Emission of spin-polarized electrons from strained InGaP and InGaAsP photocathodes

Spectra of photoemission quantum yield and electron spin polarization are measured on the wide-bandgap epitaxial InGaP and InGaAsP layers, which are promising for use as high efficient and stable sources of polarized electrons. Pseudomorphic epitaxial layers with a thickness of 100-600 nm and band gaps of 1.4-1.9 eV are grown on GaAs substrates with various magnitudes of lattice mismatch and, thus, with various in-plane compressive strains. The magnitude of the strain-induced splitting of the valence band is measured by the photocurrent spectroscopy. The surface preparation technique includes removal of oxides in the solution of HCl in isopropanol and transfer to the Mott polarimeter under nitrogen atmosphere, followed by final heat cleaning in UHV. For thin (d = 100-200 nm) highly strained pseudomorphic layers a polarization up to P = 0.66 is obtained. Due to strained-induced splitting of the valence band this value exceeds the theoretical limit P-0 = 0.5 for the unstrained material. Although the magnitudes of electron spin polarization are lower than those reported earlier for strained GaAs layers, high photoemission quantum yield Y in InGaAsP layers provide high values of the combination (PY)-Y-2 that is directly related to the figure of merit of polarized electron beam. Since March, 1995 the InGaP photocathodes are successfully explored in the polarized electron source at Mainz Microtron.