Effect of Al Electronic Configuration on the SiO2 Thin Film Growth via

Oct 10, 2013 - Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore ... In the first half, the trimethylaluminum mo...
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Effect of Al Electronic Configuration on the SiO2 Thin Film Growth via Catalytic Self-Assembling Deposition Gang Ni,† Bo Han,*,† and Hansong Cheng*,†,‡ †

Sustainable Energy Laboratory, China University of Geosciences Wuhan, 388 Lumo Road, Wuhan 430074, China Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore

‡

S Supporting Information *

ABSTRACT: A self-assembling deposition process of SiO2 thin film growth catalyzed by Al with a small silanol precursor was systematically studied using density functional theory. The full catalytic self-assembling deposition (CSD) cycle is divided into two half reactions. In the first half, the trimethylaluminum molecule undergoes a dissociation process on the hydroxylated SiO2(001) surface that results in the anchoring of an −AlCH3 species on the surface and the sequential elimination of two CH4 molecules. Subsequently, in the second half of the reaction, two reaction routes, i.e., the top-down and the bottom-up routes, were examined to address the growth mechanism of the chain extension with bis(methoxyl)-monobutoxylsilanol. Our results suggest that the bottom-up route is energetically preferred with a strong influence by the catalytic effect of the seed layer of the Al species. The sp2 electronic configuration of the Al atom allows its pz orbital to accept electron from the lone pair of the silanol precursor, which facilitates the Al−O formation. The electronic configuration of the Al atom was found to undergo sp2 → sp3 → sp2 evolution cycles along the reaction pathway, each of which produces one layer of a Si−O unit to grow the chain. Our results are consistent with the experimental observations and provide a detailed mechanistic understanding on the CSD processes. film can be obtained with a deposition rate as high as 12 nm per reaction cycle. It was reported that each Al atom is capable of reacting with more than one TBS per cycle, leading to a thicker film. Using tris(tert-pentoxy)silanol (TPS) and TMA, Burton et al. investigated the influence of temperature, silanol pressure, and silanol exposure time on the thickness of deposited SiO2 films.21 It was reported that a higher pressure of silanol, which facilitates the diffusion of the precursor molecules, and a lower substrate temperature, which enhances the surface adsorption, can be beneficial for raising the growth rate of silicon oxide thin films. The same precursors (TMA and TPS) were utilized by Won et al. to investigate the relationship between the density of the catalyst layer and the growth rate of SiO2 coatings.22 It was revealed that by reducing the catalyst coverage a growth rate of more than 20 nm per cycle can be achieved with the TMA pulse time as short as 0.08 s. The large space available for the precursor diffusion, which is provided by the low density of the catalyst, enables growth of a thicker SiO2 film per cycle. These previous studies convincingly demonstrate that there is a strong correlation between the deposition rate and the catalytic effects of Al in propelling the growth of SiO2 films. It was proposed that the Lewis-acid character of the Al atom

1. INTRODUCTION Uniformity and conformality are two essential attributes required for silicon dioxide (SiO2) thin films for high performance electronic devices. These films can be prepared via a variety of techniques, such as chemical vapor deposition (CVD),1,2 atomic layer deposition (ALD),3−5 and plasmaenhanced chemical vapor deposition (PECVD),2,6,7 which have been widely utilized in the semiconductor industry for applications such as wire grid polarizer,8 nanomaterials coating,9−11 and protective layers against diffusion.12,13 With the increasing demand for smaller feature sizes, low temperature deposition has become an essential requirement to ensure growth of high quality thin films. To this end, ALD has been recognized as the preferred technique. Unfortunately, the low deposition rate of ALD, which typically allows only less than 1 nm thin film to be coated in one ALD cycle, is deemed to be the bottleneck for large scale applications of ALD. To overcome the shortcoming, a similar technique based on catalytic self-assembling deposition (CSD) was proposed for deposition of SiO2 thin films with a substantially higher deposition rate in the presence of metal catalysts, such as Al,8−15 Hf,16−18 and Zr.17,19,20 Significant efforts have been made to address various aspects of this technique in recent years. For instance, using tris(tert-butoxyl)silanol (TBS) and trimethylaluminum (TMA) as precursors,14 Hausmann and coworkers demonstrated that a conformal amorphous SiO2 thin © 2013 American Chemical Society

