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

We present the results of an investigation of the light absorption due to i nterband and interlevel transitions and the photoconductivity in dense arra ys of Ge quantum dots (QDs) in Si formed using the effect of self-organizat ion 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 t he energy of indirect (in real space) exciton transition, which is explaine d by the spatial separation of electron and hole. Self-consistent calculati ons of the wavefunctions for electrons and holes in exciton and in the exci ton complexes showed that an electron in a single exciton is localized in t he 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, e lectrons 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 depol arization of the external electromagnetic wave due to interaction with the collective charge density oscillations in the lateral direction of the arra y of Ge nanoclusters. It is established that the hole injection into an exc ited state of QDs leads to a longwave shift of the photoconductivity peak a s a result of decay of the collective excitations and suppression of the de polarization effect. (C) 2001 MAIK "Nauka/Interperiodica".