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Amorphous deuterated carbon films irradiated by swift heavy ions: Infrared measurements and ion beam analysis
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Nuclear Instruments and Methods in Physics Research B 131 (1997) 135-140
Amorphous deuterated carbon films irradiated by swift heavy ions: Infrared measurements and ion beam analysis F. Pawlak a3*, Ch. Dufour b, A. Laurent ‘, E. Paumier ayb,J. Perriere ‘, J.P. Stoquert d, M. Toulemonde a ’ CIRIL, CEA-CNRS, B.P. 5133, F-14070 Caen Cedex 5, France h LERMAT, URA CNRS 1317. ISMRa, Bouleoard Mar&ha1 Juin, F-14050 Cam Cedex, France ’ GPS, URA 17. Universitks Paris VII et Paris VI, Tour 23,2 Place Jussieu, 75251 Paris Cedex 05, France d Groupe PHASE, CRN, 67037 Strasbourg Cedex 2, France
Abstract Mixed hydrogenated and deuterated amorphous carbon films have been irradiated at GANIL in the MeV/amu energy range with an electronic stopping power varying between 1 keV nm- ’ and 13 keV nm- ’ These films roughly contain 10% of hydrogen and 30% of deuterium. Carbon (C), hydrogen (H) and deuterium (D) contents were determined by Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA). The evolution of C-H and C-D bondings contents was determined by infrared absorption measurements. The main effects due to MeV amu-’ ion irradiations are the decrease of C-D bondings content and deuterium relative concentration (D/C atomic ratio) as a function of fluence. A long time after irradiation C-H bondings content and hydrogen relative concentration (H/C atomic ratio) increase. The hydrogen absorption cross section is equal to the deuterium effusion cross section within the experimental errors whatever is the physical characterisation (ion beam analysis or infrared absorption). Moreover, both techniques give the same cross sections. Hence the following interpretation is proposed: during irradiation hydrogen and deuterium atoms are ejected from the ion tracks and afterwards are replaced by hydrogen atoms coming from the ambient air and diffusing inside the irradiated material along the latent tracks.
1. Introduction During several years the interaction between swift heavy ions (several MeV amu-’ > and the matter has been the subject of many studies [I]. It appears that the electronic slowing down characterised by the electronic stopping power, S,, governs ion damage. Two models have been proposed, the thermal spike model [2] and the ionic spike one [3]. In each of
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author.
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them the electron mobility plays a significant role: lower is the electron mobility, higher is the sensitivity of the irradiated material. In this point of view, carbonaceous compounds are very interesting materials. The electrical conductivity may vary from lo- ” R-l cm-’ for diamond to IO3 a-’ cm -I for graphite coming back to lo- ’ 6 R- ’ cm ’ for polyethylene. Amorphous hydrogenated carbon are intermediate materials with conductivity varying between lo-l6 R-’ cm-’ and 10e7 R-’ cm-’ [4]. Hydrogenated amorphous carbon (a-C:H) films have been the subject of a great interest due to their singular
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl68-583X(97)00299-1
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ings creation [ 141 and increase of resistivity [ 191 were observed. The present paper reports the results obtained in soft amorphous deuterated carbon films irradiated with swift heavy ions over a wide range of electronic stopping power, between 1 and 13 keV nm-‘, with a beam energy ranging between 2 and 10 MeV amu ‘. Infrared measurements were used to study the C-D and C-H bondings evolution as function of fluence and time after irradiations. Ion beam analyses were performed to study the evolution of the C, D, and H atomic area1 densities as function of fluence.
