Auger lineshape analysis of carbon bonding in sputtered metal-carbon thin films

Surface Science 124 (1983) 591-601 North-Holland Publishing Company

591

AUGER LINESHAPE ANALYSIS OF CARBON BONDING SPUTI'ERED METAL-CARBON THIN FILMS S. C R A I G

IN

and G.L. HARDING

School of Physics, University of Sydney, Sydney, NSW 2006, Australia and R. P A Y L I N G

Research and Technology Centre, John Lysaght (Australia) Ltd., Port Kembla, NSW, Australia Received 19 July 1982; accepted for publication 19 October 1982

The Auger spectrum for carbon corresponds to the KLL group of transitions in which the L states are the 2s and 2p valence states, and as a consequence each type of carbon bond is associated with a distinctive Auger lineshape in dN(E)/dE mode which acts as a fingerprint for the chemical bonding state of the carbon. Transition metal-carbon composite layers with carbon-carbon and carbon-metal bonding states have carbon Auger lineshapes which are intermediate linear combinations of the amorphous carbon and transition metal-carbide lineshapes. In this work a simple mathematical technique based on peak height relationships has been used to separate the carbon-carbon (C-C) and carbon-metal (C-M) bonding state components of the carbon Auger fineshapes of sputtered stainless steel-carbon composite layers. Application of a value of 3.5 for the Auger sensitivity factor of carbon in C-M states relative to carbon in C-C states allowed relative numbers of C - M and C-C bonding states in the stainless steel-carbon composite layers to be determined. Heat treatment (at 500°C) of the composite layers resulted in alterations in the bonding configuration of the carbon atoms which were reflected in changes in the carbon Auger lineshapes. The association of changes in carbon atom bonding configuration with changes in optical extinction coefficient and electrical resistance of the stainless steel-carbon composite layers is also discussed.

1. Introduction A u g e r e l e c t r o n e m i s s i o n f r o m an a t o m is a d e - e x c i t a t i o n p r o c e s s w i t h a n u m b e r o f a l t e r n a t e t r a n s i t i o n s a n d results in an e l e c t r o n s p e c t r u m c o n s i s t i n g o f a g r o u p o f o v e r l a p p i n g lines. Q u a n t i f i c a t i o n of e l e c t r o n d a t a for a n A u g e r s p e c t r u m is b a s e d o n m e a s u r e m e n t o f the A u g e r c u r r e n t c o r r e s p o n d i n g to the i n t e g r a l o f the A u g e r p e a k in the e l e c t r o n e n e r g y d i s t r i b u t i o n N ( E ) . P e a k - t o peak heights (pph) of the normally measured d N ( E ) / d E mode spectra are p r o p o r t i o n a l to t h e A u g e r c u r r e n t [1] a n d a l l o w q u a n t i f i c a t i o n u s i n g sensitivity 0039-6028/83/0000-0000/$03.00

© 1983 N o r t h - H o l l a n d

S. Craig et al. / Auger lineshape analysis of carbon bonding

592

factors derived from elemental standards if the Auger current is proportional to the element concentration in the specimen and if the Auger electron energy distribution and transition probabilities determining peak shape and Auger current are independent of the chemical bonding state of the atom. Proportionality of the Auger current to concentration is governed by matrix sensitive factors of atomic density, electron escape depth and electron backscattering coefficient. Alteration of peak shape, and hence Auger sensitivity factor, with bonding state is observed for core-core-valence and core-valence-valence transitions in which a change in the type of chemical bonding alters the wavefunction of the valence electron states [1-5]. The Auger spectrum for carbon corresponds to the KLL group of transitions in which the L states are the 2s and 2p valence states. The variation in hybridisation of the 2s and 2p wavefunctions with the carbon atom bonding state alters the electron energy levels and transition probabilities associated with the Auger process [3,4]. Graphite and diamond are allotropes of carbon having carbon atoms linked by a network of covalent bonds with s p 2 and s p 3 hybridisation, respectively. The electronic structure of CO is represented as a triple bond with dipole moment C °- O °+ involving the C(2p) states. Transition metal-carbides are characterised by a partial ionic bond with spatial charge distribution mainly due to the transfer of metal d electrons to the C(2p)

