Multilayer Controllable Growth And Conductivity Type Modulation Of Large Size Hexagonal Boron Nitride Films

Hexagonal boron nitride(h-BN),as a new material with both regular hexagonal lattice and semiconductor features,has found many excellent properties during the evolution from bulk synthesis to thin film synthesis,such as smooth surface without dangling bonds promoting high carrier mobility in plane,atomic structure of hexagonal lattice promoting stable and high stability.It has a high potential for deep ultraviolet applications owing to its wide band gap as well as high thermal conductivity and chemical inertia,which make it a high quality and high potential material.However,there still remain many challenges in the synthesis and application issue.For example,it is difficult to synthesize single-layer films in large size.Because of the wide bandgap,it is difficult to obtain high conductivity.Towards applications in deep ultraviolet devices,it is necessary to achieve p-type and n-type conductive characteristics that intrinsic h-BN does not possess.In order to solve these problems,large area of single-layer h-BN thin films were prepared by LPCVD method.The growth of heterostructures of h-BN and graphene was studied.The multilayer h-BN thin films were grown on the surface of copper foil using a unique scheme.The theoretical models of vacancy defects and substitution defects of intrinsic h-BN monolayer were calculated by the first-principles method based on density functional theory.The concrete results are as follows:(1)The synthesis of large area monolayer h-BN films and vertical heterostructure with graphene.Firstly,in the pretreatment stage electrochemical polishing technique was used to promote the surface smoothness of Cu foil.It can effectively reduce the nucleation point and form a flat surface.Under the optimal polishing conditions,the surface is annealed in the pretreatment stage,and a longer annealing time was found to form a more favorable surface for growth.In the growth stage,we proposed a wound substrate method to obtain super large area growth of h-BN(>25 inch).According to the difference between the growth segregation growth mode of graphene and the epitaxy growth mode of h-BN,a simultaneous growth method of vertical heterostructures was proposed to obtain both growths at the same time.It was found that graphene precipitated under the h-BN.Due to the van der Waals force of h-BN,the crystal domains of graphene tend to grow well orientated.(2)Controllable growth of multilayer h-BN films.The limitation of h-BN single layer growth in LPCVD has been overcome.Under the same pressure,the growth of h-BN thin films has been changed from single layer to multi-layer by using second catalytic substrates.The growth of multilayer h-BN thin film has been systematically investigated in terms of time,temperature,distance,preheating,and heating rate.The controllable multi-layer h-BN growth has been achieved:about 10 layers of multilayer h-BN films can be grown in 8 minutes,and the controllable growth of layers can be achieved in the growth sequence of 2-8 minutes.(3)Theoretical studies of p-and n-type conductivity of h-BN films.Based on the first-principles method,the native and doped h-BN monolayer are calculated and analyzed by using VASP software.The optimal p-and n-type dopant candidates are chosen by investigating the density of states,band structures and electron density distributions.The main results show that the monlayer h-BN is a direct bandgap semiconductor(at K point)and the bandgap is Eg=4.69 eV.Deeper levels lead to higher activation energies.Four different substitution candidate atoms,Be,Mg,Zn and C,have been selected for the p-type doping.The results show that the Mg atom is the best doping source for p-type doping because of its low impurity activation energy,weak carrier localization and high in-plane mobility.On the other hand,four different candidate atoms,C,Si,O and S,are also selected for the n-type doping.The results show that the impurity level in the ON system is closer to the bottom of the conduction band and has smaller activation energy,which makes it easier to form n-type conductivity.The effect of electron localizatin is not obvious and the in-plane mobility appears high,which indicates a good property of in-plane conducting.