properties including extreme hardness, infrared transparency, chemical inertness and electrical resistivity [5]. The hydrogen concentration (up to 50%) and the ratio of single carbon bondings (sp3 coordination) to double carbon bondings (sp’ coordination) depend on deposition parameters and govern all the properties [6-81. Hydrogen stabilises dangling bonds of sp3 carbon [4] and increases the resistivity and the optical gap [9,10]. The higher the sp’/sp’ ratio, the higher the resistivity [7]. To sum up, the domain of properties of a-C:H films is bounded by those of diamond, graphite and polymers. The soft, low density (K 1 g cm -3> and highly hydrogenated films (> 35%) are called “polymerlike” whereas the hard, high density (> 1.6 g cmd3> and poorly hydrogenated films (< 35%) are called “diamond-like”. Whatever the ion energy, keV or MeV, and the material, a-C:H or polymers, the main effects of irradiation are the loss of hydrogen [ 1 l-131, the decrease of the optical gap [lo], the breaking of carbon-hydrogen bondings (C-H) [ 14,151 and the decrease of sp3/sp’ ratio [ 1 1,15,16]. Characterisations of released particles during irradiation have shown a molecular effusion (mainly H,) [17]. Up to now, studies about polymer irradiations put the stress on the modifications of chemical groups versus the fluence depending on the electronic stopping power [15,18]. Post irradiation effects have been the subject of several studies in a-C:H films where cetone bond-
2. Experiment
description
Amorphous deuterated and hydrogenated carbon films, called here after a-C:H,D,, have been deposited on n-type silicon substrates at - 35 V bias voltage by decomposition of CD, gas containing 10% of CH, in a dc multipolar plasma [20]. The density deduced from ion beam analysis and high resolution electron microscopy observation (film thickness varies between 50 and 100 nm> is 1.0 + 0.1 g cm-?. The optical index is 1.59 [20]. Table 1 gives all the experimental conditions and all the irradiation parameters. Ion irradiations (S, Ar, Nb, Sn, Pb and U> were performed at GANIL (Grand Accelerateur
Table I Irradiation parameters (ion beam, its energy, electronic stopping power Se, flux @ and fluence @,) and initial samples characteristics. For each S, value. all the samples were cut off from the same film. The samples composition was determined by RBS (carbon content) and ERDA (hydrogen and deuterium contents). The electronic stopping power St was calculated with TRIM 91 [21] assuming a density of 1 g cm-’ and a composition ofCD,, ,,,, Ion
lhs15+
JIIArlh+
‘WNb’.‘+
I I?Sn47+
zlapb’J+ /I*“‘S+
Energy (MeV/amu)
S, (keV nm-
10.1 5.3 9.2 5.5 I .8 6.4 1.2 2.2 2.6 2.6 2.0
I .o 1.5 I .3 I .7 2.5 5.5 6.7 7.9 12.3 12.3 13.0
’)
@(ion scm-‘) 2x 2x 6x 6x 6X 3.5 2.5 2.5 2x 2x 2.2
IO” IO” 10” IO’ IO* x IOX x IO” x lox IOX 10” x IOX
’
Q,,,,,$,
CH,D,
Carbon area1 density (10” C cm-‘)
Thickness (nm)
CH,,,,D,,,, CH o I ED,,.,,,
2.8 2.4 2.4 2.4 2.4 4.6 3.2 3.2 3.5 2.3 4.4
60 50 50 50 50 100 70 70 75 50 100
(ioncm-‘)
4.1 2.0 I.2 1.2 I.2 1.8 2.5 2.5 5x 5x 5x
x lOI x 10”’ x IO’? x IO’? x IO’? x IO” X IO” X IO” IO’? IO’? 10”
CH,,.I9“,,A, CH,,19D,,,,, CH 019 D 0.u CH o,?D,,,, CH,,,, D 0 Jb CH o I7I’,, 46
CH,,,,,,D,,,, CH o ,,D,,s CH ,,.I? D 011
137
F. Pawlak et al./ Nucl. Instr. and Meth. in Phys. Res. B 131 (1997) 135-140
d’Ions Lourds at Caen, France) on SME line (Sortie Moyenne Energie) with normal incidence beam. Fourier Transform Infrared spectrometry measurements were performed in transmission geometry using the Nicolet FTIR spectrometer (Magna IR, 750), with incident p-polarised light incoming at 0, = 55” with respect to the normal to the sample. This technique allows the absorption measurements of the following stretching bands: C-H (between 3100 and 2800 cm-’ ) and C-D (between 2300 and 2000 cm- ’ ) [22]. The parameter Y (cm) is defined as the absorbance band area A, (cm-’ ) normalised by the carbon area1 density N, (C cm--*) to take into account the thickness variation of films. We obtain the following relation [23]: NGTW,
y+-_ c
2.3N,
deuterium effusion was less than 10% of the total content. The surface hydrogen adsorption and the amorphous carbon film thickness were determined by a systematic comparison with a pure silicon wafer. The hydrogen surface content was less than lOi H/cm* as compared to a mean value of 4 X lOI H/cm2 for the non- irradiated amorphous carbon films which contain the least hydrogen content. In the case of deuterium, it is always negligible. Moreover, the ERDA spectra on thick samples show a decrease (increase) of the deuterium (hydrogen) plateau. All the ion beam analyses were performed at least 10 weeks after irradiation when the hydrogen absorption is stabilised.
