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oom-temperature deposition of amorphousitanium dioxide thin film with high refractive indexy a filtered cathodic vacuum arc technique

hiwei Zhao, Beng Kang Tay, and Guoqing Yu

Amorphous titanium dioxide �TiO2� thin film has been prepared by a filtered cathodic vacuum arctechnique at room temperature. It was concluded from the core level of Ti 2p3�2 �458.3 eV� and O 1s�529.9 eV� and their deviation in binding energy ��BE � 71.6 eV� that only one of Ti oxidation states, Ti4�,existed in the film and the film was of ideal stoichiometry. The film possessed high transmittance, whichcan reach as high as that of a quartz substrate, especially in the visible range, owing to its optical bandgapof 3.2 eV. The high refractive index �2.56 at 550 nm� and low extinction coefficient ��10�4 at 550 nm�suggested that the film had a high packing density and a low scattering-center concentration. Thesegood optical properties implied the film prepared by this technique was a promising candidate for opticalapplication. Besides, the film was found to transform in the structure from amorphous to anatasecrystalline when it was annealed at 300 °C, as evidenced by Raman and x-ray diffraction. © 2004Optical Society of America

OCIS codes: 310.6860, 310.1860, 120.4530.

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. Introduction

itanium dioxide �TiO2� thin film exists in differenttructural forms, namely, rutile, anatase, brookite,nd amorphous. It has a high refractive index,igh chemical stability, and excellent transparency

n the visible and near-infrared bands and is widelysed as a component of both reflective and trans-issive multilayers in optical applications,1 as a

rotective layer on lenses and optical fibers,2 inolar energy converters,3 and in highly efficient cat-lysts.4 For optical applications, films with highuality will require dense TiO2 thin film with a highefractive index and a low extinction coefficient.n some cases, an amorphous structure is alsoeeded owing to its absence of optical anisotropy.he properties of TiO2 thin film depend on the film’sicrostructure, which, in turn, are strongly af-

ected by the various deposition techniques andeposition conditions.Cathodic vacuum arc evaporation is a commonly

The authors are with the School of Electrical and Electronicngineering, Nanyang Technological University, Singapore39798. The e-mail address of B. K. Tay is ebktay@ntu.edu.sg.Received 9 May 2003; revised manuscript received 15 September

003; accepted 25 September 2003.0003-6935�04�061281-05$15.00�0© 2004 Optical Society of America

sed method for the deposition of oxide and nitridelms. This technique is characterized by a high ion-

zation rate, high ion energy �50 � 150 eV�, a higheposition rate, and flexibility of target arrange-ents. The feature of high energy could decrease

he void volume of film by increasing the packingensity of its microstructure, and this results in aense film. However, the generation of macropar-icles during the evaporation greatly influences prop-rties of deposited film and limits the use of suchechnique in growing high-quality films for opticalpplications.5 Off-plane filtered cathodic vacuumre �FCVA� is a promising technique that employslectromagnetic and mechanical filtering to elimi-ate unwanted macroparticles and neutral atoms.6y use of this technique, it is feasible to depositetal oxide thin films as optical coatings with high

uality.7,8

In this paper, TiO2 thin film with amorphous struc-ure was deposited without auxiliary heating by usef the FCVA technique. TiO2 thin film was charac-erized by x-ray photoelectronic spectroscopy �XPS�nd optical spectroscopy in terms of the chemicaltates and optical properties. Structure change waslso analyzed by Raman and x-ray diffraction �XRD�f the films annealed at 300 °C. Our results suggesthat the FCVA technique is a promising candidate forreparing high-quality optical films.

20 February 2004 � Vol. 43, No. 6 � APPLIED OPTICS 1281

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. Experimental Details

iO2 thin films were deposited by the FCVA systems described in detail elsewhere.9,10 The system in-orporates an off-plane double-bend filter, which ef-ectively removes macroparticles. A Ti cathode withpurity of 99.98% operated at an arc current of 100was used to obtain the plasma. The base pressure

f the system was 4 � 10�6 Torr. A mixture ofesearch-grade gases of oxygen and argon was intro-uced into the plasma stream within the filter regionf the FCVA system. The process pressure was fixedt 3.0 � 10�4 Torr. The films were grown on n-Si100� and quartz substrates at room temperature andithout substrate bias. Rapid thermal annealing

RTA� was performed on as-grown film deposited onilicon at 300 °C for 300 s in vacuum �2 � 10�6 Torr�y use of the Jipelec Jetstar system �Qualiflow Com-any, France�.The core-level spectra and compositional analysis

f the films were performed by XPS, by use of ahermo VG Scientific ESCALAB 250 spectrometerith a monochromatic Al K x-ray source �1486.6-eVhotons�. Ar� ions were used for sputter etching ofhe films. Transmittance and reflectance were in-estigated with a Perkin Elmer Lamda 16 UV–visiblepectrometer from 200 to 900 nm. Both optical con-tants and film thickness were obtained by our fittingptical spectra by use of Scout software.11 The Ra-an spectra were excited by use of the 514-nm line of

n Ar laser and collected in a backscattering geome-ry on a CCD camera by use of a Renishaw micro-aman System 1000 spectrometer, in which laserower on the sample was less than 2 mW. The crys-alline structures of the films were checked by XRDith a Cu K source.

