Highlights on photocathodes based on thin films prepared by pulsed laser deposition

Abstract

We review the current status of metallic photocathodes based on thin films prepared by pulsed laser deposition (PLD) and we explore ways to improve the performance of these devices. PLD seems to be a very efficient and suitable technique for producing adherent and uniform thin films. Time-resolved mass spectrometric investigations definitively suggest that the deposition of high-purity metallic thin films should be carried out in ultrahigh vacuum systems and after a deep and careful laser cleaning of the target surface. Moreover, the laser cleaning of the target surface is highly recommended not only to remove the first contaminated layers but also to improve the quality of the vacuum by reducing the partial pressure of reactive chemical species as H 2 O, H 2 , and O 2 molecules. The challenge to realize high-purity Mg and Y thin films is very interesting for the photocathode R&D due to the good photoemission properties of these metals. Photocathodes based on Mg and Y thin films have been characterized by scanning electron microscopy and x-ray diffraction techniques to derive the morphological and structural features, respectively. They were also tested in a photodiode cell to deduce the photoelectron properties. The quantum efficiency of such photocathodes was systematically improved by in situ laser cleaning treatments of the surface in order to remove the contaminated layers reaching, in this way, the quantum efficiency of the corresponding bulk materials.

Growth of poly-crystalline Cu films on Y substrates by picosecond pulsed laser deposition for photocathode applications

“…This value is in agreement with the value reported in literature for this material for a shorter wavelength [33] due to the fact that the photon energy at 355 nm is only a few tenths of eV over working function of Yttrium (3.0 eV), but lower than Copper (4.5 eV) [24]. Therefore, once found that the results reach the goal in terms of QE, a whole photocathode device with the design specifications required for an actual rf-gun shall be fabricated by using the lowest power density (in order to obtain a surface as smooth as possible).…”

“…The Cu films were deposited using a typical PLD system, described in a previous work [24]. Yttrium substrates were placed parallel to and in front of Cu targets of high purity (99.9%) at a distance of 5 cm, inside a high vacuum chamber.…”

“…From the reported studies, it has been pointed out that yttrium (Y) and magnesium (Mg), due to their superior performance [12][13][14][15]22,23], seem to be the most promising metallic materials for the fabrication of photocathodes. Inevitably, Y and Mg photocathodes suffer from surface contamination, upon exposure to atmospheric air conditions [24], imposing the removal of the oxidized layer, usually by laser cleaning, prior to the operational use of those photocathodes. However, this treatment leads to delamination, consumption and finally thinning of the metallic film.…”

Characterisation of Pb thin films prepared by the nanosecond pulsed laser deposition technique for photocathode application

“…The parameters are reported in Table 1. Equation (1) A detailed description of the experimental apparatus is described elsewhere [12], while the experimental conditions for the PLD thin film deposition are reported in Table 2. The films were grown at different substrate temperatures, ranging from 30 to 230 °C, by using an ohmic heater system.…”

Structural and morphological properties of metallic thin films grown by pulsed laser deposition for photocathode application

“…The deposition of Y and Pb thin films was performed in a typical PLD high vacuum system described elsewhere [7]. Target and substrate were placed parallel inside the vacuum chamber, while the laser beam impinged at an angle of 45° on the target surface, provoking the ablation of the target and, thus, the deposition of the ablated material on the substrate surface.…”