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S. Craig et al. / Auger lineshape analysis of carbon bonding

593

states. The density of states of the transition metal-carbides has a band of C(2s) states 10-15 eV below the Fermi level and a band of C(2p)-metal d hybridised states 0-5 eV below the Fermi level, with the changes in the density of states associated with bonding to different transition metal ions principally involving states near the Fermi level [6]. Fig. 1 shows examples of the observed alterations in the carbon KLL Auger spectrum with chemical bonding state [3,5,7]. The graphite Auger spectrum is characteristic of single crystal graphite [3,8], amorphous carbon [4,8,9], diamond surface layers subsequent to ion bombardment [4], and surface decomposition of residual gases (CO, CO 2, hydrocarbons [4]). The diamond Auger spectrum was measured with an insulating crystal and is subject to charging effects [3]. In the present work carbon atom bonding states for a range of homogeneous stainless steel-carbon composite layers were determined by analysis of the carbon K L L Auger lineshapes. Fig. 1 shows that each type of carbon bond is associated with a distinctive Auger lineshape in d N( E ) / d E mode which acts as a fingerprint for the chemical bonding state of the carbon. The carbon lineshapes of transition metal-carbon composite layers with carbon-carbon and carbon-metal bonds exhibit intermediate forms which can be reproduced by a linear combination of the amorphous carbon and transition metal-carbide lineshapes [10,11]. A simpler mathematical technique based on the peak height relationships of the carbon Auger d N ( E ) / d E spectra for amorphous carbon and iron carbide (Fe3C) is used in the present work to separate the carbon-carbon (C-C) and carbon-metal (C-M) bonding state components of the measured carbon lineshapes. Application of a value of 3.5 for the Auger sensitivity factor of carbon in C - M states relative to carbon in C - C states [1 l] allows relative numbers of C - M and C - C bonding states in the stainless steel-carbon composite layers to be determined. Heat treatment (at 500°C) of the composite layers results in alterations in the bonding configuration of the carbon atoms which are reflected in changes in the carbon Auger lineshapes. The alterations in carbon atom bonding configuration are associated with changes in optical extinction coefficient and electrical resistance of the homogeneous layers. The bonding changes largely account for the minor degradation of solar absorptance and thermal emittance resulting from 500°C bakeout during evacuation of tubular solar collectors incorporating a sputtered copper/graded stainless steel-carbon selective surface [12,13].

2. Experimental methods A post-cathode magnetron sputtering source [14] was used to deposit a range of 150 nm thick homogeneous stainless steel-carbon composite layers onto planar glass substrates pre-coated with a - 1 #m thick copper conducting

