Method of forming variable sensitivity transmission mode negative electron affinity photocathode

DETAILED DESCRIPTION The present invention is comprised of a technique for achieving a variable sensitivity transmission mode NEA photocathode by varying the backsurface recombination velocity of the photoemitter layer, the method of forming the photocathode, and the resulting variable sensitivity NEA photocathode structure.
The luminous sensitivity of such a photocathode structure can be varied, in an optimum case, by as much as a factor of three by varying the recombination velocity from approximately 10.
sup.
7 cm/second to less than 10.
sup.
5 cm/second.
The basic structure is preferably comprised of a Group III-V photoemitter on a larger bandgap Group III-V window substrate, but is not limited only to those materials.
For example, the photoemitter layer may be made from a Silicon seed crystal and the larger bandgap material may be a Silicon-Oxide transparent insulator layer and a Molybdenum transparent conductor layer.
In either of the cases, the window substrate or transparent conductor and insulator layer combinations act as a field plate and a dielectric material through which the electric field is applied and have a wider bandgap than the photoemitter material.
The photoemitter, insulator, and conductor layers are respectively chosen from the group of materials classed as metals, insulators, and semiconductors.
The method of forming the present variable sensitivity photocathode is by vapor phase epitaxial techniques and/or liquid phase epitaxial methods onto appropriate single crystal substrates in which the seed substrate may be either removed from the active region of the cathode if it is not transparent to the wavelengths of interest or the seed substrate may remain as a support window if it is transparent to the appropriate wavelengths.
The method of forming and the resulting photocathode structure can be better understood by referencing the following drawings as explained in the detailed description.