Received: June 13, 2013 Revised: October 8, 2013 Published: October 10, 2013 22705

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dictates the catalytic reaction,14 although detailed mechanisms of the surface reactive processes remain to be fully understood. In this paper, we report a simulation study based on density functional theory (DFT) to systematically address the role of Al in the CSD process using TMA and a simplified model of TBS as precursors. The main objective is to gain detailed understanding of the structural variation upon a CSD process and the associated reaction pathways. The catalytic mechanism is then examined based on the calculated electronic configurations of the catalyst center and the thermochemical energies and activation barriers.

2. COMPUTATIONAL METHODS The molecular size of TBS is relatively bulky. Computationally, it would require a large supercell to accommodate the molecular species on the surface. To simplify the computational model without losing the key chemical features of the molecule upon its reaction with TMA on the SiO2 surface, we replaced two of the three tert-butoxyl groups in the molecule with two methoxyl groups, and the reactivity of TBS toward TMA was examined with the remaining tert-butoxyl group associated with the silanol. Therefore, bis(methoxyl)-monobutoxylsilanol (BMMTS) was utilized as the silicon precursor to mimic the reactive behavior of TBS with TMA. Because the reaction with TMA occurs near the tert-butoxyl-silanol bond, the replacement is not expected to affect the reactivity significantly but allows us to use a smaller unit cell in our computational model (see Table S1, Supporting Information for detailed comparison). The fully hydroxylated SiO2 surface with the (001) orientation was selected as the substrate, on which the thin film grows. The SiO2(001) has been shown to be the preferred orientation in amorphous silica.23,24 The reconstructed and fully hydroxylated SiO2(001) surface was modeled as a slab containing six alternating layers, each of which includes two layers of O atoms and one layer of Si atoms. A rectangular surface unit cell was selected with the optimized cell parameters of a = 17.0192 Å and b = 14.739 Å. Between two neighboring slabs, an approximately 12 Å thick vacuum gap was inserted to avoid interactions between the slabs. The supercell of the selected surface contains 24 Si atoms, 60 O atoms, and 24 H atoms. Before the precursor adsorption, the surface was fully equilibrated. The surface model used in this paper has been widely accepted and validated for simulating surface reaction in many studies.23−28 The main bond parameters of the calculated surface structure, shown in Figure 1, were found to be in good agreement with the experimental values and previous DFT calculations.23,24 The electronic structure calculations were performed using DFT with the exchange-correlation functional proposed by Perdew and Wang.29 The projector augmented wave (PAW) method was used to describe the core electrons of atoms, and the valence orbitals were represented with a plane-wave basis set with a cutoff energy of 396 eV. Electronic energies were calculated with a self-consistent-field (SCF) with a tolerance of 10−4 eV. Geometry optimization was performed for all surface structures with the fixed coordinates of the bottom three layers of the substrate until the total energy of the system converged to within 10−3 eV. Only marginal changes in the surface structure were observed if the atoms of the bottom three layers were allowed to relax with the differences in bond lengths and bond angles of less than 0.02 Å and 3°, respectively. Transitionstate structures were obtained using the nudged-elastic-band (NEB) method30,31 to calculate the minimum-energy profile

Figure 1. The optimized structure of hydroxylated SiO2(001) surface: (a) side view, (b) top view. The yellow color is for Si atom, red for O atom, and white for H atom, respectively.

along the prescribed reaction pathways with the initial and final states chosen based on the optimized structures. In all cases, eight structural images were calculated along a prescribed pathway with a tolerance of 0.003 eV. The Brillouin zone integration was sampled within a 2 × 2 × 1 MonkhorstPack kpoint mesh,32 and electron smearing was employed using the Methfessel-Paxton scheme33 with a width of 0.1 eV to minimize the errors in the Hellmann−Feynman forces. All calculations were performed with the Vienna ab initio simulation package (VASP).34

