Spectral transmittance and module structure fitting for transmission-mode GaAs photocathodes

A transmission-mode GaAs photocathode includes four layers of glass, Si3N4, Ga1-xAlxAs and GaAs. A gradient-doping photocathode sample was obtained by molecular beam epitaxy and its transmittance was measured by spectrophotometer from 600 nm to 1100 nm. The theoretical transmittance is derived and simulated based on the matrix formula for thin film optics. The simulation results indicate the influence of the transition layers and the three thin-film layers except glass on the transmittance spectra. In addition, a fitting coefficient needed for error modification enters into the fitted formula. The fitting results show that the relative error in the full spectrum reduces from 19.51% to 4.35% after the formula is modified. The coefficient and the thicknesses are gained corresponding to the minimum relative error, meanwhile each layer and total thin-film thickness deviation in the module can be controlled within 7%. The presence of glass layer roughness, layer interface effects and surface oxides is interpreted on the modification.