594

s. Craig et al. / Auger lineshape analysis of carbon bonding

layer. The magnetron system is a prototype coater which deposits a sputtered copper/graded stainless steel-carbon composite solar selective surface onto twenty 1.5 m long glass tubes undergoing planetary motion about 1.7 m long copper and stainless steel post-cathodes [15]. The type 316 stainless steel cathode was reactively sputtered in argon plus acetylene and the planar substrates were attached to the centres of the glass tubes. Deposition conditions for each homogeneous composite layer were identical to a component layer coating stage of the graded stainless steel-carbon deposition [12], with carbon content controlled by adjusting the flow of C2H2 to the sputter zone. Homogeneous layer composition ranged from stainless steel (SS) to metal-free amorphous hydrogenated carbon (a-C:H). The a - C : H component of the sputtered layers is a random network of carbon atoms in tetrahedral co-ordination with nearest neighbours modified by carbon-carbon double bonds and the inclusion of order 0.5 hydrogen atom per carbon atom in monohydride and dihydride bonding configurations [16]. Heat treatment of a section of each coated substrate was performed at 500°C for 1 h in a continuously pumped vacuum furnace with pressure < 10 -3 Pa. The stainless steel-carbon composite films and a polished bulk sample of Fe3C in an Fe matrix (pearlite) were examined in a Physical Electronics 551 XPS/Scanning Auger System operating at a residual pressure < 10 -6 Pa. Sample mounts held four specimens thus enabling consecutive Auger analysis of the as-deposited and heat treated composite film pairs, and a vacuum interlock device allowed rapid changing of the sample mounts. The Multiplex unit was tuned for the elements C, O, Fe, Cr, Ni and Cu and the samples ion-milled until the element peaks were constant. The ion beam was rastered to provide uniform etching over an area of approximately 8 mm 2 with the etch rate decreasing from 4.3 nm min-1 for stainless steel to 3.9 nm min-1 for a - C : H . Removal of 10-20 nm of the composite film was normally required due to either chromium depletion and nickel enrichment at the surface or different etch rates for the chromium and nickel. A 5 kV primary electron beam of 25/~A current with 6 Vpp modulation and a large beam diameter of - 0.5 mm to reduce possible electron beam damage was used to obtain the Auger d N ( E ) / d E mode spectra. These conditions provide high sensitivity with moderate resolution. The electron beam caused a noticeable darkening in the absorbed current (normal mode) image of the a - C : H films but the carbon Auger lineshape and pph did not alter with continued exposure to the beam. Repeated AES scans of the SS-C composite layers subsequent to ion-milling showed an increase with time of the amorphous carbon component of the carbon Auger lineshapes due to electron beam induced surface decomposition of CO, CO 2 and hydrocarbons from the residual vacuum [4,11]. In order to minimise these electron beam and residual vacuum effects the carbon Auger lineshapes were recorded simultaneously with continued ion-milling of the surface and were constant with depth subsequent to the initial etching of 10-20 nm.

S. Craig et al. / Auger lineshape analysis of carbon bonding

595

3. Analysis of carbon KLL Auger lineshapes The c a r b o n K L L Auger lineshapes of a - C : H and Fe3C are shown in fig. 2 with a typical intermediate Auger lineshape for a stainless steel-carbon composite layer. The carbon Auger lineshapes of FeaC and of stainless steel F e - C r - N i mixed carbides are assumed identical. The intermediate carbon Auger lineshapes of the S S - C composite layers are arranged in fig. 3 according to metal atom concentration (determined by electron microprobe analysis table 1 of ref. [17]). The a - C : H and Fe3C lineshapes are employed as standards by determining the peak height ratios for P1, P2 and P3, and b as defined in fig. 2. The relevant equations are listed in fig. 2 and enable separation of the c a r b o n - c a r b o n ( C - C ) and c a r b o n - m e t a l ' ( C - M ) b o n d i n g state components of the intermediate S S - C carbon lineshapes by measuring the values of P2 and b.

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The equation relating P2 and b for the SS-C composite carbon lineshape to P2 M for the component Fe3C carbon lineshape derives from the negative displacement of the minimum at 262 eV for the SS-C lineshape (i.e. P2 > P2M) due to the cut-on of P1 c for the component a-C : H carbon lineshape at 262 eV (similarly for P3). Calculation of P2 M allows evaluation of bu and hence the value of P1M (pph of carbon in C-M bonding states). Parameter b is the sum of bc plus b M and consequently evaluation of b M allows determination of bc and hence P1 c (pph of carbon in C - C bonding states). Calculated values for the pph of carbon in the C-C and C - M bonding states (P1 c and Plea respectively) for the range of SS-C composite layers are listed in table 1. The similarity of carbon KLL Auger lineshapes for transition metal-carbides (figs. 1 and 2) corresponds to a similarity in basic structure of the valence band density of states [6] and thus provides a basis for the comparison of results for the SS-C composite layers with Auger analysis of carbon bonding states in other transition metal-carbon environments. Of particular relevance are Auger data comparing differential and integral mode Auger spectra for carbon adsorbed on tungsten [11]. Ishikawa and Tomida determined the relation of peak-to-peak height (A) for dEN(E)/dE mode spectra to integrated peak

