We investigate the fidelity of electron spin initialization in quantum dots utilizing nanosecond-pulsed electrical spin injection through a semimagnetic spin aligner in a spin light-emitting diode. At the onset of the electroluminescence signal, the circular polarization degree of the emitted light, corresponding to the spin polarization degree, is distinctively higher than under constant-current excitation. The observed spin-polarization dynamics are attributed to state filling effects. Additional contributions due to spin-flip mechanisms within the optically active region are identified.
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