Large-size Monolayer-controllable H-BN Film And Modulation Doping For P,n-type Conductivity

Since the discovery of graphene in 2004,the study of other new two-dimensional has aroused great research interest.So far,more than hundreds of two-dimensional materials have been discovered.Similar to graphene,large-size and high quality 2D materials are not only important for exploring new physical phenomena and properties under two-dimensional limits,but also have many practical applications in optoelectronics.In recent years,except graphene,two-dimensional materials such as hexagonal boron nitride(h-BN),transition metal sulfides,and black phosphorus have also been prepared,which have greatly expanded the applications of 2D materials.h-BN has excellent chemical stability,outstanding physical properties,atomic smooth surface,high thermal conductivity and wide band gap.The structure of h-BN is very similar to that of graphene,so it is also called "white graphene".However,due to the wide band gap of boron nitride,the limited area preparation and the immature application technology of deep ultraviolet LEDs have limited the further development of boron nitride.Therefore the preparation of a large area and high-quality h-BN,tuning of its energy band gap,and promotion of deep ultraviolet LED applications have become our main research challenges.In this thesis,a large-area and high-quality h-BN thin film was prepared by LPCVD technique.The first-principles calculations with density functional theory was used to analyze the atomic vacancy and the modulation doping for p,n-type conductivity.Finally we successfully synthesed p,n-type h-BN film,which further promoted its application in deep ultraviolet LEDs.The principal results are as follows:(1)Growth of large size layer-controllable h-BN films.Many factors could affect the growth of h-BN film such as types of precursors,the decomposition temperature of the precursors,the substrate,the flow rate of the carrier gas and the temperature of reaction chamber.On this basis,we used LPCVD system with borazane as precursor,and proposed a method of winding the Cu foil substrate to grow a high-quality,large-area two-dimensional h-BN film.The home-designed quartz fork was made to support the wound substrate,so that the substrate can be maintained in a good shape and the carrier gas can pass through smoothly during the growth process.A large-size(25-inch)h-BN film was achieved in a limited size reactor.The distance between the substrate and the precursor was found critical for controlling layer number of h-BN film.These results provide possibilities for the manufacture and development of two-dimensional novel optoelectronic devices based on h-BN.(2)First-principles calculations on p,n-type doping in h-BN.The geometric and electronic structures of vacancy defects(VN,VB)and foreign impurities(BeB,MgB,CN,ZnB,CB,SiB,ON,and SN)in h-BN monolayer was studied.The band structure showed that h-BN is an indirect band gap semiconductor with a bandgap value of 4.12 eV.For the intrinsic vacancy defects,VN was found to influence the micro-geometry of h-BN,and will form deeper impurity levels which seemed difficult to be activated.For the p-type doping of h-BN,the calculated results showed that the carriers in h-BN:MgB have a weak spatial localization and Mg could be an optimal p-type impurity.For the n-type doping of the h-BN,we selected C and Si for the substitution of B,and O and S of N.The CB and ON systems are more stable with a low formation energy.The activation energy of ON system is less than 0.02 eV,and its carrier's spatial localization appears weak,which could be an optimal n-type doping impurity.(3)p-type doping of h-BN was successfully achieved by modulation doping with Mg.Magnesium nitride was used as the impurity source.A 3-zone and 2-gas-line CVD system was setup for h-BN doping.The Mg3N2 powder was contained in a quartz boat and placed in the individual heating zone.Mg was confirmed as substituting impurity in the h-BN lattice by XPS and AES measurements.The surface current can reach 12 ?A under 10 V and the hole density is 1.7 × 1014 cm-2.On the other hand,C was selected as the impurity source for n-type doping of h-BN.A pulsed method for C doping was proposed for implementation of C in h-BN.However,n-type conductivity still remains unavailable due to high activation energy.In the near future,the preparation of p,n-type conductive h-BN films will provide wide range of application in deep-UV optoelectronic devices.