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The effect of fluence on the hardening of C60 films irradiated with He and N ions
Nuclear Instruments and Methods in Physics Research B 148 (1999) 634±638
The eect of ¯uence on the hardening of C60 ®lms irradiated with He and N ions C.E. Foerster a, F.C. Serbena a, C.M. Lepienski b, D.L. Baptista c, F.C. Zawislak a
c,*
Departamento de Fõsica, Universidade Estadual de Ponta Grossa, Al. N. de Ara ujo s/n, 84010-330 Ponta Grossa, PR, Brazil b Departamento de Fõsica, Universidade Federal do Paran a, Caixa Postal 19081, 81531-990 Curitiba, PR, Brazil c Instituto de Fõsica, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, 91501-970 Porto Alegre, RS, Brazil
Abstract The hardness (H) and the Young modulus (E) of thin C60 ®lms deposited on Si substrate and ion irradiated are studied via the nanoindentation technique. C60 ®lms 170 nm thick were irradiated with 170 keV N and 30 keV He ions. The hardness and the Young modulus were measured as a function of the ¯uence / and of the transferred nuclear (Sn ) plus electronic (Se ) energy density rt /(Sn + Se ). The results show an increase of H from 0.33 GPa for the pristine C60 ®lm to 15 GPa after the irradiations with the highest ¯uences. Similarly E raises from 23 to 180 GPa after the irradiations. It is observed that the curves of H and E follow the relation between the number of destroyed C60 molecules measured via Raman spectroscopy and the average deposited energy density rt , reaching the highest values when the ®lms are fully amorphized. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.48.+c; 62.20.Qp; 46.30.Pa Keywords: Ion irradiation; Fullerene; Hardness; Young modulus; Amorphization
1. Introduction The chemical and physical properties of C60 molecules [1] oer a great potential for various mechanical and tribological applications [2±4]. Gupta et al. [5], report an increase in hardness and wear resistance after bombarding C60 ®lms with 100 keV Ar ions at a high ¯uence of 1016 cmÿ2 . These increases are justi®ed by the authors as due
* Corresponding author. Tel.: +55 51 3166427; fax: +55 51 3191762; e-mail: [email protected].
to the presence of defects and densi®cation produced by the irradiation. Palmetshofer and Kastner [6] using Raman spectroscopy report polymerization and amorphization of C60 ®lms irradiated with H, He, C and Ar ions at energies from 60 to 600 keV and ¯uences from 1012 to 5 ´ 1016 cmÿ2 . The damage, destruction and amorphization of irradiated and implanted fullerene ®lms depend on the transferred electronic plus nuclear energy density to the C60 layer through the ionic irradiations. The results of Ref. [7] show that it is necessary an energy density transference of @5 ´ 10ÿ3 3 to start the destruction of the C60 ®lm and eV/A
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 0 0 - 3
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
3 the ®lm is for transferences larger than 0.5 eV/A totally destroyed. In this contribution we present detailed results of hardness (H) and Young modulus (E) measurements in C60 ®lms irradiated with 30 keV He and 170 keV N ions as function of the ¯uence. These data are correlated with the averaged transferred electronic and nuclear energy density by the ionic beams, and with the damage and amorphization results obtained from Raman experiments performed on the same C60 ®lms. 2. Experimental procedures The C60 ®lms were prepared by evaporation of the powder in vacuum at a temperature of 450°C, and deposition on polished (1 1 1) silicon wafers with a 30 nm layer of SiO2 . The N energy implantation was 170 keV, with the ¯uence ranging from 5 ´ 1011 to 5 ´ 1015 ions cmÿ2 ; the He energy was 30 keV, with ¯uences from 1012 to 1016 ions cmÿ2 . The electronic (Se ) and nuclear (Sn ) stopping powers and the ranges have been calculated via the TRIM simulations (version 1995) [8]. The total transferred energy density is obtained by multiplying the ¯uence by Se plus Sn : rt / Sn Se .The projected ranges (Rp ) of the N and He ions, 380 and 290 nm respectively, are larger than the ®lm