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Titanium isopropoxide as a precursor for atomic layer deposition: characterization of titanium dioxide growth process
Applied Surface Science 161 Ž2000. 385–395 www.elsevier.nlrlocaterapsusc
Titanium isopropoxide as a precursor for atomic layer deposition: characterization of titanium dioxide growth process Jaan Aarik a,) , Aleks Aidla a , Teet Uustare a , Mikko Ritala b, Markku Leskela¨ b b
a ¨ Institute of Materials Science, UniÕersity of Tartu, 18 Ulikooli Steet, 50090 Tartu, Estonia Department of Chemistry, UniÕersity of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN 00014 UniÕersity of Helsinki, Helsinki, Finland
Received 24 November 1999; accepted 17 February 2000
Abstract Atomic layer deposition ŽALD. of titanium oxide from titanium isopropoxide ŽTiŽOCHŽCH 3 . 2 .4 . and water as well as from TiŽOCHŽCH 3 . 2 .4 and hydrogen peroxide ŽH 2 O 2 . was studied. According to data of real-time quartz crystal microbalance ŽQCM. measurements, adsorption of TiŽOCHŽCH 3 . 2 .4 was a self-limited process at substrate temperatures 100–2508C. At 200–2508C, the growth rate was independent of whether water or H 2 O 2 was used as the oxygen precursor. Insufficient reactivity of water vapor hindered the film growth at temperatures 100–1508C. Incomplete removal of the precursor ligands from solid surface by water pulse was revealed as the main reason for limited deposition rate. The growth rate increased significantly and reached 0.12 nm per cycle at 1008C when water was replaced with H 2 O 2 . The carbon contamination did not exceed 1 at.% and the refractive index was 2.3 in the films grown at temperatures as low as 1008C. q 2000 Elsevier Science B.V. All rights reserved. PACS: 68.55; 81.15 Keywords: Atomic layer deposition; Thin films; Titanium oxide; Adsorption kinetics
1. Introduction Atomic layer deposition ŽALD. also known as atomic layer epitaxy w1x, molecular layering w2x and molecular layer epitaxy w3x is an appropriate method for growing uniform thin and ultra-thin films, especially in the cases when precise thickness control, high reproducibility, thickness uniformity and excellent conformity w4,5x are required. In the ALD pro-
) Corresponding author. Tel.: q372-7-375877; fax: q372-7375540. E-mail address: [email protected] ŽJ. Aarik..
cess, the film is formed via a series of subsequent self-saturating chemisorption reactions. Therefore, provided that the deposition conditions are properly chosen, the thickness increase per reaction step should be independent of modest variation of the precursor fluxes and exposure times. As a result, the film thickness should be determined by the number of reaction cycles, mainly, which makes the process control precise and convenient. However, for some precursor systems, the real process seems to be much more complex. For instance, rather contradictory results have been obtained for the ALD growth of titanium dioxide ŽTiO 2 . from titanium isopropoxide ŽTiŽOCHŽCH 3 . 2 .4 . and
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 2 7 4 - 9
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water. The growth rate as high as 0.15 nm per ALD cycle has been measured by Doring et al. w6x at the ¨ substrate temperature TS s 1508C. By contrast, at the same substrate temperature, the growth rate did not exceed 0.015 nm per cycle in the work of Ritala et al. w7x. Steric hindrance and kinetic limitation have been suggested as the main reasons for low growth rate w7x. In order to investigate the contribution of these effects, we performed a series of additional experiments. Besides methods for post-growth characterization of thin films, quartz crystal microbalance ŽQCM. method was used to study the deposition kinetics in real-time. The research was concentrated on revealing the mechanism of surface reactions and, in particular, the reaction step that limits the growth rate. Besides water, hydrogen peroxide ŽH 2 O 2 . was used as the oxygen precursor. Because of different reactivity of water and hydrogen peroxide, this enabled us to obtain more information about the role of kinetic effects. The study was stimulated by the circumstance that TiO 2 thin films have found a number of applications, for instance, in optical coatings w8x, chemical sensors w9x and solar cells w10x. Moreover, titanium oxide is a constituent of important multicomponent oxides like ŽBa,Sr.TiO 3 , PbTiO 3 and Bi 4Ti 3 O 12 . Although there are some other precursors like titanium tetrachloride w2,11,12x and titanium ethoxide w13x, which can be used for ALD growth of TiO 2 , titanium isopropoxide as an alternative to those is still worth studying. The work was additionally motivated by the fact that titanium isopropoxide had also proved to be a suitable precursor for growing SrTiO 3 and BaTiO 3 thin films w5x needed for new generation dynamic random access memories.
2. Experimental The TiO 2 thin films were grown in a hot-wall flow-type reactor w14x. N2 gas with 99.999% purity was used as a carrier leading titanium isopropoxide and the oxygen precursor ŽH 2 O or H 2 O 2 . vapors alternately to the substrate. The carrier gas pressure measured at the reactor outlet was kept at about 250 Pa. After each precursor pulse the reactor was purged with pure carrier gas in order to remove the gaseous reaction products and avoid overlapping the precur-
sor pulses. Therefore, an ALD cycle consisted of a TiŽOCHŽCH 3 . 2 .4 pulse, purge time, oxygen precursor pulse and another purge time. Titanium isopropoxide was volatilized at temperatures 26–758C. The vapor pressure was determined by the temperature of the TiŽOCHŽCH 3 . 2 .4 cell. Water and hydrogen peroxide were vaporized in containers, which were kept at 208C and 368C, respectively. The flow of oxygen precursor was controlled with a calibrated needle valve. In order to get concentrated H 2 O 2 , water was removed from the commercial 30% H 2 O 2 solution by vacuum distillation. A QCM working at the oscillation frequency of about 30 MHz was applied to investigate the deposition kinetics. The mass changes were measured with the time resolution of 0.5 s, which enabled us to record the behavior of film mass within a single ALD cycle. In order to increase the accuracy of measurements and reduce the contribution of transient effects that appeared after changing the growth conditions, the mass sensor signal was always recorded during four to five subsequent ALD cycles. In addition, the data corresponding to the first cycle were neglected when the average mass sensor response was calculated. This enabled us to diminish misleading effects, which appeared usually in the beginning of the growth process and were probably related to the changes in the surface layer properties. Before starting the measurements with a new mass sensor, a 5–10-nm buffer layer was first grown to reduce the influence of the sensor material on the deposition process The thin films for post-growth characterization were deposited on silica and Ž100.-oriented silicon substrates. The substrate temperatures used for growing the films ranged from 1008C to 3008C. The optimum precursor doses and purge times were determined from the results of QCM measurements. The pulse and purge times were chosen equal to 2 s. The temperature of the TiŽOCHŽCH 3 . 2 .4 source was set at 428C and the partial pressures of H 2 O and H 2 O 2 were kept at 20 and 10 Pa, respectively. The composition of the films grown on silicon substrates was measured with Auger electron spectrometry ŽAES.. Immediately before each AES measurement, the film surface was always cleaned by argon-ion bombardment. The reflection high-energy electron
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diffraction ŽRHEED. method was applied for structure studies. The thickness and refractive index of films grown on silica substrates were determined from optical transmission spectra w15x.
3. Results 3.1. Deposition kinetics Relatively long cycle times were used to investigate the kinetics of precursor adsorption during a single ALD cycle. The time dependence of the deposit mass recorded at substrate temperatures of 1008C, 2008C and 2758C is depicted in Fig. 1. At all substrate temperatures used, the film mass abruptly increased during the TiŽOCHŽCH 3 . 2 .4 pulse and decreased after switching on the water pulse. The mass increase was evidently due to TiŽOCHŽCH 3 . 2 .4 adsorption while the decrease was caused by desorption of reaction products formed from the surface
Fig. 1. Mass sensor signal as a function of time recorded during a single ALD cycle. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Water vapor pressure measured at reactor outlet is 19 Pa.
