Study On The First-principles Of Scandium And Erbium Doped Aluminum Nitride Thin Films

AlN piezoelectric thin film material has always been research hot spot as high frequency of surface acoustic wave（SAW）devices.With the development of wireless communication system,the increased high demand for high frequency SAW filter.There is no doubt that the AlN films are the preferred material due to its high p-wave velocity.But compared with traditional lead zirconate titanate piezoelectric film and ZnO thin films,the piezoelectric coefficient d₃₃ of AlN film and inherent is lower which limit the application AlN films in the field of high frequency and power electronic devices.It is critical to develop AlN films with specific piezoelectric properties for practical applications.In this paper wurtzite AlN film was selected as the main research object.Through the first principles calculation method based on density functional theory system,the crystal structure,piezoelectric properties,electronic structure and optical properties of Sc、Er doped AlN was studied.The piezoelectric properties and optical properties of the doping system were analysis by different doping content from 0 to 25%.And through the theoretical research of Sc\Er AlN doped crystal structure,the laws of doped AlN are concluded that rare earth doption mechanism of Sc、Er doped AlN are analysis.And combined with electronic structure the optical properties of the mixed system also are explained.Specific content this thesis of as follows:（1）CASTEP module is adopted to establish the supercell structure of the doping content（6.25%、12.5%、25%）of doping system.In order to ensure the reliability of the calculation data,conduct the convergence test of energy ecut and k point grid of the two mixed system separately concluded that the most suitable parameters for the study of doping system.The optimum cutoff kinetic energy of Sc_xAl_1-xN、Er_xAl_1-xN are 600 ev,500 ev respectively.There k point grid are 6×6×4、5×5×3。（2）Study on crystal structure and piezoelectric properties of the doped system.The calculations of crystal structure are including unit cell parameters,cell size and the bond length.The piezoelectric coefficient can be calculated by the formula d₃₃≈0)₃₃?₃₃,in which the elastic constants₃₃ can be calculated by CASTEP module while the piezoelectric constant0)₃₃ can be calculated by VASP.Through research and analysis,we find the piezoelectric properties are closely related to the crystal structure of the materials which the piezoelectric coefficient has increased by the increased doping content.And compared with doped Sc,doped Er is more effective in improve the piezoelectric properties of AlN.When doping content is25%,the piezoelectric coefficient d₃₃=8.67 pC/N was calculated in Er_xAl_1–xN system,while the piezoelectric coefficient d₃₃=7.67 pC/N was calculated in Sc_xAl_1–xN system.By analyzing the crystal structure of Sc and Er doped AlN,the piezoelectric properties of doping system are closely related to crystal structure.Because the ion radius of the doped element Sc and Er is greater than the ion radius of Al,the crystal structure of Sc\Er_xAl_1-xN doped system is distorted,thus improving the piezoelectric properties of AlN.（3）Study on electronic structure and optical properties of doping system.the band structure,density of states,complex dielectric function,absorption spectrum and reflection spectrum of Sc、Er doped AlN systems can be calculated.Compared the parameters of pure and doped AlN,the intrinsic reasons of optical properties changes after doping AlN by Sc、Er will be found.Electronic structure of doped systems is closely related to optical properties.Due to the rich electron configuration of impurity atoms Sc,Er shell layer,the band of doping system is increased and the probability of electronic transition is increased.Also the energy level scope of optical absorption is enlarged.With the increase of the doping content,the band gap of two kinds mixed system changes narrow.At the same time the absorption edge of corresponding doping system is occurred red shift.In the ultraviolet region,the absorption and reflection of doping system are stronger which can be used for ultraviolet shielding or detector.Meanwhile,the electronic energy loss decreased significantly after Er dope.Based on the research of the piezoelectric and optical properties of Sc、Er doped AlN materials,modified AlN piezoelectric materials are found.And through the crystal structure of the doped systems explain the inner mechanism of the piezoelectric performance doped AlN.Through the electronic structure of the doped systems reveal the inner reasons of the optical parameters doped systems.These provide theoretical guidance for the future study the performance of other element doped AlN.