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Atomic Layer Deposition of Thin Films Using O2 as Oxygen Source Mikael Schuisky,†,| Jaan Aarik,‡ Kaupo Kukli,*,§ Aleks Aidla,‡ and Anders Hårsta†,⊥ Uppsala University, Materials Chemistry, The Ångstro¨ m Laboratory, P.O. Box 538, SE-75121 Uppsala, Sweden, and University of Tartu, Institute of Materials Science and Institute of Experimental Physics and Technology, Ta¨ he 4, 51010, Tartu, Estonia Received February 2, 2001. In Final Form: June 1, 2001 Atomic layer deposition of TiO2 films was realized by using alternate pulses of TiI4 and O2. The film growth mechanism was studied by quartz crystal microbalance in the temperature range 200-350 °C. The adsorption of TiI4 proceeded via partial decomposition of TiI4, which resulted in an enhanced reactivity by the formation of a TiIx surface layer with x < 3. The reactivity of O2 toward this layer was sufficient to form TiO2 at an O2 pulse duration of 2 s when the substrate temperature was not lower than 235 °C. TiO2 films were also grown on Si(100) substrates at deposition temperatures between 230 and 460 °C. No residual iodine could be detected in the films grown at temperatures higher than 230 °C. Phase-pure anatase was formed in the whole temperature range except at the highest temperature where rutile was also obtained.
Introduction Properties of materials of both scientific and technological importance, such as TiO2, can depend on their synthesis route. The performance of TiO2 based devices depends on the purity of the material and the density of the crystallographic phase formed, which, in turn, depend on the precursors used in the synthesis. TiO2 has been widely used as a catalyst,1-3 but it can also be applied as a dielectric in metal oxide semiconductor devices.4,5 Commonly, the TiO2 films used in electronics are grown by chemical vapor deposition (CVD) from titanium alkoxide precursors.4,6 To minimize the interface reactions, reduction of the deposition temperature is of importance, but this may increase the carbon contamination level. Halides can be used instead of alkoxides, but if H2O, H2O2, or an alcohol is used as the oxygen precursor, acids are released which are able to deteriorate the TiO2/substrate interface.6 Therefore, investigation of hydrogen-free deposition processes7,8 is of interest in order to seek novel ways for preparation of high-quality films and devices. Among various synthesis techniques, atomic layer deposition (ALD), also referred to as atomic layer epitaxy9 †
Uppsala University. University of Tartu, Institute of Materials Science. § University of Tartu, Institute of Experimental Physics and Technology. ⊥ E-mail:
[email protected]. | Present address: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0245. ‡
(1) Haukka, S.; Lakomaa, E.-L.; Jylha¨, O.; Vilhunen, J.; Hornytzkyj, S. Langmuir 1993, 9, 3497. Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (2) Lakomaa, E.-L.; Haukka, S.; Suntola, T. Appl. Surf. Sci. 1992, 60/61, 742. (3) Schrijnemakers, K.; Impens, N. R. E. N.; Vansant, E. F. Langmuir 1999, 15, 5807. (4) Bae, G.; Song, Y.; Jung, D.; Roh, Y. Appl. Phys. Lett. 2000, 77, 729. (5) Luan, H. F.; Mao, A. Y.; Lee, S. J.; Luo, T. Y.; Kwong, D. L. Mater. Res. Soc. Symp. Proc. 1999, 567, 481. (6) Fuyuki, T.; Matsunami, H. Jpn. J. Appl. Phys. 1986, 25, 1288. (7) Schuisky, M.; Hårsta, A. J. Phys. IV (France) 1999, 9, Pr8-381. (8) Hårsta, A. Chem. Vap. Deposition 1999, 5, 191. (9) Suntola, T. Mater. Sci. Rep. 1989, 4, 261.
