Front. Chem. Frontiers in Chemistry Front. Chem. 2296-2646 Frontiers Media S.A. 1035902 10.3389/fchem.2022.1035902 Chemistry Original Research Reaction mechanism of atomic layer deposition of zirconium oxide using zirconium precursors bearing amino ligands and water Xu et al. 10.3389/fchem.2022.1035902 Xu Rui 1 Zhou Zhongchao 1 Li Jing 1 Zhang Xu 1 Zhu Yuanyuan 1 Xiao Hongping 1 * Xu Lina 1 * Ding Yihong 1 Li Aidong 2 Fang Guoyong 1 * 1 Key Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, China 2 National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing, China

Edited by: Sudip Pan, University of Marburg, Germany

Reviewed by: Jose Luis Cabellos, Polytechnic University of Tapachula, Mexico

Mahesh Kumar Ravva, SRM University, India

Ayan Datta, Indian Association for the Cultivation of Science, India

 *Correspondence: Hongping Xiao, hp_xiao@wzu.edu.cn; Lina Xu, xulina@wzu.edu.cn; Guoyong Fang, fanggy@wzu.edu.cn

This article was submitted to Theoretical and Computational Chemistry, a section of the journal Frontiers in Chemistry

 04 11 2022 2022 10 1035902 03 09 2022 06 10 2022 Copyright © 2022 Xu, Zhou, Li, Zhang, Zhu, Xiao, Xu, Ding, Li and Fang. 2022 Xu, Zhou, Li, Zhang, Zhu, Xiao, Xu, Ding, Li and Fang

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As a unique nanofabrication technology, atomic layer deposition (ALD) has been widely used for the preparation of various materials in the fields of microelectronics, energy and catalysis. As a high-κ gate dielectric to replace SiO2, zirconium oxide (ZrO2) has been prepared through the ALD method for microelectronic devices. In this work, through density functional theory calculations, the possible reaction pathways of ZrO2 ALD using tetrakis(dimethylamino)zirconium (TDMAZ) and water as the precursors were explored. The whole ZrO2 ALD reaction could be divided into two sequential reactions, TDMAZ and H2O reactions. In the TDMAZ reaction on the hydroxylated surface, the dimethylamino group of TDMAZ could be directly eliminated by substitution and ligand exchange reactions with the hydroxyl group on the surface to form dimethylamine (HN(CH3)2). In the H2O reaction with the aminated surface, the reaction process is much more complex than the TDMAZ reaction. These reactions mainly include ligand exchange reactions between the dimethylamino group of TDMAZ and H2O and coupling reactions for the formation of the bridged products and the by-product of H2O or HN(CH3)2. These insights into surface reaction mechanism of ZrO2 ALD can provide theoretical guidance for the precursor design and improving ALD preparation of other oxides and zirconium compounds, which are based ALD reaction mechanism.

 zirconium oxide atomic layer deposition reaction mechanism tetrakis(dimethylamino)zirconium density functional theory 21873073 52073142 LY22B030007 M30031 M35047 National Natural Science Foundation of China10.13039/501100001809 Natural Science Foundation of Zhejiang Province10.13039/501100004731 National Laboratory of Solid State Microstructures, Nanjing University10.13039/501100011243 1 Introduction

As an excellent nanofabrication technology, atomic layer deposition (ALD) can prepare large-area, uniform and conformal thin films at the atomic level (Klaus et al., 1997; Ritala et al., 2000; Hausmann et al., 2002; Chen et al., 2011). Meanwhile, the compositions and structures of thin films can also be controlled through varying the number of ALD cycles and precursors. ALD is a type of chemical vapor deposition (CVD) technique, namely, atomic layer chemical vapor deposition (ALCVD). It can divide the whole CVD reaction into several separate surface reactions. It can have the features of self-limitation and take full advantage of the gas-solid surface reactions. Currently, ALD has been widely used in the fields of microelectronics, nanotechnology, catalysis and energy, etc. (Zaera, 2008; Rolison et al., 2009; Marichy et al., 2012; O'Neill et al., 2015; Palmstrom et al., 2015; Asundi et al., 2019).

