Development And Performance Evaluation Of Varied-composition Varied-doping GaAlAs/GaAs Photocathode

The study of the photocathode is developing in the direction of the varied spectral response.The core of this paper is how to obtain the wide-spectrum blue-extension photocathode with specific bandwidth and peak spectral response characteristics.To achieve this purpose,we presented a design scheme of the varied-composition varied-doping GaAlAs/GaAs photocathode.A systematic study has been carried out in terms of the theory of the photoemission,the quantum efficiency model,the structure design of the material,the activated preparation process and the evaluation of the photoelectric emission performance of the cathode.According to the characteristics of the energy band structure of the varied-composition varied-doping GaAlAs/GaAs photocathode,based on the Cs and O adsorption of double dipole model on the photocathode surface,the Toping model was used to calculate and compare the adsorption and electron affinity of Cs on the GaAlAs(100)and GaAs(100)surface.Meanwhile,the optical performance calculation model of the GaAlAs/GaAs photocathode was established by the theory of thin-film optical matrix.Concerning the Spicer's photoemission "three-step physical model",the electronic ground state and excited state were analyzed by quantum mechanics.The electron energy distribution was calculated from the photoelectron stimulated transition,the photoelectron transport in the cathode to the electron transfer across the surface band bending region and then passing through the surface barrier to the vacuum.By setting the appropriate boundary conditions to solve the one-dimensional minority carrier continuous equation,the quantum efficiency formula of the wide-spectrum blue-extension and narrowband GaAlAs/GaAs photocathode with varied-composition varied-doping was deduced.Based on the electron affinity model of GaAlAs/GaAs heterojunction,the interfacial barrier was analyzed.The band structure model,the internal electric field of the varied-composition varied-doping GaAlAs/GaAs and its influence on the photoelectron were studied by analyzing the variation of the band structure caused by changing the A1 composition of GaAlAs material.For different application requirements,the structure design and material growth were implemented on the wide-spectrum blue-extension and narrowband GaAlAs/GaAs photocathode,respectively.Then the carrier distribution and surface morphology of the photocathode were tested by electrochemical capacitance-voltage(ECV)and scanningelectron microscope(SEM)respectively,and the qualities of the samples were evaluated.The surface chemical cleaning technology of GaAlAs photoemission material was studied by using X-Ray photoelectron spectrometer(XPS)and Ar+sputtering to surface test and then spectral fitting analysis.After comparing the contents of C and O impurities and the quantum efficiency of the surface with different cleaning methods,we found that the"two-step chemical cleaning method"can not only effectively remove the oxide on the surface of the sample,but also remove some impurities C.By the research of effect of high purifying heating temperature and Cs/O activated current ratio on the photocathode,it was found that high heating temperature can further remove impurities C on the surface and the photocathode was very sensitive to Cs,O activated current.No matter the Cs/O current was too large or too small,it would lead to the attenuation of photocurrent.The photocathode samples of three different structures designed and grown in this paper were prepared and tested by Negative electron affinity(NEA)photocathode activation and multi-information in situ characterization system,and then the experimental data were fitted with the parameters.The photoemission performance of the different structure photocathodes were evaluated according to the fitting results.The effect of Cs,O adsorption and recaesiation on the stability of the photocathode was studied by Cs,O activation and recaesiation experiment.It was found that the narrowband response photocathode was better than the wide-spectrum blue-extended photocathode.Meanwhile,aiming at some new problems in the field of specialty,the band-gap narrowing on the surface of photocathode were studied and analyzed by wide spectral experiments.The study of the photocathode is developing towards the wide-spectrum blue-extension photocathode with specific bandwidth and peak spectral response characteristics.To achieve this purpose,we presented a design scheme of the varied-composition varied-doping GaAlAs/GaAs photocathode.And a systematic study has been carried out in terms of the theory of the photoemission,the quantum efficiency model,the structure design of the material,the activated preparation process and the evaluation of the photoelectric emission performance of the cathode.According to the characteristics of the energy band structure of the varied-composition varied-doping GaAlAs/GaAs photocathode,based on the Cs and O adsorption of double dipole model on the photocathode surface,the Toping model was used to calculate and compare the adsorption and electron affinity of Cs on the GaAlAs(100)and GaAs(100)surface.Meanwhile,the optical performance calculation model of the GaAlAs/GaAs photocathode was established by the theory of thin-film optical matrix.Concerning the Spicer's photoemission "three-step physical model",the electronic ground state and excited state were analyzed by quantum mechanics.The electron energy distribution was calculated from the photoelectron stimulated transition,the photoelectron transport in the cathode to the electron transfer across the surface band bending region and then passing through the surface barrier to the vacuum.By setting the appropriate boundary conditions to solve the one-dimensional minority carrier continuous equation,the quantum efficiency formula of the wide-spectrum blue-extension and narrowband GaAlAs/GaAs photocathode with varied-composition varied-doping was deduced.Based on the electron affinity model of GaAlAs/GaAs heterojunction,the interfacial barrier was analyzed.The band structure model,the internal electric field of thevaried-composition varied-doping GaAlAs/GaAs and its influence on the photoelectron were studied by analyzing the variation of the band structure caused by changing the Al composition of GaAlAs material.For different application requirements,the structure design and material growth were implemented on the wide-spectrum blue-extension and narrowband GaAlAs/GaAs photocathode,respectively.At last,the carrier distribution and surface morphology of the photocathode were tested by electrochemical capacitance-voltage(ECV)and scanningelectron microscope(SEM)respectively,and the qualities of the samples were evaluated.The surface chemical cleaning technology of GaAlAs photoemission material was studied by using X-Ray photoelectron spectrometer(XPS)and Ar+sputtering to surface test.After comparing the contents of C and O impurities and the quantum efficiency of the surface with different cleaning methods,we found that the"two-step chemical cleaning method"can not only effectively remove the oxide on the surface of the sample,but also remove some impurities C.By the research of effect of high purifying heating temperature and Cs/O activated current ratio on the photocathode,it was found that high heating temperature can further remove impurities C on the surface and the photocathode was very sensitive to Cs,O activated current.No matter the Cs/O current was too large or too small,it would lead to the attenuation of photocurrent.The photocathode samples of three different structures designed and grown in this paper were prepared and tested,and then the experimental data were fitted with the parameters.The photoemission performance of the different samples were evaluated according to the fitting results.The effect of Cs,O adsorption and recaesiation on the stability of the photocathode was studied by Cs,O activation and recaesiation experiment.It was found that the narrowband response GaAlAs photocathode has better stability and repeatability due to the fact that it can easily produce the stable Al-O chemical bond.Meanwhile,the band-gap narrowing on the surface of photocathode were studied and analyzed by experiments.Based on this,the red shift of quantum effect curve of the photocathode was explained.