Evolution of Si (and SiC) nanocrystal precipitation in SiC matrix

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Thin Solid Films 516 (2008) 3824 – 3830 www.elsevier.com/locate/tsf

Evolution of Si (and SiC) nanocrystal precipitation in SiC matrix Dengyuan Song ⁎, Eun-Chel Cho, Young-Hyun Cho, Gavin Conibeer, Yidan Huang, Shujuan Huang, Martin A. Green ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney NSW 2052, Australia Received 20 October 2006; received in revised form 30 April 2007; accepted 19 June 2007 Available online 26 June 2007

Abstract Si1−xCx films with varying ratio of carbon to silicon (C/Si) were fabricated by magnetron co-sputtering from a combined C and Si target. The composition in films was changed by adjusting the ratio of sputtered target's area between C and Si. Analysis of X-ray photoelectron spectroscopy for as-deposited films shows that C/Si atomic ratios of our films have ranges of 0.33–1.02. Thermal annealing of as-deposited films was carried out at various temperatures from 800 to 1100 °C in a conventional furnace. Fourier transform infrared spectra show a shift of Si–C stretching peak towards higher wavenumbers from ∼ 737 cm− 1 to ∼ 800 cm− 1 with increasing annealing temperature. From the results of Raman spectroscopy, X-ray diffraction and transmission electron microscopy, it was found that the dominant type of nanocrystals changes from Si to SiC in the films annealed at 1100 °C when the C/Si atomic ratio increases from 0.33 to 1.02. © 2007 Elsevier B.V. All rights reserved. Keywords: Sputtering; Silicon carbide; Annealing; Silicon nanocrystal; X-ray photoelectron spectroscopy; Raman spectroscopy; Infrared spectroscopy

1. Introduction Fabrication of silicon nanocrystals embedded in a dielectric matrix has attracted considerable interest in silicon optoelectronics [1–3] and in third-generation photovoltaics [4,5]. When silicon nanocrystals are made very small (b ∼ 7 nm in diameter), they behave as quantum dots (QDs) due to three-dimensional confinement of carriers [5]. Quantum confinement causes material's effective bandgap to increase. Carriers also can tunnel between dots to produce a quantum dot superlattice when these QDs are close enough together. For photovoltaic applications, such nanocrystal materials may allow the fabrication of higher bandgap solar cells that can be used as tandem cell elements on top of normal Si cells [4,5]. To date, considerable work has been done on the growth and characterization of Si nanocrystals embedded in oxide [6,7] and nitride [8,9] dielectric matrices. Compared to Si nanocrystals in oxide and nitride, recent modeling [10] shows that Si nanocrystals in SiC matrix are a promising material for all-silicon tandem solar cells because

⁎ Corresponding author. Tel.: +61 2 9385 4454; fax: +61 2 9662 4240. E-mail address: [email protected] (D. Song). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.150

carrier transport should be easier due to a lower barrier height between neighboring nanocrystals and hence high tunneling probability. However, little has been reported on the experimental properties of Si nanocrystals embedded in SiC matrix [11]. In this work we have fabricated Si1−xCx films with varying atomic ratio of the Si to C by using a magnetron co-sputtering from a combined Si and C target. Off-stoichiometric Si1−xCx is of interest as a precursor to realize Si nanocrystals in SiC matrix, because it is thermodynamically metastable when the composition fraction is with 0 b x b 0.5. Si nanocrystals are therefore able to precipitate during a post-deposition annealing. Raman spectroscopy and X-ray diffraction (XRD) measurement were used to investigate the evolution of Si and/or SiC nanocrystals in SiC matrix with increasing carbon content and annealing temperature. Furthermore, the samples were observed by highresolution transmission electron microscopy (HRTEM), providing direct evidence for the formation of nanocrystals in the films. On the basis of Raman, XRD and TEM results, we discuss three typical cases for annealed films with a low, intermediate and high carbon-to-silicon (C/Si) ratio, with the aim of analyzing the dependence of dominant type of nanocrystals (Si and/or SiC) on the film's composition and processing conditions.

