Ab Initio Investigation of Surface Chemistry of Alumina ALD on

Article pubs.acs.org/JPCC

Ab Initio Investigation of Surface Chemistry of Alumina ALD on Hydroxylated γ‑Alumina Surface Aditya Shankar Sandupatla, Konstantinos Alexopoulos, Marie-Françoise Reyniers,* and Guy B. Marin Laboratorium voor Chemische Technologie, Universiteit Gent, Technologiepark 914, 9052 Gent, Belgium S Supporting Information *

ABSTRACT: Atomic layer deposition (ALD) of alumina using trimethyl aluminum (TMA) and water on amorphous alumina is analyzed using periodic-dispersion corrected DFT calculations. The energetics of the investigated reactions suggest that monomethyl aluminum (MMA) is the most abundant reaction intermediate at ALD operating conditions. The dominant reaction path toward the methylation of the surface is found to be adsorption of TMA on bridge oxygen via Lewis acid−base complex formation followed by ligand exchange reactions (LERs) with hydroxyls and surface water in its vicinity. Further adsorption and LERs of TMA leads to a saturated methylated surface (∼6.4 CH3 nm−2) which is in agreement with experimental observations and infrared spectra. The surface restructuring that is observed in almost all the reactions investigated seems to play an important role in the formation of conformal alumina films.

1. INTRODUCTION Atomic layer deposition (ALD) is a thin film deposition technique that offers excellent uniformity and conformality with a precise control over the film thickness and composition.1−3 In ALD, film growth proceeds through self-limiting surface reactions.3−5 These characteristics make ALD useful to produce catalysts for the chemical industry and to manufacture thin electronic chips in the microelectronic industry.6,7 Alumina ALD is typically performed using trimethyl aluminum (Al(CH3)3, TMA) and water as precursors.1,2,8 One deposition cycle consists of an exposure to a TMA pulse, a purge period, an exposure to a water pulse, followed by another purge period. Purging is done to remove excess precursors and gaseous reaction products and to avoid gas phase reactions. With TMA and water as precursors, alumina ALD can be generally represented by the overall reaction between Al(CH3)3 and water, as shown below, and is often considered to occur in two half cycles. In the first half cycle, TMA reacts with the hydroxylated surface producing a methylated surface, and in the second half cycle water reacts with the methylated surface, regenerating the hydroxylated surface. first half-cycle:

overall reaction: Al(CH3)3(g) +

During the TMA pulse, it is generally accepted8−11 that the gaseous precursor first adsorbs on the hydroxyls and bridge oxygens present on the hydroxylated surface. Depending on the temperature and the density of surface hydroxyls, adsorbed TMA can then undergo successive ligand-exchange reactions (LERs) with nearby surface hydroxyl groups, leading to the formation of Al−O bonds and the release of methane. In addition to ligand-exchange reactions, adsorbed TMA can dissociate over the bridge oxygen without methane formation. These reactions cause the formation of a methyl terminated surface. During the subsequent water pulse, water can adsorb near the methyl groups on the surface and can undergo ligandexchange reactions with them, leading to the formation of O− H groups and the release of methane. Alternatively, water may dissociate over unsaturated aluminum sites to restore the hydroxyl terminated surface. By repeating these cycles, alumina is deposited in a layer by layer manner on the substrate. However, it should be noted that alumina deposition occurs on the original substrate material at the start of the ALD process, while it can be considered to occur on an ALD grown alumina surface after several ALD cycles.3 Several theoretical models have been proposed in literature to understand the surface chemistry of alumina ALD. Small gas-

TMA (g) + hydroxylated surface → methylated surface + CH4(g)

second half-cycle: H 2O(g) + methylated surface

Received: March 11, 2015 Revised: May 19, 2015 Published: May 21, 2015

→ hydroxylated surface + CH4(g) © 2015 American Chemical Society

1 3 H 2O(g) → Al 2O3(s) + 3CH4(g) 2 2

13050

DOI: 10.1021/acs.jpcc.5b02382 J. Phys. Chem. C 2015, 119, 13050−13061

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The Journal of Physical Chemistry C

Figure 1. (a) Top view and (b) side view of the (2 × 1) supercell of the (100) hydroxylated γ-Al2O3 surface with a coverage of 8.8 OH nm−2. Active sites for adsorption of TMA are terminal hydroxyls (1, 6), surface water (2, 7), bridge oxygen (3, 4, 8, 9), and bridge hydroxyls (5, 10). Dashed line is the boundary of the (2 × 1) supercell. Color code for atoms of this surface model: aluminum = brown, oxygen = red, hydrogen = white. Bottom 16 Al and 24 O atoms were fixed, while the top atoms of the slab were relaxed.

