Study On Near-free-standing Graphene Growth And Nucleation Mechanism On The SiC Substrate

As the first two-dimensional crystal of single atomic layer,graphene has broken the classical theory of two-dimensional thermodynamic fluctuation.Graphene is a hexagonal honeycombed structure composed of carbon atoms through sp2 hybridization.This unique structure gives graphene excellent mechanical,thermal optical and electrical properties,so graphene has a very broad application prospect in scientific research,energy,military and communication fields.It has been more than a decade since graphene was first discovered,and countless researchers have devoted thenselves to the field.However,until now,the problem of large-scale industrial preparation of high-quality graphene has not been solved,and the pratical application of graphene is still on paper.Among the many graphene preparation methods discovered so far,silicon carbide(SiC)epitaxial method can realize the preparation of large size and high quality graphene at the wafer level.At the same time,in the subsequent preparation of graphene devices,new impurities and defects will not be introduced due to secondary transfer,and it can be compatible with the current semiconductor processing process,which has great potential application in the field of microelectronics.Unlike mechanical stripping and chemical vapor deposition,SiC epitaxial graphene is grown on the basis of SiC decomposition at high temperature,Si atoms preferentially sublimate,and C atoms on the surface enrichment and reconstruction to fore graphene.Limited by the grown mechanism,the graphene grown on the Si surface of the SiC substrate has a strong coupling effect with the substrate,which grately limits the play of the excellent electrical properties of graphene and hinders the practical application of graphene prepared by SiC epitaxial method.Therefore,it is necessary to start from the SiC epitaxial growth principle to study the new graphene growth equipment and the factors affecting the growth of graphene,and to explore a new method for preparing near-free-standing graphene on the SiC substrate.Guided by the preparation of high-quality near-free graphene,this paper designed and proposed a growth method of near-free-standing graphene based on the process of graphene nucleation.Starting from the development of targeted graphene growth equipment,combined with experimental characterization and mechanism research,the main research work and conclusions are as follows:1 Conventional SiC epitaxial graphene is heated by medium frequency induction,which is not good for accurate control of the nucleation process.Base on the optimization of software program,we designed and build a new type of high-temperature chemical vapor deposition furnace.In the new facility,we explored the optimal conditions for the growth of graphene,characterized it by atom force microscope,Raman spectroscopy and scanning electron microscope,and divided the substrate surface into 9 regions to study the growth and distribution of graphene.Finally,the optimal conditions for the growth of graphene were obtained as follows:the hydroeching tejperatx1re was 14501C;the growth temperature of graphene was 1650?,the pressure was 900mbar and the growth time was 30min.The quality and distribution of surface graphene were analyzed by means of Kelvin probe force microscope and macroscopically low-energy electron diffraction.Meanwhile,the growth mode of SiC epitaxial graphene was further verified,with priority nucleation at the edge of the step and gradually pushing into the platform.The effect of microtubules defects in SiC substrates on the growth of graphene was investigated.Characterized by Raman spectroscopy,scanning electron microscopy(SEM)and atomic force microscopy(AFM),and the conclusions is drawn that the steps around the microtubules have higher gibbs free energy.The step morphology of the surface will be seriously bent during hydrogen etching,and the etching pits of different shapes will be generated,which affect the uniformity of the surface step distribution;In the growth stage of graphene,the growth rate of graphene near the microtubule is too fast,and a large number of thick graphene layers of small size are generated,resulting in the uneven distribution of graphene on the substrate surface,which is not conducive to giving full play to the excellent electrical properties of graphene.2 Growth principle of graphene based on SiC epitaxial method,a new method for inducing the growth of near-free-standing graphene was proposed.This method only retained the reconstitute nuclear process of C atoms after Si atoms on SiC substrates volatilized,so as to serve as nucleation site,and then introduced carbon source in methane to grow along the nucleation site.High quality near-free-standing graphene was obtained directly on SiC substrate.The nucleation mechanism of near-free-standing graphene was explained by constructing growth model and designing contrast experiment.X-ray photoelectron spectroscopy(XPS)was used to reveal the role of nucleation and growth in the preparation of near-free-standing graphene by comparing the experimental groups that underwent different growth stages.In order to further explore the induced interation between SiC substrate and graphene in near-free-standing graphene,Raman spectroscopy was used to analyze the change of Raman displacement at peak G of different samples,and it was verified that the coupling between the induced graphene and SiC substrate significantly decreaser.The layer spacing between graphene and SiC was 3.6±0.2A directly measured by high resolution transmission electron microscope,which verified that the prepared graphene was near-free-standing graphene.