Hard Graphitic-Like Amorphous Carbon Films with High Stress and Local Microscopic Density R

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Hard graphitic-like amorphous carbon films with high stress and local microscopic density R. G. Lacerda, P. Hammer, C. M. Lepienski, F. Alvarez, and F. C. Marques

Citation: Journal of Vacuum Science & Technology A 19, 971 (2001); doi: 10.1116/1.1365130 View online: http://dx.doi.org/10.1116/1.1365130 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/19/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 143.106.1.143 On: Thu, 11 Sep 2014 17:48:59 Hard graphitic-like amorphous carbon films with high stress and local microscopic density R. G. Lacerdaa) and P. Hammer Universidade Estadual de Campinas, Unicamp, Instituto de Fı´sica ‘‘Gleb Wataghin’’-13083-970, Campinas-SP, Brazil C. M. Lepienski Universidade Federal do Parana´-UFPR, Laborato´rio de Propriedades Nanomecaˆnicas, Departamento de Fı´sica, Caixa Postal 19081, 81531-990 Curitiba-PR, Brazil F. Alvarez and F. C. Marques Universidade Estadual de Campinas, Unicamp, Instituto de Fı´sica ‘‘Gleb Wataghin’’-13083-970, Campinas-SP, Brazil ͑Received 25 August 2000; accepted 19 February 2001͒ In this work, we report unusual properties of amorphous carbon films prepared by ion beam-assisted deposition using different noble gases ͑neon, argon, and krypton͒. Independent of the noble gas ions used, the intrinsic compressive stress and plasmon energy increase sharply with the assisting ion beam energies up to 100 eV. Above this energy, the material properties depend on the mass of the ion. The highest values of stress ͑ϳ12 GPa͒ and plasmon energy associated with the C 1s core electron ͑29.5 eV͒ are of the same order of magnitude as those reported for highly tetrahedral amorphous carbon films. Structural results, however, indicate that the material is composed of a hard, highly stressed, and locally dense graphite-like network, i.e., a predominantly sp2-bonded material. It is suggested that the ion bombardment compacts the film structure by reducing the interplanar cluster distances, generating high compressive stress and high local density. The differences in the properties of the films introduced by Ne, Ar, and Kr bombarding ions are also discussed. © 2001 American Vacuum Society. ͓DOI: 10.1116/1.1365130͔

I. INTRODUCTION sp2-bonded amorphous carbon films deposited by ion beam- assisted deposition ͑IBAD͒ technique using neon, argon, and For two decades, there has been a continuous effort to krypton ions. characterize and understand the structure of amorphous carbon films produced by various methods. However, a full un- II. EXPERIMENT derstanding of the microstructure of a-C films is still a chal- The a-C films were deposited by the IBAD technique lenge mainly due to the ability of carbon to bond with using two Kauffman ion sources. A detailed description of different hybridization. For example, depending on the mix- the deposition system can be found elsewhere.5 Three sets of ing of the sp2 and sp3 bonds, it is possible to prepare mate- films were prepared using separately Arϩ,Krϩ, and Neϩ as rials with completely different structures and properties. Fur- bombarding ions. Each noble gas was introduced in both thermore, it is well known in the literature that the type of Kauffman sources, one for the sputtering of a graphite target carbon hybridization alone does not completely determine and the other for the simultaneous assisting of the growing the properties of a-C films: they are also critically dependent film. The graphite target was sputtered using ion beam en- on the deposition conditions and applied technique. Due to ergy of 1500 eV. Films of about 100 nm thick were prepared the strong in-plane bonds of graphite, a question was raised at 150 °C with ion beam-assisting energies in the range of 0 recently about the possibility of preparing graphitic-like to about 800 eV. The ion per C atom flux ratio (I/A) in- ͑ ͒ amorphous carbon films with high hardness.1 Recently, a-C creases, for the three series of films Ne, Ar, and Kr , from 0.2 to 1.8 with the assisting energy. For each assisting en- films with a graphitic-like structure and relatively high hard- ͑ ͒ ͑ ͒ ergy, the flux ratio is about the same within 10% for the ness 20–40 GPa have actually been developed and treated ͑ ͒ 2–4 three series. The ion flux I was obtained by measuring the as a new form of hard a-C films. However, the micro- assisting ion beam current density, and C atom flux ͑A͒ was structure of hard graphitic-like a-C film and its influence on determined from the deposition rate without using the assist- other properties are still unclear and a subject of intense in- ing gun. After the deposition, the samples were transferred 2 vestigation. The understanding of this hard sp microstruc- within the vacuum system to the ultrahigh vacuum chamber ture is of fundamental importance for the optimization of the ͑pressure Ͻ10Ϫ7 Pa͒ for in situ photoelectron spectroscopy material for possible technological application. In this work, measurements ͑XPS, UPS͒. No oxygen was detected. In ad- we report on the preparation of hard predominantely dition, no contaminants ͑such as hydrogen, water, etc.͒ were detected by Fourier transform infrared spectroscopy within a͒Electronic mail: gribel@ifi.unicamp.br the spectrometer resolution. After the XPS/UPS analysis, the

