Rb based alkali antimonide high quantum efficiency photocathodes for bright electron beam sources and photon detection applications L. Cultrera, 1, a) and C. Gulliford, A. Bartnik, H. Lee, I. Bazarov 1 Cornell Laboratory for Accelerator-Based Sciences and Education, Cornell University, Ithaca, New York 14853, USA (Dated: 23 December 2016) High quantum efficiency alkali antimonide photocathodes have been grown over both stainless steel and glass substrates using sequential evaporation of Sb, K, Rb and Cs. Quantum efficiencies well above 25% have been measured at 400 nm. A bi-alkali Rb-K-Sb photocathode grown on a stainless steel substrate has been installed in a high voltage DC gun at Cornell University and the intrinsic electron beam emittance measured at different photon energies. PACS numbers: 79.60.-i, 85.60.Ha Keywords: photocathodes, photon detector, photoinjector, alkali antimonides I. INTRODUCTION Semiconductor materials belonging to the alkali antimonides family have been widely used for many decades in photon detection applications because of their high Quantum Efficiency (QE) in the visible range of the spectrum 1. During the last two decades they also have demonstrated their ability to provide the high average currents required to drive Energy Recovery Linacs (ERL), as well as the high brightness required to operate future X-ray Free Electron Lasers (FEL) 2,3. More recent studies show that for applications demanding extremely bright electron beams, such as Ultrafast Electron Diffraction (UED), these photocathodes provide electrons with Mean Transverse Energies (MTE) equivalent to sub-room temperature when operated with photon energy near their photoemission threshold and at cryogenic temperatures 4,5. Most of the high efficiency photocathode materials have been discovered by trial and error and their production recipes empirically refined in the course of many years 6. The renewed interest in these materials has triggered efforts to understand the formation dynamics of these materials in an attempt to improve the smoothness of the alkali antimonides surface with the final aim of mitigating the intrinsic electron beam emittance growth due to the surface roughness 7,8.Even with this large interest not all the possible combinations of the elements belonging to the alkali metals and Sb metal have yet been explored. The work here reported was inspired by the results obtained in 1969 from M. Dvorak who showed that a multi-alkali photocathode obtained using the sequential evaporation procedure of Cs-Rb-K-Sb developed for image orthicons presented a peak QE of about 40% at 330 nm 9. The growth recipe used at that time was not optimized for operating in transmission mode and results in a photocathode thicknesses that drastically limited transmission mode photoemission over the whole visible spectral range. Despite the extremely high QE reported for this material no other experimental work related to its synthesis and photoemissive properties has been found in the literature. In order to optimize the growth process and produce multi-alkali photocathodes with maximal QE, we performed several preliminary experiments using sequential evaporation of K and Rb only over a thin layer of Sb. Several of these samples were grown over a) Electronic mail: lc572@cornell.edu.
2 polished stainless steel to optimize the growth procedure and were intended to be operated only in reflection mode. Additional samples were grown over glass substrates allowing for operation in reflection or transmission mode (the latter being preferred for light detection applications). For both substrate materials our growth procedure yielded cathodes with QE above the 20% at 400 nm. A representative cathode sample was then installed in one of Cornell University s high voltage DC guns 10 and the cathode properties characterized at different laser wavelengths: for a photon energy of 690 nm and nominal surface electric field of about 4 MV/m, this cathode delivered electron beam MTEs as low as 40 mev (corresponding to an intrinsic beam emittance of 0.28 µm) with a corresponding QE of 2.4x10 4. II. EXPERIMENTAL SETUP AND METHODS The growth of Rb based photocathodes was carried out in a UHV growth chamber (see Ref.11 for details) equipped with effusion cells hosting boron nitride crucibles that were filled