Electron spectroscopic studies of tantalum carbide

Surface Science 157 (1985) L395-L400 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS ELECTRON SPECTROSCOPIC STUDIES OF TANTALUM CARBIDE * G.R. GRUZALSKI, D.M. ZEHNER and G.W. OWNBY Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Received 21 January 1985; accepted for publication 20 February 1985

Carbon-K characteristic loss, carbon-KVV Auger, and valence-band X-ray photoelectron spectra were measured for a well-ordered, nearly stoichiometric (100) surface of tantalum carbide. These spectra compare well with curves that were generated by using a calculated total density of states.

Obtaining a better understanding of the electronic structure of transitionmetal carbides has been a long-standing objective [1]. Experimental techniques recently applied with this objective include Bremsstrahlung isochromat spectroscopy [2], AES [3,4], core-level characteristic loss spectroscopy (CLS) [5], transmission ELS [6], ellipsometry and reflectivity [7], UPS [8,9], XPS [10,11], and X-ray absorption [12] and emission [13] spectroscopies. Because only a few experimental investigations of this nature have involved tantalum carbide [3,7,14-16], and of these only the optical-properties work of Modine et al. [7] has involved single crystals, we have made KVV AES, core-level CLS, and valence-band XPS measurements on well-ordered, nearly stoichiometric (100) surfaces of tantalum carbide. In this letter we compare some of our results with theoretical results based upon the self-consistent APW calculation of Klein et al. [17] for stoichiometric tantalum carbide. Measurements were made in an ion-pumped UHV chamber having a base pressure of less than 10-s Pa and equipped with a double-pass CMA containing a coaxial electron gun, an unmonochromatic aluminum X-ray source, LEED optics, and a quadrupole mass spectrometer. The XPS data were obtained with the CMA in the retarding mode (15 eV pass energy); the CLS and AES data were obtained with the CMA in the nonretarding, first-derivative mode (1 V peak-to-peak modulation). The energy scale of the spectrometer was calibrated by setting the measured gold 4f7/2 binding energy equal to 84.1 eV with respect to the Fermi level and by using procedures similar to those described elsewhere [18]. * Research sponsored by the Division of Materials Sciences, US Department of Energy under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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G.R. Gruzalski et aL / Electron spectroscopicstudies of tantalum carbide

The tantalum carbide crystal used in this study was polished to within 0.1 o of the (100) plane to minimize the effects of steps. It was supported in the UHV chamber by a thermally insulated loop of tungsten wire (0.010 inch diameter); two slots were spark cut into opposite edges of the specimen disk to accommodate the wire. An infrared pyrometer was used to monitor the surface temperature of the crystal, which was heated by bombarding its back surface with 800 eV electrons. The surface investigated was prepared by argon ion bombardment (1000 V, 10 /zA, 10 min) followed by a 10 min anneal at 2000°C. Surfaces thus prepared exhibited sharp (1 x 1) LEED patterns and were free of significant amounts of contamination (the only contaminant detected was about 0.01 monolayer of oxygen). These surfaces may have deviated from stoichiometry, but probably by no more than a few atomic percent [19]. Eigenvalues and eigenvectors of an LCAO (Slater-Koster) Hamiltonian for stoichiometric tantalum carbide were computed on a mesh of about 600 points in the fcc irreducible Brillouin zone (IBZ). The results were integrated using the tetrahedron method [20] to obtain the total density of states (DOS); the computation included 1536 tetrahedra in the IBZ. The Slater-Koster parameters used in the computation were those determined by Klein et al., who employed a least-squares fitting procedure to ground-state eigenvalues from their self-consistent APW calculation [17]; the fit included a basis set of 13 orbitals. In fig. 1, measured valence-band XPS, carbon-K CLS, and carbonKVV AES spectra (solid curves) are compared with theoretical spectra (dashed curves) generated from the computed total DOS; in each figure the point of zero energy was chosen to correspond with the Fermi level. The uncorrected valence-band XPS spectrum and N ( E ) are shown in fig. la, where N ( E ) is the total valence-band DOS (computed) broadened with an instrument resolution function (a Gaussian with 1.2 eV FWHM). The XPS spectrum is qualitatively similar to that obtained by Ihara et al. for hot-pressed tantalum carbide [16]. Three peaks are prominent in the spectrum: the one near - 12 eV is due to carbon 2s states; the one near - 5 eV to carbon 2p and tantalum 5d states; and the one near - 0 . 6 eV to a non-zero DOS at the Fermi level, which is primarily made up of Tzg states. As seen, the overall agreement between the two curves is quite good. Nonetheless, two comments should be made regarding the agreement. First, the relatively large intensities of the experimental spectrum at energies corresponding to the bottom of the valence band are due to inelastically scattered electrons and, moreover, to a3 and o(4 satellite excitations of tantalum 4f7/2 electrons. Second, N ( E ) is not a true theoretical spectrum in that matrix-element effects have not been included; although this simplification should not significantly affect peak locations, it may be responsible for the intensities of the peaks near - 7 . 0 and - 0 . 6 eV being relatively large and small, respectively, in comparison with those in the experimental spectrum.

G.R. Gruzalski et al. / Electron spectroscopicstudies of tantalum carbide

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Fig. ]. Comparison of measured (a) valence-band XPS, (h) carbon-K CLS, and (c) carbon-KW AES spectra (solid curves) with theoretical spectra (dashed curves) generated as described in the text.

