Alkali-antimonide photocathodes were grown on Si(100) and studied by means of XPS and UHV-AFM to validate the growth procedure and morphology of this material. The elements were evaporated sequentially at elevated substrate temperatures (first Sb, second K, third Cs). The generated intermediate K-Sb compound itself is a photocathode and the composition of K2.4Sb is close to the favored K3Sb stoichiometry. After cesium deposition, the surface layer is cesium enriched. The determined rms roughness of 25 nm results in a roughness domination of the emittance in the photoinjector already above 3 MV/m
Nano-roughness limits the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. However there is an urgent need for photocathodes that can produce an order of magnitude or more lower emittance than present day systems in order to increase the transverse coherence width of the electron beam. In this paper we demonstrate a method for producing alkali antimonide cathodes with near atomic smoothness with high reproducibility.Photoemission based electron sources for the next generation x-ray high repetition rate, high brightness light sources such as Energy Recovery Linacs 1 and Free Electron Lasers 2 need to satisfy several criteria, namely: high (>1%) quantum efficiency (QE) in the visible range, smallest possible intrinsic emittance, fast (sub-ps) response time and a long operational lifetime. During the past decade, alkali-antimonides (eg. K 2 CsSb) have emerged as the only class of materials that satisfies all these requirements with a high QE >5% and a low intrinsic emittance in the range of 0.36-0.5 µm per mm rms laser spot size in green (520-545 nm) light 3-5 . Additionally, alkali-antimonides also show promise as sources of ultra-cold electrons for ultrafast electron diffraction 6 applications and Inverse Compton Scattering based Gamma ray sources 7 . Although alkali antimonides have many excellent characteristics, the synthesis process leads to relatively high levels of roughness 8 . K 2 CsSb photocathodes are typically grown as thin films over conducting substrates by thermal evaporation of ∼10-30 nm of Sb followed by sequential thermal evaporation and reaction of K and Cs respectively 3,9 . The films created by this process are not ordered and can have a root mean square (rms) surface roughness as high as 25 nm with a period of roughly 100 nm 8 . Such a surface roughness can distort the electric field near the cathode surface causing the intrinsic emittance to drastically increase. Ignoring the contribution of the slope effect 10,11 , to first order, the intrinsic emittance after accounting for this electric field effect can be given by in = 2 in0 + 2 f , where in0 is the intrinsic emittance of the cathode at near zero electric field and f is the enhancement to the intrinsic emittance at an electric field of f MV/m (typically in the range of 1-20 MV/m) at the cathode surface. In RF/SRF based electron guns, used for high bunch charge applications, the electric field at the cathode surface can be greater than 20 MV/m. In this case the electric field enhancement of the intrinsic emittance can be as high as 2 µm per mm rms laser spot size making these cathodes unusable.12 . The smallest possible intrinsic emittance is limited by the lattice temperature of the cathode to in0 = 0.22 a) Electronic mail: fjun@lbl.gov µm at room temperature and can be obtained by exciting electrons with near threshold photons 13 . However, in alkali-antimonides the smallest possible emittance is limited to a higher value even at photoemission...
Alkali antimonides have a long history as visible-light-sensitive photocathodes. This work focuses on the process of fabrication of the bi-alkali photocathodes, K2CsSb. In-situ synchrotron x-ray diffraction and photoresponse measurements were used to monitor phase evolution during sequential photocathode growth mode on Si(100) substrates. The amorphous-to-crystalline transition for the initial antimony layer was observed at a film thickness of 40 Å . The antimony crystalline structure dissolved upon potassium deposition, eventually recrystallizing upon further deposition into K-Sb crystalline modifications. This transition, as well as the conversion of potassium antimonide to K2CsSb upon cesium deposition, is correlated with changes in the quantum efficiency.
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