Sahu et al. Nanoscale Research Letters 2011, 6:178 http://www.nanoscalereslett.com/content/6/1/178 Page 5 of 10 Figure 3 FTIR transmission spectra of (a) as-deposited Si3N4/SRSN/SiO2 films, (b) sample S4 containing 33 at.% of silicon excess and annealed within the temperature range of 500-1100°C for 30 min in N2 ambient, (c) evolution of Si-N stretching band position of samples S1, S2, S3, and S4 as a function of annealing temperature. of pseudo-direct bandgap behavior. However, no sin-np/ cluster has been detected in our EFTEM analysis for these as-deposited SRN films. Thus, the role of QCE can be ruled out. The behavior of PL is in very good agreement with the findings by other authors for a-SiNx: H [30-32] and a-SiCx:H [33,34], where it has been attributed to a model based on band-tail states. In this model, the carriers radiatively recombine with the loca- lized states at the band tails of the gap. With increasing nitrogen content, the bandgap energy increases, which results in a blue shift of the PL energy. However, for band-tail mechanism, the emission generally occurs at energies lower than 1.82 eV [11]. In this study, the peak position of PL spectra is higher than this value, espe- cially for the samples with lower excess of Si. Further- more, based on Robertson’s calculated results [35,36], Ko et al. [37] have proposed possible mechanisms related to defect states to explain PL in silicon nitride films. Being amorphous, the samples have varying opti- cal band gaps depending on composition. Therefore, the energy levels are not well defined. In addition, there is a distribution of states for a given defect giving rise to broad features in the PL spectra. Some investigations have shown both theoretically and experimentally that the PL band with peak positions of 1.8-3.2 eV [38-40] is closely related to the defect states within the bandgap of amorphous SiNx materials. Indeed, large amount of defect-related gap states generally exists in non-stoichio- metric silicon nitride layers obtained by CVD process. These defect states having different energy levels contri- bute to the radiative emission by creating different chan- nels for the relaxation of electronic states. It can be noticed that a weak peak appears around 3.1 eV, whose relative intensity decreases with increasing nitrogen con- tent in the films. Wang et al. have attributed this peak to the presence of nitrogen-dangling bonds [41], whereas others have attributed it to the presence of ≡ Si0 defect sites, which give gap states at 3.1 eV, 80% localized on sites [36]. In this case, as the band becomes prominent at higher Si content, it can be safely assigned Figure 4 Roomtemperature PL spectra of as-deposited Si3N4/SRSN/SiO2 films. (a) (Color online) Room temperature PL spectra of as- deposited Si3N4/SRSN/SiO2 films having an excess of silicon from 22 to 33 at.% (samples S1, S2, S3, and S4) in the middle SRSN layer, (b) PL energies and intensities of the as-deposited films as a function of silicon excess in the SRSN layer.