Phonon Transport Properties And Thermal Conductivity In Single And Stacked Tilted Interface Systems

Silicon(Si)and Germanium(Ge)are among the best materials for microelectronics/optoelectronics and photovoltaic device applications,thus becoming the cornerstone of the semiconductor industry.Si Ge nanostructures have been increasingly used in many fields,and their thermal transport properties have attracted extensive attention.Thermal energy is mainly transferred through phonons(quantization of lattice vibrations)in Si Ge nanostructured materials.Although the reduction in thermal conductivity is a challenge with respect to the thermal management of nanoelectronic devices used in computer processors,it can be beneficial for thermoelectric(TE)energy conversion.A deeper understanding of thermal transport at the nanoscale is strategically important to achieve the optimal use of low-dimensional nanostructures in emerging applications.In this work,we systematically study phonon thermal transport in including tilted single-interface structure and multi-interface superlattice structure using nonequilibrium molecular dynamics methods.The results show that phonon transport is not the same in superlattice structures and single-interface nanostructures.For tilted singleinterface nanostructures,the calculation results show that the interface roughness reduces the sudden change of the phonon impedance at the interface,so that the phonon with a larger incident angle can be transmitted,thereby enhancing the transmission,which leads to the higher interfacial thermal conductance of the rough interface structure than that of the smooth interface structure.For the superlattice structure,the calculation results show that:(1)The change of the interface angle will cause a strong anisotropy of thermal conductivity of the superlattice structure,and the degree of anisotropy does not change monotonically with the interface angle.(2)Since the atomic arrangement at the interface of 45o and 90o structures conforms to the atomic distribution of close-packed planes in the diamond structure,coherent transport of phonons is induced under short periods.(3)In a superlattice structure with an interface angle of 90o,due to the difference in the lattice constants of Si and Ge,the interface atoms are subjected to large tensile(compressive)stress,resulting in strong phonon scattering.And many straight curves and band gaps can be found in the high-frequency region in its phonon dispersion.So it has a weakening effect on thermal conductivity.(4)In short period,the mean free path of phonon-phonon scattering of superlattice structure is greater than its period length,phonons can maintain their phase and wave properties,and carry out coherent transport,which results in a linear increase in thermal conductivity with increasing sample length.Under the long period,the thermal conductivity hardly changes with the change of the sample length.The main reason is that the period length is greater than the phonon coherence length,resulting in incoherent phonon transport.This thesis could be helpful in furthering the understanding of phonon thermal transport in low-dimensional Si/Ge nanostructures and may serve as a highly useful experimental guide in Si/Ge thermoelectric applications as well as in other thermal-related applications.