Fast and Slow Light-Emitting Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures (NSs) produce photoluminescence (PL) and electroluminescence in the desired spectral range of 1.3-1.6 mu m. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast (PL lifetime <20 ns) and hence more efficient SiGe light-emitting devices. The results presented here demonstrate that in such Si/SiGe 3D NSs with a nominal Ge concentration approaching similar to 35% the PL peaked near 0.78 eV strongly depends on the Si/SiGe heterointerface abruptness. In other Si/SiGe NS/quantum-well samples with a Ge concentration approaching similar to 40%, we find two PL bands peaked at similar to 0.8 eV and similar to 0.9 eV at low temperatures. The PL peaked at 0.8 eV rises and decays slowly, and it quickly saturates as the excitation intensity increases. In contrast, the PL peaked at 0.9 eV shows a much shorter lifetime and exhibits a linear dependence versus excitation intensity. The slow/delayed PL at 0.8 eV is attributed to carrier recombination at the SiGe NS/Si transition layer while the faster and more efficient PL at 0.9 eV is associated with SiGe quantum wells. More complicated and similarly fast (similar to 10(-7) s) decays are observed at very high excitation intensities due to electron-hole droplet formation. The physics of carrier recombination in these Si/SiGe NSs is discussed.