Theoretical Investigation on Thermal Properties of Silicon Based Nanostructures

Abstract

With the continuous decrease of fossil fuel supplies but increasing demand for energy in the world, thermoelectrics has attracted wide attention in recent years due to its ability to provide sustainable energy harvested from wasted heat. It has been challenging to increase the thermoelectric efficiency over the past five decades, until very recently exciting progresses have been achieved in this field by using semiconductor nanostructures. These recent advances are achieved mainly due to the significant reduction of thermal conductivity in these low-dimensional materials. This thesis is devoted to search for various strategies that can effectively reduce thermal conductivity of semiconductor nanostructures, which is of great interest to further enhance the thermoelectric efficiency.

To begin with, we discuss some critical aspects of molecular dynamics simulations, which are used in this study to investigate the thermal properties of silicon based nanostructures. By using silicon nanowires and silicon-germanium nanojunctions as examples, the effect of heat bath on calculated thermal properties in non-equilibrium molecular dynamics simulations has been investigated. In addition, based on crystalline silicon and germanium, different implementations of Green-Kubo formula have been examined. Furthermore, new methods to improve the accuracy of thermal conductivity calculations in equilibrium molecular dynamics simulations have been proposed.

In the second part, we demonstrate through molecular dynamics simulations various strategies that can effectively reduce thermal conductivity of silicon nanowires, including random doping, superlattice and hollow nanostructure. These approaches belong to the incoherent mechanisms that reduce thermal conductivity by enhancing the phonon scattering rate. Moreover, we discuss in core-shell nanowires an intriguing oscillation effect in heat current autocorrelation function, while the same effect is absent in pure silicon nanowires, nanotube structures and randomly doped nanowires. Detailed characterizations of the oscillation signal reveal that this intriguing oscillation is caused by the coherent resonance effect of the transverse and longitudinal phonon modes, which offers a coherent mechanism to tune thermal conductivity in core-shell nanowires.

Finally, we study thermal conductivity of silicon nanowires with different cross sectional geometries. Interestingly, a universal linear dependence of thermal conductivity on surface-to-volume ratio is found in silicon nanowires with modest cross sectional area larger than certain threshold, regardless of the specific cross sectional geometry. This offers a simple approach to tune thermal conductivity by geometry. Moreover, the physical mechanisms that cause the deviation from the universal linear relation for very thin silicon nanowires below the threshold are also discussed.