Determination of the sp3sp2 ratio in a-C:H films by infrared spectrometry analysis

Diamond and Related Materials 4 (1995) 1210-1215

Determination

of the sp3/sp2 ratio in a-C:H films by infrared spectrometry analysis

C. De Martin0

a, F. Demichelis b, A. Tagliaferro b

a Sorin Biomedica Cardio S.p.A., 13040 Saluggia (VC), Italy b Dip. Fisica, Politecnico di Torino, Corso Duca Abruzzi 24, I-10129 Torino, Italy Received 29 November 1994; accepted in final form 13 April 1995

Abstract In this paper, a method for the determination of the sp3/sp2 ratio for carbon atoms in a-C:H films is presented. First, the infrared analysis of the films is performed and the sp3/sp2 ratio for the hydrogenated carbon atoms is determined. From this value, and making different assumptions for the different types of a-C:H (graphite-, diamond- or polymer-like), the overall sp3/sp2 ratio is determined. The results obtained are in good agreement with NMR data for the types of a-C:H considered. Keywords: Hydrogenated

carbon; IR spectroscopy; sp3/sp2 determination

1. Introduction

Thin films of hydrogenated amorphous carbon (a-C:H) have attracted a great deal of interest in the last few years because of their properties. a-C:H, having good adhesion to substrate and high values of hardness and Young’s modulus, is commonly known as diamondlike hydrocarbon (DLHC or, sometimes, DLC) [1,2]. It has been argued [3] that the wide range of properties of a-C:H films is due to both the relative amount of H to C atoms and to the ratio of sp3 to sp2 coordinated carbon atoms. While the abundance of H and C (and of impurities, such as 0) can be readily found, for instance by Rutherford backscattering and nuclear reaction analysis, the value of the sp3/sp2 ratio and the degree of sp2 clustering are difficult to evaluate. As far as the sp3/sp2 ratio is concerned, several methods for determining its value have been proposed. However, many of these methods are rather sophisticated (EELS [4]), require a large amount of material and measuring time (NMR [S]) or give only relative results (optical methods [6,7]). IR spectrometry has been proposed [S] as a technique for determining the sp3/sp2 ratio for hydrogenated sites. Its main limitation [9] is that this method does not detect unhydrogenated sites and unbonded hydrogen, so that some other route is needed to estimate their concentration. A rule of thumb was recently proposed [ 101, in which the sp3/sp2 ratio determined from IR measurements is reduced by a factor of 3 to 4 and the total sp3/sp2 ratio is then estimated. 0925-9635/95/$09.500 1995Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00295-2

In this paper we propose a modification of this last method, which allows us to evaluate the total sp3/sp2 ratio. This method, as will be detailed in the following, is properly applied when the amount of unbonded hydrogen [9] is known, but it can also be used to estimate the total sp3/sp2 ratio using the total hydrogen content when information about unbonded hydrogen is lacking. The purpose of the present paper is to describe the above-quoted method and to check its validity for at least some types of a-C:H films having different properties and hydrogen contents. The comparison of the results obtained by the present method and those obtained by NMR shows that this method is reliable for most types of a-C:H and that it can be a useful tool for quickly obtaining a reliable estimate of the total sp3/sp2 ratio.

2. Experimental We have studied films deposited by various methods: glow discharge [ 11, dual ion beam sputtering [ 111 and sputter-assisted plasma CVD [ 121. We have verified that films having similar physical characteristics and grown by using different deposition methods give similar results. For this reason, we distinguish the films on the basis of their properties and not the deposition method used.

C. De Martin0 et al.lDiamond and Related Materials 4 (1995) 1210-1215

Films were deposited on optically polished crystalline silicon substrata for IR, hardness and hydrogen content measurements. For NMR measurements the films were deposited on an aluminium foil, which was chemically dissolved. For details about hydrogen content and NMR measurements see Refs. [S] and [ 131. The optical gaps of the samples were determined (by a procedure described in Ref. [ 141) from optical transmittance and reflectance measurements made by means of a Perkin-Elmer Lambda 9 UV-VIS-NIR spectrophotometer. For this analysis, films were deposited on fused silica substrata. Hardness measurements were made by means of a nanoindenter [ 93. IR transmission measurements were made in the range 2500-3500 cm-’ by using (in the transmission mode) a Perkin-Elmer FTIR 2000 spectrophotometer.

