| Semiconductor materials,especially IVA group semiconductor materials,have been widely used in many fields,such as micro-electronics,photoelectrons and thermoelectric materials,etc,due to their excellent performances.The development and application of semiconductor materials have become the important driving force for the national economy and people's life progress.It's easy to introduce interfaces in the preparation of semiconductor materials,due to the influence of preparation conditions and properties of materials.Although the order of the magnitude for a single interface thermal resistance is extremely low,when the size of devices is in nanometer level,the interface thermal resistances can't be ignored,due to the increase number of interfaces.Moreover,the existence of interfaces can not only introduce interface thermal resistance,but also influence the intragranular thermal properties sometimes.Although the effect of interfaces on thermal transmission has been confirmed,it's still puzzled whether different types of interfaces will affect thermal transport differently.Moreover,it's also of great significance for the design and application of semiconductor materials.To this end,in this paper,IVA group semiconductor materials are set as objects of study.By using the molecular dynamic simulations and first principles,the effect of different types of interfaces(homogeneous interfaces,heterogeneous interfaces,twin boundaries,and hetero-twinned interfaces)on thermal transport properties of semiconductor materials has been studied,respectively.Polycrystalline materials have been used to study the effect of homogeneous interfaces on thermal transmission.A simple theoretical model for describing thermal conductivity of polycrystalline materials has been proposed through theoretical derivation.In this model,thermal conductivity of polycrystalline materials can be easily obtained by using only its grain size,single crystal thermal conductivity,single crystal phonon mean free path and Kaptiza thermal resistance.The effectiveness of this model has been verified by using polycrystalline diamond thermal conductivity in non-equilibrium molecular dynamic(NEMD)simulations and polycrystalline Si thermal conductivity at 300 K in experiments.Based on this theoretical model,the relative importance of grain boundaries and size effect on polycrystalline diamond at 300 K and polycrystalline Si at 300 K and 500 K has also been studied.Results demonstrate that with the increase of grain size,both the effect of grain boundaries and size effect on thermal conductivity of polycrystalline diamond and Si become weaker;at the same time,the relative importance of size effect on thermal conductivity becomes stronger,while the relative importance of grain boundaries becomes weaker.When their grain size is increased to about 10000 nm,both grain boundaries and size effect almost have no effect on thermal conductivity.The novel theoretical model provides a convenient path to the calculation of polycrystalline material thermal conductivity.It's expected that it will greatly contributed to in-depth understanding of thermal properties in polycrystalline materials.Diamond/Si C polycrystalline composites have been taken as the prototype to study the effect of heterogeneous interfaces on thermal transmission.By using NEMD simulations,thermal conductivity of diamond/SiC polycrystalline composites has been calculated,and a comparison between Maxwell model and NEMD results shows that Maxwell model overestimates the polycrystalline composite thermal conductivity.By comparing the thermal resistances of heterogeneous and homogeneous interfaces,it can be obtained that when inflow of heat arises from the same materials,thermal resistance of heterogeneous interfaces is larger than that of homogeneous interfaces,namely,and this is the reason for the overestimation of Maxwell model.Phonon wave packet simulations are carried out to analyze the different scattering mechanisms at the diamond/SiC interface with a twist angle of 0°,diamond/diamond and SiC/SiC interfaces with a twist angle of 53.13°.Results show that when heat flow arises from diamond,ratio of thermal resistance between diamond/SiC and diamond/diamond interfaces ranges from 1.2 to 2.6 at different frequencies,which includes the ratio in NEMD simulation 1.6;when heat flow arises from Si C,ratio of thermal resistance between diamond/SiC and SiC/SiC interfaces ranges from 0.17 to 1.52 at different frequencies,which also includes the ratio in NEMD simulation 1.22.Thus,in diamond/SiC polycrystalline composites,when heat arises from the same materials,stronger phonon scattering at heterogeneous interfaces than the homogeneous interfaces is caused by the combined actions of energy transmission coefficients and transmission time.The twinned diamond superlattices(SLs)have been taken as the prototype to study the effect of twin boundaries on thermal transmission.NEMD simulations have been carried put to calculate the thermal conductivity of twinned diamond SLs with different twin thickness(0.62-9.92 nm).Results indicate that thermal conductivity of twinned diamond SLs is smaller than that of single crystalline diamond,and with the increase of twin thickness,the thermal conductivity is increased.In bulk twinned diamond SLs,twin boundary thermal resistance is irrelevant to twin thickness,and the twin boundary thermal resistance is about 2?10-13 m2K/W,which is three orders of magnitude lower than conventional grain boundaries' thermal resistance;the intragranular thermal resistance is the same as that of the bulk perfect diamond.The reduction of thermal conductivity is only caused by the additional intergranular thermal resistance,while it's almost unaffected by size effect.Thermal transport properties of twinned diamond SLs(D=0.62 nm)and single crystal diamond have been compared by first principles calculation,results show that heat capacities of the two structures are almost the same,ratio for square of average phonon group velocity between twinned and single crystal diamond is 0.98,and ratio for average phonon relaxation time between twinned and single crystal diamond is 0.88,demonstrating that the effect of twin boundaries on thermal transmission is irrelevant to heat capacity,and it is caused by the combined effect of reduced phonon group velocity and reduced relaxation time,the reduced relaxation time makes a primary contribution.Si/Ge hetero-twinned SLs have been taken as the prototype to study the effect of hetero-twinned interfaces on thermal transmission.By employing NEMD simulations,thermal conductivity of Si/Ge hetero-twinned SLs with different periodic lengths has been studied.The results indicate that thermal conductivity of Si/Ge hetero-twinned SLs is compared similar to that of conventional Si/Ge SLs,while it's much smaller than that of single crystal Si,twinned Si and Si0.5Ge0.5 alloy with the same periodic lengths.Phonon kinetic theory has also been applied to analyze the thermal properties of Si/Ge hetero-twinned SLs with a periodic length of 1.932 nm,as well as that of conventional Si/Ge SLs,single crystal Si,twinned Si and Si0.5Ge0.5 alloy with similar structures.Results show that heat capacities of the five structures are almost the same;the average phonon group velocity of Si/Ge SLs is 3273 m/s,and it's the lowest in the five;the average phonon relaxation time of Si/Ge SLs is 5.11 ps,which is much lower than that of single crystal Si.Thus,the effect of Si/Ge hetero-twinned interfaces is irrelevant to heat capacity,and it is caused by the combined actions of reduced phonon group velocity and phonon relaxation time.Moreover,despite of similar thermal conductivities in Si/Ge hetero-twinned SLs and conventional Si/Ge SLs,phonon scattering mechanisms in the two structures are fundamentally different that the average phonon group velocity in Si/Ge hetero-twinned SLs is lower,while the average relaxation time in Si/Ge hetero-twinned SLs is larger.Our results demonstrate a new kind of SLs with strong phonon scattering. |
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