would be computationally expensive. On the other hand, γalumina has a bulk density1 of 3.5 g cm−3 with a very poor crystalline nature indicated by its XRD showing broad and poorly resolved peaks.20 Additionally, both 4-coordinated and 6-coordinated aluminum are present in γ-alumina21 as in amorphous alumina. Therefore, we used a γ-alumina model in place of an amorphous alumina model. A bulk γ-Al2O3 unit cell was obtained by calcination of boehmite through topotactic transformation using molecular dynamics as presented in literature.22 This bulk unit cell, that inherited the crystallographic directions of boehmite, consists of 8 Al2O3 units in its lattice (a = 558.7 pm, b = 841.3 pm, c = 806.8 pm). However, the crystallographic directions of γ-alumina are typically given within the spinel orientation.20 Therefore, its thermodynamically most stable (100) surface within the spinel orientation, which corresponds to the (001) surface within the boehmite orientation, was created from the aforementioned bulk γ-Al2O3 unit cell by adding a vacuum gap in the z-direction. For the current calculations, a (2 × 1) supercell of the (100) surface of γ-alumina was used to avoid lateral interactions of the TMA unit with its periodic image. The hydroxylated surface21 with a coverage of 8.8 OH nm−2 was used for the investigation of the reactions with TMA. This configuration has terminal hydroxyl, bridge hydroxyl, bridge oxygen and adsorbed water groups on the surface (Figure 1). These fragments can be present on the surface at the end of the water pulse.10,23,24 The water present on the hydroxylated alumina surface model will be called surface water to distinguish it from the water that comes from the gas phase during the water pulse. 2.2. Periodic DFT Calculations. Spin-unpolarized periodic density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP). The package uses the projector augmented wave method and planewave basis sets to describe the electron ion interaction.25−28 The Perdew−Burke−Ernzerhof functional was used to calculate the exchange and correlation energies within the generalized gradient approximation (GGA) to account for nonlocality.29 A plane-wave energy cutoff of 400 eV and a Gaussian smearing of 0.1 eV were used.30 The atoms were relaxed to their instantaneous ground state using a conjugate gradient algorithm with a force convergence criterion of 0.05 eV Å−1.

phase clusters representative of the terminal hydroxyl and methyl group were used to compute the energetics of the TMA and water half-cycle, respectively.12 However, only (1−2) Htransfer transition states for the ligand-exchange reactions could be investigated with these cluster models, while dissociation reactions and other ligand-exchange reactions with nearby surface groups could not be described with these cluster models. In order to account for the additional interactions and reactions possible on surfaces, Elliott and co-workers13−15 investigated the reaction with TMA on a fully hydroxylated αalumina (∼15 OH nm−2) surface model using periodic DFT calculations. In addition, they showed that TMA would dissociate over bridge oxygen by studying the reaction with TMA on a bare α-alumina (0 OH nm−2) surface model. However, these surface models neglect that, under typical ALD operating conditions, the alumina surface is known to be noncrystalline and partially hydroxylated (∼8.7 OH nm−2).10 In order to have a better analysis of the surface chemistry of each half-cycle, the reactions of TMA and water were investigated in this work over a surface model that is more representative of the actual amorphous partially hydroxylated alumina surface. Since surface water, hydroxyls and bridge oxygen are located next to each other over this surface, the possibility of TMA reacting with these different active sites was investigated. As more TMA molecules react with the surface, self-termination is expected to occur when the surface methyl groups shield the TMA molecules from accessing the alumina surface.3,16 Thus, a model for the saturated methylated surface was constructed in this study using a novel heuristic approach and its consistency was verified with the reported experimental observations.16,17

2. COMPUTATIONAL DETAILS 2.1. γ-Al2O3 Surface Model. In the ALD process, amorphous alumina layers are deposited18 and under typical operating conditions, a hydroxyl coverage of 8.7 OH nm−2 (at 473 K under vacuum) was reported on these layers.10 In this work, we wanted to investigate ALD reactions on the surface of deposited amorphous alumina. Although models of amorphous alumina are available in literature,19 working with these models 13051


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Figure 2. Top views showing optimized geometries for adsorbed TMA on active sites of the hydroxylated (100) γ-Al2O3 surface with 8.8 OH nm−2: (a) 1-terminal hydroxyl, (b) 2-surface water, (c) 3-bridge oxygen, and (d) 4-bridge oxygen. Color code for atoms of TMA: aluminum = blue, carbon = black, and hydrogen = yellow. Color code for atoms of γ-Al2O3: aluminum = brown, oxygen = red, hydrogen = white. Blue dashed line indicates the boundary of the (2 × 1) supercell of the hydroxylated (100) γ-Al2O3 surface.

and final hydroxylated surface and for the saturated methylated surface by numerically solving the partial Hessian matrix. The elements in the matrix are obtained by finite differencing of the forces at different points near the optimized geometry. The forces were calculated by displacing each atom in the positive and negative directions along the x, y, and z axes by the same small distance (1 pm). Infrared intensities were calculated by making use of the gradient of the dipole moment.37

Several geometries along the reaction pathway connecting the reactant and product were optimized using the nudged elastic band (NEB) method.31,32 The transition state on this reaction pathway was obtained by the dimer method33 with the same force convergence criterion. The Monkhorst−Pack division scheme34 was chosen to generate a set of k-points within the Brillouin zone. Using a 5 × 4 × 4 k-point mesh, the cohesive energy of bulk γ-Al2O3 was converged within 1 meV. Accordingly, a 4 × 4 × 1 k-point mesh was applied for the (2 × 1) supercell of (100) γ-Al2O3 surface. A vacuum gap of ∼1.5 nm was found sufficient to separate subsequent slabs, yielding surface energies that are converged within 0.001 J m−2. For the surface calculations, no symmetry was used and a dipole correction was included. In all these calculations, the bottom 16 Al and 24 O atoms were fixed while the top atoms of the slab and all the atoms of the adsorbed species were relaxed. All the energies were calculated at 0 K, and no zero-point energy corrections were included in the reported results. All the results presented in this work include dispersion corrections. Dispersion corrections calculated by the DFT-D2 method35,36 were added to the calculated DFT energies to account for longrange dispersion interactions. In order to compare with the available experimental spectra, vibrational frequencies associated with partially relaxed atoms were calculated for the initial

3. RESULTS AND DISCUSSION 3.1. Reactions of TMA on the Partially Hydroxylated Surface. The hydroxylated surface under ALD conditions can have bridge oxygen, terminal hydroxyl, and bridge hydroxyl and may even have a small quantity of water adsorbed on the surface,24 called surface water in the present work. Our surface model has all these features (Figure 1). During a TMA pulse, there are three kinds of reactions that could occur on the hydroxylated surface, namely, adsorption of TMA, ligand exchange reactions of TMA, and dissociation of TMA. 3.1.1. Adsorption of TMA. Upon exposure of the hydroxylated surface to the TMA pulse, TMA can adsorb on bridge oxygen, terminal hydroxyl, bridge hydroxyl, and surface water. In this step, the aluminum of TMA accepts a lone pair of electrons from oxygen to form an Al−O bond, resulting in a 13052


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The Journal of Physical Chemistry C Lewis acid−base complex formation.12,13 Adsorption was investigated only on the sites O1−O5 of our surface model (Figure 1) because these sites are identical to O6−O10, as a (2 × 1) supercell was used. Adsorption geometries are shown in Figure 2, and energetics are tabulated in Table 1.

