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Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films
Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films P.A. Karaseov a,⇑, V.S. Protopopova b, K.V. Karabeshkin a, E.N. Shubina a, M.V. Mishin a, J. Koskinen b, S. Mohapatra c, A. Tripathi d, D.K. Avasthi e, A.I. Titov a a
Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia Aalto University, Espoo, Finland Guru Gobind Singh Indraprastha University, New Delhi, India d Inter University Accelerator Center, New Delhi, India e Amity University, Noida 201313, Uttar Pradesh, India b c
a r t i c l e
i n f o
Article history: Received 21 November 2015 Received in revised form 7 April 2016 Accepted 8 April 2016 Available online xxxx Keywords: Tetrahedral amorphous carbon Metal doping Swift heavy ion irradiation Conductive channel formation Conductive AFM
a b s t r a c t Thin carbon films were grown at room temperature on (0 0 1) n-Si substrate using dual cathode filtered vacuum arc deposition system. Graphite was used as a source of carbon atoms and separate metallic electrode was simultaneously utilized to introduce Ni or Cu atoms. Films were irradiated by 100 MeV Ag7+ ions to fluences in the range 1 1010–3 1011 cm 2. Rutherford backscattering spectroscopy, Raman scattering, scanning electron microscopy and atomic force microscopy in conductive mode were used to investigate film properties and structure change under irradiation. Some conductive channels having metallic conductivity type were found in the films. Number of such channels is less than number of impinged ions. Presence of Ni and Cu atoms increases conductivity of those conductive channels. Fluence dependence of all properties studied suggests different mechanisms of swift heavy ion irradiation-induced transformation of carbon matrix due to different chemical effect of nickel and copper atoms. Ó 2016 Published by Elsevier B.V.
1. Introduction Surface functionalization has become one of the most active research areas in recent times due to technological applications demand materials with specific properties. Diamond-like carbon (DLC) is a valuable material to engineer surface properties because it possesses wide variety of attractive properties, like high hardness, low friction and high wear resistance. In particular, use of different DLC coatings can strongly enhance field emission (FE) [1]. Further FE current increase can be achieved by swift heavy ion (SHI) irradiation of the applied coating. Indeed, under SHI irradiation, amorphous carbon undergoes modification of the bonding ratio along the ion trajectories. In diamond-like tetrahedral amorphous carbon (ta-C), a change to a more graphitic (hence, more conductive) phase has been found [2]. Fabrication of conductive nanowires in insulative DLC matrix using SHI irradiation with energies 1 MeV/nucleon was demonstrated [3–5]. There is a good reason to believe that doping of ta-C with different metal impurities should improve the track conductivity in compare to ones ⇑ Corresponding author. E-mail address: [email protected] (P.A. Karaseov).
formed in undoped ta-C film. Indeed, as it was shown in [6], incorporation of low concentrations of B, N, Cu, and Fe during the film growth process results in an enhancement of latent track current. Two mechanisms can be responsible for that enhancement, namely, catalytic effect of impurities, which enhances transformation of insulating diamond-like bonds to highly conductive graphite-like phase within the track, and percolation of highly conductive metallic nano- and micro-grains in the film. Which mechanism dominates in the given conditions is not understood yet. In this study we investigate the influence of doping with nickel and copper on properties of ta-C films and explore track formation in these films under 100 MeV Ag ion irradiation. 2. Experimental details Highly conductive (0.001–0.002 X cm) n-type phosphorousdoped (1 0 0) silicon 1 1 and 0.5 1 cm in size were used as substrates. The very low resistivity Si substrates ensure that no Schottky barriers at the ta-C/Si interface obscure the I–V measurements. Si wafers were cleaned by ultrasonication in acetone before placing into deposition chamber. Sample fabrication was done by dual cathode filtered vacuum arc (FCVA) consecutive deposition
http://dx.doi.org/10.1016/j.nimb.2016.04.022 0168-583X/Ó 2016 Published by Elsevier B.V.
