X-ray photoelectron spectroscopy study of thin TiO2 films cosputtered with Al Jin-Cherng Hsu, Paul W. Wang, and Cheng-Chung Lee
In this study, titanium dioxide 共TiO2兲 films were fabricated by cosputtering of a titanium (Ti) target and an aluminum (Al) slice in a smaller area by an ion-beam sputtering deposition method. The sputtered films were postannealed at 450 °C. The x-ray photoelectron spectroscopy spectra were categorized by their oxygen bonding variations, which include high-binding-energy oxygen, (HBO), bridging oxygen, low-binding-energy oxygen, and shifts of the binding energies (BEs) of oxygen (O) and Ti signals. The enhancement of HBO and higher BE shifts of the O 1s spectra as a function of cosputtered Al in the film imply the formation of an Al—O—Ti linkage. Corresponding changes in the Ti 2p spectra further confirm the modification of properties of the cosputtered film that results from the variation of the chemical bonding environment. An observed correlation between the chemical structure and optical absorption of the Al cosputtered films can be used to modify the optical properties of the film. © 2006 Optical Society of America OCIS codes: 310.1620, 310.1861, 310.6860, 310.6870.
1. Introduction
Titanium dioxide 共TiO2兲 has been extensively studied in optical coating research because it has a high refractive index and is transparent in the visible and the near-infrared regions. It is mechanically hard and environmentally stable and can be produced by many kinds of deposition methods.1 Yet high-fluence optics including optical coating has been required, and laser-induced damage of that coating has been studied.2 Minimizing the optical absorption and the grain’s columnar structure of the film is necessary because defects of the grain’s columnar structure will influence absorption and thermal and mechanical stability.3 However, ion-beam sputtering,4 the conventional means, can produce these films with excellent optical properties such as high refractive index, low absorption, and low scatter.5,6 The surface roughness and the value of extinction coefficient k are reduced to a minimum when the film is postannealed at an optimum temperature, 275 °C.7 Once the TiO2 J.-C. Hsu (
[email protected]) is with the Department of Physics, Fu-Jen Catholic University, 510 Chung-Cheng Road, Hsin-Chuang, 242 Taiwan. P. W. Wang is with the Department of Physics, Bradley University, 1501 West Bradley Avenue, Peoria, Illinois 61625. C.-C. Lee is with the Thin Film Technology Center, National Central University, Chung-Li, 320 Taiwan. Received 12 December 2005; accepted 20 January 2006; posted 15 February 2006 (Doc. ID 66569). 0003-6935/06/184303-07$15.00/0 © 2006 Optical Society of America
film has been postannealed at a higher temperature, such as 450 °C, an amorphous crystalline transformation causes the absorption to increase.8 The TiO2 film recrystallizes into anatase and rutile grain9 and easily develops oxygen vacancies or titanium interstices.10 Gluck et al.11 and Feldman et al.12 showed that changes in the structure, the optical properties, and the stress reduction of films can result from mixing two materials during deposition.13 Besides, the films mixed with oxides, deposited by magnetron sputtering and postannealed at 450 °C for 1 h, have been investigated; a reduction of optical loss and the stabilization of the film’s composition against environmental effects such as aging and exposure to humidity were shown.14 From the reasons mentioned above, we used an ion-beam sputtering method to cosputter Ti and a smaller amount of Al in ambient O, and we postannealed the as-deposited films to 450 °C for 6 h in air to investigate the changes in their heat-induced properties. Apparently the amount of cosputtered Al affects the chemical bonding of cosputtered films. Hence x-ray photoelectron spectroscopy analysis was used in this study. 2. Experiment
A schematic drawing of the experimental setup is shown in Fig. 1. The films were prepared by an ionbeam sputtering deposition (IBSD) system that had an ion source with 3 cm diameter graphite grids made by Vecco, Inc. The Ti target was mounted upon 20 June 2006 兾 Vol. 45, No. 18 兾 APPLIED OPTICS
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Fig. 2. Refractive index (n) and extinction coefficient (k) of the cosputtered specimens.