3. RESULTS AND DISCUSSION The fully hydroxylated SiO2 (001) surface contains two types of hydroxyl groups as indicated in Figure 1b. Chemically, the more exposed hydroxyl species (labeled as O1) is more reactive toward silicon precursors than the embedded one (labeled as O2) due to the high electron density and high surface exposure of the O1 atom.23,24 This structural arrangement makes the O1 site on the surface more accessible for electrophilic attack by the precursor molecules. In the present study, BMMTS and TMA were chosen to be the precursors for the CSD process to deposit a SiO2 thin film. Here, TMA serves as the catalytic center upon anchoring on the surface, and BMMTS provides a source of silanol for the growth of the SiO2 layers. The optimized TMA, BMMTS, and TBS molecules are shown in Figure 2. The replacement of the two tert-butoxyl groups in TBS, which do not directly participate in the growth of the −O−Si−O− chain, by two methoxyl groups (BMMTS) does not give rise to a significant difference (less than 0.1 Å) in the main calculated bond parameters. In addition, we note that the Al atom in TMA adopts sp2 hybridization and is thus coplanar with the three C atoms. This electronic structure makes an empty 3p orbital of the Al atom readily available to react with the lone pair of the surface hydroxyl group. To understand the detailed growth mechanisms, the selfassembling process was broken into two half-reactions, as shown in Scheme 1. In the first half (step A), the TMA precursor is anchored on the substrate through a dissociative chemisorption process by reacting with the exposed hydroxyl 22706

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Figure 2. The optimized structure of precursors: (a) TBS; (b) BMMTS; (c) TMA. The white, red, yellow, and pink balls represent H, O, Si, and Al atoms, respectively.

Scheme 1. The Proposed CSD Reaction Mechanism: (A) the First Half Reaction and (B) the Second Half Reaction

Figure 3. The initial, transition, and final structures of the partial dissociative chemisorption of TMA on hydroxylated SiO2 (001) surface to form adsorbed Al(CH3)2 intermediate. The blue ball indicates the migrating H atom.

two Al−O bonds. The reaction results in elimination of two CH4 molecules from the surface. In the first step, a gas phase TMA molecule comes into contact with the hydroxylated SiO2(001) surface driven by the strong orbital interaction between the empty 3p orbital of Al in TMA and the electron lone pair of oxygen in the hydroxyl group. As expected, the O1 type of oxygen on the surface is more reactive than the O2 type largely due to the higher exposure toward the TMA molecule.35 An Al−CH3 bond in TMA is then broken as the Al atom forms a new bond with the O1 atom, while the H atom of the hydroxyl group simultaneously migrates to the C atom of the precursor to form a methane molecule. This process undergoes a fourmembered ring transition state, consisting of the Al−C−H−O atoms, with an essentially dissociated Al−C bond in TMA and a significantly elongated O−H bond on the surface. As the Al− C distance continues to decrease, the Al−O distance is shortened from 2.237 Å to 1.764 Å. As a consequence, an −Al(CH3)2 species is adsorbed on the SiO2 (001) surface with

species of the SiO2(001) surface, resulting in elimination of a CH4 molecule with the Al atom serving as the catalytic center for silanol insertions. Subsequently, in the second half (step B), a BMMTS molecule reacts with the Al−C bond to form an Al− O−Si species by librating a CH4 molecule from the surface. This Al−O−Si species is subject to continuous attacks by the BMMTS precursor to grow Al−O−Si−O−Si− chains via elimination of a t-butanol molecule until cross-linking among the chains occurs. The chain growth is then terminated and the Al catalyst center is no longer accessible by BMMTS. As a consequence of the self-limiting process, a layer of SiO2 is formed. 3.1. TMA Decomposition and Anchoring. Mechanistically, the dissociative chemisorption of TMA on the SiO2(001) surface takes two consecutive steps to complete the anchoring of a Al−CH3 species, which serves as the catalytic center on the surface. This requires the TMA molecule to react with two neighboring surface hydroxyl groups by sequentially forming 22707