or molecular layering,10 is a method dominantly controlled by subsequent self-limited chemisorption reactions on the substrate surface. In the ALD of TiO2, highly reactive metal and oxygen precursors, such as TiCl4 and H2O,1,2,11-14 Ti(OCH2CH3)4 and H2O,15,16 Ti(OCH(CH3)2)4 and H2O,17,18 or TiI4 and H2O-H2O219-21 are alternately introduced into the reaction zone in the form of chopped fluxes. Thus, in all these processes, one deposition cycle is a sequence of the titanium precursor pulse, first purge period, oxygen precursor pulse, and second purge period. The purging is applied to remove the surplus precursors and to avoid gas-phase reactions. In the case of thermal decomposition of the metal precursor, the self-limited adsorption mechanism becomes violated even if the decomposition itself is a surface-controlled process. For instance, in the alkoxide based processes, oxide films can grow by pyrolysis due to internal oxygen of the alkoxides.15-18 During the oxygen precursor pulse, the surface reactions are selflimited, lasting until the reactive ligands of the metal precursor adsorbed on the surface are totally consumed. The oxygen sources most frequently used in ALD of oxides are H2O,1-3,11-18 H2O-H2O2,19-21 and various alcohols.22,23 Such processes involve surface OH groups, (10) Aleskovskii, V. B, J. Appl. Chem. USSR 1975, 47, 2207. (11) Ritala, M.; Leskela¨, M.; Nyka¨nen, E.; Soininen, P.; Niinisto¨, L. Thin Solid Films 1993, 225, 288. (12) Aarik, J.; Aidla, A.; Uustare, T.; Sammelselg, V. J. Cryst. Growth 1995, 148, 268. (13) Aarik, J.; Aidla, A.; Sammelselg, V.; Siimon, H.; Uustare, T. J. Cryst. Growth 1996, 169, 496. (14) Cameron, M. A.; Gartland, I. P.; Smith, J. A.; Diaz, S. F.; George, S. M. Langmuir 2000, 16, 7435. (15) Ritala, M.; Leskela¨, M.; Rauhala. E. Chem. Mater. 1994, 6, 556. (16) Aarik, J.; Aidla, A.; Sammelselg, V.; Uustare. T.; Ritala, M.; Leskela¨, M. Thin Solid Films 2000, 370, 163. (17) Ritala, M.; Leskela¨, M.; Niinisto¨, L.; Haussalo, P. Chem. Mater. 1993, 5. 1174. (18) Aarik, J.; Aidla, A.; Uustare, T.; Ritala, M.; Leskela¨, M. Appl. Surf. Sci. 2000, 161, 385. (19) Kukli, K.; Ritala, M.; Schuisky, M.; Leskela¨, M.; Sajavaara, T.; Keinonen, J.; Uustare, T.; Hårsta, A. Chem. Vap. Deposition 2000, 6, 303. (20) Schuisky, M., Hårsta, A.; Aidla, A.; Kukli, K.; Kiisler, A.-A.; Aarik, J. J. Electrochem. Soc. 2000, 147, 3319. (21) Kukli, K.; Aidla, A.; Aarik, J.; Schuisky, M.; Hårsta, A.; Ritala, M.; Leskela¨, M. Langmuir 2000, 16, 8122.
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formed as a result of surface reactions during the H2O, H2O2, or alcohol pulse.1,3,11,12,14,24,25 These OH groups are presumably active adsorption sites and assist in the subsequent adsorption of the metal precursor. Their density has been shown to affect the nucleation at the film/substrate interface.1 In principle, the ALD of thin films should be possible without participation of hydrogencontaining surface groups. In this case, the metal precursors can react with oxygen bridges and be adsorbed dissociatively on the surface of the growing oxide.1,2,11 The ALD processes without intentional hydroxylation of the oxide surface have mostly been based on organic precursors of the corresponding metals. For instance, ozone (O3) has been used to grow MgO,26 Ga2O3,27 Y2O3,28 and CeO229 from diketonates. In some cases, air pulses have been used in order to remove ligands while preparing metallic Ni/Al2O3 catalysts.30 Although exploitation of pure oxygen (O2) has been rather scarce, O2 has been applied as a supplementary oxidizer at 500 °C in the alkoxide-based ALD of Al2O3.23 However, the temperatures applied in the latter study were sufficiently high for pyrolysis of the alkoxide (Al(OCH(CH3)2)3) used in the process, actually enabling the film growth without any supplementary oxidizer. Recently, Chan et al.31 reported sequential dosing of Ta(OCH2CH3)5 and O2 on Pt substrates at 100 °C, where the formation of Ta2O5 was monitored in situ by Raman spectroscopy. In this case, the substrate surface had to be exposed to O2 during long time periods, reaching 1-2 min. In the halide-based ALD of oxides, supply of oxygen in addition to H2O might enhance the