Spatially indirect excitons in self-assembled Ge/Si quantum dots

We study the effect of the interactions of electrons and holes on the optical properties of Ge/Si type-II quantum dots (QDs). In contrast to type-I systems, the excitonic 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 with 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 quantization energies. This is the reason why the biexciton absorption is blueshifted as compared with a single exciton. The spatial separation of electrons makes it possible for a dot to trap more electrons than there are holes. As a result, the conductivity of stacked arrays of Ge/n-Si QDs is found to decrease under interband optical excitation. The negative photoeffect is explained by the trapping of mobile electrons in the quantum well created by the Hartree potential of holes photoexcited in the dots.