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Figure 5b). The energy profiles of the reactions along these two pathways were then investigated separately. First, along Path (a), the −Al(CH3)2 species reacts with the OaHa group to form an O−Al(CH3)−O species (Figure 5a). A four-membered ring structure consisting of the Al−Oa−Ha−Ca atoms is gradually formed in the course of the dissociation. At the transition state, the Al−O distance becomes slightly longer than the value in step 1 (2.246 Å vs 2.121 Å). In the optimized final structure, the electronic configuration of the Al atom still remains to be sp2 with the optimized bond angles of O−Al−O and C−Al−O to be 111.5° and 123.6°, respectively. The −Al− CH3 species points up along the surface normal. The calculated main structural parameters are summarized in Table 2. Kinetically, the secondary dissociation of TMA is not as facile as the primary dissociation with the calculated activation energy of 1.23 eV, considerably higher than the value (0.23 eV) in step 1 (Figure 4). This is largely because the anchoring of the −Al(CH3)2 species on the surface limits its degree of freedom. As a consequence, despite the Al atom still adopting a sp2 configuration, the species is incapable of aiming the pz orbital of the Al atom at the lone pair of the oxygen atom of the hydroxyl group, with which the reaction occurs, to enhance the orbital interaction and thereby to reduce the activation barrier, as in the case of step 1. The process is again highly exothermic with the calculated thermochemical energy of −1.35 eV, close to the value of −1.39 eV in step 1. The reaction results in the formation of a −Al−CH3 species on the SiO2(001) substrate and the removal of a second methane from the surface. Along Path (b), the −Al(CH3)2 species can undergo another route with the metal being attacked by the OH group residing on the same substrate Si atom, as shown in Figure 5b. The main optimized structural parameters are presented in Table 2. The transition state structure along Path (b) (TS2b) consists of a six-membered ring consisting of the Cb−Al−O−Si−Ob−Hb atoms with a Al−Ob distance of 1.983 Å, which is significantly shorter than the analogous distance along Path (a) (2.246 Å). At the final state, the calculated O−Al−O angle is 86.4° with the Al−CH3 pointing to the surface normal, indicating that the structure suffers from a strong geometric strain arising from the highly stressed surface structure of the four-membered ring formed by the Ob−Al−O−Si atoms (Figure 5b, P2b). The severe geometric stress along this dissociation pathway is thus expected to give rise to unfavorable thermodynamics and kinetics. Indeed, the calculated activation barrier of 1.65 eV is significantly higher than the value for path a (1.23 eV) and, in contrast to Path (a), the process becomes endothermic, although the calculated reaction energy of 0.09 eV is quite modest. It is interesting that, despite the strong stress in the local bonding area, only slight relaxation was found in the rest of the surface upon the secondary dissociation of TMA. Comparing the two reaction pathways considered, we conclude that the secondary dissociation of TMA on the SiO2(001) surface most likely occurs by forming an Al−O bond with the −OH group residing at the adjacent Si atom (Path (a)) to minimize the stress of both the transition state and the final structures. 3.2. Catalytic Chain Growth. With the anchoring of the catalytic center, we now consider the second half of the reaction as outlined in Scheme 1 to examine the growth of the −O− Si(OCH3)2−O− chain catalyzed by TMA. Initially, the hydroxyl group of the BMMTS molecule, which was used to mimic the function of TBS for the growth of the thin film, reacts with the −Al−CH3 species on the surface to form a

a strong covalent Al−O bond, while a methane molecule is released to the gas phase. The optimized structures of the initial, transition, and final states are shown in Figure 3, and the main structural parameters are listed in Table 1. It is important Table 1. Main Optimized Bond Lengths (Å) and Bond Angles (°) of Step 1 of TMA Chemisorption label

