Surface Kinetics of Titanium Isopropoxide in High Vacuum Chemical

Oct 6, 2015 - (3, 4) Precursor deposition efficiencies are furthermore a mean to characterize the reactivity of the precursor on the surface and hence...
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Surface Kinetics of Titanium Isopropoxide in High Vacuum Chemical Vapor Deposition Michael Reinke,†,‡ Yury Kuzminykh,†,‡ and Patrik Hoffmann*,†,‡ †

Laboratory for Advanced Materials Processing, Empa, Swiss Federal Laboratories for Materials Science and Technology, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland ‡ Laboratory for Photonic Materials and Characterization, École Polytechnique Fédérale de Lausanne, Station 17, CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: Understanding the surface kinetics of precursor decomposition during thin film formation represents a key aspect in the understanding and engineering of chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. The determination of activation energies of the surface reaction steps, however, is often challenging because it requires precise knowledge of precursor impinging rates. Afterward, the kinetics can be investigated by comparing the amount of deposited material with the absolute precursor flow. Ideally, the experimental equipment allows a distinction between gas phase and surface reactions. Both are difficult to achieve in conventional CVD processes. A high vacuum environment, however, enables the quantitative prediction of precursor impinging rates due to the ballistic nature of precursor transport; additionally precursor gas phase reactions do not occur. We investigated the surface reaction kinetics of titanium isopropoxide (TTIP) in the high vacuum CVD of titanium dioxide. Additionally, we have investigated the addition of water as a reactant to the deposition process. In this way, even surface kinetics relevant for ALD processes could be investigated. Here, we propose a comprehensive surface kinetic model of titanium isopropoxide surface reactions including hydrolytic and pyrolytic reactions. The surface kinetic model was fitted to 363 data points taken from combinatorial experiments covering a wide range of deposition parameters (substrate temperature, 175−610 °C; TTIP impinging rate, 0.1 × 1015−6.0 × 1015 cm−2 s−1; water impinging rate, 4.5 × 1016−9.0 × 1016 cm−2 s−1), which enabled us to derive activation energies for desorption, hydrolysis, and pyrolysis.

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INTRODUCTION Chemical vapor deposition (CVD) processes rely on the decomposition reactions of molecules (precursors) to form a thin film.1 The understanding of the growth process is divided into two parts: understanding the elemental reaction steps of the precursor molecules that ultimately lead to the deposition and derivation of surface kinetics and activation energies of the relevant process steps. Knowledge of activation energies of desorption and decomposition, as well as the respective preexponential factors in their corresponding rate terms is beneficial for understanding and modeling CVD and atomic layer deposition (ALD) processes. Comprehension of the temperature and impinging rate dependence of the reaction rates and deposition efficiency (the ratio of reacted precursor molecules to the number of arrived precursors) is, for example, of high interest for optimization of conformal coatings in thermal CVD2 and to optimize exposure times in ALD on complex surface geometries with high aspect ratios.3,4 Precursor deposition efficiencies are furthermore a mean to characterize the reactivity of the precursor on the surface and hence of interest for precursor development and comparison. Deposition kinetic parameters are typically derived by analyzing the dependence of precursor flux and substrate temperature on the growth rate. A kinetic model that described the process is subsequently fitted to the results. This model © XXXX American Chemical Society

consists of a rate equation (system) for the time-dependent surface occupations and is solved for the steady state case. By this methodology, reaction activation energies can be derived. In order to study surface kinetics, the experimental setup should allow precise control of precursor impinging rates and preferably allow distinguishing between gas phase reactions prior to the arrival of precursor molecules and surface reactions after their arrival on the surface. Both are challenging for conventional CVD systems. In the latter the precursor flux is dependent on relatively complex gas flow dynamics and therefore difficult to predict.5,6 Even in low pressure CVD, the relatively high density of precursor and carrier gas molecules leads to collisions of molecules, which can change the reaction order of the surface reaction7 or lead to gas phase decomposition of the precursor.8 Gas phase reactions and subsequent gas phase nucleation also limit the simultaneous use of reactive precursors as they decrease the quality of the deposit or its adhesion. We performed a surface kinetic study of titanium tetra isopropoxide (Ti(OiPr)4 or TTIP) during the deposition of titanium isopropoxide using combinatorial high vacuum Received: July 24, 2015 Revised: September 18, 2015

