Electronic structure of the activating layer in III-V:Cs-O negative-electron-affinity photoemitters

The activating layer in negative-electron-affinity III-V:Cs-O photoemitters, treated as an adsorbed film of Cs 2+x O, is nonmetallic for low x and metallic for high x, leading to physically different heterojunction (low x) and Schottky (high x) modes of activation. Discordant reports on the nature of the Cs-O layer may have their origin in the widely differing physical properties possible for different compositions and thicknesses. Conditions for optimum photoresponse, interfacial barriers, and photoemission thresholds predicted for the Schottky mode are very similar to those previously interpreted on an atomistic dipole layer model. The nature and composition of practical Cs-O coatings are discussed, and a possible limitation on long-wavelength photoresponse is suggested.