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Journal of Crystal Growth 279 (2005) 420–424 www.elsevier.com/locate/jcrysgro
Effect of substrate temperature on the growth of ternary Al–C–N thin films by reactive magnetron sputtering A.L. Ji, Y. Du, L.B. Ma, Z.X. Cao Institute of Physics, P.O. Box 603, Beijing 100080, China Received 24 January 2005; accepted 17 February 2005 Available online 2 April 2005 Communicated by R. Kern
Abstract Oxygen-free aluminum carbonitride thin films were synthesized by reactive magnetron sputtering of Al target with a gas mixture of Ar, CH4 and N2. The effect of substrate temperature varying from the room temperature to 400 1C was investigated, since the crystalline Al–C–N compounds can be modeled as the stacking of zigzag building blocks Al2C2, Al2C and AlN. Generally, film growth proceeds preferably along the [0 0 0 l] direction, but m-plane growth makes its competitive presence in the temperature range from 50 to 260 1C, as revealed by X-ray diffraction. For the samples with a typical composition of Al50C13N37, the size of crystallites, and thus the root-mean-square roughness of the film, becomes larger with increasing substrate temperature. Berkovich hardness is over 27.0 GPa for all as-deposited films and a maximum value of 33.6 GPa was measured in the sample prepared at 300 1C. These results indicated that hard Al–C–N coatings with well-controlled orientation can be fabricated by reactive magnetron sputtering at moderate substrate temperatures. r 2005 Elsevier B.V. All rights reserved. PACS: 68.55; 81.15; 68.60.B Keywords: A3. Magnetron sputtering; A3. Thin film; B1. Aluminum carbonitride; B2. Hardness
1. Introduction Carbonitrides of light elements constitute a large family of wide-gap materials which can be prepared in forms of bulk crystal, thin film coating, ceramics and so on to serve extensive Corresponding author. Tel.: +86 10 82649441.
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[email protected] (A.L. Ji).
applications in various industries including microelectronics and optoelectronics [1–3]. Remarkably, a ternary compound might allow independent or simultaneous tailoring of their atomic and electronic structures by composition regulation; thus an effective tuning of the material properties can be established in a wide range [4–7]. It can be anticipated that the ternary Al–C–N compounds will exhibit some individual characters as they
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contain metal atoms, consequently they promise versatile applications in electronics due to the excellent properties such as readiness of metallization [8] and extreme electrical insulation. However, the Al–C–N compounds as individual competitive materials have not received comparable attention they deserve. Since the determination of the atomic structures for the homologous series (AlN)nAl4C3, n ¼ 1 to 6, by Jeffrey and Wu in 1960s [9,10], there had not been any systematic research work specifically devoted to aluminum carbonitrides in the following 30 years, to the best knowledge of the authors. In the recent few years, an increasing interest has been directed toward ternary Al–C–N and also some quaternary compounds such as SiCAlN and TiAlCN [11] with Al, C and N as constituents. In the year 2001, the cyanide crystal Al(CN)3 in the simple cubic structure was synthesized for the first time using a wet chemistry method [12]. This material in the form of white powder prefigures the wide-gap nature for the like of aluminum carbonitrides. Jiang and coworkers pioneered the fabrication of Al–C–N thin films by reactive magnetron sputtering, aiming at stabilized hypothetical bC3N4 by aluminum incorporation [13,14]. Their results show that the synthesis of authentic ternary phase for Al–C–N is a nontrivial enterprise. In the present work, we report the synthesis of the ternary Al–C–N films by reactive magnetron sputtering at moderate substrate temperatures. As the structure of this ternary compound can be modeled with the stacking of the zigzag segments Al2C2, Al2C and AlN—for detailed illustration see Figs. 1 and 2 in Ref. [9]—which may be formed on the flight path to the substrate, the substrate temperature is expected to play an important role in determining the microstructure of the deposits, hence the morphology and other relevant characteristics.
2. Experiment Films were grown on Si (1 0 0) substrates by reactive magnetron sputtering of an aluminum target (4 N purity) with the gas mixture of argon, nitrogen and methane. Before mounting into the
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Fig. 1. Typical energy-dispersive X-ray spectrum for the Al–C–N deposits. The presence of oxygen contamination could be excluded owing to the featurelessness of the profile at 530 eV.
