Stepwise mechanism and H2O-assisted hydrolysis in atomic layer deposition of SiO2 without a catalyst
© Fang et al.; licensee Springer. 2015
Received: 12 November 2014
Accepted: 23 December 2014
Published: 18 February 2015
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.
All the species in ALD SiO2 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 .
Results and discussion
SiCl4 half-reaction: stepwise mechanism
Selected bond distances (in Å) of all species for SiCl 4 half-reaction
H2O half-reaction: H2O-assisted hydrolysis
In conventional SiO2 CVD, SiCl4 and H2O are introduced into the reaction chamber simultaneously. Subsequent hydrolysis and condensation lead to the formation of SiO2. Although two reactants are separately introduced into the chamber, hydrolysis and condensation also occur in SiO2 ALD. In fact, the half-reaction between water and the Cl-terminated surface exchanges Cl and -OH ligands and changes Si-Cl* species into Si-OH* species. Due to this the possible reactions of the H2O half-reaction (B) may include the formation of silanol (-Si-OH) via the exchange of ligands between Cl and -OH (reactions B1, B2, B3, B4, and B5) and the formation of -O-Si-O- bridge bonds by removing H2O (reactions B6, B8, and B10) and HCl (reactions B7 and B9), similar to the hydrolysis (-Si-OH) and condensation (-O-Si-O-) processes of SiO2 CVD.
When reviewing the full SiO2 ALD cycle, including reactions A1 to A2 and B1 to B10, we find that the free energy barrier for the H2O half-reaction is lower than that for SiCl4 half-reaction. The principal reason is that there are massive H2O molecules adsorbed on the surface, which result in H2O-assisted hydrolysis of -O2Si-Cl2 *, -O2SiOH-Cl*, -OSi-Cl3 *, -OSiOH-Cl2 *, and -OSi(OH)2-Cl* and accelerate the H2O half-reaction. Therefore, the SiCl4 half-reaction is the RDS of the full ALD cycle of SiO2 and controls the ALD growth of SiO2.
Through detailed DFT calculations, the possible reaction pathways of (A) SiCl4 half-reaction and (B) H2O half-reaction in SiO2 ALD without a catalyst have been investigated. The SiCl4 half-reaction is the RDS of SiO2 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 H2O half-reaction is a complicated process, including hydrolysis and condensation. 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 in SiO2 ALD and H2O-based ALD of other oxides, such as Al2O3, TiO2, ZrO2, and HfO2.
This work was supported by the National Natural Science Foundation of China (51202107), the State Key Program for Basic Research of China (2015CB921203 and 2011CB922104), the China Postdoctoral Science Foundation (2014 M551556), Open Project of National Laboratory of Solid State Microstructures (M27009), and Zhejiang Provincial Natural Science Foundation of China (LY13B030005). ADL is also grateful for the support of the Doctoral Fund of the Ministry of Education of China (20120091110049) and the Priority Academic Program Development (PAPD) in Jiangsu Province. We thank the High Performance Computing Center of Nanjing University for providing the computing resources.
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