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intermediate species. At TS s 1008C and 2008C, the mass increase completely saturated during the TiŽOCHŽCH 3 . 2 .4 pulse ŽFig. 1.. Thus, the adsorption products formed on the surface disabled further adsorption of TiŽOCHŽCH 3 . 2 .4 . At TS s 2758C, however, no saturation could be achieved. Correspondingly, at the latter substrate temperature, the film mass somewhat decreased during the first purge time while at 1008C and 2008C, it did not. At the substrate temperature of 1008C, a small decrease appeared in the mass sensor signal immediately after switching off the water pulse ŽFig. 1.. As the amplitude of this mass change increased with the partial pressure of water, desorption of molecular water from the film surface is the plausible reason for this effect. Fig. 1 shows that the time constant describing the steep mass increase in the beginning of TiŽOCHŽCH 3 . 2 .4 pulse decreases with increasing substrate temperature. Consequently, the adsorption process is evidently controlled by a surface reaction, which proceeds faster at higher temperatures. Fig. 1 also demonstrates that at sufficiently high temperatures, an unsaturated process occurs during the TiŽOCHŽCH 3 . 2 .4 pulse. It was revealed that the rate of the latter process rapidly increased with the substrate temperature. Therefore, the process was most probably due to thermal decomposition of TiŽOCHŽCH 3 . 2 .4 w16,17x. Indeed, Wu et al. w16x have observed a steep increase in the rate of thermal decomposition of TiŽOCHŽCH 3 . 2 .4 at 2758C approximately. This temperature very well agrees with the lowest substrate temperature, at which the unsaturated adsorption appears in our experiments. Thermal decomposition andror desorption of titanium-containing surface intermediate species both preferably appearing at elevated temperatures are the most possible reasons for the mass decrease observed at 2758C ŽFig. 1. and higher substrate temperatures during the first purge time. However, as will be shown below, thermal decomposition resulting in the growth of TiO 2 and releasing of some isopropoxide ligands or constituents of them is in better agreement with the experimental data. There were no qualitative changes in the shape of the mass sensor signal when hydrogen peroxide, instead of water, was used as the oxygen precursor. The main quantitative difference was that the values
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of D m 0 characterizing the growth rate of the ALD process ŽFig. 1. significantly increased at the substrate temperatures of 100–1508C after this replacement ŽFig. 2.. Independently of the oxygen precursor used, D m 0 significantly increased with the increase of the substrate temperature from 250–2758C to 3008C. Also, the dependence of D m 0 on the TiŽOCHŽCH 3 . 2 .4 pulse width became stronger. Increasing influence of the pulse time observed at temperatures 275–3008C is in agreement with the data shown in Fig. 1 and is obviously due to the
Fig. 2. D m 0 as a function of substrate temperature in Ža. TiŽOCHŽCH 3 . 2 .4 rH 2 O and Žb. TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 processes. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of water and hydrogen peroxide is 19 and 13 Pa, respectively.
precursor decomposition. At temperatures 100– 2008C, however, the pulse time of 2 s is a bit too short to achieve complete saturation of adsorption Žsee also Fig. 1.. In this context, it is of importance to note that neither the adsorption time constant nor the rate of unsaturated growth, observed at 275– 3008C, depended on the titanium isopropoxide pressure. We kept the TiŽOCHŽCH 3 . 2 .4 pulse time at 2 s and varied the source temperature from 268C to 468C at the substrate temperatures 1008C and 2008C and from 428C to 758C at TS s 3008C. However, no effect of the source temperature on D m 0 was recorded although the TiŽOCHŽCH 3 . 2 .4 pressure had to rise more than five times at the lower substrate temperatures and about nine times w18x at TS s 3008C. Therefore, in the present case, the insufficient exposure time cannot be compensated with higher precursor pressure. Similarly, the decomposition process appearing at higher substrate temperatures is independent of the precursor concentration in the gas phase. Consequently, the adsorption time constant as well as the precursor decomposition rate are determined by the rate of surface reactions rather than by the precursor supply from the gas phase. Another remarkable result is that even at the substrate temperatures where a significant mass decrease was recorded during the first purge time ŽFig. 1., D m 0 does not depend on the purge length used after the TiŽOCHŽCH 3 . 2 .4 pulse ŽFig. 2.. Therefore, the mass decrease cannot be explained by desorption of the titanium-containing surface intermediate species. Instead, the decomposition of those species and release of precursor ligands from the surface is a more reasonable explanation for this effect. In the process where water was used as an oxygen source, the growth rate was almost independent of substrate temperature when the latter was varied from 1508C to 2758C ŽFig. 2Ža... However, D m 0 decreased noticeably with the decrease of the substrate temperature down to 1008C ŽFig. 2Ža... The dependence of D m 0 on the substrate temperature changed markedly its shape when water was replaced with hydrogen peroxide. The growth rate increased by the factor of 2, approximately, when the substrate temperature decreased from 2508C to 1008C ŽFig. 2Žb... In addition, the influence of the TiŽOCHŽCH 3 . 2 .4 pulse length became much weaker. The last fact indicates that the surface reactions
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determining the TiŽOCHŽCH 3 . 2 .4 adsorption proceed faster on the surface treated with H 2 O 2 than on the surface treated with H 2 O. For this reason, the TiŽOCHŽCH 3 . 2 .4 pulse time of 2 s is sufficient to cover the surface with adsorption products and no considerable increase in the film mass can be obtained with the further increase of the pulse time. These data show that the reactivity of the oxygen precursor is a critical parameter determining also the TiŽOCHŽCH 3 . 2 .4 adsorption kinetics in the next ALD cycle. Additional evidence of the low reactivity of water is presented in Fig. 3. As can be seen, D m 0 as a function of water pressure ŽFig. 3Ža.. and pulse time ŽFig. 3Žb.. does not saturate at 1008C. By contrast, D m 0 satisfactorily saturates in the process where hydrogen peroxide is used as the oxygen source. At higher substrate temperatures, especially at 3008C, D m 0 as a function of oxygen precursor dose saturates in the TiŽOCHŽCH 3 . 2 .4rH 2 O process, as well ŽFig. 3.. It is possible that at 100–1508C, the further increase in the dose might compensate the low reactivity of water but in many applications, this approach is not desirable. First, prolonged pulse time results in a longer thin film deposition process. Secondly, at higher water pressures, an overlap of the precursor pulses may appear and cause an increase in the thickness gradients in thin films. It is worth noting, however, that at the doses used in this work, the overlap of precursor pulses was still insignificant when the purge times were 2 s or longer. As shown in Fig. 2, the purge time following the TiŽOCHŽCH 3 . 2 .4 pulse has no effect on the growth rate when varied from 2 to 10 s. The purge time following the water pulse had some influence. Nevertheless, at the H 2 O pressure of 20 Pa, the growth rate increased by 10%, only, when this purge time was reduced from 10 to 2 s. Further decrease of the purge time from 2 to 0.2 s, by contrast, caused a growth rate increase of about 20%, already. Thus, at short purge times, the precursor pulses evidently overlap with each other. To characterize surface reactions, the effect of process parameters on the mass change D m 2 ŽFig. 1. was also studied. Similarly to D m 0 , the value of D m 2 , which included the effects of the oxygen precursor pulse and both purge times, did not depend on the purge time following the TiŽOCHŽCH 3 . 2 .4
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Fig. 3. D m 0 as a function of Ža. oxygen precursor pressure in the reactor and Žb. oxygen precursor pulse time. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. TiŽOCHŽCH 3 . 2 .4 pulse time and following purge time are equal to 2 s. Purge time used after oxygen precursor pulse is 5 s.
pulse ŽFig. 4.. No effect was observed even at high substrate temperatures where a considerable mass decrease appeared during this purge time Župper curve in Fig. 1.. Therefore, the latter mass decrease was connected with a process that resulted in the deposition of a similar solid substance as the reaction between the oxygen precursor and surface intermediate species did. Correspondingly, as mentioned above, desorption of titanium-containing surface in-
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Fig. 4. D m 2 as a function of substrate temperature in Ža. TiŽOCHŽCH 3 . 2 .4 rH 2 O and Žb. TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 processes. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of water and hydrogen peroxide is 19 and 13 Pa, respectively.
