Structure And Preparation Of Si-N Co-Doped Diamond Films

In order to develop n-type semiconductor diamond films and improve the mechanical properties of diamond films,a new type of Si-N co-doped diamond film is proposed in this project.The first principle calculation method is used to study the crystal structure and electronic structure of Si-N co-doped diamond films.The microwave plasma chemical vapor deposition?MPCVD?process is used to prepare Si-N co-doped diamond films The structure and preparation of Si-N co-doped nano diamond films were studied from two aspects.In the aspect of theoretical calculation,in order to find the structural form of Si-N co-doped diamond film and study the influence of Si-N co-doping on the defect energy level and electronic structure of diamond,this paper studies the Si-N co-doped diamond by the first principle calculation based on the density functional theory.According to the phenomenon that 3C-SiC may appear in Si doped diamond films,two cases of Si-N co-doped diamond and n-doped 3C-SiC are calculated respectively.According to the formation energy calculation,the structure of Si-N co-doping is found,and the influence of Si-N co-doping on the electronic structure of diamond is analyzed by calculating the defect ionization energy,energy band,density of state and differential charge.In the aspect of experimental preparation,we think that Si-N co-doped diamond film may grow high?110?preferred orientation nano diamond grains,which can improve the hardness and modulus of the film.Therefore,we use MPCVD technology to prepare Si doped diamond films with TMS as silicon source,and use SEM,AFM,X-ray diffraction?XRD?,Raman spectroscopy?Raman?and nano indentation were used to characterize the surface morphology,microstructure and mechanical properties of the samples to determine the appropriate Si doping parameters.On the basis of the determined Si doping parameters,we continue to use NH₃ as nitrogen source to prepare Si-N co-doped films and to characterize the SEM morphology of the samples.Through calculation and experiment,the following results are obtained:From the calculation of formation energy,we get that the formation energy of Si-N co-dopant in diamond supercell is very large,between 6.978-13.208 eV,which shows that the doping concentration of Si-N co-dopant is very low,it is difficult to enter into diamond.The formation energy of N-substituted doped C site in 3C-SiC is very small,which is0.150 eV.It is suggested that the n-doped 3C-SiC/diamond composite film may be formed by Si-N co-doped diamond.The calculation of defect energy level shows that Si-N co-doping can make the donor defect energy level reach Ec-0.54 EV under the conduction band,improve the very deep donor energy level Ec-1.70 eV of N-doped diamond,and reduce the resistance of the film at room temperature.The calculation of energy band,density of state and differential charge shows that Si-N co-doping can change diamond into n-type semiconductor.Moreover,due to the incorporation of Si atom,the energy of N-s state in CBM decreases,and the free electron introduced by N is far away from N atom,which has a large space of motion,so the donor energy level has been greatly improved.The N-substituted doped C site in 3C-SiC is also n-type doping,and has a very shallow defect energy level of 0.025-0.149 eV under the conduction band.This shows that the composite films obtained by Si-N co-doped diamond have good semiconductor properties,and the resistance at room temperature may be good data.The experimental results show that when the CH₄ flux of Si single doped diamond film is constant,increasing the flux of TMS will refine the diamond grains in the film,and with the flux increasing,the grain size will be smaller.It is found that the proper TMS flux can increase the proportion of amorphous carbon phase in the grain boundary and increase the hardness of the film.When TMS flux is constant,increasing CH₄ flux will also make the diamond grain size smaller,but more defects will be introduced,which will reduce the surface quality and mechanical properties of the film.With the increase of NH3 flux,the size of nano grains on the surface of Si-N co-doped diamond films is gradually increasing.The effect that Si doping can increase the secondary nucleation of the film and grain refinement was weaken.This shows that the addition of N promotes the crystallization of diamond.