deposition rate. For instance, the TiO2 growth rate was increased in the TiI4-H2O2 ALD process19 in comparison to that in the TiI4-H2O process. This may, at least partially, be caused by the influence of oxygen, possibly released from the H2O2. Nevertheless, no reports on the successful application of pure O2 in fast surface reactions have been available to date. For this reason, the goal of the present study is to clarify whether it is possible to use oxygen in lowtemperature halide-based ALD of metal oxides and how the choice of oxygen precursor influences the phase composition and purity of the TiO2 thin films. Experimental Section The films were grown in a hot-wall horizontal flow-type ALD reactor.12 TiI4 (Strem Chemicals Inc.) of 99.99% purity was evaporated from a silica boat kept at 115 °C in the flow of the carrier gas. O2 (AGA, 99.999%) was led into the reaction zone through needle and solenoid valves. The partial pressure of O2 was set at about 30 Pa in the reactor. This pressure was about 4 times higher than the H2O-H2O2 pressure used earlier for growing TiO2 films from TiI4.19 N2 (AGA, 99.999%) was used as the carrier and purging gas. The total pressure in the ALD reactor was 250 Pa, approximately. (22) Tiitta, M.; Nyka¨nen, E.; Soininen, P.; Niinisto¨, L.; Leskela¨, M.; Lappalainen, R. Mater. Res. Bull. 1998, 33, 1315. (23) Hiltunen, L.; Kattelus, H.; Leskela¨, M.; Ma¨kela¨, M.; Niinisto¨, L.; Nyka¨nen, E.; Soininen, P.; Tiitta, M. Mater. Chem. Phys. 1991, 28, 379. (24) Kol’tsov, S. I. J. Appl. Chem. USSR 1969, 42, 975. (25) Tolmachev, V. A.; Okatov, M. A. Sov. J. Opt. Technol. 1984, 51, 104. (26) Putkonen, M.; Johansson, L.-S.; Rauhala, E.; Niinisto¨, L. J. Mater. Chem. 1999, 9, 2449. (27) Nieminen, M.; Niinisto¨, L.; Rauhala, E. J. Mater. Chem. 1996, 6, 27. (28) Mo¨lsa, H.; Niinisto¨, L.; Utriainen, M. Adv. Mater. Opt. Electr. 1994, 4, 389. (29) Mo¨lsa, H.; Niinisto¨, L. Mater. Res. Symp. Proc. 1994, 335, 341. (30) Lindblad, M.; Lindfors, L. P.; Suntola, T. Catal. Lett. 1994, 27, 323. (31) Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. J. Am. Ceram. Soc. 1999, 121, 9219.
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Figure 1. QCM signals as functions of time. Long single deposition cycles were studied at different temperatures indicated by labels. Labels denote also the cycle time parameters (in seconds) as sequence of TiI4 exposure time, the first purge period, and O2 exposure time. To study the deposition kinetics, a quartz crystal microbalance was applied. An AT-cut quartz crystal oscillator with 30-MHz working frequency was used as sensor to record the changes in the thin film mass. To avoid reactions between TiI4 and the Ag electrodes on the quartz crystal, a TiO2 buffer layer of 5-10 nm thickness was deposited on the sensor prior to the studies. The buffer layer was grown at a sensor temperature of 250 °C using Ti(OC2H5)4 and H2O as the precursors. As the mass sensor signal was recorded with the sampling period 0.5 s, the QCM method enabled us to record the kinetics of the mass behavior within a single ALD cycle. In addition, the optimum values of the TiI4 pulse duration, t1, first purge length, t2, O2 pulse duration, t3, and second purge time, t4, were determined from QCM measurements. The films for post-growth studies were grown onto Si(100) substrates at temperatures, TG, ranging from 235 to 457 °C. Prior to the growth, the substrates were etched in HF to remove the native oxide. The film thickness and refractive indices were evaluated from the optical transmission spectra32 measured using an Hitachi U2000 spectrophotometer. The phase content of the films grown was examined by a Philips MPD 1880 X-ray diffractometer using Cu KR radiation. The residual iodine content of the films was analyzed by X-ray fluorescence spectroscopy (XRFS) using a Spectro X-lab 2000 spectrometer. Additional elemental analyses of the films were performed by X-ray photoelectron spectroscopy (XPS) using a Perkin-Elmer PHI5500 instrument, equipped with a hemispherical energy analyzer and a monochromatic X-ray source (Al KR, 1486.6-eV radiation). The binding energy scale was calibrated by setting the C 1s peak of carbon at 284.8 eV.