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G.R. Gruzalski et al. / Electron spectroscopic studies of tantalum carbide

Shown in fig. l b is the carbon-K CLS spectrum, which has been integrated, background-corrected, and loss-deconvoluted (i.e., an attempt was made to remove features associated with inelastically scattered electrons [21]). The primary beam energy was 1100 eV for the CLS spectrum and 820 eV for the electron-loss spectrum used in the ioss-deconvolution correction; the energy of the adsorption edge was about 283 eV below the Fermi level. The point of half maximum of the absorption edge was positioned coincident with the Fermi level so that the spectrum could be compared with N(E), the total conduction-band DOS broadened with Gaussians having FWHM's equal to 0.6% of the kinetic energy of the detected electrons. Agreement between the two curves is again quite good. The peak near 9 eV, barely discernible in the N ( E ) curve, is mostly due to carbon 2p states, which probably accounts for it being more pronounced in the CLS spectrum. (Because the initial ls states are strongly localized about carbon atoms, transitions to other carbon states are expected to be more probable than those to tantalum states.) Additional discrepancies are expected at higher energies, because the LCAO fit included only 13 orbitals and simply was not intended to describe the electronic structure at these energies where other bands exist; even here, however, agreement appears to be quite good. Shown in fig. l c is the carbon-KVV spectrum, which has been integrated, background-corrected, and loss-deconvoluted. The primary beam energy was 3 keV for the Auger spectrum and 272 eV for the electron-loss spectrum used in the loss-deconvolution correction. The uncorrected spectrum (not shown) is qualitatively similar to the carbon-KVV spectrum for tantalum carbide reported by Shul'ga and Gutsev [3], although their spectrum more closely resembles those we have obtained for disordered, carbon-deficient surfaces [19]. In accordance with the simple Lander model [22,23], we have positioned the KVV spectrum in fig. lc so that it can be readily compared with N ( E ) o N ( E ) , the total valence-band DOS self convoluted and then broadened with Gaussians having FWHM's equal to 0.6% of the kinetic energy of the detected electrons. The overall agreement between the two curves is good enough to identify the sources of the most prominent features of the experimental spectrum. This may be done simply by inspection of N(E) in fig. la, keeping in mind that the Auger line shape preferentially reflects the carbon environment. The observed feature near - 2 8 eV (referred to below as ss) primarily is due to transitions involving carbon 2s electrons, the split feature near - 2 0 eV (sp) to transitions involving carbon 2s and 2p electrons, and the features near - 1 0 and - 5 eV (pp) to transitions involving carbon 2p electrons. As seen, the positions of the pp features agree reasonably well with those in the computed curve, suggesting that the corresponding holes are delocalized and that the hole-hole interactions are weak. The ss and sp features, however, appear at significantly lower energies than do their counterparts in the computed curve, which suggests that the hole-hole interactions are

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stronger in these cases. Also, the sp feature is split, presumably because singlet and triplet hole-hole states are formed. Similar trends in Auger line shapes for other transition-metal compounds have been reported by Smith and Levenson [4] and more recently by Pehrsson and Ramaker [24]. Finally, we note that the structure observed near zero relative energy is completely absent in the computed curve. This structure is less enhanced in spectra from well-annealed surfaces exposed to oxygen, and it is absent, or nearly so, in spectra from surfaces prepared by argon-ion bombardment. These observations suggest that this structure may be associated with the well-ordered surface itself. In conclusion, the DOS of stoichiometric tantalum carbide calculated by Klein et al. [17] adequately describes the overall features exhibited by valenceband XPS and carbon-K CLS spectra from a well-ordered, nearly stoichiometric (100) surface of tantalum carbide. Also, the agreement between the carbonKVV AES spectrum and the self-convoluted DOS is good enough to identify the most prominent features in the experimental spectrum; effects ascribable to final-state hole-hole localization have been discussed, and structure in the spectrum possibly associated with the surface has been noted. Thanks are due to D.A. Papaconstantapoulos for supplying the appropriate Slater-Koster Hamiltonian and to both him and D.E. Ramaker for helpful discussions.

References [1] Two relatively recent reviews are: A. Neckel, Intern. J. Quantum Chem. 23 (1983) 1317; S.T. Oyama and G.L. Hailer, in: Catalysis, Eds. G.C. Bond and G. Webb, Specialist Periodical Reports, Vol. 5 (Royal Society of Chemistry, London, 1982) p. 333. [2] F. Riehle, Th. Wolf and C. Politis, Z. Physik B47 (1982) 201. [3] Ju.M. Shul'ga and G.L. Gutsev, J. Electron Spectrosc. Related Phenomena 34 (1984) 39. [4] M.A. Smith and L.L. Levenson, Phys. Rev. B16 (1977) 1365. [5] J. Pfliager, J. Fink, G. Crecelius, K.-P. Bohnen and H. Winter, Solid State Commun. 44 (1982) 489. [6] J. Pfliiger, J. Fink, W. Weber, K.-P. Bohnen and G. Crecelius, Phys. Rev. B30 (1984) 1155; Phys. Rev. B31 (1985) 1244. [7] F.A. Modine, R.W. Major, T.W. Haywood and G.R. Gruzalski, Phys. Rev. B29 (1984) 836. [8] P.M. Stefan, M.L. Shek, I. Lindau and W.E. Spicer, Phys. Rev. B29 (1984) 5423. [9] R.D. Bringans and H. HOchst, Phys. Rev. B30 (1984) 5416. [10] H. H6chst, R.D. Bringans, P. Steiner and Th. Wolf, Phys. Rev. B25 (1982) 7183. [11] Louis Porte, Laurent Roux and Jean Hanus, Phys. Rev. B28 (1983) 3214. [12] A. Balzarotti, M. De Crescenzi and L. lncoccia, Phys. Rev. B25 (1982) 6349. [13] V.A. Gubanov, E.Z. Kurmaev and G.P. Shveikin, J. Phys. Chem. Solids 38 (1977) 201. [14] L. Ramqvist, K. Hamrin, G. Johansson, U. Gelius and C. Nordling, J. Phys. Chem. Solids 31 (1970) 2669.