Multi-wavelength Raman investigation of sputtered a-C film nanostructure

Surface & Coatings Technology 200 (2006) 5427 – 5434 www.elsevier.com/locate/surfcoat

Multi-wavelength Raman investigation of sputtered a-C film nanostructure G. Messina, S. Santangelo * INFM, Dipartimento di Meccanica e Materiali, Facolta` di Ingegneria, Universita` ‘‘Mediterranea’’, Localita` Feo di Vito, 89060 Reggio Calabria, Italy Received 23 November 2004; accepted in revised form 8 July 2005 Available online 11 August 2005

Abstract Multi-wavelength Raman spectroscopy is employed to investigate the nanostructure of amorphous carbon (a-C) films, prepared by sputtering at 20 and 400 -C, and the structural modifications produced by thermal anneling at 400 and 800 -C. The results are discussed in the light of more recent assessments on resonant Raman spectroscopy in C-based materials. High-temperature depositions and thermal annealing promote development and/or clustering of sp2 phase, with film optical transparency reduction. In both the cases, nanoclusters of larger and more uniform dimensions are formed at higher temperatures. However, annealing process favours aromaticity, while high-temperature depositions oppositely augments distortions and promotes bond disorder. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 78.30.Ly; 63.22.+m Keywords: Carbon; Amorphous; Sputtering; Nanostructure; Raman scattering spectroscopy; Annealing

1. Introduction Raman spectroscopy is widely employed as a fast, nondestructive tool for the post-growth material diagnostics. The analysis, aimed at the investigation of the film structural and bonding properties, is commonly pursued using 514.5 nm excitation wavelength. In disordered C-based materials, the interest is usually centred on the bands arising from the vibrational modes of the C sp2 atoms, the well-known Dand G-bands. For visible excitation, the former, due to the A 1g breathing modes of ring-organised C-atoms, lies approximately at 1360 cm 1, while the latter, due to the bond stretching of all the pairs of (both ring- and chainorganised) C-atoms, is detected approximately at 1570 cm 1. However, owing to the resonant nature of Raman scattering from k-bonded C-atoms [1,2], there is a growing interest on multi-wavelength Raman spectroscopy (MWRS). In the last years, MWRS has been used to study the properties of both nanocrystalline and amorphous Cphases [1– 6]. Alike in traditional analysis at 514.5 nm, in the most of MWRS studies attention is often focussed * Corresponding author. Tel.: +39 0965 875305; fax: +39 0965 875201. E-mail address: [email protected] (S. Santangelo). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.07.004

exclusively on the D- and G-bands. Moreover, the analysis is generally limited to deduce essential information, so as, except for a few of cases [6], only some of the indicators attainable by spectra decomposition are really considered [7 –10]. In this paper, MWRS is used to study the effect of deposition and annealing temperatures on the structural properties of sputter-grown a-C films, topic whose investigation, in the past years, has been faced prevailingly by single-wavelength (488 or 514.5 nm) Raman spectroscopy [11– 20]. In this work, the preliminary results are presented relative to 20 and 400 -C deposition and to 400 and 800 -C annealing temperatures. Although interest is mainly focused onto the region dominated by D- and G-bands, in order to obtain complementary information about the film structure, a spectral range (from 200 to 3600 cm 1) wider than that usually considered [2,13 – 17] is here investigated. By decomposing the spectra measured at 457.9, 514.5 and 633 nm excitation wavelengths, a large set of descriptive parameters is derived and entirely analysed. From the discussion of these results in the light of current assessments on resonant Raman spectroscopy in C-based materials, an extremely accurate picture of the film nanostructure changes is achieved.

