X-ray diffraction measurements were carried out on all prepared samples. The spectra did not show any characteristic Bragg peaks for silicon oxide, indicating the amorphous structure of this oxide. Fig. 1 contains a sample of the XRD spectra that we obtained, which is related to the sample annealed at 1200 ° C (sample F). In all measured XRD spectra, the sharp peak is related to Si (111) (CSM card no. 65-1060).
Fig. 2 shows the infrared vibrations observed in the FTIR spectra of the annealed samples. The broad and intense band at 997 - 1142 cm-1 and the peak at 640 cm-1 are attributed to the Si-O-Si stretching vibrations . The peak observed at 813 cm-1 is assigned to Si-O-Si bending vibrations [28, 29]. The absorption peak at 461 cm-1 is due to Si-O-Si rocking vibrations . The weak band located at 419 cm-1 and the peak at 619 cm-1 could be originated from Si-O pounds . The shoulder at 1190 cm-1 appears due to a splitting of longitudinal optical and transverse optical stretching motions .
We notice that, for all peaks, the intensity increases with increasing oxidation temperature, indicating that the oxidation rate increases with increasing temperature.
Fig. 3 shows the reflectance spectra of the prepared samples
We notice that, the spectra are overlapping and the effect of annealing on the reflectivity is not carried out in a specific direction and that for most wavelengths the value of the reflectivity does not exceed the reflectivity in the case of the non-annealed sample. On the other hand, despite the tendency of reflectivity to decrease with increasing wavelength, the decrease associated with the appearance of two peaks in the range λ<400 nm in all spectra and these peaks are a result of the interaction of light with silicon nanoparticles plasmons. The appearance of two plasma edges in each spectrum is due to the nanoparticles having two different sizes [31, 32]. In addition to the mentioned plasma absorption edges, we observe three additional edges in the range λ>400 nm in the spectra D (at 600 nm), E (at 405 nm) and F (at 516 nm). We believe that these edges are a consequence of defects in the silicon oxide structure caused by the phenomena of splitting of longitudinal optical and transverse optical stretching motions of the bonds Si – O - Si, which was observed during the analysis of the FIR spectra (Fig 2).
It is known that, samples with plasma edges with short wavelengths have high concentrations of charge carriers [31, 32]. By applying this rule to Fig. 4 that illustrates the position of each plasma edge as a function of annealing temperature, we conclude that plasma edge 1 in all spectra represents the case of Si nanoparticles with high charge carrier concentration.
Because the prepared films were opaque, we calculated the optical energy band gap from the reflectance spectra using Kubelk-Munk method [33, 34]. Fig. 5 shows the method of calculating the energy band for each sample. In this figure, F(R) is a function of the reflectivity R at the wavelength λ. This function represents the absorption coefficient and given by the formula [33, 34]:
The optical energy band gap (Eg) was calculated by assuming direct transitions between the valence band and the conduction band.
We observe that there are multiple absorption edges in each spectrum (more than one energy band gap). The non-annealed sample (sample A) spectrum contains three absorption edges, one of them belonging to the silicon and the others belonging to the nanoparticles. These edges appear in the rest of the samples, despite the occurrence of oxidation processes. The absorption edge of silicon oxide appears in all spectra of the annealed samples except for the curve of sample B. However, an absorption peak of silicon oxide can be observed in sample B curve in Fig. 6, which shows the absorption coefficient as a function of the wavelength for each sample. This result indicates that the oxide molecules in sample B do not form a continuous coherent structure. On the other hand, the appearance of the silicon absorption edge in all spectra in Figs. 5 and 6 can be explained by the lack of a complete oxidation of the sample surface during the thermal treatment.
The presence of the absorption edge of both silicon and silicon oxide helps to detect the degree of oxidation by comparing the values of the absorption coefficients at wavelength maxima both silicon and silicon oxide. Since the oxide layer does not have the same thickness in every sample, it is necessary to adopt the relative absorption coefficient, which equals the ratio between the absorption coefficient of the silicon oxide and the absorption coefficient of the silicon (αoxide = F(R)Si/ F(R)oxide. Fig. 7 represents the relative absorption coefficient as a function of the annealing temperature.
In this figure, the oxidation rate appears to increase with increasing annealing temperature within the region 800 – 1000 ○C. The annealing at 1100 ○C causes a sharp decrease in the αoxide parameter value. Conversely, the annealing at 1200 ○C leads to an increase in this parameter value. The reason for the decrease in both the absorption coefficient and the relative absorption coefficient of silicon oxide in the case of sample E is the presence of the edge that resulting from the defects in the silicon oxide structure (Fig 3) near the edge of the silicon oxide (Fig 6).
We mentioned that the silicon oxide formed due to oxidation processes is not amorphous; although it can be guessed that the oxide layer formed on the surface of the silicon wafer affects some of the characteristics of the silicon peak that appear in the XRD spectra. Fig. 8 illustrates the XRD silicon (111) peak as a function of the coefficient αoxide.
We notice that, with the exception of sample A that has low oxygen content, the intensity of the silicon peak increases linearly with increasing the relative absorption coefficient. This result is important because it attributes the decrease in the intensity of the silicon peak in the XRD spectra to an increase in the surface oxidation rate.
Fig. 9 shows the optical band gap of silicon oxide as a function of annealing temperature. We notice that the band gap increases with increasing annealing temperature in the range 800 – 1000 ○C. Once moving to the region of samples with a stressed silicon oxide structure (1100 – 1200 ○C), the band gap begins to decrease with the increase in the annealing temperature.
Fig. 10 shows the optical band gap of silicon as a function of annealing temperature. We notice that the band gap of silicon also increases with increasing annealing temperature in the range 800 – 1100 ○C. We also notice that, increasing the annealing temperature to 1200○C decreases the energy gap value.
It is important to assess the effect of the formation of nanoparticles on the optical properties of the prepared samples. For this purpose, we defined the relative absorption coefficients of the nanoparticles observed in the Figs 5 and 6, in a manner similar to the definition of the relative absorption coefficient of silicon oxide. Fig. 11 illustrates the relative absorption coefficients of the nanoparticles as a function of the annealing temperature.
We notice that, the effect of the formation of nanoparticles on the absorption coefficient is evident in the case of the sample C and to a lesser extent in the case of the sample E.