The refractive index and thickness of the SiOx
films as a function of the growth temperature are shown in Figure ; the refractive index of the thin SiOx
films changes with Tg. Thicker samples were obtained when the growth temperature was increased from 900°C to 1,400°C. A variation in the refractive index from 1.4 to 2.2 was measured when the growth temperature was increased from 1,150°C to 1,400°C. This variation has been related to a change of the silicon excess in SiOx
]. Therefore, we can modify the silicon excess in the SiOx
films by changing the growth temperature.
Refractive index (a) and thickness (b) as functions of growth temperature of the SiOxfilms.
FTIR absorption spectra of thin SiOx
films are shown in Figure . These spectra show the absorption peaks associated with the rocking (458 cm−1
) (peak 1), bending (812 cm−1
) (peak 2), on-phase stretching (1,084 cm−1
) (peak 4), and out-of-phase stretching (1,150 cm−1
) (peak 5) vibration modes of the Si-O-Si bonds in SiO2
]. The position of the stretching absorption peak in SiOx
films changes with the growth temperature. Tg produces changes only with regard to the microstructure of the films, where the radiative defects can be activated during the process. The on-phase stretching peak position slightly moves towards a higher wavenumber with a higher growth temperature. On the other hand, the out-of-phase stretching peak position shows similar changes with the growth temperature. The position of peak (4) depends on silicon excess. This is an evidence that the SiOx
films have a great content of sub-stoichiometric SiOx
2) phase in the as-deposited state. Peaks in the spectra at 883 cm−1
(peak 3), corresponding to Si-H bending and Si-OH, and the other one located at 2,257 cm−1
(peak 6), corresponding to Si-H stretching, are observed [17
]; these bonds are present in the films due to hydrogen incorporation during the growth process. Also, a peak centered at 2,352 cm−1
(peak 7) comes from the CO2
content in the atmosphere [20
]. Furthermore, the peak intensity changes with the growth temperature as shown in Figure . A relation between peak intensity and thickness is established; to bigger peak intensity, a bigger thickness. In Figure , we can see that if there is a high growth temperature then the thickness increases. Therefore, the peak intensity is bigger too. Moreover, the oxygen and hydrogen contents change with the growth temperature. The hydrogen and oxygen contents decrease when the growth temperature increases, as can be seen with the behavior of the peaks 3 and 7 and peaks 2, 4, and 5, respectively, as shown in Figure .
FTIR absorption spectra of the thin SiOxfilms for different growth temperature. The numbers (1 to 7) mean different vibration modes, which are described in Table .
XEDS spectra of the SiOx
films were realized for several growth temperatures; the stoichiometric ratio is determined by the atomic percentages of silicon and oxygen. The peak intensities of oxygen and silicon change with the growth temperature. The peak intensities of silicon are higher when decreasing the growth temperature, and the peak intensities of oxygen decrease when increasing the growth temperature. These variations indicated that the stoichiometry of the SiOx
films changes with the growth temperature. We can see that the oxygen content decreases with the increase of the growth temperature, and the silicon content decreases with the increase of the growth temperature. Then, with higher growth temperature, the silicon content increases and the oxygen content decreases. In Table , the composition results of the XEDS spectra are listed. Figure shows the XPS experimental spectra of the Si 2p line and the evolution of the Si 2p line of different SiOx
films. The four oxidation states, as well as the unoxidized state, can be modeled as tetrahedral bonding units, in which a central Si atom is bonded to (4
) Si atoms and n
oxygen atoms (Si-Si4 − n
) with n
0 to 4. Therefore, the 99.5 eV peak is associated with elemental silicon. SiO2
spectra increase the peak energy to 103.3 eV, corresponding to n
4. The Si 2p binding energies are normally about 99 to 103 eV. It is widely accepted that the Si 2p photoelectron peak of SiOx
contains five components, corresponding to a non-oxidized state and four different oxidation states of Si [21
]. The variation of the oxidation states of the SiOx
films leads to peak position’s shift, as shown in Table . A peak at about 99 eV accompanied by a peak at about 103 eV is present; they can be attributed to Si and SiO2
, respectively, and any variation could be attributed to sub-oxidized silicon [23
]. The increasing electro-negativity of the Si-O bound relative to the Si-Si bond results in a shift to a higher binding energy of the core level electrons in the silicon.
Compositional results (atomic percentages of oxygen (O) and silicon (Si))
Si 2p XPS spectra show the composition of the SiOxfilms.
