The III-V Photocathode: A Major Detector Development

Abstract

New photocathodes have been developed which show improvements in sensitivity of as much as ten to a hundred times over conventional cathodes in the near infrared, and useful improvements at shorter wavelengths. The development and performance of these III-V photocathodes are described. The development stems from a combination of basic knowledge of the photoemission process, gained in the 1950s, and the advancing understanding of the technology of III-V materials, in the 1960s. The superior performance of these cathodes is due to the fact that the vacuum level at the surface lies below the bottom of the conduction band in the bulk of the material. Consequently, the threshold of response is set by the III-V bandgap. The bandgap (and the threshold of response) can be varied by alloying different III-V materials together. A reduction in thermionic emission is realized with these cathodes. At present no semitransparent III-V cathodes with comparably interesting performance are available. The problems hindering further improvements, as well as the problems of placing these cathodes in practical multipliers and image tubes, are discussed briefly.

The Lick Observatory Image-Dissector Scanner

“…The tubes that we are now using do not have ultraviolettransmitting fiber-optics plates and are not useful below 4000 Â. Numerous types of image tubes can be used as the first tube; it would be particularly valuable to develop image tubes with III-V cathodes such as those described by Spicer and Bell (1972) in this meeting. If the resolution of the image tubes could be improved the size of the spectral image on the tube and the slit size of the image dissector could be reduced.…”

Automated Activation Procedure for GaAs Photocathodes at Photo-CATCH*

“…The photoelectric threshold of GaAs, i.e. the sum of the band gap energy 𝐸 g and the electron affinity 𝐸 A , is about 5.5 eV [14], corresponding to an incident wavelength of 225 nm. However, in order to attain electron emission with high spin-polarization, excitation at photon energies in the range of 𝐸 g < 𝐸 𝛾 < 𝐸 g + Δ so is required, with the spin-orbit split-off energy Δ so of GaAs [15].…”

“…The main limiting factor is the electron affinity of the clean semiconductor surface, about 4.1 eV for GaAs. It can be effectively reduced to zero and even to negative values by applying a thin surface layer of Cs and further adding O (or another oxidant such as NF 3 ) to the Cs-layer, creating an NEA photocathode [14]. This surface layer is added during the actviation process, requiring the photocathode to be placed in an ultra-high vacuum (UHV) environment with a base pressure below 10 −10 mbar and the prior…”

Optical Detectors for Spectroscopy

“…Finally, the development of negative-affinity photocathodes (Spicer and Bell 1972) with the potential for very high quantum efficiencies at visible and near-infrared wavelengths out to 9000 Â is now under way as part of the Generation III night-vision program (see, for example, Boardman and Beauvais 1983).…”