Fig. 1. Schematic drawing of the IBSD system. OPM, optical monitor.
a water-cooled polygonal turret that was 15 cm away from the ion source. The target consisted of a 10 cm diameter round disk of pure Ti metal 共99.995%兲 and a slice of Al 共99.99%兲 in various sizes suspended by a Ti wire at the center of the Ti target for cosputtering. The substrates were 25 mm ⫻ 25 mm ⫻ 1 mm Corning 7059 glass, cleaned with isopropyl alcohol in an ultrasonic bath and blown dry with dry nitrogen gas. Before the deposition, the vacuum chamber was pumped down to a base pressure of less than 7 ⫻ 10⫺7 Torr by a cryogenic pump. Oxygen used as the working gas was fed near the substrate and regulated at optimum partial pressure, 2 ⫻ 10⫺5 Torr, by a needle valve.7 During deposition, the background pressure was ⬃1 ⫻ 10⫺4 Torr. The ion-beam voltage and ion-beam current of the ion source were 1100 V and 30 mA, respectively. The ion current density was ⬃1 mA兾cm2. The deposited films with an optical thickness of ⬃0.5 (at ⫽ 650 nm) were measured by an optical monitor. Meanwhile, a quartz monitor was used to monitor the deposition rate. All specimens were labeled Al-R, where R was the area ratio of Al to Ti in the target. The area ratio was close to the composition ratio, as was confirmed by a linear relationship between the refractive index and the film’s composition. This relation is consistent with that of a previous study of another cosputtering oxide material15; i.e., the refractive index decreases as the R value increases, as shown in Fig. 2. Three specimens, Al1兾40, Al-1兾20, and Al-1兾10, were postannealed in air for 6 h in an oven at 450 °C at a heating rate of ⫹5 °C兾min and were naturally cooled to room temperature. The optical parameters of all specimens were measured by an ellipsometer (variable-angle spectroscopic ellipsometer made by the J. A. Woollam 4304
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Company). The morphologies from the grain columnar structure of the films were investigated with a Digital Instruments Nanoscope II atomic-force microscope. A typical scanning area in an atomic-force microscope was 1 m ⫻ 1 m 共256 ⫻ 256 pixels兲 on a vibration-free platform. The root-mean-square value of surface roughness was obtained by use of software that came with the instrument. XPS was used to analyze the composition and chemical states of O 1s and Ti 2p species in postannealed films containing various lesser amounts of Al. Because of a sample charging effect the binding energies were calibrated by reference to the C 1s line at 285.0 eV.16 The XPS spectra were taken on specific binding energy ranges that covered Ti 2p1兾2, Ti 2p3兾2, and O 1s. The O 1s signals were fitted mainly on the basis of the binding energies of O 1s in TiO2 and Al2O3, located at 530.1 and 531.6 eV, respectively.17 To analyze the XPS data, we applied mixed Gaussian curves to fit the spectra. The optimum positions and FWHM of the peaks were iteratively determined from least-squares fitting by with PEAKFIT software. 3. Results and Discussion