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area (S) of EN(E) mode spectra for amorphous carbon and W2C surface layers on tungsten ribbon. Plots of A versus S for amorphous carbon and W2C indicated a linear relationship with a ratio of the proportionality constant for

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carbide relative to that for amorphous carbon equal to 3.5. Consequently, for equal carbon atom number density (assumed indicated by equal S), the dN(E)/dE mode spectra of carbon in C - M bonding states has peak-to-peak height - 3.5 times that of carbon in C-C bonding states. Applying an Auger sensitivity factor of 3.5 for carbon in C-M states relative to carbon in C-C states allows determination of relative numbers of carbon atoms in C-C and C - M bonding states in the SS-C composite layers. The proportion of carbon atoms in C - M bonding states for the as-deposited and heat treated layer pairs are plotted in fig. 4. Accuracy of the analysis technique and the assumption that intermediate lineshapes are linear combinations of the a - C : H and Fe3C lineshapes were checked by comparing the measured and calculated values of P3 and by determining the atomic concentration ratio M / C c_ m of metal atoms to carbon atoms in C - M states (table 1). Excellent agreement was obtained with the ratio P3(calculated)/P3(measured) having a mean value of 1.03 + 0.05 and the ratio M//Cc-m ~ 3 for low metal atom concentration corresponding to the chemical stoichiometry M3C for Fe-Cr-Ni carbides.

S. Craig et al. / Auger lineshape analysis of carbon bonding

599

4. Discussion Alterations in the carbon atom bonding states due to heat treatment (fig. 4) are associated with changes in the optical and electrical properties of the stainless-steel carbon composite layers [17]. Table 1 lists values of extinction coefficients at wavelengths 0.5 and 2.5 /~m and DC electrical resistivity (measured at 300 K) for the nine homogeneous SS-C layer, as deposited and after heat treatment in vacuum at 500°C for - 1 h. Relative values of electrical resistivities and extinction coefficients in the layers 1-9 are generally as expected for as-deposited composite films with a decreasing metal carbide/metal fraction in an insulating matrix [17,18]. Heat treatment results in a decrease in resistivity for the stainless steel-carbon composite layers 1-8 and an increase in extinction coefficient for layers 3-9 (table 1). A contribution to the increase in extinction coefficients (Ak) at 0.5/~m in the high carbon content layers results from the a - C : H matrix due to a reduction in energy band gap consequent on hydrogen evolution during heat treatment [16]. Results for the metal-free a-C: H layer (No. 9, table 1) indicate that the maximum contribution from the matrix is Ak - 0.2 at 0.5/~m and Ak - 0.02 at 2.5 /~m. The increases in extinction coefficient at 0.5 /~m for stainless steel-carbon composite layers 3-8 are also of order 0.2 but the a-C : H matrix cannot account for the relatively large values of Ak at 2.5/~m. The increases at 2.5 /~m are largely explained by changes in bonding arrangements of metal atoms in the carbon matrix. For all layers except 1 and 2 (highest metal content) the carbon C - M state pph decreases after heat treatment (fig. 4). The corresponding increase in number density of metal atoms bonded in a metal rather than metal-carbide phase results in an increase in density of electron states in the conduction band [6] with a consequent decrease in resistivity and increase in extinction coefficient k. For layers 1 and 2, which contain relatively low levels of carbon, heat treatment results in an increase in metal-carbon bonding. Similar effects are observed for amorphous carbon deposits on the surfaces of transition metals [4]. The small decreases in resistivity for layers 1 and 2 probably result from annealing during heat treatment. Annealing may similarly contribute to resistivity changes in all the composite layers, but changes of more than an order of magnitude in the low metal content films is predominantly due to the bonding changes. Similar structural changes have been observed by Sikkens [19,20] in a study of sputtered nickel-carbon composite films by X-ray diffraction and ESCA. During heat treatment at 400°C, films of Ni~C in a carbon matrix were observed to undergo structural changes which involved detachment of C or Ni from Ni3C particles, with consequent decreases in electrical resistivity.

