The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface,thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement,which might be due to the increase in either the number or the energy of electrons reaching the surface,is not clear at present. In this paper,the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed,and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.
The resolution characteristic can be obtained by the modulation transfer function (MTF) of a GaAs/GaAlAs photocathode.After establishing the theoretical model of GaAs(100)-oriented atomic configuration and the formula for the ionized impurity scattering of the non-equilibrium carriers,this paper calculates the trajectories of photoelectrons in a photocathode.Thus the distribution of photoelectron spots on the emit-face is obtained,which is namely the point spread function.The MTF is obtained by Fourier transfer of the line spread function obtained from the point spread function.The MTF obtained from these calculations is shown to depend heavily on the electron diffusion length,and enhanced considerably by decreasing the electron diffusion length and increasing the doping concentration.Furthermore,the resolution is enhanced considerably by increasing the active-layer thickness,especially at high spatial frequencies.The best spatial resolution is 860 lp/mm,for the GaAs photocathode of doping concentration 1 × 10 19 cm 3,electron diffusion length 3.6 μm and the active-layer thickness 2 μm,under the 633-nm light irradiated.This research will contribute to the future improvement of the cathode's resolution for preparing a high performance GaAs photocathode,and improve the resolution of a low light level image intensifier.
The stability of a reflection-mode GaAs photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature.We observe the photocurrent of the cathode decaying with time in the vacuum system under the action of Cs current,and find that the Cs atoms residing in the vacuum system are helpful in prolonging the life of the cathode.We examine the evolution and analyse the influence of the barrier on the spectral response of the cathode.Our results support the double dipolar model for the explanation of the negative electron affinity effect.