Fig. 2. Profiles of the Al 2p spectral line for AlN and the ternary Al–C–N deposits. hn ¼ 1253:6 eV:
vacuum chamber, the substrates had been ultrasonically cleaned successively in alcohol and acetone, and rinsed with de-ionized water following 3 min etching in 1.0% HF solution to remove the native oxide. The base pressure in the chamber was better than 1.0 105 Pa, and the working pressure was maintained at 1.3 Pa with the total gas flow rate tuned at 4.0 sccm, of which 2.4 sccm were reserved for argon. The distance between the substrate and target was fixed at 50 mm. As a
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matter of routine, the target would be presputtered under proposed growth conditions for an hour in order to obtain a reproducible metal atom supply. The substrate temperature was varied from room temperature to 400 1C. Here, for the series of samples prepared for the study of the effect of substrate temperature, the flow rate for nitrogen is 1.44 sccm, while for methane it is only 0.16 sccm. Chemical composition of the deposits was determined in a complementary way by energydispersive X-ray spectrometry (Sirion, Fei) and Xray photoelectron spectroscopy (ESCA-lab, Mark II) using the Mg Ka line (hn ¼ 1253:6 eV). Surface morphology of the films was assessed by an atomic force microscope (AFM, Digital Instruments) operated in the contact mode. X-ray diffraction using the Cu Ka irradiation (Rigaku D/Max2400) was employed for structural characterization. In doing the hardness measurement with a Berkovich indenter (CSEM), the maximum indent depth was restricted within 80 nm to avoid the influence of the soft substrate.
3. Results and discussion The energy-dispersive X-ray spectrum in Fig. 1 demonstrates clearly an oxygen-free Al–C–N deposit. This can be ascribed to the utilization of methane as precursor—the reductive hydrogen ions can effectively eliminate the oxygen contamination. The atomic composition of the deposits was calculated based on the energy-dispersive Xray spectral data in conjunction with the X-ray photoelectron spectral measurement. As reported in our previous publication [15], as expected the aluminum content of the Al–C–N deposits in a wide range of processing parameters remains at about 50% within 1% fluctuation. For the series of samples concerned here the composition of the deposits suffers only slight modification by substrate temperature variation—it is Al50C13N37 with a deviation less than 1.0% for all the three components. The Al 2p spectral line centered at 73.7 eV with a full-width-at-half-maximum of 1.96 eV displays no obviously separate features by comparing with that from a pure AlN sample (Fig. 2), where the full-width-at-half-maximum is
only 1.60 eV resolved by the same spectrometer. This helps exclude the possibility of significant phase separation into the AlN and Al4C3, though being not strong enough to confirm the ternary nature of the deposits. X-ray diffraction provides more profound information on the microstructure of the deposits. Fig. 3 exhibits the X-ray diffraction patterns for the series of the samples. No reflection from either the AlN or the Al4C3 phase can be identified. The dominant peak at 2y 361 can be assigned to the (0 0 0 l) reflection of aluminum carbonitride—at moment without discrimination between the hexagonal (space group P63mc) and the rhombohe¯ dral (space group R3m) phases. Here ‘‘l’’ denotes the number of basal planes in a unit cell; it is an even number that can be as large as 30 as for the prototype (AlN)6Al4C3. With increasing substrate temperature, the position of this peak shifts continuously from 2y 35:561 to 2y 35:781: This is to say that at higher substrate temperature, the deposits are more compact along the c-axis, and the basal plane spacing becomes smaller owing to the adequate adjustment of the building blocks. The calculated basal plane spacing is d ( greater than the typical value of d 2:30 A ( 2:50 A; for the large crystallites of homologous series (AlN)nAl4C3 prepared at high temperature [9]. This is comprehensible since the Al–C–N samples prepared by magnetron sputtering are highly
Fig. 3. XRD pattern for the aluminum carbonitride films deposited at different substrate temperatures.
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perturbed in their microstructure, as confirmed by the direct transmission electron microscopic observation [15]. The stacking of three building blocks in a statistical way under given conditions results in severe local disorder in both chemistry and structure of the deposits. At the current stage, we cannot specify the value ‘‘n’’ in the formula (AlN)nAl4C3, consequently not the index ‘‘l’’ for the reflection (0 0 0 l) either. The local stacking disorder, however, implies an enhanced capability to prevent dislocation motion. For all the samples concerned here, a Berkovich hardness over 27.0 GPa was measured—it amounts to 33.65 GPa in the sample prepared at 300 1C. Hence, the Al–C–N compounds can be specified as a superhard material. With properly adjusted concentration of carbon, a Berkovich hardness of 53.4 GPa could be obtained [15], approaching the lower limit for the natural diamond.
