Chemical Vapor Growth Mechanism And Properties Of Two-dimemsional Hexagonal Boron Nitride Films

Two-dimensional layered materials have attracted significant interest in recent ten years since the discovery of graphene due to a number of exceptional properties and a broad range of applications.Atomically thin hexagonal boron nitride（h-BN）,known as"white graphene",is a two-dimensional graphene-like layered material and possesses excellent overall performances,including superb chemical stability at elevated temperature,low dielectric constant,high mechanical strength,good thermal conductivity and high corrosion resistance.As a result,h-BN plays an important role in structural materials and optoelectronic devices,especially in deep ultraviolet light emitters and and serving as a top dielectric layer of a field emission device.Methods of preparation and characterization of graphene also apply to two-dimensional h-BN films.However,compared to the weak van der Waals forces between the neighboring C layers of graphene,the prominent lip-lip interactions between neighboring h-BN layers due to the polar B-N bond makes it much more difficult for h-BN to form monolayer films.Therefore the preparation of two-dimensional h-BN films is more difficult and challenging,and the relevant researches are still in basic stage.In this paper,low-pressure chemical vapor deposition（LPCVD）was used to prepare h-BN films,using borazine as a precursor.Combined with theoretical analysis,we put a deep probe into the effects of deposition conditions on the growth and quality of atomically thin h-BN films.Then we further studied the optical and mechanical properties of the h-BN films,as well as its oxidation resistance.The tunneling devices were fabricated and the electrical properties of h-BN films were also investigated.The effects of deposition temperature,flow of dilution gas,total flow of precursor and carrying gas,deposition time and metal substrate on the morphology and quality of h-BN films were investigated in this paper.In the CVD process,thermally induced decomposition of the precursor and dehydrogenation of the polyborazylene derived from the polymerization of borazine take place successively above the substrate,during which the h-BN crystal edges continuously incorporate the activated boron and nitrogen species arriving at its active sites and finally form the h-BN films.In essential,the morphological evolution of the h-BN grains was ascribed to the misbalance between the concentration of boron and nitrogen species and their chemical potential,determined by the CVD conditions.The growth and morphology evolution of h-BN grains were strongly affected by the CVD parameters,which determined the physical and chemical state of the precursor on the substrate surface.During the CVD process,the dissolution of boron and nitrogen in copper takes place.The solubility of B atoms in the copper matrix,which increases when temperature goes up,is higher than that of N atoms,causing the ratio of the available activated boron and nitrogen species to deviate from the stoichiometry ratio of BN.Thus the shape of h-BN grains changed.With the temperature going up,the h-BN grain size increased,while the precursor concentration reduced the grain size due to a larger nucleation density.Dilution gas with a higher proportion of hydrogen inhibited the pyrolysis of the precursor,leading to a smaller size of the h-BN grain.On the other hand,by changing the chemical potential of the activated radicals,the increase of hydrogen proportion allowed the existence of protrusions at the grain edges which are higher in energy.In addition,the doping of impurity elements such as Si and O,etc,also changes the grain morphology.With a moderate deposition temperature and precursor concentration as well as a longer exposure time,the isolated h-BN grains started to expand and eventually merge to form a continuous high-quality single crystal h-BN film.The CVD parameters determine the type of growth kinetic of h-BN film and hence affect the quality of h-BN film.The surface morphology,thickness and crystallinity of h-BN thin films were investigated by scanning electron microscopy（SEM）,atomic force microscopy（AFM）and transmission electron microscopy（TEM）.The thickness of the h-BN films measured by AFM reflected the deposition（or growth）rate.With higher flow of the dilution gas,we found an increasing growth rate of h-BN films and the deposition process was dominated by dynamic control at which the surface of the films became smoother.Dehydrogenation of the precursor was the rate-limiting step during the growth process,and therefore with a larger proportion of precursor,the deposition rate exhibited a characteristic of first increasing and then gradually slowing down according to the growth kinetic calculation.The effect of temperature on the h-BN deposition rate was amplified because the reaction rate of each deposition step depends on temperature exponentially,and thus the thickness of the h-BN thin film increased rapidly at high temperature.The crystallinity and surface morphology of the epitaxial h-BN film were also improved.However,an excessively high temperature could make it more easily to form polycrystalline BN particles and lead to contaminations of impurities from tubular furnace structural materials.We successfully reduced the surface roughness of the metal substrate through mechanical polishing and electropolishing.A smoother substrate was more favorable for the growth of homogeneous and continuous h-BN film,which was tightly stuck to the substrate.The annealed copper substrate was almost single crystal,with the dominant crystal orientation of Cu（111）and Cu（100）,both of which could grow epitaxial h-BN films,while Cu（111）is more preferable to the h-BN growth.Here we also studied the optical,mechanical and anti-oxidation properties of the obtained h-BN film and fabricated a tunneling device to measure its electrical properties.The outcomes are as follows.Ultraviolet–visible（UV–vis）absorption spectrum showed that h-BN film was highly transparent in the visible region while a strong absorbance peak was observed at²01nm.The optical band gap of h-BN thin films with the thickness of 3⁵.5nm was between 5.89⁵.98eV,and it decreased with a thicker film.The mechanical strength of h-BN films was measured by atomic force microscopy（AFM）.The elastic modulus E^2D and breaking strengthσ_m^2D were about 504.3 N/m and68.6 N/m respectively.In terms of oxidation resistance,only 3nm h-BN films can effectively prevent the Ni substrate from being oxidized at high temperature（>1000°C）.Those oxygen molecules with sufficient energy at high temperature penetrated through the hexagonal rings of h-BN films and were adsorbed onto the interface of the metal substrate and h-BN films.Then these oxygen molecules reacted with the substrate while h-BN was almost free from oxygen erosion.With respect to the electrical properties,we fabricated the Au/h-BN/Au tunneling device on SiO₂（300nm）/Si.The two-dimensional h-BN films exhibited a typical quantum tunneling effect with a breakdown strength of2.9MV/cm.