TMA was observed in the case of cluster models12 and a fully hydroxylated (0001) α-alumina surface model13 because the lone pair electrons of the oxygen of these hydroxyls are available with less ease for the formation of an Al−O bond with the TMA. Overall, it can be concluded that the adsorption of TMA on hydroxyls and bridge oxygen is possible as long as they are not sterically hindered. 3.1.2. Ligand Exchange Reactions of TMA. Adsorbed TMA can react further with hydroxyls and surface water on the hydroxylated surface to form dimethyl aluminum (DMA), monomethyl aluminum (MMA) and unsaturated aluminum (UA) i.e., aluminum without a methyl group.10,13 These reactions are ligand exchange reactions (LERs), where a methyl group abstracts a hydrogen from the hydroxyl or surface water and is eliminated as methane. We investigated possible LERs of TMA adsorbed on O1, O2 and O3 with the hydroxyls and surface water in the close vicinity. An energy diagram of these reactions is presented in Figure S1 (Supporting Information). Since O3 is the most favorable adsorption site for TMA, we will focus our discussion on the LERs of TMA adsorbed on O3. Once the methyl group is removed as methane from TMA, DMA is formed with aluminum in a triply coordinated state. However, this DMA can rearrange in some cases to form additional Al−O bonds with nearby bridge oxygen or hydroxyl. Thus, the coordination number of the aluminum in DMA can increase after such a rearrangement. As seen from Table 2, the

Table 1. DFT Calculated Adsorption Energies (ΔEads in kJ mol−1) of TMA on Various Active Sites on the Hydroxylated (100) γ-Al2O3 Surface with 8.8 OH nm−2 adsorption site terminal hydroxyl (O1) surface water (O2) bridge oxygen (O3) bridge oxygen (O4) bridge hydroxyl (O5) data from literature on clusters representing hydroxyls (a) Al(OH)2−OH (b) Al(O−Al(OH)2)2−OH on periodic (0001) α-alumina surface on hydroxyl of fully hydroxylated surface (∼15 OH nm−2)

Figure 2a 2b 2c 2d

ΔEads (kJ mol−1) −138 −136 −213 −119 sterically hindered

−5012 −5912 −6813

Adsorption of TMA on terminal hydroxyl (O1) and on surface water (O2) yield similar adsorption energies. Adsorption of TMA on surface water (O2) causes a hydrogen transfer from this surface water (O2) to the neighboring terminal hydroxyl (O1) as can be noticed in Figure 2. Therefore, the Lewis acid−base complex formed on O2 is structurally very similar to the one formed on O1, which is the reason why similar adsorption energies are obtained for TMA adsorption on O1 and O2. On the other hand, TMA adsorption on bridge hydroxyl (O5) was not possible because the approach of TMA to this site was sterically hindered. Although structural rearrangements of the surface atoms were noticed upon adsorbing TMA on the bridge oxygen O3 (Figure 3) making this adsorption highly exothermic, no rearrangement was observed upon adsorption of TMA on O4. The Al−O bonds of O3 are longer than that of O4, which means that the Al−O bonds of O3 are weaker than that of O4. Upon adsorption of TMA on O3, one of the weaker Al−O bond breaks to form another Al−O bond with TMA. From these observations, it can be thought that the adsorption site preference is determined by the ability of the active site to undergo such structural rearrangements. Compared to the adsorption energies obtained on the partially hydroxylated surface (Table 1), a weaker adsorption of

Table 2. DFT Calculated Electronic Energies (kJ mol−1) for Ligand Exchange Reaction (LER) of Adsorbed TMA with the Nearest Hydroxyl or Surface Water on the Hydroxylated (100) γ-Al2O3 Surfacea reaction TMA adsorbed on O3 (i) reacting with terminal hydroxyl at O1 (ii) reacting with surface water at O2 (iii) reacting with bridge hydroxyl at O5 LER investigated on cluster models (i) A(OH)2−OH-TMA → Al(OH)2-O− DMA + CH4 (ii) Al(O−Al(OH)2)2−OH-TMA → Al(O− Al(OH)2)2-O−DMA + CH4 LER investigated on fully hydroxylated (0001) (∼15 OH nm‑2) ∥Al−OH-TMA → ∥Al−O-DMA + CH4

CN

Ea (kJ mol−1)

ΔEr (kJ mol−1)

5 3 4

112 77 92

−70 33 −66

3

41

−10512

3

50

−10512

α-alumina surface 4

8714

−11613

Ea = activation energy, ΔEr = reaction energy. CN is the coordination number of aluminum in the dimethyl aluminum (DMA) formed. a

Figure 3. Top view showing local surface atomic arrangements around bridge oxygen O3 and O4: (a) before TMA adsorption on O3, (b) after TMA adsorption on O3, (c) before TMA adsorption on O4, and (d) after TMA adsorption on O4. Color code for the atoms is the same as that used in Figure 2. 13053