Results Growth Kinetics. Figure 1 depicts typical behavior of the film mass within a single ALD cycle. During the TiI4 exposure, the precursor adsorption was detected as an increase in the QCM oscillation period that was proportional to the mass increment. The QCM signal increased abruptly at the beginning of the TiI4 pulse and continued to increase with much lower but nearly constant rates at all temperatures studied (Figure 1). The slope of the linear part of the QCM signal versus time curve (Figure 1) increased with the growth temperature, TG. After switching off the TiI4 pulse, a continuous mass decrease, due to desorption of some surface species, was detected. The rate of this process also increased with TG (Figure 1). The behavior of the QCM signal observed during the TiI4 pulse and the following purge pulse was similar to that described in the case of TiI4 adsorption in the TiI4-H2O-H2O2 ALD process.21 Thus, thermal decom(32) Ylilammi. M.; Ranta-aho, T. Thin Solid Films 1993, 232, 56.
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Figure 2. QCM signals recorded during four subsequent deposition cycles at different temperatures and O2 exposure times. For label key, see Figure 1.
position of surface TiIx species is the likely reason for the linear increase of film mass during the TiI4 pulse as well as for the decrease of mass during the first purge time. Indeed, the decomposition of these species and desorption of iodine creates free adsorption sites and enables additional adsorption of the metal precursor when the surface is exposed to TiI4. During the purge time, by contrast, the same processes, i.e., the decomposition of surface species and release of iodine, result in the mass decrease. The O2 pulse caused an additional mass decrease (Figure 1). This decrease was relatively slow, especially when TG ) 200 °C, but it was still faster than the decrease observed during the preceding purge pulse. The explanation for the mass decrease appearing during the O2 pulse is that the oxygen atoms, which are much lighter than the iodine atoms, replace the latter as surface species. It can be seen in Figure 1 that the rate of the reaction between O2 and the iodide-terminated surface clearly increases with the growth temperature, since at higher temperatures, the mass decreased faster and shorter time was needed to stabilize the QCM signal at a new level. Nevertheless, the stabilization of the QCM signal during the O2 pulse was significantly slower compared with the corresponding process using H2O-H2O2.21 Because of slow exchange reactions, short O2 pulses obviously did not remove all iodine from the surface, especially at low temperatures. As a result, the reactivity of the surface toward TiI4 was not completely recovered and the amount of TiI4 adsorbed in an ALD cycle decreased with increasing cycle number. This kind of effect is shown in Figure 2. As can be seen, when TG ) 200 °C and t3 ) 2 s, the mass increment ∆m1 caused by the TiI4 pulse and especially the mass increment ∆m0 describing the effect of complete the ALD cycle are remarkably higher in the first cycle than in the following cycles. The increase of t3 to 5 s resulted in almost constant ∆m1 and ∆m0 values in four subsequent ALD cycles. When TG ) 300 °C, however, the O2 pulse duration of 2 s was sufficient to obtain the same mass increment in all cycles applied. An explanation for the relatively high ∆m0 value obtained in the first cycle when TG ) 200 °C and t3 ) 2 s (Figure 2) is that
Figure 3. Dependence of (a) ∆m0 and ∆m1 and (b) ∆m0/∆m1 on growth temperature at different cycle times.
iodine was not completely removed from the surface. Hence, one could expect a considerable iodine contamination in the films grown under such conditions. To characterize the effect of process parameters on the growth rate and reaction mechanism more thoroughly, the average values of ∆m0 and ∆m1 were measured at different exposure times and substrate temperatures (Figure 3). One can see in Figure 3a that the TiI4 pulse duration has stronger effect on ∆m0 than the O2 pulse duration has. At the same time the influence of the TiI4 pulse duration on ∆m1 is relatively weak and the ∆m0/ ∆m1 ratio thus increases with the TiI4 pulse duration (Figure 3b). This indicates that the adsorbate layer formed during long TiI4 pulses contain larger relative amounts of titanium than the adsorbate layer formed at short TiI4 pulses. Figure 3 also shows that although the ∆m0/∆m1 ratio did not change considerably with the increase of TG from 225 to 350 °C, in particular at t1 ) 2 s, the values of ∆m1 and ∆m0 increased by a factor of 1.5, approximately. The values of ∆m1 and ∆m0 for three different growth temperatures are shown as a function of the O2 pulse time in Figure 4a. The ∆m1 values increased with increasing O2 pulse times. The increment was larger for pulses up to 2 s, while for pulses longer than 2 s the relative increment of the ∆m1 value decreased. The relative change of the ∆m1 value was smallest for the highest deposition temperature. The ∆m0 values showed a minor increment with increasing O2 pulse time for all three deposition temperatures. In Figure 4b the ∆m0/∆m1 ratio is plotted versus the O2 pulse duration and a strong influence can be seen for short pulse times. For O2 pulse times longer than 2 s the ∆m0/∆m1 ratio values seem to stabilize. The stabilization of the ∆m0/∆m1 ratio value occurs at shorter O2 pulse times for the higher deposition temperatures. From Figure 4 it can be seen that the ∆m0/∆m1 ratio depends considerably more on the O2 pulse duration at TG ) 225 °C. As discussed above, the reasons for the decrease
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Figure 6. X-ray diffraction patterns of TiO2 films deposited onto Si(100) substrates at different temperatures.