3. Basic assumptions As the range of physical properties of a-C:H films is wide, different assumptions have to be made for the different kinds of a-C:H (DLHC, graphite-like, polymerlike etc.). In order to avoid a one-to-one relationship between film properties and assumptions, we will group a-C:H films into three categories. Of course, some a-C:H films will not fit in any of these categories. For such films, ad hoc assumptions should be made. Here, we consider three types of a-C:H, to which most of the literature films can be assigned, as described below.

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where D is measured in gigapascals (by nanoindenter on thick films [9]) and H is the hydrogen atomic fraction. The factor F in Eq. (1) has been introduced as an ad hoc value in order to take into account the difference in hydrogen content between Ref. [ 51 samples and other samples having similar hardness, as in the case of our samples. The atomic fraction Ho of hydrogen for the sample of Ref. [S] is given by Ho=0.27+0.01D

(2)

Since the hardness is correlated with the unhydrogenated sp3 fraction, the F factor is chosen to be equal to the ratio F=-

1-H

l-Ho

(3)

Moreover, since the maximum hardness value reported in Ref. [S] is about 17 GPa of hardness, this procedure has not been used for higher values. In DLHC films the amount of unbonded hydrogen might be quite large [ 171, so that the results obtained assuming all hydrogen as bonded might be less precise. All the reasons quoted above, together with the broad shape of the IR signal in DLHC films, would suggest that less precise results should be expected for DLHC films. We can observe that, assuming (1 - H)F equals unity, the final results are very similar to those obtained properly using Eqs. (2) and (3). This fact, together with the limitations quoted above, favours the use of the simplified version of the method. In the following, then, we will quote the results obtained assuming (l-H)F=l inEq.(l).

3.1. Graphite-like a-C:H (GLHC) These films have medium Taut energy gap (< 1.8 eV), low hydrogen content (< 30 at.%) and low hardness (< 10 GPa). From NMR measurements it is known that all sp3-hybridized carbon atoms are hydrogenated (sp3;‘=0 [ 15,163). For this reason we assume sp3;‘=0.

These films have high hardness (> 15 GPa), low hydrogen content (< 30 at.%) and Taut gap in the range 1.0-1.5 eV. In a recent paper [S], the existence of a direct relationship between sp3;’ and hardness (D) values has been reported. So, for films having these characteristics, from the knowledge of hardness a value for sp3;’ can be inferred. From the quoted paper [S], we have extracted the following interpolating relationship:

x(1-H)F

These are hydrogen-rich (> 45 at.%), transparent (Taut gap > 2 eV), soft (hardness <<10 GPa) films. In this case, from Ref. [S], the following relationship can be extracted: sp3;0= 0.08( 1 - H)

3.2. Diamond-like a-C:H (DLHC)

sp3;’ = (0.031+ 1.315 x 10-2D-3.547

3.3. Polymer-like a-C:H (PLHC )

x 10-5D2) (I)

(4)

It should be noted that the assignments of some IR peaks are different from those used for DLHC film [8]. As far as sp’ hybridized carbon is concerned, as no C2H2 molecules are assumed to be present in the films, we have sptiH < sp’;‘. We assume that sp’;‘= spliH, although this is its minimum possible value. However, it can be observed that the value of spl;” normally does not exceed a few atomic percent, so that even neglecting the sp’;’ term, the precision of the result, limited by the above assumptions, will not be greatly affected. In the case of DLHC and GLHC no direct information can be obtained about more than 50% of H atoms. The