thickness (170 nm). The ion irradiations were performed at room temperature, using the 500 keV ion implanter of the Instituto de Fõsica ± UFRGS, with beam current <100 nA cmÿ2 to avoid heating of the samples. The Raman spectra were obtained using a HeNe laser of 30 mW for excitation, coupled to a microscope-spectrometer system with a CCD detector. The laser spot was about 2 lm and the sample was moved fastly in a random pattern, to avoid localized heating and photo-damage of the ®lm. The measurements of hardness and Young modulus were performed using the technique described by Oliver and Pharr [9], through the analysis of the loading/unloading curves obtained by the Nanoindenter IITM machine at the Departamento de Fõsica ± UFPR. The loads used varied from 0.5 to 16 mN in order to obtain in-
635
formation about the ®lm, the interface and the substrate. 3. Results The results are presented in two steps. In the ®rst one, we used Raman spectroscopy to study the ion induced destruction of the C60 molecules. In a second step the measurements of hardness and Young modulus are investigated to obtain information about these parameters in the ®lm, interface and substrate ± this was performed in all samples (pristine and irradiated ®lms). 3.1. Raman spectroscopy of C60 irradiated ®lms Fig. 1 shows the pristine C60 Raman spectrum for the 170 nm thick ®lm, as well as the spectra of the same ®lm irradiated at various ¯uences with 30 keV He ions. The spectra clearly reveal the eect of the ion irradiation, which results in the intensity decrease of the Raman lines. At the highest ¯uence, the eect of the irradiation is the complete destruction of the fullerene molecules, resulting in the formation of an amorphous carbon layer, indicated by the broad and asymmetric Raman line between 1000 and 1700 cmÿ1 for a ¯uence of 1016 He cmÿ2 . Similar spectra have been
Fig. 1. Raman spectra of non-irradiated and 30 keV He irradiated ®lms at three ¯uences. At 1016 He cmÿ2 the C60 ®lm is completely amorphous.
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C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
3.2. Hardness and Young modulus measurements
obtained for the ®lms irradiated with N at 170 keV. The intensities of the Raman lines measure the remaining part of the C60 molecules after the irradiation, from which the destroyed fraction is obtained. The destroyed fraction of C60 , Idestr (in %) as function of the ¯uence for both ions is plotted in Fig. 2 (a), and clearly shows that the destruction starts and is completed at dierent ¯uences for He and N ions. On the other hand we can plot the same Raman data as function of the averaged total transferred energy density rt . Fig. 2(b) displays Idestr as function of rt showing that all the He and N experimental points follow the same curve, indicating that the important parameter in the C60 ®lm destruction is the total transferred energy density.
Figs. 3(a) and (b) show the variation of hardness as a function of the contact depth for some of the irradiated ®lms and also for the pristine ®lm. The hardness of the C60 pristine ®lm at 70 nm is 0.33 GPa, which agrees with results from the literature [5]. This depth, corresponding to 40% of the ®lm thickness, is selected to avoid the in¯uence of the substrate on the hardness measurement of the ®lm. The hardness of the irradiated ®lms increases up to 12 GPa for He and 15 GPa for N irradiations at the highest ¯uences used. The Young modulus as function of the contact depth is shown in Figs. 4(a) and (b), for the same
Fig. 2. Intensity of the destroyed fraction, Idestr (in %) of C60 irradiated with He and N ions as function of: (a) the ¯uences, (b) the averaged total transferred energy density.
Fig. 3. Hardness as function of the contact depth for: (a) pristine ®lm and N irradiated ®lms, (b) for He irradiated ®lms. The lines are drawn only to guide the eye.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
637
Fig. 5. Hardness as function of the averaged total deposited energy density by the ions for N and He irradiation. The lines are drawn only to guide the eye.