Figure 4. Dependence of (a) ∆m0 and ∆m1 and (b) ∆m0/∆m1 on O2 pulse duration at different growth temperatures.
Figure 5. Dependence of growth rate and refractive index of TiO2 films on growth temperature. The films were grown on Si(100) substrates. The data were obtained from measurement made on different points on the substrate surface.
of ∆m1 and increase of the ∆m0/∆m1 ratio could be related to the incomplete removal of iodine from the thin film surface during the O2 pulse. The variation of the second purge time (t4) from 2 to 5 s did not influence the values of ∆m0, ∆m1, and ∆m0/∆m1. Therefore the surface layer formed during the O2 pulse was stable in the N2 ambient used in the reactor. Since ∆m0 was independent of the second purge length, the purge time of 2 s is sufficiently long to remove O2 gas from the reactor and to avoid the CVD-type growth, which would appear if the precursor pulses were overlapping. Film Properties. In the further studies, the precursor pulses were set at 2 s in order to obtain sufficient growth rates and minimize the nonsaturated TiI4 adsorption. The purge periods between the precursor pulses were also kept at 2 s. Figure 5 represents the growth rate and refractive index of the films deposited on silicon substrates. It can
be seen that the growth rate monotonically increases from 0.06 to 0.10 nm/cycle with the increase of temperature from 235 to 380 °C. A significantly higher growth rate, 0.20 nm/cycle, was measured for the films grown at 457 °C. Refractive index values of 2.2-2.5 were obtained in an intermediate temperature range of 275-380 °C, while at both lower and higher growth temperatures the value was much lower, which indicates a lower density of these films. No residual iodine could be detected by XRFS in any of the deposited TiO2 films. The detection limit for iodine by XRFS can be estimated to be considerably less than 1%. The chemical composition of three samples deposited at 230, 255, and 380 °C was additionally investigated by XPS. Prior to the XPS analysis the film surface was sputtered for 10 s using Ar+ ions with 3-keV beam energy and a sputtering current of 4 µA to remove any adsorbed contaminating surface species. No iodine could be detected in the films deposited at 255 and 380 °C by XPS, but a small amount of iodine (0.24%) was observed for the film deposited at 230 °C. Figure 6 demonstrates X-ray diffraction patterns taken from the films grown at different temperatures, verifying the growth of TiO2. The only phase observed in the films grown between 235 and 380 °C was anatase, which grew with a pronounced (101) orientation. For the film grown at the highest deposition temperature, 457 °C, a strong additional peak was observed belonging to the 110 reflection of rutile. Discussion The results presented in the previous section demonstrate that O2 can be successfully used as an oxygen precursor in the TiI4-based ALD growth of TiO2. Moreover, the growth rates obtained in the TiI4-O2 process were comparable to those measured earlier for the TiI4-H2OH2O2 process at comparable temperatures.20,21 This fact indicates that the reaction mechanisms could also be very similar for these two processes. As no hydroxyl groups can be formed on the oxide surface that is exposed to pure oxygen and pure nitrogen, one should consider the adsorption of TiI4 on a completely dehydrated surface. Decomposition of TiI4 on the solid surface that evidently takes place at the growth temperatures used in this work definitely contributes to more efficient bonding of TiIx species to the film surface. Therefore the most probable reaction scheme for TiI4 adsorption could be proposed as
TiI4(g) f TiIx(ads) + (4 - x)/2I2(g)
(1)
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where (g) and (ads) denote the gas phase and surface adsorbed species, respectively. Provided that the decomposition of the surface species is negligible during a relatively short purge period between the TiI4 and O2 pulses, the reaction of O2 with the surface species can be written as
TiIx(ads) + O2(g) f TiO2(s) + x/2I2(g)
(2)