C. De Martino et al./Diamond and Related Materials 4 (1995) 1210-1215

1212

reliability of the assumptions made above is, then, a crucial point in determining the accuracy of the result. As far as PLHC is concerned, where H30.45, this is not a critical point.

value must be subtracted from the total amount before the present method is applied. From Eqs. (5) and (6), and remembering that C + N = 1, one can show that the amount of sp3-hybridized hydrogenated carbon atoms is given by: 1 + 2D,,

1+

Tl2 + G2

IR13

Ib(1

4. Analysis of the IR spectra The IR absorption peaks have a shape which can be properly modelled by a mixing of gaussian and lorentzian shapes. For the sake of simplicity, gaussian shapes were used in the deconvolution of the measured spectra. As reported in Ref. [8], each of the CH absorption peaks appearing in the region 2700-3400 cm-’ can be assigned to different vibrating groups. We refer here to the peak assignment made in Refs. [ 81 and [ 141. However, because of the amorphous nature of the material, a shift in the peak position up to 10 cm-’ can occur. Attention has to be paid to this fact when peak assignment is made. Moreover, in the region 2900-2980 cm- ’ many different vibration modes occur. Care has to be taken, especially in the analysis of the 2910-2960 cm-’ region. On the one hand, the existence of an sp2 CH mode around 2950 cm-’ [17] must be carefully checked, since it can greatly affect the precision of the results. On the other hand, the peak at 2920 cm-’ can be assigned either to sp3 CH or sp3 CH2 [S]. However, the sp3 CH2 peak is paired with the 2850 cm-’ one. This means that the absence or the presence of this last peak can help in the attribution of the 2920 cm-’ peak. In the following, we will briefly recall the IR method proposed by Dischler et al. [ 81. The relationships used to determine sp3/sp2 and sp’/sp3 ratios are (for symbols refer to Appendix A): IR,,=

3;H C3H + C3H2 + C3H3 $= C2H + C2H2

1;" ClH sP IR1’ = sp3;HC3H + C3H2 + C3H3

(5)

(6)

We wish to point out that this method is based upon the assumption that the bond strength is the same for all vibrating groups, which might not be the case if local surroundings affect bond energy and strength. To proceed further, knowledge of the hydrogen content (atomic fraction of hydrogen H; see Appendix A) of the film is needed. It is clear that only the hydrogen atoms bonded to carbon atoms contribute to the IR signal. However, a certain number of unbonded hydrogen atoms might be present [14]. If information on the amount of unbonded hydrogen is available, this

+

021)

>

2+T2+3&2

(7)

From this value, one easily obtains sP

2;. _ sp3;H -z

(8)

and finally: sP

1;”= Sp3;“IR,3

(9)

Once these values are known one obtains the total amount of unhydrogenated carbon sites: sp3;o+ spz;o + spl;o =(I

-I$)-(sp3;"

+spZ"

+spy

(10)

No more information can be extracted from the analysis of the IR spectra in the range 2700-3400 cm-r and from the knowledge of the hydrogen content. Further assumptions are needed in order to determine separately the values of sp3;‘, sp2;’ and sp’;‘. These assumptions are dealt with in the previous section.

5. Results We have applied the above method to the different types of films. For each type we have analysed several samples, deposited in different ways. However, we have reported only one sample (except DLHC, see later) for each type in order to avoid heavily reading tables. This was possible because all samples analysed are properly represented by those reported in Table 1. In order to test the method, we have compared the results we obtained with the results obtained by applying the rule of thumb of Ref. [lo]. The lack of information about IR spectra and NMR analysis (which both have to be known for each test sample) has not allowed us to make a more extended use of literature data, since our purpose was to test the present method against a reliable one.

