APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 6
10 AUGUST 1998
Amorphous-tetrahedral diamondlike carbon layered structures resulting from film growth energetics M. P. Siegal,a) J. C. Barbour, P. N. Provencio, D. R. Tallant, and T. A. Friedmann Sandia National Laboratories, Albuquerque, New Mexico 87185-1421
~Received 30 March 1998; accepted for publication 8 June 1998! High-resolution transmission electron microscopy ~HRTEM! shows that amorphous-tetrahedral diamondlike carbon (a-tC) films grown by pulsed-laser deposition on Si~100! consist of three-to-four layers, depending on the growth energetics. We estimate the density of each layer using both HRTEM image contrast and Rutherford backscattering spectrometry. The first carbon layer and final surface layer have relatively low density. The bulk of the film between these two layers has higher density. For films grown under the most energetic conditions, there exists a superdense a-tC layer between the interface and bulk layers. The density of all four layers, and the thickness of the surface and interfacial layers, correlate well with the energetics of the depositing carbon species. © 1998 American Institute of Physics. @S0003-6951~98!03232-X# to ablate the graphite targets for this study were 11, 27, and 45 J/cm2. Figure 1 is a cross-sectional HRTEM image of the 11 J/cm2 film taken at lower magnification so that the entire film can be seen. A thin gold coating was deposited onto the film surface to easily denote its location. From this image, we find that the total film thickness is ;43.0 ~60.5! nm from the film surface to its interface with the Si substrate. Thicknesses of 52.0 and 58.0 nm for the films grown at 27 and 45 J/cm2, respectively, were measured from similar images. RBS analysis, using 3.5 MeV He ions, yielded carbon areal densities of each film as 6.59, 8.24, and 9.0331017 (65%) C atoms/cm2. This gives average mass densities of 3.06, 3.16, and 3.11 g/cm3, respectively, for films grown at PLD energy densities of 11, 27, and 45 J/cm2. However, this simple analysis assumes that the a-tC films are homogeneous in density. Figure 1 shows that this is not the case, that thin layers of varying contrast exist within the a-tC film at both its surface and substrate interface. The brighter region at the film surface probably results from the lower density a-tC material expected at the end of the film growth process, consistent with the subplantation model.4,18 This layer of film has not experienced the same localized high pressure as the bulk of the material. The bright layer at the film–substrate interface also likely represents a lower density material than the bulk. TRIM code calculations find that for C ion implan-
Hydrogen-free amorphous-tetrahedral diamondlike carbon (a-tC) films with properties approaching those of diamond were first reported in 1988 using two different energetic physical growth methods, cathodic filtered ion arc and pulsed-laser deposition ~PLD!.1,2 Many papers have since reported a-tC structural properties such as threefold (sp 2 -like! to fourfold (s p 3 -like! coordinated carbon bonding ratios, residual film stress, and mass density as a function of the energetics of a-tC growth.3–11 Others have correlated these structural properties to hardness, electronic transport, dielectric permittivity, optical transparency, and electron emission.12–17 Most studies assume that a-tC is homogeneous throughout the thickness of a given film. The greatest complexity typically assumed is the existence of a fewnanometer-thick region of higher threefold coordinated carbon near the film surface, consistent with a subplantation model for a-tC growth.4,18 Briefly, the model hypothesizes that for depositing carbon to form fourfold coordinated bonding, the species must grow in a state of very high local pressure. This pressure can be achieved through the creation of point defects from ion implantation into a sublayer just below the deposition surface. Since the final layer deposited does not experience further subplantation, a surface region results that is rich in threefold coordinated carbon bonding. Furthermore, since a-tC properties appear to vary with the choice of substrate material, it may also be true that an interfacial region exists which is different from the bulk of the film.19 In this letter, we use high-resolution transmission electron microscopy ~HRTEM! to observe the nature of this interface region for films grown by PLD on Si~100! substrates. In combination with carbon atom areal densities measured using Rutherford backscattering spectrometry ~RBS!, the density of each carbon region within a film can be determined. Nominally 50 nm thick a-tC films were grown at room temperature in a vacuum chamber with base pressure ,1027 Torr using 248 nm ~KrF! PLD from a pyrolytic graphite target onto Si~100! substrates. The details of deposition are described in Ref. 9. The laser energy densities used
FIG. 1. Cross-sectional TEM image of a-tC film grown by PLD using 11 J/cm2.
a!
