Spin polarized electroluminescence and spin photocurrent in hybrid semiconductor/ferromagnetic heterostructures: an asymmetric problem

The photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under forward electric bias have been measured in the same hybrid Semiconductor/Ferromagnetic metal structures (Spin-Light Emitting Diode). Systematic investigations have been performed on devices with different ferromagnetic spin injectors, consisting e.g. of MgO tunnel barriers with a CoFeB ferromagnetic layer. Though a very efficient electrical spin injection is demonstrated, very weak polarization of the photocurrent is evidenced: the photocurrent polarization measured under continuous resonant circularly polarized excitation of the quantum well excitons is below 3%. This demonstrates that the investigated devices do not act as efficient spin filters for the electrons flowing from the semiconductor part to the ferromagnetic part of these structures though these systems are very efficient spin aligners for electrical spin injection. We interpret the weak measured photocurrent polarization of the 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 non ferromagnetic electrical contact.