Research On Heat Dissipation And Related Mechanisms Of Graphene And Its Interface In LED Chips

With the continuous development of technology,high-power LED has been widely promoted and popularized in daily lighting,vehicle lighting and other fields.However,the higher power,miniaturization and functionalization for high-power LED in these fields lead to the thermal reliability problem of LED chips becoming more and more serious.LED chip is the core component of LED lighting devices.It is a multi-interface structure composed of a variety of materials.The damage and degradation of materials and interface structure caused by high temperature are directly related to the quantum efficiency,luminous flux and life of LED chip.Meanwhile,graphene has attracted much attention in the construction of new generation optoelectronic devices because of its high thermal conductivity,fast carrier mobility and high transmittance.Under this background,it is a trend to introduce graphene into high-power LED chips and try to build new LED chips with high thermal conductivity and high reliability.Aiming at the development requirements of high-power LEDs and the problem of difficult heat dissipation of LED chips,this thesis first verified the effectiveness of the introduction of graphene and other interface layer in LED chips on the temperature reduction of LED chips and analyzed its influencing factors,and then based on molecular dynamics theory,Non-equilibrium Green's function and density functional theory,and other methods to in-depth and systematically explored the thermal transport properties of defective graphene,graphene homogeneous(heterogeneous)interfaces,and new graphene-based LED interface structure,such as Si C / graphene buffer layer /Ga N.The research results obtained are as follows:1.Experimental study on heat transfer and preparation of single crystal silicon /interface layers / copper heterostructures.The graphene film was prepared by liquid exfoliation,solvent exchange and water-based self-assembly film methods in sequence,and the diamond-like carbon(DLC)film and copper film were prepared by magnetron sputtering.Finally,three heterogeneous interface structures,namely single crystal silicon / copper thin film and single crystalline silicon / graphene thin film /copper thin film and single crystal silicon / diamond-like carbon thin film / copper thin film,were constructed.Next,the effect of different interface layers and related states on the interfacial heat transfer was studied.The results show that the embedding of graphene film and DLC film can effectively reduce the thermal resistance of the heterogeneous interface structure,and the coverage area of the graphene film have significant effects on the thermal resistance of the heterogeneous interface structure.2.Thermal regulation and mechanism analysis of defective graphene nanoribbons.First,non-equilibrium molecular dynamics and thermal relaxation methods were used to study the heat transfer of nitrogen or silicon doped graphene nanoribbons containing grain boundaries.As a result,it was found that even a low doping concentration would result in a significant reduction in the heat transport capability of graphene nanoribbons.Moreover,the larger atomic mass difference of silicon carbon than nitrogen carbon leads to lower thermal transport capacity.Furthermore,the effect of the relative positions of nitrogen atoms and(Stone-Wales)SW defects on the thermal conductivity was studied.The results show that nitrogen atoms occupying different characteristic positions of SW defects would significantly affect the heat transfer of graphene nanoribbons,and the influencing factors include doping position,doping quantity and distribution of carbon nitrogen covalent bond.It is also found that the close distance of dopant atoms and the hot(cold)bath or defects facilitates the heat transfer of the graphene nanoribbons.In addition,based on isotropic non-equilibrium molecular dynamics,non-equilibrium Green's function,density functional theory and thermal relaxation methods,the thermal regulation of graphene nanoribbons with uniform and random distribution of nitrogen-doped or single-vacancy defect has been studied.The results show that the maximum thermal regulation of graphene nanoribbons by uniformly randomly distributed graphite type nitrogen atoms occurs at a lower concentration of 0.88%.And,the degree of thermal regulation of uniform and random distribution of various defects is pyridine type,pyrrole type,graphite type and singlevacancy defects in order from large to small.This is closely related to their defect composition and structural characteristics.The average thermal conductivity of random uniform distribution from large to small corresponds to the original graphene,graphite type and pyrrole type graphene nanoribbons.The thermal conductivity of pyrrole type and single vacancy defect graphene nanoribbons is similar,and the thermal conductivity of pyridine type graphene nanoribbons is slightly lower than the former two.In view of the above phenomenon,the regulation mechanism of uniform random distribution and various defects on the heat transfer was revealed from the aspects of phonon density of states,phonon transmission and energy transmission.3.Thermal regulation and mechanism analysis of two-dimensional graphenebased interface.Firstly,the applicability of interfacial thermal conductance(ITC)calculation of the isotropic non-equilibrium molecular dynamics method and the lumped heat capacity model method was analyzed and compared.The results show that ITC of bilayer graphene is almost the same near the Debye temperature,but when the temperature is lower than the Debye temperature,the neglect of the strong temperature dependence of phonon modes in the isotropic non-equilibrium molecular dynamics method results in the ITC larger than that calculated by the lumped heat capacity model method,and the maximum ratio is 3.3 at 200 K.In addition,the ITC increases exponentially with the increase of temperature,which can be confirmed by thermal relaxation time and interfacial bonding strength.Then,the thermal transport properties of methylated bilayer graphene were investigated after verifying the applicability of potential parameters through charge density difference,model configuration and interatom spacing.The results show that the introduction of methyl is not conducive to the ITC with a maximum decrease of 33.81%,and the in-plane thermal conductivity with a maximum decrease of by 27.37%.This disadvantage was analyzed by thermal relaxation time and interfacial bonding strength.In addition,the effects of methyl modification and deformation engineering on the thermal regulation of graphene/h-BN heterointerfaces were investigated by transient pump probe method.Specifically,the ITC increases first and then decrease with the increase of methyl,with the maximum increment of 54.25%.For deformation engineering,the ITC increases by 15.5% at the early stage,and then decreases to zero.The regulation mechanism of these thermal regulation methods was analyzed by interface spacing,coupling interaction energy,change of bond length and thermal relaxation time.4.Heat transfer and construction of Si C / graphene buffer layer / Ga N heterogeneous interface.Firstly,the graphene buffer layer was epitaxially grown on6H-Si C by simulated annealing method,and the Tersoff-Erhart-Albe(TEA)potential and environment dependent interatomic potential(EDIP)were analyzed by evaluating the nucleation temperature,radial distribution function,average atomic binding energy and atomic structure.The results show that TEA potential is better than EDIP potential.Furthermore,the Si C / graphene buffer layer / Ga N heterogeneous interface structure was constructed based on the model obtained by the above calculation,and the TEA potential and the transient pump probe method were used to study its heat transfer properties.The results show that the increase of the nucleation area,especially the combination of different nucleation regions,can make it easier for energy to pass through the interface,and the maximum reduction in the ITR obtained is 17.9%,which also shows that the integrity of graphene is essential for the heat transfer of the interface.Meanwhile,results also show that the single-vacancy defect of graphene can reduce the interfacial thermal resistance(ITR)by a maximum of 9.6%,which means that the ITR can be reduced through defect engineering.Finally,the study of the temperature dependence of the ITR shows that an increase in temperature(300K to 1100K)leads to a maximum decrease of 19.6%.