Interband absorption in charged Ge/Si type-II quantum dots

Using electron-filling modulation absorption spectroscopy, we study the effect of quantum dot charging on the interband excitonic transitions in type-II Ge/Si heterostructures containing pyramidal Ge nanocrystals. In contrast to type-I systems, 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 the dominance of the hole-hole interaction compared to the electron-hole interaction due to the spatial separation of the electron and hole. The large oscillator strength (0.5) and the exciton binding energy (25 meV) are determined from the experimental data. 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 inhomogeneous strain distribution in the quantum dot layer has been taken into account through modification of the confining potential. The calculations show that the electron of an indirect exciton resides in the Si near to the Ge pyramid apex due to maximum strain in this region, while the hole is confined close to the pyramid base. The electron-hole overlap is calculated to be 15%. 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.