Study On Single Grystal And Film Growth Of Hexagonal Boron Nitride With Low-pressure Chemical Vapor Deposition

Hexagonal boron nitride(h-BN),an insulator with a bandgap of 5.2-5.97 eV,is a layered material with the similar structure to graphite,which has a wide range of potential applications in microelectronics.Compared with the traditional SiO2/Si substrate,the amount of charge impurities in h-BN is lower and the surface flatness of h-BN film is closer to atomic level.As reported,the carrier mobility of graphene can be increased by at least one order of magnitude,when the h-BN films is as the supporting layer of graphene.However,how to prepare large-area,high-quality h-BN is still a big challenge.In the light of the challenges faced to h-BN fabrication,the molecular flow growth model based on LPCVD is introduced to prepare large-area,high-quality h-BN.A molecular-flow growth model,based on the precursor supply,was constructed by simply placing Cu foils on the top of quartz substrates.Based on this molecularflow model,various factors of h-BN preparation,such as the surface roughness,annealing temperature,annealing pressure and oxygen,were analyzed systematically.Finally,h-BN single crystals with a large lateral size of about 70 um were obtained under the extrem low nucleation sites of h-BN on Cu foils.We found that in the molecular-flow growth system the precursor transport on growth substrate could be changed by controling annealing pressure,which could change edge profile from positive curve to negative curve.If the Cu foils are placed on oxygen-releasing substrates,such as fresh quartz plates,aluminas,etc.,the morphologies of h-BN single crystal could be transformed from triangle shapes to rhombus or asymmetric diamond structures.Through the study of the intermediate state of the morphologies evolution of h-BN,we discovered that the morphologies evolution of h-BN was conformed to point-to-edge processes,which eventually formed a regular complementary structure.If there were etching processes at the same time,these complementary structures would form into more various and complex shapes.In addition,we also studied the etching behavior of h-BN in hydrogen,and found that h-BN single crystals and the thin films shared with similar etching behaviors.Namely,in the initial stage both of these etch processes were started from the edges,which showed an anisotropic etching behavior.However,with the etching of h-BN the etching rate is continuously accelerated,and finally the disorder etching process occurs.