Activation of the Na2KSb photocathode with Cs and O2 at lowered temperatures

Several attempts to activate the Na2KSb photocathode with Cs and O2 at RT failed due to the growth of a thick intermediate oxide layer, consisting of Sb, Na and K oxides, and thus to the formation of a highly alkali-deficient Na2KSb base layer. The spectral sensitivities in the near IR region were very low because these defects had contributed to a reduced escape depth of excited electrons and an increased photoelectric threshold energy. A novel activation at 140 K has therefore been performed in order to promote the adsorption of Cs and to prevent the harmful diffusion of O into the near-surface region of the Na2KSb and the strong diffusion of K from the interior of the Na2KSb towards its surface. During cooling-down from RT to 140 K, the photosensitivity of the Na2KSb dropped to about one third of the initial value measured at RT, due to an increase in the electrical resistivity of the Na2KSb, thus causing a space charge to arise, which attenuated photoemission. The CsO2 cycles resulted in a relatively high photosensitivity of 250 μA/lm, if its initial drop is taken into account. During heating to RT, the photosensitivity first increased to 370 μA/lm at about 210 K, and then, in the range between 210 K and RT, it unexpectedly decreased from 370 to 100 μA/lm. This behaviour was attributed to defects which caused the build-up of a surface potential barrier, such as the reduction of Cs (sub)oxides, which was enabled by the enhanced diffusion of K from the interior of the Na2KSb into the deposited Cs,O surface film at temperatures above 210 K.