4. Discussion and conclusion
Fig. 4. Young modulus as function of the contact depth: (a) for pristine ®lm and N irradiated ®lms, (b) for He irradiated ®lms. The lines are drawn only to guide the eye.
samples of Fig. 3. For the pristine ®lm, the value of E is 23 GPa. The Young modulus also shows an increase after the irradiations, rising to 140 GPa and 180 GPa for He and N bombardment at the highest ¯uences. For high loads the hardness and the Young modulus converge to the Si substrate values found in the literature. Fig. 5 shows the hardness as a function of the averaged total deposited energy density rt by the N and He ions. For low ¯uences (<3 ´ 1013 cmÿ2 or 3 ) the hardness does not a energy density <2 eV/A present a considerable increase. Fig. 5 also shows that H starts to increase at a smaller energy density transference for N than for He. The same behavior as function of rt is observed for the Young modulus.
Our Raman data show that for He 30 keV irradiation, the destruction of C60 starts for ¯uences larger than 1013 He cmÿ2 and is completed (100% destroyed) at 1015 He cmÿ2 , but the transformation of the ®lm into a completely amorphous layer is only attained at the ¯uence of 1016 He cmÿ2 , as shown in Fig. 1. The same trend is observed for the ®lm irradiated with 170 keV N ions but for lower ¯uences. The ®nal destruction and amorphization is accomplished at dierent ¯uences, but when the data are plotted as function of the total energy density transferred by the ions rt , as shown in Fig. 2 (b), the destruction is achieved at the same transfer of energy density for both 3 . ions, >0.5 eV/A As the bonding energy of each carbon atom in C60 is 7.4 eV, and the volume of a C60 molecule 3 is 3 , it is easy to estimate that 10ÿ2 eV/A @700 A the energy necessary to set free one carbon atom 3 the molecular energy density of and 0.64 eV/A C60 . As shown in Fig. 2 (b), the onset of C60 de3 and struction at a transference of @5 ´ 10ÿ3 eV/A 3 the total destruction at @0.5 eV/A for both ions are in agreement with the above estimates. The observed increase of H as function of rt (Fig. 5) has a shape similar to the C60 destruction intensity of Fig. 2, but with two important dierences: (a) the hardness data do not merge into a common curve for both ions; (b) the increase of
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C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
hardness starts at a higher deposited energy density than the energy necessary to initiate the destruction of the C60 molecules. The large increase of H by a factor of @50 only occurs at the highest ¯uences, when the C60 layers are transformed into amorphous carbon, as shown by Raman Spectroscopy. With regard to the fact that the increase of H starts at lower deposited energy density for N than for He irradiation, the reasons can be the higher total stopping cross section of N ions, or the fact that there is a larger contribution of nuclear stopping power for N than for He in the total stopping power. In summary, our experimental results show that the highest values of hardness and Young modulus of C60 ion irradiated ®lms are obtained when the C60 is transformed into an amorphous layer. Similar improvements in hardness obtained in ion irradiated polymers [10] and glassy carbon [11] could also be attributed to the amorphization produced by the irradiations. Acknowledgements This work was supported by the Brazilian Agencies FINEP/PRONEX, CAPES and FAPERGS.
References [1] N.S. Dresselhauss, G. Dresselhauss, P.C. Eklund, J. Mat. Res. 8 (1993) 2054. [2] B. Bhushan, B.K. Gupta, G.W. Van Cleef, C. Capp, J.V. Coe, Appl. Phys. Lett. 25 (1993) 3253. [3] T. Thundat, R.J. Warmackk, D. Diing, R.N. Compton, Appl. Phys. Lett. 63 (1993) 891. [4] J. Raun, B. Bhushan, J. Mat. Res. 8 (1993) 3019. [5] B.K. Gupta, B. Bhushan, C. Capp, J.V. Coe, J. Mater. Res. 9 (1994) 2823. [6] L. Palmetshofer, J. Kastner, Nucl. Instr. and Meth. B 966 (1995) 343. [7] F.C. Zawislak, D.L. Baptista, M. Behar, D. Fink, P.L. Grande, J.A.H. da Jornada, Nucl. Instr. and Meth. B, submitted. [8] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. B 174 (1980) 257. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 15564. [10] G.R. Rao, E.H. Lee, J. Mater. Res. 11 (1996) 2661. [11] D. Mclulloch, A. Homan, S. Praver, J. Appl. Phys. 74 (1993) 135.