Õ Õ Õ Õ Õ Õ 971 J. Vac. Sci. Technol. A 19„3…, May Jun 2001 0734-2101 2001 19„3… 971 5 $18.00 ©2001 American Vacuum Society 971 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 143.106.1.143 On: Thu, 11 Sep 2014 17:48:59 972 Lacerda et al.: Hard graphitic-like amorphous carbon ﬁlms 972

sample.7 In the free-electron model, the plasmon energy represents the collective excitations of valence electrons. For various materials, such as a-C, the maximum of the electron energy-loss function corresponds to the classical plasmon energy.8,9 As is well known, the square of the plasmon en- 2 ͑ ͒ ergy (E p) is proportional to the local electronic atomic density of the material.10,11 Therefore, in amorphous materials the plasmon energy can be used as a probe of the local microscopic density rather than to the global macroscopic density of the film.5,7–11 So, the variation of the plasmon energies displayed in Fig. 1͑a͒ indicates a local density modification of the films. The compressive stress reaches a maximum of ϳ10 GPa for Neϩ assisting energies of ϳ100 eV. Most probably due to the experimental uncertainties, no differences in the stress were obtained with Arϩ and Krϩ ions. It is noteworthy that both stress and plasmon energy have a similar dependence on the assisting energy. The highest ͑smallest͒ plasmon energy and stress is obtained for Arϩ ͑Neϩ) by assisting the growth with ϳ400 eV ͓Fig. 1͑a͔͒. Moreover, the highest stress values ͑ϳ12 GPa͒ and plasmon energy ͑ϳ29.5 eV͒ obtained for Arϩ assisted films are comparable to those reported for highly tetrahedral amorphous carbon films.12,13 Also, the maximum of the plasmon energy depends on the bombarding ion species. Since the deposition conditions of the three series of films ͑Ne, Ar, and Kr͒ are similar, includ- ing the same energy dependency of the flux ratio, an expla- nation of these findings is attempted. For assisting energies under 100 eV, the mechanism of material densification and stressing is independent of the ion ͑ ͒ ͑ ͒ FIG. 1. Dependence of the plasmon energy a , compressive stress b , and atomic mass ͓Figs. 1͑a͒ and 1͑b͔͒. This behavior can be un- (⌬E) energy transfer ͑c͒ of a-C films prepared by IBAD as a function of the noble gas assisting energy. derstood by the classical scattering theory. The energy transferred (⌬E) by the ions to the growing film is given by ⌬ ϭ␥ ␥ϭ ϩ 2 E (I/A)Ea where 4mcM ion /(M ion mc) , mc is the samples were analyzed by Raman spectroscopy at room tem- carbon mass, M ion is the ion mass, I/A is the ion to atom flux perature. The macroscopic density and noble gas concentra- ͑ ϭ ϩ ϩ ϩ ϭ ͒ ratio I Ne ,Ar ,orKr and A C in our case , and Ea is tions ͑of about 3% for all films͒ were determined by Ruth- the ion assisting energy. As indicated in Fig. 1͑c͒, the ener- ͑ ͒ ϩ ϩ ϩ erford backscattering spectroscopy RBS . The hardness was gies transferred by Ne ,Ar , and Kr are about the same determined using a set of different loads in the 0.5–90 mN for energies below 100 eV. This may explain why the ion range with a Nanoindenter IIs from Nano Instruments. Stress mass starts to exert more influence on the properties of the measurements were performed using an optical bending films at bombardment energies above 100 eV. Another im- 6 beam method. portant point is that the plasmon energy ͓Fig. 1͑a͔͒ is larger ͑smaller͒ for materials assisted with Arϩ ͑Neϩ), indicating a III. RESULTS AND DISCUSSION more efficient densification when Arϩ ions are used. It is Figure 1͑a͒ shows the dependence of the plasmon energy well known from classical scattering theory that the energy associated to the C 1s core electron ͑from now on ‘‘plasmon transfer is more effective between atoms of equivalent mass energy’’͒ as a function of the assisting energy. The plasmon ͑see above the expression of ⌬E͒. Among the noble gases, ϩ energy increases significantly upon increasing assisting en- Ne is the one with a mass closest to C atoms and hence has ergy. Provided that the assisting energy is below 100 eV, the most effective energy transference on ion bombardment, similar plasmon energies are obtained independently of the see Fig. 1͑c͒. This probably means that the large energy ϩ bombarding ion ͑Arϩ,Neϩ,orKrϩ͒. By using Neϩ, a maxi- transferred by Ne would sputter and damage the material mum plasmon energy value of about 28.7 eV is reached at an rather than efficiently increase its local density. On the other ϩ assisting energy of 100 eV. On the other hand, when hand, the massive Kr ions would weakly interact with the C Arϩ ͑Krϩ) is used, maximum plasmon energy of 29.5 eV atoms and a less effective material compaction would be ͑29.1 eV͒ is obtained at assisting energies of about 400 eV. achieved. Results of Marton et al. reporting TRIM simulations The plasmon energy peak, measured by XPS, is produced by of total displacements and defects created in graphite by the inelastic scattering of C 1s electrons before leaving the noble gas bombardment at various energies seems to support