3. Experimental
results
’
where N (cm-‘) is the bonding concentration, r (cm*) the average absorption strength, S (cm) the optical length and wa the central wavenumber of the band (2920 and 2150 cm- ’ for the stretching vibrations C-H and C-D respectively). Infrared analyses were performed before and after irradiation. Hydrogen absorption was followed versus time after irradiation and so the hydrogen content was seen to be stabilised 8 weeks after irradiation. Thus, all the results presented here correspond to infrared measurements made at least 10 weeks after irradiation. Within the experimental errors, deuterium content does not depend on time after irradiation. A 2.9 MeV 4HeC ion beam was used to perform Rutherford Backscattering spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) to determine carbon, hydrogen and deuterium area1 densities. Hydrogen and deuterium contents were obtained using the computer code developed by J.P. Stoquert. Absolute value of this hydrogen content was confirmed by the analysis of the hydrogen content in a polyethylene film: hydrogen effusion was followed versus helium fluence and the hydrogen content was determined by extrapolation to zero fluence. The same attention was used to measure H and D contents in the amorphous carbon films: in the present analysis conditions (a total charge of 10e6 Cb with a beam intensity of 2 nA at a grazing angle leading to a surface beam spot of 0.18 cm2) the hydrogen and
In this part, the results of the niobium irradiation are discussed, and then the results obtained by the same method with the other irradiations (S, Ar, Sn, Pb and U) are summarised. RBS analysis (Fig. 1) shows a decrease of carbon area1 density N,. The decrease of the D/C ratio and the increase of the H/C ratio as function of fluence are observed in Fig, 2. From Fig. 2, the asymptotic atomic ratios ((H/C),,=, and (D/C),,=,) at infinite fluence (@t = m) are reported in Table 2. It may be seen that these asymptotic values are independent of the ion nature and electronic stopping power. Thus, when the fluence was not high enough to determine these asymptotic ratios a mean value deduced from the measured ones has been used and noted by an asterisk in Table 2. From Fig. 2 the
“p------------l
01 0.0
0.5
1.0 (IO
”
1.5
20
Nb cm-‘)
Fig. 1. Carbon areal density evolution of a-C:D fluence during Nb irradiation at 5.5 keV nm- ’
II. ION IRRRADIATION
films versus
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138
et al./Nucl.
Instr.
and Meth.
I (IO l3 Nb cm-*)
Fig. 2. Evolution of the atomic ratios (D/C) and (H/C) obtained by RBS and ERDA measurements, versus fluence in an a-C:D film irradiated with Nb ions with electronic stopprng power of 5.5 keV nm-’ (measurements were made 10 weeks after irradiation).
cross sections are extracted lowing exponential laws: (H/C)
(Table 2) using the fol-
Q, - (H/C),,=,
(H/C),,_,
(19971
135-140
(Fig. 3a) and C-D (Fig. 3b) stretching bands, yC H and y,_, respectively. Fig. 4 shows the increase of Yc _ u and the decrease of Yc u defined in Section 2 versus fluence. The asymptotic values of Y~,~,),,, x and Yu_mQr=X estimated at infinite fluence (@I = m> are reported in Table 2. The same procedure as above is used when the fluence was not high enough to determine these asymptotic values and a mean value deduced from the measured ones has been used and noted by an asterisk in Table 2. Using the same previous exponential laws we may define the cross sections related to C-H and C-D bondings evolutions which are equal to a(, c-u) = 2.5 _t 0.5 X 1O-‘3 cm2 and u (Y C-H) = 3.5 f 1.0 X 10er3 cm* respectively quite in agreement with the ion beam analyses. The cross sections obtained for S, At-, Nb, Sn, Pb and U beam irradiations are given in Table 2.
2.0
15
1.0
0.5
in Phys. Res. B I31
- (H/C),,=,
= I - exp( - a(,,,,@t). 4. Conclusion
(D/C>@, - (D/C)@,=0
(D/C>ev= (D/C),,=, = 1 - exp( -a,,,,,@t). 1
-
For each electronic stopping power value, ion beam analysis (Table 2) shows that the deuterium effusion cross sections and the hydrogen absorption cross sections are equal within the experimental errors. This equality was already observed with the previous sulphur irradiation [24]. The same feature appears for the infrared absorption measurements for either the hydrogen or the deuterium. This observa-
obtain similar cross sections u(u,,-) = 3.0 + 1.0 1O-‘3 cm2 and a(,,,) = 3.5 + 1 X IO-l3 cm* from the evolution of H/C and D/C ratios respectively. Fig. 3 shows the evolution of the absorbance band normalised by the carbon area1 density for the C-H We x
Table 2 Irradiation parameters and cross sections obtained from infrared and ion beam analysis measurements. During the irradiations the surface scanned by the beam was about 22 cm’ (see the text for the values quoted by an asterisk) Ion
SC,(keVnm_
’ ) Deuterium (D/C),,,_
S
Ar
1.0 1.5 I .3
I .7 Nb Sn Pb u
2.5 5.5 6.7 7.9 12.3 12.3 13.0
0.27 0.23 0.26 0.26 0.26 0.25 0.26 0.26 0.30 0.2s 0.25
by heavy ions
Hydrogen
,
+ 0.05 * 0.03 & 0.05 * + 0.05 * * 0.05 * It 0.05 f 0.05 * k 0.05 * k 0.05 * 0.05 f 0.05
ulD/rl
(Yc-o)c~~,=~a,Yc-I,,
lo- Ii (cm-‘)
(IO-“’
4+3 2.5 f I 3 * I.5 5+ 1s 8*2 35 * 10 80 * 20 80 k 30 100 f 30 140 f 30 50 * 30
cm)
IO-”
(H/C)<,>,= /
(cm-‘)
2+1 311 4+ 1.5 6.5 + I.5 8*3 35* 10 90 f 30 110+30 125 f 20 14O+20 65 + 20
0.29 0.30 0.30 0.30 0.30 0.33 0.30 0.30 0.30 0.29 0.32
+ 0.05 F_0.05 f 0.05 * * 0.05 * ?z 0.05 * * 0.05 * 0.05 ’ +_0.05 * * 0.05 * + 0.05 + 0.03
%,/Cl
(Y,_H)o,=r
q,.,.,,
lo- IJ (cm-‘)
(lo-‘”
IO-” (cm-‘)
3+2 3&2
9.0 f 3.0 652
IO f 4 30* IO 100 + 20 IlO& 120&30 100 * 20 85 * 30
_
_
13+2 7+1 6.5 + 1.5 12*2 12*2 7il
25 + 5 80 k 30 110+30 125 + 20 150*20 85 & 30
cm)
