Spin filter and highly polarized electron sources

A subtle problem of spin-polarized electron emission spectroscopy is the link between the polarization of the emitted electrons and the ground state polarization within the solid. The first part of this dissertation presents experimental evidence that the excited electrons alter their polarization during their way to the surface, i.e. the excited electrons undergo a strong spin-dependent scattering in ferromagnetic materials. This could be measured on Co(0001), Co/Cu(001) and on a new semiconductor/ferromagnet system, namely on GaAs/Ag/Fe. These results are compared with a microscopic model. They allow a better understanding of any spin-polarized electron emission spectra. The second part of this thesis deals with highly polarized electron sources. Using optical spin orientation, photoelectrons with spin polarization above 50% have been extracted from strained III-V semiconductors. These highly polarized electron sources consist of an epitaxial overlayer having a smaller energy gap and a slightly larger lattice constant than the substate material. The tensile stress along the surface normal lifts the orbital degeneracy of the valence band. This lifting allows the polarization to be enhanced compared to the unstrained cubic material. Since the critical thickness of the strained layers does not exceed a few 100 A, the highest polarization could only be measured in the thinnest layers. The photoyield of these thin layers is unfortunately very small. In order to obtain high polarization together with high yield, a new photocathode with an integrated Fabri-Perot optical cavity was designed. A new resonance phenomenon has been observed which finds a natural explanation in terms of a small anisotropy of the in-plane lattice strain. Both SEM pictures of the cleaved edges and high resolution X-ray diffraction show a partly non tetragonal distortion of the unit cell in the photocathode. This general behavior makes the creation of highly polarized electron samples with high polarization and high yield, and other new electrical devices based on strayned layers extremely delicate. (author) figs., tabs., refs