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
2
P.A. Karaseov et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
of carbon and metal layers. The FCVA system was equipped with a 60° bent magnetic filter for macro particle contamination reduction. Bending magnet was switched to pass carbon and metal ions simultaneously with arc pulse switching between cathodes. This method yields a homogenous distribution of the dopant impurities throughout the grown film. The chamber was evacuated by a dryscroll vacuum pump and a cryo-pump. Total pressure during the deposition not exceeded 1 10 3 Pa. The distance between the substrate holder and the filter border was about 1 cm. Samples were placed in the thermally insulating, grounded, rotating holder (rotation velocity was 20 rpm). Part of the substrate surface was masked to get contact area for further electric measurements. Two cathodes, 99.997% graphite rod and metal rod (nickel or copper, 99.0% purity) 6.35 mm in diameter were surrounded by a cylindrical anode. The initiation of arc pulses at a specific cathode was synchronized with magnetic filter switching via National InstrumentsTM virtual instrument and with LABVIEWTM software. System interface allowed us to control film composition by variation of the number of arc pulses at particular cathode and their sequence. The arc current pulses had the amplitude of 0.7 kA and width of 0.6 ms, each pulse was triggered at 1 Hz frequency. The average deposition rate of carbon, nickel and copper was estimated by measuring of monatomic film thickness deposited by known number of pulses. Recipes to grow 100 nm thick undoped, nickel- and copper-doped samples (referred below as ta-C, ta-C: Ni and ta-C:Cu respectively) were developed as follows. 5:1 arc pulse sequence on carbon and nickel cathodes was used to get 3 at.% concentration of nickel in ta-C volume with total amount of 1440 pulses. 4:1 carbon and copper cathode pulse sequence was used to get 3 at.% copper concentration with total amount of 1500 pulses. Total number of pulses on carbon cathode was 1200 in all cases. Films were irradiated at room temperature with 100 MeV Ag7+ ions using 15 MV Pelletron accelerator at IUAC, New Delhi. Ions with this energy have electronic energy loss Se = 14 keV/nm, uniform throughout the film (calculated by TRIM 2008). This Se level is close to track formation threshold in amorphous carbon [8]. The base pressure in the vacuum chamber during the irradiation was 1 10 3 Pa and the current was about 0.5 pnA (1 particle nA = 6.25 109 ions/s). The ion beam was scanned over area of 1 1 cm2 by an electromagnetic scanner. Ion fluence was in the range 1 1010–3 1011 cm 2. Ag ions are stopped deep inside the silicon substrate and hence do not affect any film properties. Rutherford backscattering spectroscopy using 0.7 MeV He++ probing ion beam (500 keV HVEE machine) was used to determine doping level achieved and metal distribution throughout the film depth. Raman spectroscopy (Horiba Jobin-Yvon Labram HR Raman spectrometer, excitation wavelength 488 nm) was used as an effective way to investigate the sp2 cluster structure in carbon films. Spectra were normalized to silicon peak; silicon background was then subtracted; the characteristic G- and D-peaks were fitted using Gauss’ functions [7]. Surface properties were also investigated by scanning electron microscopy using Supra 55VP machine. Contact atomic force microscopy (AFM) and spreading resistance microscopy (SRM or conductive-AFM) studies were performed using scanning probe multimicroscope Ntegra Aura (NT-MDT Company). The measurements were carried out in ‘scanning by sample’ mode with the use of universal measuring head equipped with the special probe holder designed for current measurements. Diamond-coated conductive probes (HA_HP_DCP) with a typical curvature radius of 100 nm and nominal spring force constant of 16 N/m were utilized. Conductive AFM was performed in single-pass mode, which permitted simultaneous acquisition of topography maps, roughness data, current maps, current distribution, and the average current JSR flowing through the studied sample area under the voltage USR applied to the probe. The USR value
can be varied from 10 to 10 V. Film conductivity was investigated by means of current–voltage spectroscopy. During the measurements, the range of the applied voltage values varied from sample to sample and was limited by the maximum measured current of ±25 nA. 3. Results and discussion For application of ion tracks embedded in an isolating matrix, it is important to have the conductivity contrast (the ratio of in- and out-track conductivity) as high as possible. Doping with metals can increase conductivity of the ta-C matrix via (i) enhanced graphitic phase formation; (ii) formation of metal nanoclusters, or other non-uniform distribution of metallic content, and (iii) combination of both mechanisms. We perform investigation of properties of asgrown films to get insight into their structure. Fig. 1 shows RBS spectra collected from as-deposited films. Impurity peak profiles (indicated by arrows in Fig. 1) reveal uniform metal distribution throughout doped film. Signal from carbon atoms in the film is also clearly seen on the background appeared due to He ion scattering deeper in silicon substrate. Average relative atomic concentration in doped films was estimated by integration of corresponding RBS peaks [9] and is presented in Table 1. Silicon background was subtracted before carbon peak integration. Almost same doping level of about 3 at.% was achieved with both metals used. SEM images of as-grown films (see inset to Fig. 5) show smooth surface without significant features, except some, very rarely seen, small particles. We believe they appear due to non-ideal filtering of macroparticles sputtered from the electrode and not due to metal clustering in the film itself. Hence we neglect them and perform conductive AFM and other investigations over non-contaminated areas. Atomic force microscopy also revealed very smooth film surface, with roughness less than 0.5 nm. All Raman spectra (see Fig. 2) have two peaks centered at 517 and 950 cm–1 raised by scattering in silicon substrate and two more peaks centered around 1360 and 1550 cm–1 both caused by scattering in carbon films. These so-called D- and G-peaks are well established features of Raman process in amorphous carbon films [7]. Shape of that part of spectrum excited by visible wavelength depends on the configuration of the sp2-bonded clusters and not on sp3-bonded sites due to sp3 sites have 50 times lower visible light absorption cross section. Ratio of integrated intensities of
Fig. 1. RBS spectra obtained from as-grown samples. Only 1 of each 5 experimental points is shown for figure clarity. Channels collecting probing ions scattered from C, Cu and Ni atoms located at the sample surface are indicated by arrows.
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
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Table 1 Properties of as-grown films.
