Surface Purification Evaluation And Process Optimization Study Of GaAs(100) Photocathode

GaAs photocathode is widely used in the field of low-light-level night vision and other areas because of its excellent photoelectric characteristics.The surface purification of GaAs photocathode has been one of the main research hotspots.Due to the different preparation systems and growth methods of GaAs cathode materials at home and abroad,there are some differences in the research results of GaAs surface purification process.In this context,based on the‘photocathode preparation and characterization ultra-high vacuum interconnection system’,this paper conducts research on the surface purification of GaAs（100）photocathode,focusing on the adsorption of impurities on the surface of the cathode,optimization of the cleaning process,and evaluation of purification,so as to improve the photoemission performance of NEA GaAs（100）photocathode.Firstly,the adsorption models of oxygen and carbon atoms at different positions on the As-rich reconstructed GaAs（100）β₂（2×4）surface were constructed by the first-principles calculation method based on density functional theory.By analyzing the structural variation,work function,adsorption energy,and optical properties of the adsorption models,it was found that the adsorption of impurity atoms changed the electronic atomic structure of the GaAs（100）surface and affected the stability of the As dimer.The adsorption of impurity caused the increment of surface work function,and the structures at different absorption positions showed different adsorption energy,the optical properties of different adsorption surfaces were also varied.In the adsorption model of oxygen atoms,As-O-Ga structure was the most hard to be removed,and the adsorption of carbon atoms would cause the As dimer to break and form a dangling bond.Secondly,the newly developed‘photocathode preparation and characterization ultra-high vacuum interconnection system’was introduced integrating various surface analysis methods,with higher vacuum degree and more advanced technology of measurement and characterization.Combined with the system,a series of surface cleaning experiments were designed.On the basis of the original chemical cleaning method of the research group,a UV-ozone cleaning method was added.The comparative experiment of UV-ozone treatment and degreasing,along with the combined experiment of UV-ozone treatment and degreasing were designed.In the process of high temperature cleaning,comparative experiments of residual gas analysis at different temperatures and surface analysis experiments of high-temperature heating at different temperatures were designed.The application of the system surface analysis method in the experiment was introduced,especially the application of X-ray photoelectron spectroscopy（XPS）micro-area analysis.With the aid of systematic surface analysis methods,various surface cleaning experiments were evaluated.The XPS surface micro-area analysis technology was used to characterize the cleaning effect of chemical cleaning combined with UV-ozone.The results showed that the purification method combining ultraviolet ozone and chemical cleaning made the surface of GaAs（100）have the lowest content of impurities including oxides and the smallest thickness of oxide layer,and the favorable arsenic-rich surface formed during the process was more suitable for high-temperature heating cleaning.By using QMS to analyze the residual gas of high temperature heating desorption,it was found that the desorption rules of certain specific gases such as As H₃and As₂can be used as one of the reliable standards for judging the surface cleanliness of GaAs photocathodes.The XPS and UPS were used to analyze the GaAs（100）surface after heating at different temperatures.The chemical composition and valence band spectrum of surface were characterized.Finally,the activation comparison experiments were carried out on GaAs（100）photocathodes treated with different purification processes,and in-situ surface analysis was performed on the activated photocathodes.Combining the analysis of photocurrent and spectral response curves,it was verified that the optimized surface purification process could obtain the photocathode with a cleaner surface and better emission performance.