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Figure 4. Schematic representation of LER of adsorbed TMA (R1), of DMA (R2), and of MMA (R3). Ea, ΔEr, and ΔEdes are the activation energy, reaction energy, and desorption energy, respectively, expressed in kJ mol−1.

coordination number of aluminum in the DMA formed can vary depending on the active site that adsorbed TMA reacts with. DMA remains triply coordinated in the case of reaction with the surface water at O2, while it reaches respectively a 4fold and a 5-fold coordination state in the cases of reaction with the bridge hydroxyl at O5 and with the terminal hydroxyl at O1. As seen in Table 2, LERs calculated in our work are more activated than in the case of cluster models,12 while the activation energy of the LER investigated on the fully hydroxylated (0001) α-alumina surface13 falls well within the range of activation energies calculated in our work. The undercoordination of the Al atom in the clusters is mainly responsible for the higher reactivity of these models compared to the extended surfaces. Moreover, within the same adsorption site for TMA, any variation observed in the activation energies of our calculated LERs is most likely due to differences in the orientation and distance between methyl and reacting hydroxyl or surface water. Of all the LERs investigated, LER of adsorbed TMA on O3 with bridge hydroxyl at O5 appears to be kinetically as well as thermodynamically more favored (Table 2). This LER is shown schematically as R1 in Figure 4. In this reaction, adsorbed TMA on bridge oxygen O3 has bridge hydroxyl on O5 in its neighborhood (Figure 5a). After hydrogen transfer from the bridge hydroxyl on O5 to the methyl of TMA, methane and DMA forms. However, this DMA species has aluminum in a triply coordinated state and is energetically unstable, as can be seen on the nudged elastic band (NEB) profile of the reaction in the Supporting Information (image a3 in Figure S2a). Therefore, this DMA rearranges and bonds to a nearby bridge oxygen to achieve a 4-fold coordination state (image a4 in Figure S2a). DMA formed from the first LER (R1) has a surface water in the neighborhood at O2 (Figure 5b). Thus, we further

investigated the LER of DMA with this surface water. The reaction is shown schematically as R2 in Figure 4. In this reaction, a methyl of DMA abstracts a hydrogen from the surface water at O2. This leads to methane, MMA and a hydroxyl at O2. Although the MMA produced in this reaction forms a bond with the hydroxyl at O2 to retain a 4-fold coordination state, this MMA is unstable because the aluminum is in a distorted 4-fold coordination state (image b3 in Figure S2b). Therefore, MMA breaks its bond with one of the bridge oxygen and forms a bond with another bridge oxygen (image b4 in Figure S2b). The consecutive LERs (R1 and R2) cause depletion of the hydroxyl environment around MMA. Even though MMA has a hydroxyl in its neighborhood at O2, it is located at a rather large distance (Figure 5c). This LER between MMA and hydroxyl at O2 was investigated and found to be highly activated. The reaction is shown schematically as R3 in Figure 4. UA formed in this reaction bonds with another bridge oxygen to retain its 4-fold coordination state. This UA is stable but has a higher energy than MMA due to its distorted tetrahedral geometry (Figure 5d). Although the methane formed in the aforementioned LERs (R1, R2, R3) is initially physisorbed, its desorption energy is rather small (between 15 and 20 kJ mol−1) and hence it can easily desorb. Overall, the energetics of the investigated LERs of adsorbed TMA on O3 show the following trends: activation energy of LER of DMA (R2) < activation energy of LER of adsorbed TMA (R1) ≪ activation energy of LER of MMA (R3), and reaction energy of LER of DMA (R2) > reaction energy of LER of adsorbed TMA (R1) ≫ reaction energy of LER of MMA (R3). This is in good agreement with Elliott and Greer13 who reported also the same trend for the reaction energies of LERs. These trends show that the formation of MMA would be favored thermodynamically and kinetically. Therefore, depending on the density of hydroxyls, the alumina surface would most 13054


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Figure 5. Energy profile for adsorption of TMA on bridge oxygen O3, the LERs, and dissociation on the hydroxylated (100) γ-Al2O3 surface. Top view of (a) TMA adsorbed on bridge oxygen O3 and products of dissociation and/or LERs, (b,e) DMA-dimethyl aluminum, (c,f,g) MMAmonomethyl aluminum, and (d) UA-unsaturated aluminum. Color code for the atoms is the same as that used in Figure 2. Path A is the dominant path toward the methylation of the surface.

TMA pulse over the partially hydroxylated surface consists of TMA adsorption followed by consecutive LERs. 3.2. Reactions of Water with Adsorbed TMA, DMA, MMA, and UA. Water introduces the oxygen necessary for the formation of an alumina layer in the ALD process. Reactions of water with adsorbed TMA, DMA, MMA and UA were investigated. The geometries were taken from the previous subsection (subsection 3.1): adsorbed TMA (Figure 5a), DMA (Figure 5b), MMA (Figure 5c) and UA (Figure 5d). 3.2.1. Adsorption of Water. When water approaches the surface, it adsorbs prior to reacting with the methyl groups. Musgrave and Widjaja reported a Lewis acid−base complex formation of water with the aluminum of the DMA and MMA species.12 However, they used clusters of DMA (Al(OH)− (CH3)2, Al(O−Al(OH)2)−(CH3)2) and MMA (Al(OH)2− CH3, Al(O−Al(OH)2)2−CH3) to study the water reactions. The aluminum in these clusters is in a 3-fold coordination state.