Impurity concentration, at.% G-peak position, cm 1 ID/IG
ta-C
ta-C:Ni
ta-C:Cu
0 1563 0.26
3.5 1560 0.52
2.9 1550 0.54
a
b
Fig. 2. Raman spectra collected from as-grown films. D- and G- peaks on carbon – induced part of the spectra are indicated. Gauss fitting used to determine ID/IG ratio is also shown. Spectra are shifted in vertical direction and only 1 of each 15 experimental points is shown by a symbol for clarity in the figure.
these two peaks (ID/IG) and G-peak position provide indirect information about diamond-like phase of the film [7]. In the case of scattering in disordered materials (like our films) random distribution of phonon lifetimes is expected, which is well described by Gauss distribution, hence, we use two symmetric-line fitting. As it is seen in Fig. 2, cumulative fitting curves coincide well with experimental ones. Same well coinciding fits was found in the case of scattering from irradiated films. [7]. ID/IG ratios and G-peak positions obtained for different as-grown samples is presented in Table 1. It reveals similar ID/IG ratio for metal-doped films (0.53) which is twice higher than that for undoped sample (0.26). G-peak for ta-C film is positioned at 1562 cm 1. Consistent with the interpretation in the literature, the intensity ratio of the D peak to G peak and the position of the G peak are indicative for high sp3 fraction in undoped sample [7,10,11]. For ta-C:Ni and ta-C:Cu samples G peak is at 1560 cm 1 and 1550 cm 1 respectively which is the signature of higher amount of sp2-bonded fraction in nickel- and especially in copper-doped as-grown films [7,10]. Change in ID/IG ratio and G-peak shift after irradiation with 100 MeV Ag7+ swift heavy ions are shown in Fig. 3a and Fig. 3b correspondingly. It is seen that in the pure ta-C film ID/IG ratio slightly increases with fluence and G-peak position remains at almost wavelength. Irradiation of ta-C:Cu films causes slight increase in that ratio, and for ta-C:Ni film ID/IG reaches maximum of 0.9 at a fluence 5 1010 cm 2 and then decreases to 0.62 after irradiation to 3 1011 cm 2. G-peak position in copper-doped sample gradually increases from 1550 to 1558 cm 1 during irradiation, whereas in ta-C:Ni sample it decreases reaching 1552 cm 1 at fluence 5 1010 cm 2 and then restores to initial value at 3 1011 cm 2. So we conclude that nickel and copper have different effect on behaviour of carbon matrix they are embedded under SHI irradiation. Indeed, information about relative content of sp3 bonded carbon obtained from Raman data is not direct and require
Fig. 3. ID/IG ratio (a) and G-peak shift (b) for different films as indicated, as a function of irradiation fluence. Lines are to guide the eye.
further investigation by other techniques more sensitive to sp3 hybridisation. Fig. 4(a) presents set of I–V curves taken by AFM in conductive mode at different points on as-grown ta-C:Ni sample and corresponding averaged curve. It is seen that resistivity of as-grown sample is high with non-Ohmic behaviour. Same result was obtained during measurements on other as-grown films and corresponding averaged curves are shown by closed symbols in Fig. 4(d). It reveals that doping with metal increases film conductivity, and ta-C:Cu film is more conductive than ta-C:Ni film. Indeed, higher graphitic phase content found (see Fig. 2) could be the reason of such increase in sample conductivity. SHI irradiation changes electric properties of our samples. Resistivity taken at some points of samples irradiated to fluence 1 1010 cm 2 is very low with linear current-versus-voltage dependences (see Fig. 4b). I.e. some ohmictype conductive paths between film surface and substrate are formed at such fluence. Number of points where ohmic-type conductivity was found is less than number of ions impinged the target surface. Conductivity measured at other points of irradiated film remains non-ohmic is found to be just slightly increased after irradiation (Fig. 4(a and b)). Dispersion between I and V curves measured at different points of low-fluence-irradiated films also exceeds that of unirradiated samples. Average current gradually increases with fluence as illustrated in Fig. 4(c), where I–V characteristics at different fluences for undoped ta-C sample are shown. In the Fig. 4(d) averaged I–V curves for doped and undoped films irradiated to fluence 5 1010 cm 2 are presented. Pure ta-C film exhibits very slight conductivity change after irradiation. Strong difference in influence of nickel and copper doping on film conductivity change under irradiation is seen in this figure. Averaged resistivity of ta-C:Cu film increases giving tip current saturation (25 nA) at 0.5 V, which is 4 times less than in unirradiated sample. Conductivity of ta-C:Ni film increases more significantly with fluence. Indeed, Ni-doped film exhibit almost ohmic conductivity in many surface points after irradiation to 5 1010 cm 2 as revealed by averaged curve in Fig. 4(d). With fluence increase,
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
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P.A. Karaseov et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
a
b
c
d
Fig. 4. Set of I–V curves collected from unirradiated (a) and irradiated to 1 1010 cm 2 (b) ta-C:Ni sample. (c) Averaged I–V curves obtained from ta-C film irradiated to different fluences indicated in the legend in 1010cm 2. (d) Averaged I–V curves for all three samples, before (closed symbols) and after (open symbols) irradiation to a fluence 5 1010 cm 2. Only 1 of each 20 experimental points is shown by a symbol for figure clarity.
average conductivity further increases in all other cases studied. Dispersion between I and V curves measured at different points of high-fluence-irradiated films rapidly decreases during irradiation to fluences over 5 1010 cm 2. Interestingly, topography of all samples, either virgin or irradiated, is very smooth, without any significant features. We were not able to find any correlation between topographic features and areas having ohmic resistivity which is unlike to data reported earlier [3–6]. These findings bring us to the conclusion that most of ions do not form continuous tracks in carbon matrix and only few conductive channels extend from the surface to the substrate after irradiation to low fluence. These channels appear as a result of percolation of incomplete latent tracks formed by different ions. Such a track can consist of partially graphitized carbon network, which in turn can be decorated by impurity atoms or nanoclusters. AFM-tip-measured current is then determined by number, length and conductivity of transformed volumes and by barriers between them [12]. SEM studies reveal formation of light-grey spot contrast in the images acquired from copper-doped samples after SHI irradiation. Note that no specific SEM contrast was found on images acquired from all as-grown films (see inset to Fig. 5a). Image contrast from nickel-doped film is quite distinct. It consists of dark spots surrounded by lighter circle. Example of both contrasts after irradiation of ta-C:Cu and ta-C:Ni to a fluence of 5 1010 cm 2 is shown in Fig. 5(a) and (b) correspondingly. Some dark spots in
Fig. 5. SEM images of ta-C:Cu (a) and ta-C:Ni (b) samples irradiated to a fluence 5 1010 cm 2. Bar length is 5 lm. Inset shows typical image of all three virgin films and irradiated ta-C film.
