MBE Growth of Graded Structures for Polarized Electron Emitters Page: 1 of 10
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MBE Growth of Graded Structures for
Polarized Electron Emitters
Aaron Moy,a T. Maruyamab F. Zhoub and A. Brachmannb
aSVT Associates, Eden Prairie, MN.
bSLAC National Accelerator Laboratory, Menlo Park, CA
Abstract. SVT Associates, in collaboration with SLAC, have investigated two novel
photocathode design concepts in an effort to increase polarization and quantum efficiency.
AlGaAsSb/GaAs superlattice photocathodes were fabricated to explore the effect of antimony on
device operation. In the second approach, an internal electrical field was created within the
superlattice active layer by varying the aluminum composition in AlGaAs/GaAs. A 25%
increase in quantum efficiency as a result of the gradient was observed.
Keywords: Polarized Photocathode
PACS: 29.25.Bx, 29.27.Hj
Polarized electrons have been essential for high-energy parity-violating
experiments and measurements of nucleon spin structure, and polarized electron
beams will be required for all future linear colliders. Polarized electrons are readily
produced by GaAs photocathode sources. When a circularly polarized laser beam
tuned to the bandgap minimum is directed to the negative-electron-affinity (NEA)
surface of a GaAs crystal, longitudinally polarized electrons are emitted into vacuum.
The electron polarization is easily reversed by reversing the laser polarization. The
theoretical maximum polarization of 50% for natural GaAs was first exceeded in 1991
using the lattice mismatch of a thin InGaAs layer epitaxially grown over a GaAs
substrate to generate a strain in the former that broke the natural degeneracy between
the heavy- and light-hole valence bands.1
In a previous collaboration between SVT Associates and SLAC, photocathodes
were developed based on GaAso.4Po.36/GaAs superlattice. These devices
demonstrated polarizations as high as 86%, with quantum efficiencies exceeding 1%.2
Two known factors limit the polarization of these cathodes: 1) a limited band splitting;
and 2) a relaxation of the strain in the surface epilayer since the 10-nm critical
thickness for the 1% lattice-mismatch is exceeded.
In an effort to advance photocathode operating characteristics, two different device
structures were examined. The first design employed AlxGa1.xAsySbipy material to
reduce depolarization. A second design employed an AlxGa1.XAs/GaAs superlattice
where the aluminum content was varied during the growth. This graded AlxGa1.xAs
Work supported in part by US Department of Energy contract DE-AC02-76SF00515.
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MBE Growth of Graded Structures for Polarized Electron Emitters, report, August 25, 2010; [California]. (https://digital.library.unt.edu/ark:/67531/metadc1015059/m1/1/: accessed March 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.