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2. Experimental details

3. Results and discussion

Thin a-C films are synthesised by graphite sputtering in Ar atmosphere. Depositions are performed, at room temperature (RT) and 400 -C, using a conventional 13.56 MHz diode system. The (99.999% purity, 20 cm in diameter) graphite target is placed at 20 mm from the grounded electrode holding the (100)-Si substrate. No external bias is used. During deposition, r.f. power, chamber pressure and Ar flow-rate are 300 W, 20 mTorr (2.7 Pa) and 80 sccm, respectively. Under these conditions, 300 nm thick films are grown. Post-deposition annealing is carried out on films deposited at RT. Samples are annealed for 1 h in high vacuum (base chamber pressure better than 10 6 Torr) at 400 and 800 -C. After thermal treatment, the specimens are allowed to cool at RT before exposition to the ambient. As-grown (series #1) and annealed (series #2) films are characterised by means of multi-wavelength Raman spectroscopy. Unpolarised spectra are measured in air at RT by using a (Jobin Yvon Ramanor U-1000) double monochromator equipped with an electrically cooled (Hamamatsu R943-02) photo-multiplier as a detector, and photon counting electronics. A (Coherent Innova 70) Ar+ laser operating at 514.5 and 457.9 nm (2.41 and 2.71 eV) and a (Melles – Griot) He – Ne laser operating at 633 nm (1.96 eV) are used as excitation sources. The S/N ratio is improved by recording multiple scans. The use of a power density of ¨ 20 W/mm2 at the film surface prevents sample damage during measurements. Gaussian bands, superimposed to a linear background, are used to reproduce the spectra. The band frequency position, width (FWHM) and intensity are chosen by a least-square best-fit method by utilising a commercially available spectroscopic analysis software package.

3.1. Indications coming out from traditional analysis at 514.5 nm The Raman spectra of the investigated a-C films, as excited by a traditional source operating at 514.5 nm, are shown in Fig. 1. The spectra of both the sample series are largely dominated by D- and G-bands. At lower frequencies, the fingerprint of the silicon substrate at 521.5 cm 1 is always clearly visible. The weak disorder-induced bands, typical of sputter deposited a-C films [21], instead, are better observable in spectra of as-grown samples. At higher frequencies, the broad asymmetrical feature of the D- and G-band second order is detected, together with the CO2 stretching line at 2330 cm 1, indicative of a slight film oxygen contamination. The main parameters obtained by the spectra fitting are shown in Fig. 2 as a function of deposition and annealing temperatures. As the synthesis temperature (Tsyn) increases, the G-band broadens and shifts upwards, while the D-band contrarily shifts towards lower frequencies. On the other hand, as frequently observed [10,13 –17,20], the increase of annealing temperature (Tann) produces a progressive upshift and shrinking of the G band. The D-band frequency position (x D), instead, after an initial increase, drastically drops. The D- to G-band intensity ratio (I D/I G) exhibits a quite typical trend [10,13 – 18,20]: it always raises as an effect of a temperature increase. Analogous variation is found for the average size of C sp2 clusters (L C), as estimated from the I D/I G ratio via the Ferrari – Robertson relationship [22]. Oppositely, the Si-line to G-band intensity ratio (I Si/I G), currently regarded as an optical transparency indicator [23], decreases with increasing temperature. Recently, it has been pointed out [22] that manifold factors participate in determining the shape of the Raman

(d) (c)

514.5 nm

Rel. intensity (arb.un.)

(b)

1200

(a)

1200

1200

1200

2400

2400

2400

2400

3600

3600

3600

3600

Raman shift (cm-1) Fig. 1. As-measured 514.5 nm Raman spectra of typical a-C films of series #1 (as-grown) and series #2 (annealed). Deposition temperature of as-grown films is 20 -C (a) and 400 -C (b). Annealing temperature of thermally treated films is 400 -C (c) and 800 -C (d).

G. Messina, S. Santangelo / Surface & Coatings Technology 200 (2006) 5427 – 5434

as grown

annealed

(d)

1584

1,8

annealing

ISi /IG

ωG (cm-1)

(a)

1,2

1568

1,2

ID /IG

γG (cm-1)

150

120

(e)

(b) 1392

1,1

1,5

LC (nm)

ωD (cm-1)

2,4

1376

(c) 1360

0

(f) 400

800 0

400

800

1,4

temperature (ºC) Fig. 2. Dependence on deposition and annealing temperatures of the main indicators derived from the quantitative analysis of the 514.5 nm spectra: (a) G-band frequency position x G and (b) width c G, (c) D-band frequency position x D, (d) Si-line to G-band intensity ratio I Si/I G, (e) D- to G-band intensity ratio I D/I G and (f) estimated average cluster size L C. The arrows indicate the modifications produced by the thermal treatment.

