An investigation of the properties of epitaxial chromium-substituted vanadium carbide thin films

Vacuum 109 (2014) 212e215

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An investigation of the properties of epitaxial chromium-substituted vanadium carbide thin films John Carroll a, b, Robert Krchnavek a, Carl Lunk b, Theodore Scabarozi b, Samuel Lofland b, *, Jeffrey Hettinger b a b

Department of Electrical and Computer Engineering, Rowan University, Glassboro, NJ 08028, USA Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2014 Accepted 5 July 2014 Available online 1 August 2014

We report on the synthesis and physical properties of epitaxial Cr-substituted vanadium carbide (V1
xCrx)2C thin films deposited on c-axis sapphire by magnetron sputtering using a co-deposition technique. Solid solutions were found to form up to x z 0.5 over a wide range of deposition temperatures. In general, it was found that surface roughness is dependent on the degree of crystallinity as well as the deposition temperature. However, the surface friction was remarkably low, as long as the surface roughness remained below a critical value of about 2 nm. Even for relatively small amounts of Cr (x ~ 0.1), there was significantly enhanced corrosion resistance over that of V2C. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Binary carbide Sputtering Thin film

Binary carbides are frequently used for industrial applications both in bulk form and as protective coatings. This class of materials possesses a unique combination of physical properties including high electrical conductivity, high melting temperatures, excellent oxidation and corrosion resistance, low surface friction and high hardness [1e8]. These physical properties and the ability to apply these materials as coatings at relatively low temperatures allow them to be used in a variety of applications. In this paper, their potential for application in the ever-expanding field of nanotechnology is investigated. Specifically, vanadium carbide, a member of this class of materials, has been shown to grow easily in thin-film form by several synthetic approaches [6,9e14]. Resulting from the ease of processing and the maturity of micro/ nano fabrication techniques associated with silicon, most nano/ micro electromechanical (NEMs/MEMs) devices are based upon silicon technologies. Recent advances in this area have increased the need and pursuit of thin films with low friction and high hardness for use as protective coatings since silicon is very soft making wear of these devices a major limiting factor for some applications requiring physical contact. In addition to being soft, many silicon nanostructures suffer from poor corrosion and oxidation resistance [15,16] and as such require a protective layer to be used in these corrosive environments. Carbides have been

* Corresponding author. Tel.: þ1 856 256 4382; fax: þ1 856 256 4478. E-mail address: lofl[email protected] (S. Lofland). http://dx.doi.org/10.1016/j.vacuum.2014.07.008 0042-207X/© 2014 Elsevier Ltd. All rights reserved.

shown to be effective as a protective coating on microelectric devices [17] and steel [18]. Here we report on the properties of epitaxial Cr-substituted V2C thin films grown on sapphire. Synthesis was done with a custom magnetron sputtering system consisting of two one-inch cathodes for the vanadium and chromium targets and a two-inch cathode for the carbon target. Sputtering was done with RF power supplies operating at 13.1 MHz. The base vacuum pressure was approximately 2  10
8 Torr. Deposition was done with sputtering grade argon introduced at a flow rate of 40 sccm with pressure controlled at 6 mTorr by a throttle valve and capacitance manometer. The working distance between the singlecrystal epitaxially polished c-axis-oriented Al2O3 substrates and the target-substrate distance was approximately 8 cm. Synthesis was performed at substrate temperatures between 600 and 900  C. X-ray diffraction (XRD) was performed to identify the phases present in the films. XRD measurements were performed with a Panalytical Empyrean X-ray diffractometer in the Bragg-Brentano configuration with Cu-Ka radiation. Wavelength dispersive X-ray fluorescence spectroscopy (WDXRF) was done with a Rigaku Primus III to measure the composition. Standards for quantitative analysis of films were created from pellets pressed from known mixtures of commercially available VC and Cr3C2 powders. These were compared to thick films grown on both Al2O3 and graphite substrates. The surface morphology was characterized with a JEOL 5200 atomic force microscope (AFM). The AFM was outfitted with a Hysitron Triboscope configured with a 1-mm radius conical tip to

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Fig. 2. XRD data for (V1
xCrx)2C films grown at a) 600  C and b) 900  C. The addition of Cr results in a shift of the peak as expected due to substitution. Not surprisingly, the solubility at 600  C (x ~0.4e0.5) is less than that at 900  C (x > 0.7).