The specimen labeled Al-0 was pure TiO2 film deposited by use of only a pure Ti target by IBSD. After postannealing at 450 °C for 6 h, the film became polycrystalline and its surface was quite irregular, as shown in Fig. 3(a).7 The effective ionic radii of Ti2⫹, Ti3⫹, and Ti4⫹, which are 100, 81, and 65 pm, respectively,18 obviously decrease with increasing numbers of positive charges. The thermal energy from the postannealing process may stimulate Ti ions to a higher oxidation state Ti4⫹, which is smaller in radius such that part of film’s surface collapses and becomes irregular. A thin film deposited by ion-beam sputtering typically has a built-in high stress that can be reduced by cosputtering of two materials in it.13,19 The Gibbs free energy of Al2O3, TiO2, and Ti is ⫺400, ⫺200, and ⫹112 kcal mol⫺1, respectively.20 The dissociation of Ti—O bonds to form Al—O bonds results in a net loss of system energy, which reduces tensile stress in the Al cosputtered films. Even though the formation of Ti
Fig. 3. Surface morphology and roughness of Al cosputtered films postannealed at 450 °C. (a) Data of an earlier study.7
metal costs energy, namely, ⫹112 kcal mol⫺1, forming Ti—O—Al and pure Al2O3 reduces the total system energy by ⬃⫹100 kcal mol⫺1. Consequently the cosputtered film has less surface energy and a smoother surface. The collapsed area just disappears and becomes a flat surface when the minor amount of Al is increased to Al-1兾10, as shown in Fig. 3(d), where the surface roughness apparently deceases from 1.70 in Al-0 to 0.152 nm in Al-1兾10 and refractive index n simultaneously decreases from 2.54 to 2.324. Furthermore, the larger surface roughness in Al-0 permits more surface scattering and optical scattering loss.9 However, the smaller surface roughness in Al-1兾10 results in lower scattering loss. In fact, the k value can be divided into an absorption coefficient 共␣a兲 and an equivalent distributive scattering coefficient 共␣s兲 of the scattering loss, i.e., k ⫽ ␣a ⫹ ␣s.21 The low absorption of the cosputtered film after postannealing results from the oxygenation of Al atoms that bond to low-binding energy oxygen (LBO) from the TiO2 network in the film, as discussed for the XPS spectra below. The presence of Al atoms reduces the effect of amorphous-crystalline transformation owing to reorientation of Ti atoms in the cosputtered film. Although Al-1兾10 has the smallest surface rough-
ness, as shown in Fig. 3, the k value of Al-1兾10 is still larger than those of the other specimens, as shown in Fig. 2. This result cannot be explained by the reduction of surface roughness, but apparently the rearrangement of O atoms in the film plays a significant role in the absorption coefficient in Al-1兾10 film. Even though the oxygen vacancies in bulk flow to the TiO2 surface as reported by low-energy electron microscopy, i.e., the oxygen vacancies move to surface at annealing temperature between 800 and 850 °C, which is higher than that in this study.22 Therefore the flow of O vacancy from the bulk onto a surface during postannealing is not considered. A.
O 1s XPS Spectra
In general, XPS peaks from core-level electrons in an amorphous film are broad owing to the less-localized electron density distribution in an amorphous structure.17 Figure 4 shows the observed O 1s XPS spectra (solid curves). The shoulders at the higher binding energy (BE) of the O 1s signal are clearly observed. Then the spectra are decomposed into the three peaks [bridging oxygen (BO), LBO, and high-bindingenergy oxygen (HBO)] for all the specimens. According to the BE of the peaks, the main peak with the 20 June 2006 兾 Vol. 45, No. 18 兾 APPLIED OPTICS
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Fig. 4. XPS spectra of the cosputtered films at O 1s energy regions.