3 ? I.5 6i4
F. Pawlak et al./Nucl.
139
Instr. and Meth. in Phys. Res. B 131 (1997) 1352140
.002 (a)
I 0.5
1.5
1.0
2.0
(10 ” Nb cm”)
---3100
3000
2900
2800
Fig. 4. Evolution of Yc _ o and Yc _ n , obtained from RBS and IR measurements, versus fiuence in an a-C:D film irradiated with Nb ions with electronic stopping power of 5.5 keV nm-‘. The ion beam analysis were performed 10 weeks after irradiation. The infrared measurements were performed 8 days (black symbols) and 6 months (open symbols) after irradiation.
W (cm -‘)
(b)
2300
2200 0
2100
(cm -‘)
(Fig. 21, they become independent of fluence. Moreover the total content of deuterium plus hydrogen is constant versus fiuence (Fig. 2) within the experimental errors. Such a correlation supports that the two physical observations are linked and that the absorbed hydrogen atoms are bonded to carbon atoms. This convergence explains why the cross sections extracted from all the different physical characterisations are identical for the same value of the electronic stopping power within the experimental errors. It also appears that all the absorbed hydrogen atoms are bound to sp3 carbon atoms, since no creation of sp2 C-H bondings (3050 cm-’ > is observed. Fig. 5 sums up all the results: the mean radii deduced by averaging the cross section values are plotted versus the fluence.
Fig. 3. Evolution of the stretching vibration bands of C-H (Fig. 3a) and C-D (Fig. 3b) bondings. The absorption spectra were measured 10 weeks after irradiation. The absorption y (cm’) is defined as In,,,(I,,(W)/I(W))/N, where I,,(o) and I(w) are the intensities of the incident and transmitted beams at wavenumber o. The respective fluences are: (1) 0; (2) 6.3 X 10” Nb cmm2; (3) 2.5~ 10” Nb cm-’ (4) 6.3X lOi Nb cmm2; (5) 1.8X 10” Nb cm-‘.
tion strongly suggests that hydrogen atoms appear in the irradiated part of the amorphous carbon matrix. Moreover, when the Y,_, and Yc_, values (Fig. 4) are normalized by the hydrogen (H/C) and deuterium (D/C) relative concentrations respectively
I
r
I
I
I
I 10
5
15
Se (keV nm .‘) Fig. 5. Mean values of the radii versus the electronic power S,.
II. ION IRRRADIATION
stopping
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Instr. and Meth. in Phys. Res. B 131 (1997) 135-140
Acknowledgements F. Pawlak is beneficiary of a thesis contract CEA-Region Basse Normandie. The authors thank F. Gourbilleau who made the TEM observations.
References [ll M. Toulemonde, Appl. Radiat. Isot. 46 (1995) 675. [21 F. Seitz, J.S. Koeler, Solid State Phys. 2 (1956) 305. [31 R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear Tracks in Solids, Principles and Applications (University of California Press, 1975). [41 J. Robertson, Adv. Phys. 35 (1986) 317. 151 J.C. Angus, P. Koidl and S. Domitz, in: Plasma Deposited Thin Films, eds. J. Mort and F. Jansen (CRC Press, Boca Raton, 1986) p. 89. Films and [61 See for instance, Diamond and Diamond-like Coatings, Vol 266, eds. R.E. Clausing et al. (Plenum Press, New York, 1991). [71 P. Koidl, C. Wild, R. Lecher and R.E. Sah, in: Diamond and Diamond-like Films and Coatings, eds. R.E. Clausing et al. (Plenum Press, New York, 1991) p. 243. 181 N. Fourches, G. Turban, Thin Solid Films 240 (1994) 28. [91 F. Jansen, M. Machonkin, S. Kaplan, S. Hark, J. Vat. Sci. Technol. 3 (1985) 605. [lOI G. Compagnini, G. Foti, R. Reitano, G. Mondio, Appl. Phys. Lett. 57 (1990) 2546.
