Theory And Preparation Of Graded-composition And Exponential-doping Al_xGa_1xAs/GaAs Photocathodes

The graded-composition and exponential-doping structure is conducive to the photoemission performance of Al_xGa_1-xAs/GaAs photocathodes.In order to perfect the theory system and improve the preparation technologies of the graded-composition and exponential-doping Al_xGa_1-xAs/GaAs photocathodes,the systematic researches on the optical properties theory,quantum efficiency theory,optimization in cleaning and activation process of the graded-composition and exponential-doping Al_xGa_1-xAs/GaAs photocathodes were carried out in this thesis.Considering the lack of studies in optical performance of the graded-composition and exponential-doping Al_xGa_1-xAs/GaAs photocathodes,the optical property formulae were deduced based on the optical matrix theory of thin-film,in which the reflectivity,transmissivity,and absorptivity were figured out.The effect of cathode thicknesses and Al compositon changes on reflectivity,transmissivity,and absorptivity were analyzed,which provides theoretical basis for intensive study of the optical performance in graded-composition and exponential-doping Al_xGa_1-xAs/GaAs photocathodes.Based on the complex structure of the graded-composition and exponential-doping Al_xGa_1-xAs/GaAs photocathodes,the theoretical quantum efficiencies of the Al_xGa_1-xAs/GaAs photocathodes with graded-composition and exponential-doping structure were deduced,respectively,by solving the one-dimensional continuity equations.We analyzed the effect of some relevant performance parameters such as cathode thicknesses and Al compositon changes on the quantum efficiencies by using the deduced quantum efficiency models.Through comparison with the quantum efficiency models with other structures,the superiority in quantum efficiency of the graded band-gap Al_xGa_1-xAs/GaAs photocathodes was validated.Moreover,the experimental optical properties and quantum efficiencies were well fitted by using the theoretical models,and the relevant performance parameters were obtained.In order to further improve the photoemission performance of Al_xGa_1-xAs/GaAs photocathodes,the removal effect of different chemical cleaning methods was comparative studied,and a more efficient chemical cleaning method to remove the surface oxides and carbon contaminant was proposed.During the heat cleaning process,the analysis of vacuum degree and gas contents changes in the vacuum system can help evaluate the cleanliness of Al_xGa_1-xAs/GaAs photocathode surface.By comparison of the vacuum degrees during the low-temperature heat process and the quantum efficiencies after activation of cathode samples using different temperature in the low-temperature heat process,the suitable heating temperature in the"high to low temperature"process was obtained.The improvement of the chemical and heat cleaning process,is conducive to enhance the quantum efficiency of Al_xGa_1-xAs/GaAs photocathode.The photoemission performance is closely related to the activation process,so we carried out a series of activation experiments of Al_xGa_1-xAs/GaAs photocathodes.By comparative analysis of residual gases,activation process and spectral measurement results,it is found that the activation effect of O source with BaO₂ is more satisfied.To solve the problem of long preheating time,and uncontrollable vaporation rate in O source with Ag₂O,the activation process using O source with Ag₂O was improved.By using different illumination conditions during activation process,it is found that Al_xGa_1-xAs/GaAs photocathodes which are activated under red wave band monochromatic light can yield higher quantum efficiency and obtain better stability.The experimental results show that the integral sensitivity of Al_xGa_1-xAs/GaAs photocathodes can be improved by more than 40%and the stability is also optimized by using the optimized cleaning and activation process.