Reduction of surface roughness emittance of Cs3Sb photocathodes grown via codeposition on single crystal substrates

Abstract

Alkali antimonide photocathodes are capable of generating high brightness electron beams given their high quantum efficiency and low mean transverse energy (MTE). Increasing the brightness of the photoemitted electron beams beyond the current state of the art requires surface control of the photocathode at the atomic scale, since the beam brightness at the cathode is degraded by a rough, inhomogeneous surface. In this work, we grow cesium antimonide photocathodes on single crystal substrates (Al2O3, TiO2, 3C–SiC, and a control sample on Si) and study the resulting surface morphology with scanning tunneling microscopy (STM). We observe dramatic changes in surface morphology across substrates. In particular, we demonstrate 10 times larger island size and significantly reduced roughness on two samples grown on 3C–SiC(100) substrates as compared to samples on other substrates. By computing the local electric fields which these surfaces would generate in an electron accelerator source, we estimate the roughness-induced contribution to MTE. Across samples, the calculated contribution to MTE varies by a factor of 17, and the smallest value computed is 12 meV at an extraction field of 50 MV/m, which is smaller than typical values for alkali antimonides. Additionally, we show that oxidation, commonly encountered in vacuum transfer and in accelerator operation, does not affect the measured surface morphology. Our findings suggest that even in high field environments, the brightness of the photocathodes grown on 3C–SiC will be primarily determined by the material's electronic properties rather than by spurious fields generated by photocathode roughness.