Optical characterization of Si1−xGex nanodots grown on Si substrates via ultrathin SiO2 buffer layers

Abstract

Growth of Si1−xGex nanodots with x=0, 0.33, 0.67, and 1.0 was accomplished on ultrathin SiO2 buffer layers of 1–2 ML on Si(001) and Si(111) substrates using single-source gaseous precursors at 550°C. The Si1−xGex dots have diameters of ∼10nm and an areal density of ∼1011cm−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 1.0–1.1eV. The first group in the 0.8–1.0eV range shows peaks similar to those observed in Si with dislocations. The second group of peaks at the 1.0–1.1eV 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.