Nitrogen at the Si-nanocrystal / SiO2 interface and its influence on luminescence and interface defects

The influence of the high temperature annealing ambient (N2 or Ar) on size controlled Si nanocrystals (NCs) in SiO2 ranging from ~2 to ~6 nm has been investigated in detail. Generally, N2 annealing is beneficial as the dangling bond density (Pb-defects at the NC/SiO2 interface) is about half accompanied by a doubled PL intensity. The N-related PL blueshift was found to be pronounced only for the small NCs whereas it appears to be insignificant for larger NCs. The origin of this N-blueshift was previously attributed to NC growth suppression by the presence of N. However, no evidence for this assumption is found by time-resolved PL, as the luminescence decay times are similar despite considerable N-blueshift. The exact location of the N incorporated during annealing was investigated by ToF-SIMS and ESR: Besides the distinct N-enrichment in the NC-layer, the K0-center (•Si≡N3) was detected indicating the formation of an interfacial N layer at the NC/SiO2 interface. ERD analysis enabled the quantification of the incorporated N as well as the excess Si. Combined with TEM analysis (determination of NC size) the calculation of the NC-density per superlattice layer and the thickness of the interfacial N-layer were achieved. It turns out that ~ 5×1014 N-atoms cm-2 exist at the NC, which is well in accordance to the optimum value of the bulk Si/SiO2 interface. These results support our recently suggested explanation for the N-blueshift that is based on the influence of the polarity of the surface terminating groups on the bandgap of the NC.