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, w hich are promising for use as high efficient and stable sources of pol arized electrons. Pseudomorphic epitaxial layers with a thickness of 1 00-600 nm and band gaps of 1.4-1.9 eV are grown on GaAs substrates wit h various magnitudes of lattice mismatch and, thus, with various in-pl ane 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 solut ion of HCl in isopropanol and transfer to the Mott polarimeter under n itrogen 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 val ence band this value exceeds the theoretical limit P-0 = 0.5 for the u nstrained material. Although the magnitudes of electron spin polarizat ion are lower than those reported earlier for strained GaAs layers, hi gh 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 photoca thodes are successfully explored in the polarized electron source at M ainz Microtron.