We report an experimental observation of both a fast ͑ϳ10 s͒ and a slow ͑ϳ10 ms͒ photoluminescence ͑PL͒ that coexist in Ge-rich ͑x Ͼ 0.5͒ islandlike, three-dimensional Si/ Si 1−x Ge x nanostructures. We present a quantitative model that explains the observed PL lifetime dependence on carrier concentration, temperature, and detection wavelength. The PL dynamics are found to be determined by the excess carrier concentration: the fast PL is associated with a dynamic type I and the slow PL with a type II energy band alignment in Ge-rich Si/ SiGe nanostructures.

We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.

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Using polarized Raman spectroscopy, we examine different vibrational modes (i.e., Si–Si, Si–Ge, and Ge–Ge) in Si/Ge self-organized nanostructures. Here, we present unambiguous proof that multilayers of Ge nanometer-size, “dome-shaped” islands grown on a 〈100〉 Si substrate are nearly fully relaxed and that the built-in strain field is substantially localized in the surrounding Si matrix. In contrast, multilayers with “pyramid-shaped” islands do not show observable relaxation. The large strain in the Si layers of the multilayer dome samples correlates with the greater self-organization in these structures compared to the multilayer pyramid samples.

In Ge∕Si Stranski-Krastanov nanostructures grown by chemical vapor deposition, the authors find ∼30meV/decade photoluminescence (PL) spectral shift toward greater photon energies as excitation intensity increases from 0.1to104W∕cm2. The PL lifetime exhibits strong spectral dependence, and it decreases from ∼20μs at 0.77eVto200ns at 0.89eV. The authros attribute the observed PL spectral shift and shorter PL lifetime at higher photon energies to an increasing contribution from recombination between holes populating excited Ge cluster energy states and electrons in SiGe alloy cluster regions.