Analysis of the relation between spectral response and absorptivity of GaAs photocathode

In order to study the relation between spectral response and absorptivity of GaAs photocathode, two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), respectively. The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E. The thickness of GaAs emission layer is 1.6 μm or 2 μm. The Al component is 0.5 or 0.63. The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J. The thickness values of GaAs emission layer are 1.4 μm, 1.6 μm or 1.8 μm, respectively. The Al component is 0.7 or varies from 0.9 to 0. The doping concentration of the GaAs emission layer is divided into 8 sections between 1×1018 cm-3 and 1×1019 cm-3. The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer. And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer. Based on the law of energy conservation, the absorptivity curves are obtained according to the experimental reflectivity and transmittivity. In the same coordinate system, both the curves are obtained by unitary processing according to the max. A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response, and those are termed the similar I barrier and the similar Ⅱ barrier, respectively. The results indicate that for both the GaAs photocathodes, the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves. The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample, and 0.3025 eV for the MOCVD sample, respectively. The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD. In the shortwave region, the absorptivity is very large, but the spectral response cuts off nearby 500 nm. In the visible wavelength region, the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve. In the near infrared region, a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve. The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.