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precursor adsorbs dissociatively on hydrogen terminated silicon (100) substrates and on hydroxyl terminated surfaces as pointed out by Cho et al. and Rathu et al., respectively.17,18 The nature of the adsorption sites seems to be dependent on the substrate temperature, being hydroxyl groups at low temperatures and titanium dioxide at high temperatures where dehydroxylation is more pronounced.18 Fictorie et al.16 identified two competing reaction mechanisms for the decomposition reaction of adsorbed TTIP on a TiO2 surface by temperature-programmed desorption and molecular beam scattering experiments: either (a) an isopropoxyl ligand reacts with a surface hydroxyl group to form propene and water or (b) an oxygen atom of an isopropoxyl group can abstract a terminal hydrogen of another isopropoxide ligand to form propene and isopropanol. In both reactions titanium dioxide is formed on the surface. Cho et al.17 have found similar results for the decomposition of TTIP on hydrogen terminated Si(100) surfaces. The addition of water to the process lead to the hydrolysis of the adsorbed TTIP species while forming isopropanol as pointed out by Rathu et al.18 Wu et al. found that the decomposition reaction is the ratedetermining step in the surface reaction of TTIP indicating that reaction byproducts rapidly desorb.15 The influence of the desorption of precursor molecules on the growth rate was reported by Taylor et al.10 The authors use a surface occupation rate equation to derive the reaction rates of TiO2 in high vacuum conditions at high temperatures in the absence of water, similar to the model we will present later. They find that precursor desorption is necessary to rationalize the decrease of film growth toward higher temperatures. Figure 1 illustrates the overall reaction scheme of the proposed surface kinetic model. The deposition process is

chemical vapor deposition. TTIP is a well-known precursor that has been used since the seventies in thermal CVD9 and since the nineties in HV-CVD10 or in ALD11 processes in combination with water. We have recently described how TTIP can be used in combination with water in a low temperature CVD process.12 HV-CVD is not only a promising technique to deposit functional oxides at high growth rates and crystalline quality,13 it also offers unique advantages for the study of surface kinetics. In this contribution, we propose a kinetic model for the surface reactions of TTIP during titanium dioxide formation. We use experimental data to fit kinetic parameters such as activation energies and pre-exponential factors. Subsequently, we apply the model to discuss the titanium dioxide growth in absence and presence of water and discuss different growth regimes. Finally we discuss the limitations of the kinetic model.

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EXPERIMENTAL METHODS All deposition experiments were performed in our semiautomated high vacuum chemical vapor deposition reactor on (001) oriented silicon substrates with their native oxide layer. The films were grown in two different regimes, a low temperature regime in the presence of water (175−225 °C), where hydrolytic reactions dominate the surface reactions and a high temperature (310−620 °C) regime in absence of water, where the film is formed by pyrolysis. The precursor impinging rates varied between 0.1 and 6.0 × 1015 cm−2 s−1 for TTIP and 4.5 and 9.0 × 1016 cm−2 s−1 for water. Film growth was in situ monitored by reflectometry indicating a constant growth rate after nucleation. The results of both experiments have been previously published.12,14 For details concerning the experimental and analytical procedures we refer to the corresponding publications. The titanium incorporation rate was quantified by energy dispersive X-ray analysis (EDX). We have shown previously that it is possible to evaluate thin film growth rates by this technique with an error of maximal 10% compared to Rutherford backscattering (RBS) measurements.17 In total we evaluated the growth rate and chemical composition of films grown under 363 different experimental conditions, i.e., we varied precursor fluxes and substrate temperatures. The experimental data separates in 77 data points for the high temperature regime (T > 300 °C) and 286 data points for low temperature depositions in combination with water (T < 300 °C). In the next section, we describe the surface kinetic model that was fitted to our experimental data. We used Wolfram’s MATHEMATICA software package to numerically minimize the sum of the absolute difference between experimentally measured and modeled titanium incorporation rate (∑|GRTi exp − GRTi mod|) using the Nelder−Mead algorithm. Surface Kinetic Model. In this contribution, we propose a surface kinetic model comprising pyrolytic and hydrolytic surface reactions for the deposition of titanium dioxide using TTIP and water. The elemental reaction steps of TTIP adsorption and subsequent decomposition have been subject of various publications; the most relevant for our experimental conditions have been published by Wu et al.,15 Fictorie et al.,16 Taylor et al.,10 Cho et al.,17 and Rathu et al.18 All publications confirm a rapid uptake of TTIP molecules on the surface upon exposure. The reported sticking coefficient, however, varies between 0.03210 and “close to unity”.15 The

