MBE Growth of Graded Structures for Polarized Electron Emitters Page: 6 of 10
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simulations and verified experimentally.4s,6 The decrease of the transport time by an
order of magnitude not only increases the current density that is required for an RF
gun by the same amount but it also increases the polarization. In the case of thin films
(<100nm) the polarization can increase by decreasing the transport time of the
electrons and decreasing the total number of scattering events (equation 1).
2 _ sl
P 1+ em 1 sa z nj Eq. (1)
Zsl S1 Zeml
tem: time of electron emission in vacuum
P: electron polarization upon excitation with circularly polarized light
s: total number of scattering events.
Based on the simple model of Equation (1), an increase of 5-10% in polarization can be
achieved with accelerating the electrons.
The main objective of these experiments was to demonstrate an internal bias using
an AlxGa1.xAs/GaAs superlattice structure. Figure 5 shows the reference sample
layout, where the active layer consisted of 19 periods of superlattice layers, with 11
monolayers of AlxGa1.XAs and 7 monolayers of GaAs in each pair. For the reference
sample, the aluminum composition x was held constant at 0.35. An internal bias was
introduced into subsequent photocathodes by varying the aluminum fraction, x,
throughout the active layer. By slightly varying the aluminum fraction between each
pair through the superlattice, a graded field was created. This type of design,
however, adds complexity to the epitaxial growth, as atomic mole fraction must be
quickly and precisely varied.
n7. nm Ga ap Al 35%)GaAs 311 nrr
7.rm ~ AS~GaAs 1 98 nm
1 Repeated for a
1I ayer : total of 19 pairs
AI(35%)GaAs 311 nrr
GaAs 1 98 nm
D.5 gm Al G C 5 As
Figure 5. Schematic structure of the reference A1GaAs/GaAs superlattice photocathode.
The superlattice band structure was studied using the k-p envelope function
method. Figure 6(a) shows the mini-band energies as a function of the superlattice
well width for a fixed barrier width (7 monolayers). The two highest energy valence
bands correspond to the heavy- and light-hole mini-bands, and the maximum energy
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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/m1/6/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.