Study Of Phase Transformation, Optical And Electrical Properties Of C-BN Thin Films

In boron nitride system,cubic boron nitride(c-BN) is a superhard and wide band-gap semiconductor material with the unification of assorted excellent physical and chemical properties.The c-BN crystal synthesized by high pressure high temperature method(HPHT) have been widely used for cutting and milling tools due to its extreme hardness(～70GPa) and high thermal conductivity which are only second to diamond,outstanding oxidation resistance in air at high temperature,and high chemical inertness against ferrous metals.All these properties make c-BN more desirable than diamond in numerous tooling applications,considering its excellent mechanical properties,c-BN is very attractive in optical and electronic applications as a wide band gap semiconductor material,especially in special environment.Moreover, becouse c-BN has the widest optical band gap(＞6.4 eV) inâ…¢-â…¤compound semiconductors,it exhibits good transmittance over a wide range from UV to visible and up to infrared spectra.Thus,c-BN is suitable for a broad range of optical applications,such as UV detectors,short wavelength emitter or detector,and photovoltaic cell,as well as super-hard protective coatings for optical components and optical windows.Further,c-BN can be doped for both n- and p-type conductivity.easily,so c-BN is a very promising material for fabrication of high temperature,high power,high frequence and transparent devices operating in harsh environment.However,the small size of c-BN crystals synthesized by HTHP method has limited BN applications as a promising material in optical,electrinic components and devices,and super-hard super-hard coatings for cutting tools as well.Based on the great developments of thin film science and technology,to fully exhibit the excellent properties of c-BN,the studis of preparation methods and properties of high- quality c-BN films have been reaearch focus and difficulty in the scientific fields. Considering issues about the present developing state of c-BN thin film,the dissertation is aimed at the convenient preparation,mechanism and pathway of phase transformations,optical properties,and doping of c-BN films.The main research processes achieved have been summed up as following: 1) Phase transitions among the polytypes of boron nitrides during preparation of c-BN thin film are studied from energy and structure aspects.C pathway from h-BN to c-BN is analyzed,namely:h-BN→r-BN→c-BN.It is showed the energy barrier is very high in transformation from h-BN to r-BN,but it is very low from r-BN to c-BN. In fact,defects and impurities in c-BN thin films might favorably drive the transformation from h-BN to c-BN.Defects and impurities can reduce the energy barrier for the transformation from h-BN to r-BN.2) Based on further understanding of mechanism and pathway of phase transformations and division of nucleation and growth process in the c-BN film deposition,we adjust experimental parameter in nucleation stage to develop a new and improved method(three-step approach) to achieve repeatable preparation of c-BN films with high cubic phase content and enhanced adhesion.3) Two groups of RF-sputtered BN films(pure hexagonal phase and approx. 27.5%cubic content,respectively) were annealed at 600 to 1000(?) under nitrogen at atmospheric pressure after deposition.FTIR spectroscopy indicates a reversible transformation from hexagonal phase to cubic phase,and again hexagonal phase.The most effective temperature for h-BN converting to c-BN is 900(?).Further,the indirect transformation by a stepwise path from h-BN to explosive BN(E-BN) and then to c-BN employing metastable E-BN as an intermedium was observed.In addition,we tentatively put forward that the existence of defective h-BN and the plane interstitial defects play a key role on h-BN to c-BN transformation.4) The h-BN thin films with a wide optical band gap E_g(5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency bias magnetron sputtering and post-deposition annealing at 700℃.The optical absorption behavior of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting by the Urbach tail model.The high temperature still has significant effect on the optical absorption properties,refractive index n,and optical conductivity s of h-BN thin films.The blue-shift of optical absorption edge and the increase of E_g probably result from the stress relaxation in the film under high temperature.In addition,it is found that refractive index clearly exhibits different trends in visible and ultraviolet region.Previous calculational results of optical conductivity of the h-BN films are confirmed in our experimental results. 5) The temperature dependence of the cubic phase content and optical properties of boron nitrogen(BN) thin films was studied in this section.The optical absorption edge shift to the high energy with annealing temperature increase indicates that the film optical band gap Eg becomes large.The change in the optical absorption properties result from the stress relaxation and phase transformation owing to the high temperature annealings.The dependence of a on the photon energy is fitted by the Urbach tail model in order to determine the Urbach energy E₀.In addition,it is found that refractive index n exists clearly different dependences on temperature in visible and ultraviolet region,and the optical conductivity s threshold moves to high energy with increasing annealing temperatures in the threshold region.6) The c-BN thin films were deposited on Si substrates using the conventional radio-frequency sputtering system.The c-BN thin films are doped by implanting Be ions into them.The surface resistivity of BN thin films that were embedded and annealed is lower 3-6 orders of magnitude than that of intrinsic c-BN thin films.The surface resistivity of BN thin films increases with the increasing of implantation dose in c-BN thin films or the ascent of annealing temperature or the decreasing of cubic phase content in BN thin films.In the same condition,the resistivity of h-BN thin films is lower 2 orders of magnitude than that of c-BN thin films(cubic phase content is not equal to zero).