Many-particle effects in excitonic transitions in type-II Ge/Si quantum dots

The effect of quantum dot charging on the interband excitonic transitions has been studied in type-II Ge/Si heterostructures containing pyramidal Ge nanocrystals using electron-filling modulation absorption spectroscopy. The ground state absorption is found to be blueshifted when exciton–hole and exciton–exciton complexes are formed. For a positively charged dot, we argue that this is the consequence of dominance of the hole–hole interaction compared to the electron–hole interaction due to the spatial separation of the electron and hole. The results are explained by effects of the electron and hole localization and by electron wave function leakage in the dots. The electronic structure of spatially indirect excitons is calculated self-consistently in the effective-mass approximation for pyramidal-shaped Ge/Si quantum dots. The calculations show that the electron of an indirect exciton resides in the Si near the Ge pyramid apex due to maximum strain in this region, while the hole is confined close to the pyramid base. When two excitons are excited in the dot, the electrons are found to be spatially separated and have different single-particle quantization energies. We argue that this is the reason why the biexciton absorption is blueshifted as compared to a single exciton. A satisfying agreement is found between theoretical and experimental data.