Sahu et al. Nanoscale Research Letters 2011, 6:178 http://www.nanoscalereslett.com/content/6/1/178 Page 3 of 10 Figure 1 Atomic fraction of Si, N, and H versus gas flow ratio in SRSN films deposited on Si substrates, as measured by RBS and ERDA techniques. are clearly observed. No lattice fringes have been detected in the HREM analysis, suggesting amorphous nature of these nanoclusters. The inset to Figure 2 indi- cates that the size distribution of Si-nps is centered at 3-nm diameter with a standard deviation of 0.6 nm. However, for other samples with less Si excess, no Si-np was detected in the EFTEM/HREM analysis. It is specu- lated that even if silicon precipitation has occurred dur- ing annealing, the Si cluster size might be below the detection limit of EFTEM (1.5 nm). Infrared spectroscopy FTIR spectra of as-deposited Si3N4/SRSN/SiO2 struc- tures are shown in Figure 3a. These spectra exhibit the characteristic features of Si-N asymmetric stretching (820 cm-1), Si-O-Si stretching (1060 cm-1), and Si-H stretching modes (2170 cm-1) [19,20]. The peak present around 450 cm-1 can be ascribed to an overlapping of Si-O-Si rocking mode and Si-N breathing mode [21]. No clear feature of N-H stretching mode around 3350 cm-1 is observed. It is speculated that the total amount of N-H bonds (if any) is below the detection limit of FTIR. Previously, Xu et al. [22] have reported that at higher MW power, N-H bonds are less possible to form, survive, and remain in the silicon nitride as they are most likely to be dissociated at a higher MW power [22]. However, the MW power in this study is lower Table 1 Selected samples for investigation Sample notation Gas flow ratio (R = NH3/SiH4) Si atomic excess S4 1.5 33 S3 1.64 29 S2 1.79 25 S1 1.93 22 than that used in [22]. Furthermore, Martinez et al. have explained this behavior based on the following che- mical reaction [21]: Si − Si + N − H =Si − H + Si − N This is a well-known network bond process favored by the tendency to chemical order, whereby formation of Si- H and Si-N bonds is favored at the expenses of Si-Si and N-H bonds. The process is exothermic with a favorable energy balance of 0.25 eV [23]. Also, it can be observed that the intensity of Si-N and Si-H stretching mode increases with increasing Si content of the film following the above reaction kinetics. With increasing Si content, the Si-N peak shifts toward higher wavenumber, indicat- ing enrichment of nitrogen atoms in silicon nitride phase. Figure 3b shows the FTIR spectra of sample S4 contain- ing 33 at.% of excess silicon and annealed up to 1100°C. FTIR spectra of other samples follow similar trends after the post-deposition annealing treatment. It can be noticed that Si-H bands progressively decreases up to an annealing temperature of 700°C, indicating desorption of hydrogen from the films. At an annealing temperature of 900°C, the Si-H band completely disappears. It can be noticed from Figure 3c that the position of Si-N remains unchanged up to an annealing temperature of 700°C. For higher anneal- ing temperatures, the Si-N peak shifts significantly toward higher wavenumbers. This shift indicates that the number of nitrogen atoms bonded to Si is increasing. Since high temperature annealing process broke the Si-H and N-H bonds and hydrogen effused from the film, the remaining unbonded Si and N atoms could bond together to form new Si-N bonds. Therefore, with increasing annealing temperature, there is enrichment of nitrogen atoms in the silicon nitride phase approximating it to stoichiometric Si3N4. In addition, temperature-induced evolution of sub-