As the core of microelectronics technology, the development of large-scale integrated circuits obeys Moore’s law. Since the beginning of the 21st century, the thickness of SiO2 gate dielectrics in MOSFET devices has continuously decreased. However, the tunneling effect of electrons leads to significant leakage and power consumption and seriously affects the stability and reliability of MOSFET devices. Currently, using high-κ gate dielectrics to replace SiO2 is an effective method for solving the problem. Because of the high dielectric constant and thermodynamic stability, zirconium oxide (ZrO2) has been used as gate dielectrics via the ALD method for MOSFET devices (Gaskell et al., 2007; Dezelah IV et al., 2008; Kaipio et al., 2014; Jung et al., 2015; Kanomata et al., 2016; Mahuli et al., 2021; Xu et al., 2021).

In general, the prerequisite and key to the success of ALD technology require suitable precursors. For ZrO2 ALD, the zirconium precursors include these linked by alkyl, halide and alkoxy ligands, such as Zr(Cp)2, ZrCl4 and Zr(OEt2)4 (Williams et al., 2002; Yoshii et al., 2002; Niinistö et al., 2005; Knapas and Ritala, 2008). Subsequently, the zirconium precursor bearing amino ligands is also a candidate for ZrO2 ALD. Because of good volatility, thermal stability and high reactivity, tetrakis(dimethylamino)zirconium (TDMAZ, Zr(NMe2)4) has been studied (Provine et al., 2016). Different precursors have different effects on the overall ALD reaction. Experimentally, thermal ALD of ZrO2 can be performed using Zr(NMe2)4 as the zirconium source and H2O as the oxygen source. It can be written as two separate reactions as follows: ( A )   ZrO 2 − OH * + Zr ( NMe 2 ) 4 → ZrO 2 − O − ZrNMe 2 * + HNMe 2 ( B )   ZrO 2 − O − ZrNMe 2 * + H 2 O → ZrO 2 − O − ZrOH * + HNMe 2 where an asterisk designates a surface species.

To obtain more insight into the ALD reaction mechanism of various materials, many theoretical calculations have been performed (Elliott, 2012; Hu et al., 2015; Elliott et al., 2016). These works include density functional theory (DFT) calculations, molecular dynamics and Monte Carlo simulations. For example, the ALD mechanism of oxides and nitrides, such as SiO2, Si3N4, Al2O3, TiO2, ZrO2 and HfO2, have been widely explored (Mukhopadhyay et al., 2008; Ren et al., 2008; Han et al., 2012; Huang et al., 2013; Huang et al., 2014). To date, only a few mechanisms of ZrO2 ALD using precursors with halide and alkyl ligands, such as ZrCl4 and ZrCp2Me2, have been studied (Brodskii et al., 2002; Jeloaica et al., 2003; Jeloaica et al., 2005; Ren et al., 2011; Zhou et al., 2013). However, the investigation of more effective zirconium precursors bearing amino ligands and their roles and reaction mechanism for ZrO2 ALD is still lacking.

Herein, we investigated the reactions of Zr(NMe2)4 and H2O on surfaces to gain more insight into the reaction mechanism of ZrO2 ALD using DFT. The whole reaction of ZrO2 ALD includes the TDMAZ half-reaction (A1 and A2) and H2O half-reaction (B1–B10), as shown in Figure 1. The results show that both TDMAZ and H2O can react with the hydroxyl and amino groups on the surface. These insights into the reaction mechanism of ZrO2 ALD can improve precursor design and ALD growth for other oxides and zirconium compounds and boost the further development of ALD chemistry.

Possible pathways of ZrO2 ALD using TDMAZ and H2O as precursors.

 2 Computational details

To model the surface reaction of ZrO2 ALD, the cluster model Si63H48(OH)16 was adopted and shown in Figure 2. In general, silicon is used as a substrate material in microelectronic devices. The cluster model is based on the hydroxylated Si(001) surface with four layers of silicon atoms and sixteen hydroxyl (–OH) groups. Our previous and other works both proved that when the size of the surface is larger than the size of precursor molecules, the cluster and slab models can give similar results (Mukhopadhyay et al., 2008; Fang and Ma, 2013). The suspended bonds of the model are saturated with H atoms. To model the surface, the three layers of Si atoms at the bottom are fixed and sixteen Si atoms and hydroxyl groups on the surface are relaxed. The precursors include TDMAZ and H2O as shown in Figure 2.

Surface model for Si63H48(OH)16, TDMAZ and H2O. The yellow, red, white, blue and light blue balls represent Si, O, H, N and Zr atoms, respectively.

