The optical second-harmonic generation (SHG) spectra of silicon nanocrystals (Si NCs) prepared by implanting Si ions uniformly into silica substrates, then annealing, are compared and contrasted to their ellipsometric and photoluminescence excitation (PLE) spectra. Three resonances—two close in energy to $E$${}_1$ ($3.4$ eV) and $E$${}_2$ ($4.27$ eV) critical-point resonances of crystalline silicon (c-Si), and a broad resonance intermediate in energy between $E$${}_1$ and $E$${}_2$—are observed in all three types of spectra. These features are observed in conjunction with a sharp 520 cm${}^{-1}$ Raman peak characteristic of c-Si. While the ellipsometric and PLE spectra differ only slightly between samples with average NC diameter $〈d_{\text{NC}}〉=3$ and 5 nm, the SHG spectrum changes dramatically from a nearly featureless spectrum dominated by the non-bulk-like intermediate resonance for $〈d_{\text{NC}}〉=3$ nm, like the SHG spectrum of amorphous Si (a-Si), to a featured spectrum with pronounced resonances at $3.4$, $3.73$, and $4.8$ eV for $〈d_{\text{NC}}〉=5$ nm. The results suggest that SHG is uniquely sensitive to a nanointerfacial transition region containing a-Si and suboxide that is most prominent for small $〈d_{\text{NC}}〉$, while ellipsometric and PLE spectra are more sensitive to the c-Si core of the NCs. The persistence of an a-Si tail in the Raman spectrum of annealed samples supports the interpretation.

• Received 17 February 2011

DOI:https://doi.org/10.1103/PhysRevB.84.165316