Research On Photoemission Mechanism And Material Design Of Graded Doping/Graded Composition GaN-based Nanowire Arrays Electron Source Assisted By Electric Field

Electronic sources play a crucial role in applications such as short wavelength high gain free electron lasers,and photocathodes are an important component of electronic sources.The electric field assisted graded doping/graded component GaN based nanowire array electron source can improve the emission performance of the photocathode through the combined effect of internal and external electric fields.This paper conducts in-depth research on the photoelectric emission theory,optical absorption characteristics,material structure,and cathode stability of electric field assisted graded doping/garded component GaN based nanowire cathodes.The main research work and innovative points are as follows:In order to solve the transverse emission of GaN nanowire array photocathode and the shielding effect caused by the“secondary absorption”of adjacent nanowires,a method of combining internal and external electric fields is proposed.A field assisted nanowire array cathode photoemission model is established,and the influence of cathode structural parameters on quantum efficiency and collection efficiency is analyzed.The results show that the highest quantum efficiency of the graded doping cathode（46.5%）is lower than the highest quantum efficiency of the graded component cathode（57.3%）.During the process of enhancing the external electric field,the collection efficiency of the graded Al component cathode increased by 140%.The finite-difference time-domain method（FDTD）method is used to analyze the Al_xGa_1-xN nanoarrays with graded Al component from the aspects of optical absorption properties and photogenerated carrier generation rate.The results indicate that the use of nanoarray structures can enhance the light capture ability in the ultraviolet band,achieve ultra-low reflectivity and ultra-high absorption,effectively concentrate light at the center of the nanostructure,and achieve high sensitivity characteristics of cutoff wavelength.GaN nanowires are analyzed from the atomic scale in terms of formation energy,adsorption energy,atomic structure,band structure and optical properties by using the first principles.The results show that GaN nanowires are the most stable under the action of an electric field in the（001）direction,and the N_Ga substitution defect is the surface defect with the lowest formation energy in GaN nanowires.In Al_xGa_1-xN superlattice nanowires with adjustable components,the continuous internal electric field makes Al_xGa_1-xN superlattice nanowires have a smaller bandgap.The structure of transition metal particle-doped nanowires is studied,Mn doped GaN nanowires have 100%spin polarization characteristics.In order to systematically study the surface stability of negative electron affinity GaN nanowire array photocathodes obtained by different activation methods,the optoelectric property of the adsorption surface of residual gas molecules are calculated using first principles.The optimal coverage of Cs and O atoms（NF₃ molecules）in the first process and Cs in the second process is 3:1（2）:4.Cs and NF₃ molecules can form stronger connections.In the high concentration residual gas atmosphere,the Cs/Li/NF₃ alternately activated surface is more stable than the pure P-type surface.However,high concentrations of H₂O and CO₂ gas have a destructive effect on the activated layer,and even cause the negative electron affinity of the activated surface to be lost.