with pure metals Rb (99.9% Strem Chemicals Inc.), K (99.95% Strem Chemicals Inc.), Cs (99.5% Strem Chemicals Inc.) and Sb 99.999% (Sigma Aldrich Co.). Recently the growth chamber has been upgraded. Previously, temperature cross talk between the effusion cells was observed, particularly during Sb evaporation when the furnace is heated to well above 450 C. In order to mitigate this effect a water cooled shroud surrounding the effusion cells has been added to the chamber. Additionally, in the new configuration, the furnace shutters which trigger the alkali metal vapor flux on and off have been redesigned, improving the sealing of the furnaces (Fig.1). This improved control over the vapor fluxes becomes important when evaporating Rb and Cs metals due to their very high vapor pressures already present at room temperature (roughly 10 5 Pa for both metals 12 ). All growth and characterization experiments have been performed in UHV environments with partial pressures of oxygen and water vapors in the 10 11 Torr or lower ensuring that surface oxidation effects can be neglected. Figure 2 displays examples of the two different photocathodes: a Rb-K-Sb grown on stainless steel substrate for operation in reflection mode, and a semitransparent Cs-Rb-K- Sb photocathode grown over a Borofloat 33 glass substrates that can be operated in either reflection or transmission mode. Currently the existing substrate holder limits the substrate size to 30 mm, the chamber itself has been designed to provide uniform deposition profiles up to 100 mm. Quantum efficiency as function of the wavelength has been deduced by measuring the photocurrent extracted from a negatively biased photocathode when monochromatic light generated by a system of lamp and monochromator was used to illuminate the cathode surface either in reflection mode (with the light directly impinging on the photocathode material) or in transmission mode (with the light illuminating the photocathode through the glass substrate). In order to remove any spurious signals, the photocurrent was measured using a mechanical chopper to modulate the intensity of light and a lock-in amplifier locked at the modulating frequency. The existing light delivery system limits us to wavelengths longer than 400 nm. The growth methods used in the present experiments were derived from the recipes previously reported to synthesize other bi-alkali photocathodes based on the sequential evaporation of pure Sb and alkali metals over a suitable substrate (either stainless steel or Borofloat 33 glass) 11,13. The photocurrent produced from the negatively biased thin film under illumination with light at 532 nm is continuously measured during the synthesis procedure. Initial growth of either 10 or 20 nm Sb base layer and exposure of it to K vapors was performed with a roughly constant substrate temperature of about 140 C. The cooling down to room temperature of the substrate, obtained only by radiative losses, was initiated only once photocurrent first peaked (see Fig.3) and during exposure to Rb vapors by turning off the substrate heater.the growth of the Cs-Rb-K-Sb photocathode similarly follows the Rb-K-Sb procedure and is completed with exposure of the photocathode to Cs vapors once
3 FIG. 1. Picture of the UHV growth chamber used to synthesize the photocathodes. Insets show a schematic and picture of the water cooled shroud surrounding the furnaces, as well as the pneumatically operated shutters used to trigger the deposition (color online). FIG. 2. Picture of two different photocathodes: on the left side a Rb-K-Sb grown over a stainless steel substrate; on the right side a semitransparent Cs-Rb-K-Sb photocathode grown over a Borofloat 33 glass substrate (color online). the photocurrent extracted during Rb exposure no longer increased, as reported in figure 4. The electron beam measurements have been performed using a high voltage (up to 400 kv) DC electron gun. A selected Rb-K-Sb photocathode was moved from the growth chamber by means of a UHV transport chamber and installed in the gun. Electron beams were generated at different gun voltages (150, 250 and 350 kv) by illuminating the photocathode with light produced by several laser diode modules emitting light at 405, 532, 638, 650 and 690 nm. For each gun voltage and laser wavelength four different pinhole diameters (0.3, 0.5, 0.7 and 1 mm) were used to select the central part of the laser beam that was imaged over the cathode surface. Beam emittances downstream of the gun were computed from direct phase space measurements using our Emittance Measurement System14 and the intrinsic emittance of the photocathode for a defined wavelength and gun voltage was derived from the linear regression of the emittance as function of the initial beam spot size. Additionally, the QE as function of wavelength and cathode surface electric field was