R

TS

P

Al−C C−H O−H Al−O C−Al−O Al−O−H O−H−C H−C−Al

1.983 2.591 0.990 2.237 91.532 95.873 106.735 65.834

2.115 1.384 1.341 2.121 78.646 60.629 160.215 60.508

2.837 1.098 3.314 1.764 95.557 58.876 168.032 18.522

to note that the Al−O bond formed in the final state is significantly shorter than the Al−O bond in normal alumina (1.95 Å), indicating a strong interaction between the Al and the O1 atoms. The optimized surface structure of the final state reveals that the Al still maintains the sp2 configuration, which facilitates electron sharing between Al and O1. The process was found to be strongly exothermic with the calculated reaction energy of −1.39 eV, as shown in the first part of the calculated minimum energy profile in Figure 4. The primary decom-

Figure 4. The calculated potential energy profiles along the step 1 of the prescribed first half reaction pathways.

position of TMA is also kinetically facile with a relatively modest activation barrier of 0.23 eV. Only a small change in the surface geometry to accommodate the dissociative chemisorption of TMA is observed. Before the secondary dissociation of TMA to break up another Al−CH3 bond (the second step), which occurs in a similar fashion to the primary dissociation, the as-deposited surface species must reorient itself to adopt an optimal configuration to react with the adjacent −OH groups. Two possible reaction sites for the secondary dissociation of the surface species are then identified with an energy difference of 0.01 eV. One is for the Al atom to bond with the O atom (labeled as Oa in Path (a), R2a in Figure 5a) of the hydroxyl group residing on the adjacent Si atom, and another is for the Al atom to be attached to the O atom of the −OH species located on the same Si atom (labeled as Ob in Path (b), R2b in 22708

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Figure 5. The initial, transition, and final structures of the dissociative chemisorption of step 2 of TMA chemisorption. (a) Path a. (b) Path b. The blue balls indicate the migrating H atom.

Table 2. The Main Optimized Bond Lengths (Å) and Bond Angles (°) of Step 2 of TMA Dissociation Path (a) labels

R2

TS2a

P2a

a

Al−C Oa−Ha Ca−Ha Al−Oa Ca−Al−Oa Al−Oa−Ha Oa−Ha−Ca Ha−Ca−Al

1.949 0.994 3.063 3.400 66.202 42.973 86.635 62.151

2.133 1.023 1.659 2.246 73.766 62.633 156.584 63.963 Path (b)

4.216 3.160 1.093 1.704 73.989 75.019 143.276 15.862

labels

R2b

TS2b

P2b

b

1.956 0.994 3.224 3.292 71.856 108.121 83.481 88.388

2.112 1.327 1.363 1.983 78.909 64.184 150.979 59.572

3.616 2.598 1.096 1.775 78.397 74.159 177.687 29.586

Al−C Ob−Hb Cb−Hb Al−Ob Cb−Al−Ob Al−Ob−Hb Ob−Hb−Cb Hb−Cb−Al

a

Figure 6. The calculated potential energy profiles along the step 2 of the prescribed first half reaction pathways. The energies of the reactants are shifted to zero to facilitate the comparison.

CH4 elimination. The plane formed by the Al, O, and Si atoms is nearly perpendicular to the surface with the sp2 configuration of the Al atom essentially maintaining intact. Detailed analysis of the role of Al indicates that orbital interactions play a significantly role in the dissociation process. Prior to the silanol attack, the sp2 hybridized Al atom forms three σ-bonds with two O atoms on the surface and the C atom of the methyl group. On top of that, there is rather weakly delocalized π-conjugation among the Al atom and the two associated O atoms (Π43). To visualize the charge variation on the Al atom, we performed analysis on the differential charge density along the reaction pathway. The calculated differential charge density on the Al atom is displayed in the structures of the reactant, transition state, and the final product in Figure 8. Throughout the reaction, the Al atom maintains a σ-bond with the −CH3 species. The results clearly show that the Al atom loses electrons to the O atoms it bonds with. Here, the blue color indicates electron loss, and the yellow one represents charge gaining. The sp2 configuration of Al is turned into sp3 at the transition state as the lone pair on the O atom of the hydroxyl group of BMMTS donates charges to the empty pz orbital of Al, leading to a significantly enhanced electron density between these two atoms. This gives rise to an