spectra of disordered and amorphous C-films: the ratio of tetrahedral- to trigonal-bonds, the clustering of the sp2 phase, the bond disorder and the organisation of C sp2 atoms in rings or chains. In particular, the D-band intensity is sensitive to the variations in the sp2 phase clusterisation degree. Instead, the G-band may downshift as an effect of the enhanced sp3/sp2 bonding fraction, the diminished clustering, and the increased bond-angle disorder, while it may upshift due to the presence of C sp2 atom chains. In the light of these assessments, the observed variation of the G-band frequency position (x G) might find explanation in more concomitant factors. Nevertheless, the I D/I G increase signals that both higher deposition temperature and thermal annealing result in a clustering enhancement. Such a structural modification is consistent with the indications coming out from the I Si/I G decrease. Since at a given photon energy, a higher absorption coefficient is measured in a-C than in ta-C films [24], the optical transparency diminishing reveals a development of the sp2 phase. Robertson has recently evidenced [25,26] that two diverse kinds of disorder affect a-C films: an homogeneous disorder, related with the bond-angle distortions (BAD), and an inhomogeneous disorder, related with the statistical distribution of sp2 clusters and chains of different shape

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and size. As the width (c G) of the G-band, in spectra recorded at fixed excitation wavelength, gives a measure of the homogeneous disorder, the variation observed suggests that the increase of Tsyn and Tann has opposite effects. Higher Tsyn result in a BAD enhancement, while, contrarily, treating films at increasing Tann progressively reduces homogeneous disorder. On the other hand, in accordance with current assessments [19,25], the average cluster size is found to increase with increasing temperature. Thus, larger and more distorted clusters are formed at higher Tsyn; larger, but increasingly ordered, clusters are formed at higher Tann. Actually, a recent study, employing visible Raman spectroscopy, evidences that the microstructure changes, produced in magnetron sputtered a-C films by deposition at different temperatures and annealing at various temperatures, have diverse nature [20]. Moreover, it has been shown that postdeposition annealing increases the size of sp2 clusters, embedded in the sp3 matrix, until the sp3 matrix disappears completely and the film transforms into nanocrystalline graphite [19]. Present results on treated samples agree well with this finding. The D-band downshift [21] observed at higher Tsyn is consistent with the c G indications. The annealing process at lower temperature produces an x D increase supporting [21] the evolution towards a more ordered film structure. A nonmonotonic x D dependence on Tann has been already observed in a-C films prepared by pulsed laser deposition [17] and by ion beam sputtering [16]. The x D drop occurring at Tann > 400 -C has been understood in terms of changes in bonding [16], due to the thermally induced conversion from sp3 to sp2 occurring at higher temperatures [15]. On the other hand, as well known [13 – 17], the graphitisation process of a-C and a-C:H, ta-C and ta-C:H starts above 300 -C in films with low sp3-content [13]. In particular, in hydrogen-free a-C films, the thermal stability has been clearly shown to decrease with decreasing sp3 bonding fraction [13]. Recently, the existence of a correlation has been evidenced between sp3 content and position of the Raman G-band in 514.5 nm spectra of C-based amorphous films [27]. On the basis of this empirical relationship, in samples synthesised at 20 -C, approximately the 25% of C-bonds should be tetrahedral. As discussed, instead, the remaining films could have even lower sp3 bonding fractions. Such sp3 contents would locate RT-grown films in the third stage of the amorphisation trajectory [22,26], while the remaining ones should belong to the second stage of the ordering trajectory [7,22]. Nonetheless, since, for visible excitation, clustering and ordering always raise the G-band position in both the stages [7], the x G variation here observed is consistent with the indications provided by the remaining spectra fitting parameters. In order to collect information further supporting or refuting above conclusions, the spectroscopic analysis is extended at different excitation wavelengths. The results obtained are presented and discussed below.

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(d)

Rel. intensity (arb.un.)

457.9 nm

(c) 1200

(b) (a)

1200

1200

1200

2400

2400

2400

2400

3600

3600

3600

3600

Raman shift (cm-1) Fig. 3. As-measured 547.9 nm Raman spectra of a-C films deposited at RT (a) and 400 -C (b), and deposited at RT and subsequently annealed at 400 -C (c) and 800 -C (d).

