Negative affinity 3–5 photocathodes: Their physics and technology

Negative electron affinity (NEA) photocathodes are defined by the relationship between the potential barrier at the surface and the bottom of the conduction band in the bulk of the material. If the bottom of the conduction band liesabove the potential barrier at the surface, the device is said to have a negative electron affinity. In practice this condition is obtained by heavyp-doping of the semiconductor (to encourage downward band bending at the surface) and by adding a thin film (several atomic layers) of cesium richcesium oxide on the clean semiconductor surface. The physics, development, fabrication, and applications of the NEA cathode are reviewed. The threshold of response of a NEA photocathode is set by the semiconductor bandgap. By alloying to form ternary or quaternary 3–5 compounds (3–5 compounds are formed from elements of the 3rd and 5th columns of the periodic table), the bandgap (and thus the threshold) can be placed at any desired photon energy within certain limits. The most important limit is that at about 1.1 eV which is the lowest limit achieved for NEA cathodes. This limit is set by the point at which the bandgap of the 3–5 material becomes less than the surface potential barrier. Fundamental work aimed at understanding the 3–5: cesium oxide “interfacial” barrier which sets this limitation is briefly discussed. Because of the “interfacial” barrier, the quantum yield of NEA cathodes decreases as the threshold of response moves to lower photon energy. Field assisted photocathodes provide a means of extending the threshold of response beyond 1.1 eV. Two different approaches to field assisted photocathodes and recent achievements are discussed. A major advancement has been the achievement of semi-transparent NEA photocathodes by sealing GaAs to glass. This makes possible practical NEA image tubes. The thermionic emission from 3-5 NEA cathodes can be orders of magnitude lower than that from conventional photocathodes. The reasons for this are discussed. Yield and dark current data are given on 3-5 NEA cathodes in operating photomultipliers. Work supported in part by the Advanced Research Projects Agency of the Department of Defense and monitored by Night Vision Laboratory, U.S Army Electronics Command, under Contract No. DAAK 02-74-C-0069.