Nano)Structure of a-SiC:H

Article pubs.acs.org/JPCC

Effect of Substrate Temperature on (Micro/Nano)Structure of a‑SiC:H Thin Films Deposited by Radio-Frequency Magnetron Sputtering. Mohsen Daouahi†,‡ and Najeh Rekik*,§,∥ †

Physics Department, Faculty of Science, University of Hail, Kingdom of Saudi Arabia Laboratoire de Physique de la Matière Condensée, Département de Physique, Faculté des Sciences de Tunis, Campus universitaire, 1060 Tunis, Tunisia § Department of Chemistry, University of Alberta, Gunning/Lemieux Chemistry Centre, 11227 Saskatchewan Drive, Edmonton, Canada T6G 2G2 ∥ On leave from: Laboratoire de Physique Quantique, Faculté des Sciences de Monastir, route de Kairouan, 5000 Monastir, Tunisia ‡

ABSTRACT: The nature of the hydrogen bonds and their influence on film (micro/nano)structure has been investigated as a function of substrate temperature TS (200−500 °C) in hydrogenated amorphous silicon carbide (a-SiC:H) thin films. These films were prepared by radio-frequency magnetron sputtering at a common pressure and high hydrogen dilution in the gas phase mixture (Ar + 80% H2). Infrared absorption and Raman spectroscopy experiments have been carried out to characterize the (micro/nano)structure of different series as a function of TS. The results of Fourier transform infrared spectroscopy indicate that all series are characterized by the presence of the low Si−Hn and C−Hn bond concentration and no absorption band at 2000 cm−1 attributed to isolated monohydride Si−H can be detected, which suggests that the hydrogen is predominantly bonded as polyhydride groups (Si−H2)n, giving the band at 2090 cm−1. The concentration of the Si−C bond and the crystalline fraction, which increase with increasing TS, show an improvement in the crystallinity of the films. The influence of TS on the growth of crystalline nanograins is studied by Raman spectroscopy. At temperatures TS ≥ 200 °C, these films contain Si−C crystallites in addition to the Si nanocrystals distributed in the amorphous network. The Si crystallite size is unaffected by the increase of TS but the size of graphitic domains is reduced and their distribution becomes more disordered. Results of Raman spectroscopy show also that the high values obtained for the line width of transverse acoustic TA-like band toward low frequencies justifies the deconvolution of infrared spectra taking into account the doublet (845 and 910 cm−1). The results of this paper therefore shed light on the origin of amorphous and nanocrystalline structure in the a-SiC:H thin films, which has been a subject of debate for decades. enhanced chemical vapor deposition technique (PECVD),13,14 very high frequency glow discharge technique (VHFGD),15,16 remote hydrogen microwave plasma CVD from a triethylsilane precursor, 1 7 , 1 8 hot-wire chemical vapor deposition (HWCVD),19−21 and inductively coupled plasma chemical vapor deposition (ICP-CVD).22,23 Indeed, most studies dealing with a-SiC:H films use radio-frequency “RF” plasma enhanced chemical vapor deposition-like technique.13,14 Only a few

1. INTRODUCTION Amorphous silicon carbide has attracted much attention in recent years. Due to its chemical stability, hardness, interesting optical and electronic properties, it can be used in many applications such as solar cells,1−7 thin film transistors,8−10 light emitting diodes,11,12 etc. It is well established by now that among the various semiconductor thin films, hydrogenated amorphous and nanoor microcrystalline silicon (a-Si:H and nc- or μc-Si:H) films have much potential for fabrication of large area thin film solar cells and other devices.1−12 Though device quality amorphous and microcrystalline silicon films are mainly prepared by plasma © 2012 American Chemical Society

Received: August 11, 2012 Revised: September 6, 2012 Published: September 11, 2012 21018