3.2. Complementary indications emerging from analysis at 457.9 and 633 nm

Excepting for the position in as-deposited films, for each series, the difference between various samples diminishes going towards shorter k exc. At 457.9 nm, the G-band widths of as-grown films converge to the common value of ¨ 140 cm 1, while x G and c G of annealed films reach ¨ 1590 cm 1 and ¨100 cm 1, respectively. In all the samples, increasing k exc results in a D-band downshift and in an optical transparency enhancement (Fig. 6). As a general trend, the differences in x D and I Si/I G values relative to the various samples progressively increase moving towards longer k exc. The D- to G-band intensity ratio, raising with increasing k exc (Fig. 7a), exhibits the behaviour peculiar of sputtered a-C, microcrystalline graphite and annealed taC:H [6]. In series #1 (#2), a more pronounced I D/I G increase is observed in films deposited at lower Tsyn (treated at higher Tann). In order to obtain additional easily readable information, the dispersion of the Raman indicators shown in Figs. 5– 7

Figs. 3 and 4 show the Raman spectra measured in the investigated a-C films using 457.9 and 633 nm excitation sources, respectively. As can be seen, at lower excitation energy (E exc), the intensity of Si-line is strongly enhanced in all the spectra. Contrarily, the broad second-order structure flattens, while the D-band gains intensity relative to the Gband. At higher E exc, the trend is reversed. The dependence on excitation wavelength (k exc) of the main parameters obtained by the spectra fitting is shown in Figs. 5– 7. In all the samples, the G-band is found to upshift and shrink moving towards shorter k exc (Fig. 5). In literature, similar x G and gG trends are reported for sputtered a-C and annealed ta-C:H [6,7]. At any wavelength, x G and c G values attained in films of series #1 are always sharply different from those measured in series #2.

(d)

633 nm

Rel. intensity (arb.un.)

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1200

2400

3600

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1200

1200

2400

Raman shift

(cm-1)

2400

2400

3600

3600

3600

Fig. 4. As-measured 633 nm Raman spectra of a-C films deposited at RT (a) and 400 -C (b), and deposited at RT and subsequently annealed at 400 -C (c) and 800 -C (d).

G. Messina, S. Santangelo / Surface & Coatings Technology 200 (2006) 5427 – 5434 as-grown 20ºC as-grown 400ºC annealed 400ºC annealed 800ºC

as-grown 20ºC as-grown 400ºC annealed 400ºC annealed 800ºC

1,8 1584

(a)

(a)

1,5

ID /IG

ωG (cm-1)

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1,2 1568 0,9 160 140 120

LC (nm)

γG (cm-1)

2,0

(b)

(b)

1,6 1,2

100 450

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450

excitation wavelength (nm)

1400

ωD (cm-1)

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1380 1360

(b) 6 4

annealed 4

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(a) 4 2

3 2

annealing

0 9

1

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6 3

2 0 3

(c)

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2

7

6

1

LC disp. (10-3)

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ID /IG disp. (10-3 nm-1)

as-grown 20ºC as-grown 400ºC annealed 400ºC annealed 800ºC

ISi /IG

600

ISi/IG disp. (10-2 nm-1)

is further calculated. The average variation rate of the Kth parameter ( P K ) is attained as the ratio DP K /Dk, with Dk = k max k min = 175.1 nm denoting the width of the k exc range explored and DP K = |P K (k max) P K (k min)| standing for the variation correspondingly undergone by P K . The results achieved are shown in Fig. 8 as a function of temperature. A Tsyn increase causes x G, c G and x D to

5 0

400

800 0

400

800

temperature (ºC)

2 0

550

Fig. 7. Dependence on excitation wavelength of (a) the D- to G-band intensity ratio I D/I G and (b) the estimated average cluster size L C.

ωD disp. (10-1 cm-1nm-1) γG disp. (10-2 cm-1nm-1) ωG disp. (10-2 cm-1nm-1)

Fig. 5. Dependence on excitation wavelength of the G-band frequency position x G (a) and width c G (b).

1340 8

500

excitation wavelength (nm)

450

500

550

600

650

excitation wavelength (nm) Fig. 6. Dependence on excitation wavelength of (a) the D-band frequency position x D and (b) the film optical transparency indicator I Si/I G.

