Deposition of epitaxial Ti2AlC thin films by pulsed ...

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J. Rosén,a) L. Ryves, P. O. Å. Persson, and M. M. M. Bilek. School of Physics, The University of Sydney, New South Wales 2006, Australia. Received 26 October ...

JOURNAL OF APPLIED PHYSICS 101, 056101 共2007兲

Deposition of epitaxial Ti2AlC thin films by pulsed cathodic arc J. Rosén,a兲 L. Ryves, P. O. Å. Persson, and M. M. M. Bilek School of Physics, The University of Sydney, New South Wales 2006, Australia

共Received 26 October 2006; accepted 8 January 2007; published online 2 March 2007兲 A multicathode high current pulsed cathodic arc has been used to deposit Ti2AlC thin films belonging to the group of nanolaminate ternary compounds of composition Mn+1AXn. The required stoichiometry was achieved by means of alternating plasma pulses from three independent cathodes. We present x-ray diffraction and transmission electron microscopy analysis showing that epitaxial single phase growth of Ti2AlC has been achieved at a substrate temperature of 900 °C. Our results demonstrate a powerful method for MAX phase synthesis, allowing for phase tuning within the Mn+1AXn system. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2709571兴 The MAX phases are a family of nanolaminate ternary compounds with the general composition Mn+1AXn, where M is an early transition metal, A is a p element 共usually groups IIIA and IVA兲, and X is carbon or nitrogen.1,2 Following their initial discovery in the 1960s,1 these compounds have gained increased attention because of their unique combination of metallic and ceramic properties.3 This mix of properties stems from the highly anisotropic hexagonal crystal structure, wherein the M and X atoms form octahedral edge sharing building blocks interleaved by layers of the A element. Ti3SiC2, the most well-characterized MAX phase 共see Ref. 4 and references therein兲, is reported to have good electrical and tribological properties, including low friction, good resistance to oxidation and corrosion, as well as high tolerance to damage and thermal shock. These characteristics are ideal for thin film applications in, for example, sliding electrical contacts operating in high-temperature environments. MAX phases from the Ti-Al-C system show similar mechanical and electrical properties, while oxidation studies reveal a superior oxidation resistance.5 MAX phases have previously been synthesized primarily by bulk synthesis techniques such as hot isostatic pressing, and as thin film materials using chemical vapor deposition 共CVD兲. Physical vapor phase condensation 共PVD兲 offers the advantage of comparatively low-temperature synthesis, as well as a more controlled growth environment. Over the last few years, growth of MAX phase thin films at 900 °C using magnetron sputtering has been reported 共see, for example, Ref. 6兲. The Tin+1AICn MAX alloys in particular have so far been synthesized using sintering processes for bulk samples7 and magnetron sputtering to produce thin films.6,8–10 Although the cathodic arc is an attractive PVD method due to its high level of ionization 共close to 100%兲,11 there are as yet no reports of MAX phase synthesis using this technique. The high level of ionization in cathodic arc plasmas represents an extra degree of freedom, allowing control over ion energy through electric and magnetic fields, to achieve enhanced control of microstructural evolution and hence thin film properties. In order to achieve a high level of control over the depositing species flux ratios required for MAX phase syna兲

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thesis, we have developed a three cathode high-current cathodic arc deposition system.12 Here we report on its use for the deposition of a MAX phase, and show evidence for single phase epitaxial films of Ti2AlC in this article. The films were deposited using a high current, 900 A, triple cathode plasma source, mounted in a vacuum chamber with a base pressure of 1 ⫻ 10−6 mbar. Center triggering was used to initiate discharges on each cathode in turn. The center triggering method was selected because it ensures a repeatable flux of ions to the substrate over large numbers of pulses.13 To accommodate different arc spot velocities, the Ti, Al, and C cathodes were set to deliver alternating plasma pulses of 350, 300, and 850 ␮s length, respectively, in the ratio 15 Ti to 1 C to 10 Al, at a rate of 10 Hz. The pulse ratios were attained though an initial estimation, based on individual deposition rates measured for the different cathodes, of the plasma flux mix 共number of pulses兲, required to achieve the correct film stoichiometry. These pulse ratios were subsequently tuned according to the results of a few iterations of material synthesis and x-ray diffraction 共XRD兲 analysis. With each pulse delivering a submonolayer amount of the cathode material, the fine control over stoichiometry required to produce MAX phase was attainable. An internally mounted curved magnetic filter was used to remove macroparticles from the plasma plume. Filter fields 共generated in the solenoid by a separate power supply兲 of 31 mT 共for Ti兲, 12 mT 共for Al兲, and 27 mT 共for C兲 were used to optimize the plasma transport through the filter for each pulse. These conditions were identified separately for each species by studying the effect of field strength on the substrate current and the uniformity of the flux distribution at the substrate, measured using a collector plate divided into four quadrants. Prior to deposition, the ␣-Al2O3共0001兲 substrates were rinsed in ethanol and then degassed in the chamber at 900 °C for ⬃5 min under high-vacuum conditions. Under isothermal conditions, interface layers of ⬃10 nm Ti and ⬃10 nm TiC0.75 were deposited onto the substrate, the latter was previously reported to promote crystallization.10 The MAX phase composition was subsequently laid down with a deposition rate of approximately 5 Å s−1. The films were characterized using XRD and transmission electron microscopy 共TEM兲. ␪-2␪ scans as well as 1°

101, 056101-1

© 2007 American Institute of Physics

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056101-2

J. Appl. Phys. 101, 056101 共2007兲

Rosen et al.