Figure 1. Illustration of the surface kinetic model. The adsorption probability of TTIP and water molecules is described by a Langmuir isotherm and sticking coefficients; both adsorbed species may desorb prior to reaction. Titanium dioxide is formed either by pyrolysis of adsorbed TTIP or by water induced hydrolysis.

divided into two parts: adsorption of the precursor and water molecules and subsequent surface reaction of the adsorbed species. We assume that the process can be described applying a Langmuir isotherm for the adsorption of TTIP and water. Our experimental methodology does not allow to identify the precise nature of the surface species; in the case of TTIP it might be either the entire molecule or, according to above given references, a reacted fragment of the precursor molecule where one or two isopropoxyl ligands were dissociated during the adsorption process. For water, it might be either adsorbed water or surface hydroxyl groups. B

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Scheme 1. Rate Equations Describing Surface Kinetics of Titanium Dioxide Growth Utilizing TTIP and Water as Precursor Paira

Γ represents the surface coverages per unit area (Γ0x , total available surface sites; ΓTTIP, adsorbed TTIP molecules; ΓH2O, adsorbed water), JTTIP and JH2O the TTIP and water impinging rates, and k the reaction rate of the respective surface species (des, desorption). a

deposition process either mere TTIP or coexposed with water. Finally, we outline limitations of the surface kinetic model. Surface Kinetic Parameters. The surface kinetic parameters that fit the surface kinetic model best to our experimental findings are listed in Table 1. Figure 2 compares the measured

The maximum number of adsorbed molecules in one monolayer Γ0x (x = TTIP or H2O) is dependent on the respective molecular diameter. For TTIP, we assume 1.5 × 1014 cm−2, which is motivated by the estimation of the molecular diameter of TTIP of 7.9 Å by Sinha et al.19 The covered surface area of a single TTIP molecule is then 4.9 × 10−15 cm2. If the precursor molecules arrange in a hexagonal closest package, Γ0Ttip follows as given above. The molecular diameter of water is 2.5 Å,20 which leads to a maximal number of adsorbed water molecules Γ0H2O of 1.5 × 1015 cm−2. Our model implies that TTIP and water molecules can coexist on the substrate surface and that surface coverage of TTIP does not influence the adsorption or desorption of the water and vice versa. Analogue to Taylor et al.10 the kinetic model comprises the desorption of this adsorbed species. We believe that this reaction could be the reaction of previously liberated water (or impinging water) with the surface to break a Ti−O−Ti bond and the subsequent desorption of Ti−OH containing surface species (most probably still comprising some iPrO ligands). The adsorbed precursor species can decompose pyrolytically, which leads to the formation of titanium dioxide and the generation of a new adsorption site. We describe the surface reactions kinematically by a first and a second order reaction term with respect to the surface coverage because we found previously that the pyrolytic surface reaction of TTIP molecules cannot be solely explained assuming first order reactions.14 Furthermore, an adsorbed precursor molecule can be hydrolyzed by adsorbed water molecules. Scheme 1 presents the rate equations for the surface concentration of adsorbed TTIP species (ΓTTIP) and adsorbed water (ΓH2O). In a steady state CVD process the surface densities are constant, i.e., dΓx/dt = 0. The temperature dependence of the reaction rates is taken into account by an Arrhenius rate expression of the form k = A exp(−ΔH/RT), where ΔH are the enthalpies of the respective reactions, A the corresponding frequency or pre-exponential factors, T the absolute temperature, and R the universal gas constant. The growth rate corresponds to the number of incorporated titanium atoms into the thin film. It is calculated by

