Abstract: A photocathode structure containing a photocathode material, comprising a plate of single crystal gallium indium phosphide having major surfaces and relative proportions of gallium and indium such that the lattice parameter thereof is substantially the same as that of said photocathode material, and, an epitaxial layer of photocathode material located on a first said major surface of said crystal, the thickness of said layer of photocathode material being of the order of the diffusion length of electrons therein and at least part of a second said major surface of the gallium indium phosphide plate being substantially free from contact by solid material.