Nalini et al. Nanoscale Research Letters 2011, 6:156 http://www.nanoscalereslett.com/content/6/1/156 fabricated by sputtering the Si cathode and simulta- neously introducing nitrogen into the Ar plasma. The nitrogen rate rN was kept at 10% while the total flow rate was fixed at 10 sccm. The pressure in the chamber was chosen as 2 mTorr for SiNx layers. The temperature of deposition was maintained at 500°C for all the cases. The thickness of the SRSO sublayer was fixed to be 3.5 nm in order to be within the quantum confinement regime. In order to understand the influence of SiNx matrix, two different thicknesses of the SiNx sublayer (3.5 and 5 nm) were chosen. The FTIR spectra of these samples were recorded in absorption configuration using Nicolet Nexus spectro- meter at Brewster’s angle (65°). The photoluminescence (PL) spectra of the annealed samples were obtained in the visible range using Jobin Yvon monochromator in the wavelength range 550-1100 nm. The excitation wavelength of 488 nm (Ar laser) was used for measurements. Results and discussions FTIR spectroscopy Figure 1 shows the FTIR spectra obtained for the non- annealed SRSO/SiO2, SiNx/SiO2, and SRSO/SiNx multi- layers. The spectra were recorded at the Brewster angle of 65° that enables the detection of the LO3 mode of silica at about 1250 cm-1 in addition to the TO3 mode located near 1080 cm-1. In SRSO/SiO2 around 1225 and 1080 cm-1 we notice the LO3 and TO3 peak from the Si-O stretching, the TO4-LO4 doublet between the 1100-1200 cm-1 and the TO2-LO2 asymmetric stretching of Si-O from SiO2 at 810 and 820 cm-1, respectively [13]. The presence of Si- nc is attested by the intensity of the LO3 peak which is representative of the Si-O bond at the interface [14] Figure 1 FTIR spectra of the multilayer structures at Brewster’s angle. Page 2 of 5 between silicon and silica while the TO3 vibration mode at about 1080 cm-1 is the signature of the volumic silica. The SiNx/SiO2 film has a broad peak in the 1250-950 cm-1 region which can be due to the contributions of both LO and TO modes from SiO2 and Si-N stretching mode [15-17]. The absorption band located around 860 cm-1 could be attributed to the Si-N asymmetric stretch- ing mode. In the case of SRSO/SiNx films, the shoulder around 1190 cm-1 may be due either to N-H bond [16,18] or to a contribution of the LO3 mode of Si-O-Si bonds at 180° [13]. Such a result is the signature of the Si nano- particles formation within either the SiNx [19] and/or the SRSO sublayer [13]. Between 1050 and 1070 cm-1 lies the LO peak of a-SixNyHz from Si-N as it has been observed in the SiNx/SiO2 spectrum adding the contri- bution of the TO Si-O mode. PL spectra The PL emission spectra of the annealed multilayer structures were measured using 488 nm excitation wavelength and the spectrum was recorded in the visible range. Two different annealing treatments were chosen for the study–1 min-1000°C (rapid thermal annealing– RTA) and 1 h-1100°C under N2 atmosphere, the latter being the classical annealing treatment used for recover- ing defects in SiO2 matrix to favor luminescence from Si-nc [3]. Figure 2 shows the effect of the annealing treatment on the PL intensity of the three kinds of mul- tilayer structures. All the curves are normalized to a total thickness of 100 nm. Since the number of periods and the sublayer thickness remains the same for each of these films, i.e., Nperiods(tsublayer1/tsublayer2) = 50(3.5/3.5 nm), it becomes possible to make a comparative analysis from the PL spectrum of these three different multilayer structures. The interference effect in PL intensity has been investigated by the method proposed by Holm et al. [19] for all the spectra presented in this article. This method gives us the PL intensity versus layer and sub- strate parameters (refractive indices, thicknesses). We assume and homogenous density of emitting centers, an average refractive index within the thickness of multi- layer. For measurements on Figure 2 no important change in PL has been found due to interference. It can be noticed from the spectrum that when the multilayers are subjected to the classical annealing treat- ment of 1 h-1100°C, there is no emission from the SRSO/SiNx while the SRSO/SiO2 structure shows a strong PL signal and has a wide range of emission spec- trum. At the same time, it is interesting to note a very weak PL signal in the case of SiNx/SiO2. The PL peaks appear in a region usually related to the optical transi- tions in the SiO2 matrix due to the presence of defects [3,17]. The lower part of Figure 2 shows the PL