  The emission of circularly polarized light from a single quantum dot relies
on the injection of carriers with well-defined spin polarization. Here we
demonstrate single dot electroluminescence (EL) with a circular polarization
degree up to 35% at zero applied magnetic field. The injection of spin
polarized electrons is achieved by combining ultrathin CoFeB electrodes on top
of a spin-LED device with p-type InGaAs quantum dots in the active region. We
measure an Overhauser shift of several $\mu$eV at zero magnetic field for the
positively charged exciton (trion X$^+$) EL emission, which changes sign as we
reverse the injected electron spin orientation. This is a signature of dynamic
polarization of the nuclear spins in the quantum dot induced by the hyperfine
interaction with the electrically injected electron spin. This study paves the
way for electrical control of nuclear spin polarization in a single quantum dot
without any external magnetic field.
