In this paper, we demonstrate a very efficient electrical spin injection into an ensemble of InAs/InGaAs quantum dots at zero magnetic field. The circular polarization of the electroluminescence coming from the dots, which are embedded into a GaAs-based Spin Light Emitting diode reaches a value as large as 20% at low temperature. In this device, no external magnetic field is required in order to inject or read spin polarization thanks to the use of an ultrathin CoFeB electrode (1.1 nm), as well as p-doped quantum dots (with one hole per dot in average) as an optical probe. The electroluminescence circular polarization of the dots follows the hysteresis loop of the magnetic layer and decreases as a function of bias for large voltages. In a reverse way, we have also investigated the possibility to use such a device as a photodetector presenting a photon helicity-dependent photocurrent. We observe a weak asymmetry of photocurrent under right and left polarized light that follows the hysteresis cycle of the magnetic layer, and the effect decreases for increasing temperatures and can be controlled by the bias.
In p+ GaAs thin films, under excitation by a tightly-focused laser, the spatial profile of the spin polarization is monitored as a function of excitation power. It is found that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle which causes a concentration dependence of the spin stiffness. Thermoelectric currents are also predicted to depend on spin under degeneracy (spin Soret currents), but these currents play a relatively small role in this case. The spin dependence of the mobility is also found weak. Conversely, ambipolar coupling with holes increases the steady-state photo-electron density at the place of excitation and therefore the amplitude of the degeneracy-induced polarization decrease at the place of excitation.
Degeneracy of a photoelectron gas is shown to strongly affect spin polarized electron transport since the Pauli
principle dictates a concentration dependence of the spin stiffness and of the mobility. This causes a spin
dependence of the diffusion constant D. A spin-dependence of D as large as 50 % is measured using polarized
microluminescence imaging in p+ GaAs thin films, revealing a novel spin filter effect. The charge diffusion
constant also depends on spin via a second order effect.