All species in ZrO2 ALD reactions were optimized using DFT with the M06-2X functional. The M06-2X functional is one of the most suitable functionals to describe the interactions between the precursors and the surface (Zhao and Truhlar, 2008a; Zhao and Truhlar, 2008b). This functional was also tested using the precursor reaction on the hydroxylated surface in previous work (Fang and Ma, 2013). By comparing different density functionals (M06-2X, PBEPBE and B3LYP) with the MP2 method, it was found that the M06-2X functional is appropriate for ALD surface reactions (Fang and Ma, 2013). Meanwhile, the dispersion correction for weak interactions was performed using Grimme dispersion method with the original D3 damping function (GD3) (Grimme et al., 2010; Grimme et al., 2011). To balance the computational accuracy and cost, the 6-311G(d,p) basis set was used for the relaxed atoms and adsorbates on the surface and the LANL2DZ basis set was used for the Zr atom. Other atoms of the substrate at the bottom were described using the 6-31G basis set. All stationary points and transition states were verified using frequency and intrinsic reaction coordinate (IRC) calculations. The Gibbs free energies were also calculated from the partition functions, as well as the enthalpy and entropy terms at different temperatures (298.15 and 473.15 K) and pressures (1 atm and 0.2 Torr) (Baletto and Ferrando, 2005; Levine, 2008; Li and Truhlar, 2014). Notably, the precursor molecules in the gas phase have three motions of rotation, translation, and vibration. When the precursors are adsorbed on the surface, the rotation and translation motions are lost and new vibrations are produced. In other words, the entropy of the surface has no contribution from translation and rotation, and only has the contribution from the vibrations (Zhou et al., 2022). All optimization, frequency and IRC calculations were performed with the Gaussian 09 program (Frisch et al., 2013). The corresponding thermodynamic properties were calculated by shermo program (Lu and Chen, 2021).

 3 Results and discussion 3.1 Tetrakis(dimethylamino)zirconium reaction on the hydroxylated surface 3.1.1 The elimination of the first amino ligand of tetrakis(dimethylamino)zirconium <italic>via</italic> the A1 reaction

When the precursor tetrakis(dimethylammonium)zirconium (TDMAZ) approaches the hydroxylated surface, it can undergo two steps (A1 and A2) of the substitution and elimination of amino ligands. The Gibbs free energy profiles of the elimination reaction (A1) of the first amino ligand are shown in Figure 3. First, TDMAZ can be adsorbed on the hydroxylated surface to form intermediate Im1 A1 with the adsorption energy ( E ads ) of 31.9 kcal/mol. Then, it can undergo a four-membered ring (4MR) transition state (TS A1 ) with very low activation energy ( E a ) and the imaginary frequency of 373 cm−1. In TS A1 , the Zr atom of the precursor can attack the O atom of the hydroxyl group on the surface. At the same time, the H atom of the hydroxyl group can be transferred to the N atom of the amino ligand of the precursor. Later, the intermediates Im2 A1 and dimethylamine (HNMe2) can be generated. Eventually, the product P A1 (–OZr(NMe2)3) can be formed and the HNMe2 molecule can be released from the surface, in which the desorption energy ( E des ) of dimethylamine is 21.4 kcal/mol. The bond length changes at the reaction center are listed in Table 1. In the reaction process, the lengths of the O–H and Zr–N bonds increase from 0.980 to 2.072 Å in Im1 A1 to 2.536 and 2.368 Å in Im2 A1 , respectively. The lengths of the Zr–O and H–N bonds decrease from 2.481 to 2.119 Å in Im1 A1 to 2.076 and 1.019 Å in Im2 A1 , respectively. All these indicate that O–H and Zr–N bonds are broken and Zr–O and H–N bonds are formed.

Gibbs free energy profiles (ΔG, kcal/mol) of the A1 section of the TDMAZ reaction on the hydroxylated surface.

Bond lengths (Å) at the reaction center in the A1 section of the TDMAZ reaction on the hydroxylated surface.