the Fig. 5(b) have very small bright dots appeared close to spot center. Earlier we have identified similar dots as nickel clusters obtained in carbon matrix [13]. However, in this study we are not aware that metal nanoclusters were formed under irradiation. Further investigation is needed to clarify this question. Thus, all experimental results suggest that type of impurity metal used, i.e. Cu or Ni, plays important role in swift-heavyion-induced matrix transformation. The main reason of differences found in manner they behave under irradiation is chemical interactions of Ni and Cu with carbon atoms. More research is needed to further clarify that difference.
4. Conclusions In conclusion, tetrahedral carbon films with different metal doping were grown on Si substrate using dual cathode filtered vacuum arc deposition technique. As grown film properties and their change under 100 MeV Ag7+ swift heavy ion irradiation were studied. Atomic force microscopy did not reveal formation of any irradiation-related peculiarities on the surface topography. Few conductive channels were found in the films by current–voltage measurements after irradiation at low fluence regime. More continuous conductive channels are formed with fluence increase. Ion-irradiation-induced conductivity change grows more efficiently in films doped with Ni than with Cu. Scanning electron microscopy has shown irradiation-induced formation of specific contrast on metal-doped samples. This contrast is strong in the case of nickel doping, it is very slight in copper doped film and
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
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we found no such spots on undoped samples. Formation of metal nanoparticles in as-grown metal-doped films or under irradiation was not found yet. Fluence dependence of all properties studied suggests different chemical effect of nickel and copper on transformation of carbon matrix under irradiation. Acknowledgements Authors are grateful to Dr. C. Trautmann for helpful discussion and appreciate valuable comments of two anonymous reviewers, whose kind attention helped us to improve quality of that manuscript. V.P. acknowledges Micronova Nanofabrication Centre and Department of Applied Physics at Aalto University, Espoo, Finland for the provision of facilities and technical support. Authors are also thankful to IUAC Pelletron group for providing stable ion beam due to which, it was possible to irradiate the samples. Work was partially supported by Russian Foundation for Basic Research and by DST of India. References
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[2] M. Waiblinger, Ch. Sommerhalter, B. Pietzak, J. Krauser, B. Mertesacker, M.CH. Lux-Steiner, S. Klaumünzer, A. Weidinger, C. Ronning, H. Hofsäß, Appl. Phys. A 69 (1999) 239. [3] N. Koenigsfeld, H. Hofsass, D. Schwen, A. Weidinger, C. Trautmann, R. Kalish, Diamond Relat. Mater. 12 (2003) 469. [4] M. Waiblinger, Ch. Sommerhalter, B. Pietzak, J. Krauser, B. Mertesacker, M.Ch. Lux-steiner, et al., Appl. Phys. A 69 (1999) 239. [5] A. Kumar, D.K. Avasthi, A. Tripathi, L.D. Filip, J.D. Carey, J.C. Pivin, J. Appl. Phys. 102 (2007) 044305. [6] J. Krauser, A.-K. Nix, H.-G. Gehrke, H. Hofsass, C. Trautmann, A. Weidinger, Nucl. Instr. Meth. Phys. Res. B 272 (2012) 280. [7] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [8] J. Krauser, A. Weidinger, M. Waiblinger, V. Hoffmann, C. Trautmann, B. Schultrich, H. Hofsäss, C. Ronning, Proceedings of the Sixth Applied Diamond Conference/Frontier Carbon Technology Joint Conference, 2001, p. 400 (Auburn, Alabama). [9] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, 1986 (North Holland, New York, p. 63). [10] A. Anders, N. Pasaja, S. Sansongsiri, Rev. Sci. Instr. 78 (2007) 063901. [11] W. Ren, A. Iyer, J. Koskinen, A. Kaskela, E.I. Kauppinen, K. Avchaciov, K. Nordlund, J. Appl. Phys. 118 (2015) 194306. [12] A. Kumar, D.K. Avasthi, A. Tripathi, D. Kabiraj, F. Singh, J.C. Pivin, J. Appl. Phys. 101 (2007) 014308. [13] E.N. Shubina, P.A. Karaseov, M.V. Mishin, V.S. Protopopova, A.Ya. Vinogradov, N.N. Karasev, A.V. Arkhipov, A.L. Shakhmin, O.A. Podsvirov, A.I. Titov, St. Petersburg State Polytech. Univ. J. Phys. Math. 4–2 (182) (2013) 115.
[1] S.C. Ray, S.K. Ghosh, Z. Chiguvare, U. Palnitkar, W.-F. Pong, I.-N. Lin, P. Papakonstantinou, A.M. Strydom, Jpn. J. Appl. Phys. 49 (2010) 111301.