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FIG. 3. Valence band spectra UPS ͑He II͒ of a-C films prepared by IBAD FIG. 2. Typical Raman spectra of a-C film prepared under argon assisting using Ne, Ar, and Kr as working gases. The top of the valence band of a energy of 400 eV. ta-C film ͑Ref. 15͒ is also included for comparison.

14 ϩ this interpretation. These researchers show that Ne is the 284.3 eV for graphitic structures, while the respective posi- one which produces the largest total atom displacements and 15,18 ϩ tion for diamond is around 285.0 eV. The C 1s spectrum, vacancies in graphite. Conversely, the Kr ion is less effec- shown in Fig. 4, was deconvoluted using three Gaussians tive in generating displacements ͑i.e., compression͒ in graph- 2 ͑ ͒ 3 ͑ϳ ϩ components representing the sp 284.3 eV , sp 285.0 ite, indicating the poor interaction between Kr and carbon eV͒ bonding structures, and a small background component. atoms. In short, we suggest that there is an optimum ion This procedure has already been used to estimate the relative assisting energy window for maximum material densifica- 3 2 15,18 ϩ concentration of sp and sp sites. By using the relative tion. This is probably the reason why Ar , an ion having an 2 3 ratio of the areas ͓A1(sp )/A2(sp )͔, we estimate a concen- intermediate atomic mass, compacts the material more effi- tration of about 80% of sp2 sites in our films, which is in ciently. agreement with the UPS results. Moreover, the low optical The plasmon energy has been widely used to estimate the band gap ͑0–0.5 eV͒ obtained for all samples is consistent 3 degree of sp bonding in pure amorphous carbon with a material with a high number of sp2 sites. Using an 12,13,15 ϳ films. Values of about 30 eV have been reported for empirical relation between band gap and sp2 concentration 3 12,13,15 films with high concentration of sp sites (ta-C). In reported by Ferrari, Chowalla and co-workers,17,19 a content ͑Ͼ addition, these films also show very high stress 7 higher than 80% of sp2 sites is expected. ͒ 13,16 GPa . It is to be noted that similar values of plasmon Figure 5 shows the nanohardness measurement for the energy and stress of those of ta-C were also found in our hardest a-C film prepared under Arϩ bombardment as a ͑ ͒ films see Fig. 1 . As will be seen, however, a careful analy- function of the indentation depth. As expected, the hardness sis of the film structure shows no evidence of a highly tetrahedral carbon phase. Figure 2 shows a Raman spectrum of the film with the highest plasmon energy. The disorder ͑D͒ band ͑ϳ1340 cmϪ1͒ and graphitic ͑G͒ band ͑ϳ1540 cmϪ1͒ are clearly observed. The ratio between these two bands is commonly used to obtain structural information of a-C 17 films. All films reported here have a high ID /IG ratio of about 2. This high ID /IG ratio indicates a strong presence of sp2 clusters in the film structure, in contrast with the low ͑ϳ ͒ 17 ID /IG ratio 0.1 found in ta-C films. This result is also supported by UPS measurements. Figure 3 depicts the UPS spectra of films prepared with the three noble gases used. A shoulder at the top of the valence band ͑0–4 eV͒ is characteristic for C–C ␲ bonds, indicating the predominant presence of sp2 sites in the material. No significant variation of these states was observed as a function of the assisting energy and the noble gas used. For comparison purposes, the top of the valence band of a ta-C film was also included in 15 FIG. 4. XPS measurements of the C 1s core level peak of a-C films pre- Fig. 3. In addition, the analysis of the C 1s core level spec- pared using Ne, Ar, and Kr as working gases. The C 1s peak was deconvo- trum of our films ͑Fig. 4͒ also indicates the predominance of luted using three Gaussians representing the sp2 (ϳ284.3 eV), sp3 sp2 sites. It is known that the C 1s peak is located at around (ϳ285.0 eV) bonding structures, and a small background component.