dx.doi.org/10.1021/jp3079937 | J. Phys. Chem. C 2012, 116, 21018−21026

The Journal of Physical Chemistry C

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studies have been devoted to the “RF” magnetron sputtering,24,25 which presents several advantages. One of them is that we can vary the H2 percentage (i.e., H2 dilution) in the gas phase mixture (Ar + X% H2) in a large range independently from the (RF) power. This technique has already been used for the elaboration of amorphous and polycrystalline silicon materials with quite interesting properties.26,27 Moreover, the magnetron cathode induces a confinement of the plasma at the silicon target which increases the ionization process. Thus “RF” magnetron sputtering appears to be an alternative technique to deposit a-SiC:H films with interesting optoelectronic properties. The growth temperature, temperature distribution, and pressure in a growth chamber can significantly affect the local growth rate along the growth interface. A nonuniform temperature distribution can also induce thermal stresses in the material, leading to generation and propagation of dislocations and/or other defects. It is, therefore, important to understand the underlying interrelationship between the aSiC:H growth and thermal conditions. However, this understanding is difficult to achieve solely from the experiments because in situ measurement of temperature and pressure in a growth chamber remains a difficult task. Severe constraints exist on measurement in a-SiC:H system primarily because of high temperature. Controlling the substrate temperature during deposition is one of the most important factors involved in the changes in structure of an a-SiC:H thin film. However, the variations in the structure of vapor-deposited a-SiC:H thin films with substrate temperature using RF plasma enhanced chemical vapor deposition like-technique have not been studied systemically. In addition, it is well established that optoelectronics properties depend on the preparation conditions.13,24 The deposition parameters such as substrate temperature,7,11 hydrogen dilution, etc. play an important role in the quality of a-SiC:H thin films. The relation between the preparation parameters and the structure properties has not been elucidated yet. Great efforts have been devoted to the optimization of the deposition parameters to produce a high quality a-SiC:H samples. Depending on the deposition conditions, different Si− H groupings can be incorporated and strongly influence the optoelectronics properties of the films. The nature of hydrogen bonding configuration, especially the polyhydrides groups {SiH2, SiHn, (SiH2)n} present in the films, play a key role in the formation of structural inhomogeneities and disorder. In this paper, we report the preparation and spectroscopic studies of four series of a-SiC:H thin films prepared by varying substrate temperature (TS) in the range 200−500 °C. Various structural studies reveal that the films deposited at low TS are amorphous in nature, whereas crystallinity develops at elevated TS (TS ≥ 200 °C).14,28 In this work, we will focus our attention on the hydrogen bonding configurations of a-SiC:H thin films obtained by RF magnetron sputtering at high hydrogen H2 dilution and their influence on the crystallization process for different values of the substrate temperature TS. The a-SiC:H films were characterized at different TS using infrared “IR” absorption and Raman spectroscopy techniques. According to the variation of TS, the effects of the microstructure on the crystallization of the a-SiC:H films are discussed.

gas phase mixture (Ar + 80% H2) at different substrate temperatures TS ranging between 200 and 500 °C and at a common pressure. The plasma is initiated between the cathode and the anode at a pressure of 50 mTorr by the application of a high “RF” voltage. The plasma is sustained by the ionization caused by secondary electrons emitted from the cathode due to ion bombardment which are accelerated into the plasma across the cathode sheath. Various numbers of carbon chips, cut from a highly pure graphite rod, have been placed on the surface of 10 cm diameter silicon target to vary the carbon to silicon area ratio. The ratio of the sputtered areas of both silicon and carbon was adjusted to obtain quasi-stoichiometric SiC layers.29 Finally, it is interesting to mention that five samples in each series were analyzed to get reproducible results. The IR absorption spectra of all series were obtained by Nicolet 510 Fourier transform infrared (FTIR) spectrometer (in the range 400−4000 cm−1) with a resolution of 2 cm−1 and averaged over 500 scans. To determine the nature of hydrogen bonding and the dependence of these bonds on the substrate temperature, the IR absorption spectra of all series were computed after baseline subtraction and thickness normalization. Raman measurements were carried out at room temperature with Dilor Z24 Raman spectrophotometer equipped with a triple monochromator, a water-cooled photomultiplier, and a photon counting system. The measurements were done in backscattering configuration using the 514.5 nm excitation wavelength from an argon ion laser. The spot was focused on a diameter of about 80 μm on the sample surface and the excitation power density was kept below 1 mW/cm2 to avoid any heating effect or crystallization of the films by laser beam.