. Results and Discussion

. Chemical States and Compositions

igure 1�a� shows the Ti 2p core-level spectrum. Cs binding energy of 284.6 eV is used as a reference toalibrate the XPS system. The binding energiesBEs� of two spin-orbit components, Ti 2p1�2 and Tip3�2, were located at 464 and 458.3 eV, respectively,n good agreement with the values for TiO2 �Ti4�� inhe literature.12 It is widely known that there areainly three kinds of oxidation state of titanium:i2� �455.1 eV�, Ti3� �456.7 eV�, and Ti4� �458.7 eV�,hich correspond to TiO, Ti2O3, and TiO2, respective-

y.13 Figure 1�b� presented the O 1s spectrum,hich is asymmetric. It was well decomposed into

wo peaks centered at 529.9 and 531.3 eV withWHM of 2.07 and 3.7 eV, respectively. The lowerE component corresponded to O 1s in TiO2 of thelm. Also, it has been accepted that deviation of theore-level O 1s from that of Ti 2p3�2��BE� is a sensi-ive indicator of the oxidation state of Ti.12 In ourase, �BE was 71.6 eV and consistent with the valueor TiO2 in the literature.12 The �BE for TiO andi2O3 are 75.0 0.2 and 73.1 0.2 eV, respectively.12

herefore it was concluded that Ti in the film shouldxist in the form of Ti4� rather than Ti2� and Ti3�.

282 APPLIED OPTICS � Vol. 43, No. 6 � 20 February 2004

he higher BE component �531.3 eV� in the O 1s XPSpectrum was attributed to the defective oxides531.5 0.5 eV�.14 The O�Ti ratio was calculated toe nearly 2 from the peak area of O 1s and Ti 2p3�2ombined with their sensitivity factors.

The surface morphology of film is known to be oneactor affecting its optical properties. Therefore theorphology of the film observed by an atomic forceicroscope is given in Fig. 2. Clearly, the surfaceas rather smooth, and rms roughness is approxi-ately 0.5 nm within 2 �m � 2 �m. This smooth

urface, as a result of the high migration ability ofondensing particles due to their high energy as men-ioned above, tends to contribute to the suppression ofurface light scattering loss and therefore results inigh transmittance of the film.

. Optical Properties

igure 3 shows the transmittance of the films depos-ted on quartz as well as of uncoated quartz. Theeaks in the transmission spectra are due to the con-tructive and destructive interference caused by theifference in refractive index between the film andhe substrate when the incident light passes through

ig. 1. �a� Ti 2p XPS spectrum of TiO2 thin film on n-Si�100�. �b�1s XPS spectrum of TiO2 thin film on n-Si�100�.

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he film and quartz substrate. It was observed thathe film was transparent in the visible and near-nfrared range. The transmittance was as high ashat of quartz, especially in the longer wavelength,emonstrating that there was almost no light loss.n wavelengths less than 400 nm, however, the trans-ittance approaches zero. As shown later, the op-

ical energy bandgap of the film is 3.2 eV �equal to 390m�, so the absorption of light, greater than 400 nm

n wavelength, can be neglected. Hence the scatter-ng of light would be the main reason of light loss, ifny. This slight light loss in longer wavelengths isot due to absorptive defects as depicted later by the

ow value of the extinction coefficient and suggestshat there is little scattering-center concentration inhe film. It is believed that the above-mentionedmooth surface also contributes to the reduction ofhe surface scattering-center concentration.

The film refractive index and extinction coefficientre shown in Fig. 4. It was found that the refractivendex was 2.56 at 550 nm, higher than those preparedy other techniques as shown in Table 1. Usually, aigher refractive index implies a higher packing den-ity19 or metal-rich film. Here it is not metal-rich

ig. 3. Transmittance of the stoichiometric TiO2 thin film as aunction of wavelength.

ecause of the low extinction coefficient as shown inable 1, which was as low as 10�4, indicating the

deal stoichiometry in the oxide film20 and consistentith the XPS results. Thus TiO2 film exhibits highacking density as a result of bombardment of depos-ted species with high energy, similar to the case inon-assisted beam deposition.21 The good transmit-ance and better values in optical constants stronglyhows the potential applications for optical coatingseposited by the FCVA.Optical absorption spectra of as-grown TiO2 film is

hown in Fig. 5 as �h��1�2 versus photon energyelectron volts�. The absorption coefficient above thehreshold of fundamental absorption follows theh� � Eg�2 energy dependence corresponding to indi-ect transitions with absorption or emission of pho-ons. The optical bandgap �Eg� value was found toe 3.2 eV, which was close to that of the amorphousiO2 films reported by Takikawa et al.2

