Thermal Conductivity Of Shell-doped Silicon Nanowires By Molecular Dynamics Simulation

Energy conversion between heat and electricity based on thermoelectric effect hasbeen attracted much attention recently due to its power of waste-heat recovery andenvironmentally friendly refrigeration. Thermoelectric efficiency is described by thethermoelectric figure of merit ZT=S2σT/k, where T is the absolute temperature, S isthe Seebeck coefficient, σ is the electrical conductivity, and k is the total thermalconductivity, respectively. This sparks the possibility of improving ZT by engineeringmaterials for reduced κ, without much affecting other parameters that could contributeto ZT.It is very important to develop inexpensive materials that can supply sustainableand clean energy to meet the needs of the future. Although silicon is the most widelyused material in the semiconductor industry with a low-cost and high-yield processingcapability, it has long been considered to be a very inefficient thermoelectric materialwith ZT~0.01because of its high k. Fortunately, it is found that nanometer treatmentcan significantly reduce the thermal conductivity of silicon. Experiments haveprovided direct evidence that an approximately100-fold improvement of the ZTvalues over bulk Si is achieved in silicon nanowires (Si NWs).Recently, the core-shell structure of Si and Ge nanowires (NWs) with a bettertransconductance and higher carrier mobility comes to the spotlight of research. Theadvantage that the electronic and phonon transport in the core-shell structure NWs aredecoupled is very favorable for thermoelectric purposes. The thermal conductivity ofboth Si/Ge and Ge/Si core-shell NWs have been demonstrated to be reducedsignificantly. In previous studies of the thermal conductivity in core-shell NWs, thecomponent in the shell is unique, and the reduction of thermal conductivity stemsfrom the interface effects. We suppose that combining the fine-tunings of interfaceand impurity scattering, the thermal conductivity of Si/Ge core-shell NWs can befurther reduced by modulating the component in the shell region. In this work,non-equilibrium molecular dynamics (NEMD) simulation is performed to investigatethe thermal conductivity of Si/GexSi1-xcore-shell NWs. The results show that the Gedopant concentration in the shell region influences the thermal conductivityintensively, and the thermal conductivity of Si/Ge0.6Si0.4core-shell NWs is markedlylower than that of Si/Ge core-shell NWs. In order to reveal the origin of the reductionof thermal conductivity, we perform the vibrational eigenmodes analysis and find the strong localization of phonon modes from1.0THz to2.0THz and9.0THz to16.0THz in shell-doped Si NWs. Furthermore, we evaluate the spatial distribution oflocalized modes and demonstrate that the strong localization of phonons in theshell-doped region suppresses thermal transport in shell-doped Si NWs greatly and isresponsible for the significant reduction of thermal conductivity. Both helpful todecrease the thermal conductance and increase the electrical conductance, we suggestthat shell-doped Si NWs with large dopant concentration will be a very wonderfulcandidate for thermoelectric applications.