3. RESULTS AND DISCUSSION The IR absorption spectra obtained for different series at different substrate temperatures TS are plotted in Figure 1. All

Figure 1. FTIR absorption spectra of samples deposited at different substrate temperature.

spectra exhibit three main bands. These bands show mainly the presence of a broad absorption band in the region 500−1100 cm−1 and two less intense bands located in the 2000−2200 and 2800−3100 cm−1 ranges. To investigate the individual contributions of absorption modes, we have fitted the absorption spectra for each of the interest region with Gaussian centered peaks. The absorption band between 500 and 1100 cm−1 was considered to be a mixed band of several modes. Generally, this band was deconvoluted with four main signals centered respectively at 640−680 cm−1 for Si−H wagging,30 at

2. EXPERIMENTAL DETAILS Four series of a-SiC:H thin films, labeled by their corresponding substrate temperature, were deposited by magnetron reactive cosputtering of carbon and silicon in the 21019



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Figure 2. Deconvolution of the infrared spectra of samples deposited at different substrate temperature in the region 500−1200 cm−1. Gaussian and Lorentzian contributions at Si−C peak are indicated. Theoretical envelopes (sum of the Gaussians) are given by the red lines. The experimental results are given by the black lines.

800 cm−1 for Si−C stretching,31 at 890−900 cm−1 for Si−H bending, and at 980−1000 cm−1 for C−Hn wagging-rocking vibrational modes.32 In our case, we opted for a deconvolution of this band into five spectral contours where the peak centered at 890 cm−1 was replaced by the doublet centered at 840−855 and 890−900 cm−1, characteristic of the presence of polyhydride groups like (Si−H2)n. It is important to note that in a-Si:H thin films, the presence of two peaks (doublet) centered near 845 and 890−910 cm−1 is characteristic of the presence of polyhydride (Si−H2)n complexes, which are known to favor the formation of structural inhomogeneities.33,34 The deconvolution choice, using five spectral contours, is based on the fact that it is well-known that the films obtained by “RF” magnetron sputtering were characterized by the presence of (Si−H2)n comparatively to those obtained by PECVD.35 It has been also reported that the nc-Si:H films are characterized by the presence of (Si−H2)n chains structures.36 The best deconvolution of the band in the spectral region 500−1100 cm−1 was obtained with five Gaussians reproducing the contributions of different modes of vibration. It is important to note that the absorption band centered at 780 cm−1 is attributed to the Si−C vibrational mode.30 The decomposition of all series is shown in Figure 2. The absorption peak, centered at 780 cm−1, is detected in all series and represents about 50% of the total band area. The absorption band, attributed to Si−C, is thus considered as the predominant absorption peak. Figure 3 shows the variation of Si−C peak areas for different series by taking the corresponding area of integrated absorption of the same modes for series TS = 200 °C as the unit. As we can see from

Figure 3. Normalized peak area of the wagging, stretching modes of Si−H and Si−C as a function of the substrate temperature TS.

the figure, this variation was elucidated by a linear fit. The figure confirms also that the Si−C bonds density is enhanced when TS increases, allowing us to elucidate that the number of carbon atoms incorporated into the films, to form Si−C bonds, increases with TS. This finding illustrate that the carbon atom is preferentially bonded to silicon which, responsible for the increase in the Si−C bond density. Figure 4a shows the variation of the full width at half-maximum (fwhm) of Si−C peak as a function of TS. The fwhm of Si−C band decreases 21020



