Co2FeSi/GaAs/(Al,Ga)As spin light-emitting diodes:: Competition between spin injection and ultrafast spin alignment

Electrical injection from the Heusler alloy Co(2)FeSi into (Al,Ga)As is investigated for different growth temperatures T(G) of the injector layer. Depending on T(G), the spin polarization of injected electrons in the semiconductor is determined by two competing mechanisms: actual spin injection at the Co(2)FeSi/(Al,Ga)As interface and ultrafast spin alignment in the (Al,Ga)As layer. This layer is strongly affected by the thermally activated diffusion of Co, Fe, and Si during the growth of the Co(2)FeSi layers. Despite the electrical compensation and magnetic transformation in the underlying semiconductor structure, a spin-injection efficiency of at least 50% is achieved as deduced from the analysis of electroluminescence and time-resolved photoluminescence data.