Table 1. Surface Kinetic Parameters Fitted to the Surface Kinetic Modela frequency factor [s−1] decomposition desorption sticking coefficients

hydrolysis pyrolysis TTIP water TTIP water

4.7 1.3 2.0 5.0

× × × ×

activation energy [kJ mol−1]

10−7 cm2 103 cm2 1010 109

86 185 114 81 0.30 0.91

a The fitting is based on 363 measurement points consisting of substrate temperature, TTIP, and water impinging rate and measured growth rate.

growth rates with the values that we calculated using the fitted parameters. Generally, the kinetic model matches the experimental results quite well. We found that the contribution of film formation due to first order pyrolytic surface reactions of adsorbed TTIP species is negligible as compared to the second order contribution or to the hydrolytic reaction rate. In consequence, we used only the second order reaction term for the final fitting. This is surprising since Wu et al.,15 Fictorie et al.,16 and Taylor et al.10 found a better agreement between experiment and model assuming first order kinetics. The results are, however, in accordance to previous evaluations of our experiments where we concluded that second order reactions are of importance in the decomposition reaction.14 The apparent activation energy for the second order pyrolytic reaction is 185 kJ/mol; due to the different reaction order a comparison with previously determined energies (85 or 135 kJ/mol)10,15,16 assuming first order kinetics is not possible. The activation energy for the hydrolytic reaction is 86 kJ/mol; this value is within the error range of the previously determined value (87 ± 7) kJ/mol.12 The pre-exponential factor for the pyrolytic reaction (1.3 × 103 cm s−1) is significantly higher than the frequency factor for the hydrolytic reaction (4.7 × 10−7 cm s−1), which is partially explained by the different number of assumed adsorption sites for TTIP and water. The sticking coefficient of TTIP was fitted with 0.30, the one of water with 0.91. Herman et al. reported a sticking coefficient of unity for the adsorption of water on titanium dioxide.21 We

(1) (2) Ninc = k pyrolysis ΓTTIP + k pyrolysis ΓTTIP 2 + k hydrolysis ΓTTIPΓH2O

(1)

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RESULTS AND DISCUSSION In the following section, we present the results of the fitting procedure. Then, we discuss the surface kinetics of TTIP in the C

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ylation of titania (78 kJ/mol).22 The pre-exponential factors are very similar (2.0 × 1010 and 5.0 × 109 s−1 for TTIP and water, respectively.) Figure 3 compares experimental with modeled incorporation rates for different precursor fluxes as a function of temperature

Figure 3. Titanium incorporation (growth) rate as the function of substrate temperature and TTIP precursor impinging rate. Experimental data (points) and surface kinetic model (lines) show excellent agreement, indicating the high validity of the surface kinetic parameters for modeling film growth kinetics at high temperatures.

in the high temperature regime. The average error for the high temperature depositions is 8%. Less good is the agreement for water assisted low temperature depositions, where the average absolute deviation is 26%; the average deviation for all the fitted points is 22%. The presence of water increases the complexity of the process as more reaction pathways are present; not all of them might be well described by the surface kinetic model. Because of the higher surface occupation at low temperatures also multilayer adsorption might occur, which is currently not considered in the model. Despite the assumptions and simplifications made, we are confident that the experimental growth rates of titanium dioxide are well described by the surface kinetic model. Surface Kinetics without Water Exposure. In this and the following sections, we use the surface kinetic model to analyze the experimental data and to make quantitative predictions of deposition rates of titanium dioxide. Furthermore, we investigate the growth limiting factors. We start with the simple case, where the substrate is only exposed to TTIP. This situation represents the classical high temperature CVD of titanium dioxide. Figure 4a depicts the growth rate and surface coverage for different TTIP impinging rates as a function of the substrate temperature. The lower temperature limit for film growth (and precursor decomposition) is at about 250 °C, which is comparable to previously described values in ALD literature.18 The growth rate increases with increasing substrate temperature. It reaches its maximum with highest incorporation rate where the increasing pyrolysis rate is not yet counterbalanced by the increasing desorption rate. Further increase of the temperature reduces the growth rate due to dominating precursor desorption. At low substrate temperatures, the growth rate is independent of the precursor impinging rate, indicating that the chemical reaction rate is the limiting factor.

