Characterization of Si+ ion-implanted SiO2 films and silica glasses

We report here electron spin resonance (ESR), Raman scattering, photoluminescence (PL), and absorption studies of ${\mathrm{Si}}^{\mathrm{+}}$ ions implanted into silica glasses and thermally grown ${\mathrm{SiO}}_{\mathrm{2}}$ films on Si wafers. The aim is to understand the defect formation and luminescence mechanism as the films were annealed at several temperatures. In as-grown Si-implanted films, paramagnetic defects in the form of $\mathrm{Si}{\mathrm{–E}}^{\prime }$ and nonbridging oxygen hole centers (NBOHC) were detected by ESR. A luminescence band, centered at 650 nm, was observed in these samples. The $E^{\prime }$ and NBOHC defects were annealed out in samples annealed above $\text{500\hspace{0.17em}°}\mathrm{C},$ but Si dangling bonds in the form of $P_b$ centers were observed. In films annealed above $\text{800\hspace{0.17em}°}\mathrm{C},$ Si nanocrystals of varying size between 1 and 5 nm were formed, as detected by transmission electron microscopy. A red PL band (>700 nm) and a Si-phonon band at 510 ${\mathrm{cm}}^{\text{−1}},$ gradually evolved as a function of anneal temperature. The observation of the Si-phonon mode with a 15 ${\mathrm{cm}}^{\text{−1}}$ shift in the peak position from the bulk affirms the presence of Si clusters in the annealed ion-implanted ${\mathrm{SiO}}_{\mathrm{2}}$ films and silica glasses. We attribute this red PL band to excitonic recombination in Si nanocrystals. The peak position of this PL band was found to shift with excitation wavelength. The shift of the PL spectra as a function of excitation energy was interpreted as the emission from Si particles of certain sizes that are excited by the incident energy. An energy band gap distribution due to a size distribution was used to model the PL spectra at each excitation energy.