[ 1 I] G. Compagnini, L. Calcagno, G. Foti, Phys. Rev. Lett. 69 ( 1992) 454. [12] W.E. Wallace, T.T. Chiou, J.B. Rothman, R.J. Composto, Nucl. Instr. and Meth. B 103 (1995) 435. [ 131 F. Abel, V. Quillet, M. Schott, Nucl. Instr. and Meth. B 105 (1995) 86. [ 141 F. Fujimoto, M. Tanake. Y. Iwata, A. Ootuka, K. Komaki, M. Haba, K. Kobayashi, Nucl. Instr. and Meth. B 33 (1988) 792. [I51 N. Betz, A. Le Mdl, E. Balanzat, J.M. Ramillon, J. Lamotte, J.P. Gallas, G. Jaskierowitz, J. Polym. Sci., Polym. Phys. B 32 (1994) 1493. [ 161 M.J. Paterson, K.G. Orrman-Rossiter. S. Bhargave, A. Hoftinan, J. Appl. Phys. 75 (1994) 792. [17] W.L. Brown, L.J. Lanzerotti, J.E. Bower, K.J. Marcantonio, Nucl. Instr. and Meth. B 24/25 (1987) 512. [18] E. Balanzat, N. Betz, S. Bouffard, Nucl. Instr. and Meth. B 105 (1995) 46. [19] W.J. Wang, C. Jing, T.M. Wang, Nucl. Instr. and Meth. B 95 (1995) 339. [20] J. Spousta, These Universite de Paris (VI/VII) Jussieu t 1994). [21] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping Power of Ions in Solids (Pergamon, New York, 1980). [22] B. Dischler, R.E. Sah, P. Koidl, W. Fluhr and A. Wokaun, ISPC-7 Eindhoven (1985) p. 45. [23] M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev. B 16 (1977)3556. [24] F. Pawlak, E. Balanzat, Ch. Dufour, A. Laurent, E. Paumier, J. Perriere, J.P. Stoquert and M. Toulemonde, Nucl. Instr. and Meth. B 122 (1997) 579.
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Amorphous deuterated carbon films irradiated by swift heavy ions: Infrared measurements and ion beam analysis F. Pawlak a3*, Ch. Dufour b, A. Laurent ‘, E. Paumier ayb,J. Perriere ‘, J.P. Stoquert d, M. Toulemonde a ’ CIRIL, CEA-CNRS, B.P. 5133, F-14070 Caen Cedex 5, France h LERMAT, URA CNRS 1317. ISMRa, Bouleoard Mar&ha1 Juin, F-14050 Cam Cedex, France ’ GPS, URA 17. Universitks Paris VII et Paris VI, Tour 23,2 Place Jussieu, 75251 Paris Cedex 05, France d Groupe PHASE, CRN, 67037 Strasbourg Cedex 2, France
Abstract Mixed hydrogenated and deuterated amorphous carbon films have been irradiated at GANIL in the MeV/amu energy range with an electronic stopping power varying between 1 keV nm- ’ and 13 keV nm- ’ These films roughly contain 10% of hydrogen and 30% of deuterium. Carbon (C), hydrogen (H) and deuterium (D) contents were determined by Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA). The evolution of C-H and C-D bondings contents was determined by infrared absorption measurements. The main effects due to MeV amu-’ ion irradiations are the decrease of C-D bondings content and deuterium relative concentration (D/C atomic ratio) as a function of fluence. A long time after irradiation C-H bondings content and hydrogen relative concentration (H/C atomic ratio) increase. The hydrogen absorption cross section is equal to the deuterium effusion cross section within the experimental errors whatever is the physical characterisation (ion beam analysis or infrared absorption). Moreover, both techniques give the same cross sections. Hence the following interpretation is proposed: during irradiation hydrogen and deuterium atoms are ejected from the ion tracks and afterwards are replaced by hydrogen atoms coming from the ambient air and diffusing inside the irradiated material along the latent tracks.
1. Introduction During several years the interaction between swift heavy ions (several MeV amu-’ > and the matter has been the subject of many studies [I]. It appears that the electronic slowing down characterised by the electronic stopping power, S,, governs ion damage. Two models have been proposed, the thermal spike model [2] and the ionic spike one [3]. In each of
* C:orresponding [email protected]
author.
Fax:
+ 33-3145-4714:
email:
them the electron mobility plays a significant role: lower is the electron mobility, higher is the sensitivity of the irradiated material. In this point of view, carbonaceous compounds are very interesting materials. The electrical conductivity may vary from lo- ” R-l cm-’ for diamond to IO3 a-’ cm -I for graphite coming back to lo- ’ 6 R- ’ cm ’ for polyethylene. Amorphous hydrogenated carbon are intermediate materials with conductivity varying between lo-l6 R-’ cm-’ and 10e7 R-’ cm-’ [4]. Hydrogenated amorphous carbon (a-C:H) films have been the subject of a great interest due to their singular
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl68-583X(97)00299-1
II. ION IRRADIOATION
AND TRACKS
136
F. Pawlak et al. /Nucl.
Instr. and Meth. in Phys. Res. B 131 (1997) 135-140
ings creation [ 141 and increase of resistivity [ 191 were observed. The present paper reports the results obtained in soft amorphous deuterated carbon films irradiated with swift heavy ions over a wide range of electronic stopping power, between 1 and 13 keV nm-‘, with a beam energy ranging between 2 and 10 MeV amu ‘. Infrared measurements were used to study the C-D and C-H bondings evolution as function of fluence and time after irradiations. Ion beam analyses were performed to study the evolution of the C, D, and H atomic area1 densities as function of fluence.