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Appl. Phys. Lett., Vol. 73, No. 6, 10 August 1998
FIG. 3. ~a! Layer thickness and ~b! density of interface ~I and II!, bulk ~III!, and surface ~IV! layers of a-tC films grown as a function of PLD energy density.
FIG. 2. High-resolution TEM images of a-tC/Si(100) interfaces from films grown at ~a! 11 J/cm2 and ~b! 45 J/cm2.
tation energies up to 400 eV, the intermixing of C and Si atoms is 80%.3,5,6,19,22–24 The presence of the lower density surface layer is not a surprise. However, the existence of a two-layer, pure C interfacial region is not intuitively obvious. First, note that the density of these layers increases with growth energy; indeed, the density of all the layers are dependent on a-tC growth energetics. Second, the first interface layer is of low density. Finally, the second interface layer is ‘‘superdense.’’ The densities of crystalline graphite and diamond are 2.25 and 3.51 g/cm3, respectively. Since the density of glassy carbon, or nanocrystalline graphite can be as low as 1.8 g/cm3, well below that of graphite, it is astounding to find that a thin layer occurs in amorphous carbon films that can actually be similar to that of diamond. How is this possible? Ion bombardment of a low-energy, low-mass atom upon a heavier mass substrate atom will result in partial film deposition and partial backscattering away from the substrate. Furthermore, as a carbon film begins to form, a wave of energetic recoiled C atoms and interstitials can also reflect from the Si substrate. The 10’s-to-100’s eV energies of the C species in both PLD and filtered arc deposition are in this classification. Therefore, it is unlikely to attain a high, localized pressure necessary for the creation of a high percentage of fourfold coordinated C bonds when a large fraction of the incoming species is being backscattered. This results in a first layer region of nominally low density. As this first region develops in thickness, the implanting C species experience increasing interaction with a growing C film rather than with the Si substrate. Backscattering of C atoms decreases in frequency, allowing a film with greater density to grow on top of the first layer. For films grown using the highest energetics, some depositing species will penetrate the thin C layer and be backscattered off the Si substrate interface. This unique occurrence of C species being deposited into a layer from both directions can result in the creation of extreme localized pressure. It is apparent from the HRTEM image in Fig. 2~b! and the data presented in Fig. 3~b! that this results in the development of a thin region of super-high density. Finally, as the film thickness continues to grow, implantation of C species to the depths of the substrate becomes untenable, and all further film growth occurs in the manner understood as subplantation. A relatively uniform film then grows close to the film surface until deposition is terminated. The TEM images taken at lower magnification, such as Fig. 1, typically show a monotonic brightness increase from
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the substrate interface to the film surface region. This may suggest that a-tC films become slowly less dense as a function of film thickness. However, interpretation of such contrast variations over the extent of the entire film thickness is also highly dependent on the thickness uniformity of the cross-sectional HRTEM sample preparation, and, therefore, creates an uncertainty in the contrast-density relationship. In summary, the results of this letter are of clear importance to understanding the properties of a-tC films, especially those properties that may be highly dependent on the existence of multilayers of varying density, i.e., threefold to fourfold coordinated C atom ratios. These results are also relevant to correctly performing analysis of structural measurements that are affected by the entire film, such as Raman spectroscopy and x-ray reflectivity. The authors thank J. Sullivan, E. Stechel, and P. Schultz for the many interpretative discussions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE under Contract No. DEAC04-94AL85000.
