Evidence of Multiple No-Phonon Photoluminescence Peaks in Si_1-xGe_x/Si Multiple Quantum Wells

We observe band edge photoluminescence from Si_1-xGe_x/Si multiple quantum wells (MQW's) grown at a minimum growth rate (R_Ge =3D 0.01 Ås and R_Si =3D 0.04 Ås for 600 s) and a substrate temperature of 710^oC. The samples consist of five quantum wells with 30 Åwell widths and 300 Åbarrier widths with 20% Ge in the alloy. We observe intense quantum confined photoluminescence in the Si_1-xGe_x/Si MQW samples with the addition of a new peak approximately 11 meV below the no-phonon line. This new peak has a TO phonon replica associated with it about 11 meV below the TO phonon replica of the no-phonon line. Temperature dependent data shows that the new peak behaves in a manner consistent with the no-phonon line. A posible mechanism for this new peak is islanding of the Ge during the growth of the alloy. A calculation of the confinement of these wells indicates that the shift in the peak of 11 meV corresponds to an island approximately 2 monolayers thick.