propably have an abundance of MMA species when exposed to TMA. 3.1.3. Dissociation of TMA. Elliott and Greer reported a spontaneous dissociation of TMA over a bridge oxygen on the bare α-alumina surface.13 Therefore, instead of adsorbed TMA directly undergoing LERs (Path A in Figure 5), we have also investigated reaction pathways that start with the dissociation of TMA adsorbed on O3 (Paths B and C in Figure 5). Upon dissociation, DMA gets bridged between O3 and O4 (Figure 5e), while the methyl group adsorbs on top of an aluminum on the surface. Subsequently, the methyl of DMA formed from the dissociation of TMA can be eliminated by LERs. Pathways considering LER with surface water O2 (Path C) and LER with bridge hydroxyl O5 (Path B) were investigated (Figure 5). However, based on the energy profiles of Figure 5, Path A is overall more favorable than Path B and Path C. Therefore, it can be concluded that the initial reaction pathway during a 13055


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Figure 6. Top views of TMA-modified surfaces before/after adsorption of water: (a) next to adsorbed TMA, (b) next to DMA, (c) next to MMA, and (d) on UA. Color code for atoms of adsorbed water: oxygen = green, hydrogen = orange. Color code for the rest of the atoms is the same as that shown in Figure 2. Hydrogen bonds are shown by yellow dotted lines. Energetics mentioned are expressed in kJ mol−1.

Figure 7. Schematic representation of the adsorption of water on the surface and LER with adsorbed TMA (R4), with DMA (R5), and with MMA (R6). ΔEads, Ea, ΔEr, and ΔEdes are the adsorption energy, activation energy, reaction energy, and desorption energy, respectively, expressed in kJ mol−1.

adsorbed TMA (Figure 6a). This surface aluminum achieves a stable octahedral configuration with adsorption of water. In the case of DMA, adsorption of water can occur by hydrogen bonding as shown in Figure 6b. This adsorption of water next to DMA causes a rearrangement of the DMA species. DMA rearranges and forms an additional bond with O2 (Figure 6b). Also, in the case of MMA, adsorption of water can occur by hydrogen bonding as shown in Figure 6c. Therefore, in the case of associative adsorption of water, water either forms a Lewis acid−base complex with the surface aluminum or hydrogen bonds in the neighborhood of the methyl groups.

Therefore, water binds with this aluminum of DMA and MMA more readily to gain a 4-fold coordination state. These clusters have terminal methyl groups with no neighboring hydroxyl groups. Therefore, the likelihood of hydrogen bonding of water could not be observed as a possible mode of adsorption in their study. In our case, aluminum in adsorbed TMA, DMA, and MMA is in a stable tetrahedral configuration (Figure 6). Therefore, water cannot form a Lewis acid−base complex with the aluminum of these species as in the reported cluster calculations.12 Instead, water was found to form a Lewis acid−base complex with the surface aluminum present close to 13056


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activation energy (158 kJ mol−1). This re-emphasizes that MMA is the most stable surface species during both half-cycles of ALD. 3.3. Model for Saturated Methylated Surface. As mentioned in subsection 3.1, the dominant path toward the methylation of the surface consists of TMA adsorption and ligand-exchange reaction steps. Therefore, an attempt was made to construct a model for the saturated methylated surface using the following heuristic approach. Starting from the end-point of the dominant path described in subsection 3.1 (Figure 5d), TMA was first adsorbed on an available active site. Then LERs were investigated only if the carbon of the methyl group was near to a hydrogen of hydroxyl or surface water by less than 250 pm. Otherwise, another TMA was allowed to adsorb on an available active site. This procedure was continued to the point that addition of TMA was sterically hindered by the methyls on the surface. 3.3.1. Adsorption of Second TMA Molecule on Bridge Oxygen O3 in the Presence of UA. We began our investigation with a surface having UA as a starting surface model (Figure S4a). This UA is bonded to the bridge oxygen O3. This structure was optimized with an additional TMA initially bonded to the bridge oxygen O3. However, it was found that this TMA adsorbs dissociatively with the methyl group of the TMA bridged between UA and aluminum of TMA (Figure S4b). Interestingly, a similar dissociative adsorption of TMA was reported by Elliott and Greer in the case of adsorption of TMA on a bare α-alumina surface.13 Since no hydroxyls were close to methyls of this dissociated TMA, adsorption of a third TMA on an available active site of this surface was considered. 3.3.2. Adsorption of Third TMA Molecule on Hydroxyl O6 and Subsequent LERs. As a next step, adsorption of another TMA was considered on O6 (hydroxyl) as shown in Figure S4c. Adsorption occurs via a Lewis acid−base complex formation between aluminum of TMA and oxygen of O6. Here, a methyl of adsorbed TMA and a hydroxyl at O7 are close enough to allow a LER. In this LER, methyl abstracts hydrogen from hydroxyl at O7 and is eliminated as methane. After methyl elimination, TMA is transformed to DMA (Figure S4d). Additionally, two hydrogen transfers on the surface during the LER were noticed. Transfer of hydrogen was observed from O1 (surface water) to O7 and another hydrogen transfer occurred from O6 (hydroxyl) to O2 (bridge oxygen). The DMA formed initially attains a metastable state with aluminum in a 3-fold coordination state and rearranges spontaneously to a tetrahedral configuration by forming a bond with O7. Desorbing methane from the surface did not result in any structural change of the surface (Figure S4e). The remaining methyls were not close enough to hydroxyls to allow a LER. Therefore, adsorption of a fourth TMA on this surface was investigated. 3.3.3. Adsorption of Fourth TMA Molecule on Hydroxyl O1 and Subsequent LERs. At this stage, adsorption of a fourth TMA was possible at O1 (hydroxyl). This adsorption also involved a Lewis acid−base complex formation between aluminum of TMA and oxygen of O1 (hydroxyl). This adsorption led to surface restructuring which involved complete dissociation of the methyl group that was previously bridged between Al1 and Al2 (Figure S4f). The remnant DMA fragment is bridged between O3 and O5. This surface restructuring involved also a transfer of hydrogen from O2 (hydroxyl) to O6 (bridge oxygen). In addition, a methyl group of DMA (bridged between O6 and O7) moved close enough to