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S. Craig etaL /Augerlineshape analys~ofcarbon bonding

5. Conclusion The carbon K L L Auger dN/dE mode lineshapes of sputtered stainless steel-carbon composite layers are linear combinations of the a-C : H and Fe3C Auger spectra. Carbon peak-to-peak heights for C - C and C - M state components of the carbon Auger lineshapes have been determined by a simple technique involving peak height relationships, and application of a relative Auger sensitivity factor of 3.5 has allowed quantitative determination of the relative numbers of carbon atoms in C - C and C - M bonding states. The similarity of the carbon Auger lineshapes for transition metal-carbides suggests that variations of the peak height analysis technique for carbon bonding states can be applied generally to transition metal-carbon composite materials. Analysis of the relative numbers of C - C and C - M states in as-deposited and heat treated SS-C composite layers generally indicates a decrease in C - M bonding states due to heat treatment. The increase in number density of metal atoms bonded in a metal rather than metal-carbide phase corresponding to the decrease in C - M bonding states largely explains alterations in the solar absorptance and thermal emittance associated with 500°C bakeout of sputtered copper/graded stainless steel-carbon films incorporated as the selective surface in all-glass evacuated collectors. The relevance of this work to degradation of sputtered metal-carbon selective surfaces is discussed in more detail elsewhere [17].

Acknowledgements The authors wish to thank John Lysaght (Australia) Limited (Research and Technology Centre), Port Kembla, Australia, for provision of the Auger analysis facilities. Financial support for this work has been provided by the New South Wales State Government and by His Royal Highness Prince Nawaf Bin Abdul Aziz of the Kingdom of Saudi Arabia through the Science Foundation for Physics within the University of Sydney.

References [1] P.H. Holloway,in: Scanning Electron Microscopy/1978,Ed. O'Johari (SEM, AMF O'Hara, IL) p. 361. [2] F. Pons, J. Le H6ricy and J.P. Langeron,SurfaceSci. 69 (1977) 565, 547. [3] T.W. Haas, J.T. Grant and G.J. Dooley III, in: Adsorption-Desorption Phenomena, Ed. F. Pie.ca (Academic Press, New York, 1972)p. 359. [4] T.W. Haas, J.T. Grant and G.J. Dooley III, J. Appl. Phys. 43 (1972) 1853. [5] M.P. Hooker and J.T. Grant, SurfaceSci. 62 (1977) 21. [6] L. Ramqvist, J. Appl. Phys. 42 (1971) 2113.

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[7] T.W. Haas and J.T. Grant, Appl. Phys. Letters 16 (1970) 172. [8] C.C. Chang, in: Characterization of Solid Surfaces, Eds. P.F. Kane and G.G. Larrabee (Plenum, New York, 1974) p. 535. [9] R. Ducros, G. Piquard, B. Weber and A. Cassuto, Surface Sci. (1976) 513. [10] M.A. Smith, S. Sinharoy and L.L. Levenson, J. Vacuum Sci. Technol. 16 (1979) 462. [11] K. Ishikawa and Y. Tomida, J. Vacuum Sci. Technol. 15 (1978) 1123. [12] G.L. Harding and B. Window, J. Vacuum Sci. Technol. 16 (1979) 2101. [13] G.L. Harding and S. Craig, Solar Energy Mater. 5 (1981) 149. [14] J.A. Thornton and A.J. Penfold, in: Thin Film Processes, Eds. J.L. Vossen and W. Kern (Academic Press, New York, 1978) p. 76. [15] G.L. Harding, B. Window, D.R. McKenzie, A.R. Collins and C.M. Horwitz, J. Vacuum Sci. Technol. 16 (1979) 2105. [16] S. Craig and G.L. Harding, Thin Solid Films, to be published. [17] S. Craig and G.L. Harding, Thin Solid Films, to be published. [18] 1.T. Ritchie and G.L. Harding, Thin Solid Films 57 (1979) 315. [19] M. Sikkens, Solar Energy Mater. 6 (1982) 403. [20] M. Sikkens, Solar Energy Mater. 6 (1982) 415.