bis(methoxyl)-mono-butoxylSi species and to eliminate a methane molecule from the surface.36 Subsequently, the silanol molecules may sequentially react with the bis(methoxyl)-monobutoxylSi species on the SiO2 substrate, leading to the continuous growth of the Si−O chain on the surface. In the first step of the catalytic reaction, the O atom of the hydroxyl group in BMMTS aims at the Al−CH3 bond with the O atom attacking the Al atom and the H atom of hydroxyl forming a methane molecule. This leads to the formation of a −Al−O−Si−R species, where R = O-tBu(OCH3)2. The calculated structures of the initial, transition, and final states of the first layer growth by silanol catalyzed by Al are shown in Figure 7, and the main optimized bond parameters are listed in Table 3. As the silanol molecule approaches to the catalytic center, both the Al−C and O−H bonds are weakened and the Al−O distance decreases. A four-membered ring structure is then formed at the transition state to minimize the activation energy with the reduced distance between Al and O of 2.014 Å. At the final state, both the Al−C bond and the O−H bond are broken, leading to the decomposition of the silanol and the 22709

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Figure 7. The initial, transition, and final structure of the Si precursor chemisorption process. The blue ball was the migrating H atom.

kinetically nearly barrierless (Figure 8). The reaction results in the formation of a stronger bond between Al and O and the cleavage of the weaker bond between Al and C and thus is expected to be thermodynamically favorable. The sp2 hybridization in Al is recovered upon completion of the reaction with the Al atom forming three σ bonds and one delocalized π-bond (Π64) with three O atoms. The process was found to be strongly exothermic with the calculated reaction energy of −1.44 eV (Figure 8). The results demonstrate that deposition of the first layer of silanol on the surface can be achieved with favorable thermodynamics and kinetics. 3.3. Chain Growth with Silanol. We next examine the growth of the −O−Si(OCH3)2−O− chain, which is necessary to grow a thicker film. Again, since the following reactions only focus on the Si−O bond of the −Al−O−Si(OCH3)2(O-tBu) species formed in the previous reaction, the tBu group was replaced by a methyl group to minimize our computational model. Admittedly, the replacement may reduce the steric hindrance between the as-deposited species and the precursor molecule. However, this effect is expected to be negligible due to the relatively long distance between the reaction center and the tBu group (larger than 2.7 Å). There are two possible reaction pathways for the precursor to react with the surface species. In the first pathway, the −OH group of the silanol molecule reacts with the methoxy group of the surface species from the top (the top-down route) with the chemical environment in the proximity of Al remaining intact. The optimized structure of the initial, transition, and final states are depicted in Figure 9, and the main structural parameters are shown in Table 4. At the initial state, the silanol molecule is attracted by the strong H-bonding interaction between the H atom in the precursor and the O1 atom of the adsorbed species on the surface, yielding an adsorption energy of −0.17 eV (Figure 10). Both the Si−O1 and O2−H bonds are gradually weakened as the Si−O2 distance decreases. A distorted fourmembered ring structure is then formed at the transition state with a significantly shortened Si−O2 distance of 2.181 Å. At the

Table 3. The Main Optimized Bond Length (Å) and Bond Angles (°) of Si Precursor Chemisorption labels

R

TS

P

Al−C C−H O−H Al−O C−Al−O Al−O−H O−H−C H−C−Al

1.933 2.901 0.970 4.209 60.252 60.372 135.616 102.722

1.993 2.018 0.994 2.014 84.990 85.192 124.328 65.489

4.559 1.096 2.837 1.658 52.286 98.590 144.108 11.740

Figure 8. The calculation potential energy profile of Si precursor chemisorption. The differential charge density of R, TS, and P on the Al atom is displayed on the optimized structures. The blue color represents charge loss and the yellow color stands for charge gain.

additional Al−O bond. The strong orbital interaction therefore facilitates the transition state formation. Indeed, the calculated activation barrier of 0.08 eV indicates that the process is

Figure 9. The optimized initial, transition state, and final structures along the top-down growth pathway. The blue ball represents the migrating H atom. 22710

