Observation of strain-enhanced electron-spin polarization in photoemission from InGaAs.

Electron spin polarization in excess of 70% has been observed in photoemission from a 0.1 pm-thick epitaxial layer of In,Gar-,As with ~~0.13 grown on a GaAs substrate. Under these conditions, the epitaxial layer is expected to be highly strained by the 0.9% lattice mismatch. The electron polarization and the quantum efficiency have been measured as a function of the excitation photon energy from 1.25 to 2.0 eV. A significant enhancement of the electron polarization occurs in the vicinity of 1.33 eV where the expected strain induced level splitting permits optical excitation of a single band transition. PACS numbers: 29.25.Bx, 29.75.+x, 73.60.Br, 79.60.Eq 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 I’ 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.2 The structures which have been investigated for GaAs include GaAs-AlGaAs superlattices and strained GaAs produced by epitaxial growth of thin GaAs layers on a Si substrate. No significant enhancement of emitted electron spin polarization has yet been achieved.3 This letter reports the first significant enhancement of electron spin polarization above 50% from a NEA photocathode. The sample for this experiment was a single heterojunction of InGaAs epitaxially grown on a GaAs substrate under conditions to create pseudomorphic strain by lattice mismatch. Strain induced changes of band structure in crystals have been extensively studied theoretically as well as experimentally.4 A suitably thin epitaxial layer of InGaAs grown on a GaAs substrate incorporates a biaxial compressive strain in the plane of the interface and a tensile strain along the growth direction. Full strain is realized when