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2. Experimental details

3. Results and discussion

The samples were deposited by radio-frequency (rf) magnetron sputtering in an argon plasma. Si-(100) wafer and quartz substrates were used simultaneously for different measurement purposes. Si wafer substrates were used for Fourier transform infrared (FTIR), TEM and XRD measurements. The quartz plates were used for Raman spectroscopy measurement. A carbon target (4 inch diameter, thickness 3 mm, purity 99.999% from Williams Ltd) was co-sputtered with a silicon target (4 inch wafer, thickness 0.5 mm, on top of a carbon target). The Si target had been pattered into a grid-like square open array, made by cutting the wafer with a Q-switched Nd:YAG laser scriber followed by a chemical etching in a NaOH solution at 80 °C to remove the residuals due to laser cutting and to smooth edges of the pattern. The C/Si ratio in films was controlled by changing the open area of the Si target. The vacuum chamber had a base pressure of less than 2.5 × 10− 4 Pa. Sputtering was carried out at a working gas pressure of 1.2 Pa in a pure argon gas with the rf power of 150 W. All the samples were not intentionally heated during the deposition process. Targets were pre-sputtered for 10 min to remove any contaminants on the surface. Film deposition time was 45 min and the thicknesses of films were in the range of 250–400 nm. As-deposited films were then annealed at various temperatures from 800 to 1100 °C in a conventional furnace in a nitrogen ambient. Details of annealing process are 800 °C for 25 min, 1000 °C for 12 min, and 1100 °C for 9 min. A determination of chemical composition was performed by X-ray photoelectron spectroscopy (XPS), which was carried out using a Fisons ESCALAB 220i-XL with a monochromatic Al Kα (1486.5 eV) X-ray source and a hemispherical energy analyzer. The working pressure was below 5 × 10− 7 Pa. X-ray source power is 10 kV × 12 mA, and the analysed area of samples is ∼ 0.3 mm2. FTIR absorption spectra were measured by a Nicolet 5700 spectrometer with resolution set at 4 cm− 1. Raman spectra were measured by a micro-Raman spectrometer (Renishaw, RM2000) in backscattering configuration, with a 50× optical microscope objective. The laser light comes from an Ar ion laser with a wavelength of 514.4 nm, and is applied to the air-side surface of the films. Before measurement, the system is calibrated with a single crystal Si wafer, which has a clear peak at around 520 cm− 1. The crystalline structure of the annealed samples was determined by grazing incidence X-ray diffraction (GI-XRD) using a Philips's X'Pert Pro materials research diffraction system at a voltage of 45 kV and a current of 40 mA, using Cu Kα radiation (λ = 1.540562 Å). The glancing angle between the incident X-ray and sample surface was 0.7°. As a rough estimate for the concentration of crystalline phases of Si and SiC in annealed films, a semi-quantitative analysis according to XRD data was performed by XRD pattern analysis software (X'Pert High Score, PANalytical Ltd [12]). In this tool, a method of quantitative analysis by XRD, referred to as normalized reference intensity ratio, was used [13]. Cross-sectional HRTEM observations were performed with a JEOL-3000F operated at 300 kV. The TEM samples were prepared by a standard mechanical thinning technique followed by Ar ion milling polishing at 3 KV.

3.1. Compositional characteristics

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XPS analysis of as-deposited films was performed by measuring C 1s and Si 2p spectra to determine the sample's chemical composition. Before measurement, Ar+ bombardment etching to a depth of ∼ 10 nm (etching rate ∼ 1.6 Å/s for 60 s) was used to remove surface contamination. Fig. 1 shows a typical widescan XPS spectrum of an as-deposited sample. From the magnified C 1s and Si 2p spectra in the inserts, the presence of Si–C bonds is clearly identified by the peaks C 1s centered at ∼ 283 eVand Si 2p centered at ∼ 100 eV [14]. Besides the expected carbon and silicon, oxygen and nitrogen were also detected in the XPS spectra. These impurities possibly were caused by the residual atmosphere gases in the chamber and/or residual surface contamination. Note that the argon peaks (Ar 2p and Ar 2s) in Fig. 1 are due to argon ions implanted during the argon beam bombardment etching, and not from the film itself. The Eclipse software package (VG Scientific) was used for quantitative data analysis. XPS spectra were fitted by Gaussian function and background was removed by Shirley subtraction method. The sensitivity factors for quantitative compositional calculation of carbon, oxygen, silicon, and nitrogen are 1.0, 2.93, 0.82 and 1.80, respectively. The accuracy of the XPS quantitative analysis, as derived for duplicate analysis, was less than ± 5%. Table 1 lists near-surface chemical composition for a set of samples with a different C/Si atomic ratio from Si-rich to near stoichiometric. It was found that film's chemical composition (C, Si) can well be controlled by changing the sputtered area ratio of C to Si target. 3.2. Structural characteristics 3.2.1. FTIR spectroscopy FTIR absorption spectra were measured for as-deposited and annealed films on silicon wafer substrate in the region of 400–2200 cm− 1 to investigate a bonding configuration of the

Fig. 1. Typical wide-scan XPS spectrum of one of the samples. The inserts show the magnified C 1s and Si 2p spectra.