termediate species could not cause this mass decrease. Differently from the growth rate, D m 2 did not increase with the TiŽOCHŽCH 3 . 2 .4 pulse time varied from 2 to 5 s ŽFigs. 2 and 4.. Consequently, the surface concentration of removable precursor ligands saturates at shorter TiŽOCHŽCH 3 . 2 .4 pulses than that of titanium-containing surface intermediate species. It is remarkable that D m 2 did not depend on the pulse time even at the substrate temperatures 275– 3008C where the unsaturated mass increase was observed during the TiŽOCHŽCH 3 . 2 .4 pulse. Thus, after saturation of the surface with precursor ligands, additional adsorption of TiŽOCHŽCH 3 . 2 .4 is evidently possible provided that some of these ligands first desorb from the surface. At higher temperatures, this kind of desorption can be caused by thermal
decomposition of surface intermediate species. At lower temperatures, however, different types of hydroxyl groups w19x that probably reside on the surface exposed to H 2 O or H 2 O 2 may react with adsorbing TiŽOCHŽCH 3 . 2 .4 and in this way, also cause the ligand desorption. One can see in Fig. 4 that D m 2 depends on the substrate temperature as well as on the oxygen precursor chosen. Higher D m 2 values correspond to the TiŽOCHŽCH 3 .4rH 2 O 2 process ŽFig. 4.. It should be noted that D m 2 as a function of substrate temperature has a maximum at about 2008C ŽFig. 4Žb.. where the growth rate is close to its lowest value ŽFig. 2Žb... Another interesting result is that at substrate temperatures below 2008C, D m 2 decreases with decreasing temperature independently of the oxygen precursor used. The value of D m 0 , in turn, increases with decreasing temperature in the TiŽOCHŽCH 3 .4rH 2 O 2 process ŽFig. 2Žb.. while it decreases in the TiŽOCHŽCH 3 .4rH 2 O process ŽFig. 2Ža... It has been demonstrated Žsee e.g. Ref. w14x. that relative mass changes corresponding to different reaction steps ŽFig. 1. enable one to characterize the ligand exchange per surface intermediate species participating in formation of the thin film material. For instance, both D m 0 and D m 2 are proportional to the abundance of the surface intermediate species reacting with the oxygen precursor. Consequently, the D m 2rD m 0 ratio does not depend on the concentration of the reacting species any more but describes the average mass change in those. Fig. 5 shows that the D m 2rD m 0 ratio is somewhat higher at the TiŽOCHŽCH 3 . 2 .4 pulse time of 2 s than at the pulse time of 5 s. Although the difference is close to the experimental error, it occurs at all temperatures and in cases of both oxygen precursors used. Thus, the longer the TiŽOCHŽCH 3 . 2 .4 pulse, the smaller the amount of ligands that stay in the surface intermediate species and can be removed during the following oxygen precursor pulse. Fig. 5 also demonstrates that the D m 2rD m 0 ratio decreases with the increase of substrate temperature from 2508C to 3008C. Such decrease is evidently caused by thermal decomposition, which appears at these temperatures. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, the D m 2rD m 0 ratio decreases with the decrease of sub-
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H 2 O processes may be due to the fact that in the latter process, the D m 2rD m 0 ratio is rather low at 2008C ŽFig. 5.. On the other hand, the reactivity of H 2 O is evidently too low to complete the exchange reactions at 1008C ŽFig. 3Žb.., and create as high abundance of hydroxyl groups as H 2 O 2 does at this temperature. 3.2. Properties of thin films
Fig. 5. D m 2 rD m 0 ratio as a function of substrate temperature. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of H 2 O and H 2 O 2 is 19 and 13 Pa, respectively.
strate temperature from 2008C to 1008C ŽFig. 5.. In this temperature range, adsorption of TiŽOCHŽCH 3 . 2 .4 completely saturates. For this reason, the possible role of unsaturated processes, such as thermal decomposition, can be excluded. Instead, exchange reactions between adsorbing titanium isopropoxide and surface hydroxyl groups, preferably appearing at low temperatures w19x, should be considered as factors influencing the D m 2rD m 0 ratio. The data presented in Fig. 6 support the conclusion about the possible role of hydroxyl groups. This figure shows that the D m 2rD m 0 ratio decreases with increasing H 2 O 2 pulse time. Therefore, higher H 2 O 2 dose results in more significant ligand release during the following TiŽOCHŽCH 3 . 2 .4 pulse rather than during the current H 2 O 2 pulse. This kind of effect is possible, if the H 2 O 2 pulse length influences surface abundance of hydroxyl groups that, in turn, controls the ligand exchange in the following TiŽOCHŽCH 3 . 2 .4 adsorption step. It should be noted in this connection that at 100–2008C, the D m 2rD m 0 ratio does not depend remarkably on the substrate temperature in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O process ŽFig. 5.. Such difference between the TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 and TiŽOCHŽCH 3 . 2 .4 r
RHEED studies revealed that the films grown at 100–1508C were amorphous while those grown at 1808C and higher substrate temperatures were polycrystalline. Anatase was the only crystalline phase detected by RHEED independently of the substrate temperature and oxygen precursor used in the deposition process. The background corresponding to amorphous phase decreased with increasing growth temperature and a well-developed texture with preferred orientation of crystallites in the w110x direction appeared in the films grown at 250–3008C. According to AES measurements, the OrTi ratio was close to 2.0 when the growth temperature was varied from 1508C to 3008C in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O process and from 1008C to 3008C in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process. In the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at 1008C, the values of the OrTi ratio ranged from 1.9 to 2.0 and scattered significantly when measured at different points on the sample. The concentration of carbon residues measured by AES
Fig. 6. D m 2 rD m 0 as a function of H 2 O 2 pulse time. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of H 2 O 2 is 13 Pa.
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in the films grown at 150–3008C did not exceed 0.2–0.5 at.%. In the films grown at 1008C, the carbon concentration was a bit higher, and depended on the oxygen precursor. The films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at this temperature contained 1.3–1.6 at.% of carbon. At the same time, the carbon concentration was still 0.6–1.0 at.%, only, in the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O 2 . The refractive index of the films deposited from TiŽOCHŽCH 3 . 2 .4 and H 2 O 2 at 1008C was 2.3 while the growth rate calculated from the optical thickness reached 0.12 nm per cycle. The latter value compared rather well to the growth rate obtained by Doring et al. w6x for the TiŽOCHŽCH 3 . 2 .4rH 2 O ¨ process at the same substrate temperature. At the substrate temperatures of 150–2508C, the growth rate was estimated to be about 0.05–0.06 nm per cycle in the TiŽOCHŽCH 3 . 2 .4rH 2 O process. This was remarkably higher than the growth rate obtained earlier for the same process in the same temperature range w7x. The improvement was mainly due to optimization of the H 2 O dose.
4. Discussion As described above, adsorption of TiŽOCHŽCH 3 . 2 .4 on the TiO 2 surface treated with H 2 O or H 2 O 2 is a self-limited process at temperatures ranging from 1008C to 2508C. Nevertheless, even in this relatively narrow temperature range, different concurrent mechanisms determine the thin film growth. As a result, the growth rate depends on the substrate temperature as well as on the oxygen precursor used. However, in case of both TiŽOCHŽCH 3 . 2 .4rH 2 O and TiŽOCHŽCH 3 . 2 .4rH 2 O 2 processes, there is a temperature range where the main requirement for self-controlled growth is fulfilled, i.e. the growth rate as a function of both titanium and oxygen precursor pressure and pulse time completely saturates. Also, the side effects such as desorption of intermediate adsorption products from the solid surface andror formation of non-stoichiometric solid material are insignificant in this range. The TiŽOCHŽCH 3 . 2 .4r H 2 O process is self-limited at substrate temperatures ranging from 2008C to 2508C, approximately. In the case of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, the self-limited growth can be obtained at 100–2508C. It
is interesting to note that although the latter process is completely self-controlled at 100–2508C, there is no typical ALD window, i.e. the region where the growth rate does not depend on the substrate temperature. As will be shown below, this is mainly because a temperature-dependent steric hindrance limits the growth rate in this process. Naturally, the steric effect depends on the sizes of surface intermediate species adsorbed on a titanium site, and for this reason on the exchange reactions accompanying TiŽOCHŽCH 3 . 2 .4 adsorption. Indeed, due to the ligand sizes, the effective diameter of a TiŽOCHŽCH 3 . 2 .4 molecule is significantly larger than that of a titanium site on the surface of TiO 2 w13x. Therefore, the smaller the amount of ligands adsorbed with each titanium atom, the higher the possible surface abundance of titanium adsorbed in each ALD cycle. The average number of ligands adsorbed with each titanium atom can be estimated from the data presented in Fig. 5. Using the D m 2rD m 0 ratio Žsee also Fig. 1. and the molar mass of film material, Mf , one can find the mass exchange in a surface intermediate species participating in the film formation. If the gaseous reaction products released during TiŽOCHŽCH 3 . 2 .4 adsorption do not cause additional exchange reactions with TiO 2 , then the mass of ligands andror ligand constituents released in the TiŽOCHŽCH 3 . 2 .4 adsorption process can be calculated as: M1 s M TIP y Ž 1 qD m 2rD m 0 . Mf y m H
Ž 1.
where M TIP s 283.9 amu is the molar mass of TiŽOCHŽCH 3 . 2 .4 and m H expresses the mass of hydrogen replaced in the exchange reactions between hydroxyl groups and adsorbing TiŽOCHŽCH 3 . 2 .4 . In general, m H is not known but it ranges from 0 to 4 amu Žsee e.g. Ref. w14x.. One can easily see that in case of D m 2rD m 0 ratios obtained in this work ŽFig. 5., the value of M TIP y Ž1 q m 2rD m 0 . Mf exceeds m H by the factor of 15 as minimum. Thus, an approximation can be used to estimate M1: M1 ( M TIP y Ž 1 q D m 2rD m 0 . Mf .
Ž 2.
