International Journal of Nanoscience Vol. 3, No. 6 (2004) 797 802 c World Scientific Publishing Company XPS STUDY OF DIAMOND-LIKE CARBON-BASED NANOCOMPOSITE FILMS S. ZHANG,Y.Q.FU,X.L.BUIandH.J.DU School of Mechanical and Aerospace Engineering Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 Advanced Materials for Micro and Nano Systems Programme Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576 msyzhang@ntu.edu.sg X-ray photoelectron spectroscopy (XPS) analysis of amorphous carbon (a-c)-based nanocomposite films has been carried out. Four bonding types of carbon are revealed from the C 1s spectra: C C sp 2 and sp 3 bonding, C O and C Ti bonding. With the introduction of Ti and Al into the a-c matrix, the carbon sp 3 /sp 2 ratio decreases significantly. XPS results also confirm that the Al doped in the film basically exists as an elemental Al in the a-c matrix, and the titanium is mostly bonded with carbon to form nc-tic. Keywords: Nanocomposite; diamond-like carbon; sputtering; XPS. 1. Introduction Nanocomposite films have recently attracted increasing interest because of the possibilities of synthesizing the materials with unique physical chemical properties. 1 4 Presently, extensive theoretical and experimental efforts are made to synthesize and study these nanocomposite films. 5 7 To maximize hardening and toughening effects, it is common to use ternary, quaternary or even more complicated composition systems. One popular design method is to imbed nanocrystalline phases in an amorphous matrix. 8,9 Diamond-like carbon (DLC) or amorphous carbon (a-c) has been recognized as the primary candidates for the amorphous matrix, but a-c has a problem of high residual stress and poor toughness. The incorporation of metals, for example, Al, into hydrogen free DLC matrix effectively reduces the residual stress and improves toughness. In Ref. 10, a multi-target magnetron sputtering system was used to prepare four types of films: pure a-c (a-c without doping), amorphous carbon with Al doping, nanocrystalline TiC embedded in pure amorphous carbon and nanocrystalline TiC embedded in amorphous carbon doped with Al, i.e., a-c, a-c(al), TiC/a-C and TiC/a-C(Al). The results showed that without Al doping, the amorphous carbon Corresponding author. 797
798 S. Zhang et al. films possess a large residual stress of over 4 GPa. After doping with Al, the residual stress drastically decreases to a mere 0.3 GPa. However, the hardness drastically decreased from about 32 GPa to about 9 GPa. After TiC was embedded into a-c(al) matrix in the form of nanosized crystals, the hardness is largely restored. nc-tic/a-c(al) has a hardness of 20 GPa and residual stress of 0.5 GPa. 10 However, the detailed chemical bonding structures for the nc-tic/a-c(al) films have not been fully understood. This information can be obtained by X-ray photoelectron spectroscopy (XPS). In this study, detailed XPS analysis was performed on those films to study the chemical bonding states, based on which relationship between the bonding state and the mechanical response is established. 2. Experimental X-ray photoelectron spectroscopy (XPS) analysis was performed on the sample surface using a Kratos-Axis spectrometer with monochromatic Al Kα (1486.71 ev) X-ray radiation (15 kv and 10 ma) and a hemispherical electron energy analyzer. The base vacuum of the chamber was 2 10 9 Torr. Survey spectra in the range of 0 1100 ev were recorded in 1 ev steps for each sample, followed by high-resolution spectra over different elemental peaks in 0.1 ev steps, from which the detailed composition was calculated. Curve fitting was performed after a Shirley background subtraction by nonlinear least square fitting using a mixed Gauss/Lorentz function. To remove the surface contamination layer, Ar ion bombardment was carried out for 600 seconds using a differentially pumped ion gun (Kratos MacroBeam) with an accelerating voltage of 4 kev and a gas pressure of 1 10 7 Torr. The bombardment was performed at an angle of incidence of 45 with respect to the surface normal. 3. Results and Discussion Figure 1 shows XPS carbon core level spectra (C 1s) of a-c, a-c(al), TiC/a-C and TiC/a-C(Al) films. For a-c film, the full width at half maximum (FWHM) of the C 1s spectra is about 2.1 ev, which is much larger than those of graphite (FWHM = 0.7 ev) and diamond samples (FWHM = 1.0 ev). The peaks are fitted using binding energy interval values from the literature 11,12 and then Gaussian decomposition results in three kinds of bonds: C C sp 2 bonding at 284.7 ev, sp 3 bonding at 285.7 ev and C O bonding at 288.3 ev. The concentrations of the different bonds are calculated and the sp 3 /sp 2 hybridization ratio derived from the relative content and listed in Table 1. For the a-c(al) film, three bonds (C C sp 2, C C sp 3 and C O bonds) are detected. No formation of aluminum carbide is found since there is no peak at the Al C energy of 281.5 ev. After inclusion of Al into the a-c matrix, the sp 3 /sp 2 ratio decreases from 68% to 19%, i.e., a drastic increase in the sp 2 fraction occurs, which is the reason why there is a drastic decrease in both film hardness and residual stress. 10 Analyzing C 1s spectra from TiC/a-C and TiC/a-C(Al) films, a new bond is observed at about 282 ev, which is the C Ti bond.