Nevertheless, water is also found to adsorb dissociatively over UA (Figure 6d). This UA is in a distorted tetrahedral configuration which makes it very reactive. Water from the gas phase dissociates over this UA spontaneously producing hydroxyl on UA and on O2. 3.2.2. Ligand Exchange Reactions of Water. Water adsorbed in the neighborhood of adsorbed TMA, DMA and MMA can participate in a LER to remove surface methyls. During such a LER, one hydrogen from water is transferred to the methyl group, which is eliminated from the surface as methane. The remnant hydroxyl from water that remains on the surface is responsible for regenerating the hydroxylated surface. The investigated LERs of water with adsorbed TMA, DMA and MMA are discussed below. 3.2.2.1. LER of Water with Adsorbed TMA. Water adsorbed next to adsorbed TMA has one of its hydrogen atoms pointing toward a methyl of TMA (Figure S3 a1). The LER between this hydrogen of adsorbed water and the methyl of adsorbed TMA in its close proximity was investigated. The aluminum of the DMA formed at the end of this reaction is triply coordinated with a trigonal planar geometry (Figure S3 a4). The remnant hydroxyl from water remains adsorbed to the surface aluminum. This reaction is schematically represented as R4 in Figure 7. 3.2.2.2. LER of Water with DMA. Water adsorbed next to DMA on the surface can participate in a LER. This reaction is schematically represented as R5 in Figure 7. As we can see in Figure S3 b1, one hydrogen of the adsorbed water is oriented toward one of the methyl groups of DMA. The methyl group receives this hydrogen and is eliminated as methane. The remnant hydroxyl part of the reacting water abstracts a hydrogen from the surface water present at O1 (Figure S3 b2). As a result, another water molecule is formed on the surface (Figure S3 b3). This formed water is adsorbed on the surface with the formation of a Lewis acid−base complex with the surface aluminum. Although the MMA formed out of this reaction step is in a distorted tetrahedral configuration (Figure S3 b3), this MMA further rearranges. The aluminum of MMA breaks its bond with one surface oxygen and forms a bond with another surface oxygen (Figure S3 b4). This rearrangement leads to a more stable tetrahedral structure of MMA. 3.2.2.3. LER of Water with MMA. Water adsorbed next to MMA has its hydrogen atom oriented toward a methyl group of MMA (Figure S3 c1). The LER involving this hydrogen and methyl group was investigated. This reaction is schematically represented as R6 in Figure 7. UA and methane are formed upon abstraction of hydrogen by the methyl group. Upon elimination of the methyl group, the UA forms a bond with the bridge oxygen O3 to obtain a distorted tetrahedral configuration (Figure S3 c3). At the same time, the remnant hydroxyl from the reacting water abstracts a hydrogen from the surface water on O6 to form another water molecule. However, as seen from the reaction sequence R6, this formed water can further dissociate with an activation barrier of 17 kJ mol−1. As seen from the final surface formed after this reaction step (Figure S3 c5), the hydroxyl part binds to the UA, while the remnant hydrogen part of the dissociated water binds to O6. At the same time, UA breaks its bond with the bridge oxygen O3. From the energetics of these reaction steps, we see that the reaction of water with MMA requires a high activation energy (155 kJ mol−1). Nevertheless, as investigated in subsection 3.1, the reaction of hydroxyl with MMA required also a high 13057


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Figure 8. (a) Top view of the saturated methylated surface. (b) Top view of the hydroxylated surface formed after replacing the methyls of the saturated methylated surface with hydroxyls. Color code for atoms introduced by hydroxyls: oxygen = green, hydrogen = orange. Color code for the rest of the atoms is the same as that used in Figure 2. Aluminum deposited by first, second, third, and fourth TMA molecules are indicated as Al1, Al2, Al3, and Al4 respectively.

Puurunen et al.16 exposed the hydroxylated alumina surface to a TMA pulse at 473 K. The surface had a hydroxyl coverage of 8.7 OH nm−2 similar to that used in our study. At a reaction temperature of 473 K, they reported around 6 CH3 nm−2 at the end of the TMA pulse and an aluminum deposition of 4.6 Al nm−2. In our reported saturated methylated surface, there are six methyls and four aluminum atoms deposited on an area of 0.94 nm2. This means ∼6.4 CH3 nm−2 and ∼4.3 Al nm−2 which agrees with the reported values (Table 3). This methylated

