Spatially Indirect Excitons in Self-Assembled Ge/Si Quantum Dots.

摘要

Using eletron 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 blue shifted 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 and electron-electron interactions conmpared to the electron-hole interaction due to the spatial separation of the electron and hole. When two excitons are excited in the dot the electrons are found to be spatially separated and have different single-particle quantization energies. This is the reason why the biexciton absorption is blue shifted as compared to a single exciton.