Enhanced photocathode performance through optimization of film thickness and substrate

Abstract

It is shown that the efficiency of photoelectron emission may be enhanced, several-fold, through optimization of photocathode film thickness and appropriate substrate configuration. Such an enhancement is based on a careful consideration of wave interference effects in the film and the consequent modulation of the absorption profiles and electron emission probabilities. The inadequacy of the well-known Lambert-Beer law for modeling photon absorption in thin films is also discussed.

Quantum Efficiency Enhancement of Bialkali Photocathodes by an Atomically Thin Layer on Substrates

“…A scientific implication is that there could be a novel optical interaction mechanism to enhance the QEs of semiconductor photocathodes using atomically thin coatings. This could open up a new pathway in the ongoing approach of engineering substrates to enhance the QE of deposited semiconductor photocathodes 13,15 . A technological implication could be that atomically thin layer coating eliminate time-consuming optimizations of substrate preparations.…”

“…We covered photocathode thickness of 15-25 nm, incident angle of 0-89 degrees as well as substrate materials of iron and silicon. All of these parameters have strong effects on optical absorption of photocathodes when conditions are in interference effects regime 13 . The fact that they did not alter the absorption by photocathodes in our case suggests that it is not the dominant mechanism to explain our results.…”

“…Only limited literature is available on this topic, but the consensus indicates benefits from electrochemically etching the surfaces to smoothen morphology (i.e., electropolishing) and further chemical passivation by exposing them to gases at elevated temperatures, and so on. More recent approaches in this direction include the use of semiconductor single crystals such as silicon and gallium arsenide for clean and atomically controlled surfaces, the use of different metal surfaces for reflectivity enhancement, and implementation of nanostructures …”

“…electropolishing) and further chemical passivation by exposing them to gases at elevated temperatures, etc. More recent approaches in this direction include use of semiconductor single crystals such as silicon and gallium arsenide for clean and atomically controlled surfaces [10][11][12] , the use of different metal surfaces for reflectivity enhancement 13 , and implementation of nanostructures 14,15 .…”

Perspectives on Designer Photocathodes for X-ray Free-Electron Lasers: Influencing Emission Properties with Heterostructures and Nanoengineered Electronic States

“…Photon absorption is a necessary first step in photoemission, and higher QEs are mostly attributable to the use of semiconductor materials having well-defined bandgaps and relatively low electron affinities that promote electron emission into the vacuum. For ultrafast photocathodes, work is underway to utilize waveinterference effects to modulate the absorption profile (e.g., , absorption as a function of depth 32 ) in a way that also enhances emission probability several-fold 117 . While largely theoretical in nature, early studies indicate experimental confirmation of these effects are likely.…”