the bridge hydroxyl O10 to consider studying a subsequent LER. LER between methyl of DMA and bridge hydroxyl O10 leads to MMA with aluminum in a 3-fold coordination state (Figure S4g). Desorbing methane from the surface does not lead to any surface restructuring (Figure S4h). At this point, the methyl group of TMA adsorbed on O1 is close enough to a hydroxyl (O7) to consider a subsequent LER. This LER triggers major surface restructuring upon methane formation (Figure S4i). DMA formed during this LER on O1 is metastable because aluminum is in a 3-fold coordinated state. Therefore, DMA rearranges and bonds with a bridge oxygen (O7), and one of the methyl group of this DMA dissociates from aluminum. This methyl group then bonds with the aluminum of MMA (Al3) bridged between O6 and O7. No further surface restructuring took place upon desorption of methane from the surface (Figure S4j). Any further LER could not be considered because none of the methyls present on the surface were close enough to the hydroxyls present on the surface. Addition of a fifth TMA molecule on this surface was sterically hindered by the methyls on the surface. Hence, a state of self-termination of the reactions with TMA was reached at a methyl coverage of ∼6.4 CH3 nm−2. The resultant surface (Figure 8a) could be considered as a representative model for the saturated methylated surface at the end of the TMA pulse. The LERs investigated in this section while constructing the methylated surface required lower activation energies in the range of 12−26 kJ mol−1 (Figure S4) as compared to the LERs investigated in subsection 3.1.2 (Figure 5) which had activation energies in the range of 66−158 kJ mol−1. This shows that LERs become kinetically favored once methyls in the neighborhood are formed from reactions with TMA. 3.4. Comparison of Calculated and Observed Properties. In the pursuit of forming a saturated methylated surface (Figure 8a), four TMA molecules reacted with the initial hydroxylated surface. These four TMA molecules possess 12 methyl groups of which 6 methyls remain on the methylated surface while the other 6 methyls are eliminated as methane molecules. This means that 50% of methane would desorb during the TMA pulse and the remaining 50% of methane would desorb during the water pulse. The same finding was also reported by Rahtu et al.17 using a quadruple mass spectrometer.

Table 3. Characteristics of the Saturated Methylated Surface (Figure 8a) 2

methyls per nm at saturation coverage aluminum deposited per nm2 methane desorption during TMA pulse

present study

literature

6.4 ∼4.3 50%

∼6.016 ∼4.616 50%17

surface still has hydroxyls on the surface. Nevertheless, any approach of additional TMA to react with these hydroxyls was sterically hindered by the methyls. Also, Dillon et al.8 observed the presence of unreacted hydroxyls while performing alumina ALD on the γ-alumina surface at 500 K and 40 Pa. Experimental investigations have followed the ALD process by taking FTIR difference spectra of the surface after every pulse with the surface before the pulse.24,38 In this way, the consumption of hydroxyls during the TMA pulse and the generation of hydroxyls during the water pulse could be monitored. Infrared spectra of these hydroxylated and methylated surfaces were calculated, and difference spectra were produced for comparison with those reported by experiments, namely, Figure 9. As mentioned earlier, the saturated methylated surface constructed in this study has methyls along with few unreacted hydroxyls. The spectra of this methylated surface (Figure 9a) show positive peaks at 3480 (less intense peak) and 3147 (O− H stretch hindered by methyl groups), 3125−2917, 1430− 1373, and 1207−1157 cm−1 which can be attributed to O−H stretches of unreacted hydroxyls, C−H stretches, asymmetric C−H bending, and symmetric C−H bending, respectively. These calculated frequencies correspond well with those reported in the literature at 3000−2800, 1464, and 1216 13058


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Figure 9. (a) Difference IR spectra of methylated surface (c) with preceding hydroxylated surface (b). Dotted line is based on calculations in this work. Solid line was reported after 40 ALD cycles on LDPE particles.24 Dashed line was reported after 20 ALD cycles on mesoporous silica substrate.38 Color code for the atoms is the same as that used in Figure 8.

Figure 10. (a) Difference IR spectra of hydroxylated surface (c) with preceding methylated surface (b). Dotted line is based on calculations in this work. Solid line was reported after 40 ALD cycles on LDPE particles.24 Dashed line was reported after 20 ALD cycles on mesoporous silica substrate.38 Color code for the atoms is the same as that used in Figure 8.

cm−1 attributed to C−H stretching vibrations, asymmetric C− H bending, and symmetric C−H bending, respectively.24,38−40 In the second half-cycle of ALD, the saturated methylated surface formed at the end of the TMA pulse will react with water to regenerate the hydroxylated surface. Therefore, the methyls of the saturated methylated surface (Figure 8a) were replaced with hydroxyls and the surface was reoptimized to create a hydroxylated surface (Figure 8b). This hydroxylated surface formed does not have the same configuration of hydroxyls as the initial hydroxylated surface (Figure 1). The initial hydroxylated surface has surface water while the hydroxylated surface formed has only hydroxyls.

The infrared peaks for the initial hydroxylated surface were calculated at 3780−3650, 1910−1330, and 700−600 cm−1. These peaks observed as negative peaks in Figure 9a are attributed to OH stretches, vibrations of surface water, and lattice vibrations of alumina, respectively. These calculations are consistent with infrared peaks reported at 3800−2600, 2000− 1250 and 700−400, and 1000−500 cm−1 corresponding to OH stretches, vibrations of water, and lattice vibrations of alumina, respectively.8,24,38,41 Also, as shown in Figure 10a, the infrared peaks for the final hydroxylated surface were calculated at 3835−2830 cm−1 corresponding to OH stretches consistent with reported values. 13059