Fig. 8. Dispersion of the main indicators derived from the quantitative analysis of the spectra as a function of deposition and annealing temperatures: (a) G-band frequency position x G and (b) width c G, (c) Dband frequency position x D, (d) Si-line to G-band intensity ratio I Si/I G and (e) D- to G-band intensity ratio I D/I G. The spread in the average size L C of the clusters probed is also shown (f ). The arrows indicate the modifications produced by the thermal treatment.

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become more dispersive. The same occurs for x D if Tann is raised. Upon thermal treatment, instead, the dispersion of x G diminishes, while the c G dispersion exhibits a nonmonotonic behaviour. Finally, in both the series of films, moving towards higher temperatures, I Si/I G and I D/I G become less dispersive. In monocrystalline graphite, the G-line does not disperse [3,28]. In spite of the loss of three-dimensional ordering, no x G dispersion is yet observed in nanocrystalline graphite and glassy carbon [25,28 – 31]. The G-band becomes dispersive in more disordered C-films. The existence of dispersion in these materials has been recently related with the presence of a range of sp2 configurations or clusters having different local band gaps and, correspondingly, diverse vibration frequencies [6]. Accordingly, the dispersion of the G-band has been understood in terms of a resonant selection of sp2 configurations: with increasing E exc, the configurations with wider k gap, vibrating at higher frequencies, are excited. In this frame, the x G dispersion gives a measure of the degree of disorder affecting the film structure [6,7]. On the light of these assessments, the data of Figs. 5a and 8a confirm the indications coming out from the 514.5 nm analysis. Increasing Tsyn causes the formation of more disordered sp2 configurations. On the contrary, thermal annealing reduces disorder. In accordance with results relative to annealed ta-C:H [6,7], the extent of disorder reduction is larger at higher Tann, where the dispersion drastically drops and the G-band position approaches the values peculiar of nanocrystalline graphite and glassy carbon. This fairly agrees with the long-time known crystallization to graphite of sp2 a-C above 600/700 -C [6,14]. In all the investigated films, the D-band frequency position (Fig. 6a) shows a dependence on wavelength analogous to that observed in microcrystalline graphite [2,6], in glassy carbon [25,32] and in ta-C:H [6] and polyparaphenilene [32] annealed at very high temperatures. It has been pointed out [6] that the D-band, showing the maximum dispersion in micro- and nanocrystalline graphite, behaves oppositely to the G-band, so as its dispersion gives a measure of the degree of aromaticity of the film structure. Thus, the trends of Fig. 8c are interpreted as the indication that a temperature enhancement always promotes the development of a ring-organised sp2 phase. Relative to this, significant indications emerge from the wavelength dependence of the I Si/I G ratio. The optical transparency enhancement, observed in all the samples, at longer k exc (Fig. 6b), is consistent with the absorption coefficient lowering occurring in C-films at lower photon energies [24]. The difference in absorption coefficients (a) measured in the various materials reduces with increasing photon energy [24]. This reflects onto I Si/I G ratios converging to a common value at shorter k exc. The decline in I Si/I G dispersion (with k exc), attained at higher temperatures (Fig. 8d), relies on the raising of the a variation rate (with photon energy), observed going from