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high temperature.41 The fact that FC increases with TS has been reported for layers of a-SiC:H obtained by sputtering in a plasma composed of 100% hydrogen for temperature ranging from 200 to 600 °C.24 Our values found for FC as a function of TS for films elaborated with 80% H2 are close to those found for the films elaborated with 100% H2 but the fundamental difference is the clear linear variation of FC with TS for our films. The same result has been obtained for films elaborated by others deposition techniques.28 Hydrogen bonding in the films was studied by analyzing the different absorption bands associated with the different vibrational modes. One has to distinguish between the stretching, bending, and wagging modes, localized respectively in the 2000−2200, 840−900, and 630−680 cm−1 ranges. A contribution due to the wagging modes of Si−H bonds is observed in all series. In Figure 3, we plot the variation of peak areas corresponding to the wagging modes for different series by taking the corresponding surface wagging mode for series TS = 200 °C as the unit. As we can see, an exponential decay of the integrated absorption of the wagging band with TS as depicted in Figure 3 by the exponential fit. This observation shows a progressive decrease in the concentration of hydrogen atom bound to Si when TS increases. In addition, the frequency of Si−H wagging mode is shifted from 640 to 670 cm−1 with increasing TS. This result is in good agreement with results reported in refs 42 and 43. The low intensity of the absorption band centered at 2090 cm−1 corresponding to the vibration mode for the clustered Si− H bond, including (SiH2, SiHn, (SiH2)n)44 is detected for all series. This finding is in good agreement with earlier FTIR results.45 This absorption band was deconvoluted into a single Gaussian centered near 2090 cm−1 showing the absence for all series of the Gaussian component at 2000 cm−1 attributed to isolated monohydride Si−H. The absence of the 2000 cm−1 peak could also indicate that hydrogen is bonded to silicon mainly as polyhydride. This behavior has been observed for aSiC:H films obtained by sputtering and PECVD.46,47 Figure 3 shows also that the intensity of wagging and stretching bands undergoes the same decrease in temperature range 200−400 °C. However, between 200 and 400 °C, the increase of TS induces a strong decrease in the intensity of the stretching band comparatively to wagging band. This result shows that increasing temperature substrate causes a decrease of the concentration of polyhydride groups (SiH2)n. This trend is more pronounced from 400 to 500 °C. It is interesting to note also that the low concentration of polyhydride groups is liable to reduce the microvoids density and consequently improves the compactness of a-SiC:H films. Thus, by RF sputtering, it is possible to obtain a dense films of a-SiC:H only by increasing substrate temperature. Figure 5 shows that from 200 to 400 °C the Si−H2 stretching mode frequency shift to higher wavenumber values as has been reported for a-SiC:H films elaborated by other methods.48 Generally, this effect is related to the incorporation of carbon where the electronegativity of Carbon element is higher than Si element.49 Note that there has been a debate on the cause of this frequency shift which can be associated to void formation.50 Interestingly, it is observed that in the 400−500 °C range temperature, the SiH stretching mode frequency shifts to lower wavenumber values, indicating that the frequency shift is not only due to C incorporation. In our case, the downshift at this temperature range can be also attributed to the presence of the local strain like tensile stress as reported in ref 51. Concerning

Figure 4. (a) FHWM of the Si−C stretching peaks and (b) crystalline fraction FC as a function of the substrate temperature TS.

from 106 cm−1 at TS = 200 °C to 75 cm−1 at TS = 500 °C. The low values of fwhm obtained for all series, especially for series obtained at TS = 500 °C indicate that a limitation of the angular distortion of bonds has occurred. It is well-known that fwhm of Si−C peak is related to disorder of the network.37,38 The decrease of fwhm of Si−C band likely indicate that a phase transition from amorphous to crystalline state of the film took place when TS increases. Consequently, an improvement in the crystallinity of films with the increase of TS is observed. To determine the crystalline fraction FC of the different series, the Si−C absorption peak centered at about 780 cm−1 was deconvoluted into two contributions, Gaussian and Lorentzian, after subtraction of the contributions of the Si− H2 and C−H vibrational modes located at 840, 900 (doublet), and 1000 cm−1 (singlet), respectively. FC was deduced from the relationship given by24,39 FC = AL /(AL + A G)

where AL and AG are the area under the Gaussian and Lorentzian components respectively and where AG (respectively AL) can be assumed to be proportional to the amount of Si−C bonds in the amorphous (respectively, crystalline) phase. Figure 4b shows the variation of FC as a function of the deposition temperature. As we can see, when TS varies from 200 to 500 °C, FC undergoes a rapid increase (21−43%). It is important to underline that the value of FC at 300 °C is relatively important compared to recent results obtained by other techniques.40 We should note that obtaining a crystalline fraction of 30% at low temperature (TS ≤ 300 °C) is a good prospect for achieving crystalline layers compared to those obtained by thermal annealing, where crystallization occurs at 21021



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Figure 5. Peak position of Si−H2 as a function of substrate temperature TS.

Figure 6. Raman spectra of the different series as a function of substrate temperature TS under visible excitation (514.5 nm).