The requirement that the gaseous reaction products released during TiŽOCHŽCH 3 . 2 .4 adsorption should not cause secondary exchange reactions is well fulfilled in the present case because organic
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compounds with low reactivity are formed from TiŽOCHŽCH 3 . 2 .4 during its adsorption on TiO 2 w16x. Therefore, using the D m 2rD m 0 values shown in Fig. 5 and the molar mass of TiO 2 as Mf , one can find from Eq. Ž2. that at 200–2508C, M1 ranges from 130 to 150 amu in the TiŽOCHŽCH 3 . 2 .4rH 2 O process and from 70 to 100 amu in the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process. The mass of a single –OCHŽCH 3 . 2 ligand is equal to 59 amu. Therefore, at temperatures 200–2508C, in average 2.2–2.5 and 1.2–1.7 ligands are released from each TiŽOCHŽCH 3 . 2 .4 molecule during its adsorption on the surfaces treated by H 2 O and H 2 O 2 , respectively. In the TiŽOCHŽCH 3 . 2 .4rH 2 O process, the ligand release does not change significantly when the substrate temperature decreases down to 1008C. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, by contrast, it becomes more effective and up to 2.6–2.9 ligands are released from a TiŽOCHŽCH 3 . 2 .4 molecule adsorbing at 1008C on the surface exposed to H 2 O 2 . At the substrate temperatures above 2508C, a rather similar increase in the ligand release appears with increasing substrate temperature. In this temperature range, however, the process is less sensitive to the oxygen precursor used. At 3008C, the estimated number of released ligands is 2.8–3.0 and 2.2–2.7 in the TiŽOCHŽCH 3 . 2 .4 rH 2 O and TiŽOCHŽCH 3 . 2 .4 r H 2 O 2 processes, respectively. The results presented above demonstrate that the number of isopropoxide ligands, which stay in the surface intermediate species together with each titanium atom, depends remarkably on the substrate temperature. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process performed at 200–2508C, for instance, up to 2.8 ligands per titanium atom stay in the surface layer after the TiŽOCHŽCH 3 . 2 .4 pulse. At 1008C, this number is 1.1–1.4 only. Comparing these values with the changes in D m 0 ŽFig. 2., one can see that the growth rate is almost inversely proportional to the ligand to titanium ratio in the surface intermediate layer. A nearly similar result can be obtained from comparison of the growth rates and ligand exchange at temperatures 250–3008C. These facts confirm that steric hindrance really influences the growth rate. Significant release of ligands during TiŽOCHŽCH 3 . 2 .4 adsorption is probably the main reason why the growth rate obtained in this work is
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even higher than the growth rate in the TiŽOCH 2 CH 3 .4rH 2 O process w13x. In fact, a TiŽOCH ŽCH 3 . 2 . 4 molecule is larger than a TiŽOCH 2 CH 3 .4 molecule w13x. Therefore, in case of the same reaction mechanism, the application of TiŽOCHŽCH 3 . 2 .4 instead of TiŽOCH 2 CH 3 .4 should result in a lower growth rate. However, real-time mass spectrometry studies have revealed that practically no ligands are released during TiŽOCH 2 CH 3 .4 adsorption on the surface of TiO 2 w20x. The release of ligands from an adsorbing metal precursor obviously depends on the precursor stability. On the other hand, as discussed above, the surface hydroxyl groups probably formed during the oxygen precursor pulse should also contribute to the ligand release, especially at lower temperatures. The increase of the equilibrium surface abundance of hydroxyl groups with decreasing temperature w19x, for instance, well explains why the growth rate of a completely self-controlled process still depends on the substrate temperature. Increased density of hydroxyl groups has been discussed as a possible reason for the enhanced rate of the InCl 3rH 2 O 2 and SnCl 4rH 2 O 2 atomic layer epitaxy processes compared with the rate of the InCl 3rH 2 O and SnCl 4rH 2 O processes w21x. The results of the present study indicate, however, that the role of oxygen precursor obviously depends on the oxide and on the substrate temperature used. At 1008C, for instance, H 2 O 2 seems to create more hydroxyl groups on TiO 2 than H 2 O does. On the contrary, at 200–2508C, a smaller amount of ligands is released from an adsorbing titanium isopropoxide molecule in the TiŽOCH ŽCH 3 . 2 .4 rH 2 O 2 process than in the TiŽOCHŽCH 3 . 2 .4rH 2 O process. Thus, in the latter temperature range, the abundance of hydroxyl groups formed by H 2 O 2 at open adsorption sites is evidently lower than the abundance obtained after exposure to a similar H 2 O dose. It should be noted that in the TiŽOCHŽ CH 3 . 2 . 4 rH 2 O process, the exchange of –OCHŽCH 3 . 2 ligands during TiŽOCHŽCH 3 . 2 .4 pulse is not the only factor determining the growth rate. One can see in Fig. 5 that at TS s 1508C, the D m 2rD m 0 ratio and thus, the mass exchange in the reacting surface intermediate species are almost independent of whether H 2 O or H 2 O 2 is used as the oxygen precursor. At the same time, the growth rate
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of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process is about 1.5 times higher than that of the TiŽOCHŽCH 3 . 2 .4rH 2 O process ŽFig. 2.. An explanation for this difference is that although H 2 O and H 2 O 2 create the same abundance of hydroxyl groups on the open TiO 2 surface, the H 2 O pulse does not remove all isopropoxide ligands. The ligands that stay on the surface reduce the number of adsorption sites for TiŽOCHŽCH 3 . 2 .4 and in this way, hinder the growth process. To explain the observed difference in the growth rate, one should assume that after the H 2 O pulse, about 30% of the adsorption sites are covered with ligands preventing adsorption of TiŽOCHŽCH 3 . 2 .4 . Nevertheless, the concentration of carbon was as low as 0.2–0.5 at.% in the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at 1508C. Consequently, most of the ligands that stay on the surface after the H 2 O pulse and hinder the further film growth are not incorporated in the film material. This result, however, is not surprising provided that TiŽOCHŽCH 3 . 2 .4 evidently does not adsorb on the ligands residing on the film surface.
steps still indicated that the reaction mechanism depended on the oxygen precursor. The relative amount of isopropoxide ligands released during titanium isopropoxide adsorption is larger in the process where water is used as the oxygen precursor than in the process where water is replaced by H 2 O 2 . The structure of deposited films, however, did not depend on the oxygen precursor choice. The films grown at 100–1508C were amorphous while those deposited at 1808C and higher substrate temperatures contained polycrystalline anatase.
Acknowledgements The authors are grateful to Alma-Asta Kiisler and Kaupo Kukli for assistance in growth experiments. The work was partly supported by Finnish National Technology Agency ŽTEKES. and Estonian Science Foundation ŽResearch Grant No. 1878..
References 5. Conclusions QCM studies performed in this work revealed that thermal decomposition of titanium isopropoxide at temperatures above 2758C, and insufficient reactivity of water at temperatures below 2008C are the main reasons which cause deviations from the self-limited ALD growth of titanium dioxide from titanium isopropoxide and water. However, adsorption of titanium isopropoxide on the titanium dioxide surface is a self-saturated process at substrate temperatures of 100–2508C. At these temperatures, the completely self-controlled growth was realized in the process where hydrogen peroxide instead of water was used as a oxygen precursor. In this kind of process, the growth rate was as high as 0.12 nm per cycle at 1008C, while the carbon contamination of the films grown at this temperature did not exceed 1 at.% and the refractive index reached 2.3. At temperatures 200–2508C, the growth rate ranged from 0.05 to 0.06 nm per cycle and was almost independent of the oxygen precursor used. The mass changes measured for different reaction
w1x T. Suntola, Mater. Sci. Rep. 4 Ž1989. 265. w2x V.B. Aleskovskii, V.E. Drozd, Acta Polytech. Scand., Chem. Technol. Ser. 195 Ž1990. 155. w3x J. Nishizawa, H. Abe, T. Kurabayashi, J. Electrochem. Soc. 132 Ž1985. 1197. w4x M. Ritala, M. Leskela, ¨ J.-P. Dekker, C. Mutsaers, P.J. Soininen, J. Scarp, Chem. Vap. Deposition 5 Ž1999. 7. w5x M. Vehkamaki, M. Ritala, M. ¨ T. Hatanpaa, ¨¨ T. Hanninen, ¨ Leskela, ¨ Electrochem. Solid-State Lett. 2 Ž1999. 504. w6x H. Doring, K. Hashimoto, A. Fujishima, Ber. Bunsen-Ges. ¨ Phys. Chem. 96 Ž1992. 620. w7x M. Ritala, M. Leskela, ¨ L. Niinisto, ¨ P. Haussalo, Chem. Mater. 5 Ž1993. 1174. w8x T. Honda, K. Yanashima, J. Yoshino, H. Kukimoto, F. Koyama, K. Iga, Jpn. J. Appl. Phys. 33 Ž1994. 3960. w9x H. Tang, K. Prasad, R. Sanjines, ´ F. Levi, ´ Sens. Actuators, B 26–27 Ž1995. 71. w10x B. O’Regan, M. Gratzel, Nature 353 Ž1991. 737. ¨ w11x M. Ritala, M. Leskela, P. Soininen, L. Niinisto, ¨ E. Nykanen, ¨ ¨ Thin Solid Films 225 Ž1993. 288. w12x H. Kumagai, M. Matsumoto, K. Toyoda, M. Obara, M. Suzuki, Thin Solid Films 263 Ž1995. 47. w13x M. Ritala, M. Leskela, ¨ E. Rauhala, Chem. Mater. 6 Ž1994. 556. w14x J. Aarik, A. Aidla, A.-A. Kiisler, T. Uustare, V. Sammelselg, Thin Solid Films 340 Ž1999. 110. w15x R. Swanepoel, J. Phys. B: Sci. Instrum. 16 Ž1983. 1214.
