Physical Mechanism Of Energy Transport In Nanostructures And Interfaces

Moore’s law stated that semiconductor performance would double every 18 months.In the past few decades,the development of the semiconductor industry has always complied with Moore’s law.But accompanied with the increasing power density of integrated circuits,chip heating has become a major obstacle to the further improvement of semiconductor integration.On the one hand,the rises of temperature in electronics would cause false performance readouts or failures.On the other hand,a higher electrical resistance of the interconnecting metals would be induced by the temperature rises,leading to slower switching times of semiconductor devices and decreased circuit speed.Thus,studying the mechanism of nanoscale energy transport not only becomes a foundation to solve the frontier issue of scientific research,but also becomes a benefit for the further development of the semiconductor industry.In the electronic devices,electron-electron scattering,electron-phonon scattering,phonon-phonon scattering become important factors of the heating problem.Meanwhile,with the gradual decrease of the material size,the interfacial thermal conductance of material is becoming increasingly important as well.At nanoscale,with the increase of the interfaces per unit length,the main factor that affects the thermal transport is no longer the thermal resistance of the material itself,but also the interfacial thermal conductance.In this situation,the development of thermal interfacial materials for thermal management of devices with high power density has become particularly important.This dissertation combines molecular dynamics,Green’s function and Monte Carlo simulations to investigate the physical mechanism for phonon transport in nanostructure.Also,the effects of electron-phonon scattering on the thermal conductivity and electrical resistivity are explored theoretically and experimentally.Finally,this dissertation studies the effect of phonon mean free path on the interfacial thermal conductance,and rises a more convenient and efficient approach to improve heat conduction at the interface.The main work of this dissertation includes:1.A theoretical model is reported to describe the enhanced kink effects induced by phonon focusing effects in kinked nanowires.The theoretical model is used to predict the additional thermal resistance in kinked graphite nanowire,which is consistent with the molecular dynamics simulations.It is demonstrated that such high kink-induced resistance can be attribute to the strong phonon focusing effects in graphite nanowires,which makes the phonons more easily to be backscattered by the boundary without transporting across the kinked region,and reduces the probability of phonon scattering to change the directions of the phonons in the kinked region.By increasing interlayer interactions to enhance the phonon focusing effects in kinked graphite nanowires,it is demonstrated that the additional thermal resistance induced by kink also increases,verifying the correctness of the assumption.According to the theoretical model,when it comes to thick kinked graphite nanowires with phonon mean free path of ～200nm,the additional thermal resistance for kinked region with single layer is up to ～300 times larger than that of a straight wire segment of equivalent length.Meanwhile,phonon transport in van der Waals nanowires behaves strong anisotropic and size-dependent properties.With the help of molecular dynamics simulation,nonequilibrium Green’s function,and Monte Carlo simulation methods,it is demonstrated that when the structure size is smaller than the phonon mean free paths,the classical anisotropic model derived from the Boltzmann transport equation fails to predict the thermal conductivity.For thin van der Waals nanowires,the cross-plane phonon modes dominate the heat transfer process along arbitrary crystalline directions except the in-plane direction,which makes phonons transport in a stair-like way.Increasing the structure size,the in-plane phonon modes gradually dominate the phonon transport along arbitrary crystalline directions except the out-plane direction.2.Electron-phonon scattering plays a non-negligible role in the phonon transport.This dissertation shows rather high thermal conductivity of iridium dioxide that is mainly attributed to phonon transport.Analysis indicates that the large lattice contribution results from the strong interatomic bonding and large difference in the atomic mass between iridium and oxygen.Interestingly,it is found that electron-phonon scattering plays a significant role and leads to a remarkable reduction in the lattice thermal conductivity.Furthermore,this dissertation studies the effects of electron-phonon scattering on the electron transport process in metal nanowires.Through combined experimental measurements and numerical modeling,non-monotonic variations of the boundary scattering rate are showed for free electrons in metal nanowires as temperature escalates.This observation is attributed to the change of electron-phonon scattering angle as temperature reduces,which alters the surface scattering rate.At low temperatures,electrons traveling along the wire axis have to be first relaxed by electron-phonon scattering before they collide with the nanowire surface.Theoretical analysis indicates a transition temperature of 0.29 times Debye temperature.A theoretical model considering the effects of scattering angle is proposed that can fit the measured experimental data for both copper and silver nanowires over a wide temperature range.3.The interfacial thermal conductance as an accumulation function of phonon mean free path is rigorously derived from the thermal conductivity accumulation function.Based on the theoretical model,the interfacial thermal conductance accumulation function between Si/Ge is calculated.The results show that the range of mean free paths for phonons contributing to the interfacial thermal conductance is far narrower than that contributing to the thermal conductivity.The interfacial thermal conductance is mainly contributed by phonons with shorter mean free paths,and the size effects can be observed only for an interface constructed by nanostructures with film thickness smaller than the mean free paths of those phonons mainly contributing to the interfacial thermal conductance.Molecular dynamics simulation is also employed to verify the proposed model.4.To control the interfacial thermal conductance,this dissertation studies how hydrogenation of graphene affects thermal transport behaviors.The phonon transmission and interfacial thermal conductance at the interfaces of monolayer graphene(Gr)/double-sided hydrogenated graphene(DHGr)/single-sided hydrogenated graphene(SHGr)encased by metals(Cu and Ni)are investigated using the density functional theory and the nonequilibrium Green’s function method.The results demonstrate that,the interfacial thermal conductance across metal/Gr/metal interfaces depends on the interaction between graphene and metal.Chemisorption between metal and graphene can enhance the interfacial thermal conductance,while physisorption suppresses the conductance.Besides,single-sided hydrogenation can change the type of interaction from physisorption to chemisorption due to the transfer of massive electrons,contributing to the interfacial thermal conductance.Furthermore,to experimentally modulate the interfacial thermal conductance,a convenient and efficient approach is reported to improve heat conduction across the metal/graphite interface.It is demonstrated that the interfacial thermal conductance between Al and graphite can be enhanced by a factor of ～5 after milling the graphite with focused ion beam.Such enhancement is attributed to the decreased Fermi level of the milled graphite compared with the pristine counterpart.Once the graphite is milled with the focused ion beam,surface defects are formed that induce the redistribution of electrons at the interface between Al and graphite.The formation of enormous dipoles on the milled graphite/Al interface leads to the conversion of the interfacial interaction from physisorption to chemisorption,which is beneficial to phonon transmission across the interface.Based on the measured Fermi level difference,nonequilibrium Green’s function method predicts that the interfacial interaction strength in the Al/milled graphite is increased by 4 fold compared with the Al/ pristine graphite,which causes the increase of the interfacial thermal conductance.The theoretical model also predicts that the interfacial thermal conductance does not increase monotonically with the interaction strength.Once the interaction strength exceeds a critical value,the interface thermal conductance will decrease.