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ing energy ͑Fig. 6͒ is different from those IBAD samples obtained by Andre et al., which found decreasing density values at energies as low as 200 eV.20 This can be understood due to the low values of ion/C flux ratio (I/A) used in their work ͑Ͻ0.2͒ compared to the higher I/A(ϳ1.8) values used in our work. The analysis presented above strongly suggests the exis- tence of a hard graphitic-like structure with high stress and high local density. In short, these films are composed of a matrix of strained and locally dense sp2 clusters, probably cross linked by a small fraction of sp3 sites. Usually, hard a-C films with a high concentration of sp3 sites (ta-C) are prepared by the direct bombardment of carbon species (Cϩ).12,13 The relatively high stress ͑Ͼ7 GPa͒ and plasmon energy ͑ϳ30 eV͒ observed for these films are generally in- ϩ 16,21 FIG. 5. Nanohardness vs indentation depth for a-C film deposited under Ar terpreted by the subimplantation model. In this model, bombardment. the carbon ions arriving within an optimum energy range ͑50–600 eV͒, are implanted beneath the film surface increas- increases as a function of the indentation depth reaching ing the local density and generating high compressive stress 3 about 30 GPa, decreasing afterwards to hardness slightly by the preferential formation of sp C–C bonds. Thus, the higher than that of the silicon substrate for indentation depth high stress and local density are directly related to the for- 3 comparable to the film thickness ͑100 nm͒. It is noteworthy mation of the sp C–C bonds in the film during growth. In to mention that even though the films have a mainly this work, however, we report the formation of a mainly graphitic-like structure their hardness are relatively high, an graphitic-like a-C structure with both high stress and local unusual characteristic for an sp2-rich network. density comparable to those obtained for ta-C films. There- The high microscopic ͑local͒ density contrasts to the low fore, it is evident that the ͑standard͒ subimplantation model macroscopic density obtained by RBS, Fig. 6. The values of cannot be applied to these experimental findings. The reason about 2.3 g/cm3 are similar to the density of graphite but are can be related to the difference in the deposition process, relatively small compared to ϳ3 g/cm3 reported for ta-C since we used noble gases to assist the film during growth, films.12,19 This result indicates that our films have indeed a instead of an ion beam of carbon atoms. graphitic-like structure, as also supported by the results of In an attempt to understand how such a structure could be UPS, XPS, and Raman. The difference between the high formed, it is interesting to analyze the effects of high pres- microscopic density and the low macroscopic density can be sure on the lattice parameters of polycrystalline graphite. By explained in terms of the presence of voids in the material applying different hydrostatic pressures on powdered graph- structure, which reduces the average macroscopic density. ite, Lynch and Drickamer reported a decreasing interplanar On the other hand, as already mentioned, the plasmon energy distance with increasing pressure, see Fig. 7.22 On the other is sensitive to the local carbon environment and is not affect hand, these authors also reported a nonsignificant variation in by the presence of voids.10 Another important point is that the in-plane distance of the graphite powder. As indicated in the macroscopic density dependence on the noble gas assist- Fig. 1, there is a correlation between the plasmon energy and the compressive stress. To compare these results with those of Lynch and Drickamer, we have plotted the relative plasmon energy, i.e., the relative local density as a function of compressive stress of our films ͑Fig. 7͒. It is worth noting that the range of hydrostatic pressure used by Lynch and Drickamer is equivalent to the compressive stress of our films. As observed in the plot, the behavior of the local relative density on increasing stress has a very similar dependence to that of the graphite interplanar distance reduction as a function of the applied pressure. Therefore, it is probable that a similar reduction of the distance between planes of the graphitic-like clusters would be the cause of the increasing local density in our samples. The suggested reduction of the interplanar distance of graphitic-like domains, generating high stress and high local density, is probably related to an implantation process in-

