Spin-polarized electroluminescence and spin-dependent photocurrent in hybrid semiconductor/ferromagnetic heterostructures: An asymmetric problem

We have measured in the same hybrid semiconductor/ferromagnetic (FM) metal structures the photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under forward electric bias (spin light-emitting diode operation). The systematic investigations have been performed on devices with different ferromagnetic spin injectors: tunnel barrier of Al2O3 surmounted by a thin Co ferromagnetic layer or MgO tunnel barriers with a CoFeB FM layer. The semiconductor part of the device is composed of an AlGaAs diode with a GaAs/AlGaAs quantum well embedded in the intrinsic region. Though a very efficient electrical spin injection is demonstrated with a measured circular polarization of the electroluminescence up to 30% for an external field of 0.8 T, very weak polarizations of the photocurrent are evidenced whatever the nature of the device is. The maximum photocurrent polarization obtained under continuous resonant circularly polarized excitation of the quantum well excitons is about 3%. This demonstrates that the investigated devices do not act as an efficient spin filter for the electrons flowing from the semiconductor part toward the ferromagnetic part of these structures though these layers are very efficient spin aligners for electrical spin injection. We interpret the weak measured polarization of the photocurrent in the percent range as a consequence of the Zeeman splitting of the quantum well excitons which yields different absorption coefficients for the polarized excitation laser with different helicities. This leads to different intensities of photocurrent collected for the two different circularly polarized excitations. This interpretation is confirmed by an experiment exhibiting the same results for photocurrent measured on a device with a nonferromagnetic electrode.