Optical characterization of Si1-x Gex nanodots grown on Si substrates via ultrathin Si O2 buffer layers

Growth of Si1-x Gex nanodots with x=0, 0.33, 0.67, and 1.0 was accomplished on ultrathin Si O2 buffer layers of 1-2 ML on Si(001) and Si(111) substrates using single-source gaseous precursors at 550 °C. The Si1-x Gex dots have diameters of ∼10 nm and an areal density of ∼ 1011 cm-2. Raman spectroscopy conducted on the nanodots shows that they are relaxed and their compositions correlate closely with the molecular ratios in the precursors used in their fabrication. Photoluminescence (PL) spectra were taken with reduced laser power density which enhanced the PL contribution from the nanodots while suppressing the PL contribution from the Si substrate. Two groups of PL peaks were observed, in the ranges of 0.8-1.0 and 1.0-1.1 eV. The first group in the 0.8-1.0 eV range shows peaks similar to those observed in Si with dislocations. The second group of peaks at the 1.0-1.1 eV range shows an increase in intensity with increasing Ge concentration in the dots. However, both groups of peaks appear to be reproducible in Si substrates after conventional flash cleaning at 1150 °C with no nanodots present. Since there is no evidence that the defect density in Si has increased after flash cleaning, the reason for their appearance is as yet undetermined.