The response time and spin relaxation time of thin unstrained and strained III/V-semiconductor photocathodes installed in sources of polarized electrons have been investigated. Cathodes of various active layer thicknesses have been studied. An upper limit for the response time of a 150 nm thick strained layer photocathode has been found to be 2.5 ps. As a consequence, the average depolarization during transport in the conduction band to the surface is estimated to be lower than 3% and does not contribute substantially to the upper limit of about 80% for the spin polarization of the emitted electrons. The results indicate a high surface recombination velocity of SϾ1.2 ϫ10 7 cm/s at the surface band bending region.
Photocathode for highly polarized electron emission has been developed, fabricated, and studied. The photocathode is based on short-period strained AlInGaAs∕AlGaAs superlattice grown by molecular beam epitaxy. Deformation of AlInGaAs quantum well results in 87meV energy splitting between heavy hole and light hole minibands. Electron emission from the developed photocathode demonstrates maximal polarization of 92% with quantum efficiency of 0.85% at room temperature.
Spin-polarized electron emission from the first superlattice photocathodes developed with strain compensation is investigated. An opposite strain in the quantum well and barrier layers is accomplished using an InAlGaAs/GaAsP superlattice structure. The measured values of maximum polarization and quantum yield for the structure with a 0.18 µm-thick working layer are close to the best results reported for any strained superlattice photocathode structure, demonstrating the high potential of strain compensation for future photocathode applications. An analysis of the photoemission spectra is used to estimate the parameters responsible for the polarization losses.
A new resonance phenomenon has been observed in the spin polarization of optically oriented photoelectrons emitted from strained semiconductor photocathodes.The strained photocathode is part of an all-integrated Fabry-Perot optical cavity. The resonance of the electron spin polarization have the same period on the wavelength scale as the oscillations of the optical reflectivity. They find a natural explanation in terms of a small anisotropy of the in-plane lattice strain which has been verified directly by high-resolution x-ray rocking curve diffraction.
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