The eect of ¯uence on the hardening of C60 ®lms irradiated with He and N ions C.E. Foerster a, F.C. Serbena a, C.M. Lepienski b, D.L. Baptista c, F.C. Zawislak a
c,*
Departamento de Fõsica, Universidade Estadual de Ponta Grossa, Al. N. de Ara ujo s/n, 84010-330 Ponta Grossa, PR, Brazil b Departamento de Fõsica, Universidade Federal do Paran a, Caixa Postal 19081, 81531-990 Curitiba, PR, Brazil c Instituto de Fõsica, Universidade Federal do Rio Grande do Sul, Caixa Postal 15051, 91501-970 Porto Alegre, RS, Brazil
Abstract The hardness (H) and the Young modulus (E) of thin C60 ®lms deposited on Si substrate and ion irradiated are studied via the nanoindentation technique. C60 ®lms 170 nm thick were irradiated with 170 keV N and 30 keV He ions. The hardness and the Young modulus were measured as a function of the ¯uence / and of the transferred nuclear (Sn ) plus electronic (Se ) energy density rt /(Sn + Se ). The results show an increase of H from 0.33 GPa for the pristine C60 ®lm to 15 GPa after the irradiations with the highest ¯uences. Similarly E raises from 23 to 180 GPa after the irradiations. It is observed that the curves of H and E follow the relation between the number of destroyed C60 molecules measured via Raman spectroscopy and the average deposited energy density rt , reaching the highest values when the ®lms are fully amorphized. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 61.48.+c; 62.20.Qp; 46.30.Pa Keywords: Ion irradiation; Fullerene; Hardness; Young modulus; Amorphization
1. Introduction The chemical and physical properties of C60 molecules [1] oer a great potential for various mechanical and tribological applications [2±4]. Gupta et al. [5], report an increase in hardness and wear resistance after bombarding C60 ®lms with 100 keV Ar ions at a high ¯uence of 1016 cmÿ2 . These increases are justi®ed by the authors as due
* Corresponding author. Tel.: +55 51 3166427; fax: +55 51 3191762; e-mail: [email protected].
to the presence of defects and densi®cation produced by the irradiation. Palmetshofer and Kastner [6] using Raman spectroscopy report polymerization and amorphization of C60 ®lms irradiated with H, He, C and Ar ions at energies from 60 to 600 keV and ¯uences from 1012 to 5 ´ 1016 cmÿ2 . The damage, destruction and amorphization of irradiated and implanted fullerene ®lms depend on the transferred electronic plus nuclear energy density to the C60 layer through the ionic irradiations. The results of Ref. [7] show that it is necessary an energy density transference of @5 ´ 10ÿ3 3 to start the destruction of the C60 ®lm and eV/A
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 0 0 - 3
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
3 the ®lm is for transferences larger than 0.5 eV/A totally destroyed. In this contribution we present detailed results of hardness (H) and Young modulus (E) measurements in C60 ®lms irradiated with 30 keV He and 170 keV N ions as function of the ¯uence. These data are correlated with the averaged transferred electronic and nuclear energy density by the ionic beams, and with the damage and amorphization results obtained from Raman experiments performed on the same C60 ®lms. 2. Experimental procedures The C60 ®lms were prepared by evaporation of the powder in vacuum at a temperature of 450°C, and deposition on polished (1 1 1) silicon wafers with a 30 nm layer of SiO2 . The N energy implantation was 170 keV, with the ¯uence ranging from 5 ´ 1011 to 5 ´ 1015 ions cmÿ2 ; the He energy was 30 keV, with ¯uences from 1012 to 1016 ions cmÿ2 . The electronic (Se ) and nuclear (Sn ) stopping powers and the ranges have been calculated via the TRIM simulations (version 1995) [8]. The total transferred energy density is obtained by multiplying the ¯uence by Se plus Sn : rt / Sn Se .The projected ranges (Rp ) of the N and He ions, 380 and 290 nm respectively, are larger than the ®lm thickness (170 nm). The ion irradiations were performed at room temperature, using