properties including extreme hardness, infrared transparency, chemical inertness and electrical resistivity [5]. The hydrogen concentration (up to 50%) and the ratio of single carbon bondings (sp3 coordination) to double carbon bondings (sp’ coordination) depend on deposition parameters and govern all the properties [6-81. Hydrogen stabilises dangling bonds of sp3 carbon [4] and increases the resistivity and the optical gap [9,10]. The higher the sp’/sp’ ratio, the higher the resistivity [7]. To sum up, the domain of properties of a-C:H films is bounded by those of diamond, graphite and polymers. The soft, low density (K 1 g cm -3> and highly hydrogenated films (> 35%) are called “polymerlike” whereas the hard, high density (> 1.6 g cmd3> and poorly hydrogenated films (< 35%) are called “diamond-like”. Whatever the ion energy, keV or MeV, and the material, a-C:H or polymers, the main effects of irradiation are the loss of hydrogen [ 1 l-131, the decrease of the optical gap [lo], the breaking of carbon-hydrogen bondings (C-H) [ 14,151 and the decrease of sp3/sp’ ratio [ 1 1,15,16]. Characterisations of released particles during irradiation have shown a molecular effusion (mainly H,) [17]. Up to now, studies about polymer irradiations put the stress on the modifications of chemical groups versus the fluence depending on the electronic stopping power [15,18]. Post irradiation effects have been the subject of several studies in a-C:H films where cetone bond-
2. Experiment
description
Amorphous deuterated and hydrogenated carbon films, called here after a-C:H,D,, have been deposited on n-type silicon substrates at - 35 V bias voltage by decomposition of CD, gas containing 10% of CH, in a dc multipolar plasma [20]. The density deduced from ion beam analysis and high resolution electron microscopy observation (film thickness varies between 50 and 100 nm> is 1.0 + 0.1 g cm-?. The optical index is 1.59 [20]. Table 1 gives all the experimental conditions and all the irradiation parameters. Ion irradiations (S, Ar, Nb, Sn, Pb and U> were performed at GANIL (Grand Accelerateur
Table I Irradiation parameters (ion beam, its energy, electronic stopping power Se, flux @ and fluence @,) and initial samples characteristics. For each S, value. all the samples were cut off from the same film. The samples composition was determined by RBS (carbon content) and ERDA (hydrogen and deuterium contents). The electronic stopping power St was calculated with TRIM 91 [21] assuming a density of 1 g cm-’ and a composition ofCD,, ,,,, Ion
lhs15+
JIIArlh+
‘WNb’.‘+
I I?Sn47+
zlapb’J+ /I*“‘S+
Energy (MeV/amu)
S, (keV nm-
10.1 5.3 9.2 5.5 I .8 6.4 1.2 2.2 2.6 2.6 2.0
I .o 1.5 I .3 I .7 2.5 5.5 6.7 7.9 12.3 12.3 13.0
’)
@(ion scm-‘) 2x 2x 6x 6x 6X 3.5 2.5 2.5 2x 2x 2.2
IO” IO” 10” IO’ IO* x IOX x IO” x lox IOX 10” x IOX
’
Q,,,,,$,
CH,D,
Carbon area1 density (10” C cm-‘)
Thickness (nm)
CH,,,,D,,,, CH o I ED,,.,,,
2.8 2.4 2.4 2.4 2.4 4.6 3.2 3.2 3.5 2.3 4.4
60 50 50 50 50 100 70 70 75 50 100
(ioncm-‘)
4.1 2.0 I.2 1.2 I.2 1.8 2.5 2.5 5x 5x 5x
x lOI x 10”’ x IO’? x IO’? x IO’? x IO” X IO” X IO” IO’? IO’? 10”
CH,,.I9“,,A, CH,,19D,,,,, CH 019 D 0.u CH o,?D,,,, CH,,,, D 0 Jb CH o I7I’,, 46
CH,,,,,,D,,,, CH o ,,D,,s CH ,,.I? D 011
137
F. Pawlak et al./ Nucl. Instr. and Meth. in Phys. Res. B 131 (1997) 135-140
d’Ions Lourds at Caen, France) on SME line (Sortie Moyenne Energie) with normal incidence beam. Fourier Transform Infrared spectrometry measurements were performed in transmission geometry using the Nicolet FTIR spectrometer (Magna IR, 750), with incident p-polarised light incoming at 0, = 55” with respect to the normal to the sample. This technique allows the absorption measurements of the following stretching bands: C-H (between 3100 and 2800 cm-’ ) and C-D (between 2300 and 2000 cm- ’ ) [22]. The parameter Y (cm) is defined as the absorbance band area A, (cm-’ ) normalised by the carbon area1 density N, (C cm--*) to take into account the thickness variation of films. We obtain the following relation [23]: NGTW,
y+-_ c
2.3N,
deuterium effusion was less than 10% of the total content. The surface hydrogen adsorption and the amorphous carbon film thickness were determined by a systematic comparison with a pure silicon wafer. The hydrogen surface content was less than lOi H/cm* as compared to a mean value of 4 X lOI H/cm2 for the non- irradiated amorphous carbon films which contain the least hydrogen content. In the case of deuterium, it is always negligible. Moreover, the ERDA spectra on thick samples show a decrease (increase) of the deuterium (hydrogen) plateau. All the ion beam analyses were performed at least 10 weeks after irradiation when the hydrogen absorption is stabilised.
