Research On Auto-activation And Structure Design Of GaAs Photocathode

As a kind of night vision instrument, low light level image intensifier plays an important role in modern society. GaAs photocathode is the key part of low light level image intensifier, so how to prepare high performance GaAs photocathode is always the hot spot in night vision field. In order to increase the performance of GaAs photocathode, some thorough studies in the photoemission process, surface cleanness judgment technique, automatic activation technique, varied-doped GaAs photocathode' structure design and experiment were performed.Electron's excitation process from valence band to conduction band and transportation process from bulk to surface was studied and escape probability equation was solved from Schrodinger equation. Difference of electron's escape probability among high-temperature Cs activation process, high-temperature (Cs.O) activation process, low-temperature Cs activation process and low-temperature (Cs.O) activation process was compared. It was found that the reason why low-temperature (Cs.O) activated GaAs cathode's performance is higher than high-temperature (Cs.O) activated GaAs cathode is GaAs surface is adjusted during low temperature annealing process.Surface cleanness judgment system that could judge GaAs material's surface cleanness by analyzing the vacuum pressure curve of high-temperature annealing process was developed. Two high-temperature annealing processes about GaAs grown by molecular beam epitaxy (MBE) were analyzed by the system. Highest temperature of the two annealing processes were 600℃and 640℃. respectively. It was found that H₂O desorbed from surface of GaAs at about 100℃, AsO desorbed at about 290℃. AS₂ desorbed at about 380℃, and Ga₂O desorbed at about 610℃. The result shows that atomic cleanness surface of GaAs grown by MBE couldn't be achieved by high-temperature annealing process with highest temperature of 600℃.Computer-automated system that could activate the GaAs photocathode intelligently by the standardized technique was developed. Spectral response evaluation, quantum efficiency evaluation and manual-activation were also supported by the system. The time evolution of the photocurrent during the manual activation and auto-activation of GaAs grown by vertical pulling method was compared. In manual-activation, photocurrent increases more slowly, peak photocurrent decrease faster, and (Cs.O) alternation number was fewer as compared with the auto-activation because of some mis-operation. In manual-activation, the (Cs.O) alternation number of 6, a peak photocurrent of 43μA and an integrated sensitivity of 796μA/lm were observed, whereas in auto-activation, the (Cs.O) alternation number of 9, a peak photocurrent of 65μA and an integrated sensitivity of 1100μA/lm. The result shows that the system could avoid mis-operation and increase rate of qualified GaAs photocathode effectively. Development of the system laid a foundation for mass-production of high performance GaAs photocathode.In order to improve the advantage of varied-doping structure, reflection mode gradient-doping GaAs cathode, half-reflection half-transmission mode gradient-doping GaAs cathode and transmission mode exponential-doping GaAs cathode were designed. These cathodes were grown by MBE. Difference of band structure between reflection mode uniform-doping GaAs cathode and reflection mode gradient-doping GaAs cathode was compared. Diversity of response for incident light between reflection mode gradient-doping GaAs cathode and half-refection half-transmission mode gradient-doping GaAs cathode was analyzed. Optimized thickness of transmission mode exponential-doping GaAs cathode was emulated and the emulated result was proved by experiment. Reflection mode gradient-doping GaAs cathode with integrated sensitivity of 2421μA/lm and transmission mode exponential-doping GaAs cathode with integrated sensitivity of 1547μA/lm were prepared.