Depth distribution of luminescent Si nanocrystals in Si implanted SiO2 films on Si

Depth-resolved measurements of the photoluminescence of Si implanted and annealed SiO₂ films on Si have been performed to determine the depth distribution of luminescent Si nanocrystals. Si nanocrystals with diameters ranging from ~ 2 to 5 nm were formed by implantation of 35 keV Si ions into a 110-nm-thick thermally grown SiO2 film on Si(100) at a fluence of 6 × 10¹⁶ Si/cm², followed by a thermal anneal at 1100 °C for 10 min. The photoluminescence spectrum is broad, peaks at l = 790 nm, and contains contributions from both recombination of quantum confined excitons in the nanocrystals and ion-implantation-induced defects. By chemical etching through the SiO₂ film in steps and analyzing the changes in the photoluminescence spectrum after each etch step, the depth from which each of the two luminescence features originate is determined. The etch rate of the oxide, as derived from Rutherford backscattering spectrometry data, varies from 1.3 nm/s in the regions of small excess Si to 0.6 nm/s at the peak of the concentration profile (15 at. % excess Si). It is found that the defect luminescence originates from an ~ 15-nm-thick near-surface region. Large nanocrystals luminescing at long wavelengths (l = 900 nm) are mainly located in the center of the film, where the Si concentration is highest (48 at. %). This is corroborated by transmission electron microscopy that shows a high density of Si nanocrystals in the size range of 2–5 nm in the center of the film. The largest density of small luminescent nanocrystals (l = 700 nm), not detectable by electron microscopy is found near the SiO₂ surface and the SiO₂/Si interface. This is attributed to either the fact that the surface and the SiO₂/Si interface affect the Si nanocrystal nucleation kinetics in such a way that small nanocrystals are preferentially formed in these regions, or an optical interaction between nanocrystals of different sizes that quenches the luminescence of small nanocrystals in the center of the film.