Modern theory and applications of photocathodes

Abstract

Over the last thirty years, the Spicer Three-Step model has provided a very useful description of the process of photoemission for both fundamental and practical applications. By treating photoemission in terms of three successive steps-optical absorption, electron transport, and escape across the surface-this theory allows photoemission to be related to parameters of the emitter, such as the optical absorption coefficient, electron scattering mechanisms, and the height of the potential barrier at the surface. Using simple equations and established parameters, the Three-Step model predicts the performance of cathodes and provides detailed understanding of the unexpected phenomena that appear when photocathodes are pushed into new practical domains. As an example, time responses are estimated for existing cathodes, and are found to cover a range of six orders of magnitude. Further, the time response is found to be directly related to the sensitivity (i.e., quantum efficiency) of the cathode. The quantum yield systematically decreases with the time response. Thus, metals are predicted to have the shortest time response (as little as lo-l5 set) and the smallest quantum efficiency (as little as 100~ electrons per photon), whereas the negative affinity photocathodes have high yield (as high as 0.6 electrons per photon) but long response times ( as long as 10Vg set). Other applications of the Three-Step model are discussed.

Study of Negative Electron Affinity Property for Varied Doping GaN Photocathode

“…The photoemission mechanism of NEA GaN photocathode can be explained with Spicer photoemission "3-step model": light absorption, photoelectron transmission and photoelectron emission [11] . NEA GaN photocathode is sensitive to the UV light under 365nm.…”

Polarization and charge limit studies of strained GaAs photocathodes

“…Direct-bandgap semiconductors of the III-V family are especially good electron emitters due to their high photon absorption rate, long diffusion length, and their ability to achieve negative electron affinity (NEA). [2] In p-type materials that have been activated to NEA levels, electrons photoexcited to the conduction band can escape easily because there is virtually no potential barrier at the surface (the vacuum level lies below the conduction band minimum). In particular, GaAs is the most widely used photoemitter because it has a large direct bandgap (which leads to a high photon absorption rate) and is able achieve large NEA levels.…”

“…[2] According to this simple model, the photoemission process consists of the following three steps: the photoexitation of electrons into the conduction band, the transport of electrons to the surface, and emission of electrons into the vacuum. Figure 2.6 shows a semiconductor undergoing the three steps of photoemission.…”

“…The improvement of QE at low photocathode temperams has been observed before. [2] To understand the behavior of the QE at low temperatures, it is first necessary to take a careful look at the photoemission process. As discussed in section 2.2.1, the QE of The QE behavior of thin photocathodes at low temperatures is the opposite of the behavior of bulk photocathodes, the QE decreases with lower temperatures.…”

“…[2] The photoemission efficiency of a photocathode, also known as the quantum efficiency (QE), is defined as the number of emitted electrons per incident photon. The QE depends strongly on the wavelength of illumination and on the negative-lectron-affinity (NEA) levels at the surface of the photocathode (see section 2.2).…”