  Spin-polarized photoemission from wurtzite and zinc-blende gallium nitride
(GaN) photocathodes has been observed and measured for the first time. The
p-doped GaN photocathodes were epitaxially grown and activated to negative
electron affinity (NEA) with a cesium monolayer deposited on their surfaces. A
field-retarding Mott polarimeter was used to measure the spin-polarization of
electrons photoemitted from the top of the valence band. A spectral scan with a
tunable optical parametric amplifier (OPA) constructed to provide low-bandwidth
light revealed peak spin polarizations of 17% and 29% in the wurtzite and
zinc-blende photocathodes, respectively. Zinc-blende GaN results are analyzed
with a spin-polarization model accounting for experimental parameters used in
the measurements, while possible mechanisms influencing the obtained spin
polarization values of wurtzite GaN are discussed.