FIG.6.Macroscopic density of a-C ﬁlms as a function of the noble gases duced by the intense noble gas bombardment during growth. assisting energy. The energetic noble gases ions striking the ﬁlm surface can

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compact the film structure by the reduction of the distance between the sp2 clusters, inducing both high compressive stress and local ͑microscopic͒ density. The achievement of highly stressed and locally dense films with low concentration of sp3 sites can also be understood with the ideas of the subimplantation process. Finally, it is shown that the plasmon energy alone cannot be used as a measure of the degree of sp3 bonding in pure a-C films. ACKNOWLEDGMENTS This work was partially sponsored by FAPESP, CNPq, and PADCT. The authors are in debt to Dr. F. L. Freire, Jr. for the RBS measurements.

1M. Coˆte´, J. C. Grossman, M. L. Cohen, and S. G. Loui, Phys. Rev. B 58, 664 ͑1998͒. 2G. A. J. Amaratunga, M. Chhowalla, C. J. Keily, I. Alexandru, and R. M. Devenish, Nature ͑London͒ 383, 321 ͑1996͒. 3 ͑ ͒ FIG. 7. Normalized interplanar distance ͑C/Co͒ of graphite ͑*͒ as a function R. G. Lacerda and F. C. Marques, Appl. Phys. Lett. 73,617 1998 . 4 of the applied pressure from Ref. 22 and relative local density increase I. Alexandru, H.-J. Scheibe, C. J. Kiely, A. J. Papworth, G. A. J. Ama- ͑᭝͒ ratunga, and B. Schultrich, Phys. Rev. B 60, 10 903 ͑1999͒. (P0 /P)ofa-C films as a function of the compressive stress using neon , 5 argon ͑᭺͒, and Kr ͑᭿͒ as working gases. C represents the graphite inter- F. Alvarez, M. C. dos Santos, and P. Hammer, Appl. Phys. Lett. 73,3521 0 ͑ ͒ planar distance under a pressure of about 1 GPa, and P represents the 1998 . 0 6M. M. de Lima Jr., R. G. Lacerda, J. Vilcarromero, and F. C. Marques, J. plasmon energy squared for the three sets of films with a compressive stress ͑ ͒ value of about 1 GPa. Appl. Phys. 86, 4936 1999 . 7See, for instance, F. Wooten, Optical Properties of Solids ͑Academic, New York, 1972͒. 8H. R. Philipp and H. Ehrenreich, Phys. Rev. 129, 1550 ͑1963͒. generate a ‘‘knock-on’’ process, forcing the implantation of 9H. Roth, K. Lang, M. Ebelshauser, D. Lang, and H. Schmoranzer, Phys. carbon atoms without necessarily creating sp3 bonds. There- Lett. A 235, 551 ͑1997͒. 