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(4) Klaus, J. W.; Sneh, O.; George, S. M. Growth of SiO2 at Room Temperature with the Use of Catalyzed Sequential Half-Reactions. Science 1997, 278, 1934−1936. (5) Ritala, M.; Kukli, K.; Rahtu, A.; Räisänen, P. I.; Leskelä, M.; Sajavaara, T.; Keinonen, J. Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources. Science 2000, 288, 319−321. (6) Leskelä, M.; Ritala, M. Atomic Layer Deposition (ALD): From Precursors to Thin Film Structures. Thin Solid Films 2002, 409, 138− 146. (7) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2009, 110, 111−131. (8) Dillon, A. C.; Ott, A. W.; Way, J. D.; George, S. M. Surface Chemistry of Al2O3 Deposition Using Al(CH3)3 and H2O in a Binary Reaction Sequence. Surf. Sci. 1995, 322, 230−242. (9) Lakomaa, E. L.; Root, A.; Suntola, T. Surface Reactions in Al2O3 Growth from Trimethylaluminium and Water by Atomic Layer Epitaxy. Appl. Surf. Sci. 1996, 107, 107−115. (10) Puurunen, R. L. Correlation between the Growth-Per-Cycle and the Surface Hydroxyl Group Concentration in the Atomic Layer Deposition of Aluminum Oxide from Trimethylaluminum and Water. Appl. Surf. Sci. 2005, 245, 6−10. (11) Travis, C. D.; Adomaitis, R. A. Modeling Alumina Atomic Layer Deposition Reaction Kinetics During the Trimethylaluminum Exposure. Theor. Chem. Acc. 2014, 133, 1−11. (12) Widjaja, Y.; Musgrave, C. B. Quantum Chemical Study of the Mechanism of Aluminum Oxide Atomic Layer Deposition. Appl. Phys. Lett. 2002, 80, 3304−3306. (13) Elliott, S. D.; Greer, J. C. Simulating the Atomic Layer Deposition of Alumina from First Principles. J. Mater. Chem. 2004, 14, 3246−3250. (14) Elliott, S. D.; Pinto, H. P. Modelling the Deposition of High-k Dielectric Films by First Principles. J. Electroceram. 2004, 13, 117−120. (15) Elliott, S. D. Predictive Process Design: A Theoretical Model of Atomic Layer Deposition. Comput. Mater. Sci. 2005, 33, 20−25. (16) Puurunen, R. L.; Lindblad, M.; Root, A.; Krause, A. O. I. Successive Reactions of Gaseous Trimethylaluminium and Ammonia on Porous Alumina. Phys. Chem. Chem. Phys. 2001, 3, 1093−1102. (17) Rahtu, A.; Alaranta, T.; Ritala, M. In Situ Quartz Crystal Microbalance and Quadrupole Mass Spectrometry Studies of Atomic Layer Deposition of Aluminum Oxide from Trimethylaluminum and Water. Langmuir 2001, 17, 6506−6509. (18) Adiga, S. P.; Zapol, P.; Curtiss, L. A. Structure and Morphology of Hydroxylated Amorphous Alumina Surfaces. J. Phys. Chem. C 2007, 111, 7422−7429. (19) Momida, H.; Hamada, T.; Takagi, Y.; Yamamoto, T.; Uda, T.; Ohno, T. Theoretical Study on Dielectric Response of Amorphous Alumina. Phys. Rev. B 2006, 73, 054108. (20) Alvarez, L. J.; León, L. E.; Sanz, J. F.; Capitán, M. J.; Odriozola, J. A. Surface Structure of Cubic Aluminum Oxide. Phys. Rev. B 1994, 50, 2561−2565. (21) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. Use of DFT to Achieve a Rational Understanding of Acid−Basic Properties of γ-Alumina Surfaces. J. Catal. 2004, 226, 54−68. (22) Krokidis, X.; Raybaud, P.; Gobichon, A. E.; Rebours, B.; Euzen, P.; Toulhoat, H. Theoretical Study of the Dehydration Process of Boehmite to γ-Alumina. J. Phys. Chem. B 2001, 105, 5121−5130. (23) Matero, R.; Rahtu, A.; Ritala, M.; Leskelä, M.; Sajavaara, T. Effect of Water Dose on the Atomic Layer Deposition Rate of Oxide Thin Films. Thin Solid Films 2000, 368, 1−7. (24) Ferguson, J. D.; Weimer, A. W.; George, S. M. Atomic Layer Deposition of Al2O3 Films on Polyethylene Particles. Chem. Mater. 2004, 16, 5602−5609. (25) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (26) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for OpenShell Transition Metals. Phys. Rev. B 1993, 48, 13115−13118.

CONCLUSIONS Periodic dispersion-corrected DFT calculations with a partially hydroxylated (100) γ-Al2O3 surface model have allowed to obtain insight in the surface chemistry of alumina ALD. The energetics of the investigated reactions suggest that MMA is the most abundant reaction intermediate at ALD operating conditions. The dominant reaction path toward the methylation of the surface is found to be adsorption of TMA on bridge oxygen by Lewis acid−base complex formation followed by LERs with hydroxyls and surface water in its vicinity. Further adsorption and LERs of TMA leads to a saturated methylated surface (∼6.4 CH 3 nm−2) that is in agreement with experimental observations. Surface restructuring is observed in almost all the reactions investigated due to the fact that aluminum has a drive to be in a stable tetrahedral or octahedral configuration. These surface rearrangements can be essential for the conformal growth of alumina layers deposited by ALD on the amorphous alumina substrate.

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ASSOCIATED CONTENT

S Supporting Information *

Energy profiles for adsorption and ligand exchange reaction of TMA on different active sites, NEB profiles of ligand exchange reactions, and top views of intermediate surface states during methylation of surface. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02382.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 32 (0)9 331 1735. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Fund for Scientific Research Flanders (FWO), the Long Term Structural Methusalem Funding by the Flemish Government, and the Interuniversity Attraction Poles Programme − Belgian State − Belgian Science Policy. The computational resources (Stevin Supercomputer Infrastructure) and services used in this work were provided by Ghent University.

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ABBREVIATIONS ALD, atomic layer deposition; DFT, density functional theory; DMA, dimethyl aluminum; FTIR, Fourier transform infrared; GGA, generalized gradient approximation; LDPE, low density polyethylene; LER, ligand exchange reaction; MMA, monomethyl aluminum; NEB, nudged elastic band; TMA, trimethyl aluminum; UA, unsaturated aluminum; VASP, Vienna ab initio simulation package

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REFERENCES

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