3. Experimental
results
’
where N (cm-‘) is the bonding concentration, r (cm*) the average absorption strength, S (cm) the optical length and wa the central wavenumber of the band (2920 and 2150 cm- ’ for the stretching vibrations C-H and C-D respectively). Infrared analyses were performed before and after irradiation. Hydrogen absorption was followed versus time after irradiation and so the hydrogen content was seen to be stabilised 8 weeks after irradiation. Thus, all the results presented here correspond to infrared measurements made at least 10 weeks after irradiation. Within the experimental errors, deuterium content does not depend on time after irradiation. A 2.9 MeV 4HeC ion beam was used to perform Rutherford Backscattering spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) to determine carbon, hydrogen and deuterium area1 densities. Hydrogen and deuterium contents were obtained using the computer code developed by J.P. Stoquert. Absolute value of this hydrogen content was confirmed by the analysis of the hydrogen content in a polyethylene film: hydrogen effusion was followed versus helium fluence and the hydrogen content was determined by extrapolation to zero fluence. The same attention was used to measure H and D contents in the amorphous carbon films: in the present analysis conditions (a total charge of 10e6 Cb with a beam intensity of 2 nA at a grazing angle leading to a surface beam spot of 0.18 cm2) the hydrogen and
In this part, the results of the niobium irradiation are discussed, and then the results obtained by the same method with the other irradiations (S, Ar, Sn, Pb and U) are summarised. RBS analysis (Fig. 1) shows a decrease of carbon area1 density N,. The decrease of the D/C ratio and the increase of the H/C ratio as function of fluence are observed in Fig, 2. From Fig. 2, the asymptotic atomic ratios ((H/C),,=, and (D/C),,=,) at infinite fluence (@t = m) are reported in Table 2. It may be seen that these asymptotic values are independent of the ion nature and electronic stopping power. Thus, when the fluence was not high enough to determine these asymptotic ratios a mean value deduced from the measured ones has been used and noted by an asterisk in Table 2. From Fig. 2 the
“p------------l
01 0.0
0.5
1.0 (IO
”
1.5
20
Nb cm-‘)
Fig. 1. Carbon areal density evolution of a-C:D fluence during Nb irradiation at 5.5 keV nm- ’
II. ION IRRRADIATION
films versus
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138
et al./Nucl.
Instr.
and Meth.
I (IO l3 Nb cm-*)
Fig. 2. Evolution of the atomic ratios (D/C) and (H/C) obtained by RBS and ERDA measurements, versus fluence in an a-C:D film irradiated with Nb ions with electronic stopprng power of 5.5 keV nm-’ (measurements were made 10 weeks after irradiation).
cross sections are extracted lowing exponential laws: (H/C)
(Table 2) using the fol-
Q, - (H/C),,=,
(H/C),,_,
(19971
135-140
(Fig. 3a) and C-D (Fig. 3b) stretching bands, yC H and y,_, respectively. Fig. 4 shows the increase of Yc _ u and the decrease of Yc u defined in Section 2 versus fluence. The asymptotic values of Y~,~,),,, x and Yu_mQr=X estimated at infinite fluence (@I = m> are reported in Table 2. The same procedure as above is used when the fluence was not high enough to determine these asymptotic values and a mean value deduced from the measured ones has been used and noted by an asterisk in Table 2. Using the same previous exponential laws we may define the cross sections related to C-H and C-D bondings evolutions which are equal to a(, c-u) = 2.5 _t 0.5 X 1O-‘3 cm2 and u (Y C-H) = 3.5 f 1.0 X 10er3 cm* respectively quite in agreement with the ion beam analyses. The cross sections obtained for S, At-, Nb, Sn, Pb and U beam irradiations are given in Table 2.