Oxidation states of the SiOxfilms obtained by means of the convolution of the XPS curves
AFM images of the SiOx
films in Figure are presented. All images exhibit a rough surface. It can be seen that the surface exhibits different characteristics depending on the growth temperature, which influences the size of the grains (roughness), their form, and composition. Average roughness decreases by decreasing the Tg and thickness [8
]. The roughness analysis is shown in Figure . It is observed that the surface roughness of SiOx
films with lower Tg is less than that with higher Tg, except for Tg
1,150°C. The high roughness of the sample grown at 1,400°C is probably the cause of the index variation with a not clear tendency, and the roughness could be due to the big nanocrystals embedded in the SiOx
Figure 4 3D AFM images of SiOxfilms deposited on silicon substrate at different Tg. Scanned area is 4×4 μm2.
Figure 5 Average roughness (Sa) as a function of Tg for SiOxfilms. Scanned area is 4×4 μm2.
On the other hand, the structure of SiOx films was analyzed using the HRTEM technique. Figure shows the HRTEM images for the SiOx films, which indicate the presence of Si-ncs embedded in the SiOx films. Some of them are semi-elliptical, and some other ones have an enlarged shape. The agglomeration process takes place between the nearest Si-nanoclusters forming Si-nc. All micrographs show that the SiOx matrix contains small clusters, which on the basis of the selected area electron diffraction (SAED) analysis can be identified as Si-nc. SAED is referred to as ‘selected’ because, in the micrograph, it can easily choose which part of the sample we obtain the diffraction pattern; in our case, only on the Si-nc. About ten micrographs were obtained to each sample; with them, a statistical analysis of the distribution of the Si-nc diameter sizes was realized. A great dispersion of diameter sizes is observed; the diameter size goes from 1 up to 9 nm, being the average diameter size around 5.5, 4, and 2.5 nm for 1,150°C, 1,050°C, and 900°C, respectively, as indicated in the histograms of Figure a,b,c.
The plain-view HRTEM images and histograms of the SiOxfilms. For samples with Tg at (a) 1,150°C, (b) 1,050°C, and (c) 900°C.
From AFM images, the samples grown with lower Tg look more homogeneous than those grown with higher Tg. As shown in the HRTEM images, the silicon excess agglomerates to create Si-ncs; then, the roughness observed in AFM measurements can be associated with Si-ncs and compounds. In addition, FTIR spectra show a phase separation (Si and SiO2), which is deduced from the shift of the Si-O stretching vibration mode towards the SiO2 frequency value, and it is corroborated with both XPS and HRTEM. Therefore, elemental Si, SiOx, and SiO2 phases with Tg are present, and depending on Si excess, the roughness, size of Si-nc, oxidation states, and vibration modes of the Si-O-Si bonds, some of these phases could be dominant. This indicates that a direct correlation between the roughness, size of Si-nc, oxidation states, and vibration modes of the Si-O-Si bonds exists. In other words, the roughness is produced by the formation of Si-ncs and oxidation states. The diffusion of excess silicon at high Tg produces Si-ncs in the SiOx films, i.e., the silicon particles diffuse to create silicon agglomerates around the nucleation sites when the SiOx is grown at high Tg.
Figure shows the PL response of SiOx films corresponding to different growth temperatures. At all samples, a wide PL spectrum is observed. At the growth temperatures of 1,150°C, 1,050°C, and 900°C, the PL peaks are at 558, 546, and 534 nm, respectively. At the highest growth temperature (1,400°C), the PL has the weakest intensity, and the PL peak has the longest wavelength of 678 nm. Moreover, the PL bandwidth and intensity increase for growth temperatures lower than 1,400°C. Therefore, the PL also depends on Tg, size of Si-nc, roughness, oxidation states, and vibration modes of the Si-O-Si bonds, as shown previously.
PL spectra of the SiOxfilms with different Tg. Inset shows the convolution realized to PL spectra.
The optical bandgap of SiOx
films is obtained with transmittance spectra measurements. Transmittance spectra for SiOx
films deposited on quartz are shown in Figure a. The transmittance of all these films is relatively high (>80%) between 600 and 1,000 nm, as shown in the figure, and reduces to zero for wavelengths below 600 nm. The growing temperature produces a clear change of the curves and a shift towards lower wavelengths related to a silicon excess change of the SiOx
]. In Tauc’s plot, an increase in the energy bandgap (Eg
) has been detected when the growth temperature decreases, as shown in the inset of Figure a. The values of the optical bandgap Eg
can be estimated from the following equation known as the Tauc plot [27
Figure 8 UV–vis transmittance spectra, absorption coefficient versus energy, and (αhν)1/3versus energy (hν). (a) UV–vis transmittance spectra and the inset show the energy optical bandgap from SiOx films as a function of (more ...)
where Eg is the optical bandgap corresponding to a particular transition in the film; A, a constant; ν, the transmission frequency, which multiplied by the plank constant h we have photon energy hv, and the exponent n characterizes the nature of band transition. The absorption coefficients α(λ) were determined from transmission spectra with the following relation:
) is the transmittance, and d
is the thickness of the SiOx
is shown in Figure b. On the other hand, values of n
1/2 and 3/2 correspond to direct-allowed and direct-forbidden transitions; n
2 and 3 are related to indirect-allowed and indirect-forbidden transitions, respectively [28
]. From a plot (αhν
, the bandgap can be extrapolated from a straight line to hν
0. For all different growth temperatures, the best straight line is observed for n
3 (Figure c), indicating an indirect-forbidden transition mechanism.