largest area is assigned to BO (an O atom bonded in Ti—O linkage) because O in Ti–O linkage is a dominant species and a binding energy of O 1s in TiO2 was observed at 530.1 eV.17 The peak with the lower binding energy at 528.3 eV is assigned to LBO, in which the O atom does not bond to a Ti atom and may have an extra electron attached to it that is similar to the nonbridging-oxygen (NBO) defect in silica glass.23,24 These loosely bonded O defects are similar to the NBO hole centers that are well known in amorphous SiO2, in which LBO 1s peak was observed by XPS.25 The peak with the higher binding energy assigned to HBO refers to the O atom bonded in the Al—O linkage because the binding energy of O 1s in Al2O3 was observed at 531.6 eV.17 The peak area of HBO grows with Al content in the film, as the XPS data show in Table 1. It is expected that more O atoms will bond to Al atoms when the Al content increases. Indeed, the ability of an Al atom to attract electrons is better than that of a Ti atom as the value of electron affinity of an Al atom is 48 kJ mol⫺1 and that of a Ti atom is 37.7 kJ mol⫺1.26 That is why the main peak, BO, is somewhat shifted to a higher BE from 530.09 to 530.36 and then to 530.56 eV as the Al content increases, as is also shown in Table 1. So the larger the R value of the film, i.e., the more cosputtered Al, the more apparent shifts in the BEs of BO. Moreover, the FWHM of BO, ⬃1.5 eV, is smaller than that of LBO, i.e., ⬃1.6 eV because BO has a more oriented electron distribution than LBO, as is also shown in Table 1. In as much as the Al-0 film is LBO rich, i.e., more unbonded or more loosely bonded O is at the surface, the surface should be more open and rough, as shown in Fig. 3(a). Noted the reduction of LBO, whose peak is located at 528.4 eV, where the area of the LBO signal decreases from 5.2% to 1.9% and then to 0.7% when the R values increase from 1兾40 to 1兾20 and then to 1兾10, as shown in Fig. 4. The BO area in Al-1兾20, 75.7%, practically equals that in Al-1兾40, 75.8%, listed in Table 1. From the changes in concentration of LBO and BO, it is obvious that the cosputtered Al atoms first form a chemical bond, Al—O*— Ti, where O* represents LBO that has an extra attached electron to form the chemical bond easily. Moreover, because LBO bonds loosely, the electron density is broadly distributed such that the Al–O* bond is formed easily. Furthermore, the decrease of LBO owing to the presence of Al atoms is a consequence of the ability of Al atoms to attract electrons being better than that of Ti atoms.26 LBO peak areas
Table 1. XPS Data for O 1s
High-Binding-Energy Oxygen, HBO
Binding-Energy Oxygen, BO
Low-Binding-Energy Oxygen, LBO
Specimen
BE (eV)
Area (%)
FWHM (eV)
BE (eV)
Area (%)
FWHM (eV)
BE (eV)
Area (%)
FWHM (eV)
Al-1兾40 Al-1兾20 Al-1兾10
531.61 531.60 531.61
18.0 22.5 33.1
2.11 2.10 2.19
530.09 530.36 530.56
75.8 75.7 66.0
1.52 1.50 1.59
528.43 528.41 528.48
5.2 1.9 0.7
1.59 1.58 1.59
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Fig. 5. XPS spectra of the cosputtered films at Ti 2p1兾2 and Ti 2p3兾2 energy regions.
decrease as more cosputtered Al atoms exhaust LBO when the R value is increasing. Until the BO area deceases to 66% in an Al-1兾10 film from which LBO has been nearly exhausted, Al atoms start to form Al—O bonds in which the O atoms originate from a TiO2 network. Then a substoichiometric oxide, Ti2O3, is created from the network and increases the optical absorption of the film.8 B.
