Alkali vapour pressure variations during Na 2KSb(Cs) photocathode synthesis

Abstract

A highly efficient semi-transparent multialkali antimonide Na 2KSb(Cs) photocathode was synthesized in situ by a standard procedure. The synthesis steps leading to p-doping of the Na 2KSb base layer, the creation of n-type K and Cs surface states, the creation of a Cs surface dipole layer and, finally to the formation of a Cs-Sb surface dipole layer or incorporation of Cs and Sb into an n-type surface compound are identified and explained. The addition and substitution reactions of alkali (sub) antimonide compounds, taking place during the synthesis of the homogeneous base layer, are considered thermodynamically using fairly reliable estimates of thermochemical functions. Alkali vapour pressure variations influencing the synthesis of the highly efficient photocathode are discussed. Excess Na in the residual atmosphere reacts with the evaporated Sb, giving an undoped heterogeneous base layer. Excess Cs reacts with the evaporating Sb, preventing strong p-doping of the base layer and leading to an unfavourable p-type Cs 3Sb saturated compound in the surface layer.