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thermochemical energy of −0.18 eV, as depicted in Figure 10, indicating an elevated temperature is needed to activate this process. Alternatively, growth of the −Si−O− chains may also adopt a bottom-up approach, in which the −OH group of the silanol attacks the Al atom on the surface from the bottom (the bottom-up route), as shown in Figure 11a. The main optimized structural parameters are listed in Table 5. In this process, the

Table 4. The Main Optimized Bond Lengths (Å) and Bond Angles (Degree) of the Initial, Transition State and Final Structures along the Top-Down Growth Route labels

R

TS

P

Si−O1 Si−O2 O2−H O1−H O2−Si−O1 Si−O1−H O1−H−O2 H−O2−Si

1.645 3.600 0.980 2.122 58.506 98.565 165.097 36.477

1.733 2.181 1.022 1.575 68.762 87.358 120.160 82.678

3.583 1.628 2.168 0.977 61.027 33.451 170.609 94.864

Table 5. The Main Optimized Bond Lengths (Å) and Bond Angles (°) of the Bottom-Up Chain Growth Pathway labels a

Al−O Al−Ob Ob−H Si−Ob Si−Oc Si−Oa Oc−H Si−Ob−Al Ob−Al−Oa Al−Oa−Si Oa−Si−Ob

R

TS

P

1.669 4.169 0.985 1.675 1.632 5.404 2.559 137.808 88.637 85.359 47.744

2.360 1.723 2.913 1.621 3.205 1.744 0.984 110.115 72.628 82.325 94.485

4.230 1.670 4.776 1.624 4.551 1.637 0.984 161.092 11.924 43.650 106.899

Ob atom from the −ObH group of the BMMTS attacks the Al atom of the surface, while the H atom from the same −OH group aims at the Oc atom of the tert-butoxyl group of the precursor. At the initial state (R), the BMMTS molecule is attracted by the −OH group of the surface via the strong Hbonding interaction with the calculated H-bond distances ranging from 1.747 Å to 1.817 Å, which lowers the energy of the reactant by 0.69 eV (Figure 10). The interaction between the lone pair of the Ob atom and the empty 3p orbital of Al gives rise to a strong Al−Ob bond with a distance of 1.723 Å at the transition state (TS), at which a four-membered ring is formed with the Al−Oa bond elongated from 1.669 Å to 2.360 Å. Indeed, the calculated differential charge density of the transition state structure confirms that the electron density of the Al−Oa bond is modestly reduced, while the electron density of the newly formed Al−Ob bond is also enhanced (Figure 11b, TS). As a result, the Ob−H bond is broken with the calculated

Figure 10. The calculated potential energy profiles of the two possible thin film growth pathways. Here, the asterisk (*) represents the interaction energy between the surface and a precursor molecule at infinite separation.

final state, both the Si−O1 and H−O2 bonds are broken, leading to the elimination of a tertiary butanol from the surface and the formation of a new Si−O2 bond (1.628 Å), which extends the −Si−O− chain by one unit. It is worth noting here that the Al atom acts largely as a spectator, instead of a catalytic center, during the reaction process. As a result, this process needs an activation barrier of 0.89 eV with the calculated

Figure 11. (a) The optimized initial, transition state, and final structures. (b) Differential charge density of the Al atom displayed on the optimized structures for the bottom-up chain growth pathway. The blue color represents the charge loss, and the yellow color stands for the charge gain. 22711

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routes, i.e., the top-down route and the bottom-up route, were systematically examined to address the growth mechanism of the chain extension. Our results suggest that the bottom-up route is strongly promoted by the catalytic effect of the sp2hybridized Al species and thus energetically more favorable than the top-down route. It was found that the sp2 electronic configuration of the Al atom enables its pz orbital to accept electron from the lone pair of the silanol precursor to facilitate the Al−O formation. As the precursor approaches the catalytic site and goes through the transition state to form the final product, the electronic configuration of the Al atom undergoes a sp2 → sp3 → sp2 evolution cycle. Each cycle produces one layer of a Si−O unit to grow the chain. The calculated results are consistent with the experimental phenomena and provide detailed mechanistic understanding on the CSD processes.