the absorption band at 2800−3000 cm−1, which is due to stretching mode of CHn bonds, it was found that it is very weak.52 Moreover, the percentage of the spectral area of the bands (2800−3200 cm−1) relative to the total area of the entire IR spectrum did not exceed 3% for all series and rapidly decreased with TS compared to Si−Hn peak. This decrease in the area under CHn can be attributed to the decrease in the voids, as has been recently shown in ref 53. The fact that our series of a-SiC:H contain a small amount of CHn is a good result because it is known that the presence of CHn in the films is one of the most important factors for structural distortion in the Si network.54 The rapid decrease of CHn with TS agrees well with the experimental fact considering that C−H bonds in the a-SiC:H films begins to dissociate from 400 °C by annealing Ta.55 This result implies also there is no preferential attachment of hydrogen to carbon. It seems that the simultaneous decrease in the concentration of CHn and SiHn bonds with increasing TS from 200 to 500 °C promotes the formation of Si−C bonds. This decrease in the concentration of CHn and SiHn bonds makes difficult the estimation of the concentration of Si−H and C−H bonds from their stretching peaks centered respectively at 2080 and 2900 cm−1. Besides, the interesting features of IR spectra are the near absence of CH3 stretching (2800−3000 cm−1) as well as CH3 bending and rocking (1200−1600 cm−1) modes, which clearly indicate that carbon and hydrogen are mainly bonded to silicon. This is a very interesting result because the Si−CH3 mode has been correlated to the deterioration of the photoelectric properties.56 Micro-Raman spectroscopy was performed on the same series analyzed by IR absorption measurements. To study the influence of TS on the growth of crystalline nanograins, we give in Figure 6 the Raman spectroscopy results of different series of a-SiC:H thin films. These spectra present two main broad bands in the 100−600 and 1100−1600 cm−1 spectral regions. Concerning the 1100−1600 cm−1 spectral region, the Raman spectrum for series TS = 200 °C (Figure 7) shows a broad band between 1300−1600 cm−1, which was deconvoluted into two Gaussians centered at 1358 and 1535 cm−1. It is known that these bands are attributed to the D band (1330 cm−1) and to the G band (1580 cm−1) of graphitic coordination modes of graphite carbon.57 This splitting is associated with the graphitization process and is usually observed in pure a-C:H where a broad band between 1300−1600 cm−1 results from the superposition of the two well-known G and D bands.57 With respect to the series TS = 300, 400, and 500 °C the shape of Raman spectra has undergone a substantial change indicative of

a structural rearrangement. Also, the Raman spectra for the carbons does not follow the vibration density of states.58 For series TS = 300 °C, the Raman band between 1300−1600 cm−1 is deconvoluted into one symmetrical Gaussian band centered at 1455 cm−1, illustrating the absence of D and G -like bands as shown in Figure 7. The presence of the symmetric peak centered at 1455 cm−1, in contrast to the double band observed for TS = 200, corresponds to the band typically observed in amorphous carbon;57 a sign of a partial crystallization of carbon clusters. The symmetric band around 1450 cm−1 (TS = 500 °C) could be ascribed to the presence of very small and highly disordered carbon clusters59,60 or isolated sp2 bonds dispersed through the a-SiC:H films.57 In comparison, for series TS = 400 °C and TS = 500 °C, their Raman bands between 1300−1600 cm−1 were deconvoluted into several Gaussians peaks,28 as shown in Figure 7. The position of the Raman carbon peak (G band) for these series is much lower than that observed for crystalline graphite (1580 cm−1). This large shift could be due to the decreased size of graphitic clusters.61 Furthermore, for series TS = 500 °C a shift toward lower frequencies was observed along with an increase in the fwhm for all three Gaussians peaks, as compared to series TS = 400 °C. A decrease in the size of graphitic clusters for series TS = 500 °C compared to series TS = 400 °C was determined. The increase of the width is due to the distribution of small clusters for series TS = 500 °C becoming more disordered than for series TS = 400 °C. On the basis of these observations, when TS increases from 400 to 500 °C, the size of graphitic domains is reduced and their distribution becomes more disordered. It is important to note that Figure 7 shows also the presence of a band centered at ≈775 cm−1, attributed to the presence of Si−C. This band is clearly seen for series TS = 200 °C and TS = 300 °C whereas in a previous study conducted by Choi et al.,62 this band was not detected in their Raman spectra. This result confirms that a large amount of Si−C is present in our series and it is consistent with FTIR results. For the 100−600 cm−1 spectral region, the first-order Raman spectra obtained for series TS = 200 °C is shown in. Figure 8. For series TS = 200 °C and TS = 300 °C, the Raman spectrum shows only a broad peak centered at around 480 cm−1 characteristic of a predominant amorphous structure. It is very important to note that for series TS = 400 °C and TS = 500 °C, the best deconvolution of the band, in the spectral region 400−600 cm−1, was obtained with two Gaussian peaks centered respectively at (483 and 556 cm−1) or 501 cm−1 as shown by Figure 8. The presence of the peak centered at 483 cm−1 is 21022



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Figure 7. Deconvolution of the Raman spectra in the region 600−1800 cm−1. Theoretical envelopes (sum of the Gaussians) are given by the red circles. The experimental results are given by the black lines.

