DFT Investigation into Alumina ALD Growth Inhibition on Hydroxylated

Article pubs.acs.org/JPCC

DFT Investigation into Alumina ALD Growth Inhibition on Hydroxylated Amorphous Silica Surface Aditya Shankar Sandupatla, Konstantinos Alexopoulos, Marie-Françoise Reyniers,* and Guy B. Marin Laboratorium voor Chemische Technologie, Universiteit Gent, Technologiepark 914, B-9052 Gent, Belgium S Supporting Information *

ABSTRACT: Alumina (Al2O3), a suitable replacement for silica (SiO2) as gate oxide in metal oxide semiconductor field effect transistors (MOSFET), is deposited on the amorphous silica layer of the semiconductor substrate by atomic layer deposition (ALD) using trimethylaluminum (TMA) and water as precursors. A computationally efficient model for the hydroxylated amorphous silica surface is obtained by means of molecular dynamics and is used to investigate the reason behind the observed growth inhibition during alumina ALD. The reactions of TMA are investigated by periodic DFT calculations on surfaces with hydroxyl coverage of 3.38 OH nm−2 and 5.07 OH nm−2. The formation of SiCH3 surface species is found to be possible only on the less hydroxylated surface during the first TMA half-cycle, while the subsequent reaction of water with the SiCH3 surface species is found to be highly activated (Ea = 196 kJ mol−1). Since these SiCH3 surface species are rather unreactive toward water, fewer hydroxyls are regenerated during this first water half-cycle, resulting in the observed initial growth inhibition. Moreover, alumina growth can continue over the aluminum surface species, trapping the unreactive SiCH3 species at the interface between deposited alumina and silica. Such carbon impurities at the interface should be avoided nonetheless, since they can create undesirable tunneling currents in MOSFETs. high tunneling currents.10 However, there are various instances of carbon impurities being reported at the interface of deposited alumina and silica (Figure 1). These are SiCH3

1. INTRODUCTION Silica acts as a gate insulator in metal oxide semiconductor field effect transistors (MOSFET). With miniaturization of electronic devices, this gate oxide has to be thinned to 1 nm thickness which is equal to four to five atomic layers.1 However, at this thickness, a tunneling current can run through silica, which causes the device to fail.2 Alumina has a higher dielectric constant than silica and is thermodynamically stable in contact with silica.3 Therefore, the deposition of a few layers of alumina on the silica substrate would yield a gate oxide that satisfies the required capacitances in miniature MOSFETs while minimizing the undesirable tunneling currents.4 To achieve this goal, thin layers of alumina can be deposited on the silica surface in MOSFET by atomic layer deposition (ALD).5 ALD is a thin-film deposition method which proceeds through self-limiting surface reactions.6−8 This technique offers highly conformal films and enables precise control over film thickness and composition at an atomic level.9 A complete alumina ALD cycle consists of an exposure to an aluminum precursor, i.e., trimethylaluminum (TMA), a purge period, an exposure to an oxygen precursor, i.e., water, and another purge period.5,7 The overall reaction between TMA and water yields an alumina layer on the surface and byproduct gaseous methane. Methane along with excess reactants are expelled from the ALD reactor during the purge periods. In the case of gate oxides in MOSFET, continuous thin films with no impurities are desired. Such impurities would result in © XXXX American Chemical Society

Figure 1. Schematic representation of ALD cycles on hydroxylated silica.

surface species as reported in investigations of TMA reactions on a silica surface.11−15 Moreover, growth inhibition of alumina has been reported on silica substrates with a hydroxyl coverage of less than 4 OH nm−2.15 In practice, the silica substrate is annealed to enhance its structural stability. Experimentally, it has been shown that increasing the annealing temperature of the silica substrate causes dehydroxylation which gives rise to SiCH3 surface species during the first TMA pulse.12 Since these SiCH3 species are not removed during the next water pulse, the initial hydroxyl coverage is not restored. This decreased hydroxyl Received: June 2, 2015

