Design And Research On Alkali Evaporation Source Based On Glass Tube Sealing Technology

Alkali metal doping plays a very important role in the research of thin film materials grown by molecular beam epitaxy.As the core device of alkali metal doping,alkali metal evaporation source is mainly used to evaporate alkali metals such as lithium,sodium,potassium,rubidium and cesium.Alkali metal doped thin films have been widely used in semiconductor dielectric thin films,solar cells and other research fields because they can improve the structural stability of the whole doped system and optimize its electrochemical performance.Most of the existing alkali metal evaporation sources use alkali metal release agents as raw materials,and the amount of alkali metal obtained by evaporation is small,which cannot meet the demand for alkali metal content in the process of multiple doping during the epitaxial growth of semiconductor materials.Therefore,an evaporation source for obtaining a large amount of alkali metal elements was obtained by glass tube sealing technology.The alkali metal simple substance technology for vacuum seal of that glass tube comprises the follow steps: place the alkali metal simple in a glass tube with a needle valve at the opening in the glove box,take out and connect a vacuum pump group for pumping treatment,and then rotate and heat the glass tube to complete the wire drawing sealing.The alkali metal content can reach 1.5 g～2.5 g,which is greatly improved compared with the existing alkali metal evaporation source(5 mg).The specific research contents and results of this paper are as follows:(1)According to the relationship between saturated vapor pressure and temperature of alkali metal materials,two kinds of alkali metal evaporation sources with high melting point for lithium,sodium and potassium metals and with low melting point for rubidium and cesium metals are designed.According to the working principle of the two alkali metal evaporation sources,the design and research scheme of the alkali metal evaporation source is determined.(2)According to the working principle and design scheme of two alkali metal evaporation sources,the structure of high and low melting point alkali metal evaporation sources was designed respectively.Based on the finite element analysis software ANSYS,the thermal radiation simulation analysis of the heating filament in the evaporation source was carried out,which proves the uniformity and safety of the thermal radiation of the glass tube and the feasibility of the structural design of the high melting point alkali metal evaporation source.The accuracy and reliability of the simulation results are verified by the actual temperature test.The heat conduction simulation analysis of the cooling effect of the semiconductor refrigeration sheet was carried out.The results show that during the intermission of the doping experiment,the temperature in the glass tube can be reduced to below the melting point of the alkali metal,so that it can be stabilized in the solid form to avoid waste,and the structural design of the low melting point alkali metal evaporation source is proved to be reasonable.(3)Combining nonlinear tuning PID control with genetic algorithm optimization,the temperature controller of alkali metal evaporation source is designed.The temperature control experiment of high melting point alkali metal evaporation source was carried out,the response time is about 5 min,and the temperature error of evaporation source was controlled within ± 0.1 °C in 5 min,and the accuracy could reach ± 0.05 °C after stabilization.The stable temperature can improve the stability of the molecular beam,thereby improve the uniformity of the molecular beam epitaxial growth.(4)Finally,the performance of the two alkali metal evaporation sources was studied,and the coating test of the high melting point alkali metal evaporation source was carried out.Through the observation during the test and the analysis of X-ray energy spectrometer,it was proved that the alkali metal evaporation source could evaporate the alkali metal elemental film.The refrigeration experiment of low melting point alkali metal evaporation source proves that the temperature of glass tube can be reduced to below the melting point of alkali metal material,so that it can be stabilized in solid form.The research on the performance of the alkali metal evaporation source proves that the two alkali metal evaporation sources have achieved the expected research and development goals and fully meet the requirements of molecular beam epitaxial growth in ultra-high vacuum environment.