Figure 8. Deconvolution of the Raman spectra in the range 100−600 cm−1. Theoretical envelopes (sum of the Gaussians) are given by the red ceircles. The experimental results are given by the black lines.

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calculated the bond angle deviation for different series. Our calculations show that Δθ decreases from 10.3 to 6.6° with increasing TS. The reduction of Δθ is an indication of improved short-range-order (SRO). The variation of ΓTO/2 shows that SRO increases with TS, whereas our IR results indicate a decrease in the concentration of Si−H and CH groupings with TS. Thus, a correlation between ΓTO/2 and the concentration of these polyhydride groups like Si−H and CH may be established. The presence of SiHn and CHn contribute to the degradation of SRO. It is found that with increasing TS from 200 to 500 °C the ΓTO shifts by 22 cm−1. The gradually narrowed width of the TO mode presents the more ordered structure of silicon grains. This is indicative of confinement effects in nanocrystalline silicon; that is, the Raman shift of optical phonon mode depends on the diameter of Si nanocrystallites. In our case, not only the existence of amorphous structure is verified but also the Raman scattering presents a small size effect. Finally, it is important to note that, on the basis of quantitative calculations of vibrational density of states for aSi,70 the decrease in the microvoids (SiH2)n size and concentration is accompanied by a decrease in the line width of TA-like bands toward low frequencies (Figure 9). The width

associated with the amorphous phase whereas the peak centered at 501 cm−1, for TS = 500 °C, is due to the presence of nanocrystalline Si within the film. This result confirms a transition of film microstructure from amorphous to mixed phase at TS ≥ 200 °C63 as has been reported for nanocrystalline silicon.64 The downshift of the peak position is a clear indication of the reduction in the size of the silicon crystallites and due to the localization of phonons at quasi-isolated silicon crystallites.65 Our results show that the formation of nanocrystalline silicon (nc-Si) occurs when the substrate temperature is higher than 200 °C, which is in good agreement with the results reported recently by Cheng et al.66 In addition, the presence of a broad Gaussian peak observed in series TS = 400 °C and TS = 500 °C, centered respectively at 483 and 501 cm−1, shows a relaxed structure. It is important to note that the size of the nanocrystallites is determined using the equation of confinement model.67 The confinement effects in reduced dimension systems such as Si nonocrystals lead to major modification in vibrational, electronic, and nonlinear properties. This is crucial for a better understanding of nanocrystallite properties for further development. The vibrational properties of the Si nonocrystals are sensitive not only to their dimensionality but also to the growth-related parameters and to the shape. The shapes of nanocrystallites affect the shift and the broadening of the Raman signal in different ways. Thus, changes in the line broadening of the optical phonon mode in the Raman spectra can be used as excellent measurement to determine the dimensionality of nanocrystallites. From the Raman shift, the average crystallite size is calculated by using the following formula:67 dRaman = 2π B /Δω

where Δω is the shift of the nanocrystalline Raman peak from the 520 cm−1 peak corresponding to the C−Si band and B = 2.0 cm−1 × nm2. The value of the Raman crystallite size dRaman remains constant for all series (≃2 nm), indicating the presence of small Si crystallites in an amorphous network. The crystallite size appears unaffected by the increase of TS, and therefore hydrogen dilution seems to be an important factor to determine the crystallite size. For our case, we conclude that the substrate temperature, TS, determines when the formation of the crystallites takes place and the dilution of hydrogen determines the size of crystallites. The large frequency downshift and the small crystalline particle size cannot be explained by phonon confinement alone. These observations are likely due to the dominance of crystalline structure for these series and/or homogeneous size distribution of Si nanocrystallites in the amorphous structure.68 It is well-known that the fwhm of the optic band Γ(TO)/2 is associated with the structural angular distortion in a-Si:H and is directly proportional to the mean bond angle aberration.69 Keep in mind that the increase in the short-range disorder is accompanied by an increase in the line width of the TO likeband. The mean dispersion of the Si−Si bonding angle Δθ is estimated from the fwhm of the TO peak at 480 cm−1 according to

Figure 9. Deconvolution procedure into Gaussian component of the Raman spectra of series TS = 200 °C, which shows the line width toward the low frequencies of the TA-like band.

of TA-like bands for all series was determined. The values for different widths are nearly twice those calculated in the case of a−Si:H films obtained by RF sputtering.71 This result shows the presence of polyhydride groups (SiH2)n and justifies the deconvolution of the IR spectra by taking into account the doublet (845 and 910 cm−1).