4 Substrate temperature (C) Thickness (nm) Photocurrent (A) 160 140 120 100 80 10 7 10 8 10 9 10 10 10 11 80 60 40 K Rb Sb 20 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Time (s) x 10 4 FIG. 3. Substrate temperature, photocurrent and elemental film thickness as measured from Quartz Crystal Microbalance frequency shift during one of the Rb-K-Sb photocathode growth experiment (color online). FIG. 4. QE as measured during the growth of a semitransparent Cs-Rb-K-Sb photocathode over a Borofloat 33 substrate (color online). measured by measuring the laser power and the electron beam current collected into the Faraday cup placed along the beamline downstream of the electron gun. III. RESULTS AND DISCUSSION While we cannot provide any information on the chemical composition and crystal structure of the Rb-K-Sb photocathodes, recent state-of-the-art density functional theory calculations indicate that a bi-alkali antimonide compound K 2 RbSb has a direct band gap with strong absorption coefficient in the visible and ultraviolet region of the spectrum 15. Multiple growth experiments have been performed resulting in production of these bi alklai photocathodes with QEs as large as 20% at 400 nm. The growth chamber produces photocathodes with a high degree of uniformity and it was not difficult to reproducibly obtain cathodes that operated in reflection mode with QEs of a few percent at 532 nm. Figure 5 shows a QE map measured by scanning a focused laser beam (100 µm laser spot) at 532 nm over a relatively large area of about 2.2x2.2 cm 2 (this area of measurement being limited by the actual measurement setup). The resulting map has an average QE of 4.1% with a standard deviation of about 0.2%. When operated in the high voltage DC gun the photocurrent collected by the downstream Faraday cup was observed to increase with the voltage of the gun and hence the electric
5 20 15 10 5 5 10 15 20 X (mm) 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 FIG. 5. QE map measured by scanning a focused laser beam at 532 nm over a 2.2x2.2 cm 2 area of a Rb-K-Sb photocathode: The resulting map has an average QE of 4.1% with a standard deviation of about 0.2% (color online). 10 1 λ = 406 nm λ = 532 nm 10 2 λ = 638 nm 10 3 λ = 650 nm λ = 690 nm 10 4 1.5 2 2.5 3 3.5 4 Cathode Field (MV/m) FIG. 6. QE of the Rb-K-Sb sample as measured in the high voltage DC gun as a function of the nominal electric field for different laser wavelengths. field at the cathode surface. Figure 6 summarizes the effect on QE as function of the nominal electric field at the cathode surface. The increase of the QE is expected from the Schottky induced lowering of the photoemission threshold. It s worth noting that while the relative increase of QE at the shorter wavelengths is almost negligible when the cathodes are operated near the photo emission threshold with the longest laser wavelength, at 690 nm the quantum efficiency of the photocathode almost doubled, from 1.4x10 4 to 2.6x10 4, while the intensity of the electric field was increased from 1.6 to 3.8 MV/m. For a given photon energy, hν, and nominal electric field intensity at the cathode surface, E, the maximum excess energy of photoelectrons, E, is given by the following expression E = hν ϕ + α βe (1) where α = e/(4πϵ 0 ), β is the electric field enhancement factor and ϕ is the photoemission threshold. Typical Rb-K-Sb photocathodes show photoemission thresholds similar to other bi-alkali antimonides materials at about 720 nm (corresponding to a photon energy of about 1.72 ev). The intrinsic emittance in the transverse x direction ϵ x,i defined as ϵ x,i = ϵ x /σ x where ϵ x is the RMS normalized emittance of the beam and σ x the RMS value of laser spot size was obtained from the linear fit of the emittance measurements with respect to the initial
