K2CsSb Cathode Development Page: 4 of 7
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METHODS
The cathodes are created in a vacuum system with a base pressure of ~0.02nTorr.
This vacuum is achieved using a 4001/s ion pump in conjunction with a titanium
sublimation pump. The deposition geometry is vertical - the sources evaporate up onto
the substrate, which is moved via a linear manipulator into position above each source
sequentially. The antimony source is a tantalum boat packed with antimony pellets.
The potassium and cesium sources are "V" sources from alvatech. The source to
substrate distance is 4cm. A crystal monitor can be used to monitor the deposition rate
of antimony and potassium. The cesium deposition rate is not measured; instead the
QE is monitored during deposition by illuminating the cathode with a green laser
(532nm), and the deposition ended when the QE stops increasing. In addition to the
laser port used for deposition, the system has two horizontally aligned optical ports.
After deposition, the cathode can be rotated to face one of these ports, with the back of
the cathode facing the other. This arrangement allows the optical transmission of a
transparent portion of the cathode substrate to be measured to check the deposition
thickness, and it allows the QE to be measured in both transmission and reflection
mode. Spectral response measurements are made through the reflection port by using a
lamp source (DH2000 from ocean optics) with a fiber-coupled monochromator
(Edmond Optics) to illuminate the cathode. This source has a 2nm bandwidth, and a
5x5mm2 spot on the cathode. Note that measurements with the lamp source require all
external light sources to be off, as even the light from the filament of an ion gauge
produces more current than the lamp. A 532nm CW laser can be used for lifetime and
high current measurements.
For high current tests, the cathode can be moved to a position containing a wire
mesh anode, with an electrode separation of 2.5cm. The anode can be biased to 5kV,
allowing higher currents to be extracted from the cathode without space charge
limitation. The parallel geometry of the cathode and anode in this case allows the
electric field on the cathode to be estimated. The wire mesh is coarse enough to allow
100% transmission of the 532nm laser.
The copper cathode substrate is 2.5cm in diameter, polished with 1 m diamond
polishing compound to a mirror finish. Portions of the copper substrate can be covered
with shims made of other materials, allowing comparison of the QE for cathodes
deposited on different substrates (to date, only copper and stainless steel have been
measured). A 6mm diameter hole in the substrate exposes a glass slide covered with a
transparent conductor. For the current cathodes, 20-40nm of sputtered copper was
used; indium-tin oxide (ITO) coated slides will be tested in the future. This 6mm
region can be illuminated from either side, allowing the QE to be compared in
reflection and transmission modes, and allowing the optical absorption of the cathode
layer to be measured. The cathode is electrically isolated from the vacuum translation
arm, enabling the cathode to be biased and the charge leaving the cathode to be
measured. The arm includes a vacuum heater capable of raising the substrate to 150C,
and liquid nitrogen (LN2) cooling lines capable of cooling the cathode to -SOC.
The cathode is deposited sequentially, following the general method of D. Dowell
[1]. The substrate is at 150C for the antimony deposition, and the rate is set to
~0.5nmn/s. 20nm of antimony is deposited, after which the substrate is cooled to ~140C
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Smedley, J.; Rao, T. & Wang, E. K2CsSb Cathode Development, article, October 1, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc928414/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.