Fig. 1. XRD data for (V1
xCrx)2C films for a) x ¼ 0, b) x z 0.08 and c) x z 0.30 grown at 600e900  C. The large peak is the (002) reflection data while the weak peak at ~ 37.5 is due to a Kb reflection from the substrate. In all cases, films grow c-axis oriented.

measure the surface friction of the films. The normal force used in the friction measurements was varied between 500 mN and 5000 mN without permanent deformation of the film. Measurements were made over lateral distances of 10 mm. Microstructure was also investigated by scanning electron microscopy (SEM) (LEO 1350 VP). Energy dispersive x-ray spectroscopy was done with an EDAX PHOENIX to confirm the V/Cr ratio and check for compositional uniformity. Finally cyclic voltammetry was used to measure the corrosion resistance of the films. An AutoLab 302N Potentiostat was used with the sample placed in a bath of 3 M HCl along with a KBr cathode and a Pt counter electrode. Subsequent Tafel analysis was used to determine the resistance to polarization. Fig. 1 shows the XRD pattern of (V1
xCrx)2C thin films grown at various temperatures. The largest diffraction peak (Fig. 1a) for x ¼ 0 for the textured samples grown here occurs near 39.4 which corresponds to the (002) reflection (ICCD No. 73-1320), suggesting that the films are predominately c-axis oriented. Subsequent offaxis XRD phi scans (not shown) indicated that the films were epitaxial with [1 1 0] of the substrate parallel to [1 0 0] of the carbide. At the lower deposition temperatures, weak peaks appeared around 42e44 likely indicating other vanadium carbide structures beginning to form. In general, the films become less textured with lowering deposition temperature. Results of XRD with increasing x

(x ¼ 0.08 and 0.30 are shown in Fig. 1b and c respectively) the range of deposition temperatures for best film epitaxy narrowed, with the optimum always at 900  C. Fig. 2 shows the (002) diffraction peaks of films of various composition deposited at 600 and 900  C. In general, these results confirm that the Cr, as determined from the elemental analysis, was incorporated in the lattice, since the peak continued to shift with increasing x, although at 600  C the solubility limit is at x ~0.4. The roughly linear dependence of the lattice spacing with x (Fig. 3) is in agreement with Vegard's law [19]. The scatter in the data may be related to carbon content as found in the pseudo-ternary system (V,Cr)2GeC [20]. The physical properties of select films deposited at either 600 or 900  C were studied. Fig. 4 shows the surface morphology for

Fig. 3. d spacing of the (002) reflection of (V1
xCrx)2C as a function of x for films grown under various conditions. The line represents a linear fit to the data and is in accord with Vegard's law, suggesting that Cr is substituting for V.

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Fig. 4. (aed) The topology of films (V1
xCrx)2C grown at 900  C a) x ¼ 0, Ra ¼ 0.236 nm b) x z 0.15, Ra ¼ 0.962 nm c) x z 0.40, Ra ¼ 2.63 nm d) x z 0.51, Ra ¼ 3.83 nm. The samples produced at 900  C invariably have larger surface features than those produced at 900  C (not shown).

samples deposited at 900  C. In general, lower deposition temperature (600  C) and lower Cr concentrations produced smoother films with the roughness at x ¼ 0 being effectively independent of deposition temperature. Presumably, the addition of Cr results in nucleation of misfit dislocations or variation in surface energy anisotropy which gives rise to StranksieKrastanov-type film growth, which could explain the increasing roughness with deposition temperature. Roughness measurements for samples grown at 900  C are 0.236 nm (x ¼ 0), 0.962 nm (x ¼ 0.15), 2.63 nm (x ¼ 0.40), and 3.83 nm (x ¼ 0.51) and at 600  C (morphology data not shown) are 0.251 nm (x ¼ 0), 0.450 nm (x ¼ 0.15), 1.16 nm (x ¼ 0.40), and 1.36 nm (x ¼ 0.51). Subsequent experiments were done to determine surface friction (Fig. 5). For films grown at 900  C, the friction correlated to the surface roughness; however, for films grown at 600  C, there appeared to be a small reduction in the friction even when the

Fig. 5. x dependence of corrosion resistance (solid line markers) and friction (open line markers) of films (V1
xCrx)2C grown at 600 (dashed lines) and 900  C (solid lines). The correlation appears to be that all films smoother than a roughness of 2 nm have very low friction (~0.04).