J. Aarik et al.r Applied Surface Science 161 (2000) 385–395 w16x Y.-M. Wu, D.C. Bradley, R.M. Nix, Appl. Surf. Sci. 64 Ž1993. 21. w17x K.L. Siefering, G.L. Griffin, J. Electrochem. Soc. 137 Ž1990. 814. w18x K.L. Siefering, G.L. Griffin, J. Electrochem. Soc. 137 Ž1990. 1206.
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w19x S. Burgeois, F. Jomard, M. Perdereau, Surf. Sci 279 Ž1992. 349. w20x M. Ritala, M. Juppo, K. Kukli, A. Rahtu, M. Leskela, ¨ J. Phys. IV 9 Ž1999. Pr8-1021. w21x M. Ritala, T. Asikainen, M. Leskela, ¨ Electrochem. Solid-State Lett. 1 Ž1998. 156.
Titanium isopropoxide as a precursor for atomic layer deposition: characterization of titanium dioxide growth process Jaan Aarik a,) , Aleks Aidla a , Teet Uustare a , Mikko Ritala b, Markku Leskela¨ b b
a ¨ Institute of Materials Science, UniÕersity of Tartu, 18 Ulikooli Steet, 50090 Tartu, Estonia Department of Chemistry, UniÕersity of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN 00014 UniÕersity of Helsinki, Helsinki, Finland
Received 24 November 1999; accepted 17 February 2000
Abstract Atomic layer deposition ŽALD. of titanium oxide from titanium isopropoxide ŽTiŽOCHŽCH 3 . 2 .4 . and water as well as from TiŽOCHŽCH 3 . 2 .4 and hydrogen peroxide ŽH 2 O 2 . was studied. According to data of real-time quartz crystal microbalance ŽQCM. measurements, adsorption of TiŽOCHŽCH 3 . 2 .4 was a self-limited process at substrate temperatures 100–2508C. At 200–2508C, the growth rate was independent of whether water or H 2 O 2 was used as the oxygen precursor. Insufficient reactivity of water vapor hindered the film growth at temperatures 100–1508C. Incomplete removal of the precursor ligands from solid surface by water pulse was revealed as the main reason for limited deposition rate. The growth rate increased significantly and reached 0.12 nm per cycle at 1008C when water was replaced with H 2 O 2 . The carbon contamination did not exceed 1 at.% and the refractive index was 2.3 in the films grown at temperatures as low as 1008C. q 2000 Elsevier Science B.V. All rights reserved. PACS: 68.55; 81.15 Keywords: Atomic layer deposition; Thin films; Titanium oxide; Adsorption kinetics
1. Introduction Atomic layer deposition ŽALD. also known as atomic layer epitaxy w1x, molecular layering w2x and molecular layer epitaxy w3x is an appropriate method for growing uniform thin and ultra-thin films, especially in the cases when precise thickness control, high reproducibility, thickness uniformity and excellent conformity w4,5x are required. In the ALD pro-
) Corresponding author. Tel.: q372-7-375877; fax: q372-7375540. E-mail address: [email protected] ŽJ. Aarik..
cess, the film is formed via a series of subsequent self-saturating chemisorption reactions. Therefore, provided that the deposition conditions are properly chosen, the thickness increase per reaction step should be independent of modest variation of the precursor fluxes and exposure times. As a result, the film thickness should be determined by the number of reaction cycles, mainly, which makes the process control precise and convenient. However, for some precursor systems, the real process seems to be much more complex. For instance, rather contradictory results have been obtained for the ALD growth of titanium dioxide ŽTiO 2 . from titanium isopropoxide ŽTiŽOCHŽCH 3 . 2 .4 . and
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 2 7 4 - 9
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water. The growth rate as high as 0.15 nm per ALD cycle has been measured by Doring et al. w6x at the ¨ substrate temperature TS s 1508C. By contrast, at the same substrate temperature, the growth rate did not exceed 0.015 nm per cycle in the work of Ritala et al. w7x. Steric hindrance and kinetic limitation have been suggested as the main reasons for low growth rate w7x. In order to investigate the contribution of these effects, we performed a series of additional experiments. Besides methods for post-growth characterization of thin films, quartz crystal microbalance ŽQCM. method was used to study the deposition kinetics in real-time. The research was concentrated on revealing the mechanism of surface reactions and, in particular, the reaction step that limits the growth rate. Besides water, hydrogen peroxide ŽH 2 O 2 . was used as the oxygen precursor. Because of different reactivity of water and hydrogen peroxide, this enabled us to obtain more information about the role of kinetic effects. The study was stimulated by the circumstance that TiO 2 thin films have found a number of applications, for instance, in optical coatings w8x, chemical sensors w9x and solar cells w10x. Moreover, titanium oxide is a constituent of important multicomponent oxides like ŽBa,Sr.TiO 3 , PbTiO 3 and Bi 4Ti 3 O 12 . Although there are some other precursors like titanium tetrachloride w2,11,12x and titanium ethoxide w13x, which can be used for ALD growth of TiO 2 , titanium isopropoxide as an alternative to those is still worth studying. The work was additionally motivated by the fact that titanium isopropoxide had also proved to be a suitable precursor for growing SrTiO 3 and BaTiO 3 thin films w5x needed for new generation dynamic random access memories.
2. Experimental The TiO 2 thin films were grown in a hot-wall flow-type reactor w14x. N2 gas with 99.999% purity was used as a carrier leading titanium isopropoxide and the oxygen precursor ŽH 2 O or H 2 O 2 . vapors alternately to the substrate. The carrier gas pressure measured at the reactor outlet was kept at about 250 Pa. After each precursor pulse the reactor was purged with pure carrier gas in order to remove the gaseous reaction products and avoid overlapping the precur-
sor pulses. Therefore, an ALD cycle consisted of a TiŽOCHŽCH 3 . 2 .4 pulse, purge time, oxygen precursor pulse and another purge time. Titanium isopropoxide was volatilized at temperatures 26–758C. The vapor pressure was determined by the temperature of the TiŽOCHŽCH 3 . 2 .4 cell. Water and hydrogen peroxide were vaporized in containers, which were kept at 208C and 368C, respectively. The flow of oxygen precursor was controlled with a calibrated needle valve. In order to get concentrated H 2 O 2 , water was removed from the commercial 30% H 2 O 2 solution by vacuum distillation. A QCM working at the oscillation frequency of about 30 MHz was applied to investigate the deposition kinetics. The mass changes were measured with the time resolution of 0.5 s, which enabled us to record the behavior of film mass within a single ALD cycle. In order to increase the accuracy of measurements and reduce the contribution of transient effects that appeared after changing the growth conditions, the mass sensor signal was always recorded during four to five subsequent ALD cycles. In addition, the data corresponding to the first cycle were neglected when the average mass sensor response was calculated. This enabled us to diminish misleading effects, which appeared usually in the beginning of the growth process and were probably related to the changes in the surface layer properties. Before starting the measurements with a new mass sensor, a 5–10-nm buffer layer was first grown to reduce the influence of the sensor material on the deposition process The thin films for post-growth characterization were deposited on silica and Ž100.-oriented silicon substrates. The substrate temperatures used for growing the films ranged from 1008C to 3008C. The optimum precursor doses and purge times were determined from the results of QCM measurements. The pulse and purge times were chosen equal to 2 s. The temperature of the TiŽOCHŽCH 3 . 2 .4 source was set at 428C and the partial pressures of H 2 O and H 2 O 2 were kept at 20 and 10 Pa, respectively. The composition of the films grown on silicon substrates was measured with Auger electron spectrometry ŽAES.. Immediately before each AES measurement, the film surface was always cleaned by argon-ion bombardment. The reflection high-energy electron
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diffraction ŽRHEED. method was applied for structure studies. The thickness and refractive index of films grown on silica substrates were determined from optical transmission spectra w15x.
3. Results 3.1. Deposition kinetics Relatively long cycle times were used to investigate the kinetics of precursor adsorption during a single ALD cycle. The time dependence of the deposit mass recorded at substrate temperatures of 1008C, 2008C and 2758C is depicted in Fig. 1. At all substrate temperatures used, the film mass abruptly increased during the TiŽOCHŽCH 3 . 2 .4 pulse and decreased after switching on the water pulse. The mass increase was evidently due to TiŽOCHŽCH 3 . 2 .4 adsorption while the decrease was caused by desorption of reaction products formed from the surface
Fig. 1. Mass sensor signal as a function of time recorded during a single ALD cycle. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Water vapor pressure measured at reactor outlet is 19 Pa.