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Table 1 Chemical composition for as-deposited SiC films used in this study as determined by XPS Sample

S1 S2 S3 S4

at.%

C/Si

C

Si

O

N

23.7 32.8 39.1 48.5

72.2 62.7 55.1 47.8

3.6 3.9 5.1 3.1

0.5 0.6 0.7 0.6

0.33 0.52 0.71 1.02

samples. Fig. 2(a) shows typical FTIR spectra as a function of annealing temperatures. The presented FTIR spectra are taken from sample S2 with C/Si ratio of 0.52, and similar results were found from other samples (S1, S3 and S4). As shown in Fig. 2 (a), the spectrum of as-deposited film consists of a broad band centered at ∼ 737 cm − 1 , and peak shape can be fitted by a Gaussian line. The Gaussian contribution is attributed to the disordered silicon-carbon bonds [15] in amorphous SiC. Clearly, upon increasing annealing temperature, a shift of peak position to higher wavenumber is observed. Finally, absorption peaks are located at ∼ 800 cm − 1 when the samples were annealed at above 1000 °C. This corresponds to the stretching mode of the Si–C bond in the SiC crystalline phase [16]. For films annealed at ≥ 1000 °C, the bands cannot be fitted only by a Gaussian distribution and is best fitted by a combination of a Lorentzian and a Gaussian. Lorentzian contribution is at about 50% for the film annealed at 1000 °C, and it increases to about 60% as the film annealed at 1100 °C. The shift of peak along with the change of band shape is due to the formation of more Si–C bonds from dangling C and Si bonds [17]. In addition, the weak peaks at ∼ 1070 cm− 1 were found for samples annealed at more than 1000 °C, corresponding to the Si–O stretching band in the films [18]. This is due to the films' surface oxide that formed during high temperature annealing. Furthermore, Fig. 2(b) shows the FTIR absorption spectra of the films deposited at varying C/Si ratio for other process parameters constant and annealed at 1100 °C. It can be seen that an increase of C/Si ratio results in a narrowing Si–C stretching peak at ∼ 800 cm− 1 . This phenomenon can be explained by a change in the film microstructure from an amorphous phase to a nanocrystalline phase, i.e., there is an improved fraction of crystallized SiC in the films when the C/Si ratio is raised [19]. 3.2.2. Raman, XRD and TEM To demonstrate an evolution of Si nanocrystals in SiC matrix with varying C/Si ratio and post-deposition annealing, Raman, XRD and TEM measurements were used to characterize the films in three typical cases: i) a low C/Si ratio of 0.33 (sample S1), ii) an intermediate C/Si ratio of 0.71 (sample S3), and iii) a high C/Si ratio of 1.02 (sample S4). Fig. 3(a) shows Raman spectra for the sample S1 grown on a quartz substrate as a function of annealing temperatures. The broadened Raman peaks at ∼ 470 cm− 1 in both as-deposited and 800 °C annealed samples are due to the transverse optical (TO) mode of amorphous silicon (a-Si) [20], with the absence of a peak around 520 cm− 1 indicating the absence of a Si crystalline phase in the films. However, a significant change is

observed in Raman spectra when annealing temperatures are at ≥ 1000 °C. An intense peak at around ∼ 508cm− 1 appears, which is assigned to the contribution from Si nanocrystals [21]. The frequency downshift of this Raman peak with respect to ∼ 520 cm− 1 of bulk silicon is considered to be caused by grain size related effects in small-grained nanostructure and compressive stress in the films [22,23]. The increase in peak intensity with annealing temperature from 1000 to 1100 °C shows that crystallized quality of Si nanocrystals benefits from raised temperature. In addition, we also observed that the weak broad peaks at ∼ 940 cm− 1 are developed when the samples were annealed above 1000 °C. Considering that the amorphous SiC vibrational density of states in Raman spectrum is up to ∼ 900 cm− 1 [24], as well as the maximum optical phonon energy of any of the crystalline polytypes of SiC is

Fig. 2. FTIR spectra of SiC films on silicon wafer substrate as a function: (a) annealing temperature, and (b) C/Si ratio. Annealing temperature and value of C/Si ratio are indicated on the related curves. The solid lines in plots are the measured curves, while the vertical dotted lines are drawn at wavenumber of 800 cm− 1 and are guides to the eye.

