Study On The Thermal Regulation And Mechanism Of Graphene-based Heterointerface On High-power LED Devices

Light Emitting Diode(LED)lighting has became the top beam pillar of solid-state lighting with the development of science and technology and the background of industry 4.0.The failure of LED becomes more and more serious with the development of high power,high brightness and large size.A large amount of heat will be generated during the process of electro-optical conversion,which will directly cause thermal failure of LED structure.Therefore,The thermal design of interface is very important of LED chip.In recent years,the two-dimensional(2D)materials have attracted extensive attention owing to excellent thermal properties,graphene is one of the most popular two-dimensional materials among them,which has become an star material in the next generation of thermal design of LED structure.The graphene will contact with P-N junction or substrate due to the limitation of LED luminescent mechanism and package structure,which will creates the heterointerface of graphene and other materials.Therefore,it is necessary to study the thermal characteristics and mechanism of graphene based heterogeneous interface.Based on this,relying on the application prospect of graphene based two-dimensional materials in LED devices,the thermal characteristics and mechanism of graphene based heterointerface are studied with the help of molecular dynamics in our research,which aims to explore the methods to regulate and control the interface heat conduction and its mechanism.The contents can be divided into the following parts:1.Study on the thermal transfer mechanism of graphene/hexagonal boron nitride heterogeneous interface.Due to its high thermal conductivity and non-conductive properties,hexagonal boron nitride can be used as both thermal interface material and substrate material in LED chip.Therefore,the thermal transfer properties of graphene/hexagonal boron nitride are studied by molecular dynamics method.On this basis,the surface of graphene and hexagonal boron nitride was modified by defect engineering,and the effects of defects on the thermal transfer of graphene/hexagonal boron nitride are further studied.The results reveal that the interfacial thermal resistance of different stack forms presents a significant downward trend with the existence of point defects.This counterintuitive behavior is due to the defects increase the number of graphene out-of-plane low-frequency phonons,thus enhancing the phonons coupling between graphene and hexagonal boron nitride layer.The interfacial thermal resistance of graphene/hexagonal boron nitride is further reduced by 50% with the defect rate increases from 0% to 5% and that is reduced by 65% with the temperature rises from 200 K to 700 K.Besides,It is found that the defective graphene/hexagonal boron nitride exists thermal rectification characteristic and that is positively related to temperature and defect rate.Our study provides a practical way for the application of defects in graphene and provides a new approach for the design of thermal rectifier devices.2.Study on the regulate and control of the thermal transfer of graphene/silicon heterointerface by the methods of surface modification.Taking silicon as a substrate material and build the graphene/silicon heterointerface,which is very popular in LED substrate material.The thermal transport between graphene and silicon heterointerface are studied by molecular dynamics.In order to regulate and control the thermal transfer of graphene/silicon,the system was modified by adding holes on the surface of silicon substrate and attaching methyl groups on the surface of graphene.The results indicates that the ITR of Gr/Si can be reduced by 13.3% through adding holes on the silicon surface.The interfacial distance and interaction force indicates that it is attributed to the enhancement of interface coupling.On this basis,we also found that the methyl group attached to graphene layer can further reduce the ITR by 35% for the first time.Besides,compared to the presence of methyl groups inside silicon hole,the ITR of heterointerface can be reduced by 15% when the methyl groups outside the silicon hole.The vibration density of states shows that the methyl group exists a strong peak at low frequency,which enhances the phonons coupling between graphene and silicon,thus reducing the phonons mismatch and decreasing the ITR.Our study provides novel measures for manipulating the ITR of Gr/Si heterointerface through surface modification.3.Study on the in-plane and out-of-plane thermal transfer mechanism of the nitrogen doped graphene/silicon heterointerface.Doping is a frequently-used method of material modification.In this part,the effects of different types of nitrogen doping,the concentration of nitrogen doping and the temperature on the planar thermal conductivity and interfacial thermal resistance of graphene/silicon are researched by molecular dynamics.The results reveal that the planar thermal conductivity will be decreased with the increase of nitrogen concentration when the doping type is random,which can be attributed to the mismatch between graphene phonons in the presence of nitrogen atoms.On the contrary,The planar thermal conductivity will be first reduced and then improved suddenly while the doping type is regular.This abnormal phenomenon can be attributed to the coupling of nitrogen phonons in regular nitrogen doping,which provides a new heat dissipation channel for heat transfer.In addition,either regular nitrogen doping or random nitrogen doping can reduce the interfacial thermal resistance of graphene/silicon,which will further reduced with the increase of the concentration of nitrogen doping.The results of vibration density of states(VDOS)reveal that the phonon matching rate of nitrogen and silicon phonons is much greater than that of graphene and silicon phonons,which provides a new thermal transfer channel for thermal transfer between graphene and silicon layer,thus reducing the interfacial thermal resistance.Finally,it is found that the temperature inhibits the planar thermal transport of graphene/silicon heterointerface and promotes the interfacial thermal transfer of graphene/silicon.Based on the research background of thermal reliability of LED devices,the heat transfer characteristics and mechanism of graphene based thermal interfacial materials were studied by molecular dynamics.Which provide new measures and ideas for the application of graphene in the key heat dissipation interface of LED chips,and provide new methods and theoretical basis for the thermal regulate and control between two-dimensional materials and LED substrate materials,thus realizing the construction of key heat transfer interface of LED high-power devices and playing an important role in promoting and guiding the regulation and management of LED internal thermal transfer.