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films P.A. Karaseov a,⇑, V.S. Protopopova b, K.V. Karabeshkin a, E.N. Shubina a, M.V. Mishin a, J. Koskinen b, S. Mohapatra c, A. Tripathi d, D.K. Avasthi e, A.I. Titov a a
Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia Aalto University, Espoo, Finland Guru Gobind Singh Indraprastha University, New Delhi, India d Inter University Accelerator Center, New Delhi, India e Amity University, Noida 201313, Uttar Pradesh, India b c
a r t i c l e
i n f o
Article history: Received 21 November 2015 Received in revised form 7 April 2016 Accepted 8 April 2016 Available online xxxx Keywords: Tetrahedral amorphous carbon Metal doping Swift heavy ion irradiation Conductive channel formation Conductive AFM
a b s t r a c t Thin carbon films were grown at room temperature on (0 0 1) n-Si substrate using dual cathode filtered vacuum arc deposition system. Graphite was used as a source of carbon atoms and separate metallic electrode was simultaneously utilized to introduce Ni or Cu atoms. Films were irradiated by 100 MeV Ag7+ ions to fluences in the range 1 1010–3 1011 cm 2. Rutherford backscattering spectroscopy, Raman scattering, scanning electron microscopy and atomic force microscopy in conductive mode were used to investigate film properties and structure change under irradiation. Some conductive channels having metallic conductivity type were found in the films. Number of such channels is less than number of impinged ions. Presence of Ni and Cu atoms increases conductivity of those conductive channels. Fluence dependence of all properties studied suggests different mechanisms of swift heavy ion irradiation-induced transformation of carbon matrix due to different chemical effect of nickel and copper atoms. Ó 2016 Published by Elsevier B.V.
1. Introduction Surface functionalization has become one of the most active research areas in recent times due to technological applications demand materials with specific properties. Diamond-like carbon (DLC) is a valuable material to engineer surface properties because it possesses wide variety of attractive properties, like high hardness, low friction and high wear resistance. In particular, use of different DLC coatings can strongly enhance field emission (FE) [1]. Further FE current increase can be achieved by swift heavy ion (SHI) irradiation of the applied coating. Indeed, under SHI irradiation, amorphous carbon undergoes modification of the bonding ratio along the ion trajectories. In diamond-like tetrahedral amorphous carbon (ta-C), a change to a more graphitic (hence, more conductive) phase has been found [2]. Fabrication of conductive nanowires in insulative DLC matrix using SHI irradiation with energies 1 MeV/nucleon was demonstrated [3–5]. There is a good reason to believe that doping of ta-C with different metal impurities should improve the track conductivity in compare to ones ⇑ Corresponding author. E-mail address: [email protected] (P.A. Karaseov).
formed in undoped ta-C film. Indeed, as it was shown in [6], incorporation of low concentrations of B, N, Cu, and Fe during the film growth process results in an enhancement of latent track current. Two mechanisms can be responsible for that enhancement, namely, catalytic effect of impurities, which enhances transformation of insulating diamond-like bonds to highly conductive graphite-like phase within the track, and percolation of highly conductive metallic nano- and micro-grains in the film. Which mechanism dominates in the given conditions is not understood yet. In this study we investigate the influence of doping with nickel and copper on properties of ta-C films and explore track formation in these films under 100 MeV Ag ion irradiation. 2. Experimental details Highly conductive (0.001–0.002 X cm) n-type phosphorousdoped (1 0 0) silicon 1 1 and 0.5 1 cm in size were used as substrates. The very low resistivity Si substrates ensure that no Schottky barriers at the ta-C/Si interface obscure the I–V measurements. Si wafers were cleaned by ultrasonication in acetone before placing into deposition chamber. Sample fabrication was done by dual cathode filtered vacuum arc (FCVA) consecutive deposition
http://dx.doi.org/10.1016/j.nimb.2016.04.022 0168-583X/Ó 2016 Published by Elsevier B.V.
