In-situ synchrotron x-ray characterization of K2CsSb photocathode grown by ternary co-evaporation

Abstract

K2CsSb is a promising photocathode candidate to serve as an electron source in next-generation light sources such as Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL). As the traditional recipe for creation of K2CsSb photocathodes typically results in a rough surface that deteriorates electron beam quality, significant effort has been made to explore novel growth methods for K2CsSb photocathodes. In this paper, a method of ternary co-evaporation of K, Cs, and Sb is described. By using in-situ synchrotron X-ray techniques, the quality of the photocathode is characterized during and after the growth. K2CsSb photocathodes grown by this method on Si (100) and MgO (001) substrates show strong (222) texture, and the two photocathodes exhibit 1.7% and 3.4% quantum efficiencies at a wavelength of 530 nm, with a rms surface roughness of about 2–4 nm. This represents an order of magnitude reduction in roughness compared to typical sequential deposition and should result in a significant improvement in the brightness of the generated electron beam.

Temperature-dependent quantum efficiency degradation of K-Cs-Sb bialkali antimonide photocathodes grown by a triple-element codeposition method

“…A different grain structure has been observed based on the acquired diffraction pattern from the codeposited as-grown K-Cs-Sb photocathode which has a predominant (2 2 2) peak and weak (2 2 0) peak, when compared with the powder diffraction pattern acquired from polycrystalline K 2 CsSb samples prepared by traditional recipe which has a predominant (2 2 0) peak [22,23]. Such a result is in good agreement with the previous study on the characterization of codeposited K-Cs-Sb photocathodes [5] and this indicates that a highly textured K-Cs-Sb thin film is formed by the codeposition method which contrasts with the polycrystalline film formed by the sequential deposition.…”

“…The simulation yields a rms roughness of 1.15 nm. The simulated thickness and roughness of the photocathode film grown by the three-element codeposition method are on a comparable level to those reported in a previous study [5]. After the decay is finished, the period of the XRR oscillations appears to be slightly larger, indicating a reduced film thickness.…”

“…X-ray reflectivity X-ray reflectivity (XRR) measurements were also performed before and after the decay process. The details of the measurement setup and data processing are given elsewhere [5]. The experimental and simulated XRR curves for the K-Cs-Sb photocathode before and after the decay are shown in Fig.…”

“…This recipe yields cathodes with ∼5% QE at 532 nm and provides a better reproducibility compared to the traditional sequential deposition recipe. This solves the roughness related emittance growth issue and enables the use of these photocathode materials in high-field electron guns [4,5]. So far, over a dozen of K-Cs-Sb photocathodes prepared by the codeposition method have been employed in the APEX VHF gun and have successfully demonstrated operational capabilities required by high repetition-rate FELs like LCLS-II [6].…”

Towards the operation of Cs-K-Sb photocathodes in superconducting rf photoinjectors

“…Models suggest that at the envisaged gradients for SRF photoinjectors (15)(16)(17)(18)(19)(20) the surface roughness of the photocathode must be reduced to values of R q ≤ 10 nm to maintain a roughness induced emittance contribution that is below the intrinsic emittance [9,10]. From recent works of Schubert et al and Ding et al it is apparent that the final roughness of alkali antimonide photocathodes depends strongly on the growth procedure and the crystallization processes involved [11,12]. It has been shown that the sequential growth process delivers very rough photocathodes due to the multiple phase transitions, while smoother photocathodes can be grown using new growth procedures such as triple evaporation of Cs-K-Sb using pure alkali metal sources and magnetron sputtering [13][14][15][16].…”