Ti 2p XPS Spectra
The XPS spectra of Ti 2p of cosputtered films are shown in Fig. 5, where the solid curves represent the as-recorded data. The Ti 2p peak is fitted by the sum
of two peaks located at ⬃464.4 and ⬃458.6 eV, which are photoelectrons from Ti 2p1兾2 and Ti 2p3兾2 core electron levels,17 respectively. The line shape changes in the XPS spectra owing to the cosputtering effect. Two shoulders appear at lower binding energies beside two main peaks in the spectrum. One of the main peaks is the XPS signal of Ti 2p1兾2 from TiO2 and the associated shoulder subpeak, Sub-1, located at 462.2 eV. Similarly, for the Ti 2p3兾2 spectrum the main peak is referred to as Ti 2p3兾2 from TiO2 and the other subpeak, Sub-2, is located at 456.5 eV. For all the main peaks and subpeaks, the BEs increase with the R values, as shown in Table 2. The same trend of higher BE shifts of BO in the O 1s spectra from a cosputtered specimen occurs in the Ti 2p spectra. The BE of Ti 2p1兾2 shifts from 464.07 to 464.30 and then to 464.78 eV, and the BE of Ti 2p3兾2 from 458.63 to 458.68 and then to 458.86 eV, as the R value increases from 1兾40 to 1兾20 and then to 1兾10. The presence of Al atoms results in the formation of a Ti—O—Al bond where the electron cloud about Ti4⫹ is pulled by the Al atoms. That is, the electron cloud of Ti atoms is pulled further toward the O—Al bond than toward the Ti—O bond in a TiO2 network. Therefore the binding energies of Ti 2p become higher as the R value increases. The subpeaks, Sub-1 and Sub-2, are assigned to Ti3⫹ from a substoichiometic oxide, Ti2O3, as reported in previous XPS studies.27,28 That is, the TiO2 network loses O atoms in the film because the Al atoms bond and oxidize with LBO. The consequence of this competition of O atoms with Al and Ti atoms results in the formation of Ti3⫹ and O—Al bonds at the same time. However, the observed peak positions of Sub-1 and Sub-2 are higher by approximately 0.2–1 eV than those of the oxidized Ti film studied by Cai et al.28 Because once more Ti—O—Al bonds form as the R value increases, fewer O atoms are available in the TiO2 network, which results in the formation of Ti3⫹. Hence lower BE shoulders of Ti are observed. Because of the influence of nearby Al atoms, the BE of Sub-1 shifts higher, from 462.21 to 462.54 and then to 463.43 eV, and the BE of Sub-2 from 456.55 to 456.60 and then to 457.24 eV. The formation of Ti2O3 from the TiO2 network is also evident from the increasing areas of the two subpeaks with the increased R values shown in Table 2. The FWHM of Sub-1 and Sub-2 also increases as Ti3⫹ grows. The broadening of Sub-1 is from 1.25 to 1.90 and to 1.98 eV, and that of Sub-2 is from 1.12 to 1.53 and to 1.63 eV as shown in Table 2. This clearly implies that a stronger chemical interaction between Ti3⫹ and the TiO2兾Al2O3 network. The reduction of the FWHM of the main peaks, Ti 2p1兾2 and Ti 2p3兾2, from 2.61 to 2.19 and 2.02 eV, and from 2.47 to 2.32 and 1.69 eV, respectively, results from the better quality of TiO2 network that is a consequence of the formation of Ti3⫹ and the elimination of LBO from TiO2 network. It should be noted that the difference in BE 共⌬兲 for Ti 2p1兾2 from that of Ti 2p3兾2 varies from 5.44 to 5.62 to 5.92 eV with increasing R values, as 20 June 2006 兾 Vol. 45, No. 18 兾 APPLIED OPTICS
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Table 2. XPS Data for Ti 2p1兾2 and 2p3兾2
2p1兾2
Sub-1
2p3兾2
Sub-2
Specimen
BE (eV)
Area (%)
FWHM (eV)
BE (eV)
Area (%)
FWHM (eV)
BE (eV)
Area (%)
FWHM (eV)
BE (eV)
Area (%)
FWHM (eV)
⌬ (eV)a
Al-1兾40 Al-1兾20 Al-1兾10
464.07 464.30 464.78
22.1 17.7 16.8
2.61 2.19 2.02
462.21 462.54 463.43
1.6 6.0 10.7
1.25 1.90 1.98
458.63 458.68 458.86
71.9 68.3 62.3
2.47 2.32 1.69
456.55 456.60 457.24
3.9 8.3 10.2
1.12 1.53 1.63
5.44 5.62 5.92
⌬ denotes the variation of the BE from Ti 2p1兾2 to Ti 2p3兾2.
