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Appl. Phys. Lett., Vol. 85, No. 13, 27 September 2004
Maruyama et al.
fer matrix method.15 While the changes in the band structure
differentially affected the polarization spectra for energies
above the polarization peak, the maximum polarization re-
mained constant at about 86%, indicating that the valence-
band splitting for this range of well thicknesses was suffi-
cient.
In conclusion, we have investigated polarized photo-
emission from strained GaAs/GaAsP superlattice structures
by systematically varying the superlattice parameters. The
heavy- and light-hole excitations have been observed for the
first time in the QE spectra, enabling direct measurements of
the heavy- and light-hole energy bands. Spin polarization as
high as 86% is reproducibly observed with the QE exceeding
1%. The superlattice structures presented here have superior
polarization and QE compared to the single strained-layer
structures of GaAs/GaAsP photocathodes.
FIG. 3. Peak polarization (solid circles), QE anisotropy (open circles), and
strain relaxation (triangles) as a function of the superlattice period.
This work was supported by Department of Energy
Small Business Innovation Research Grant No. DE-FG02-
01ER8332 and Contract Nos. DE-AC03-76SF00515
(SLAC), and DE-AC02-76ER00881 (UW).
pear constant at 85.5%, and 1.5%, respectively, for less than
15 periods. At more than 20 periods, however, the peak po-
larization decreases and the QE anisotropy increases rapidly.
The QE and polarization spectra can be understood in
terms of the superlattice band structure. In particular, the
band structure is very sensitive to the well width. Using
samples with the same barrier thickness ͑3 nm͒ and phos-
phorus fraction ͑x=0.36͒, the well thickness was varied
while the number of periods was adjusted to keep the same
total superlattice thickness. Figure 4 shows the spin polariza-
tion as a function of the excitation photon energy for
Samples 9, 10, and 11. As the well thickness was increased,
the polarization spectra shifted towards lower energy. The
peak polarization was, however, independent of the well
thickness. The HH energy relative to the conduction band
was measured to be 1.61, 1.57, and 1.55 eV for Samples 9,
10, and 11, respectively, which compares favorably with en-
ergies of 1.64, 1.60, and 1.57 eV calculated using the trans-
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128.114.34.22 On: Wed, 03 Dec 2014 07:55:18