Photoelectron spectroscopy of CsK2Sb photocathode at Synchrotron Radiation Facility using vacuum transport system

As accelerators and electron microscopes become more advanced, high-performance photocathodes are required. In particular, Cesium potassium antimonide (CsK2Sb) photocathode is of interest because of its low emittance, excitability in visible light, and high quantum efficiency (QE). The challenge is its high susceptibility to environment that lead to low operating vacuum pressure and short lifetime/low extraction charge. To resolve these issues, it is necessary to understand the molecular structure of the cathode and its degradation mechanism. In this study, we transported CsK2Sb photocathode to a beamline of synchrotron radiation facility using a vacuum transport system for molecular structure analysis. Specifically, the cathode was deposited in an evaporation system at Nagoya University. We transported it to Aichi Synchrotron Radiation Center (Aichi SR) located 15 km away, and analyzed it in the depth direction by X-ray photoelectron spectroscopy (XPS) at BL7U. Based on the results, we quantitatively evaluated the composition ratios and stoichiometry of the cathode elements (Sb, K, Cs). A Cs ex-cess state of surface was observed at the surface, and it is consistent with previous studies. The intended atomic structure of CsK2Sb was formed only at a few nanometres of the surface on the Mo substrate. On the other hand, the CsK2Sb cathode structure on the graphene substrate was preserved further in the depth direction.


Introduction
Metal photocathodes are the most robust type known, but the QE is generally in about 10-3% with an exception of cleaned magnesium (Mg) that can reach as high as 0.3% at ~250 nm and cannot be driven by a high-power green laser [1].Cesiated NEA (Negative electron affinity) gallium arsenide (GaAs) is an alternative, however, extreme ultrahigh vacuum of ~ 10-9 Pa is necessary to maintain the QE [2].Cesium potassium antimonide (CsK2Sb) is one of the highest performing alkali (multi) antimonide photocathodes, achieving quantum efficiency that exceeds 10% at 532 nm and operation pressure is at 10-7 Pa (Figure 1) [3,4,5].Record-high beam current of 60 mA in a DC injector with 30 hours 1/e lifetime has also been demonstrated [5].The remaining technical challenge for implementing CsK2Sb photocathode in accelerators is its high susceptibility to environment that lead to low operating vacuum pressure and short lifetime/low extraction charge.To resolve these issues, it is necessary to understand the molecular structure of the cathode and its degradation mechanism.
In this study, we transported CsK2Sb photocathode to a beamline of synchrotron radiation facility using a vacuum transport system for analysis without destroying the cathode structure.Specifically, we made the CsK2Sb photocathodes using two types of substrates and compared their chemical bonding states in the depth direction.The stoichiometry was quantitatively analyzed by XPS using sputtering at a beamline.QE maps for CsK2Sb photocathodes on two types of substrates were also measured.

Cathode deposition
We prepared a half graphene-coated Mo substrate.Graphene is produced on a Cu substrate by the chemical vapor deposition (CVD) method and transferred to Mo substrate using a polymer support [7].Even after solvent-based removal of the Polymethyl methacrylate (PMMA) support using acetone, <1   IOP Publishing doi:10.1088/1742-6596/2687/3/0320353 nm thick residue remains on the graphene surface.The residue was further removed via heat cleaning method under ultrahigh vacuum.We use the 10×10 mm Mo (99.95%) substrate (thickness: 0.5 mm) by the Nilaco Corporation.The cathode deposition method we used was sequential evaporation of Sb, K, and Cs.Cathode deposition was performed at Nagoya University.The experimental setup and cathode deposition techniques are described in Ref. [1,2,8]. Figure 2 shows the QE mapping for a CsK2Sb photocathode on half graphene-coated Mo substrate.The laser spot size is about ϕ0.4 mm and the cathode was biased at −100 V.The typical photocurrent was 25 μA.The photocurrent can be detected with a resolution of 0.1 nA. Figure 2 shows the QE mapping for a CsK2Sb photocathode on half graphene-coated Mo substrate.The QE on graphene shows a satisfactory value as high as 5.4% at 532 nm (hυ = 2.33 eV), and lower 2.3% at the same photon energy on Mo.These QEs are consistent with previous research [1,2,5,7].Cathode on graphene showed a high and uniform distribution of QE that falls within 5% or less.This result shows that a better cathode was deposited on the graphene substrate compared to that on the Mo substrate.