Figure 2. Comparison of experimental titanium incorporation rates on the left (results published previously12,14) and the results of the surface kinetic model on the right demonstrates that the model is capable of describing the actual thin film deposition experiments adequately in the covered parameters range.

believe this discrepancy is due to an overestimation of the water impinging rate, which is then counterbalanced by the small decrease of the sticking coefficient. One reason could be a slight misalignment of the substrate during the experiments. Another possibility is the overestimation of the water partial pressure in our precursor delivery system, for example, by residuals of dissolved nitrogen or oxygen despite rigorous outgassing. Consequently, the water impinging rate might be actually lower than measured. In our model, the sticking coefficient is temperature independent. It is also possible to fit the model using a two step adsorption process consisting of a physisorption and a subsequent activated adsorption step (chemisorption). The sticking coefficient is then dependent on the desorption rate from the physisorbed state kdes TTIP,phys and the chemisorbing rate kadsorb: s = kadsorb/(kadsorb + kdes TTIP,phys). Fitting of the data leads to comparable activation energies for desorption and chemisorption. If the activation energies are comparable, their absolute value cannot be reliably determined and the sticking coefficient de facto stays constant. As a consequence, we decided to describe the adsorption process with a temperature-independent sticking coefficient. The activation energy for the desorption of TTIP was derived as 114 kJ/mol and is lower than reported previously by Taylor et al. (240 kJ/mol).10 For the desorption of water, the fitted value of 81 kJ/mol corresponds to the reported activation energy of dehydroxD

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Figure 5. Area plots showing temperature dependence of the amount of the TTIP precursor molecules following certain reaction paths after arriving at the substrate for a precursor flux of 1015 TTIP cm−2 s−1.

Figure 4. (a) Titanium incorporation rate curves and (b) TTIP surface occupation curves as a function of substrate temperature at three TTIP impinging rates of 0.5 × 1015, 1 × 1015, and 3 × 1015 molecules/cm2 in the absence of water are presented, where the growth is driven by pyrolysis of the adsorbed TTIP species. For convenience in (a) the right vertical axis shows an equivalent growth rate, assuming a film density of 4 g cm−3.

probabilities of a TTIP molecule to pyrolyse, hydrolyze, desorb, etc. At low substrate temperatures the substrate is fully occupied by adsorbed TTIP species; therefore, all impinging precursor molecules could not be accommodated and rebounce directly from the precursor ligand terminated surface without interaction with the actual film surface. As temperature increases, the surface coverage starts to decrease due to onset of desorption and thermal decomposition of the adsorbed species. While most of the precursors still cannot be accommodated due to the occupied surface, more and more impinging molecules directly interact with the substrate surface. Here, they do not stick due to the low sticking coefficient (0.3), they decompose and form the film by a pyrolytic surface reaction, or they desorb and leave the substrate. Of special interest in CVD processes is the transition between chemical reaction and mass transport/desorption controlled regimes. It is, however, challenging to precisely define the borders of these regimes. We assume that the steady state surface coverage of adsorbed precursor species is a useful way to differentiate the regimes. Figure 6 depicts the experimental conditions that lead to different surface coverages (1%, 10%, 50%, 90%, and 99%). For very low surface coverages, i.e., at low precursor fluxes and high temperatures, the growth is clearly limited by the amount of precursors that can arrive and interact at the substrate surface. Here the growth is mass