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intermediate species. At TS s 1008C and 2008C, the mass increase completely saturated during the TiŽOCHŽCH 3 . 2 .4 pulse ŽFig. 1.. Thus, the adsorption products formed on the surface disabled further adsorption of TiŽOCHŽCH 3 . 2 .4 . At TS s 2758C, however, no saturation could be achieved. Correspondingly, at the latter substrate temperature, the film mass somewhat decreased during the first purge time while at 1008C and 2008C, it did not. At the substrate temperature of 1008C, a small decrease appeared in the mass sensor signal immediately after switching off the water pulse ŽFig. 1.. As the amplitude of this mass change increased with the partial pressure of water, desorption of molecular water from the film surface is the plausible reason for this effect. Fig. 1 shows that the time constant describing the steep mass increase in the beginning of TiŽOCHŽCH 3 . 2 .4 pulse decreases with increasing substrate temperature. Consequently, the adsorption process is evidently controlled by a surface reaction, which proceeds faster at higher temperatures. Fig. 1 also demonstrates that at sufficiently high temperatures, an unsaturated process occurs during the TiŽOCHŽCH 3 . 2 .4 pulse. It was revealed that the rate of the latter process rapidly increased with the substrate temperature. Therefore, the process was most probably due to thermal decomposition of TiŽOCHŽCH 3 . 2 .4 w16,17x. Indeed, Wu et al. w16x have observed a steep increase in the rate of thermal decomposition of TiŽOCHŽCH 3 . 2 .4 at 2758C approximately. This temperature very well agrees with the lowest substrate temperature, at which the unsaturated adsorption appears in our experiments. Thermal decomposition andror desorption of titanium-containing surface intermediate species both preferably appearing at elevated temperatures are the most possible reasons for the mass decrease observed at 2758C ŽFig. 1. and higher substrate temperatures during the first purge time. However, as will be shown below, thermal decomposition resulting in the growth of TiO 2 and releasing of some isopropoxide ligands or constituents of them is in better agreement with the experimental data. There were no qualitative changes in the shape of the mass sensor signal when hydrogen peroxide, instead of water, was used as the oxygen precursor. The main quantitative difference was that the values
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of D m 0 characterizing the growth rate of the ALD process ŽFig. 1. significantly increased at the substrate temperatures of 100–1508C after this replacement ŽFig. 2.. Independently of the oxygen precursor used, D m 0 significantly increased with the increase of the substrate temperature from 250–2758C to 3008C. Also, the dependence of D m 0 on the TiŽOCHŽCH 3 . 2 .4 pulse width became stronger. Increasing influence of the pulse time observed at temperatures 275–3008C is in agreement with the data shown in Fig. 1 and is obviously due to the
Fig. 2. D m 0 as a function of substrate temperature in Ža. TiŽOCHŽCH 3 . 2 .4 rH 2 O and Žb. TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 processes. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of water and hydrogen peroxide is 19 and 13 Pa, respectively.
precursor decomposition. At temperatures 100– 2008C, however, the pulse time of 2 s is a bit too short to achieve complete saturation of adsorption Žsee also Fig. 1.. In this context, it is of importance to note that neither the adsorption time constant nor the rate of unsaturated growth, observed at 275– 3008C, depended on the titanium isopropoxide pressure. We kept the TiŽOCHŽCH 3 . 2 .4 pulse time at 2 s and varied the source temperature from 268C to 468C at the substrate temperatures 1008C and 2008C and from 428C to 758C at TS s 3008C. However, no effect of the source temperature on D m 0 was recorded although the TiŽOCHŽCH 3 . 2 .4 pressure had to rise more than five times at the lower substrate temperatures and about nine times w18x at TS s 3008C. Therefore, in the present case, the insufficient exposure time cannot be compensated with higher precursor pressure. Similarly, the decomposition process appearing at higher substrate temperatures is independent of the precursor concentration in the gas phase. Consequently, the adsorption time constant as well as the precursor decomposition rate are determined by the rate of surface reactions rather than by the precursor supply from the gas phase. Another remarkable result is that even at the substrate temperatures where a significant mass decrease was recorded during the first purge time ŽFig. 1., D m 0 does not depend on the purge length used after the TiŽOCHŽCH 3 . 2 .4 pulse ŽFig. 2.. Therefore, the mass decrease cannot be explained by desorption of the titanium-containing surface intermediate species. Instead, the decomposition of those species and release of precursor ligands from the surface is a more reasonable explanation for this effect. In the process where water was used as an oxygen source, the growth rate was almost independent of substrate temperature when the latter was varied from 1508C to 2758C ŽFig. 2Ža... However, D m 0 decreased noticeably with the decrease of the substrate temperature down to 1008C ŽFig. 2Ža... The dependence of D m 0 on the substrate temperature changed markedly its shape when water was replaced with hydrogen peroxide. The growth rate increased by the factor of 2, approximately, when the substrate temperature decreased from 2508C to 1008C ŽFig. 2Žb... In addition, the influence of the TiŽOCHŽCH 3 . 2 .4 pulse length became much weaker. The last fact indicates that the surface reactions
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determining the TiŽOCHŽCH 3 . 2 .4 adsorption proceed faster on the surface treated with H 2 O 2 than on the surface treated with H 2 O. For this reason, the TiŽOCHŽCH 3 . 2 .4 pulse time of 2 s is sufficient to cover the surface with adsorption products and no considerable increase in the film mass can be obtained with the further increase of the pulse time. These data show that the reactivity of the oxygen precursor is a critical parameter determining also the TiŽOCHŽCH 3 . 2 .4 adsorption kinetics in the next ALD cycle. Additional evidence of the low reactivity of water is presented in Fig. 3. As can be seen, D m 0 as a function of water pressure ŽFig. 3Ža.. and pulse time ŽFig. 3Žb.. does not saturate at 1008C. By contrast, D m 0 satisfactorily saturates in the process where hydrogen peroxide is used as the oxygen source. At higher substrate temperatures, especially at 3008C, D m 0 as a function of oxygen precursor dose saturates in the TiŽOCHŽCH 3 . 2 .4rH 2 O process, as well ŽFig. 3.. It is possible that at 100–1508C, the further increase in the dose might compensate the low reactivity of water but in many applications, this approach is not desirable. First, prolonged pulse time results in a longer thin film deposition process. Secondly, at higher water pressures, an overlap of the precursor pulses may appear and cause an increase in the thickness gradients in thin films. It is worth noting, however, that at the doses used in this work, the overlap of precursor pulses was still insignificant when the purge times were 2 s or longer. As shown in Fig. 2, the purge time following the TiŽOCHŽCH 3 . 2 .4 pulse has no effect on the growth rate when varied from 2 to 10 s. The purge time following the water pulse had some influence. Nevertheless, at the H 2 O pressure of 20 Pa, the growth rate increased by 10%, only, when this purge time was reduced from 10 to 2 s. Further decrease of the purge time from 2 to 0.2 s, by contrast, caused a growth rate increase of about 20%, already. Thus, at short purge times, the precursor pulses evidently overlap with each other. To characterize surface reactions, the effect of process parameters on the mass change D m 2 ŽFig. 1. was also studied. Similarly to D m 0 , the value of D m 2 , which included the effects of the oxygen precursor pulse and both purge times, did not depend on the purge time following the TiŽOCHŽCH 3 . 2 .4
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Fig. 3. D m 0 as a function of Ža. oxygen precursor pressure in the reactor and Žb. oxygen precursor pulse time. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. TiŽOCHŽCH 3 . 2 .4 pulse time and following purge time are equal to 2 s. Purge time used after oxygen precursor pulse is 5 s.
pulse ŽFig. 4.. No effect was observed even at high substrate temperatures where a considerable mass decrease appeared during this purge time Župper curve in Fig. 1.. Therefore, the latter mass decrease was connected with a process that resulted in the deposition of a similar solid substance as the reaction between the oxygen precursor and surface intermediate species did. Correspondingly, as mentioned above, desorption of titanium-containing surface in-
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Fig. 4. D m 2 as a function of substrate temperature in Ža. TiŽOCHŽCH 3 . 2 .4 rH 2 O and Žb. TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 processes. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of water and hydrogen peroxide is 19 and 13 Pa, respectively.