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DOI: 10.1021/acs.jpcc.5b05261 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(SCF) cycle was iterated until a convergence level of 10−6 au was achieved. A simulation was run for 4000 steps with each time step set at 0.5 fs. NVT conditions were imposed by a Nosé−Hoover thermostat with a target temperature of 2000 K. This MD procedure allows for overcoming potential barriers that simple relaxation would encounter, making it more likely to identify near-ground-state configurations. The configuration obtained after the NVT simulation was annealed at a rate of 1000 K ps−1 and further subjected to a geometry optimization resulting in a model of the hydroxylated amorphous silica surface. The obtained surface model is described in section 3.1, and coordinates of the structure are given in the Supporting Information (Table S1). Geometry optimization of the slab and of the reactants and products of reactions of TMA and water were performed by spin-unpolarized periodic density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP). This package uses the projector augmented wave method and plane-wave basis sets to describe the electron ion interaction.30−33 The Perdew−Burke−Ernzerhof functional was used to calculate the exchange and correlation energies within the generalized gradient approximation (GGA) to account for nonlocality.23 A plane-wave energy cutoff of 400 eV and a Gaussian smearing of 0.1 eV were used.34 The atoms were relaxed to their instantaneous ground state using a conjugate gradient algorithm with a force convergence criterion of 0.02 eV Å−1. Several geometries along the reaction coordinate connecting the reactant and product were optimized with a force convergence criterion of 0.1 eV Å−1 using the nudged elastic band (NEB) method.35,36 The transition state on this reaction coordinate was obtained by the dimer method37 with a force convergence criterion of 0.02 eV Å−1. The Monkhorst−Pack division scheme38 was chosen to generate a set of k-points within the Brillouin zone. Using a 3 × 3 × 1 k-point mesh, the cohesive energy of the hydroxylated amorphous silica model was converged within 1 meV. For the surface calculations, no symmetry was used and a dipole correction was included. In all these calculations, about half of the bottom atoms were fixed and the remaining atoms of the slab and the atoms of the adsorbed species were relaxed. The atoms of the slab that were fixed are reported in the Supporting Information (Tables S1 and S2). 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 calculated by the DFT-D2 method24,39 to account for longrange dispersion interactions. In order to compare with the available literature data,16,40,41 vibrational frequencies associated with relaxed atoms were calculated for the initial hydroxylated surface and for the surface containing SiCH3 surface species by numerically solving the partial Hessian matrix. The elements in this matrix were obtained by finite differencing of the forces at different points near the optimized geometry. These forces were calculated by displacing each atom in the positive and negative directions along the x-, y-, and z-axis by the same small distance (1 pm).

coverage causes growth inhibition during the second TMA cycle.15 Nevertheless, a molecular-level understanding of the effect of hydroxyl coverage on growth inhibition during alumina ALD over amorphous silica is still missing. Several models of the hydroxylated amorphous silica substrate obtained by density functional theory (DFT) calculations have already been reported. Tielens et al.16 presented a model with 117 atoms in total. The main limitation of this model is its small slab thickness which can result in a severe surface deformation upon TMA adsorption. The thickness of the substrate is not uniform ranging between 0.5 and 0.8 nm. At the thinnest point, the upper side is separated from the lower one by just one HO−Si−O−Si−OH link.16,17 On the other hand, the model presented by Ugliengo et al.18 considers 222 atoms with a slab thickness of 1.45 nm, and the slab of Ewing et al.19 has around 300 atoms with a slab thickness of 1.125 nm. Although the considered slab thickness in these models is adequate for studying adsorption of TMA, the high number of atoms makes it computationally expensive to work with these models. In this paper, a reliable and computationally efficient model of the hydroxylated amorphous silica surface was constructed that allows to investigate the reactions occurring during the initial half-cycles of alumina ALD. The reactions of TMA were investigated on a surface with 3.38 and 5.07 OH nm−2 to find the mechanism that leads to the formation of these SiCH3 surface species and to investigate whether the formation of these species can be controlled or avoided by varying the hydroxyl coverage.