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
2
P.A. Karaseov et al. / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
of carbon and metal layers. The FCVA system was equipped with a 60° bent magnetic filter for macro particle contamination reduction. Bending magnet was switched to pass carbon and metal ions simultaneously with arc pulse switching between cathodes. This method yields a homogenous distribution of the dopant impurities throughout the grown film. The chamber was evacuated by a dryscroll vacuum pump and a cryo-pump. Total pressure during the deposition not exceeded 1 10 3 Pa. The distance between the substrate holder and the filter border was about 1 cm. Samples were placed in the thermally insulating, grounded, rotating holder (rotation velocity was 20 rpm). Part of the substrate surface was masked to get contact area for further electric measurements. Two cathodes, 99.997% graphite rod and metal rod (nickel or copper, 99.0% purity) 6.35 mm in diameter were surrounded by a cylindrical anode. The initiation of arc pulses at a specific cathode was synchronized with magnetic filter switching via National InstrumentsTM virtual instrument and with LABVIEWTM software. System interface allowed us to control film composition by variation of the number of arc pulses at particular cathode and their sequence. The arc current pulses had the amplitude of 0.7 kA and width of 0.6 ms, each pulse was triggered at 1 Hz frequency. The average deposition rate of carbon, nickel and copper was estimated by measuring of monatomic film thickness deposited by known number of pulses. Recipes to grow 100 nm thick undoped, nickel- and copper-doped samples (referred below as ta-C, ta-C: Ni and ta-C:Cu respectively) were developed as follows. 5:1 arc pulse sequence on carbon and nickel cathodes was used to get 3 at.% concentration of nickel in ta-C volume with total amount of 1440 pulses. 4:1 carbon and copper cathode pulse sequence was used to get 3 at.% copper concentration with total amount of 1500 pulses. Total number of pulses on carbon cathode was 1200 in all cases. Films were irradiated at room temperature with 100 MeV Ag7+ ions using 15 MV Pelletron accelerator at IUAC, New Delhi. Ions with this energy have electronic energy loss Se = 14 keV/nm, uniform throughout the film (calculated by TRIM 2008). This Se level is close to track formation threshold in amorphous carbon [8]. The base pressure in the vacuum chamber during the irradiation was 1 10 3 Pa and the current was about 0.5 pnA (1 particle nA = 6.25 109 ions/s). The ion beam was scanned over area of 1 1 cm2 by an electromagnetic scanner. Ion fluence was in the range 1 1010–3 1011 cm 2. Ag ions are stopped deep inside the silicon substrate and hence do not affect any film properties. Rutherford backscattering spectroscopy using 0.7 MeV He++ probing ion beam (500 keV HVEE machine) was used to determine doping level achieved and metal distribution throughout the film depth. Raman spectroscopy (Horiba Jobin-Yvon Labram HR Raman spectrometer, excitation wavelength 488 nm) was used as an effective way to investigate the sp2 cluster structure in carbon films. Spectra were normalized to silicon peak; silicon background was then subtracted; the characteristic G- and D-peaks were fitted using Gauss’ functions [7]. Surface properties were also investigated by scanning electron microscopy using Supra 55VP machine. Contact atomic force microscopy (AFM) and spreading resistance microscopy (SRM or conductive-AFM) studies were performed using scanning probe multimicroscope Ntegra Aura (NT-MDT Company). The measurements were carried out in ‘scanning by sample’ mode with the use of universal measuring head equipped with the special probe holder designed for current measurements. Diamond-coated conductive probes (HA_HP_DCP) with a typical curvature radius of 100 nm and nominal spring force constant of 16 N/m were utilized. Conductive AFM was performed in single-pass mode, which permitted simultaneous acquisition of topography maps, roughness data, current maps, current distribution, and the average current JSR flowing through the studied sample area under the voltage USR applied to the probe. The USR value
can be varied from 10 to 10 V. Film conductivity was investigated by means of current–voltage spectroscopy. During the measurements, the range of the applied voltage values varied from sample to sample and was limited by the maximum measured current of ±25 nA. 3. Results and discussion For application of ion tracks embedded in an isolating matrix, it is important to have the conductivity contrast (the ratio of in- and out-track conductivity) as high as possible. Doping with metals can increase conductivity of the ta-C matrix via (i) enhanced graphitic phase formation; (ii) formation of metal nanoclusters, or other non-uniform distribution of metallic content, and (iii) combination of both mechanisms. We perform investigation of properties of asgrown films to get insight into their structure. Fig. 1 shows RBS spectra collected from as-deposited films. Impurity peak profiles (indicated by arrows in Fig. 1) reveal uniform metal distribution throughout doped film. Signal from carbon atoms in the film is also clearly seen on the background appeared due to He ion scattering deeper in silicon substrate. Average relative atomic concentration in doped films was estimated by integration of corresponding RBS peaks [9] and is presented in Table 1. Silicon background was subtracted before carbon peak integration. Almost same doping level of about 3 at.% was achieved with both metals used. SEM images of as-grown films (see inset to Fig. 5) show smooth surface without significant features, except some, very rarely seen, small particles. We believe they appear due to non-ideal filtering of macroparticles sputtered from the electrode and not due to metal clustering in the film itself. Hence we neglect them and perform conductive AFM and other investigations over non-contaminated areas. Atomic force microscopy also revealed very smooth film surface, with roughness less than 0.5 nm. All Raman spectra (see Fig. 2) have two peaks centered at 517 and 950 cm–1 raised by scattering in silicon substrate and two more peaks centered around 1360 and 1550 cm–1 both caused by scattering in carbon films. These so-called D- and G-peaks are well established features of Raman process in amorphous carbon films [7]. Shape of that part of spectrum excited by visible wavelength depends on the configuration of the sp2-bonded clusters and not on sp3-bonded sites due to sp3 sites have 50 times lower visible light absorption cross section. Ratio of integrated intensities of
Fig. 1. RBS spectra obtained from as-grown samples. Only 1 of each 5 experimental points is shown for figure clarity. Channels collecting probing ions scattered from C, Cu and Ni atoms located at the sample surface are indicated by arrows.
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Table 1 Properties of as-grown films.
