Contribution of the electron-electron interaction to the optical properties of dense arrays of Ge/Si quantum dots

Abstract

We present the results of an investigation of the light absorption due to interband and interlevel transitions and the photoconductivity in dense arrays of Ge quantum dots (QDs) in Si formed using the effect of self-organization during molecular-beam heteroepitaxy. It was found that the formation of charged exciton complexes composed of two holes and one electron, as well as of the be-exciton complexes in QDs of type II, leads to an increase in the energy of indirect (in real space) exciton transition, which is explained by the spatial separation of electron and hole. Self-consistent calculations of the wavefunctions for electrons and holes in exciton and in the exciton complexes showed that an electron in a single exciton is localized in the region of maximum stress for Si in the vicinity of the Ge pyramid apex, while a hole is localized near the pyramid base. In a be-exciton complex, electrons exhibit repulsion leading to their spatial separation. As a result, the second electron is bound at the boundary between Si and a continuous Ge layer in which the pyramid bases reside. The experimental data show that an increase in the charge carrier concentration in the ground state of QDs leads to a shortwave shift of the interband resonance and to the narrowing and shape change of the light absorption band, which is explained by depolarization of the external electromagnetic wave due to interaction with the collective charge density oscillations in the lateral direction of the array of Ge nanoclusters. It is established that the hole injection into an excited state of QDs leads to a longwave shift of the photoconductivity peak as a result of decay of the collective excitations and suppression of the depolarization effect.