5.1. GLHC A typical IR spectrum, with the deconvoluted peaks, is shown in Fig. 1. We can observe that the single vibration modes contribute with narrow gaussian peaks, so that in the shape of the total curve the different contribution can be easily identified. The sp3/sp2 ratio

C. De Martino et ul./Diumond and Retated Materials 4 (I 995) 12IO-1215

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Table I Material properties Type

Hardness (Gpa)

E, (ev)

H (atomic fraction)

sp3/spz (H bonded)

spS/spZ b

sp3/sp2 (NMR)

sps b

sP3 (NMR)

spj c

GLHC DLHC DLHC PLHC Ref. [9]

5.1 15.0 16.4 3.6 -

1.45 1.00 1.18 2.75

0.28 0.26 0.28 0.64 0.37

5.02 5.41 10.98 3.30 18

0.23 0.74 0.95 2.50 0.47

0.19 0.95 1.10 2.30 0.67

0.13 0.32 0.35 0.26 0.20

0.11 0.36 0.38 0.25 0.25

0.11 0.43 0.53 0.16 0.54

a Evaluated using the method reported in Ref. [S]. b Evaluated using the method reported in the present paper. ’ Evaluated using the method reported in Ref. [lo].

200

80

150

100

50

0 2750

0

2850

2950

w

3050

3150

(cm-‘)

2700

2850 0

3000

3150

(cm-i>

Fig. 1. IR spectrum and deconvoluted peaks of a graphite-like a-C : H (GLHC) film.

Fig. 2. IR spectrum and deconvoluted peaks of a diamond-like a-C : H (DLHC) film.

for hydrogenated sites turns out to be 5.02, while the total ratio (measured by NMR) was equal to 0.19 [S]. Applying the above-quoted procedure, knowing the hydrogen content to be 28 at.%, we obtain a sp3/sp2 total ratio of 0.23, which is very close to the NMR value. The precision of the method can be better appreciated by looking at the total sp3 amount (see Table 1). Similar results were obtained for other films of GLHC.

samples are given in Table 1. These values must be compared with the value obtained by NMR on samples deposited using similar conditions [16]: see Table 1. The difference (around 10% underestimation of sp3 content) between the values obtained by the present method and by the NMR one has been observed for most of the DLHC films having hydrogen content around 25 at.% and hardness about 15 GPa. In our view, it can be traced back to the following reasons: l The values obtained by Eq. (4) are affected by an error of wO.O5(1 -H) [S]. l The results of Ref. [S] were obtained for films having more than 30 at.% of hydrogen, though the hardness was about the same order of magnitude. l The presence of unbonded hydrogen was not considered. l The very smooth shape of the IR peak (see Fig. 2) makes it difficult to be sure that the contributions have been properly identified. A more detailed study, similar to that performed in

5.2. DLHC A typical spectrum of a DLHC film is shown in Fig. 2. It can be observed that the peaks have high values for the linewidth. This indicates that a high level of disorder is present in the films. As a consequence, an overall smooth line arises. This requires a more careful treatment of data in order to identify all peaks, and still represents a source of error. The results of the above analysis for two DLHC

C. De Martino et aLlDiamond and Related Materials 4 (1995) 1210-1215

1214

obtained, while the method of Ref. [lo] gives a number quite far from the real one.

6. Conclusions

0

(cm-‘)

Fig. 3. IR spectrum and deconvoluted peaks of a polymer-like a-C : H (PLHC) film.

Ref. [S], is needed in order to get a new version of Eq. (7), more suitable for poorly (< 30 at.%) hydrogenated films.

5.3. PLHC Much higher peaks are observed, as a consequence of the large H content, in this material (see Fig. 3). The values of the linewidth are lower than in the case of DLHC, as a consequence of the floppy nature of this material. A sample deposited using similar conditions was measured using the NMR technique [ 161: the value obtained for sp3/sp2 was 2.30. This value is quite close to that obtained by the present analysis (2.50). The fairly good agreement between the two techniques can be seen looking at the data reported in Table 1. In the last column of Table 1, the results obtained applying the method of Ref. [lo] are reported. It can be observed that the results obtained by that method are similar to those reported in Ref. [lo]. It is clear that our method, although more complicated to use, gives more accurate results. In general, our method gives results closer to those obtained by NMR for all samples we have tested, and we expect this to be true for all samples, especially for GLHC, PLHC and DLHC having more than 30 at.% hydrogen. Moreover, we have performed the deconvolution of the IR spectrum reported in Ref. [9] (Fig. 1). Having no information on the hardness of the film, we have assumed, as stated in Ref. [9], a value spXo=O. The results of the analysis are reported in the last line of Table 1. It can be seen that a good approximation is