2. COMPUTATIONAL METHODOLOGY The methodology used to generate the hydroxylated amorphous silica surface model is inspired by the protocol reported by Tielens et al.16 A large model of SiO2 glass20 was truncated to form a slab containing 27 SiO2 units, after changing its lattice constants to match the density of amorphous silica (2.2 g cm−3). This structure was made periodic by manually joining the Si and O atoms along the x and y directions taking care that no defects were formed in the bulk. A vacuum gap of 1.5 nm was introduced in the z direction. Water molecules were added on the top and bottom surface of this structure, such that dangling Si and O atoms formed OH groups. The total number of atoms of the hydroxylated surface model was 99 (Si27O60H12), and its unit cell parameters were a = 1.072 nm, b = 1.104 nm, c = 2.986 nm, and α = β = γ = 90°. This hydroxylated silica model was subjected to molecular dynamics (MD) using the CP2K-Quickstep package.21 In the CP2K-Quickstep package, the density functional implementation is based on a hybrid Gaussian plane wave (GPW) scheme.22 The gradient-corrected Perdew−Burke−Ernzerhof (GGA-PBE) functional23 along with DFT-D2 dispersion corrections was used in the simulations.24 Atom-centered Gaussian-type double-ζ plus polarization (DZVP) basis sets25 were used to describe the valence electrons, while analytic Goedecker−Teter−Hutter (GTH) pseudopotentials26,27 were used to describe the core electrons. The auxiliary plane wave basis set for the electron density was cut off at 300 Ry. The wave function optimization was performed using an orbital transformation minimizer, and a preconditioner for the minimization was constructed from a Cholesky inversion of the overlap and kinetic energy matrices.28 An Always Stable Predictor-Corrector (ASPC) algorithm in third order was used for wave function extrapolation.29 Each self-consistent field

3. RESULTS AND DISCUSSION 3.1. Hydroxylated Amorphous Silica Model. As seen in Table 1, the model of the hydroxylated amorphous silica (5.07 OH nm−2) constructed in this work describes very well the observed features and is consistent with other theoretical studies. As shown in Figure 2, half of the hydroxyls, viz. O1, O2, B


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seen in Table 2, the formation of a Lewis acid−base complex of TMA with the bridge oxygen O7 is strongly endothermic and

Table 1. Comparison of the Characteristics of the Hydroxylated Amorphous Silica Model (5.07 OH nm−2) Constructed in This Work with Those Reported by Experiments and Other Modeling Work this work av Si−O bond length (pm) av Si−O−Si angle (deg) av O−Si−O angle (deg) ρ (g cm−3) no. of atoms slab thickness (nm) OH nm−2 vibrational freq (cm−1) H-bonded hydroxyls isolated hydroxyls adsorption energy of water (kJ mol−1)

other DFT

experiment

164

16619

161,43 16244

139.1

142−150,16 141.919

144,44 15345

109.3

111,

16

16

109.4

19

19

Table 2. Adsorption Energy (kJ mol−1) of TMA on Various Hydroxyls and Bridge Oxygen Present on the Hydroxylated Amorphous Silica Surface with Coverages of 5.07 and 3.38 OH nm−2

