Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.