. Structure

he Raman spectrum of the as-grown TiO2 film ishown in Fig. 6�a�. No peak, characteristic of crys-alline TiO2, was observed. This again confirmedhat the as-grown film was amorphous. After it wasnnealed at 300 °C, the film exhibited obvious peakss shown in Fig. 6�b�. It is well known that anataseitania with two formula units per unit cell has sixaman-active vibrational modes �A1g � 2B1g � 3Eg�,hereas rutile titania with two units has fouraman-active vibrational modes �A1g � B1g � B2g �

g�. The frequency and assignment of the Ramanands of anatase and rutile titania are shown in Ta-le 2. Hence it was concluded that Raman-activeibrational modes were dominated by the anatasehase. The intense 520 cm�1 longitudinal-opticalLO�-phonon line together with the weak broad peakt 300 cm�1 originated from the silicon substrate.ll of the other LO-phonon lines corresponding toaman-allowed vibrational frequencies of anatase ti-

ania �see Table 2� were obviously observed except forhe E mode at 515 cm�1 suppressed by the strong

ig. 2. Atomic force microscope image for TiO2 thin film depositedt room temperature.

ig. 4. Refractive index and the extinction coefficient as a func-

g

20 February 2004 � Vol. 43, No. 6 � APPLIED OPTICS 1283

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O-phonon line of silicon at 520 cm�1. The strongntense peaks around 395 and 636 cm�1 were as-igned to anatase B1g and Eg LO-phonon modes, re-pectively. Similarly, the other weak peaksentered at 144 and 198 cm�1 were due to anataseB1g, Eg� and �B1g, A1g� LO-phonon modes, respec-ively. No Raman bands from the rutile phase werebserved in the Raman spectra.XRD patterns for the as-grown and annealed film

re shown in Fig. 7. No diffraction peak can be ob-

Fig. 5. Plot of �h��1�2 versus photon energy �electron volts�.

ig. 6. Raman spectrum of TiO2 thin film: �a� as-grown film andb� the film after RTA at 300 °C for 300 s in vacuum.

Table 1. Comparison of Refractive Index an

Technique Refractive Index

Evaporation15 2.38Plasma-enhanced chemical vapor

deposition16

2.51

1.92.38

dc reactive magnetron sputtering17 2.242.46

rf magnetron sputtering18 2.38FCVA 2.56

284 APPLIED OPTICS � Vol. 43, No. 6 � 20 February 2004

erved for the as-grown film. However, some peaksppeared for the annealed film. The strong peak at � 25.4° and other weak peaks were attributed tohe �101� plane and the �200�, �004�, and �211� planesf the anatase TiO2. No peak associated with theutile phase was observed. These results are in goodgreement with the above Raman results.It is interesting to note that the as-grown TiO2 film

repared by use of a similar technique by Bendavid etl.24 is anatase crystalline and not amorphous as inur case. Such a discrepancy could be ascribed to a

ig. 7. XRD patterns of TiO2 thin film: �a� as-grown film and �b�he film after RTA at 300 °C for 300 s in vacuum.

tinction Coefficient by Different Techniques

Extinction Coefficient Remark

�4 � 10�3 Without ion-beam source�4 � 10�3 Using ion-beam source

— Deposited at room temperature— Deposited at 300 °C

�1 � 10�3 Deposited at room temperature�9 � 10�3 Deposited at 400 °C

�10�3 rf power was 1.9 W cm�2

�10�4 Current research

Table 2. Frequency and Assignment of the Raman Bands of Anataseand Rutile Titaniaa

Phase Frequency �cm�1� Assignment

Anatase 144 B1g

147 Eg

198 B1g, A1g

398 B1g

515 Eg

640 Eg

Rutile 143 B1g

448 Eg

612 A1g

827 B2g

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rocess pressure different from ours. In our case,he pressure �10�4� is approximately 1 order of mag-itude lower. Therefore the collisions between con-ensed species and gas are considerably reduced.orrespondingly, the deposited particles possessigher kinetic energy, and the resulting film is amor-hous. This is again verified by the fact that the filmeposited is amorphous when the substrate is biasedo �50 V.24

. Conclusion

morphous titanium dioxide thin films of high qual-ty and ideal stoichiometry have been deposited by anff-plane FCVA technique at room temperature.PS spectra showed that Ti existed in the film in the

orm of Ti4�, and the Ti-to-O atomic ratio of the filmas nearly 1:2. The film has high transmittance,nd its optical bandgap was approximately 3.2 eV.he high refractive index �2.56 at 550 nm� and lowxtinction coefficient ��10�4� suggested the film pos-essed a high packing density and a low scattering-enter concentration. These results indicate thatCVA is a promising technique in the preparation ofptical coatings. The film annealed rapidly at00 °C was of the anatase phase, as revealed by Ra-an and XRD analysis.

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