Abstract
taking into account the contribution from the quantum confinement effect of Si nanocrystals and from the native defects in silicon nitride matrix, such as N-and K-centers.
Introduction
Over the past two decades, a quantum confinement effect in silicon-based nanostructures is of great interest because of its potential for promoting the integration of optoelectronic devices with silicon very large scale integration technology. A variety of techniques has been exploited to obtain low-dimensional silicon structures, such as silicon nanocrystals (Si-NCs) synthesized by ion implantation [1] [2] , sputtering [3] , and plasma-enhanced chemical vapor deposition (PECVD) [4] [5] [6] , silicon nanopillars and nanowires formed by dry etch [7] [8] and chemical sythesis [9] [10] .
Introduction
Initially, efforts of researchers were focused on the light emission of Si-NCs embedded in silicon oxide matrix. However, it has been found out that Si-NCs would have unsatisfactory electroluminescence characteristics due to the difficulty of carrier tunneling through a wide energy bandgap of silicon oxide matrix [10] [11] . Theoretical and experimental investigations predict a high potential barrier (≈ 8.5 eV) for silicon oxide with silicon nanocrystals [12] . Silicon nitride films with Si-NCs provide a lower barrier (≈ 2.0 eV) for carriers and more intense light emission than those of silicon oxide [13] . That is why, silicon nitride has been considered as a preferable matrix material for preparation of Si-NCs-related devices.
Introduction
Strong visible photo-and electroluminescence from SiN x films with Si-NCs, also tunable by adjusting the silicon content, has been widely reported with different interpretations [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] .
Introduction
However, the origin of the luminescence is still an object of controversy, primarily due to a lack of distinction between luminescence originating from silicon precipitates and from silicon nitride matrix. Often, a visible luminescence is explained by quantum confinement effect of crystalline [17, 20, [26] [27] or amorphous [16] Si-NCs. In other cases, the luminescence has been ascribed to
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defect states [28] [29] , surface states at the nanoparticle-matrix interface [30] , or radiative recombination between localized band-tail states [18] [19] .
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A number of researchers reported on visible PL of silicon nitrides fabricated via PECVD.
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Such SiN x films contain a large concentration up to (20 -40%) of hydrogen because of using -amorphous state is metastable, so the device parameters can be subjected to degradation; -usually, the rate of nonradiative transitions in crystalline clusters is lower than that in the amorphous clusters, so the PL intensity for nanocrystals is, generally, higher than that for amorphous nanoclusters.
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In comparison with conventional methods to study nanomaterials and nanostructures, such as transmission electron microscopy and X-ray diffraction, micro-Raman scattering spectroscopy is much preferred for its nondestructive measuring, small specimen quantity and short measurement time. Moreover, a line shape analysis of the first-order Raman spectrum provides A C C E P T E D M A N U S C R I P T
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with the information on the average size of nanoprecipitates as well as crystalline or amorphous nature of the material [34] .
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The aim of this paper is to study phase transformations (formation and crystallization of Si clusters) in LPCVD -Si-rich SiN x films during the high temperature treatment and to reveal a correlation between these transformations and PL of nitride films.
Experimental details
The SRSN films were deposited on n-type (100) The structural properties of Si nanoclusters in SRSN films were studied using Raman scattering technique on both as-deposited and annealed samples. RS measurements were carried out by micro-Raman setup Nanofinder. The samples were excited with a laser beam (λ = 473 nm) and the scattered light was detected in back-scattering geometry at room temperature. PL spectra were recorded at room temperature in the spectral region of 350 -800 nm using a HeCd laser source with λ= 325 nm.
Results and discussion

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The values of thickness, refractive index, stoichiometric parameter x and amount of excessive Si in SiN x for the as-deposited films SiN 0.46 and SiN 1.0 are summarized in Table 1 .
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As shown in Table 1 , the same refractive index and similar thickness are recorded for SiN 0. 46 and SiN 1.0 samples. Refractive index n was used by some authors in order to evaluate the stoichiometric parameter x of SiN x films [16] . The SiN 1.33 composition is believed to be characterized with n between 1.8 and 2.0 depending on the growth procedure. The SRSN with stoichiometric parameter x < 1.3 is characterized with n >2.0. The more excessive (over stoichiometric) Si atoms are incorporated in SRSN films, the more high value of n is associated to them [36] . Nevertheless, in our experiment the RBS data reveal that two samples with the same n, SiN 0.46 and SiN 1.0 , differ by the total quantity of excessive Si. It can be explained as follows. The optical constants of SiN x layers are determined not only by the value of x, which depends on the concentration of excessive Si atoms, but also on the spatial localization of these excessive atoms. The excessive silicon can be randomly distributed over the atomic network or aggregated in clusters of different sizes in the SiN x matrix [37] .
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To confirm the trends, we also performed RS experiments on these samples. All samples exhibit the narrow peak at 520 cm -1 . It arises from the light scattering by the Si (100) substrate. The spectra of as-deposited SiN 0.46 and SiN 1.0 samples contain also a broad band at 480 cm -1 . This band is attributed to amorphous silicon [38] . Thus, we can conclude that there are amorphous silicon clusters in both as-deposited samples (SiN 0.46 and SiN 1.0 ). A partially phaseseparated structure, constituted by a mixture of a silicon reach phase and a phase containing more nitrogen atoms is then plausible.