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S. S. Wagal, E. M. Juengerman, and C. B. Collins, Appl. Phys. Lett. 53, 187 ~1988!. 2 P. J. Martin, S. W. Filipczuk, R. P. Netterfield, J. S. Field, D. F. Whitnall, and D. R. McKenzie, J. Mater. Sci. Lett. 7, 410 ~1988!. 3 J. Kulik, Y. Lifshitz, G. D. Lempert, J. W. Rabalais, and D. Marton, J. Appl. Phys. 76, 5063 ~1994!. 4 D. R. McKenzie, D. Muller, and B. A. Pailthorpe, Phys. Rev. Lett. 67, 773 ~1991!. 5 E. Grossman, G. D. Lempert, J. Kulik, D. Marton, J. W. Rabalais, and Y. Lifshitz, Appl. Phys. Lett. 68, 1214 ~1996!. 6 Z. Y. Chen, Y. H. Yu, J. P. Zhao, X. Wang, X. H. Liu, and T. S. Shi, J. Appl. Phys. 83, 1281 ~1998!. 7 M. Chhowalla, C. A. Davis, M. Weiler, B. Kleinsorge, and G. A. J. Amaratunga, J. Appl. Phys. 79, 2237 ~1996!. 8 H. C. Ong and R. P. H. Chang, Phys. Rev. B 55, 13 213 ~1997!. 9 M. P. Siegal, T. A. Friedmann, S. R. Kurtz, D. R. Tallant, R. L. Simpson, F. Dominguez, and K. F. McCarty, Mater. Res. Soc. Symp. Proc. 349, 507 ~1994!. 10 L. J. Martinez-Miranda, J. P. Sullivan, T. A. Friedmann, M. P. Siegal, T. W. Mercer, and N. J. DiNardo, Mater. Res. Soc. Symp. Proc. 383, 459 ~1995!. 11 M. P. Siegal, D. R. Tallant, L. J. Martinez-Miranda, N. J. DiNardo, J. P. Sullivan, P. P. Newcomer-Provencio, J. C. Barbour, T. A. Friedmann, and R. L. Simpson ~unpublished!. 12 J. Robertson, Phys. Rev. Lett. 68, 220 ~1992!. 13 V. I. Merkulov, J. S. Lannin, C. H. Munro, S. A. Asher, V. S. Veerasamy, and W. I. Milne, Phys. Rev. Lett. 78, 4869 ~1997!. 14 R. Lossy, D. L. Pappas, R. A. Roy, J. J. Cuomo, and V. M. Sura, Appl. Phys. Lett. 61, 171 ~1992!. 15 N. Missert, T. A. Friedmann, J. P. Sullivan, and R. G. Copeland, Appl. Phys. Lett. 70, 1995 ~1997!. 16 J. P. Sullivan, P. B. Mirkarimi, K. F. McCarty, T. A. Friedmann, M. P. Siegal, and M. L. Lovejoy ~unpublished!. 17 B. S. Satyanarayana, A. Hart, W. I. Milne, and J. Robertson, Appl. Phys. Lett. 71, 1430 ~1997!. 18 Y. Lifshitz, S. R. Kasi, and J. W. Rabelais, Phys. Rev. Lett. 62, 1290 ~1989!. 19 J. J. Cuomo, D. L. Pappas, J. Bruley, J. P. Doyle, and K. L. Saenger, J. Appl. Phys. 70, 1706 ~1991!. 20 TRIMRC computer code by D. K. Brice, Albuquerque, NM. 21 C. A. Davis, G. A. J. Amaratunga, and K. M. Knowles, Phys. Rev. Lett. 80, 3280 ~1998!. 22 P. H. Gaskell, A. Saeed, P. Chieux, and D. R. McKenzie, Phys. Rev. Lett. 67, 1286 ~1991!. 23 D. L. Pappas, D. L. Saenger, J. Bruley, W. Krakow, J. J. Cuomo, T. Gu, and R. W. Collins, J. Appl. Phys. 71, 5675 ~1992!. 24 F. Xiong, Y. Y. Wang, V. Leppert, and R. P. H. Chang, J. Mater. Res. 8, 2265 ~1993!.
