  We report the first experimental demonstration that interface microstructure
limits diffusive electrical spin injection efficiency across heteroepitaxial
interfaces. A theoretical treatment shows that the suppression of spin
injection due to interface defects follows directly from the contribution of
the defect potential to the spin-orbit interaction, resulting in enhanced
spin-flip scattering. An inverse correlation between spin-polarized electron
injection efficiency and interface defect density is demonstrated for
ZnMnSe/AlGaAs-GaAs spin-LEDs with spin injection efficiencies of 0 to 85%.
