Observation of Electron Polarization above 80 % in Photoemission from Strained III-V Compounds*

Spin-polarized electron photoemission has been investigated for strained III-V compounds; 1) strained In,Ga I-~As epitaxially grown on a GaAs substrate, and 2) strained GaAs grown on a GaAsl-,P, buffer layer. The lattice mismatched heterostructure results in a highly strained epitaxial layer, and electron spin polarization as high as 90% has been observed. Polarized electron sources have wide applications in many branches of physics!” The use of polarized electron sources with linear electron accelerators is a mature field generally requiring high intensity sources. For example, the SLAC Stanford Linear Collider requires peak currents of about 16A in a 2.5 nsec pulse at 120 Hz. These requirements can be met using photoemission from Negative-ElectronAffinity (NEA) G a A s, and this is the technique adopted for linear electron accelerators. The symmetry of GaAs is such that the maximum polarization is limited to 50% due to the valence band degeneracy of the heavyand light-hole bands at the P point. Much effort has been devoted to achieving higher polarization. The basic approaches involve removing the degeneracy of the valence bands and selectively exciting a single transition for the GaAs type materials or using crystals such as the ternary chalcopyrites which already have the appropriate band structure!’ This paper reports significant enhancement of electron polarization above 50 % from NEA photocathodes. The samples for this experiment were 1) a heterostructure of InGaAs epitaxially grown on a GaAs substrate:’ and 2) a heterostructure of GaAs epitaxially grown on a GaAsP buffer layer. Under these conditions, the epitaxial layer is expected to be highly strained by the lattice mismatch. Strain induces a valence band splitting which permits optical excitation of a single band transition, thus leading potentially to 100% polarization of the photoemitted electrons. The theory relating strain to band structure has been discussed extensively in the literature!’ The band structure of the strained layer is altered such that the heavy-hole and light-hole valance bands are no longer degenerate in energy at the P point, and the energy splitting is then proportional to the strain. A