termediate species could not cause this mass decrease. Differently from the growth rate, D m 2 did not increase with the TiŽOCHŽCH 3 . 2 .4 pulse time varied from 2 to 5 s ŽFigs. 2 and 4.. Consequently, the surface concentration of removable precursor ligands saturates at shorter TiŽOCHŽCH 3 . 2 .4 pulses than that of titanium-containing surface intermediate species. It is remarkable that D m 2 did not depend on the pulse time even at the substrate temperatures 275– 3008C where the unsaturated mass increase was observed during the TiŽOCHŽCH 3 . 2 .4 pulse. Thus, after saturation of the surface with precursor ligands, additional adsorption of TiŽOCHŽCH 3 . 2 .4 is evidently possible provided that some of these ligands first desorb from the surface. At higher temperatures, this kind of desorption can be caused by thermal
decomposition of surface intermediate species. At lower temperatures, however, different types of hydroxyl groups w19x that probably reside on the surface exposed to H 2 O or H 2 O 2 may react with adsorbing TiŽOCHŽCH 3 . 2 .4 and in this way, also cause the ligand desorption. One can see in Fig. 4 that D m 2 depends on the substrate temperature as well as on the oxygen precursor chosen. Higher D m 2 values correspond to the TiŽOCHŽCH 3 .4rH 2 O 2 process ŽFig. 4.. It should be noted that D m 2 as a function of substrate temperature has a maximum at about 2008C ŽFig. 4Žb.. where the growth rate is close to its lowest value ŽFig. 2Žb... Another interesting result is that at substrate temperatures below 2008C, D m 2 decreases with decreasing temperature independently of the oxygen precursor used. The value of D m 0 , in turn, increases with decreasing temperature in the TiŽOCHŽCH 3 .4rH 2 O 2 process ŽFig. 2Žb.. while it decreases in the TiŽOCHŽCH 3 .4rH 2 O process ŽFig. 2Ža... It has been demonstrated Žsee e.g. Ref. w14x. that relative mass changes corresponding to different reaction steps ŽFig. 1. enable one to characterize the ligand exchange per surface intermediate species participating in formation of the thin film material. For instance, both D m 0 and D m 2 are proportional to the abundance of the surface intermediate species reacting with the oxygen precursor. Consequently, the D m 2rD m 0 ratio does not depend on the concentration of the reacting species any more but describes the average mass change in those. Fig. 5 shows that the D m 2rD m 0 ratio is somewhat higher at the TiŽOCHŽCH 3 . 2 .4 pulse time of 2 s than at the pulse time of 5 s. Although the difference is close to the experimental error, it occurs at all temperatures and in cases of both oxygen precursors used. Thus, the longer the TiŽOCHŽCH 3 . 2 .4 pulse, the smaller the amount of ligands that stay in the surface intermediate species and can be removed during the following oxygen precursor pulse. Fig. 5 also demonstrates that the D m 2rD m 0 ratio decreases with the increase of substrate temperature from 2508C to 3008C. Such decrease is evidently caused by thermal decomposition, which appears at these temperatures. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, the D m 2rD m 0 ratio decreases with the decrease of sub-
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H 2 O processes may be due to the fact that in the latter process, the D m 2rD m 0 ratio is rather low at 2008C ŽFig. 5.. On the other hand, the reactivity of H 2 O is evidently too low to complete the exchange reactions at 1008C ŽFig. 3Žb.., and create as high abundance of hydroxyl groups as H 2 O 2 does at this temperature. 3.2. Properties of thin films
Fig. 5. D m 2 rD m 0 ratio as a function of substrate temperature. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of H 2 O and H 2 O 2 is 19 and 13 Pa, respectively.
strate temperature from 2008C to 1008C ŽFig. 5.. In this temperature range, adsorption of TiŽOCHŽCH 3 . 2 .4 completely saturates. For this reason, the possible role of unsaturated processes, such as thermal decomposition, can be excluded. Instead, exchange reactions between adsorbing titanium isopropoxide and surface hydroxyl groups, preferably appearing at low temperatures w19x, should be considered as factors influencing the D m 2rD m 0 ratio. The data presented in Fig. 6 support the conclusion about the possible role of hydroxyl groups. This figure shows that the D m 2rD m 0 ratio decreases with increasing H 2 O 2 pulse time. Therefore, higher H 2 O 2 dose results in more significant ligand release during the following TiŽOCHŽCH 3 . 2 .4 pulse rather than during the current H 2 O 2 pulse. This kind of effect is possible, if the H 2 O 2 pulse length influences surface abundance of hydroxyl groups that, in turn, controls the ligand exchange in the following TiŽOCHŽCH 3 . 2 .4 adsorption step. It should be noted in this connection that at 100–2008C, the D m 2rD m 0 ratio does not depend remarkably on the substrate temperature in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O process ŽFig. 5.. Such difference between the TiŽOCHŽCH 3 . 2 .4 rH 2 O 2 and TiŽOCHŽCH 3 . 2 .4 r
RHEED studies revealed that the films grown at 100–1508C were amorphous while those grown at 1808C and higher substrate temperatures were polycrystalline. Anatase was the only crystalline phase detected by RHEED independently of the substrate temperature and oxygen precursor used in the deposition process. The background corresponding to amorphous phase decreased with increasing growth temperature and a well-developed texture with preferred orientation of crystallites in the w110x direction appeared in the films grown at 250–3008C. According to AES measurements, the OrTi ratio was close to 2.0 when the growth temperature was varied from 1508C to 3008C in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O process and from 1008C to 3008C in case of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process. In the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at 1008C, the values of the OrTi ratio ranged from 1.9 to 2.0 and scattered significantly when measured at different points on the sample. The concentration of carbon residues measured by AES
Fig. 6. D m 2 rD m 0 as a function of H 2 O 2 pulse time. TiŽOCHŽCH 3 . 2 .4 source temperature is 428C. Pressure of H 2 O 2 is 13 Pa.
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in the films grown at 150–3008C did not exceed 0.2–0.5 at.%. In the films grown at 1008C, the carbon concentration was a bit higher, and depended on the oxygen precursor. The films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at this temperature contained 1.3–1.6 at.% of carbon. At the same time, the carbon concentration was still 0.6–1.0 at.%, only, in the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O 2 . The refractive index of the films deposited from TiŽOCHŽCH 3 . 2 .4 and H 2 O 2 at 1008C was 2.3 while the growth rate calculated from the optical thickness reached 0.12 nm per cycle. The latter value compared rather well to the growth rate obtained by Doring et al. w6x for the TiŽOCHŽCH 3 . 2 .4rH 2 O ¨ process at the same substrate temperature. At the substrate temperatures of 150–2508C, the growth rate was estimated to be about 0.05–0.06 nm per cycle in the TiŽOCHŽCH 3 . 2 .4rH 2 O process. This was remarkably higher than the growth rate obtained earlier for the same process in the same temperature range w7x. The improvement was mainly due to optimization of the H 2 O dose.
4. Discussion As described above, adsorption of TiŽOCHŽCH 3 . 2 .4 on the TiO 2 surface treated with H 2 O or H 2 O 2 is a self-limited process at temperatures ranging from 1008C to 2508C. Nevertheless, even in this relatively narrow temperature range, different concurrent mechanisms determine the thin film growth. As a result, the growth rate depends on the substrate temperature as well as on the oxygen precursor used. However, in case of both TiŽOCHŽCH 3 . 2 .4rH 2 O and TiŽOCHŽCH 3 . 2 .4rH 2 O 2 processes, there is a temperature range where the main requirement for self-controlled growth is fulfilled, i.e. the growth rate as a function of both titanium and oxygen precursor pressure and pulse time completely saturates. Also, the side effects such as desorption of intermediate adsorption products from the solid surface andror formation of non-stoichiometric solid material are insignificant in this range. The TiŽOCHŽCH 3 . 2 .4r H 2 O process is self-limited at substrate temperatures ranging from 2008C to 2508C, approximately. In the case of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, the self-limited growth can be obtained at 100–2508C. It
is interesting to note that although the latter process is completely self-controlled at 100–2508C, there is no typical ALD window, i.e. the region where the growth rate does not depend on the substrate temperature. As will be shown below, this is mainly because a temperature-dependent steric hindrance limits the growth rate in this process. Naturally, the steric effect depends on the sizes of surface intermediate species adsorbed on a titanium site, and for this reason on the exchange reactions accompanying TiŽOCHŽCH 3 . 2 .4 adsorption. Indeed, due to the ligand sizes, the effective diameter of a TiŽOCHŽCH 3 . 2 .4 molecule is significantly larger than that of a titanium site on the surface of TiO 2 w13x. Therefore, the smaller the amount of ligands adsorbed with each titanium atom, the higher the possible surface abundance of titanium adsorbed in each ALD cycle. The average number of ligands adsorbed with each titanium atom can be estimated from the data presented in Fig. 5. Using the D m 2rD m 0 ratio Žsee also Fig. 1. and the molar mass of film material, Mf , one can find the mass exchange in a surface intermediate species participating in the film formation. If the gaseous reaction products released during TiŽOCHŽCH 3 . 2 .4 adsorption do not cause additional exchange reactions with TiO 2 , then the mass of ligands andror ligand constituents released in the TiŽOCHŽCH 3 . 2 .4 adsorption process can be calculated as: M1 s M TIP y Ž 1 qD m 2rD m 0 . Mf y m H
Ž 1.
where M TIP s 283.9 amu is the molar mass of TiŽOCHŽCH 3 . 2 .4 and m H expresses the mass of hydrogen replaced in the exchange reactions between hydroxyl groups and adsorbing TiŽOCHŽCH 3 . 2 .4 . In general, m H is not known but it ranges from 0 to 4 amu Žsee e.g. Ref. w14x.. One can easily see that in case of D m 2rD m 0 ratios obtained in this work ŽFig. 5., the value of M TIP y Ž1 q m 2rD m 0 . Mf exceeds m H by the factor of 15 as minimum. Thus, an approximation can be used to estimate M1: M1 ( M TIP y Ž 1 q D m 2rD m 0 . Mf .
