Charge limit effects in emission from GaAs photocathodes at high optical excitation intensities

The stationary characteristics of a photocathode and the characteristic times of transient processes accompanying instantaneous switching of the illumination on and off under the conditions of high excitation intensity are examined in a nonlinear diffusion model, with allowance for the photovoltage dependence of the parameters of the skin layer. Analytical expressions for the quantum yield as a function of the photovoltage and radiation intensity are obtained for the stationary case. The critical value of the illumination intensity corresponding to a transition into the emission charge limit regime is found. It is shown that the time of emergence into a stationary state depends on the magnitude of the photovoltage established on the barrier and is determined mainly by the equilibration time between the electron and hole fluxes on surface recombination centers. At high illumination intensities the equilibration time is inversely proportional to the illumination intensity. At intensities corresponding to complete blocking of the cathode, this results in an emission charge limit. The photovoltage relaxation time can reach microseconds. The photovoltage dependence of the tunneling transmittance of the activation layer can be reconstructed by comparing with experiment the computational results obtained for the quantum yield as a function of the photovoltage.