10L. Ley, in Topics in Applied Physics, Vol 56: The Physics of Hydroge- fore, the implanted carbon atoms compress the film structure, nated Amorphous Silicon II, edited by J. D. Jonapoulos and G. Lucovsky 2 increasing the local density and maintaining the sp bonding ͑Springer, Berlin, 1984͒,p.67. structural character. The reduction of the interplanar distance 11Y. Katayama, T. Shimada, and K. Usami, Phys. Rev. Lett. 17, 1146 2 ͑1981͒. and the random distribution of the sp clusters, crosslinked 12 3 Y. Lifshitz, G. D. Lempert, E. Grossman, I. Avigal, C. Uzan-Saguy, R. by a small concentration of sp sites, can be responsible for Kalish, J. Kulik, D. Marton, and J. W. Rabalais, Diamond Relat. Mater. 4, the relatively high hardness of the films. These results are 318 ͑1995͒. also in agreement with the recent finding obtained by Monte 13P. J. Fallon, V. S. Veerasamy, C. A. Davis, J. Robertson, G. A. J. Ama- ratunga, W. I. Milne, and J. Koskinen, Phys. Rev. B 48,4777͑1993͒. Carlo simulations, showing that it is possible to form highly 14 23 D. Marton, K. J. Boyd, T. Lytle, and J. W. Rabalais, Phys. Rev. B 48, compressed threefold coordinated sites. Also, the im- 6757 ͑1993͒. planted noble gases incorporated into the film structure ͑ϳ3 15C. Ronning, E. Dreher, J.-U. Thiele, P. Oelhafen, and H. Hofsass, Dia- at. %͒ may be partially responsible for the observed stress. mond Relat. Mater. 6, 830 ͑1997͒. 16 ͑ ͒ Indeed, stress of the order of 1 GPa has been reported for J. Robertson, Diamond Relat. Mater. 2,984 1993 . 17A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14 095 ͑2000͒. implanted noble gas into amorphous semiconductors and me- 18J. Schafer, J. Ristein, R. Graupner, L. Ley, U. Stephann, Th. Frauenheim, tallic films.24 V. S. Veerasamy, G. A. J. Amaratunga, M. Weiler, and H. Ehrhardt, Phys. Rev. B 53, 7762 ͑1996͒. 19M. Chhowalla, J. Robertson, C. W. Chen, S. R. P. Silva, C. A. Davis, G. IV. CONCLUSIONS A. J. Amaratunga, and W. I. Milne, J. Appl. Phys. 81, 139 ͑1997͒. 20B. Andre, F. Rossi, A. van Veen, P. E. Mijnarends, H. Scht, and M. P. In summary, we presented novel results in graphitic-like Delplancke, Thin Solid Films 241, 171 ͑1994͒. a-C films with high stress, local density, and hardness by 21S. Uhlmann, Th. Frauenheim, and Y. Lifshitz, Phys. Rev. Lett. 81, 641 ͑ ͒ intense noble gases bombardment of the film during growth. 1998 . 22R. W. Lynch and H. G. Drickamer, J. Chem. Phys. 44,181͑1966͒. It is suggested that the strong ion bombardment knock-on 23M. Fyta and P. C. Keriles, J. Non-Cryst. Solids 266,760͑2000͒. carbon atoms beneath the surface. The implanted carbons 24H. Windschmann, Crit. Rev. Solid State Mater. Sci. 17,547͑1992͒.

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