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concluded that Si nanocrystals are dominant in the sample S1 due to a low C/Si ratio. Additionally, the grain size g can be estimated using the Scherrer formula [27] g¼

kk Dð2hÞcosh

ð1Þ

where λ is the wavelength of the X-rays, θ is the Bragg diffraction angle at the peak position in degrees, and Δ(2θ) is the full width at half maximum (FWHM) in radian, and k is a correction factor. The value of k is usually chosen to be 0.9 for Si films [28]. The nanocrystal size was calculated by Eq. (1) from the different Si diffraction peaks. This gives the average size of 10 ± 0.5 nm. To obtain direct evidence for the shape and distribution of nanocrystals in amorphous matrix, HRTEM investigation was performed. Fig. 4 shows a cross-sectional HRTEM image of sample S1 annealed at 1100 °C. It was found that nanocrystals were embedded in amorphous matrix and lattice fringe spacing is equal to the spacing of the Si {111} planes, consistent with a dominant crystalline phase expected by the XRD data. The shape of most of nanocrystals appears close to spherical, and size obtained from HRTEM images is in the range of ∼ 3–7 nm. It should be noted that the nanocrystal size determined by TEM is slightly smaller than that determined above by XRD. A possible explanation for the deviation is due to a spatial nonuniformity of nanocrystal sizes. The probed sample region in cross-sectional HRTEM is typically very small (30 nm × 30 nm in Fig. 4), and thus TEM does not give the spatial information of nanocrystal size. XRD has a probed sample area of 10 mm × 20 mm and a penetration depth up to the film's thickness in our measurement, which is much bigger than in the case of TEM. Therefore, XRD

Fig. 3. Raman (a) and XRD (b) spectra of the sample S1. The major peaks are labeled and annealing temperature of films is indicated on the related curves.

972 cm− 1 [25], the Raman bands at ∼ 940 cm− 1 for our samples can be explained by changes in the SiC bonding states from amorphous to crystalline with increasing annealing temperature. This is supported by the XRD results (shown later) that show the presence of crystalline β-SiC phase in the annealed films. The Raman bands of ∼ 940 cm− 1 were also observed in Ref. [26], where they have been attributed to the longitudinal optical (LO) phonon band of microcrystalline SiC. A further structural investigation was performed by means of XRD. Fig. 3(b) shows XRD spectrum of sample S1 annealed at 1100 °C. It was found that a strong diffraction peak appeared at around 2θ = 28.4°, assigned to the Si {111} plane. Other relatively weak peaks correspond to the β-SiC {111}, Si {220}, Si {311} and Si {331} planes. XRD results are in agreement with our previous Raman measurement (see Fig. 3(a)). Semiquantitative analysis from XRD spectra gives that concentration of crystalline phase of Si and SiC in this film is around 76% and 24%, respectively. Combined with Raman and XRD data, it is

Fig. 4. Cross-sectional HRTEM image of the sample S1 annealed at 1100 °C. The several white doted circles indicate the location of nanocrystals. Lattice fringe spacing corresponds to Si {111} planes.

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peaks are observed in the range of low, medium and high frequency. At low frequency, the broad band between 250– 600 cm− 1 can be deconvoluted four peaks, which correspond to three a-Si bands at ∼ 310 cm− 1 (longitudinal acoustic mode), ∼ 410 cm− 1 (longitudinal optical mode) and ∼ 475 cm− 1 (TO mode) [30] as well as a nanocrystal band at ∼ 504 cm− 1. As frequency increases into the medium region, two broad bands centered at ∼ 740 and ∼ 940 cm− 1 are similar to β-SiC Raman scattering peaks at 796 cm− 1 for TO modes and 972 cm− 1 for LO modes [31]. A shift to lower frequency of both bands with respect to crystalline SiC is probably due to the small crystallites and presence of the amorphous SiC phase [26]. Because the intensity of peaks increases with annealing temperature, it indicates an increased fraction of SiC crystalline phase with a higher annealing temperature. At higher energy,

Fig. 5. Raman (a) and XRD (b) spectra of the sample S3. Annealing temperature of films is indicated on the related curves and the major peaks are labeled. The doted lines on the top of the plot (a) give three zones related to the different Si– Si, Si–C and C–C vibration modes.

