Inherent paramagnetic defects in layered nanocrystalline Si/SiO2 superstructures

With the view to fully disclose the nature of occurring paramagnetic defects, a detailed electron spin resonance (ESR) study has been carried out on entities comprised of regular arrays of Si nanoparticles (np's) (size ∼2 nm) embedded in an SiO2 matrix, obtained by the SiO/SiO2 superlattice method. The approach encompasses high-sensitivity first- and second-harmonic low-temperature X, K and Q-band ESR in combination with computer simulations. This enabled disentanglement of the common Si dangling-bond (DB) signal, observed in the as grown state as being composed solely of Si/SiO2 interface-specific powder patterns of Pb(0) and Pb1 defects, indicating that the majority, if not all, of the np's are crystalline. The inferred densities are in the range of standard values obtained for thermal SiO2 grown on Si and remain unchanged over different (V)UV irradiation treatments. Yet, upon (V)UV irradiation, SiO2-specific defects (Eγ′ and EX) were activated, in numbers demonstrating standard SiO2 quality. Only ∼71% of the Si nanocrystals (nc's) house a Pb-type center, indicating the structure to be comprised of two subsystems, which may hence reflect in different defect-sensitive properties, such as, e.g., photoluminescence. Relying on the known properties of Pb(0) and Pb1 defects in standard microscopic Si/SiO2, the data would comply with Si nc's, in average, predominantly bordered by (1 1 1) and (1 0 0) facets.