Stepwise mechanism and H2O-assisted hydrolysis in atomic layer deposition of SiO2 without a catalyst

Atomic layer deposition (ALD) is a powerful deposition technique for constructing uniform, conformal, and ultrathin films in microelectronics, photovoltaics, catalysis, energy storage, and conversion. The possible pathways for silicon dioxide (SiO2) ALD using silicon tetrachloride (SiCl4) and water (H2O) without a catalyst have been investigated by means of density functional theory calculations. The results show that the SiCl4 half-reaction is a rate-determining step of SiO2 ALD. It may proceed through a stepwise pathway, first forming a Si-O bond and then breaking Si-Cl/O-H bonds and forming a H-Cl bond. The H2O half-reaction may undergo hydrolysis and condensation processes, which are similar to conventional SiO2 chemical vapor deposition (CVD). In the H2O half-reaction, there are massive H2O molecules adsorbed on the surface, which can result in H2O-assisted hydrolysis of the Cl-terminated surface and accelerate the H2O half-reaction. These findings may be used to improve methods for the preparation of SiO2 ALD and H2O-based ALD of other oxides, such as Al2O3, TiO2, ZrO2, and HfO2.


Background
Atomic layer deposition (ALD) is a powerful deposition technique for constructing uniform, conformal, and ultrathin films in microelectronics, photovoltaics, catalysis, energy storage, and conversion [1,2]. Compared to other fabrication techniques, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), ALD is capable of accurately controlling the thickness of thin films at the atomic scale [1]. Essentially, the principle of ALD is similar to that of CVD, except that ALD breaks the CVD reaction into two half-reactions and retains two precursors separately during the reaction [2]. Taking silicon dioxide (SiO 2 ) as an example, SiO 2 CVD using silicon tetrachloride (SiCl 4 ) and water (H 2 O) can be divided into two halfreactions, A and B, of SiO 2 ALD [3][4][5][6]: where an asterisk designates the surface species.
In order to get more insight into the reaction mechanism of SiO 2 ALD, theoretical calculation has been performed that illustrates the reaction pathways [7]. It was proposed that on the Si(001) surface, the substituted halfreaction of SiCl 4 with surface hydroxyl group (−OH) proceeds through a concerted pathway via a four-membered ring (4MR) transition state (TS), forming Si-O and H-Cl bonds while simultaneously breaking Si-Cl and O-H bonds [7]. Unlike the Lewis acids, such as AlCl 3 , TiCl 4 , ZrCl 4 , and HfCl 4 , however, SiCl 4 seems to have no strong nucleophilicity [8][9][10]. Furthermore, it was found experimentally that the rate-determining step (RDS) of the full cycle of SiO 2 ALD is the SiCl 4 half-reaction, not the H 2 O half-reaction [3][4][5][6]. To date, the reaction mechanism of the full SiO 2 ALD process on the actual SiO 2 surface has remained unclear. In this work, we have performed detailed density functional theory (DFT) calculations to investigate the reaction mechanism of the full cycle of SiO 2 ALD, involving the SiCl 4 half-reaction (A1 and A2) and H 2 O half-reaction (B1 to B10), as shown in Figure 1

Methods
In order to model the two half-reactions of SiO 2 ALD, we adopt the cluster model Si 23 O 40 H 40 , as shown in Figure 2, which is based on a hydroxylated α-SiO 2 (0001) surface. The cluster model consists of three layers of SiO 2 (Si 23 O 40 ), and 40 hydrogen atoms which are used to saturate the dangling bonds. To stimulate the surface, the lower two layers of the SiO 2 atoms of the two models were fixed in optimized geometries.
All the species in ALD SiO 2 reactions were optimized using the M06-2X functional within the framework of DFT [11,12]. In order to gain a compromise between accuracy and computational cost, the 6-31G basis set was used for the fixed atoms of the substrate and the 6-31G(d, p) basis set was employed for other atoms on the surface. For each stationary point on the potential energy surface, a frequency calculation was carried out to determine if it is a minimum or a TS. All the transition states were verified by intrinsic reaction coordinates (IRC) calculations. Gibbs free energies of all species were estimated from the partition functions, and the enthalpy and entropy terms at 600 K. The energies reported here include zero-point energy (ZPE) corrections. We note that the solid surface lacks translational and rotational freedom, and the entropy of the surface only has a vibrational contribution. In other words, after being adsorbed onto the surface, the gas molecules lose translational and rotational momenta and produce new vibrational modes. All calculations in this work were performed with Gaussian 09 program [13].