should give a spatially averaged nanocrystal size. In addition, we cannot find lattice fringes corresponding to the spacing of the β-SiC {111} planes in the TEM image. It is possible that nanocrystalline SiC was formed in a very small size and/or very poor quality. Based on results above, evidence from Raman, XRD and TEM shows that, upon annealing above 1000 °C, Si atoms in Sirich SiC films can precipitate and form dominantly Si nanocrystals in the films. Fig. 5(a) shows Raman spectra from sample S3 on a quartz substrate, which has an intermediate C/Si ratio of 0.71 and was annealed at various temperatures. The dotted lines on the top of Fig. 5(a) indicate roughly three expected regions related to the different Si–Si, Si–C and C–C vibration modes [29]. We can note that there is an obvious difference between Raman spectra of samples S1 and S3. The different characteristics of Raman

Fig. 6. Raman (a) and XRD (b) spectra of the sample S4. Annealing temperature of films is indicated on the related curves and the major peaks are labeled. The doted lines on the top of the plot (a) give three zones related to the different Si– Si, Si–C and C–C vibration modes.

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Fig. 7. Cross-sectional HRTEM image of the sample S4 annealed at 1100 °C. The inset shows a magnified view of an ordered nanocrystal. The several white arrowheads indicate the location of nanocrystals. Lattice fringe spacing corresponds to β-SiC {111} planes.

the presence of the three bands (centered at near 1100 cm− 1, 1350–1400 cm− 1 and 1590 cm− 1 respectively) is caused by C– C vibrational mode. The peak centered at ∼ 1100 cm− 1 for annealed samples is attributed to small fullerenes having fused pentagons [29]. A broad band located at ∼1400 cm− 1 appears in the as-deposited sample, related to the amorphous carbon [32]. Moreover, the intensity of the C–C band at ∼ 1400 cm− 1 diminishes and develops into weak double humps centered at ∼ 1350 and ∼ 1590 cm− 1 after annealing above 1000 °C. It is known [33,34] that these bands are attributed to D band (∼ 1330 cm− 1) and G band (1580 cm− 1) of graphitic coordination attributed A1 and E2g2 modes of graphite carbon, respectively. Fig. 5(b) shows XRD spectra for the sample S3 annealed at 1100 °C. Two prominent peaks at 2θ = 28.3 and 35.9° were observed, which are due to diffraction from Si {111} and β-SiC {111} crystalline planes, respectively. The additional weak peaks of diffraction are primarily from Si {220}, Si {311}, βSiC {220} and β-SiC {311} planes. The concentration of crystalline phase of Si and SiC in sample S3 is ∼ 42% and ∼ 58%, respectively, estimated by semi-quantitative analysis of XRD [12]. Using the Scherrer equation [27], the average Si and SiC nanocrystal size was around 7.5 ± 0.5 nm and 5.1 ± 0.4 nm, respectively. Therefore, we conclude that nanocrystals of both Si and SiC are present in sample S3 because of the increased C/ Si ratio in the film. Fig. 6(a) shows Raman spectra taken from sample S4, which has a high C/Si ratio of 1.02 and was annealed at different temperatures. In contrast to the samples S1 and S3, due to the C/ Si ratio being close to stoichiometric, strong Raman scattering from stretching vibrational modes of Si–C and C–C bonds can be easily identified. In the medium region of the frequency, the