distance increasing from 0.985 Å initially to 2.913 Å at the transition state, leading to the migration of the H atom from the Ob−H group to the tert-butoxyl group of the BMMTS. This reaction is facilitated by the evolution of orbital hybridization of the Al atom from sp2 at the initial state to sp3 at the transition state with the calculated activation barrier and reaction energy of 0.52 eV and −0.12 eV, respectively (Figure 10). The slight difference in the calculated surface reaction energies between the two reaction routes (−0.18 eV for the top-down route vs −0.12 eV for the bottom-up route) results from the different final products with methanol produced in the top-down route and isobutanol generated in the bottom-up route. At the final state (P), the four-membered ring is completely broken with the Al−Oa distance increased from 2.360 Å to 4.230 Å, leading to a strong Si−Oa covalent bond with the calculated bond distance of 1.670 Å. It is interesting to note that the orbital hybridization of Al is then fully recovered from sp3 at the transition state to sp2 at the final state following the cleavage of the Al−Oa σ bond and the formation of the conjugated π bond (Π64), as evidenced by the completely vanished electron density between the Al and Oa atoms and the significantly enhanced electron density of the Al−Ob bond. Comparing the two thin film growth pathways, it is clear that the bottom-up route is energetically more favorable than the top-down route. The catalytic participation of Al in the bottom up film growth route is essential to effectively lower the activation barrier. The orbital hybridization of Al undergoes a sp2 → sp3 → sp2 evolution cycle, each of which produces one layer of Si−O unit. By repeating this process, the film may grow sequentially with continuous infusion of silanol precursors. However, the bottom-up growth mechanism naturally allows only limited diffusion of the precursors from the gas phase to the active reaction sites as the chain length increases. It is expected that the growth of the silanol chains on the surface may become increasingly difficult. As a consequence, the film growth would become self-limited as condensation reactions between adjacent chains occur, which produce a SiO2 film as well as ethers liberated to the gas phase.

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ASSOCIATED CONTENT

S Supporting Information *

Comparison between the calculated energies of the catalytic chain growth reaction (Scheme 1B) using BMMTS and TBS is summarized in Table S1. Structural information including the BMMTS, TBS, and TMA precursors and the hydroxylatedSiO2(001) surface is also provided. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(B.H.) E-mail: [email protected] Tel: (86)27-6788-3431. *(H.C.) E-mail: [email protected] Tel: (65)6516-7761. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support of the research by a NUS start-up grant, a Singapore National Research Foundation POC grant, and the Singapore-Peking-Oxford Research Enterprise, COY-15-EWI-RCFSA/N197-1. Support from the National Natural Science Foundation of China (Nos. 20873127, 21203169, and 21233006), and the Fundamental Research Funds for the Central Universities, China University of Geosciences, are also gratefully acknowledged. We thank Professor Roy Gordon of Harvard University for stimulating discussions on the reaction mechanisms.

4. SUMMARY Catalytic self-assembling deposition is an efficient method for developing conformal SiO2 thin films with a significantly higher deposition rate than the film growth rate offered by atomic layer deposition technique. The quality of the films can be controlled through the design of a seed catalyst layer with a proper concentration and judicious choice of silanol precursors. Understanding the detailed deposition processes is important for effective control of thin film growth. In the present study, we performed systematic first principles simulations to understand the role of Al in the CSD process to grow a SiO2 thin film using TMA and BMMTS as the precursors of Al and silanol, respectively. The full selfassembling reaction cycle is divided into two half reactions. In the first half, the TMA molecule undergoes a dissociation process on the hydroxylated SiO2(001) surface that results in the anchoring of an −AlCH3 species on the surface and the consecutive elimination of two CH4 molecules. The −AlCH3 species forms two Al−O bonds with the hydroxyl groups from the two Si atoms adjacent to the reaction site to maintain a maximum orbital overlap between the Al and O atoms. We demonstrate that the reaction is energetically favorable. Subsequently, in the second half of the reaction, two reaction

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REFERENCES

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