Ž 2.
The requirement that the gaseous reaction products released during TiŽOCHŽCH 3 . 2 .4 adsorption should not cause secondary exchange reactions is well fulfilled in the present case because organic
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compounds with low reactivity are formed from TiŽOCHŽCH 3 . 2 .4 during its adsorption on TiO 2 w16x. Therefore, using the D m 2rD m 0 values shown in Fig. 5 and the molar mass of TiO 2 as Mf , one can find from Eq. Ž2. that at 200–2508C, M1 ranges from 130 to 150 amu in the TiŽOCHŽCH 3 . 2 .4rH 2 O process and from 70 to 100 amu in the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process. The mass of a single –OCHŽCH 3 . 2 ligand is equal to 59 amu. Therefore, at temperatures 200–2508C, in average 2.2–2.5 and 1.2–1.7 ligands are released from each TiŽOCHŽCH 3 . 2 .4 molecule during its adsorption on the surfaces treated by H 2 O and H 2 O 2 , respectively. In the TiŽOCHŽCH 3 . 2 .4rH 2 O process, the ligand release does not change significantly when the substrate temperature decreases down to 1008C. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process, by contrast, it becomes more effective and up to 2.6–2.9 ligands are released from a TiŽOCHŽCH 3 . 2 .4 molecule adsorbing at 1008C on the surface exposed to H 2 O 2 . At the substrate temperatures above 2508C, a rather similar increase in the ligand release appears with increasing substrate temperature. In this temperature range, however, the process is less sensitive to the oxygen precursor used. At 3008C, the estimated number of released ligands is 2.8–3.0 and 2.2–2.7 in the TiŽOCHŽCH 3 . 2 .4 rH 2 O and TiŽOCHŽCH 3 . 2 .4 r H 2 O 2 processes, respectively. The results presented above demonstrate that the number of isopropoxide ligands, which stay in the surface intermediate species together with each titanium atom, depends remarkably on the substrate temperature. In the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process performed at 200–2508C, for instance, up to 2.8 ligands per titanium atom stay in the surface layer after the TiŽOCHŽCH 3 . 2 .4 pulse. At 1008C, this number is 1.1–1.4 only. Comparing these values with the changes in D m 0 ŽFig. 2., one can see that the growth rate is almost inversely proportional to the ligand to titanium ratio in the surface intermediate layer. A nearly similar result can be obtained from comparison of the growth rates and ligand exchange at temperatures 250–3008C. These facts confirm that steric hindrance really influences the growth rate. Significant release of ligands during TiŽOCHŽCH 3 . 2 .4 adsorption is probably the main reason why the growth rate obtained in this work is
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even higher than the growth rate in the TiŽOCH 2 CH 3 .4rH 2 O process w13x. In fact, a TiŽOCH ŽCH 3 . 2 . 4 molecule is larger than a TiŽOCH 2 CH 3 .4 molecule w13x. Therefore, in case of the same reaction mechanism, the application of TiŽOCHŽCH 3 . 2 .4 instead of TiŽOCH 2 CH 3 .4 should result in a lower growth rate. However, real-time mass spectrometry studies have revealed that practically no ligands are released during TiŽOCH 2 CH 3 .4 adsorption on the surface of TiO 2 w20x. The release of ligands from an adsorbing metal precursor obviously depends on the precursor stability. On the other hand, as discussed above, the surface hydroxyl groups probably formed during the oxygen precursor pulse should also contribute to the ligand release, especially at lower temperatures. The increase of the equilibrium surface abundance of hydroxyl groups with decreasing temperature w19x, for instance, well explains why the growth rate of a completely self-controlled process still depends on the substrate temperature. Increased density of hydroxyl groups has been discussed as a possible reason for the enhanced rate of the InCl 3rH 2 O 2 and SnCl 4rH 2 O 2 atomic layer epitaxy processes compared with the rate of the InCl 3rH 2 O and SnCl 4rH 2 O processes w21x. The results of the present study indicate, however, that the role of oxygen precursor obviously depends on the oxide and on the substrate temperature used. At 1008C, for instance, H 2 O 2 seems to create more hydroxyl groups on TiO 2 than H 2 O does. On the contrary, at 200–2508C, a smaller amount of ligands is released from an adsorbing titanium isopropoxide molecule in the TiŽOCH ŽCH 3 . 2 .4 rH 2 O 2 process than in the TiŽOCHŽCH 3 . 2 .4rH 2 O process. Thus, in the latter temperature range, the abundance of hydroxyl groups formed by H 2 O 2 at open adsorption sites is evidently lower than the abundance obtained after exposure to a similar H 2 O dose. It should be noted that in the TiŽOCHŽ CH 3 . 2 . 4 rH 2 O process, the exchange of –OCHŽCH 3 . 2 ligands during TiŽOCHŽCH 3 . 2 .4 pulse is not the only factor determining the growth rate. One can see in Fig. 5 that at TS s 1508C, the D m 2rD m 0 ratio and thus, the mass exchange in the reacting surface intermediate species are almost independent of whether H 2 O or H 2 O 2 is used as the oxygen precursor. At the same time, the growth rate
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of the TiŽOCHŽCH 3 . 2 .4rH 2 O 2 process is about 1.5 times higher than that of the TiŽOCHŽCH 3 . 2 .4rH 2 O process ŽFig. 2.. An explanation for this difference is that although H 2 O and H 2 O 2 create the same abundance of hydroxyl groups on the open TiO 2 surface, the H 2 O pulse does not remove all isopropoxide ligands. The ligands that stay on the surface reduce the number of adsorption sites for TiŽOCHŽCH 3 . 2 .4 and in this way, hinder the growth process. To explain the observed difference in the growth rate, one should assume that after the H 2 O pulse, about 30% of the adsorption sites are covered with ligands preventing adsorption of TiŽOCHŽCH 3 . 2 .4 . Nevertheless, the concentration of carbon was as low as 0.2–0.5 at.% in the films grown from TiŽOCHŽCH 3 . 2 .4 and H 2 O at 1508C. Consequently, most of the ligands that stay on the surface after the H 2 O pulse and hinder the further film growth are not incorporated in the film material. This result, however, is not surprising provided that TiŽOCHŽCH 3 . 2 .4 evidently does not adsorb on the ligands residing on the film surface.
steps still indicated that the reaction mechanism depended on the oxygen precursor. The relative amount of isopropoxide ligands released during titanium isopropoxide adsorption is larger in the process where water is used as the oxygen precursor than in the process where water is replaced by H 2 O 2 . The structure of deposited films, however, did not depend on the oxygen precursor choice. The films grown at 100–1508C were amorphous while those deposited at 1808C and higher substrate temperatures contained polycrystalline anatase.
Acknowledgements The authors are grateful to Alma-Asta Kiisler and Kaupo Kukli for assistance in growth experiments. The work was partly supported by Finnish National Technology Agency ŽTEKES. and Estonian Science Foundation ŽResearch Grant No. 1878..
References 5. Conclusions QCM studies performed in this work revealed that thermal decomposition of titanium isopropoxide at temperatures above 2758C, and insufficient reactivity of water at temperatures below 2008C are the main reasons which cause deviations from the self-limited ALD growth of titanium dioxide from titanium isopropoxide and water. However, adsorption of titanium isopropoxide on the titanium dioxide surface is a self-saturated process at substrate temperatures of 100–2508C. At these temperatures, the completely self-controlled growth was realized in the process where hydrogen peroxide instead of water was used as a oxygen precursor. In this kind of process, the growth rate was as high as 0.12 nm per cycle at 1008C, while the carbon contamination of the films grown at this temperature did not exceed 1 at.% and the refractive index reached 2.3. At temperatures 200–2508C, the growth rate ranged from 0.05 to 0.06 nm per cycle and was almost independent of the oxygen precursor used. The mass changes measured for different reaction
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