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The analysis of RS spectra transformation after the annealing reveals two similar features for the both sets of samples. Firstly, when increasing the annealing temperature, the band at 480 cm -1 disappeared and a low-energy tail of the band at 520 cm -1 was observed. This lowenergy tail can be assigned to the light scattering from silicon nanocrystals. It should be noted
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that similar modifications of Raman spectra were observed in Ref [39] for Si-rich silicon oxide films with embedded Si nanoparticles. In this paper, the spectra of the SiO x films annealed at 1300 K for 60 min exhibit two obvious components. The sharp peak at 520 cm -1 was assigned to the silicon substrate, and the low-energy tail was due to the Si nanocrystals. The presence of Si nanocrystals was confirmed by high resolution electron microscopy [39] .
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Thus, the spectral transformations, manifested in Fig. 1 Secondly, a similar feature of the RS spectra of two sets of samples is revealed in the dependence of the intensity of narrow "substrate" band at 520 cm -1 on annealing temperature (Fig. 1) . Usually, the light scattering from Si substrate is an undesirable obstacle in the RS measurements of SRSN films. The strong signal from bulk crystalline Si at 520 cm -1 prevents the observation of weak bands of amorphous and crystalline precipitates in the region of (450 -520 cm -1 ). Though, one can get useful information on phase composition of the nitride films from the analysis of "substrate" peak intensity when the annealing temperature increases. It can be seen in Fig. 2 where the intensity of "substrate" band as a function of the annealing
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temperature for the both sets of samples is presented. The intensity of this band increases significantly after the annealing at 1200 °C for both samples SiN 0.46 and SiN 1.0 . It can be explained by the restoration of host silicon nitride matrix via the annealing of structural defects.
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On the other hand, the crystallization of Si clusters can result in the increase of intensity at 520 cm -1 , too. The inset in Fig. 2 shows the absorption spectra of crystalline and amorphous silicon adopted from Ref. [41] . The vertical line at 473 nm in the inset indicates the exciting laser line used in our RS measurements. As can be seen from Fig. 2 , the absorption at 473 nm in amorphous Si is stronger than in crystalline Si. Therefore, the light transmittance of the SiN x layer with crystalline Si clusters should be higher in comparison with the SiN x layer contained amorphous clusters. It is obvious, if the top layer becomes more transparent, the registered light scattering from the substrate increases. Thus, the significant increase of the intensity of 520 cm -1 band after the annealing at 1200 °C can be explained by the crystallization of initial amorphous
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Si clusters in addition to the annealing of defects in the host silicon nitride matrix.
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However, there is an obvious difference between two curves in Fig. 2 . With the increase of annealing temperature to 1000 °C, the intensity of 520 cm ( Fig.1) . One can suggest that this PL arises from Si nanocrystals. Figure 3a shows the PL spectra of SiN 0.46 and SiN 1.0 samples annealed at 1200 °C for 3 min.
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These PL spectra can be deconvoluted into three fitted Gaussians. For the both samples, the blueviolet PL band at 440 nm is the most intensive one. Also, the red PL with maximum at ~640 nm is observed for both samples, but the PL spectrum of SiN 0.46 film is much more intense in the red region. Between these two bands there is a third one with different maximum positions for two samples. So, for SiN 0.46 film this band maximum is localized in the green range (at 510 nm) while for SiN 1.0 film it is found at 480 nm (in the blue range). It is reasonable to assume that different emission mechanisms are responsible for these PL bands.
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Let us begin with the discussion of luminescence mechanism for the most intensive band at 440 nm. Over the past two decades, different native defect-related centers of radiative recombination in silicon nitride have been revealed and studied [28] [29] [42] [43] [44] [45] . Among of them the N-center is a two-fold coordinated nitrogen atom (Si 2 =N•). According to Desphande et al. [28] , the presence of such defects can lead to light emission at 3  0.1 eV. In Ref. [46] electron
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spin resonance measurements reveal that N-centers may exist in the as-deposited films, but high temperature annealing (>700 °C) creates much more N-centers. In our case, the annealing at 1200 °C can result in the formation of N-centers. Thus, the PL band at 440 nm can be attributed to the radiative recombination of joshuamartinez@example.net.
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There is other native defect called K-center (a silicon atom bonded to three nitrogen atoms (N 3 ≡Si•)) in silicon nitride. The presence of K-centers in silicon nitride matrix can result in green PL in the range of 2.5  0.1 eV [28, 44] . In SiN 0.46 samples, the amount of excess silicon atoms distributed over the silicon nitride matrix, and avoiding clustering, is larger than in SiN 1.0 films.
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Hence, a high concentration of silicon dangling bonds (K-centers) is expected in SiN 0.46 , and the green PL (510 nm) can be attributed to the K-centers, in this case.
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As regarding a nature of the red PL band (640 nm) for SiN 0.46 sample and the blue band (480 nm) for SiN 1.0 sample, we ascribe them to emitting Si nanocrystals. The average size of silicon nanocrystals can be estimated from the position of first-order Raman peak using the following formula [20] :  amount to 520 and 3.5 cm -1 , respectively.
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In our case, it is difficult to determine the average diameter of silicon nanocrystals and size distribution by the Raman spectra due to a strong light scattering from the substrate. However, one can estimate that in the case of SiN 1.0 sample low-energy tail, ascribed to silicon nanocrystals, is stretching approximately from 508 to 514 cm -1 (Fig. 3b) . 
Conclusions
We have studied the influence of annealing at (800 -1200) °C on the phase transformation 

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