Evaluation of the chemical bonding of CsK2Sb cathode in the depth direction
We transported the cathode to Aichi Synchrotron Radiation Center (Aichi SR) located 15 km away using a vacuum transport system, and analyzed it in the depth direction by XPS (BL7U).The vacuum transport system with a transfer rod and a 50 l/s Non Evaporable Getter (NEG) pump was used, and it maintained the vacuum at the level of 10-7 Pa during the transportation.In addition, it was designed with a weight of 13 kg such that it can be lifted by one person.
We first measured XPS for the as-deposited cathode surfaces on graphene and Mo substrates.And then we performed 4 separate sputtering and observed the cathode structure after each of them.Ar+ was used for sputtering.Table 1 shows the conditions of sputtering for 4 separate sputtering.①~④ of numbers are given in the order of sputtering.The sputtering was performed from low energy.Figure 3 shows the XPS results of K 2p, Cs 4d and Sb 4d peaks for CsK2Sb photocathode on half graphene-coated Mo substrate (hυ = 400 eV).The spot size of X-ray was about ϕ0.1 mm.The information depth of the analyzed region is about 0.5-1 nm, which is determined by the electron inelastic mean-free path length and the take-off emission angle of 45°.The spectra were fit by the Doniach-Sunjic function after subtracting a Shirley background.We used CasaXPS software for the analysis after converting the kinetic energy of data to binding energy.It is clear from figure 3 that the K 2p is at 297.3 eV and 294.6 eV, the Cs 4d is at 79.8 eV and 77.5 eV, and the Sb 4d is at 33.3 eV and 32.1 eV.In addition, the difference in intensity of the Cs 4d peaks on graphene and Mo is evident.The sputtering was shallow such that the substrate element of carbon (C 1s @ 284.2 eV) or molybdenum (Mo 3d @ 231.1 eV and 227.9 eV) was not observed after sputtering.Table 2 summarizes the composition percentage of K 2p, Cs 4d and Sb 4d for CsK2Sb photocathode on graphene and Mo substrates under sputtering conditions of ①~④ and without sputtering.The numbers ①~④ correspond to the conditions in table 1. results are normalized to the composition of Sb.From the composition atomic % of as-deposited surface, the stoichiometry of cathode on graphene was Cs2.28K2.00Sb,and the cathode on Mo was Cs0.20K2.04Sb.The stoichiometry of the as-deposited cathode surface on graphene substrate indicated that the surface was in a Cs-excess.On the other hand, there was almost no Cs on Mo substrate.In comparison with the QE results, it is suggested that the amount of Cs has an effect on the QE.This result is consistent with previous research in that a Cs-excess surface is necessary to achieve high QE [9,10].
For sputtered surfaces, we found that the cathode structure after sputtering of ① and ② did not change significantly, and K and Cs decreased with sputtering of ③ and ④ for both graphene and Mo substrates.However, there was a large difference in the degree of decrease on two substrates.Specifically, for cathode on graphene on substrate, K and Cs after 4 separate sputtering remained to 1/2 and 3/4 of their original values, respectively.In sharp contrast, K and Cs were almost eliminated to only 0.07 and 0, respectively for cathode on Mo after 4 separate sputtering.The structure of Cs-K-Sb on graphene after sputtering contained more alkali metals than that on Mo.This result suggests that the Sb crystal structure formed on graphene maybe more permeable to alkali metals compared to that on Mo.We also found that QE can reach as high as few % even if the cathode structure in depth direction is not ideal.This maybe explained by the fact that electrons are extracted only near the surface several nanometres at 532 nm laser.Consistent trend was observed for different measurement points on the same sample.

Summary
We measured the chemical bonding states in the depth direction of the CsK2Sb photocathode.We deposited the cathode on half graphene-coated Mo substrate at Nagoya University, transported it to Aichi SR using a vacuum transport system, and analyzed structure in the depth direction by XPS at beamline BL7U.We found that the cathode on graphene has a Cs-excess surface, and it is not the case for Mo.Because the cathode on graphene had higher QE compared to that on Mo, the result indicates that a Cs-excess surface leads to high QE.We found that K and Cs decreased with sputtering.On the other hand, the structure of Cs-K-Sb on graphene after sputtering contains more alkali metals than that on Mo.This result suggests that the Sb crystal structure formed on graphene maybe more permeable to alkali metals than that on Mo.In the future, we plan to test the cathode at high RF field.

Figure 1 :
Figure 1: Evolution of QE for CsK2Sb photocathodes as a function of air exposure.Red line indicates an air exposure, at which the QE starts to decrease drastically.[6]

Figure 2 :
Figure 2: The QE mapping for a CsK2Sb photocathode on half graphene-coated Mo substrate.The black dotted circle indicates the evaporation region of the CsK2Sb.The inset is a photograph of typical half graphene-coated Mo substrate used in this study.

Figure 3 :
Figure 3: The XPS results of K 2p, Cs 4d and Sb 4d peaks under different sputtering conditions.Light blue, black, blue, green, and red lines are for cathode surfaces after sputtering under conditions ①~④, respectively.(a) is for cathode on graphene.(b) is for cathode on Mo.The sputtering conditions ①~④ correspond to those in table 1.

Table 1 :
The conditions of sputtering for 4 separate sputtering

Table 2 :
The composition atomic % of K 2p, Cs 4d and Sb 4d for CsK2Sb photocathode on half graphene-coated Mo substrate under different sputtering conditions