FIG. 1. XRD spectra shown on a logarithmic scale including 共a兲 a film ␪-2␪ scan revealing theTi2AlC 兵000l其 peaks, 共b兲 a ␪-2␪ substrate scan, and 共c兲 a film GIXRD scan.

grazing incidence 共GI兲 scans were performed with a Siemens D5000 diffractometer using Cu K␣ radiation. Cross-sectional specimens for TEM were prepared using mechanical grinding and polishing followed by argon ion milling. Images were recorded using a JEOL 3000F transmission electron microscope, operated at 300 kV. A series of experiments showed how alternating use of the three cathodes in different plasma pulse sequences could be used to tune the film stoichiometry toward a composition allowing for MAX phase synthesis. XRD analysis of the resulting Ti2AlC film is shown in Fig. 1. Epitaxial growth is evident from the ␪-2␪ diffractogram of Fig. 1共a兲, with 40° −44° omitted to avoid the 共0006兲 substrate reflection. Excluding contributions from the substrate 关see Fig. 1共b兲兴, more than 97% of the integrated peak intensity can be attributed to Ti2AlC 兵000l其 type peaks. The additional two minor peaks around 36° and 76° are indicative of TiC 兵lll其, originating partly from the interface layer. A GIXRD scan was also performed to exclude the presence of polycrystalline material, see Fig. 1共c兲. The TEM micrographs shown in Fig. 2, reveal a 500 nm thick layered film including the interface layers. The orientation of the Ti2AlC can be identified in relation to the Al2O3 from the Fourier transformations in Fig. 2共a兲 共obtained from a second image兲 which reveal an 具001典共Ti2AIC兲 // 具0001典共Al2O3兲, and 兵11− 20其共Ti2AlC兲 // 兵11− 20其共Al2O3兲 epitaxial relationship. The high-resolution image in Fig. 2共b兲 shows the nanolaminated hexagonal nature of the Ti2AlC phase. Rare cubic precipitates can be seen 共from other images兲 in agreement with the XRD data.

FIG. 2. Cross-sectional TEM images revealing the nanolaminated structure of the Ti2AlC film and its epitaxial relationship to the substrate.

In conclusion, we have demonstrated that the cathodic arc is a method suitable for MAX phase synthesis, and report MAX phase thin film produced in this way. Using a triple cathode plasma source, epitaxial thin films of Ti2AlC were deposited at 900 °C, showing that utilizing multiple cathodes and center triggering, predefined compositions corresponding to the MAX phases can readily be achieved. We acknowledge financial support from the Australian Research Council and the Sydney University Visiting Research Fellowships Scheme. Technical assistance from John Pigott, Phil Denniss and Rob Davies is also gratefully acknowledged. V. H. Nowotny, Prog. Solid State Chem. 5, 27 共1971兲. M. W. Barsoum, Prog. Solid State Chem. 28, 201 共2000兲. 3 M. W. Barsoum and T. El-Raghy, Am. Sci. 89, 334 共2001兲. 4 J.-P. Palmquist et al., Phys. Rev. B 70, 165401 共2004兲. 5 X. H. Wang and Y. C. Zhou, Corros. Sci. 45, 891 共2003兲. 6 O. Wilhelmsson, J.-P. Palmquist, T. Nyberg, and U. Jansson, Appl. Phys. Lett. 85, 1066 共2004兲. 7 W. Zhou, B. Mei, J. Zhu, and X. Hong, J. Mater. Sci. 40, 3559 共2005兲. 8 H. Högberg, L. Hultman, J. Emmerlich, T. Joelsson, P. Eklund, J. M. Molina-Aldareguia, J.-P. Palmquist, O. Wilhelmsson, and U. Jansson, Surf. Coat. Technol. 193, 6 共2005兲. 9 C. Walter, C. Martinez, T. El-Raghy, and J. M. Schneider, Steel Res. Int. 76, 225 共2005兲. 10 O. Wilhelmsson et al., J. Cryst. Growth 291, 290 共2006兲. 11 A. Anders and G. Y. Yushkov, J. Appl. Phys. 91, 4824 共2002兲. 12 T. W. H. Oates, J. Pigott, D. R. McKenzie, and M. M. M. Bilek, Rev. Sci. Instrum. 74, 4750 共2003兲. 13 B. K. Gan, M. M. M. Bilek, D. R. McKenzie, P. D. Swift, and G. McCredie, Plasma Sources Sci. Technol. 12, 508 共2003兲. 1 2

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