Spin-polarized electrons from the surfaces of GaAsP strained films

When a lattice-mismatched layer is grown on a substrate, the misfit between the layers at the interface is accommodated in the epitaxial layer and lifts the orbital degeneracy of the levels at the valence band maximum. Hence, the polarization of optically oriented electrons may be enhanced compared to unstrained cubic material. We report the first results of high spin polarization in electron photoemission from Semiconductor heterostructures with strained GaAs/sub 1-x/P/sub x/ (x=0.05;0.1 and 0.3) overlayers on GaAs/sub 1-y/P/sub y/ buffer (y=0.32;0.35 and 0.65 respectively). The polarization of photoelectrons emitted from negative electron affinity surface of the samples was measured at residual pressure 3E-11 Torr with LEED detector. Statistical error were 1% at quantum yield smaller than 1E-3 and 0.5% at quantum yield bigger than 1E-2. The light beam energy width was 15nm. Both the quantum yield and polarization were measured in the light energy range 1.3-2.7 eV. Negative electron affinity was achieved by activation of atomically clean surfaces with cesium and oxygen. All samples were grown in the horizontal MOCVD reactor. The properties of the layers were checked by photoluminescence and X-ray diffraction measurements. At room temperature we measured the polarization up to P=84% (x=0.05, overlayer thickness d=140 nm), P=83% (x=0.3, d=140 nm), P=72% (x=0.1, d=70 nm). It shows that high residual strain exists in the layers even when their thickness is about ten times more than the critical one. We have shown that by varying the phosphorus fraction in the overlayer the highest polarization value can be tuned to AlGaAs laser diode wavelength.