Study On Thermal Conductivity Of Si/Ge Nanostructured Materials

The decrease of global energy sources and the increase of human demand for energy threaten our sustainable development.Nowadays,the efficiency of energy utilization is quite low because a large portion of energy is wasted as heat.Thermoelectric conversion offers the possibility for waste heat to be reused.At the same time,the density of device for integrated circuit has been improved rapidly during the past decades,which results in the large enhancement of heating power.The problem of self-heating is prominent in electronic chips with high power-densities,which decreases the performance of the electronic circuits and shorten the lifespan of the device.Owing to the progress of nanotechnology,people can solve the problems of energy waste heat utilization and device heat dissipation to a greater extent.In this paper,study the thermal conductivity of Si/Ge nanostructured materials not only has important scientific significance but also can provide new approaches to solve the above problems.Si/Ge superlattices have shown tremendous promise as effective thermoelectric materials due to their remarkable thermal insulating performance over bulk counterparts.The period length of the Si/Ge superlattices can effectively tailor the phonon’s transport behavior and controls their thermal conductivity.We have calculated the thermal conductivity of Si/Ge superlattices under the different period length distributions.The simulation results revealed that:(i)The thermal conductivities of superlattices with gradient and random period significantly decrease at relatively low temperature and they weakly depends on the sample total length compared to that of the uniform period superlattices,which is induced by the destroying of coherent phonons;(ii)The thermal conductivity of the gradient and random superlattices is not sensitive to temperature.Their temperature dependence of the thermal conductivity is reminiscent of that for the amorphous solid,nearly constant with the temperature.At the same time,the competition of phonons localization weakness and phonon-phonon scattering enhancement also reduces its temperature sensitivity;(iii)The thermal conductivities of the gradient and random superlattices demonstrate the same dependence on period length,sample total length and temperature.It indicates that the variation of period length distribution has little effect on the phonons transport mechanisms in the aperiodic superlattices.Considering the high price of Ge,we proposed the idea of embedding Ge film in Si matrix to reduce the thermal conductivity of the material.We have calculated the Kapitza resistance through the Si-Ge and the Ge-Si interface,and the effects of the number of Ge film and temperature on the thermal conductivity were investigated.The simulation results revealed that:(i)The thermal conductivity initially decreases,and eventually saturates as the number of Ge films increases.This is mainly due to the increase probability of phonon scattering and the suppression of model spatial behavior of phonon.(ii)Thermal conductivity of the embedded structure weakly depends on temperature.