XPS Study of Diamond-Like Carbon-Based Nanocomposite Films 799 Fig. 1. XPS C 1s core level spectra of a-c, A-C(Al), TiC/a-C and TiC/a-C(Al) films. Table 1. The atomic concentration of C 1s peaks obtained from XPS analysis. Film C C sp 2 C C sp 3 C O C Ti sp 3 /sp 2 a-c 55.8 37.9 5.3 0.68 a-c(al) 74.99 14.17 10.83 0.19 nc-tic/a-c 65.64 12.36 3.86 15.09 0.19 nc-tic/a-c(al) 61.36 10.73 4.85 16.96 0.18 The sp 3 /sp 2 ratio in these two films are about the same and are maintained at a low value of 0.19 and 0.18, respectively (see Table 1). An increase in the hardness is observed and is believed to come from the inclusion of the nanocrystalline TiC structures. 10 To understand the state of the Al in the a-c(al) and nc-tic/a-c(al) films, the Al 2p spectra are plotted in Fig. 2. The peaks located at around 74.2 and 72.7 ev are those of the elemental Al 2p 1/2 and Al 2p 3/2 spectra. The small tail at the highenergy side (above 75 ev) indicates the presence of the Al O bond because Al is extremely reactive. Comparing the XPS profiles of a-c(al) and that of nc-tic/a- C(Al), they are basically identical. This indicates that: (1) there is no evidence of the formation of Al C bonds. (2) Al doping basically exists as elemental Al in the a-c matrix and it is unlikely that Al goes into TiC in the case of nc-tic/a- C(Al). This explains the XRD results of a-c(al), nc-tic/a-c and nc-tic/a-c(al) (see Fig. 3) where only TiC peaks are detected with or without Al doping in the deposition process. 10 Figure 4 shows the Ti 2p core level spectra for nc-tic/a-c and nc-tic/a- C(Al) films. Similarly, there is not much difference between these two profiles. With
800 S. Zhang et al. Fig. 2. XPS Al 2p spectra of a-c(al) and TiC/a-C(Al) films. TiC[111] TiC[200] TiC[220] Si substrate Intensity [arb. units] TiC/a-C TiC/a-C(Al) a-c(al) 30 40 50 60 70 2θ ( 0 ) Fig. 3. XRD spectra of TiC/a-C, TiC/a-C(Al) and a-c(al) films. 10 reference to Refs. 11 and 12, the features at 454.7 ev and 460.6 ev are assigned as TiC (2p 1/2 and 2p 3/2 ), and those at 458.3 ev and 464 ev are TiO 2.Therearetwo other broad peaks at about 456.2 ev and 462 ev, which can be assigned as a Ti X peak (a combination of a sub-stoichiometric TiC x (x<1) and TiO x (or TiC x O y ), as well as a Ti 2p intrinsic satellite peak 13 ). Titanium satellite peaks are commonly observed, and these are loss peaks related to the screening effect of conduction electrons, or an electronic effect brought by atomic reordering. 14 The Ti 2p 1/2 and 2p 3/2 peaks of the TiC phase are situated at higher positions compared with the 453.8 and 459.9 ev peaks in elemental Ti 2p, due to a chemical shift resulting from charge transfer. The peak intensity ratios of TiC, Ti X and TiO 2 are 9:6:1, or the titanium in the films is mostly bond with carbon forming nc-tic, at the same time,
XPS Study of Diamond-Like Carbon-Based Nanocomposite Films 801 Fig. 4. Ti 2p XPS spectra of nc-tic/a-c and nc-tic/a-c(al) films. a relatively large amount of nonstoichiometric TiC (i.e., TiC x ) exists together with minute amounts of nonstoichiometric titanium oxide (TiO x ). 4. Summary X-ray photoelectron spectroscopy has been used to analyze a-c-based nanocomposite films prepared via co-sputtering using Ti, Al and graphite targets in an argon atmosphere. Four carbon bonds are identified: C C sp 2 bonding, C C sp 3 bonding, C O bonding and C Ti bonding. Al doping in the a-c matrix results in a sharp decrease in C C sp 3 bonding and an increase in C C sp 2 bonding or an overall decrease in the sp 3 /sp 2 ratio. This explains the reduction in hardness. XPS results also confirmed that the Al doped exists as elemental Al in the a-c matrix instead of bonding with C or Ti. Titanium in the films mostly bonds with carbon to form nanocrystalline TiC or nc-tic. Under the experimental conditions, the XPS peak intensity ratios of TiC, Ti X and TiO 2 are 9:6:1. Acknowledgment The authors are indebted to Nanyang Technological University for AcRF funding RG12/02. References 1. J. Musil, Surf. Coat. Technol. 125, 322 (2000). 2. S. Veprek and S. Reiprich, Thin Solid Films 268, 64 (1995). 3. S. Veprek and A. S. Argon, Surf. Coat. Technol. 146 147, 175 (2001). 4. S. Zhang, D. Sun, Y. Q. Fu and H. J. Du, Surf. Coat. Technol. 167, 113 (2003).
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