Species O–H Zr–N Zr–O H–N
Im1 A1 0.980 2.072 2.481 2.119
TS A1 1.088 2.164 2.394 1.506
Im2 A1 2.536 2.368 2.076 1.019

In general, the temperatures and pressures have a certain effect on the ALD surface reaction. As shown in Figure 3, at 298.15 K and 1 atm, the Gibbs activation energy (G a) of the A1 reaction is very low at only 1.3 kcal/mol, which indicates that the reaction can easily occur at room temperature. The desorption of dimethylamine is also easy and requires a low desorption energy of about 6.0 kcal/mol. As a whole, the A1 reaction is exoergic by 30.0 kcal/mol. At 473.15 K and 0.2 Torr, the free energies of the intermediates, Im1 A1 and Im2 A1 , and transition state TS A1 further increase because of temperature and entropy effects. The Gibbs activation energy of the A1 reaction increases to 12.4 kcal/mol. All of these indicate that the A1 reaction of TDMAZ on the hydroxylated surface is thermodynamically and kinetically favorable at the experimental condition of 473.15 K and 0.2 Torr.

 3.1.2 The elimination of the second amino ligand of tetrakis(dimethylamino)zirconium <italic>via</italic> the A2 reaction

The product P A1 (–OZr(NMe2)3) of the A1 section can react further with adjacent hydroxyl groups. In the A2 step, the elimination reaction of the second amino ligand of TDMAZ, there are four available hydroxyl groups on the hydroxylated surface in different directions, a, b, c and d, to form Im1 A2 , shown in Figure 4. Similar to the A1 step, the A2 reaction pathway also goes through a 4MR transition state TS A2 to obtain the bridged product P A2 (–OZr(NMe2)2O–) and release the small molecule dimethylamine. The corresponding Gibbs activation energies are 7.5, 6.6, 5.9 and 8.7 kcal/mol in the four directions relative to the product P A1 (–OZr(NMe2)3), indicating that the elimination of the second amino ligand of TDMAZ can occur easily. This is different from the first amino ligand elimination. In A1 reaction, the precursor adsorption leads to the reduction of the entropy and requires higher energy barrier of the first amino ligand elimination at certain temperature. In A2 reaction, the precursor has been anchored on the surface and the change of the entropy has little effect on the reaction barrier. As a result, the A2 reaction of the second amino ligand is also exoergic by about 20 kcal/mol. The Gibbs free energy of intermediate Im1 A2−a in the a direction is lower than that of transition state TS A2−a , which is caused by the harmonic frequency overestimating the thermal correction of intermediate Im1 A2−a , which is more stable than TS A2−a in terms of electronic energy.

Gibbs free energy profiles (ΔG, kcal/mol) of the A2 section of the TDMAZ reaction with the hydroxylated surface at 473.15 K and 0.2 Torr. Symbols a, b, c and d represent four different directions of –OZr(NMe2)3 with adjacent hydroxyl groups.

According to Table 2, the atomic distances of Zr, O, H and N in the reaction center have similar changes, indicating the breakage of O–H and Zr–N bonds and the formation of Zr–O and H–N bonds. As a whole, all the bond changes in the four directions are similar to each other. Because the product P A2−a has the lowest energy and the most stable structure, the product in the a direction is used as the initial structure for the next reaction.

Bond lengths (Å) at the reaction center in the A2 section of the TDMAZ reaction on the hydroxylated surface.

Species O–H Zr–N Zr–O H–N
Im1 A2−a 0.970 2.119 2.463 2.283
TS A2−a 1.065 2.236 2.360 1.541
Im2 A2−a 3.102 2.470 2.021 1.019
Im1 A2−b 0.966 2.104 2.435 2.328
TS A2−b 1.030 2.208 2.406 1.633
Im2 A2−b 3.159 2.465 2.021 1.018
Im1 A2−c 0.968 2.100 2.522 2.230
TS A2−c 1.042 2.200 2.402 1.597
Im2 A2−c 2.507 2.491 2.038 1.020
Im1 A2−d 0.964 2.072 2.392 2.524
TS A2−d 1.058 2.212 2.315 1.576
Im2 A2−d 2.637 2.439 2.041 1.021
3.2 H<sub>2</sub>O reaction with the aminated surface 3.2.1 H<sub>2</sub>O reaction with the aminated surface <italic>via</italic> the B1 and B2 reactions

After the TDMAZ reaction with the hydroxylated surface, H2O can be pumped into the reactor and react with the aminated surface. The H2O reaction on the surface is more complex than the TDMAZ reaction with the hydroxylated surface and involves 10 reaction pathways (B1 to B10) to eliminate dimethylamine and water molecules, as shown in Figure 1.