a
shown in Table 2. Moreover, the ⌬ value of pure TiO2 just equals 5.92 eV.17 This increasing trend toward differences in BE (⌬) also supports the theory that the quality of the TiO2 network is improved as Al atoms are cosputtered onto TiO2 films. C. Optical Absorption
The k value of Al-1兾10 film is higher than those of Al-1兾40 and Al-1兾20, as shown in Fig. 2, even though the surface roughness of Al-1兾10 is the smallest among all the specimens shown in Fig. 3, as indicated in Subsection 3.A. The sole improvement in surface roughness of Al-1兾10 cannot warrant the decrease of the k value in that film. It is believed that the formation of Ti2O3 in the film results in the increase in absorption. Moreover, the surface roughness of Al1兾40 is almost the same as that of Al-1兾20, as shown in Fig. 3. The decrease in k of these two specimens is really caused by an improvement in network structure; i.e., Al-1兾20 has a better-quality network structure owing to the reduction of LBO bonds and an increase in the ⌬ value in the film, as described in Subsections 3.A and 3.B, respectively. That is why the k value of Al-1兾20 is smaller than that of Al-1兾40, as shown in Fig. 2. There are three factors that affect the k values: surface roughness, network structure, and Ti2O3 in the film. The k value decreases when the surface is getting smoother and the network structure is getting better. In other words, the k value increases owing to higher optical absorption of Ti2O3, which results from the presence of more Ti3⫹ ions and Ti— O—Al bonds formed in more Al cosputtered films after postannealing, as discussed in Subsection 3.B. 4. Conclusions
In summary, cosputtering Al into a TiO2 film is beneficial to the film in IBSD. The optical absorption, heat resistance, and microstructure of cosputtered films were obviously improved, even after postannealing to 450 °C. TiO2 film had a smaller optical absorption when the film was cosputtered with 1兾20 Al. The surface roughness was greatly improved, even after postannealing to 450 °C when the film was cosputtered with 1兾10 Al, as Al-1兾10. These results can be explained by the formation of Al—O*—Ti and Al—O—Ti linkages from the variations of O bonding, which include HBO, BO, LBO, and the BE shifts of O and Ti, shown in the XPS spectra. LBO decreases, 4308
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HBO increases, and the BE of BO shifts a little higher when more Al is cosputtered into the TiO2 film. Moreover, the corresponding changes of the Ti 2p XPS spectra further confirm the modification of the cosputtered film that resulted from variation of the chemical bonding environment. The BE of Ti 2p, like that of O 1s, shifts higher when Al is cosputtered onto a TiO2 film. But Ti2O3 is formed simultaneously in the network and results in an increase of optical absorption. The changes in chemical environment, i.e., the breaking and formation of chemical bonds and the better quality of the TiO2 network, improve the stability and heat resistance of the cosputtered film. We thank Kung-Chi Hsiao for discussions. This research was supported by the National Science Council under contract NSC94-2215-E-030-002 and by the Office of Research and Development of Fu-Jen Catholic University. References 1. J. M. Bennett, E. Pelletier, G. Albrand, J. P. Borgagno, B. Lazarides, C. K. Carniglia, R. A. Schnell, T. H. Allen, T. TuttleHart, K. H. Guenther, and A. Saxer, “Comparison of the properties of titanium dioxide films prepared by various techniques,” Appl. Opt. 28, 3303–3317 (1989). 2. R. Chow, M. Runkel, and J. R. Taylor, “Laser damage testing of small optics for the national ignition facility,” Appl. Opt. 44, 2327–3531 (2005). 3. M. R. Kozlowski, “Damage-resistant laser coating,” in Thin Films for Optical Systems, F. R. Flory, ed. (Marcel Dekker, 1995), pp. 521–549. 4. D. T. Wei, “Ion beam interference coating for ultralow optical loss,” Appl. Opt. 28, 2813–2816 (1989). 5. A. Kalb, “Neutral ion beam sputter deposition of high-quality optical film,” Opt. News 12(8), 13–17 (1986). 6. J. R. Sites, H. Demiryont, and D. B. Kerwin, “Ion-beam sputter deposition of oxide films,” J. Vac. Sci. Technol. A 3, 656 (1985). 7. J. C. Hsu and C. C. Lee, “Single- and dual-ion-beam sputter deposition of titanium oxide films,” Appl. Opt. 37, 1171–1176 (1998). 8. L. S. Hsu, R. Rujkorakam, J. R. Sites, and C. Y. She, “Thermally induced crystallization of amorphous-titanium films,” J. Appl. Phys. 59, 3475–3480 (1986). 9. P. Löbl, M. Huppertz, and D. Mergel, “Nucleation and growth in TiO2 films prepared by sputtering and evaporation,” Thin Solid Film 251, 72–79 (1994). 10. P. Kofstad, “Thermogravimetric studies of the defect structure of rutile (TiO2),” J. Phys. Chem. Solids 23, 1579 –1586 (1962). 11. N. S. Gluck, H. Sankur, J. Heuer, J. Denatale, and W. J. Gunning, “Microstructure and composition of composite SiO2兾 TiO2 thin films,” J. Appl. Phys. 69, 3037–3045 (1991). 12. A. Feldman, E. N. Farabaugh, W. K. Haller, D. M. Sanders, and R. A. Stempniak, “Modifying structure and properties of
13.
