Optical properties of germanium monolayers on silicon

Photoluminescence and Raman spectra of thin germanium layers grown on silicon at a low temperature (250°C) have been studied. In structures of this kind, in contrast to those grown at high temperatures, luminescence from quantum wells is observed at germanium layer thicknesses exceeding ～9 monolayers (ML). With the development of misfit dislocations, the luminescence lines of quantum wells are shifted to higher energies and transverse optical (TO) phonons involved in the luminescence are confined to a quasi-2D germanium layer. Introduction of an additional relaxed Si_0.95Ge_0.05 layer into the multilayer Ge/Si structure leads to a substantial rise in the intensity and narrowing of the luminescence line associated with quantum dots (to 24 meV), which points to their significant ordering.