109.5,

44

109.4

Lewis acid−base complex

45

with bridge oxygen O7 O8 with hydroxyls O1 O2 O3 O4 O5 O6

43

2.20 99 1.5

1.70, 2.15 117,16 30019 0.5−0.8,16 1.119

2.20

5.07

5.8,16 2.1−8.119

4.015

3304−3656

3447−377516

352040

3700−3795 −69

3831−392516 −5016

3747−375040 −44 to −9042

ΔEads (nOH = 5.07 OH nm−2) 213

−106 −102 −110 −103 −109 −68

ΔEads (nOH = 3.38 OH nm−2) 239 −85

−111 −61 −93 −88

therefore unlikely to take place. On the other hand, our study suggests that adsorption of TMA on the hydroxyls (R1 in Figure 3) can readily occur and is equally favorable on these sites except for the hydroxyl on O6 (Table 2) because the relaxation of adsorbed TMA on O6 is sterically hindered by the hydroxyl at O5. In a next step, the adsorbed TMA on the hydroxyl can undergo a ligand exchange reaction (LER), in which the methyl of TMA is eliminated by abstracting a hydrogen from a nearby hydroxyl. Several possibilities for this reaction have been investigated as shown in Supporting Information (Figure S2). Depending on the orientation of the methyl and its location with respect to the hydroxyl, the activation energies for the LER range between 42 and 85 kJ mol−1 (Table S3). In addition, the respective reaction energies vary between −155 and −78 kJ mol−1, since the dimethylaluminum (DMA) formed after methyl elimination can undergo structural relaxation in some cases. Of all the LERs investigated, the LER of adsorbed TMA on O2 with hydroxyl at O3 is thermodynamically and kinetically most favorable (Figure S2). This LER is shown schematically as R2 in Figure 3. The aluminum in DMA formed after elimination of methyl from TMA is initially triply coordinated. In order to gain a stable tetrahedral coordination for aluminum, DMA rearranges structurally and bonds with the oxygen O3 of the hydroxyl in its close vicinity (Figure S3a). As seen from Table 3, a good agreement is obtained between the calculated energetics of TMA adsorption and subsequent LER on our hydroxylated silica surface (5.07 OH nm−2) with the ones reported on a Si(001) surface with a slightly higher hydroxyl coverage (6.8 OH nm−2). The consecutive reaction of the resulting DMA with a vicinal hydroxyl is investigated next (R3 in Figure 3). The LER of DMA with the hydroxyl at O1 (Figure S3b) leads to monomethylaluminum (MMA). This MMA rearranges structurally to retain a tetrahedral coordination by binding to the oxygen at O1 (Figure S3b). Subsequently (R4 in Figure 3), the MMA reacts by abstracting a hydrogen from O2 (Figure S3c) to form unsaturated aluminum (UA). UA formed remained triply coordinated and did not undergo any structural rearrangement (Figure S3c). Although the methane formed in the aforementioned LERs (R2, R3, R4) is initially physisorbed, its desorption energy is rather small (between 17 and 36 kJ

Figure 2. Top and side view of hydroxylated amorphous silica model with 5.07 OH nm−2. Color code for the atoms: silicon = blue, oxygen = red, and hydrogen = white. Hydrogen bonds are shown by yellow dotted lines. Oxygen atoms where TMA can be adsorbed are numbered.

and O5, are involved in H bonds, and the remaining ones, viz. O3, O4, and O6, are terminal, while there is also a bridge oxygen, viz. O7. The calculated frequencies are divided in two domains from 3304 to 3656 cm−1 corresponding to H-bonded hydroxyls and from 3700 to 3795 cm−1 corresponding to terminal hydroxyls. These frequencies are in very good agreement to those reported in the literature.16,40 The average Si−O distance, Si−O−Si and O−Si−O angles, and adsorption energy of water on the surface agree well with the reported values in the literature (Table 1). In the investigated configurations for adsorption of water, water binds to the surface by making H bonds with hydroxyls (Figure S1). The adsorption energy was found to be on average −69 kJ mol−1, which is in the range of −40 to −90 kJ mol−1 reported by experiments.42 3.2. Reactions of TMA on a Silica Surface with 5.07 OH nm−2. It is known from earlier theoretical work that the first elementary step in the reactions of TMA with the hydroxylated surface is the adsorption of TMA on the surface.46−48 Adsorption of TMA is largely known to take place by Lewis acid base complex formation with the oxygens exposed on the surface in the form of hydroxyls or bridge oxygen. However, as C


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Figure 3. Schematic representation of TMA adsorption and subsequent LERs on the hydroxylated amorphous silica surface with coverage of 5.07 OH nm−2 (Figure 2). ΔEads, Ea, ΔEr, and ΔEdes are the adsorption energy, activation energy, reaction energy, and desorption energy expressed in kJ mol−1.

Table 3. Comparison of Energetics (kJ mol−1) of the Reactions of TMA on Hydroxylated Silica Surface in This Work with That Investigated on Si(001) Surface49,50 adsorption OH nm−2 5.07 6.80 3.38 3.40

(this work) (Kim et al.) (this work) (Kim et al.)

reactions of TMA, water is removed from the two closest hydroxyls at O1 and O3 and a bridge oxygen O8 is formed (Figure 5) after reoptimizing the surface. The role of this bridge

subsequent LER

ΔEads

Ea

ΔEr

−102 −11249 −111 −9649

35 4849 74 6049

−155 −14450 −82 −5449

Figure 5. Top view of hydroxylated amorphous silica model (a) with 5.07 OH nm−2 and (b) with 3.38 OH nm−2. Color code for atoms is the same as shown in Figure 2.

