Development of Polarized Photocathodes for the Linear Collider Page: 2 of 6
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thickness. Thirdly, superlattice structures also have the additional advantage that they can
overcome the inherent critical thickness limitation of single heterostructures, permitting a
much thicker active layer for photoemission. The superlattice structures studied to date have
all been designed with a high doping profile in a thin (10 nm) layer near the surface. The high
surface doping density is necessary to achieve high QE and to reduce the the surface-charge
limit problem, while the lower doping density in the remaining 100 nm of the superlattice
is required to reduce depolarization to a minimum. The surface-charge-limit problem was
serious for the machines with warm accelerating structures which have a short bunch spacing
of the order of nanoseconds. The relatively long bunch spacing of 300 ns for the cold ILC
greatly reduces the surface-charge-limit problem and since there is an indication that the high
surface doping density is limiting the peak polarization, the high-gradient-doping profile is
Technical Progress and Final Results
The items below describe progress and results from superlattice parameter studies, the devel-
opment and first results for measuring spin relaxation times using a newly developed Faraday
Rotation apparatus and progress on developing a polarized source laser system with the ILC
GaAs/GaAsP Superlattice: Study of Superlattice Parameters
The best results to date for a polarized source have come from a GaAs/GaAsP superlattice.
In this study the superlattice parameters were systematically varied to optimize the photoe-
mission characteristics. The heavy-hole and light-hole transitions were reproducibly observed
in the quantum efficiency spectra, enabling direct measurement of the band energies and and
energy splitting. Eleven samples obtained from SVT Associates were obtained with varying P
fraction, number of superlattice periods and GaAs well and GaAsP barrier width parameters.
The P fraction determines the lattice mismatch between the well and barrier components of
the superlattice. The superlattice period studies were performed to determine the maximum
thickness of the superlattice before significant strain relaxation. The well and barrier thick-
ness studies resulted in a shifting of the wavelength of the peak polarization. Over a wide
range of parameters the spin polarization remained constant at about 86%, indicating that
the valence heavy-hole light-hole band splitting for a range of well thicknesses was sufficient.
The complete results have been published in Ref. .
The goal in the design of this structure was to study the effect of lower spin-orbit coupling
parameters. Samples were obtained through an SBIR award to SVT Associates. Three wafers
were grown, two with a strained barrier, and one with a strained well. The highest polariza-
tion achieved was only 70% showing no improvement compared to our best photocathodes.
The photocathode performance was found highly dependent on the superlattice parameters
and in particular, the superlattice electron confinement energy appears to be an important
parameter. The results are described in Ref. .
GaAs/GaAsP Superlattice: Surface Layer Doping Concentration
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Prepost, Richard. Development of Polarized Photocathodes for the Linear Collider, report, December 22, 2009; United States. (digital.library.unt.edu/ark:/67531/metadc926629/m1/2/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.