Sahu et al. Nanoscale Research Letters 2011, 6:178 http://www.nanoscalereslett.com/content/6/1/178 silicon-oxide-nitride-oxide-silicon (SONOS), metal- nitride-nitride-silicon (MNNOS) structures have remained as the state-of-art techniques for NVMs [16]. SiNx-based memories are inexpensive, highly inte- grated, and can be expanded to store two bits of data per memory cell [17]. Recently, Mine et al. [18] have proposed the use of SRSN layer as the charge-trapping layer instead of stoichiometric Si3N4 layer with enhanced memory properties. With further scaling down of the device dimensions, the SiNx/SiO2 interface plays the vital role for the large amount of captured charges, and these can be attributed to the excess of silicon at these interfaces, which can capture both elec- trons and holes. Furthermore, with suitable choice of excess silicon and annealing treatment, the formation and evolution of silicon nanoparticles (Si-nps) in the SRSN charge-trapping layer can be possible. The pre- sence of well-separated Si-nps can act as discrete charge storage nodes, thereby improving the leakage current and retention time. The fabrication of uniform, reproducible, and tunable Si nanostructures by simple and flexible technique, compatible with existing CMOS technology, is extremely important. In this regard, one of the best methods used for the evolution of Si-ncs in silicon nitride/oxide-based dielectrics is the high-tem- perature annealing of these films containing excess amount of Si. One of the major issues is the inevitable interface states at the crystalline silicon/dielectric interface. For the improvement of the storage proper- ties, these interface states should be minimized. In addition, the size, density, and separation between the Si-nps should be optimized. In this study, SRSN layers with different excess amount of Si were utilized for the fabrication of Si3N4/ SRN/SiO2/Si (NNOS) structures. The effect of Si-excess and post-deposition annealing temperature on photolu- minescence (PL) and charge storage mechanism in these stack layers were studied in detail. Experimental details Silicon oxide/silicon-rich silicon nitride/stoichiometric silicon nitride (SiO2-SRSN-Si3N4) stacks were made on p-Si (100) substrates. Before SRSN deposition, 10 nm of thermal oxide was grown on p-Si substrate. Subse- quently, SRSN layers of 50 nm were deposited by elec- tron-cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) under the flows of silane (SiH4) and ammonia (NH3) at 300°C under a microwave (MW) power of 300 W. In this experiment, the flow of SiH4 was fixed at 14 sccm, while the chemi- cal compositions of the films were controlled by varying the NH3 flow. The top stoichiometric silicon nitride films were deposited with the same ECR-PECVD system under a MW power of 500 W and an RF power of 8 W Page 2 of 10 at 300°C. Rutherford backscattering (RBS) and elastic recoil detection analysis (ERDA) techniques were used to determine the Si/N ratio and hydrogen content in the films, respectively. Fourier transform infrared (FTIR) spectra were recorded using Bruker-EQUINOX55 spec- trophometer. PL spectra were recorded at room tem- perature using a 355-nm exciton wavelength of a Nd: YVO4 laser. The emitted light was detected by a BWTek BCR 112E spectrometer coupled with a Sony ILX511 CCD linear image sensor. The response of the detection system was precisely calibrated with a tung- sten wire calibration source. Energy-filtered transmission electron microscopy (EFTEM) and high-resolution transmission electron microscopy (HREM) observations were performed using a JEOL2100F microscope operat- ing at 200 kV equipped with a corrector for spherical aberration and the last generation of Gatan-imaging fil- ter. The EFTEM is formed by the electrons which are selected by a slit placed in the energy dispersive plane of the spectrometer at 17 eV with a width of ± 2 eV. Al- Si3N4-SRSN-SiO2-Si (MNNOS) memory capacitor struc- tures were fabricated from the samples by evaporating Al electrodes of 0.8-mm diameter with a shadow mask and Al rear-side contact after scratching the back sur- face. Capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were carried out using HP4192A impedance analyzer through a LABVIEW interface. Results and discussion Composition of MW-CVD SRSN films Figure 1 depicts the variation of atomic content of sili- con, nitrogen, and hydrogen, as estimated from RBS and ERDA measurements on 50 nm of SRSN layers depos- ited on Si-substrates with different gas flow ratios. In particular, the SiNx samples deposited at a gas flow ratio (R = NH3/SiH4) of 2.07 (or higher) are close to stoichio- metric, whereas samples deposited using lower R values are silicon rich, being characterized by excess of Si ran- ging from 50 to 20 at.% in this study. As evidenced from ERDA measurements, the total amount of hydro- gen content decreases from 16 to 6 at.% upon increasing R from 1 to 1.93. In the present investigation, the authors have focused on four sets of samples having an excess of Si in the range 33-22 at.%. The samples are defined with notations S1, S2, S3, and S4, where higher numbers refer to higher Si excess in the SiNx matrix (see Table 1). The samples were further sandwiched between a bottom thermal oxide and a top stoichio- metric Si3N4 for further study. TEM observations of SRSN films Figure 2 shows the EFTEM image of 50-nm thick SRSN layer (sample S4) with 33 at.% of Si excess after anneal- ing at 1100°C. High densities of nearly spherical Si-nps