mol−1), and hence it can easily desorb. Therefore, methane is not expected to accumulate at the interface of alumina and silica. Overall, based on the energy profile of TMA adsorption and consecutive LERs with the hydroxyls in the vicinity (Figure 4), MMA is the most stable surface species. 3.3. Reactions of TMA on a Silica Surface with 3.38 OH nm−2. To study the effect of surface dehydroxylation on the

oxygen created due to dehydroxylation on the reactions of TMA is investigated next. The coordinates of this dehydroxylated surface are given in the Supporting Information (Table S2). The adsorption of TMA is investigated first on the bridge oxygen of the dehydroxylated surface. Similar to TMA adsorption on the bridge oxygen O7 of the surface with 5.07 OH nm−2, the adsorption of TMA on the same site of the surface with 3.38 OH nm−2 is strongly endothermic and thus unlikely to take place (Table 2). However, a Lewis acid−base complex with TMA can readily be formed on the bridge oxygen O8 (R8 in Figure 6) formed from the dehydroxylation of the surface. In addition, the investigation of TMA adsorption on hydroxyls shows that the adsorption is not equally favorable on all hydroxyls due to steric interactions. Of all the sites, adsorption of TMA on O2 (R5 in Figure 6) is found to be most exothermic (Table 2). Therefore, LER are investigated only for adsorbed TMA on O2. Starting from adsorbed TMA on O2, a LER can occur where the methyl of TMA abstracts a hydrogen from the hydroxyl at O2 (Figure S4a). Although the DMA formed is initially triply coordinated (Figure S4a2), it rearranges and binds with bridge oxygen O8 to gain a 4-fold coordination (Figure S4a3). This DMA has a hydroxyl in its vicinity on O4. LER of DMA with

Figure 4. Energy profile of adsorption of TMA and its subsequent LERs forming dimethylaluminum (DMA), monomethylaluminum (MMA) and unsaturated aluminum (UA) on a silica surface with 5.07 OH nm−2. Color code for the atoms of TMA: aluminum = brown, carbon = black, hydrogen = yellow; color code for the atoms of amorphous silica is the same as in Figure 2. D


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Figure 6. Schematic representation of TMA adsorption and subsequent reactions on the hydroxylated amorphous silica surface with coverage of 3.38 OH nm−2 (Figure 5). ΔEads, Ea, ΔEr, and ΔEdes are the adsorption energy, activation energy, reaction energy, and desorption energy expressed in kJ mol−1.

Figure 7. Energy profile of the reactions of TMA that can produce SiCH3 surface species on a silica surface with 3.38 OH nm−2. In Scheme 1, adsorbed TMA (a) reacts with hydroxyl at O2 to form triply coordinated DMA (b) that reacts further with bridge oxygen at O8 to form SiCH3 and MMA (c). In Scheme 2, adsorbed TMA at bridge oxygen O8 (d) dissociates to form DMA and SiCH3 (e); DMA further reacts with hydroxyl at O4 to form MMA (f). Color code for atoms is the same as in Figure 4. Scheme 1 is the dominant path toward the formation of SiCH3 and MMA surface species.

S4b3). Since there are no hydroxyls in the vicinity of MMA, the formation of UA cannot be considered on this less hydroxylated silica surface. The reactions responsible for the formation of the SiCH3 and MMA surface species are summarized in Scheme 1 of Figure 6. As seen from Table 3, the calculated energetics of TMA adsorption and subsequent LER on the less hydroxylated silica

this hydroxyl is found to be slightly endothermic (ΔEr = 6 kJ mol−1) with an activation energy of 97 kJ mol−1. On the contrary, DMA dissociation over the bridge oxygen O8 to form SiCH3 and MMA is highly exothermic and less activated (Figure S4b). Although the MMA formed is initially triply coordinated (Figure S4b2), it rearranges and binds with another bridge oxygen to gain a 4-fold coordination (Figure E


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Figure 8. Schematic representation of water adsorption and subsequent reaction with the adsorbed TMA, DMA, MMA, and UA surface species formed on reaction of TMA with a silica surface with 5.07 OH nm−2. The geometries of adsorbed TMA, DMA, MMA, and UA were taken from Figure 4. ΔEads, Ea, ΔEr, and ΔEdes are the adsorption energy, activation energy, reaction energy, and desorption energy expressed in kJ mol−1.