At elevated temperatures, higher impinging rates lead to higher growth rates. We could not identify experimental conditions, where the growth rate is nearly independent of the substrate temperature. In conventional CVD this regime is commonly referred to as mass flow controlled regime.1 In HV-CVD such a regime is not visible because desorbed precursors have no possibility to reenter the process and interact multiple times with the surface. In CVD, readsorption of the precursor molecules is possible, and hence, desorption of the precursor molecules has less impact on the growth kinetics. Therefore, the reaction rate is decreasing with increasing temperature in HV-CVD and the growth rate becomes limited not only by the mass flow but also by the precursor desorption. The relative surface coverage of adsorbed TTIP species is depicted in Figure 4b. It is dependent on the relation between precursor impinging rate, desorption rate, and surface reaction rate. At low temperatures, desorption and reaction rates are low compared to the precursor flow; hence, the surface is fully occupied and the growth is chemical reaction limited. As temperature increases, surface occupation decreases due to higher decomposition and desorption. When increasing the precursor impinging rate, the substrate temperature threshold that leads to a reduction of the surface occupation increases as well. Consequently increasing the precursor flux increases the transition temperature between chemical reaction limited and desorption limited regimes. Investigating the limiting factors during the process allows a better understanding of the different growth regimes. We can classify all precursor molecules in the system into four groups: (1) molecules that reacted to form the thin film, Ninc from eq 1; (2) molecules that desorb from the surface before decomposition, Ndes = kdes TTIPΓTTIP; (3) molecules that could not be accommodated on the surface because of high surface coverage and rebounce directly, Nrb = JTTIP(ΓTTIP/Γ0TTIP); and (4) molecules that hit the empty site but do not stick due to the less than unity sticking probability, Nns = JTTIP(1 − sTTIP). These four groups are illustrated in Figure 5. It is visible that the fraction of precursor molecules that actually contribute to the film growth is relatively small. If normalized to the precursor impinging rate, the values can also be regarded as the

Figure 6. TTIP deposition parameter ranges for pure chemical reaction limited and mass transport/desorption limited regimes assuming 99% (or 90%) and 1% (or 10%) surface coverage thresholds, respectively. Gradual transitions between regimes occur in between the assumed threshold pairs. Surface coverage of 50% can be used to roughly separate the regimes. E

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previous section. At even higher temperature, the growth rate is limited by the increasing desorption of the precursor itself. In Figure 7b, we illustrate the surface coverage of TTIP molecules. It is visible that the surface coverage generally decreases upon water exposure due to the enhanced reactivity of the adsorbed species. We can interpret the exposure of the substrate with water during the deposition as addition of chemical energy to the process, comparable to an increase of substrate temperature. In other words, the addition of water decreases the threshold temperature of the titanium dioxide formation; it allows the growth of films at lower temperatures. The comparison of surface occupation indicates furthermore that the transition between chemical reaction limited and desorption limited regime shifts toward lower temperatures in the presence of water. While decreasing the substrate temperature is interesting for the deposition of adhering functional films, optimal growth conditions for the desired film functionality still need to be investigated for each specific application. Film properties may differ significantly between water enhanced TTIP decomposition and purely pyrolytic deposition in absence of water. In Figure 7c, we categorize the impinging TTIP molecules into different reaction pathways similar as we did in Figure 5. Generally, we observe the same trend: only a relatively small fraction of precursor molecules actually contribute to the formation of the thin film. Due to the lower temperature threshold for titania deposition when coexposing the substrate with water, the precursor can be used more efficiently at low temperatures. Limitations of the Kinetic Model. Here, we review briefly the limitations of the kinetic model. We have determined kinetic parameters based on the experimental data within the following range of conditions: substrate temperature, 175−620 °C; TTIP impinging rate, 0.05−5.9 × 1015 cm−2 s−1; and water impinging rate, 4.5−9.0 × 1016 cm−2 s−1. The prediction of the growth rate far outside of this parameter zone might not be precise due to increased importance of processes, which were not considered in the model or not apparent enough to be properly fitted by the kinetic model. Furthermore, we made two major assumptions: (a) the process follows Langmuir kinetics excluding multilayer adsorption and (b) surface diffusion is not taken into consideration. Making these assumptions enabled us to postulate a rate equation system that we could fit to our experimental data, but implicitly neglects other possible reaction pathways. The restriction that only adsorbed TTIP molecules can react with adsorbed water molecules neglects direct reactions of impinging molecules with adsorbed species. It is not clear to what extent a direct hydrolysis reaction has to be taken into account. Such a hydrolysis could contribute to the deposition rate, and therefore, the hydrolysis reaction rate might be overevaluated during fitting due to the negligence of other reaction paths. The model does not comprise surface diffusion. Though in practice, surface diffusion of water on hydroxyl groups could enable hydrolytic reaction of distant chemisorbed TTIP precursors. In consequence we might overestimate the quantity of water on the surface or underestimate the water desorption rate.

