Growth and stoichiometry of the Cs xSb surface film of a high-efficiency, multialkali-antimonide photocathode

Abstract

The heterojunction model of a highly efficient, semitransparent, multialkali-antimonide Na 2KSb(Cs,Sb) photocathode, consisting of a homogeneous, polycrystalline, p-type Na 2KSb base layer and an n-type Cs x Sb surface film (0 < x ≤ 3), has been investigated. The thin Cs x Sb film was deposited onto the Na 2KSb base layer by alternating introductions of Cs vapour and evaporations of Sb at elevated temperatures in an ultrahigh vacuum established in the standard image intensifier tube. An estimation of this film's thickness, which amounts to between 2.5 and 8.5 nm for the stoichiometric ratio 1 ≤ x ≤ 3, was obtained using a hypothetical growth model of a thin film barrier one cluster-size thick on the photosensitive base layer surface while analysing the photocurrent behaviour both during the introductions of Cs vapour and during the evaporations of Sb. This model was based on the growth of regular pyramids with a square net base of 4 × 4 atoms and equilateral triangle sides, on the apparent surface, and on a quantum mechanics scattering cross-section for elastic scattering of photoelectrons on the base boundary atoms. Comparisons of our calculated Cs x Sb-Cs dissociation vapour pressure relations with the reported P Cs( T) expressions for Cs desorption from the Na 2KSb(Cs) photocathodes and of our estimated Sb MNN Auger signal asymmetry of the Cs x Sb compounds with the reported Sb MNN line shape of the highly efficient Na 2KSb(Cs) photocathodes were made, both leading to x = 1.5. The thermodynamic considerations of the synthesis reactions existing in the Sb-Cs binary system which gave their Cs equilibrium pressures were performed using thermodynamic functions related to real solid solutions. The Sb MNN asymmetry of the Cs x Sb compounds was obtained by means of a thermodynamically estimated valence charge transfer in Cs x Sb, supposing their mutual linear dependence.