Tailoring the emissive properties of photocathodes through materials engineering: Ultra-thin multilayers

Abstract

We report on an experimental verification that emission properties of photocathodes can be manipulated through the engineering of the surface electronic structure. Ultrathin multilayered MgO/Ag(001)/MgO films were grown by pulsed laser deposition, tuning the thickness n of the flanking MgO layers to 0, 2, 3, and 4 monolayers. We observed an increase in quantum efficiency and simultaneous decrease in work function with layer thickness. The scale and trend direction of measurements are in good but not excellent agreement with theory. Angle resolved photoemission data for the multilayered sample n = 3 showed that the emission profile has a metallic-like momentum dispersion. Deviations from theoretical predictions [K. Németh et al., PRL 104, 046801 (2010)] are attributed to imperfections of real surfaces in contrast with the ideal surfaces of the calculation. Photoemissive properties of cathodes are critical for electron beam applications such as photoinjectors for Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL). An ideal photoemitter has a high quantum efficiency, low work function, low intrinsic emittance and long lifetime. It has been demonstrated here that emission properties may be systematically tailored by control of layer thickness in ultrathin multilayered structures. The reproducibility of the emission parameters under specific growth conditions is excellent, even though the interfaces themselves have varying degrees of roughness.

Work function and quantum efficiency study of metal oxide thin films on Ag(100)

“…The addition of a finite number of MgO layers (1-5MLs) on Ag(100) is predicted to increase the QE relative to the metal substrate by two orders of magnitude for a single MgO monolayer and one order of magnitude for five MgO layers. The results of these calculations are in good agreement with an experimental measurement of four MgO layers growth on Ag(100), where the QE was found to be seven times higher than that of the clean Ag(100) [14,28]. It is notable that the addition of more layers (6-8) continues to reduce the WF but does not produce any further change in the QE.…”

Photocathode quantum efficiency of ultrathin Cs2Te layers on Nb substrates

“…We note that MgO coatings can produce a dipolar layer and enhance the QE of metals such as Ag. [26] Even if the coating were to reduce the QE by a factor of 3 or so, one would still be dealing with a value ~ 2%, still much higher than typical superconductors. This seems like a promising direction for future research.…”

“…There are numerous possibilities, including an insulating oxide or nitride coating to serve as a suitable protective layer. We note that MgO coatings can produce a dipolar layer and enhance the QE of metals such as Ag [26]. Even if the coating were to reduce the QE by a factor of 3 or so, one would still be dealing with a value ∼2%, still much higher than typical superconductors.…”

Morphology and structure of Pb thin films grown on Si(111) by pulsed laser deposition

“…Quantum emission measurements of Mg and Y thin film photocathodes revealed a value of 1.8×10-3 for the Mg photocathode and 3.3×10-4 for the Y photocathode [35]. However, even with these metals, the low quantum efficiency of a metal used in superconducting cavities, such as niobium, is still a problem [11][12][13]. The photoemission performance of those cavities could be improved by using photocathodes made from other metals or metal alloys.…”

“…Many techniques have been used for growth of Pb thin films for photocathode applications, such as electroplated deposition, arc deposition, sputtering, evaporation and pulsed laser deposition (PLD) [9][10][11][12][13][14][15][16][17][18][19]. The formation of thin films with different 2 morphology and crystalline structure is of utmost importance for the development in those applications, and there are ongoing experimental investigations into the characterization of these morphologies [7].…”