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It is worth noting that for the samples prepared in the temperature range from 50 to 260 1C, a reflection from the prism plane ð1 0 1¯ 0Þ also appears at 2y 33:051; corresponding to a planar ( This m-plane growth makes its spacing d 2:71 A: presence in a competitive way with regard to the cplane growth, and the temperature window is rather narrow. This behavior can be primarily explained with the difference in activation energy for the three building blocks and the difference in surface energy for the two planes. For a persuasive atomistic explanation, one needs the exact value of these energies, but such data for aluminum carbonitride are still unavailable. In Fig. 4, the AFM images revealing the morphology of the deposits are displayed. The surface is generally smooth and compact. From the AFM images it is clearly seen that the films are composed of small distinct crystallites in a typical
Fig. 4. AFM height images in a range of 2 2 mm deposited at (a) room temperature; (b) 200 1C; (c) 300 1C; and (d) 400 1C.
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dimension of 100 nm. This is consistent with the results by X-ray diffraction where the half width for the (0 0 0 l) reflection is nearly 0.401. The particularly small size for the Al–C–N crystallites arises from the fact that the nucleation and growth proceed with three building blocks. The nuclei grow into many small grains due to the scarcity of surface mobility at the given, moderate substrate temperatures. With increasing substrate temperature, AFM image tells that the crystallite grows bigger, accordingly the surface turns rougher. As the substrate temperature was raised from the room temperature to 400 1C, the averaged grain size increases from 83.3 to 101.5 nm, and the surface roughness changes from 4.22 to 6.60 nm.
4. Conclusion In summary, ternary aluminum carbonitride thin films without oxygen contamination were grown by reactive magnetron sputtering at substrate temperatures varying from the room temperature to 400 1C. Generally, the deposits are cplane-oriented, but the m-plane growth occurs in a competitive way in the temperature range from 50 to 260 1C. The local stacking disorder of the zigzag building blocks confers the films excellent mechanical properties, and the deposits can be termed superhard. With very compact surface morphology and the grain sizes below 100 nm, these widegap films are expected to be highly transparent to visible light. Deposition of aluminum carbonitride films on various glasses is being carried out in our laboratory for the further optical investigation of this material, to which the adhesion problem needs specific attention.
Acknowledgments This work was supported by the National Science Foundation of China Grant nos. 60306009 and 10404034, and the National High Technology Research and Development Program of China Grant No. 2003AA302170.
References [1] D. Williams, B. Pleune, J. Kouvetakis, M.D. Williams, R.A. Andersen, J. Am. Chem. Soc. 122 (2000) 7735. [2] C.M. Zettering, M. Ostling, K. Wongchotigul, M.G. Spencer, X. Tang, J. Appl. Phys. 82 (1997) 2990. [3] J.H. Xu, L.H. Yu, I. Kojima, J. Appl. Phys. 94 (2003) 6827. [4] G. Lehmann, P. Hess, J.J. Wu, C.T. Wu, T.S. Wong, K.H. Chen, Phys. Rev. B 64 (2001) 165305. [5] J. Vlcek, M. Kormunda, J. Czek, Z. Soukup, V. Perina, J. Zemek, Diam. Relat. Mater. 12 (2003) 1287. [6] G. Radnoczi, G. Safran, Z. Czigany, T. Berlind, L. Hultman, Thin Solid Films 41–44 (2003) 440. [7] V.L. Solozhenko, D. Andrault, G. Fiquet, Appl. Phys. Lett. 78 (2001) 1385. [8] M. Eizenberg, K. Littau, S. Ghanayem, A. Mak, Y. Maeda, M. Chang, A.K. Sinha, Appl. Phys. Lett. 65 (1994) 2416. [9] G.A. Jeffrey, V.Y. Wu, Acta Cryst. 16 (1963) 559. [10] G.A. Jeffrey, V.Y. Wu, Acta Cryst. 20 (1966) 538. [11] R. Roucka, J. Tolle, A.V.G. Chizmeshya, P.A. Crozier, C.D. Poweleit, D.J. Smith, I.S.T. Tsong, J. Kouvetakis, Phys. Rev. Lett. 88 (2002) 206102. [12] D. Williams, B. Pleune, K. Leinenweber, J. Kouvetakis, J. Solid State Chem. 159 (2001) 244. [13] N. Jiang, S. Xu, K.N. Ostrikov, Int. J. Modern Phys. B 16 (2002) 1132. [14] N. Jiang, S. Xu, K.N. Ostrikov, Thin Solid Films 385 (2001) 55. [15] A.L. Ji, L.B. Ma, L. Liu, C.R. Li, Z.X. Cao, Appl. Phys. Lett. 86 (2005) 021918.
