Mechanism of photocurrent variation during coadsorption of Cs and O on GaAs (100)

The photocurrent curves and spectral response curves of GaAs photocathodes are measured by the multi-information measurement system, and the photocurrent variation has been investigated as a function of Cs/O current ratios. The identical Zn doped (1×1019cm-3) p-type GaAs (100) wafers, identical methods of chemical cleaning and heat cleaning of wafers are used in the performed three experiments. From the experimental results, we find the envelopes of three photocurrent curves approximately satisfy parabola after the exposure to oxygen, while the detailed variation process and the ultimate photocurrent of them are different. The photocathode activated with the smallest Cs/O current ratio has the least consumed time and the largest photocurrent during the first exposure to cesium, and the most alteration times. The photocathode activated with the moderate ratio has the most rapid increase of photocurrent during the first exposure to oxygen, and has the highest quantum efficiency and stability after activation. The photocathode activated with the largest ratio has the fewest alteration times and the lowest quantum efficiency. These phenomena have a close relationship with the coadsorption mechanism of cesium and oxygen on GaAs, and in which the oxygen plays an important role. Due to the exposure to oxygen, the cesium atoms adsorbed on the surface becomes Cs+, their radius decrease to 1.67Å from 2.71Å, and form the dipoles with O-2, this is the main reason of above phenomena appeared.