MBE Growth of Graded Structures for Polarized Electron Emitters Page: 4 of 10
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X_ray diffraction was performed on the device wafer to confirm and measure the
superlattice (Fig. 2). A simulation of the structure was also calculated assuming 100%
strain in the structure. The superlattice peaks are observed in the measured data, as
expected from the superlattice layers. However, the location of the sideband peaks is
slightly shifted, indicating that some relaxation in the material may have occurred.
The wafers were sent to SLAC where they were cut into dies and cesium-activated.
Prior to installation in the test system, the sample is degreased in a boiling solution of
trichloroethane. After the protective oxide layer is removed in ammonium hydroxide,
the sample is rinsed in distilled water and methanol. The cathode activation method
used to obtain a negative-electron-affinity (NEA) surface consists of heat cleaning to
6000 C for 1 hour, cool-down for an hour, followed by application of cesium until the
photo-yield peaks, and then cesium and nitrogen-trifluoride co-deposition until the
photo-yield is again maximized. The heat-cleaning temperature is monitored via an
infrared pyrometer. The cathode is activated while monitoring the photo-yield with a
white light and a 670 nm diode laser. Once a red response is observed from the diode
laser, the white light is turned off and the diode laser is used to complete the
activation. The absolute QE is measured using the diode laser at a photon wavelength
of 670 nm. A tungsten lamp and a monochrometer are used to measure the relative
QE as a function of photon wavelength, and these measurements are then normalized
to the diode laser measurement at 670 nm
- QE T
10 - -
650 700 750 800 850 900
Figure 3. Quantum efficiency and polarization measurements from the AlGaAsSb/GaAs superlattice
photocathodes with 5 % aluminum composition.
Measurement of the quantum efficiency (QE) from the 10% aluminum sample was
very low, -0.01% at 650 nm excitation wavelength. Such emission was too low to
measure polarization. The 5 and 15 % aluminum composition samples did yield
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MBE Growth of Graded Structures for Polarized Electron Emitters, report, August 25, 2010; [California]. (digital.library.unt.edu/ark:/67531/metadc1015059/m1/4/: accessed September 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.