6 350 300 250 MTE (ev) 200 150 100 50 0.5 1 1.5 lectron Excess Energy (ev) 0 0 0.5 1 1.5 Electron Excess Energy (ev) FIG. 7. QEs and MTEs of the Rb-K-Sb sample as measured in the high voltage DC gun as a function of the excess electron energy as derived from Eq.1 assuming ϕ = 1.72eV and β = 1. beam size in the x direction. The mean transverse energy (MTE) of the electron beam can be calculated using the following equation under the assumption of isotropic emission with no correlation between position and momentum MT E = m e c 2 ϵ 2 x,i (2) In the estimate of intrinsic emittances and derived mean transverse energies, the surface roughness induced contributions are included. The results obtained from the QE and MTE measurements as function of the maximum electron excess energy estimated using Eq.1 with ϕ = 1.72eV and β = 1 are reported in Fig.7. Along with the experimental points the dashed curves report the QE measured at low field from another Rb-K-Sb photocathode and the MTE results obtained from a simplified 1D Monte Carlo simulation performed using the code used in Ref. 16 with the parameters reported in Table I. It s worth noting that the Monte Carlo model does not include any of the surface roughness mechanisms that can contribute to an increase in the measured intrinsic emittance. The Schottky lowering induced by the external electric field is sufficient to explain the increase of the QE measured as the gun voltage is increased from 150 to 350 kv. Relatively small changes in the values of mean free path, phonon energy and cathode thickness used in Monte Carlo are not expected to substantially change the results of the MTEs simulations reported in Fig. 7. Indeed, the asymptotic limit observed at lower excess energies is expected to be 25 mev because the electrons emitted are excited from the tails of the Fermi Dirac distribution and have mean transverse energies equal to kt (with k equal to Boltzmann s constant) as their temperature T is in equilibrium with the lattice. As the excess energies of electrons increase their MTEs that in absence of scattering can be estimated to be 1/3 of their excess energy will be lowered because of the energy losses due to scattering with phonons. The experimental results reported in Fig. 7 seem to indicate a substantial deviation from this model especially evident for the measurements performed at the highest electric field intensities and lowest laser wavelengths. While the values of relevant parameters for the Rb-K-Sb material are likely to differ from the ones reported in Table I the intent of the Monte Carlo simulations is to point out that the simple increase of the excess energy as predicted by the Schottky lowering of the emission threshold as the electric field intensity increases is not sufficient alone to explain the scaling of the MTEs deduced from the electron beam measurements. This indicates that contributions to the intrinsic emittance from the electric field intensity and surface roughness cannot be ruled out as it can be deduced by the asymptotic behavior of the experimental data at the lowest electron excess energies. The growth of the Cs-Rb-K-Sb was performed on Borofloat 33 glass substrates. The initial Sb base layer thickness was set to be 10 nm to produce thinner photocathodes better
7 TABLE I. Parameters used in the 1D Monte Carlo simulation to estimate the MTE of electron beams as function of the electron excess energy.mean free path, phonon energy and cathode thickness values used in Monte Carlo simulations to estimate MTEs are arbitrarily chosen to be the same used in Ref. 16. Parameter Value Mean free path 25 nm Phonon energy 22 mev Cathode thickness 150 nm ϕ 1.72 ev β 1 0 1 Cs Rb K Sb Na 2 KSb:Cs Na 2 KSb CsK 2 Sb 2 3 4 400 500 600 700 800 900 Wavelength (nm) FIG. 8. Spectral response of a multi-alkali Cs-Rb-K-Sb photocathode is compared with typical spectral responses of other alkali antimonides grown in our laboratory. It has to be noted that other materials have been grown using a different growth chamber or configuration of the evaporation sources. suited for transmission mode operation. The spectral response of one of these Cs-Rb-K- Sb photocathode is reported in Fig. 8 along with the curves obtained for other cathodes of the alkali antimonides family grown in our