First principle study on thermoelectrical properties of silicon nanostructures

Nanostructuring and carrier concentration optimization have been widely employed to improve thermoelectrical properties. Nanostructures can enhance phonon-boundary scattering, and adjusting carrier concentration can affect the phonon-electron scattering. In this work, we investigated the structure, electron and phonon properties of silicon thin films to obtain the dimensionless figure of merit (ZT value). The first-principle calculations were employed to obtain all the electron and phonon properties with Quantum Espresso package. The electron-phonon scattering rates were obtained based on an interpolation scheme using maximally localized Wannier functions with the aid of EPW package. We performed frequency-dependent simulations of thermal transport in silicon thin films by numerically solving the phonon Boltzmann Transport Equation with the Discrete Ordinate Method. The Seebeck coefficient and electrical conductivity are calculated by solving the electron Boltzmann Transport Equation with the BoltzTrap package using relaxation time approximation (RTA). The results show that the ZT value of silicon increases effectively as the feature size of silicon thin films goes down. The lattice thermal conductivity reductions due to the electron-phonon scattering, decrease as the feature size of silicon thin films fall and could be ignored at low feature sizes (20 nm for silicon thin films). The results also provide an optimal carrier concentration at which the highest ZT value can be achieved. The nanostructuring technology and carrier concentration optimization method can be chosen or combined more reasonably for reducing the thermal conductivity and increasing the ZT value of thermoelectric materials.