14.
15. 16.
17.
18.
19.
20.
optical thin films by coevaporation,” J. Vac. Sci. Technol. A 4, 2969 –2974 (1986). B. J. Pond, J. I. DeBar, C. K. Carniglia, and T. Raj, “Stress reduction in ion beam sputtered mixed oxide film,” Appl. Opt. 28, 2800 –2805 (1989). R. Rabady and I. Avrutsky, “Titania, silicon dioxide, and tantalum pentoxide waveguides and optical resonant filters prepared with radio-frequency magnetron sputtering and annealing,” Appl. Opt. 44, 378 –383 (2005). H. Demiryont, “Optical properties of SiO2–TiO2 composite films,” Appl. Opt. 24, 2647–2650 (1985). E. McCafferty and J. P. Wightman: “An x-ray spectroscopy sputter profile study of the native air-formed oxide film on titanium,” Appl. Surf. Sci. 143, 92–100 (1999). C. Wagner, W. Riggs, L. Davis, J. Moulder, and G. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy (Perkin-Elmer, 1979), pp. 66 and 124. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A 32, 751–767 (1976). R. N. Castellano, M. R. Notis, and G. W. Simmons, “Composition and stress state of thin films deposited by ion beam sputtering,” Vacuum 27, 109 –117 (1977). D. R. Lide, ed., CRC Handbook of Chemistry and Physics (CRC, 1991).
21. W. H. Wang and S. Chao, “Annealing effect on ion-beamsputtered titanium dioxide film,” Opt. Lett. 23, 1417–1419 (1998). 22. K. E. McCarty and N. C. Bartelt, “Role of bulk thermal defects in the reconstruction dynamics of the TiO2 (110) surface,” Phys. Rev. Lett. 90, 461041 (2003). 23. D. L. Griscom and E. J. Friebele, “Fundamental defect centers in glass: 29Si hyperfine structure of the nonbridging oxygen hole center and the peroxy radical in a-SiO2,” Phys. Rev. B 24, 4896 – 4898 (1981). 24. D. L. Griscom, “Self-trapped holes in amorphous silicon dioxide,” Phys. Rev. B 40, 4224 – 4227 (1989). 25. P. W. Wang, L. Zhang, Y. Tao, and C. Wang, “Thermal behavior of silver in ion-exchanged soda lime glasses,” J. Am. Ceram. Soc. 80, 2285–2293 (1997). 26. R. S. Berry, “Small free negative ions,” Chem. Rev. 69, 533– 542 (1969). 27. M. Jobin, M. Taborelli, and P. Descouts, “Structural characterization of oxidized titanium surfaces,” J. Appl. Phys. 77, 5149 –5155 (1995). 28. K. Cai, M. Müller, J. Bossert, A. Rechtenbach, and K. D. Jandt, “Surface structure and composition of flat titanium thin films as a function of film thickness and evaporation rate,” Appl. Surf. Sci. 250, 252–267 (2005).
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