SiCl 4 half-reaction: stepwise mechanism
The reaction pathway for the SiCl 4 half-reaction between SiCl 4 precursor and the surface hydroxyl (-OH) is shown in Figure 3. Due to high levels of hydroxyls on the SiO 2 surface after H 2 O half-reaction, SiCl 4 and hydroxyl may exchange ligands twice in the SiCl 4 half-reaction. Firstly, reaction A1 between SiCl 4 and -OH goes through a rotation transition state, TS1 A1 , with a Gibbs free energy barrier (G a ) of 34.5 kcal mol −1 and forms a pentacoordinated intermediate, Im2 A1 . Subsequently, the unstable intermediate undergoes the second transition state, TS2 A1 , forming the product -OSiCl 3 * , P A1 and accompanied by the release of HCl. Secondly, -OSiCl 3 can further react with another adjacent hydroxyl (-OH) on the surface to form the bridged product -O 2 SiCl 2 * , P A2 . Similar to reaction A1, reaction A2 between -OSiCl 3 * and -OH also undergoes two transition states, TS1 A2 and TS2 A2 , and a pentacoordinated intermediate, Im1 A2 . The overall SiCl 4 half-reaction is exergonic by 24.0 kcal mol −1 . The highest activation free energy of the SiCl 4 half-reaction is 44.5 kcal mol −1 (TS2 A1 ), indicating that the SiCl 4 halfreaction is very difficult. This difficulty can be overcome by the introduction of Lewis base catalysts, such as ammonia, pyridine, and aminosilane [14][15][16][17][18][19][20][21][22].
In Figure 3, TS1 A1 and TS1 A2 , with imaginary frequencies of 42i and 57i cm −1 , respectively, represent the formation of a Si-O bond accompanied by the rotation of SiCl 4 and -SiCl 3 . The pentacoordinated intermediates, Im2 A1 and Im1 A2 , have a trigonal bipyramidal (TBP) geometry with five ligands of four Cl atoms and one O atom or three Cl atoms and two O atoms. The TS2 A1 and TS2 A2 , with imaginary frequencies of 129i and 458i cm −1 , respectively, represent cleavages of the Si-Cl and O-H bonds and the formation of a H-Cl bond. As listed in Table 1, the Si · · · O and H · · · Cl distances in reaction A1 gradually decrease  In conventional SiO 2 CVD, SiCl 4 and H 2 O are introduced into the reaction chamber simultaneously. Subsequent hydrolysis and condensation lead to the formation of SiO 2 . Although two reactants are separately introduced into the chamber, hydrolysis and condensation also occur in SiO 2 ALD. In fact, the half-reaction between water and the Cl-terminated surface exchanges Cl Figure 3 Gibbs free energy profile of the SiCl 4 half-reaction of SiO 2 ALD. The inset shows the structures of four transition states, TS1 A1 , TS2 A1 , TS1 A2 , and TS2 A2 , two pentacoordinated intermediates, Im2 A1 and Im1 A2 , and two products, P A1 and P A2 .    half-reaction via the hydrogen bonding interaction of H 2 O…H 2 O may be termed as H 2 O-assisted hydrolysis, which is similar to Lewis-base catalysis in SiO 2 ALD through the OH…N hydrogen bond [14,23,24]. As a matter of fact, there are H 2 O-assisted reactions in nature, such as hydrolysis or solvolysis [25][26][27][28], tautomerization or proton transfer [29][30][31][32][33][34], decomposition [35][36][37], and catalysis [38,39]. H 2 O-assisted hydrolysis and solvolysis facilitate the exchange and dissociation of Cl ligand in HfO 2 ALD using HfCl 4 and H 2 O [40].

and -OH ligands and changes
Secondly, another Cl-terminated surface (-OSiCl 3 * ) can also hydrolyze step-by-step and go through pathways, B3, B4, and B5, shown in Figure 5. Three ligand exchange reactions undergo two transition states, TS1 B3 and TS2 B3 , TS1 B4 and TS2 B4 , and TS1 B5 and TS2 B5 . Similar to the SiCl 4 half-reaction, the first represents the formation of Si-O bonds and the second represents the cleavages of Si-Cl and O-H bonds and the formation of H-Cl bond. It is found that the activation free energies of -OSiCl 3 * hydrolysis are lower than that of -O 2 SiCl 2 * hydrolysis. Unlike the rigid -O 2 SiCl 2 group, the -OSiCl 3 group is more flexible. As shown in TS1 B3 of Figure 5, the hydroxyl (-OH) on the surface can interact with H 2 O through hydrogen bonding, HOH…OH, and cause the rotation of the -OSiCl 3 group, which can accelerate the hydrolysis of -OSiCl 3 and H 2 O exchange with the Cl ligand. The first hydrolysis of -OSiCl 3 * requires a low activation free energy of 23.3 kcal mol −1 ; however, the hydrolysis of -OSiOH-Cl 2 * and -OSi(OH) 2 -Cl * require slightly higher activation free energies. The reason for this may be that the direction of the hydrolyzed Cl atom of -OSiCl 3 * is more downward than that of -OSiOH-Cl 2 * or -OSi(OH) 2 Figure 5.    Figure 7. The corresponding activation free energies of HCl removal are 18.6 and 12.6 kcal mol −1 , respectively. During H 2 O or HCl removal, the two condensations both lead to the formation of the O-Si-O bridge bond, which is the elementary unit of SiO 2 and ensures its ALD growth.
When reviewing the full SiO 2 ALD cycle, including reactions A1 to A2 and B1 to B10, we find that the free energy barrier for the H 2 O half-reaction is lower than that for SiCl 4 half-reaction. The principal reason is that there are massive H 2 O molecules adsorbed on the surface, which result in H 2 O-assisted hydrolysis of -O 2 Si-Cl 2 * , -O 2 SiOH-Cl * , -OSi-Cl 3 * , -OSiOH-Cl 2 * , and -OSi(OH) 2 -Cl * and accelerate the H 2 O half-reaction. Therefore, the SiCl 4 half-reaction is the RDS of the full ALD cycle of SiO 2 and controls the ALD growth of SiO 2 .

Conclusions
Through detailed DFT calculations, the possible reaction pathways of (A) SiCl 4 half-reaction and (B) H 2 O halfreaction in SiO 2 ALD without a catalyst have been investigated. The SiCl 4 half-reaction is the RDS of SiO 2 ALD. It may proceed through a stepwise pathway, first forming a Si-O bond and then breaking Si-Cl and O-H bonds and forming a H-Cl bond. The H 2 O half-reaction is a complicated process, including hydrolysis and condensation. In the H 2 O half-reaction, there are massive H 2 O molecules adsorbed on the surface, which can result in H 2 O-assisted hydrolysis of the Cl-terminated surface and accelerate the H 2 O half-reaction. These findings may be used in SiO 2 ALD and H 2 O-based ALD of other oxides, such as Al 2 O 3 , TiO 2 , ZrO 2 , and HfO 2 .