MBE Growth of Graded Structures for Polarized Electron Emitters Page: 5 of 10
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higher currents, with QE of 0.2% and 0.1%, respectively. Polarization was measured
from the 5% aluminum sample, but demonstrated polarization of less than 50% (Fig.
Unfortunately, the device results for antimony-based AlxGaixAso.82Sbo.is/GaAs
photocathode were not as anticipated. Both quantum efficiency and polarization
values were very low. This could be a result of this superlattice design and not wafer
quality. The hypothesis is that the confinement energy around the quantum wells was
too high, trapping electrons inside and hampering their contribution to the
photoemission. Another possibility is that the compressively-strained AlGaAsSb
layers, which are normally grown at temperatures below 500 C, could have been
damaged during thermal removal of surface native oxide at 600 C before surface
GRADED ALUMINUM GALLIUM ARSENIDE SUPERLATTICE
Another approach to increasing quantum efficiency is to create an electrical field
internal to the photocathode active layer to accelerate electrons toward the negative
electron affinity (NEA) surface. Figure 4 displays the concept, where the conduction
band of the superlattice has a sloped gradient field to influence electron flow, as
opposed to a typical design where the barrier heights exist at a constant level. To
achieve such a gradient, AlXGaiXAs alloys were used as barriers. This AlXGaiXAs
material permitted wide latitude in changing the bandgap energy by varying the
composition x during growth, without creating undo changes to the overall lattice
C. B.min V
Figure 4. Diagram of the concept to create an accelerating field internal to the photocathode structure
in order to drive electrons to the surface for increased quantum efficiency.
In the presence of such accelerating field, the electron transport time decreases by
an order of magnitude to less than ips as it has been estimated by Monte Carlo
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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/m1/5/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.