transport limited. At high surface coverages, the chemical reaction rate is the most important parameter. Independent of the assumed transition surface coverage, it is evident that the transition temperature increases with increasing TTIP impinging rate. Surface Kinetics with Water Exposure. The additional exposure with water enables the hydrolytic reaction path for decomposition of the adsorbed precursor. Figure 7 illustrates

Figure 7. (a) Titanium incorporation rate, (b) steady state surface coverage, and (c) precursor reaction pathways as a function of substrate temperature at constant TTIP impinging rate (1015 cm−2 s−1) and different water impinging rates. With increasing water flux, the hydrolytic contribution to the growth rate increases.

(comparable to Figures 4 and 5) the deposition rate (GR), surface coverage, and precursor molecule reaction paths as a function of temperature for different water impinging rates but constant TTIP flux. The reason for the dominant hydrolytic reaction pathway at low temperature is the substantially lower activation energy of the process (86 kJ/mol compared to 185 kJ/mol). The growth rates of titanium dioxide for two different water fluxes are shown in Figure 7a. To efficiently support the surface decomposition of TTIP, the substrate has to be exposed to significantly higher water amount as compared to TTIP. If the water exposure supersedes the exposure of TTIP by only a factor of 10, the growth does not significantly increase, while adding 100 times more water, the TTIP dose efficiently supports the growth rate. At low temperatures, the hydrolysis reaction and the incorporation rates are limited by the hydrolytic reaction rate as can be seen in Figure 7a. With increasing temperature, the growth rate increases until the residence time of water decreases significantly due to increased desorption. In consequence, the hydrolytic contribution of the growth rate decreases and the pyrolytic reaction dominates. This water desorption limited regime is comparable to the TTIP desorption limited regime that we discussed in the F

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CONCLUSIONS We investigated the surface kinetics of titanium dioxide deposition using titanium tetra isopropoxide (TTIP) and water in chemical vapor deposition processes at substrate temperatures between 175 and 610 °C by combinatorial high vacuum chemical vapor deposition (HV-CVD). We proposed a surface kinetic model to describe the deposition process and derived surface reaction activation energies for hydrolysis, pyrolysis, desorption, and chemisorption of TTIP by fitting the model to experimental data, which we reported earlier. We found that a second order reaction term is necessary and sufficient in order to fit the experimental results for the pyrolytic decomposition. Furthermore, the film growth rates, surface occupations, fractions of precursor molecules following certain reaction paths, and transitions between different growth regimes were illustrated and analyzed. We also discussed the influence of water on the process. The presence of water shifts the threshold of deposition to lower substrate temperatures. Accordingly, it also shifts the transition between the above-mentioned two regimes toward lower temperatures. Effectively, adding water during the process is equivalent to the increase of substrate temperature in terms of reaction rates and adds further degrees of freedom for the engineering of material properties. The derived activation energies could represent important results for the optimization of CVD and ALD processes, allowing more exact prediction of conformality for the first and better understanding of the required exposure times for the latter one. The proposed kinetic model together with kinetic parameters can also be used for the simulation of precursor behavior in CVD and ALD processes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07177. Experimental data from our HV-CVD experiments that we used to fit the surface kinetic model (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: patrik.hoff[email protected]. Phone: +41 58 765 62 62. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the partial financial support of the Swiss National Science Foundation under contract 200021_13504.

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ABBREVIATIONS CVD, chemical vapor deposition; ALD, atomic layer deposition; HV-CVD, high vacuum chemical vapor deposition; TTIP, titanium tetra isopropoxide

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REFERENCES

(1) Jones, A. C.; Hitchman, M. L. Chemical Vapour Deposition: Precursors, Processes and Applications; Royal Society of Chemistry: London, U.K., 2009. (2) Yanguas-Gil, A.; Yang, Y.; Kumar, N.; Abelson, J. R. Highly Conformal Film Growth by Chemical Vapor Deposition. I. A G

DOI: 10.1021/acs.jpcc.5b07177 J. Phys. Chem. C XXXX, XXX, XXX−XXX