the 500 keV ion implanter of the Instituto de Fõsica ± UFRGS, with beam current <100 nA cmÿ2 to avoid heating of the samples. The Raman spectra were obtained using a HeNe laser of 30 mW for excitation, coupled to a microscope-spectrometer system with a CCD detector. The laser spot was about 2 lm and the sample was moved fastly in a random pattern, to avoid localized heating and photo-damage of the ®lm. The measurements of hardness and Young modulus were performed using the technique described by Oliver and Pharr [9], through the analysis of the loading/unloading curves obtained by the Nanoindenter IITM machine at the Departamento de Fõsica ± UFPR. The loads used varied from 0.5 to 16 mN in order to obtain in-
635
formation about the ®lm, the interface and the substrate. 3. Results The results are presented in two steps. In the ®rst one, we used Raman spectroscopy to study the ion induced destruction of the C60 molecules. In a second step the measurements of hardness and Young modulus are investigated to obtain information about these parameters in the ®lm, interface and substrate ± this was performed in all samples (pristine and irradiated ®lms). 3.1. Raman spectroscopy of C60 irradiated ®lms Fig. 1 shows the pristine C60 Raman spectrum for the 170 nm thick ®lm, as well as the spectra of the same ®lm irradiated at various ¯uences with 30 keV He ions. The spectra clearly reveal the eect of the ion irradiation, which results in the intensity decrease of the Raman lines. At the highest ¯uence, the eect of the irradiation is the complete destruction of the fullerene molecules, resulting in the formation of an amorphous carbon layer, indicated by the broad and asymmetric Raman line between 1000 and 1700 cmÿ1 for a ¯uence of 1016 He cmÿ2 . Similar spectra have been
Fig. 1. Raman spectra of non-irradiated and 30 keV He irradiated ®lms at three ¯uences. At 1016 He cmÿ2 the C60 ®lm is completely amorphous.
636
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
3.2. Hardness and Young modulus measurements
obtained for the ®lms irradiated with N at 170 keV. The intensities of the Raman lines measure the remaining part of the C60 molecules after the irradiation, from which the destroyed fraction is obtained. The destroyed fraction of C60 , Idestr (in %) as function of the ¯uence for both ions is plotted in Fig. 2 (a), and clearly shows that the destruction starts and is completed at dierent ¯uences for He and N ions. On the other hand we can plot the same Raman data as function of the averaged total transferred energy density rt . Fig. 2(b) displays Idestr as function of rt showing that all the He and N experimental points follow the same curve, indicating that the important parameter in the C60 ®lm destruction is the total transferred energy density.
Figs. 3(a) and (b) show the variation of hardness as a function of the contact depth for some of the irradiated ®lms and also for the pristine ®lm. The hardness of the C60 pristine ®lm at 70 nm is 0.33 GPa, which agrees with results from the literature [5]. This depth, corresponding to 40% of the ®lm thickness, is selected to avoid the in¯uence of the substrate on the hardness measurement of the ®lm. The hardness of the irradiated ®lms increases up to 12 GPa for He and 15 GPa for N irradiations at the highest ¯uences used. The Young modulus as function of the contact depth is shown in Figs. 4(a) and (b), for the same
Fig. 2. Intensity of the destroyed fraction, Idestr (in %) of C60 irradiated with He and N ions as function of: (a) the ¯uences, (b) the averaged total transferred energy density.
Fig. 3. Hardness as function of the contact depth for: (a) pristine ®lm and N irradiated ®lms, (b) for He irradiated ®lms. The lines are drawn only to guide the eye.
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
637
Fig. 5. Hardness as function of the averaged total deposited energy density by the ions for N and He irradiation. The lines are drawn only to guide the eye.