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two broad bands from Si–C vibrational modes are seen at ∼ 740 cm− 1 (TO band) and 940 cm− 1 (LO band) as in the sample S3. We note that there is a clear dependence of scattering intensity of these two peaks on annealing temperature. When the annealing temperature was raised, we observe an increase in intensity of both the TO and LO bands. This feature shows the configuration of as-deposited films changed into that of crystalline SiC during annealing. The higher value of the annealing temperature corresponds to an increased amount of the crystalline SiC phase formed in the annealed films. At high energy, the Raman spectra present similarities to those of sample S3, but there is a higher scattering intensity. The presence of a broad feature at ∼ 1400 cm− 1 in as-deposited films results from free carbon. This band develops into two bands at ∼ 1360 (D band) and 1590 cm− 1 (G band) after annealing above 1000 °C, indicating formation of amorphous graphitic carbon [33]. In addition, as mentioned above in the sample S3, we also observed a very weak peak centered at ∼ 1100 cm− 1 for annealed samples [29]. In the low energy range of b 600 cm− 1, broad peaks centered at ∼ 320, and 465 cm− 1 are associated with amorphous Si [35], while the peak observed at ∼ 510 cm− 1 arises from like-atom Si–Si bonds in amorphous SiC [24]. Gorman et al. [25] have attributed this large shift to higher energy of the Si–Si peak with respect to the typical Raman peak of amorphous Si (∼ 480 cm− 1) from coupling to the lighter carbon atoms. It was noted that such a peak of Si–Si bonds at ∼ 510 cm− 1 disappeared when the sample was annealed at 1100 °C. This may be because the density of like-atom Si–Si bonds was reduced significantly due to producing crystalline SiC [25]. Fig. 6(b) shows XRD pattern for sample S4 annealed at 1100 °C. The dominant peak in the plot is from β-SiC {111} and other SiC peaks include diffraction from β-SiC {220}and β-SiC {311} planes. Noted that a very weak peak at 2θ = 28.3° arises from the diffraction of Si {111} planes. Semi-quantitative calculation of XRD data revealed that the concentration of phase Si and SiC in this film is around 10% and 90%, respectively [12]. Scherrer equation [27] gives an estimated nanocrystal size of 7.3 ± 0.4 nm according to the FWHM of their diffraction peaks. Furthermore, Fig. 7 shows the cross-sectional HRTEM image of sample S4 annealed at 1100 °C and the inset shows a magnified view of an ordered nanocrystal. As shown in Fig. 7, crystalline phases are dispersed in an apparently amorphous matrix. A measurement of the lattice fringes in the HRTEM image revealed that they correspond to β-SiC {111} lattice planes. This is in agreement with the XRD results. In contrast to the sphere-like shape of Si nanocrystals in the sample S1 (see Fig. 4), SiC nanocrystals are anisotropically shaped and nonuniformly distributed in the matrix. Some nanocrystals join together to form an extended crystal. The dimensions of the lattice images are very irregular, and with obscure nanocrystal boundaries. This finding in the shape of SiC nanostructure crystallized from a near-stoichiometric precursor layer agrees with report in the literature [36]. The nanocrystal size from TEM images is in the range of 3–10 nm, which is roughly in agreement with the size from XRD. The difference of TEM nanocrystal shapes between the S1 and S4 may be attributed to

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different composition in their precursor layer. In the case of Sirich carbide as a precursor layer, nanocrystals were produced by means of precipitation of Si excess in matrix during annealing, and surface energy minimization favors the precipitation of Si into a spherical shape [5]. For the precursor of a nearstoichiometric composition, a mechanism of relatively large nanocrystals of random shapes seems to be favored. 4. Conclusions Si1−xCx films with varying ratio of Si to C were prepared by magnetron co-sputtering from a combined Si and C target. Precipitation of Si and SiC nanocrystal in the SiC matrix with varying C/Si ratio and post-deposition annealing process was investigated. XPS shows that films' chemical composition can be well controlled by changing the ratio of the area of C to Si in the target. The stretching mode of the Si–C bonds in FTIR spectra shows a shift towards higher wavenumbers from ∼ 737 cm− 1 to ∼ 800 cm− 1 with increasing annealing temperature due to the formation of more Si–C bonds from dangling C and Si bonds. An increase of carbon incorporation by increasing C/Si ratio results in a narrowing of the Si–C stretching peak, indicating that there is an improved fraction of crystallized SiC in the films when the C/Si ratio was raised. Nanocrystals of Si and/or SiC were produced in the annealed samples, and the dominant nanocrystals (Si and/or SiC) depend on the value of the C/Si ratio. By means of Raman, XRD and TEM measurements, three distinctively different cases were identified for the 1100 °C annealed samples. For the low C/Si ratio the dominant nanocrystals in the films correspond to Si. For the case of the intermediate C/Si ratio, nanocrystals of both SiC and Si are observed in the films. For the case of the high, near-stoichiometric C/Si ratio, the dominant nanocrystals in films correspond to SiC. Since the study of Si nanocrystals in SiC matrix is at an early stage, it should be recognized that size distribution of the Si nanocrystals is still wide although some of nanocrystals may act as QDs (b ∼ 7 nm in diameter). Further work is now underway to observe clear quantum confinement by optimizing size of Si nanocrystals in SiC matrix. Acknowledgments The authors thank the members of the Third Generation Group at the ARC photovoltaics Centre of Excellence for their contributions to this project. This work was supported by Stanford University's Global Climate and Energy Project (GCEP) as well as by the Australian Research Council (ARC) via its Centres of Excellence scheme. References [1] P.J. Walters, G.I. Bourianoff, H.A. Atwater, Nat. Matters 4 (2005) 143. [2] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440.

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