4. CONCLUSIONS In this paper, we present the preparation as well as spectroscopic studies of a-SiC:H thin films. Four series of aSiC:H were deposited by “RF” plasma-assisted magnetron sputtering by varying substrate temperature TS from 200 to 500 °C. Analysis of different results, obtained by FTIR and Raman spectroscopy techniques, shows that the substrate temperature affects the growth of a-SiC:H films. It is found that the degree of crystallinity increases with TS and reaches 43% at 500 °C. Our results show the presence of Si−Si and C−C clusters. By increasing TS, the decrease of polyhydrides groups SiHn and CHn enhances the formation of Si−C bond density and favors the formation of nanocritstalline silicon. The influence of TS on the growth of crystalline nanograins of different series was performed using Micro-Raman spectros-

ΓTO/2 = 7.5 + 3 × Δθ

where Γ(TO)/2 is expressed in cm−1 and Δθ in degrees. Raman results show that ΓTO decreases from 77 to 55 cm−1 for series of TS from 200 to 500 °C. This implies that the structural angular distortion decreases with TS. We have 21024



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would like to gratefully acknowledge the financial support from the Scientific Research Deanship. The authors also thank the anonymous referees for their careful reading of the manuscript and their fruitful comments and suggestions.

copy technique. The Raman spectra present two main broad bands localized respectively at 100−600 and 1100−1600 cm−1. The analyses of the results in these two regions allow us to reflect that in (100−600 cm−1) spectral region, the Raman spectrum of series TS = 200 °C and TS = 300 °C shows only a broad peak centered around 480 cm−1 characteristic of the predominance of the amorphous structure. The peaks centered respectively, at 483 and 501 cm−1, are attributed to the amorphous phase. The band at 501 cm−1 highlights the presence of nanocrystalline Si within the film. A transition of the film microstructure from amorphous to mixed phase at TS ≥ 200 °C is observed, and the downshift of the peak position shows a reduction in the size of the silicon crystallites. The formation of nanocrystalline silicon (nc-Si) occurs when the substrate temperature is higher than 200 °C. For the second spectral range lying between 1100 and 1600 cm−1, the shape of Raman spectra of series TS = 300, 400, and 500 °C has undergone a substantial change, showing that a structural rearrangement took place. For series TS = 300 °C, the presence of the symmetric peak centered at 1455 cm−1 corresponds to the band observed in amorphous carbon and gives rise to a partial crystallization of carbon clusters. The band centered at 773 cm−1, attributed to the presence of Si−C, confirms that a large amount of Si−C is present in all series, which is consistent with the FTIR results. For series TS = 400 °C and TS = 500 °C, the position of the Raman carbon peak is much lower than that observed for crystalline graphite. Raman spectroscopic results show also that when TS increases from 400 to 500 °C, the size of graphitic domains is reduced and their distribution becomes more disordered. The deconvolution of the band in the spectral spectral range of 500−1200 cm−1 is approved by quantitative calculations of vibrational density of states for a-Si, which justify the deconvolution of the IR spectra by taking into account the doublet (845 and 910 cm−1). Finally, several refinements may need to be made to the present experimental study. For example, this study may be assisted by accurate theoretical calculations such as solid state density functional technique (DFT) modeling.72,73 The interpretation of the Raman and IR spectra may be assisted by periodic DFT calculations for better understanding of nanocrystallite properties, particularly the changes in vibrational frequencies/intensities of different modes observed as function of substrate temperature. The structural features of the hydrogen bonds detected in the films (i.e., the donor−acceptor distance and the position of hydrogen) can be also elucidated by DFT. Furthermore, the present work can be extended to study the effect of the composition by varying the atomic fractional concentrations. The influence of these concentrations in determining the properties of the films will be studied using different techniques such as X-ray photoelectron spectroscopy, electron probe microanalysis and nuclear magnetic resonance spectroscopy.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1 780-492-8231. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the University of Hail, Kingdom of Saudi Arabia. The authors 21025



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Nano)Structure of a-SiC:H

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