The optical properties such as the energy bandgap and the PL bands between (400 to 700 nm) are usually some of the important characteristics of these materials. The PL of SiOx
films has been extensively studied in the literature [1
]. Two major mechanisms for PL in this kind of materials are generally accepted: quantum confinement effects in the Si-ncs and defect-related effects, as defects at the Si/SiOx
interface and defects associated with oxygen vacancies in the film. The first mechanism of light emission that we can consider in the SiOx
material is related to some kinds of defects produced during the growth process, as shown in the EDS, XPS, and FTIR spectra, where we have bonding such as neutral charged oxygen vacancies (NOV) (Si-Si bonds), non-bridging oxygen hole center (NBOHC), positively charged oxygen vacancies (E’ centers), interstitial oxygen molecules and peroxide radicals [13
], which can form Si-nps or E’ centers. Therefore, the increase of PL with the Tg is due to the activation of some of these radiative defects. In this study, the 550-nm PL band has been associated with silicon excess in the film in the NOV defects and E’ centers [2
] types. These bands appear well defined only if the film has been grown with temperatures within 900°C and 1,150°C. If the film was grown at a higher temperature, the band at 700 nm appears with its maximum PL emission.
On the other hand, as a second mechanism of emission, the luminescence peak of SiOx
films shows a blueshift when the Tg decreases; this behavior is ascribed to quantum confinement effect in the Si-nc. Therefore, in this case the PL spectra are analyzed in terms of a quantum confinement model [1
which corresponds to the radiative recombination of electron–hole pairs in the Si-nc, where d and EN are the diameter and energy of the Si-nc, respectively, and λ (nm) is the wavelength of the Si-nc emission. Table shows the theoretical values of average size of Si-nc calculated from the PL spectra, where the size of the Si-nc reduces and the energy band gap increases with decreasing the growth temperature, similar to an effect of quantum confinement. Note that unlike as stated in the literature for the quantum confinement effect, PL spectra are very wide which indicates that two possible mechanisms are involved.
Why is it possible that two mechanisms be involved? When a deconvolution to the PL spectra is made, as shown in the inset of Figure , different peaks are defined; some of them are related to different kinds of defects, as listed in Table . Therefore, the high-energy PL peaks are associated with quantum confinement effects in Si-ncs, while the low-energy PL peaks are associated with defects. Such a behavior is described in the schematic diagram of the band structure (Figure ); a similar behavior has been reported previously [31
]. This diagram represents the radiative transition giving rise to the emission peaks. For the more energy occurs the generation of electron–hole pairs within the Si-nc core, followed by a thermal relaxation within the conduction band of the Si-nc which in turn suggests the recombination of carriers. In the case of the less energy peaks, phonon relaxation involves more energy because of the transitions between the states of interfacial defects.
Peak position obtained by deconvolution from PL spectra and defect types relationated with the peak position
Schematic representation of the band structure and mechanisms responsible for PL from SiOxfilms.
The existence of Si-ncs in the SiOx films was corroborated with the HRTEM measurements. The diffusion of Si excess due to the deposit at high temperature, i.e., when the SiOx films are being deposited, could produce Si-ncs. The silicon particles diffuse themselves to create silicon agglomerates around the nucleation sites. If the Si excess is high enough, the Si agglomerates will be crystallized to form Si-ncs. A decrease in the Si-nc diameter has been detected when the growth temperature reduces. The high growth temperature induces the formation of crystals as the statistical analysis of the crystal size distribution, obtained from the HRTEM images, shown. Therefore, the mean diameter of Si-ncs depends on the growth temperature.
Then, two transition mechanisms are possible as the above results and discussion showed widespread bandgap transitions induced by quantum confinement and interface state transitions associated with defects in the oxide. The widespread transitions in Si-nc may bring about high energy peaks (blueshifted PL peaks), and if this energy decays between defects (NOV, NBOHC, and E’ center-related interface states), it can give place to low energy peaks (redshifted PL peaks). All these data indicate that light emission from the films is due to the Si-ncs embedded in the amorphous SiOx matrix and defects. Accordingly, we have proposed a combination of mechanisms to explain the photoluminescence in the films.