Influence of exponential-doping structure on photoemission capability of transmission-mode GaAs photocathodes

In order to improve the cathode performance, an exponential-doping structure is applied to the preparation of transmission-mode GaAs photocathodes, in which a forward-directed induced electric field is formed to facilitate photoexcited electrons movement toward surface. To verify the actual effect of the exponential-doping structure, two types of transmission-mode GaAs photocathode samples were grown by molecular beam epitaxy, in which one is exponentially doped and the other is uniformly doped. For the exponential-doping sample, the beryllium doping concentration in the active-layer decreased quasi-exponentially from bulk to surface ranging from 1*1019 cm"3 to 1*1018 cm"3. Following the chemical cleaning and heat cleaning, the activation experiments of the cathode module of 18 mm in diameter was performed in an ultrahigh vacuum chamber (base pressure <; 10" Pa) using the improved activation technique. After activation, the cathode module was transferred to the seal vacuum chamber and indium sealed into the intensifier tube, then the spectral response curve of the sealed image tube was measured by the spectral response measuring system. The results show that the first photocurrent peak of the exponential-doping sample was lower than that of the uniform-doping one during the initial stage of Cs exposure activation in the high-low temperature activation process. However, with the increase of activation time, the exponential-doping sample gradually exceeded the uniform-doping one in the photocurrent peak. Besides, the spectral response measuring results show that the exponential-doping photocathode exhibits a higher spectral response with a distinct inflexion in the long-wavelength threshold region. Through the spectral response curve fit, the fitted electron diffusion length of the exponential-doping sample is larger than the uniform-doping one. All results indicate that the exponential-doping photocathodes can obtain higher cathode performance and better photoem- ssion capability.