed. The proposed temperature computing formula provides compatible results to classical dynamics, and through the MD simulation, testifies the applicability of Fourier law in the steady heat transfer phenomena in thin films.By simulating the steady heat transfer processes in single-crystal silicon film with thickness of 2~42nm under room temperature (300K), it is found that: 1) in-plane thermal conductivity is about 5~75W/mK, which is below its bulk value at the same temperature,so that the size effect is evident; 2) in-plane conductivity decreases with decreasing of thickness, and when the thickness is less than 20nm, it decreases more quickly with thickness reduction; 3) in-plane conductivity is about 2 times the out-of-plane conductivity ( along the direction perpendicular to film surface) of those films with the same thickness,which indicates that thermal conductivity of thin-films has outstanding anisotropy. By fitting the simulation data of thermal conductivity and calculating the extrapolated values, it is found that the computing results are consistent with existing experimental data when films with thickness between 50~800nm is in consideration.The analysis of Boltzmann transport theory suggests that the size effect of thermal conductivity in thin films could be attributed to the diminishing of relax time or mean free path of phonons (lattice vibration) in films. Summarizing the mechanism of phonon scattering and computing the factual relax time of film phonons, the theoretical values of in-plane conductivity in silicon films are computed with help of Holland model. Theoretical estimation has proved the law that the thermal conductivity resulted from MD simulation changes with thickness and further proved the merits of utilizing MD technology to study the characteristics of thermal conductivity in films with thickness immeasurable directly through experiment.