Impurity concentration, at.% G-peak position, cm 1 ID/IG
ta-C
ta-C:Ni
ta-C:Cu
0 1563 0.26
3.5 1560 0.52
2.9 1550 0.54
a
b
Fig. 2. Raman spectra collected from as-grown films. D- and G- peaks on carbon – induced part of the spectra are indicated. Gauss fitting used to determine ID/IG ratio is also shown. Spectra are shifted in vertical direction and only 1 of each 15 experimental points is shown by a symbol for clarity in the figure.
these two peaks (ID/IG) and G-peak position provide indirect information about diamond-like phase of the film [7]. In the case of scattering in disordered materials (like our films) random distribution of phonon lifetimes is expected, which is well described by Gauss distribution, hence, we use two symmetric-line fitting. As it is seen in Fig. 2, cumulative fitting curves coincide well with experimental ones. Same well coinciding fits was found in the case of scattering from irradiated films. [7]. ID/IG ratios and G-peak positions obtained for different as-grown samples is presented in Table 1. It reveals similar ID/IG ratio for metal-doped films (0.53) which is twice higher than that for undoped sample (0.26). G-peak for ta-C film is positioned at 1562 cm 1. Consistent with the interpretation in the literature, the intensity ratio of the D peak to G peak and the position of the G peak are indicative for high sp3 fraction in undoped sample [7,10,11]. For ta-C:Ni and ta-C:Cu samples G peak is at 1560 cm 1 and 1550 cm 1 respectively which is the signature of higher amount of sp2-bonded fraction in nickel- and especially in copper-doped as-grown films [7,10]. Change in ID/IG ratio and G-peak shift after irradiation with 100 MeV Ag7+ swift heavy ions are shown in Fig. 3a and Fig. 3b correspondingly. It is seen that in the pure ta-C film ID/IG ratio slightly increases with fluence and G-peak position remains at almost wavelength. Irradiation of ta-C:Cu films causes slight increase in that ratio, and for ta-C:Ni film ID/IG reaches maximum of 0.9 at a fluence 5 1010 cm 2 and then decreases to 0.62 after irradiation to 3 1011 cm 2. G-peak position in copper-doped sample gradually increases from 1550 to 1558 cm 1 during irradiation, whereas in ta-C:Ni sample it decreases reaching 1552 cm 1 at fluence 5 1010 cm 2 and then restores to initial value at 3 1011 cm 2. So we conclude that nickel and copper have different effect on behaviour of carbon matrix they are embedded under SHI irradiation. Indeed, information about relative content of sp3 bonded carbon obtained from Raman data is not direct and require
Fig. 3. ID/IG ratio (a) and G-peak shift (b) for different films as indicated, as a function of irradiation fluence. Lines are to guide the eye.
further investigation by other techniques more sensitive to sp3 hybridisation. Fig. 4(a) presents set of I–V curves taken by AFM in conductive mode at different points on as-grown ta-C:Ni sample and corresponding averaged curve. It is seen that resistivity of as-grown sample is high with non-Ohmic behaviour. Same result was obtained during measurements on other as-grown films and corresponding averaged curves are shown by closed symbols in Fig. 4(d). It reveals that doping with metal increases film conductivity, and ta-C:Cu film is more conductive than ta-C:Ni film. Indeed, higher graphitic phase content found (see Fig. 2) could be the reason of such increase in sample conductivity. SHI irradiation changes electric properties of our samples. Resistivity taken at some points of samples irradiated to fluence 1 1010 cm 2 is very low with linear current-versus-voltage dependences (see Fig. 4b). I.e. some ohmictype conductive paths between film surface and substrate are formed at such fluence. Number of points where ohmic-type conductivity was found is less than number of ions impinged the target surface. Conductivity measured at other points of irradiated film remains non-ohmic is found to be just slightly increased after irradiation (Fig. 4(a and b)). Dispersion between I and V curves measured at different points of low-fluence-irradiated films also exceeds that of unirradiated samples. Average current gradually increases with fluence as illustrated in Fig. 4(c), where I–V characteristics at different fluences for undoped ta-C sample are shown. In the Fig. 4(d) averaged I–V curves for doped and undoped films irradiated to fluence 5 1010 cm 2 are presented. Pure ta-C film exhibits very slight conductivity change after irradiation. Strong difference in influence of nickel and copper doping on film conductivity change under irradiation is seen in this figure. Averaged resistivity of ta-C:Cu film increases giving tip current saturation (25 nA) at 0.5 V, which is 4 times less than in unirradiated sample. Conductivity of ta-C:Ni film increases more significantly with fluence. Indeed, Ni-doped film exhibit almost ohmic conductivity in many surface points after irradiation to 5 1010 cm 2 as revealed by averaged curve in Fig. 4(d). With fluence increase,
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022
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a
b
c
d
Fig. 4. Set of I–V curves collected from unirradiated (a) and irradiated to 1 1010 cm 2 (b) ta-C:Ni sample. (c) Averaged I–V curves obtained from ta-C film irradiated to different fluences indicated in the legend in 1010cm 2. (d) Averaged I–V curves for all three samples, before (closed symbols) and after (open symbols) irradiation to a fluence 5 1010 cm 2. Only 1 of each 20 experimental points is shown by a symbol for figure clarity.
average conductivity further increases in all other cases studied. Dispersion between I and V curves measured at different points of high-fluence-irradiated films rapidly decreases during irradiation to fluences over 5 1010 cm 2. Interestingly, topography of all samples, either virgin or irradiated, is very smooth, without any significant features. We were not able to find any correlation between topographic features and areas having ohmic resistivity which is unlike to data reported earlier [3–6]. These findings bring us to the conclusion that most of ions do not form continuous tracks in carbon matrix and only few conductive channels extend from the surface to the substrate after irradiation to low fluence. These channels appear as a result of percolation of incomplete latent tracks formed by different ions. Such a track can consist of partially graphitized carbon network, which in turn can be decorated by impurity atoms or nanoclusters. AFM-tip-measured current is then determined by number, length and conductivity of transformed volumes and by barriers between them [12]. SEM studies reveal formation of light-grey spot contrast in the images acquired from copper-doped samples after SHI irradiation. Note that no specific SEM contrast was found on images acquired from all as-grown films (see inset to Fig. 5a). Image contrast from nickel-doped film is quite distinct. It consists of dark spots surrounded by lighter circle. Example of both contrasts after irradiation of ta-C:Cu and ta-C:Ni to a fluence of 5 1010 cm 2 is shown in Fig. 5(a) and (b) correspondingly. Some dark spots in
Fig. 5. SEM images of ta-C:Cu (a) and ta-C:Ni (b) samples irradiated to a fluence 5 1010 cm 2. Bar length is 5 lm. Inset shows typical image of all three virgin films and irradiated ta-C film.
