Study On The Key Problems Of Low Temperature Growth Of SiC-based Graphene

Graphene has excellent electrical,optical,thermal and mechanical properties,and shows excellent application prospects in the fields of transistors,sensors,transparent electrodes,catalysis,and clean energy.The basis for the realization of these application prospects is the ability to produce high-quality,large-area graphene materials at low cost.Among the many preparation methods of graphene,Si C-based graphene represented by high-temperature thermal decomposition of silicon carbide substrate is the most promising high-end electronic material.However,this method requires ultra-high temperature(greater than 1400?),high cost,poor control,and the application is limited.Therefore,it is very important to study the low-temperature growth of Si C-based graphene,which is also a current research hot spot.There are two main mechanisms for preparing Si C-based graphene at low temperature:(1)CVD(chemical vapour deposition)method,which is based on the CVD method on metal substrates and uses a gaseous carbon source to deposit on the surface of Si C substrates to form graphene.(2)CDC(carbide-derived carbon)method,which uses chlorine gas to selectively react with Si atoms in Si C at high temperature(below 1000?),the formed gaseous silicon chloride is separated from the Si C substrate,and the C atoms left behind are reconstructed to form the graphene structure.This thesis studies the key issues in the growth of the above two methods,including the following three parts:(1)The kinetic process of growing graphene on the C-face of 4H-Si C substrate using propane as a carbon source was studied,and describing in detail the growth mechanism of CVD graphene at the atomic level.First,the molecular dynamics method was used to simulate the decomposition of propane.At 1250°C and oxygen-free conditions,propane will decompose into small groups such as CH₃,CH₂,and CH.Then the dehydrogenation process of CH₃,CH₂,CH and other small groups on the C-face of 4H-Si C C(2×2)reconstruction surface was studied,and the transition state atomic structure,activation energy and the energy change of the dehydrogenation process are given.It is found that the dehydrogenation of the CH₂ group has the highest activation energy among the three dehydrogenation reactions,thus the corresponding reaction rate is also the smallest.Finally,the synthesis reactions of the C_x structure on the surface of the C-face of 4H-Si C are studied,and the transition state atomic structure,activation energy,and energy change for each synthesis reaction are given.It is found that all the synthesis reactions were exothermic.The reaction for the synthesis of a C2 structure from two C atoms has the highest activation energy and the lowest reaction rate.(2)The release of strain in graphene at the grain boundary is studied,and a strain-release model is proposed.It is found in the experiment that the relationship between the 2D-peak frequency and the reciprocal of the grain size 1/L_a is linear,and the blue-shift of the 2D-peak frequency relative to free graphene represents the existence of compressive strain in graphene,so the relationship between the strain in graphene and 1/L_a is linear.In order to explain this linear relationship,a strain-release model by diffusion at the grain boundary is proposed:atoms diffuse both along the grain boundary and diffusion into the grain in the direction perpendicular to the grain boundary.The diffusion of atoms will lead to the release of compressive strain,and the strain released is proportional to the number of atoms diffused.The number of atoms diffused has a minimum in the center of the grain,and it increases exponentially from the center to the boundary.Therefore,the strain has a maximum value at the center of the grain,and decreases exponentially from the center to the boundary,and becomes zero at the grain boundary.This exponential decay relationship leads to a linear relationship between the average strain in the grain and 1/L_a,which explains the linear relationship between the 2D-peak frequency and 1/L_a.(3)The initial stage of the reaction between chlorine and Si C in the CDC method was studied,the corrosion mechanism of chlorine at high temperature was found,and the mechanism behind the conformal property of the reaction between chlorine and Si C at high temperature was revealed.Early stage of reaction between hydrogen etched single-crystalline Si C and chlorine under high tempterature(900?)was studied.Semicircular cavities are formed during hydrogen etching and the depth of the cavities increases during chlorination,which results from the higher chlorination rate in the cavity than out of it,and the shrinkage process of the chlorination reaction.Another important observation is that the semicircular shape of the cavity keeps the same after chlorination.In order to explain this conformal carbide-to-carbon transformation,the macroscopic chlorine corrosion model is proposed to prove that this phenomenon is caused by a phenomenon that only C-face of the Si C crystal can be corroded by chlorine.Then,the chlorine corrosion model is explained from the different atom arrangements in different lattice planes.Finally,the corrosion model is verified by the reaction experiment of chlorine with Si-face Si C substrate.The research in this thesis clarifies the growth mechanism of graphene in the CVD method,proposes a strain release model through the grain boundary of graphene,reveals the mechanism behind the conformal property of the reaction between chlorine and Si C in the CDC method,and has a strong guiding significance for the low temperature preparation of Si C-based graphene.