4. Discussion and conclusion
Fig. 4. Young modulus as function of the contact depth: (a) for pristine ®lm and N irradiated ®lms, (b) for He irradiated ®lms. The lines are drawn only to guide the eye.
samples of Fig. 3. For the pristine ®lm, the value of E is 23 GPa. The Young modulus also shows an increase after the irradiations, rising to 140 GPa and 180 GPa for He and N bombardment at the highest ¯uences. For high loads the hardness and the Young modulus converge to the Si substrate values found in the literature. Fig. 5 shows the hardness as a function of the averaged total deposited energy density rt by the N and He ions. For low ¯uences (<3 ´ 1013 cmÿ2 or 3 ) the hardness does not a energy density <2 eV/A present a considerable increase. Fig. 5 also shows that H starts to increase at a smaller energy density transference for N than for He. The same behavior as function of rt is observed for the Young modulus.
Our Raman data show that for He 30 keV irradiation, the destruction of C60 starts for ¯uences larger than 1013 He cmÿ2 and is completed (100% destroyed) at 1015 He cmÿ2 , but the transformation of the ®lm into a completely amorphous layer is only attained at the ¯uence of 1016 He cmÿ2 , as shown in Fig. 1. The same trend is observed for the ®lm irradiated with 170 keV N ions but for lower ¯uences. The ®nal destruction and amorphization is accomplished at dierent ¯uences, but when the data are plotted as function of the total energy density transferred by the ions rt , as shown in Fig. 2 (b), the destruction is achieved at the same transfer of energy density for both 3 . ions, >0.5 eV/A As the bonding energy of each carbon atom in C60 is 7.4 eV, and the volume of a C60 molecule 3 is 3 , it is easy to estimate that 10ÿ2 eV/A @700 A the energy necessary to set free one carbon atom 3 the molecular energy density of and 0.64 eV/A C60 . As shown in Fig. 2 (b), the onset of C60 de3 and struction at a transference of @5 ´ 10ÿ3 eV/A 3 the total destruction at @0.5 eV/A for both ions are in agreement with the above estimates. The observed increase of H as function of rt (Fig. 5) has a shape similar to the C60 destruction intensity of Fig. 2, but with two important dierences: (a) the hardness data do not merge into a common curve for both ions; (b) the increase of
638
C.E. Foerster et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 634±638
hardness starts at a higher deposited energy density than the energy necessary to initiate the destruction of the C60 molecules. The large increase of H by a factor of @50 only occurs at the highest ¯uences, when the C60 layers are transformed into amorphous carbon, as shown by Raman Spectroscopy. With regard to the fact that the increase of H starts at lower deposited energy density for N than for He irradiation, the reasons can be the higher total stopping cross section of N ions, or the fact that there is a larger contribution of nuclear stopping power for N than for He in the total stopping power. In summary, our experimental results show that the highest values of hardness and Young modulus of C60 ion irradiated ®lms are obtained when the C60 is transformed into an amorphous layer. Similar improvements in hardness obtained in ion irradiated polymers [10] and glassy carbon [11] could also be attributed to the amorphization produced by the irradiations. Acknowledgements This work was supported by the Brazilian Agencies FINEP/PRONEX, CAPES and FAPERGS.
References [1] N.S. Dresselhauss, G. Dresselhauss, P.C. Eklund, J. Mat. Res. 8 (1993) 2054. [2] B. Bhushan, B.K. Gupta, G.W. Van Cleef, C. Capp, J.V. Coe, Appl. Phys. Lett. 25 (1993) 3253. [3] T. Thundat, R.J. Warmackk, D. Diing, R.N. Compton, Appl. Phys. Lett. 63 (1993) 891. [4] J. Raun, B. Bhushan, J. Mat. Res. 8 (1993) 3019. [5] B.K. Gupta, B. Bhushan, C. Capp, J.V. Coe, J. Mater. Res. 9 (1994) 2823. [6] L. Palmetshofer, J. Kastner, Nucl. Instr. and Meth. B 966 (1995) 343. [7] F.C. Zawislak, D.L. Baptista, M. Behar, D. Fink, P.L. Grande, J.A.H. da Jornada, Nucl. Instr. and Meth. B, submitted. [8] J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. B 174 (1980) 257. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 15564. [10] G.R. Rao, E.H. Lee, J. Mater. Res. 11 (1996) 2661. [11] D. Mclulloch, A. Homan, S. Praver, J. Appl. Phys. 74 (1993) 135.