2.0
15
1.0
0.5
in Phys. Res. B I31
- (H/C),,=,
= I - exp( - a(,,,,@t). 4. Conclusion
(D/C>@, - (D/C)@,=0
(D/C>ev= (D/C),,=, = 1 - exp( -a,,,,,@t). 1
-
For each electronic stopping power value, ion beam analysis (Table 2) shows that the deuterium effusion cross sections and the hydrogen absorption cross sections are equal within the experimental errors. This equality was already observed with the previous sulphur irradiation [24]. The same feature appears for the infrared absorption measurements for either the hydrogen or the deuterium. This observa-
obtain similar cross sections u(u,,-) = 3.0 + 1.0 1O-‘3 cm2 and a(,,,) = 3.5 + 1 X IO-l3 cm* from the evolution of H/C and D/C ratios respectively. Fig. 3 shows the evolution of the absorbance band normalised by the carbon area1 density for the C-H We x
Table 2 Irradiation parameters and cross sections obtained from infrared and ion beam analysis measurements. During the irradiations the surface scanned by the beam was about 22 cm’ (see the text for the values quoted by an asterisk) Ion
SC,(keVnm_
’ ) Deuterium (D/C),,,_
S
Ar
1.0 1.5 I .3
I .7 Nb Sn Pb u
2.5 5.5 6.7 7.9 12.3 12.3 13.0
0.27 0.23 0.26 0.26 0.26 0.25 0.26 0.26 0.30 0.2s 0.25
by heavy ions
Hydrogen
,
+ 0.05 * 0.03 & 0.05 * + 0.05 * * 0.05 * It 0.05 f 0.05 * k 0.05 * k 0.05 * 0.05 f 0.05
ulD/rl
(Yc-o)c~~,=~a,Yc-I,,
lo- Ii (cm-‘)
(IO-“’
4+3 2.5 f I 3 * I.5 5+ 1s 8*2 35 * 10 80 * 20 80 k 30 100 f 30 140 f 30 50 * 30
cm)
IO-”
(H/C)<,>,= /
(cm-‘)
2+1 311 4+ 1.5 6.5 + I.5 8*3 35* 10 90 f 30 110+30 125 f 20 14O+20 65 + 20
0.29 0.30 0.30 0.30 0.30 0.33 0.30 0.30 0.30 0.29 0.32
+ 0.05 F_0.05 f 0.05 * * 0.05 * ?z 0.05 * * 0.05 * 0.05 ’ +_0.05 * * 0.05 * + 0.05 + 0.03
%,/Cl
(Y,_H)o,=r
q,.,.,,
lo- IJ (cm-‘)
(lo-‘”
IO-” (cm-‘)
3+2 3&2
9.0 f 3.0 652
IO f 4 30* IO 100 + 20 IlO& 120&30 100 * 20 85 * 30
_
_
13+2 7+1 6.5 + 1.5 12*2 12*2 7il
25 + 5 80 k 30 110+30 125 + 20 150*20 85 & 30
cm)
3 ? I.5 6i4
F. Pawlak et al./Nucl.
139
Instr. and Meth. in Phys. Res. B 131 (1997) 1352140
.002 (a)
I 0.5
1.5
1.0
2.0
(10 ” Nb cm”)
---3100
3000
2900
2800
Fig. 4. Evolution of Yc _ o and Yc _ n , obtained from RBS and IR measurements, versus fiuence in an a-C:D film irradiated with Nb ions with electronic stopping power of 5.5 keV nm-‘. The ion beam analysis were performed 10 weeks after irradiation. The infrared measurements were performed 8 days (black symbols) and 6 months (open symbols) after irradiation.
W (cm -‘)
(b)
2300
2200 0
2100
(cm -‘)
(Fig. 21, they become independent of fluence. Moreover the total content of deuterium plus hydrogen is constant versus fiuence (Fig. 2) within the experimental errors. Such a correlation supports that the two physical observations are linked and that the absorbed hydrogen atoms are bonded to carbon atoms. This convergence explains why the cross sections extracted from all the different physical characterisations are identical for the same value of the electronic stopping power within the experimental errors. It also appears that all the absorbed hydrogen atoms are bound to sp3 carbon atoms, since no creation of sp2 C-H bondings (3050 cm-’ > is observed. Fig. 5 sums up all the results: the mean radii deduced by averaging the cross section values are plotted versus the fluence.
Fig. 3. Evolution of the stretching vibration bands of C-H (Fig. 3a) and C-D (Fig. 3b) bondings. The absorption spectra were measured 10 weeks after irradiation. The absorption y (cm’) is defined as In,,,(I,,(W)/I(W))/N, where I,,(o) and I(w) are the intensities of the incident and transmitted beams at wavenumber o. The respective fluences are: (1) 0; (2) 6.3 X 10” Nb cmm2; (3) 2.5~ 10” Nb cm-’ (4) 6.3X lOi Nb cmm2; (5) 1.8X 10” Nb cm-‘.
tion strongly suggests that hydrogen atoms appear in the irradiated part of the amorphous carbon matrix. Moreover, when the Y,_, and Yc_, values (Fig. 4) are normalized by the hydrogen (H/C) and deuterium (D/C) relative concentrations respectively
I
r
I
I
I
I 10
5
15
Se (keV nm .‘) Fig. 5. Mean values of the radii versus the electronic power S,.
II. ION IRRRADIATION
stopping
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Instr. and Meth. in Phys. Res. B 131 (1997) 135-140
Acknowledgements F. Pawlak is beneficiary of a thesis contract CEA-Region Basse Normandie. The authors thank F. Gourbilleau who made the TEM observations.
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