the Fig. 5(b) have very small bright dots appeared close to spot center. Earlier we have identified similar dots as nickel clusters obtained in carbon matrix [13]. However, in this study we are not aware that metal nanoclusters were formed under irradiation. Further investigation is needed to clarify this question. Thus, all experimental results suggest that type of impurity metal used, i.e. Cu or Ni, plays important role in swift-heavyion-induced matrix transformation. The main reason of differences found in manner they behave under irradiation is chemical interactions of Ni and Cu with carbon atoms. More research is needed to further clarify that difference.
4. Conclusions In conclusion, tetrahedral carbon films with different metal doping were grown on Si substrate using dual cathode filtered vacuum arc deposition technique. As grown film properties and their change under 100 MeV Ag7+ swift heavy ion irradiation were studied. Atomic force microscopy did not reveal formation of any irradiation-related peculiarities on the surface topography. Few conductive channels were found in the films by current–voltage measurements after irradiation at low fluence regime. More continuous conductive channels are formed with fluence increase. Ion-irradiation-induced conductivity change grows more efficiently in films doped with Ni than with Cu. Scanning electron microscopy has shown irradiation-induced formation of specific contrast on metal-doped samples. This contrast is strong in the case of nickel doping, it is very slight in copper doped film and
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we found no such spots on undoped samples. Formation of metal nanoparticles in as-grown metal-doped films or under irradiation was not found yet. Fluence dependence of all properties studied suggests different chemical effect of nickel and copper on transformation of carbon matrix under irradiation. Acknowledgements Authors are grateful to Dr. C. Trautmann for helpful discussion and appreciate valuable comments of two anonymous reviewers, whose kind attention helped us to improve quality of that manuscript. V.P. acknowledges Micronova Nanofabrication Centre and Department of Applied Physics at Aalto University, Espoo, Finland for the provision of facilities and technical support. Authors are also thankful to IUAC Pelletron group for providing stable ion beam due to which, it was possible to irradiate the samples. Work was partially supported by Russian Foundation for Basic Research and by DST of India. References
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[2] M. Waiblinger, Ch. Sommerhalter, B. Pietzak, J. Krauser, B. Mertesacker, M.CH. Lux-Steiner, S. Klaumünzer, A. Weidinger, C. Ronning, H. Hofsäß, Appl. Phys. A 69 (1999) 239. [3] N. Koenigsfeld, H. Hofsass, D. Schwen, A. Weidinger, C. Trautmann, R. Kalish, Diamond Relat. Mater. 12 (2003) 469. [4] M. Waiblinger, Ch. Sommerhalter, B. Pietzak, J. Krauser, B. Mertesacker, M.Ch. Lux-steiner, et al., Appl. Phys. A 69 (1999) 239. [5] A. Kumar, D.K. Avasthi, A. Tripathi, L.D. Filip, J.D. Carey, J.C. Pivin, J. Appl. Phys. 102 (2007) 044305. [6] J. Krauser, A.-K. Nix, H.-G. Gehrke, H. Hofsass, C. Trautmann, A. Weidinger, Nucl. Instr. Meth. Phys. Res. B 272 (2012) 280. [7] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [8] J. Krauser, A. Weidinger, M. Waiblinger, V. Hoffmann, C. Trautmann, B. Schultrich, H. Hofsäss, C. Ronning, Proceedings of the Sixth Applied Diamond Conference/Frontier Carbon Technology Joint Conference, 2001, p. 400 (Auburn, Alabama). [9] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, 1986 (North Holland, New York, p. 63). [10] A. Anders, N. Pasaja, S. Sansongsiri, Rev. Sci. Instr. 78 (2007) 063901. [11] W. Ren, A. Iyer, J. Koskinen, A. Kaskela, E.I. Kauppinen, K. Avchaciov, K. Nordlund, J. Appl. Phys. 118 (2015) 194306. [12] A. Kumar, D.K. Avasthi, A. Tripathi, D. Kabiraj, F. Singh, J.C. Pivin, J. Appl. Phys. 101 (2007) 014308. [13] E.N. Shubina, P.A. Karaseov, M.V. Mishin, V.S. Protopopova, A.Ya. Vinogradov, N.N. Karasev, A.V. Arkhipov, A.L. Shakhmin, O.A. Podsvirov, A.I. Titov, St. Petersburg State Polytech. Univ. J. Phys. Math. 4–2 (182) (2013) 115.
[1] S.C. Ray, S.K. Ghosh, Z. Chiguvare, U. Palnitkar, W.-F. Pong, I.-N. Lin, P. Papakonstantinou, A.M. Strydom, Jpn. J. Appl. Phys. 49 (2010) 111301.
Please cite this article in press as: P.A. Karaseov et al., Swift heavy ion irradiation of metal containing tetrahedral amorphous carbon films, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.04.022