We have presented a method which allows us to estimate the sp3/sp2 ratio in a-C:H films in short times using an easy to use technique such as IR analysis. To apply this procedure, knowledge of the IR spectra in the 2700-3400 cm-’ range, of the hydrogen abundance, of the hardness value and of the Taut gap is needed. This last quantity is only needed in order to classify a-C:H (GLHC, DLHC or PLHC), so that one can choose the right assumption to be used. The present method is properly used when the amount of bonded hydrogen is known, but it can be used to get an estimate of the sp3/sp2 ratio when only the total hydrogen content is known. It must be observed that the results obtained when the hydrogen content is below about 20 at.% are not reliable. This is readily understood, since IR analysis gives information only about hydrogenated carbon atoms. This means that no information is obtained about more than 60 at.% of the material, and in such a case the results depend strongly upon the assumption made. We have applied the method to different kinds of a-C:H and shown that it can give fairly good results, better than previously reported methods based upon IR analysis. The assumptions made for each a-C:H type considered, although reasonable (as shown above), are quite general. Since a-C:H films can have a wide range of properties, it may happen that some of them could not be conveniently classified as above. In such a case, the method has to be tailored by making different (although justified) assumptions about unhydrogenated sites. Among the different types of a-C:H considered, the results obtained for DLHC films have turned out to be less precise (- 10% underestimation of sp3 content). This is due to the fact that the more critical assumptions were made about DLHC films (see Section 5).

Acknowledgements

We wish to acknowledge W. Gissler and J. Haupt (JRC, Ispra, Italy) for performing hardness measurements and preparing the films deposited by dual ion beam sputtering. We also wish to acknowledge A. Hammerschmidt (Siemens, Erlangen, Germany) for kindly providing the samples deposited by glow discharge and the results of NMR measurements.

C. De Martin0 et al./Diamond and Related Materials 4 (1995) 1210-1215

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Appendix A In the test the following abbreviations have been used: sp3C-H sp3C-H, sp3C-H, spZC-H sp2C-H, sp’C-H

C3H C3H2 C3H3 C2H C2H2 CIH

Intensity Intensity Intensity Intensity Intensity Intensity

T32 T12 T31 D21

Ratio Ratio Ratio Ratio

c

Abundance of carbon Abundance of hydrogen Total amount of hydrogenated sp3 carbon sites Total amount of unhydrogenated sp3 carbon sites Total amount of hydrogenated sp’ carbon sites Total amount of unhydrogenated sp2 carbon sites Total amount of hydrogenated sp’ carbon sites Total amount of unhydrogenated sp’ carbon sites

H sp3;H

sp3;o sp2;” sp2;o 1;” sP sp’;O

1;t= sp’;O+ spl;H sP spZt = sp2” + spZ;H sp3;1= sp3;o + sp3;H

of IR peaks of IR peaks of IR peaks of IR peaks of TR peaks of IR peaks

due due due due due due

to to to to to to

Group Group Group Group Group Group

vibration vibration vibration vibration vibration vibration

of sp3CH, to sp3CH, sites of sp3CH to sp3CH, sites of sp%H, to sp3CH sites of sp2CH, to spZCH sites (Atomic (Atomic (Atomic (Atomic (Atomic (Atomic (Atomic (Atomic

fraction) fraction) fraction) fraction) fraction) fraction) fraction) fraction)

Total amount of sp’ hybrid. carbon atoms Total amount of sp2 hybrid. carbon atoms Total amount of sp3 hybrid. carbon atoms

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