ta-C to a-C, and is thus interpreted as due to the development of the clustered sp2 phase at disfavour of the transparent sp3 matrix. Thinking of the G-band width as a measure of the homogeneous disorder [25,26], a broader distribution of bond angles is found to pertain to as-grown than to annealed films at any wavelength (Fig. 5b). In general, the wider k gap sp2 configurations, excited at higher E exc, exhibit narrower distributions of bond angles. As increasing Tsyn results in increased c G dispersion (Fig. 8b), a wider and less uniform distribution of bond angles is achieved in films deposited at 400 -C. Conversely, as Tann increases, bond angles approach optimal value and their distribution becomes more uniform. This confirms that high-temperature deposition favours bond disorder, while thermal annealing oppositely promotes order. The I D/I G trend of Fig. 7a suggests that at shorter k exc the probability of probing ring-organised C sp2 atoms diminishes. On the other hand, according to the traditional analysis, the increase of temperature produces an sp2 phase development and/or a clustering enhancement (Section 3.1). Hence, the dispersion diminishing (Fig. 8e) is interpreted as the indication that, at higher temperatures, the organisation of C sp2 atoms in rings, within the trigonally bonded phase, becomes more uniform. In order to get more insight onto the film structural evolution, the average size of the clusters, probed at various excitation wavelengths, is estimated from the D- to G-band intensity ratio. The L C values, shown in Fig. 7b, are obtained by: (i) assuming that the Tuinstra – Koenig relationship [33], as recently modified by Matthews et al. [32] to account for the varying excitation wavelength, L C = C large(k) I I D/I G 1 (with C large(k) = 12.6 + 0.033k for L C and k expressed in nm), holds for large clusters (L C > 2 nm), while the Ferrari –Robertson relationship [22], L C = [I D/I G/C small(k)]1/2, applies to nanoclusters; (ii) taking into account that, with increasing E exc, the maximum of I D/I G vs. L C shifts to smaller L C [22]; (iii) imposing the continuity condition between the two empirical relationships in order to explicitly derive C small(k); and (iv) further requiring that, at 514.5 nm, the value (0.55) entering the Ferrari –Robertson relationship [22] is obtained for C small. The results show that, as expected [8], at longer k exc, increasingly larger clusters are probed. The widest range of cluster sizes is obtained in films deposited at RT. The spread in L C values reduces in films grown at 400 -C and in annealed samples. From results of Fig. 8f, it is argued that, at higher temperatures, rings, more regularly distributed within the sp2 phase, have more uniform dimensions. All these hints suggest the occurrence of a gradual conversion from a disordered amorphous sp2 carbon material to a more ordered graphitic amorphous and nanocrystalline graphite. Bearing in mind that graphite is the most thermodynamically stable carbon allotrope, present findings are understood as the result of the cooperative action of all the possible relaxation mechanisms (sp3 to sp2

G. Messina, S. Santangelo / Surface & Coatings Technology 200 (2006) 5427 – 5434 as-grown 20ºC as-grown 400ºC annealed 400ºC annealed 800ºC

2nd order int. (a.u.)

x104 10

5

2,1

2,4

2,7

excitation energy (eV) Fig. 9. Dependence of the second-order D- and G-band integrated intensity on excitation energy.

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(iii) on the other hand, thermal treatment favours aromaticity, while high-temperature depositions oppositely promotes bond disorder. Increasing annealing T results in a progressive reduction of homogeneous disorder: bond angles approach optimal value and their distribution becomes gradually more uniform. On the contrary, increasing deposition T augments distortions: wider and less uniform bond-angle distributions are attained. The described structural and bonding modification should reflect onto changes in hardness and in rigidity of the graphitic dense planes. This is the reason why we intend to extend the analysis to a larger set of samples and perform hardness and elasticity measurements, aiming at deducing exhaustive information from the comparative discussion of the results achieved from these and different material diagnostics techniques (such as IR and XPS).

thermal conversion, sp2 clustering and, eventually, clustering of graphite nanocrystallites [13]) leading to the C matrix stabilisation. Finally, for the sake of completeness, the dependence of the second-order D- and G-band integrated intensity on excitation energy is investigated (Fig. 9) and compared with that of graphite [3]. The trends found are consistent with the graphite-like nature of the samples under study.

Acknowledgements

4. Conclusions

References

The structural properties of a-C films, prepared by graphite sputtering in Ar atmosphere, are investigated by multi-wavelength Raman spectroscopy. The changes produced in the film nanostructures by the use of diverse deposition temperatures (20 and 400 -C) and by postgrowth annealing at 400 and 800 -C are evidenced by quantitatively analysing the spectral features detected in the range from 200 to 3600 cm 1. A large set of descriptive parameters is derived by fitting the spectra measured at 457.9, 514.5 and 633 nm excitation wavelengths. The dependence of such indicators on wavelength is interpreted in the light of current assessments on resonant Raman spectroscopy in C-based materials. An extremely accurate picture of the effect of deposition and annealing temperatures on the a-C film nanostructure is achieved. What emerges from the present study is:

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(i) first of all, a confirmation that both high-temperature depositions and thermal annealing promote development and clustering of sp2 phase, with consequent film optical transparency reduction. In addition, it is stated that: (ii) in both the cases, the cluster-size distribution of is ruled by temperature (T). Increasing T leads to the formation of nanoclusters having, on average, larger and more uniform dimensions;

We wish to thank Prof. A. Tagliaferro and Dr. G. Fanchini of Physics Department, Polytechnics of Torino, who kindly provided a-C films.

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