laboratory. The spectral response obtained from this multi-alkali antimonides photocathode show a clear advantage in terms of QE with respect to other bi-alkali photocathodes. At 400 nm (the shortest wavelength we can operate which is imposed by limitation of our optical setup) we measured QE of about 25% thus comparable with other high efficiency bi-alkali antimonides photocathodes. On the other hand in the spectral region between 650 and 750 nm, where most of our bi-alkali antimonides shows their photoemission threshold, the efficiency of the multi-alkali Cs-Rb-K- Sb is more than a factor 10 larger then our convention bi-alkali antimonides. The spectral response in the case of Cs-Rb-K-Sb photocathodes extends further towards the infrared part of the spectrum (as compared to other bi-alkali cathodes) up to roughly 800 nm, but for wavelengths longer than 750 nm the Na 2 KSb:Cs photocathode still shows the largest sensitivities. IV. CONCLUSIONS We reported on the growth and characterization of high efficiency bi- and multi-alklai antimonides photocathodes which included Rb in their chemical composition. Photocathode materials obtained by sequential evaporation of Rb-K-Sb and Cs-Rb-K-Sb from pure elements under UHV have been characterized for possible applications in photon detection and in electron beam generation in high voltage DC gun. The multi-alkali Cs-Rb-K-Sb shows QE significantly larger than other bi-alakli for wavelengths longer than 450 nm with photo-sensitivity extending close to 800 nm. Intrinsic electron beam emittance measurements show a measurable electric field depen-
8 dence typical of a rough photocathode surface. Nevertheless MTEs as low as 40 mev can be produced from Rb-K-Sb operated near the photoemission threshold at 690 nm with QEs larger than 1x10 4. This work has been funded by the National Science Foundation (Grant No. PHY-1416318) and Department of Energy (Grants No. DE-SC0014338 and No. DE-SC0011643) 1 Hamamatsu Photonics K. K., Photomultiplier Tubes: Basics and Applications, third edition ed., Hamamatsu Photonics K. K. (2007). 2 C. Gulliford, A. Bartnik, I. Bazarov, L. Cultrera, J. Dobbins, B. Dunham, F. Gonzalez, S. Karkare, H. Lee, H. Li, Y. Li, X. Liu, J. Maxson, C. Nguyen, K. Smolenski, and Z. Zhao Phys. Rev. ST Accel. Beams 16, 073401 (2013). 3 C. Gulliford, A. Bartnik, I. Bazarov, B. Dunham, L. Cultrera, Appl. Phys. Lett. 106, 094101 (2015). 4 L. Cultrera, S. Karkare, H. Lee, X. Liu, B. Dunham, I. Bazarov, Phys. Rev. ST Accel. Beams 18, 113401 (2015). 5 T. Van Oudheusden et al., J. Appl. Phys. 102, 093501 (2007). 6 A.H. Sommer, Proc. SPIE 2022, 2 (1993). 7 J. Feng, S. Karkare, J. Nasiatka, S. Schubert, J. Smedley, H. Padmore, https://arxiv.org/abs/1610.04288 (2016). 8 M. Ruiz-Oss, S. Schubert, K. Attenkofer, I. Ben-Zvi, X. Liang, E. Muller, H. Padmore, T. Rao, T. Vecchione, J. Wong, J. Xie and J. Smedley APL Mat. 2, 121101 (2014). 9 M. Dvorak, Adv. Electronics Electron Phys. 28A 61 (1969). 10 J. Maxson, I. Bazarov, B. Dunham, J. Dobbins, X. Liu, K. Smolenski, Rev. Sci. Inst. 85, 093306 (2014). 11 L. Cultrera, H. Lee and I. Bazarov, J. Vac. Sci. Technol. B 34, 011202 (2016). 12 R. E. Honig, D. A. Kramer Vapor Pressure Data for the Solid and Liquid Elements, RCA Laboratories, (1969). 13 I.V. Bazarov, L. Cultrera, A. Bartnik, B. Dunham, S. Karkare, Y. Li, X. Xianghong, J. Maxson, W. Roussel, Appl. Phys. Lett. 98, 224101 (2011). 14 J. Maxson, L. Cultrera, C. Gulliford, I. Bazarov, Appl. Phys. Lett. 106, 234102 (2015). 15 G. Murtaza, M. Ullah, N. Ullah, M. Rani, M. Muzammil, R. Khenata, S. M. Ramay and U. Khan, Bull. Mater. Sci. 39, 1581 (2016). 16 H. Lee, L. Cultrera, I.V. Bazarov, Appl. Phys. Lett. 108, 124105 (2016).
Substrate temperature (C) 160 140 120 100 80 10 7 Photocurrent (A) 10 8 10 9 10 10 10 11 Thickness (nm) 80 60 40 K Rb 20 Sb 0 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Time (s) x 10 4
20 15 4.5 4 3.5 3 Y (mm) 10 2.5 2 5 1.5 1 0.5 5 10 15 20 X (mm) 0
10 1 λ = 406 nm λ = 532 nm QE 10 2 10 3 λ = 638 nm λ = 650 nm λ = 690 nm 10 4 1.5 2 2.5 3 3.5 4 Cathode Field (MV/m)
10 0 350 10 1 300 250 QE 10 2 10 3 MTE (ev) 200 150 10 4 100 50 0 0.5 1 1.5 Electron Excess Energy (ev) 0 0 0.5 1 1.5 Electron Excess Energy (ev)
10 0 10 1 Cs Rb K Sb Na 2 KSb:Cs Na 2 KSb CsK 2 Sb 10 2 QE 10 3 10 4 400 500 600 700 800 900 Wavelength (nm)