stretches are observed at 1249 and 1197 cm−1. These values are consistent with the reported experimental values of 1275 and 1214 cm−1, respectively.41 The calculated C−H and O−H stretches are in the range of 2900−3055 and 3510−3700 cm−1, respectively. These values agree with the ranges of 2800−3000 cm−1 and around 3750 cm−1 observed experimentally for the C−H and O−H stretches, respectively.41 3.4. Reactions of Water with Adsorbed TMA, DMA, MMA, UA, and SiCH3 Surface Species. As reported in the previous sections, adsorbed TMA, DMA, MMA, UA, and SiCH3 surface species could form upon reaction of TMA with the hydroxylated silica surface. The subsequent reaction of water with these surface species should regenerate the hydroxylated surface. Hence, the reactions of water with these surface species have been investigated next in order to probe their reactivities during the water half-cycle. The geometries of these surface species were taken from adsorbed TMA (Figures 4a and 7a), DMA (Figures 4b and 7b), MMA (Figures 4c and 7c), UA (Figure 4d), and SiCH3 (Figure 7c). Water first adsorbs on the surface and then reacts with these surface species. Upon adsorption, water can either form a Lewis acid−base complex with the aluminum or hydrogen bond with the hydroxyls on the surface (Figures 8 and 9). Water is found to form a Lewis acid−base complex with the aluminum of DMA, MMA and UA (R13, R15, R17 in Figure 8 and R21, R23 in Figure 9), while water adsorption is found to occur via hydrogen bonding with the hydroxyls next to adsorbed TMA and SiCH3 surface species (R11 in Figure 8 and R19, R25 in Figure 9). These modes of water adsorption have also been identified in our previous work while studying the reactions of

surface are comparable to the ones reported on a Si(001) surface with 3.4 OH nm−2.49 In agreement with Kim et al.,49 decreasing the hydroxyl coverage results in an increase in the activation energy of the LER of adsorbed TMA and a significant decrease in the respective reaction energy. Starting from adsorbed TMA on O8, another possibility exists for the formation of SiCH3 and MMA surface species (Scheme 2 of Figure 6). Adsorbed TMA on O8 has a hydroxyl on O4 in its close vicinity. LER between adsorbed TMA and this hydroxyl is found to have an activation barrier of 48 kJ mol−1 and a reaction energy of −115 kJ mol−1. Hence, rather than undergoing this LER with the hydroxyl on O4, TMA adsorbed on bridge oxygen O8 can more easily dissociate over it with a much smaller energy barrier (Ea = 6 kJ mol−1) forming SiCH3 surface species and DMA in a tetrahedrally coordinated state (Figure S4c). This DMA has a vicinal hydroxyl at O4. LER between DMA and hydroxyl on O4 forms MMA in a triply coordinated state (Figure S4d). However, this slightly endothermic reaction (R10 in Figure 6) is significantly more activated as compared to the reactions of Scheme 1. Therefore, by comparing the energy profiles of Schemes 1 and 2 (Figure 7), it can be observed that the reactions of Scheme 1 have lower activation barriers and lead to a product (namely SiCH3 and MMA) that is energetically more stable. Overall, it can be concluded that the dehydroxylation produces bridge oxygen that allow the formation of SiCH3 and MMA surface species upon reaction with TMA, while the formation of UA surface species does not occur on the less hydroxylated silica surface. Vibrational frequencies have also been calculated for the product of Scheme 1 (Figure 7c). Si−CH3 and Al−CH3 F


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Figure 9. Schematic representation of water adsorption and subsequent reaction with the adsorbed TMA, DMA, MMA, and SiCH3 surface species formed on reaction of TMA with a silica surface with 3.38 OH nm−2. The geometries of adsorbed TMA, DMA, MMA, and SiCH3 were taken from Figure 7. ΔEads, Ea, ΔEr, and ΔEdes are the adsorption energy, activation energy, reaction energy, and desorption energy expressed in kJ mol−1.

Figure 10. Energy profile of the reaction of water with the adsorbed TMA, DMA, MMA, UA, and SiCH3 surface species formed from the reaction of TMA on a hydroxylated silica surface with (a) 5.07 OH nm−2 and (b) 3.38 OH nm−2.

water with surface species present on the alumina surface.48 In addition, Musgrave and Widjaja reported a Lewis acid−base

complex formation of water with the aluminum of the DMA and MMA species using cluster models.46 G


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surface could avert the formation of SiCH3, thus avoiding the initial growth inhibition and carbon impurities at the interface.