roughness increased. However, close examination reveals that friction can be really placed into two categories e relatively high (~0.12) for films of roughness greater than 2 nm and low (~0.04) for smoother films. Resistance to polarization, or corrosion resistance, was also measured (Fig. 5). The films without Cr had the lowest resistance with the sample having better epitaxy showing somewhat higher resistance. As expected the addition of Cr significantly increased the corrosion resistance although the effect was surprisingly rather insensitive to the amount. Data for smaller x values might be warranted since it is known that in stainless steel, alloying of 10e12% of Cr is sufficient for good resistance to corrosion. It may be that, like stainless steel, the enhanced resistance may be related to the presence of a chromium oxide passivation layer. In conclusion, we have grown epitaxial c-axis oriented films of (V1
xCrx)2C on c-axis sapphire substrates over a wide range of temperatures. Epitaxy is best at highest temperature investigated 900  C. A solubility gap seems to appear with lowering temperature even though at 600  C, solid solution formation takes place up to x ~0.4. Higher deposition temperature and increased substitution lead to increased surface roughness, yet the friction remains low (0.04) for films with roughness less than 2 nm. The addition of even minimal Cr leads to enhanced corrosion resistance. This work was supported in part by NSF Grant CHE-0641523. Any opinions, findings, conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of NSF. References [1] Suetin DV, Shein IR, Ivanovskii AL. Structural, elastic and electronic properties and formation energies for hexagonal (W0.5Al0.5)C in comparison with binary carbides WC and Al4C3 from first-principles calculations. Physica B 2008;403:2654. [2] Goldschmidt HJ. Interstitial alloys. London: Butterworths; 1967. [3] Toth LE. Transition metal carbides and nitrides. New York: Academic Press; 1971. [4] Kosolapva TYa. Carbides properties, production, and application. New York: Plenum; 1971. [5] Lee J, Sohn K, Hyeon T. Fabrication of novel mesocellular foams with uniform ultralarge mesopores. J Am Chem Soc 2001;123:5146. [6] Kim BI, Lee S, Guenard R, Fernandez Torres LC, Perry SS. Chemical modification of the interfacial frictional properties of vanadium carbide through ethanol adsorption. Surf Sci 2001;481:185. [7] Oelerich W, Klassen T, Bormann R. Comparison of the catalytic effects of V,V2 O5, VN, and VC on the hydrogen sorption of nanocrystalline Mg. J Alloy Comp 2001;322:L5e9. [8] Portolan E, Amorim CLG, Soares GV, Aguzzoli C, Perottoni CA, Baumvol IJR, et al. Carbon occupancy of interstitial sites in vanadium carbide films deposited by direct current reactive magnetron sputtering. Thin Solid Films 2009;517:6493. [9] Delplancke MP, Vassileris V, Winand R. Structure and composition of hydrogenated TixCy thin films prepared by reactive sputtering. J Vac Sci Tech 1995;13:1104. [10] Monteiro OR, Delplancke-Ogletree MP, Brown IG. Tungsten-containing amorphous carbon films deposited by pulsed vacuum arc. Thin Solid Films 1999;342:100. [11] Monteiro OR, Delplancke-Ogletree MP, Lo RY, Winand R, Brown IG. Synthesis and characterization of thin films of WCx produced by mixing W and C plasma streams. Surf Coat Tech 1997;94e95:220. [12] Berndt H, Zeng A-Q, Stock H-R, Mayr P. Vanadium carbide films produced by plasma-assisted metal-organic chemical vapour deposition. J Phys IV 1993;3. C3e313. [13] Sessler WJ, Donley MS, Zabinski JS, Walck SD, Dyhouse VJ. Tribological behavior of TiC thin films grown by pulsed laser deposition (PLD). Surf Coat Tech 1993;56:125. [14] Vetter J, Rochotzki R. Tribological behaviour and mechanical properties of physical-vapour-deposited hard coatings: TiNx, ZrNx, TiCx, TiCx/i-C. Thin Solid Films 1990;192:253. [15] Ledermann N, Baborowski J, Muralt P, Xantopoulos N, Tellenbach JM. Sputtered silicon carbide thin films as protective coating for MEMS applications. Surf Coat Tech 2000;125:246. [16] Malyshev VV, Shakhnin DB. Corrosion resistance of nanopowders of borides and carbides of the metals of group IV-VIB in nickel-plating electrolytes. Mat Sci 2013;49(3).

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