Single phase Si1-xGex nanocrystals and the shifting of the E1 direct energy transition

We report preparation and characterization of Si1-xGex alloys with varied composition x of a large range from 0-1. The materials have been obtained by co-sputtering, followed by a heat treatment process at 600, 800, and 1000 degrees C for 30 min in a nitrogen gas atmosphere. X-ray diffraction data have revealed the formation of single-phase nanoparticles in the face-centered cubic (FCC) structure of Si1-xGex alloys. We found that lattice constant a of the Si1-xGex alloys increased linearly with the composition parameter x. Average diameters of the single-phase nanoparticles were estimated to be between 3-10 nm. Further evidence of FCC single-phase Si1-xGex nanoparticles has been obtained by high resolution transmission electron microscopy. From absorption spectra, the gradual shift of the direct phononless transition identified for the E-1 point in the Brillouin zone of bulk Ge is observed in single-phase Si1-xGex nanoparticles as a function of the composition parameter x.