In the case of R13 and R15, the Lewis acid base complex formation of water with aluminum in DMA and MMA causes aluminum to attain a 5-fold coordination. While in the case of R21 and R23, adsorption of water on DMA and MMA causes a structural rearrangement of the aluminum in order to retain its tetrahedral coordination. As aluminum in tetrahedral coordination is more stable, R21 and R23 are more exothermic than R13 and R15. Meanwhile, the adsorption of water next to adsorbed TMA (R11 and R19) and SiCH3 (R25) by hydrogen bonding is equally exothermic (ca. −60 kJ mol−1). On the other hand, water adsorption is found to be the strongest on UA (−131 kJ mol−1) due to its unsaturated nature. Moreover, water adsorbed on UA can further dissociate to produce two hydroxyl groups (Figure S5d). Subsequent ligand exchange reactions (LERs) of adsorbed water with the methyls of adsorbed TMA, DMA, MMA, and SiCH3 (Figures 8 and 9) have also been investigated. The NEB profiles of these reactions are shown in Figures S5 and S6. In these LERs, one hydrogen from water is transferred to the methyl group. Thus, the methyl group is eliminated as methane, while the remnant hydroxyl from water regenerates the hydroxylated surface. Although the activation barriers of the LER of adsorbed water with DMA (R14 and R22) and MMA (R16 and R24) are comparable on both silica surfaces (60−78 kJ mol−1), the LER of adsorbed water with adsorbed TMA is significantly more activated on the silica surface with 3.38 OH nm−2 (R20) compared to the one with 5.07 OH nm−2 (R12) because water is adsorbed farther from adsorbed TMA on the less hydroxylated surface. Overall, by comparing the energy profiles of the investigated surface reactions with water (Figure 10), we notice that the LER of water with adsorbed TMA, DMA, MMA and dissociation over UA is significantly less activated as compared to the LER of water with SiCH3 surface species. Therefore, alumina growth will continue over the aluminum species because hydroxyls can be regenerated at these sites, while the regeneration of hydroxyls is expected to be very difficult at the SiCH3 surface species. The LER of water with the SiCH3 surface species is highly activated (Ea = 196 kJ mol−1) as also reported by Delabie et al.15 (Ea = 215 kJ mol−1), who studied the hydrolysis of SiCH3 using a cluster model. Since this reaction is kinetically not favored, these SiCH3 surface species will not likely react with water during the water pulse. This is in agreement with the experimental observations15,41 which suggest that these groups are highly unreactive toward water during the water pulse. Therefore, these SiCH3 surface species are responsible for possible carbon impurities at the interface and can cause growth inhibition during alumina ALD on a partially dehydroxylated silica surface (nOH = 3.38 OH nm−2). 3.5. Conclusions. A computationally efficient model of the hydroxylated amorphous silica (5.07 OH nm−2) consistent with experimental reported data is constructed in this theoretical work. The reactions of TMA on this surface do not lead to the formation of SiCH3 surface species. However, dehydroxylation of the surface produces bridge oxygen that are reactive with TMA. These reactive bridge oxygen open up a potential reaction path that forms SiCH3 surface species. Since these SiCH3 surface species have a very high activation barrier toward reaction with water, they are expected to remain on the surface during the subsequent water pulse, leading in carbonaceous impurities in the interface between the ALD deposited alumina and the silica substrate. Overall, the calculated reaction energetics suggest that having a densely hydroxylated silica

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ASSOCIATED CONTENT

S Supporting Information *

Coordinates of the hydroxylated amorphous model, geometries for adsorbed water on silica surface, energetics of reaction of TMA on silica surface and NEB profiles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05261.

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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, application no. G004511N), 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, dimethylaluminum; FTIR, Fourier transform infrared; GGA, generalized gradient approximation; LER, ligand exchange reaction; MMA, monomethylaluminum; MOSFET, metal oxide semiconductor field effect transistors; NEB, nudged elastic band; TMA, trimethylaluminum; UA, unsaturated aluminum; VASP, Vienna ab initio simulation package.

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REFERENCES

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