Theoretical Study Of The Electronic Property Tuning Of C₃N Bilayers And The Controlled Etching Of H-BN Dielectric Material

With the continous miniature of electronic devices,the semiconductor microelectronic technology is facing more challenges.Searching for low dimensional semiconductors with superior properties to that of traditional silicon devices is an urgent task.Carbon nanomaterials have promising applications in high performance devices due to their exceptional physical and chemical properties including high mechanical strength,electrical and thermal conductivity.However,the utilization of carbon-based devices has been hampered by technical obstacles for decades.Recently,sucessessful synthesis of two dimensional C₃N in experiment offers a new opportunity for carbon nanomaterial's broad applications in optoelectronic devices.C₃N consists of a honeycomb lattice with a homogeneous distribution of nitrogen atoms.Monolayer C₃N is a semiconductor with a bandgap of?1.2 e V and an on–off ratio of?10⁸,which is regarded as an ideal material for next generation carbon-based devices.Compared with graphene,studies related to C₃N are limited due to its late synthesis in experiment.In this thesis,based on first-principles density functional theory(DFT)calculations,a systematic study about the thermal stability,electronic properties and transpert properties of bilayer C₃N was carried out.Our results demonstrated that bilayer C₃N can be a potential material for carbon-based electronics.Given that the bilayer C₃N in Field Effect Transistor(FET)devices needs an atomic-level flat insulating dielectric layer,we explored the effect of h-BN material on the electronic properties of C₃N bilayer and found that h-BN could be an ideal dielectric layer for C₃N bilayer FET.The application of h-BN in C₃N FETs requires the fabrication of high-quality h-BN with smooth and chirality-controllable edges.Currently,the synthesis of high-quality h-BN sheet has made great progesses but fabrication of h-BN with chirality-controllable edges still faces great challenges in experiment.In the thesis,we studied the mechanism of h-BN etching caused by Ni catalyst under different H₂ pressure,and selective etching of h-BN via controlling the H₂ pressure was revealed by our theoretical calculations and realized by our experimental collaborators.The main research results and conclusions are listed below:1.It is found that the thermal stability,significant bandgap engineering,and high carrier mobility of C₃N bilayer offer a new opportunity for its application in high performance carbon-based electronics.Our researches indicate that the C₃N bilayers can vary from semiconductor to metal by controlling the stacking order.In comparison with the monolayer C₃N which has a bandgap of 1.23 e V,the bandgap of C₃N bilayers can be classified as three types:1)C₃N bilayers with AA or AA'stacking are almost metallic;2)C₃N bilayers with AB or AB'stacking have 30%decrease in bandgap value;3)C₃N bilayers with moire pattern stacking have the same bandgap as that of monolayer C₃N.Based on Tight-Binding model and DFT calculations,we found such a bandgap change is caused by p_z orbital coupling between the top and bottom C₃N layer,which leads to energy splitting of the energy bands near the Fermi level.Considering the interlayer coupling is similar for all stacking structures,the difference of bandgap reduction is mainly attributed to the different numbers of overlapped p_z orbitals.The overlapped p_z orbitals of AA/AA'doubled to that of AB/AB'stacking,thus the energy splitting of the bilayer with AA/AA'stacking is almost twice that of the bilayer with AB/AB'stacking.For the twisted C₃N bilayer,there is very limited overlap of the p_zorbitals,which suggests that the bandgap changes for these structures are small.We also found that changing the interlayer distance could be another useful technique to engineer the bandgaps of C₃N bilayers with AA,AA',AB and AB'stacking,the bandgaps of these structures increase with the decrease of interlayer distance.Moreover,the application of external electric field can be another effective means to tune the bandgap of C₃N bilayers.Under an electric field of?1.4 V/nm,a bandgap reduction of nearly 0.6 e V in the AB'structure was predicted by calculations.Our theoretical predictions are confirmed by experimental results.2.h-BN could be an ideal dielectric insulating layer for C₃N bilayer based electronic devices.By studying the binding energy and electronic properties of AA'and AB'C₃N bilayers supported on h-BN dielectric substrate,we revealed the thermal stability of C₃N bilayers on h-BN substrate.Moreover,it is found that the electronic properties of C₃N bilayers are not affected by the supporting substrate.Therefore,the h-BN can be a ideal dielectric materials for the fabrication of C₃N FET.3.Selective etching of h-BN edge can be achieved by using Ni catalyst and controlling the hydrogen pressure.By calculating the formation energies of various armchair and zigzag edges of h-BN,together with their interactions with the Ni catalysts,we revealed the selective etching of h-BN edge under different hydrogen pressure.Under a low H₂partial pressure(1450 K,P_H2 is lower than 10^-2 Bar),the etching of h-BN by Ni leads to zigzag edges.Under a midium H₂ partial pressure(1450 K,P_H2 is 10^-2?10⁴Bar),the etching of h-BN leads to a mixture of armchair and zigzag edges.Under a very high H₂ partial pressure(P_H2above 10⁴ Bar),no selective etching of h-BN can be obtained and the TM clusters will be separated from the h-BN by etching holes with irregular edges.Our theoretical predictions are further valid by experimental results.