where (s) denotes the solid phase formed. If the desorption of TiIx species during the purge and O2 pulses is insignificant and stoichiometric TiO2 without considerable iodine contamination is formed in the complete ALD cycle, the ∆m0/∆m1 ratio obtained from the QCM measurements (Figures 1 and 2) is equal to the molar mass ratio of TiO2 and TiIx (eq 2). Figure 1 demonstrates that after switching off the TiI4 pulse the film mass decreases, possibly indicating a slow desorption of TiIx, I2, or I species. The mass decrease was less than 10% within 4-7 s, which was the total duration of the purge and O2 pulses used in the experiments that were employed for determination of the ∆m0/∆m1 ratio. This means that the temperatureinduced desorption of surface species would not influence the ∆m0/∆m1 ratio to a large extent. Therefore using the experimental ∆m0/∆m1 ratio, one can estimate the I/Ti ratio for the surface intermediate species formed under the conditions which result in the growth of stoichiometric TiO2, i.e., when TG g 235 °C and t3 g 2 s. Estimations made on the basis of data depicted in Figure 3b revealed that the I/Ti ratio was about 2.6 in the surface species formed during a 2 s long TiI4 pulse in the growth temperature range of 250-350 °C. At a TiI4 pulse duration of 5 s, however, a I/Ti ratio of 1.8 was obtained. These data confirm that TiI4 considerably decomposes, when adsorbed on the surface of TiO2, and that the decomposition continues on the surface, making more adsorption sites available thereby allowing a higher surface density of titanium atoms. The ∆m0/∆m1 ratios and the I/Ti ratios in the surface intermediate species are very close to the corresponding values determined earlier for the TiI4-H2O-H2O2 process at similar growth temperatures. Thus the relative mass changes do not significantly depend on whether the surface was previously exposed to a H2O-H2O2 mixture or to pure oxygen. Of course, even when using O2 of 99.999% purity as the oxygen precursor and N2 of the same purity as the carrier gas, the partial pressure of H2O could reach up to 10-3 Pa in the reactor because of water vapor that might contaminate the N2 and O2 gases. This residual H2O could, in principle, form hydroxyl groups on the oxide surface during the O2 pulse and the following purge. Nonetheless, as can be seen in Figure 1, there is no implication of any abrupt mass decrease due to reactions between water and
the TiIx surface during the first purge time. Therefore, O2 still had a dominant role in the growth process, forming TiO2 from the adsorbed TiIx species. A distinct difference between the TiI4-O2 and the TiI4H2O-H2O2 processes is that the anatase-rutile transformation appeared at much lower temperatures (300 °C) in the H2O2-assisted process.20,21 Furthermore, the refractive index of rutile films grown from TiI4 and H2O-H2O2 was as high as 2.75.21 By contrast, the refractive index of the films grown from TiI4 and O2 was lower than in the H2O2-assisted process and decreased dramatically with the appearance of rutile in the films (Figures 5 and 6). This is an unexpected result because the density and refractive index of pure rutile are higher than the corresponding parameters of anatase. Some possible explanations for the decrease in refractive index with increasing rutile content are that the films simultaneously become more porous or that a reaction with the Si substrate has taken place. Diffusion of Si into titanium oxide may also assist in the creation of solid solution intermediate layers, reducing the mean refractive index of the oxide layer. The small amount of iodine (0.24%) detected by XPS in the film grown at 230 °C was expected, since the initial QCM studies had shown that longer O2 pulses than 2 s were needed for complete removal of the iodine at low temperatures. However, no residual iodine could be detected in the films grown at 255 °C or higher temperatures. Conclusions We have demonstrated that O2 can be used as an oxygen precursor for growing TiO2 in a halide-based ALD process. TiI4 is a suitable metal precursor of sufficient reactivity that can be used in such a process. It appears that O2 is able to oxidize the adsorbed TiIx species in the temperature range of 230-460 °C. Iodine-free TiO2 films were formed at temperatures as low as 255 °C. However, at still lower deposition temperatures, longer reaction times are required, because of the slower exchange rates of the reactions between surface TiIx species and O2. Anatase was the dominating phase in the films grown on silicon substrates throughout the temperature range studied, except at the highest temperatures close to 460 °C, where both rutile and anatase were formed. Acknowledgment. We are indebted to Ms. Alma-Asta Kiisler for technical assistance during experiments and to Dr. Mikko Ritala and Prof. Markku Leskela¨ at the University of Helsinki for access to optical and XRD measurement techniques. The study has been partially supported by the Estonian Science Foundation (Grant 4205) and the ESF ALENET Grant (2000). LA010174+