As shown in Figure 5, a H2O molecule and the surface P A2 (–OZr(NMe2)2O–) can undergo ligand exchange reaction B1 with the P A2 (–OZr(NMe2)2O–) surface. It can sequentially pass through the intermediate Im1 B1, the 4MR transition state TS B1 and intermediate Im2 B1 . Eventually, the HNMe2 molecule can be released and the product P B1 (–OZr(NMe2)(OH)O–) can be formed. The B1 reaction is exoergic by 20.5 kcal/mol. The G a of the reaction is 14.8 kcal/mol. From Supplementary Table S1, it can be seen that the distance between H and O atoms increases from 0.962 Å in Im1 B1 to 2.282 Å in Im2 B1 , the distance between Zr and N atoms increases from 2.085 to 2.436 Å, whereas the distance between H and N atoms decreases from 2.890 to 1.021 Å, the distance between O and Zr atoms decreases from 2.393 to 2.015 Å. All these indicate that H–O and Zr–N bonds are cleaved and H–N and O–Zr bonds are formed in the B2 reaction.

Gibbs free energy (ΔG, kcal/mol) profiles of the B1 section of the H2O reaction with the aminated surface.

As shown in Figure 6, the H2O molecule can further react with the product P B1 (–OZr(NMe2)(OH)O–). Similar to B1, the B2 reaction is also a ligand exchange reaction between the hydroxyl group and the amino ligand. First, the water molecule can interact with the product P B1 (–OZr(NMe2)(OH)O–) surface to form the intermediate Im1 B2 . Subsequently, it can go through a 4MR transition state TS B2 to form the intermediate Im2 B2 . Finally, the HNMe2 can be released and the product P B2 (–OZr(OH)2O–) can be obtained. The B2 reaction is exoergic by 17.8 kcal/mol and the Gibbs activation energy is 11.8 kcal/mol. Supplementary Table S2 lists the changes in the bond lengths at the reaction center of the B2 reaction. Similar to the B1 reaction, H–O and Zr–N bonds are gradually broken and H–N and O–Zr bonds are gradually formed during the process of the B2 reaction. As a whole, B1 and B2 reactions are both exoergic and require low energy barriers, indicating that H2O and the aminated surface can easily react with each other.

Gibbs free energy (ΔG, kcal/mol) profiles of the B2 section of the H2O reaction with the aminated surface.

 3.2.2 H<sub>2</sub>O reaction with the aminated surface <italic>via</italic> the B3 to B5 reactions

As mentioned above, the TDMAZ reaction on the hydroxylated surface can form another product P A1 (–OZr(NMe2)3), which can also react directly with a water molecule. As shown in Figure 7, H2O can react with the product P A1 (–OZr(NMe2)3) via the B3 pathway. A water molecule can be adsorbed on the –OZr(NMe2)3 surface to form an intermediate Im1 B3 . It can undergo a 4MR transition state TS B3 with the Gibbs activation energy of 14.2 kcal/mol and an intermediate Im2 B3 . Finally, HNMe2 can be released and the product P B3 (–OZr(NMe2)2(OH) can be formed, releasing the small molecule. The B3 reaction is also exoergic by 13.5 kcal/mol. As listed in Supplementary Table S3, the lengths of the H–O and Zr–N bonds gradually increase and the lengths of the H–N and O–Zr bonds gradually decrease. All these indicate that the H–O and Zr–N bonds at the reaction center are broken and the H–N and O–Zr bonds are formed during the B3 reaction.

Gibbs free energy (ΔG, kcal/mol) profiles of the B3 section of the H2O reaction with the aminated surface.

Similarly, other water molecules can further attack the Zr atom and react with the aminated surface. As shown in Figure 1, the reaction processes of the B4 and B5 pathways are similar to that of the B3 reaction. They can form the intermediates Im2 B4 and Im2 B5 and undergo 4MR transition states TS B4 and TS B5 , respectively. Lastly, HNMe2 can be released and the products P B4 and P B5 can be obtained, as shown in Figures 8, 9. The B4 and B5 reactions are exoergic by 27.0 and 12.8 kcal/mol and the corresponding Gibbs activation energies are 12.4 and 12.3 kcal/mol, respectively. However, different from the desorption of HNMe2 in the B4 reaction, the release of HNMe2 in the B5 reaction requires a low energy of 2.3 kcal/mol. Supplementary Tables S4, S5 list the changes in the bond lengths at the reaction centers, which indicate that the H–O and Zr–N bonds are cleaved and the H–N and O–Zr bonds are formed.

Gibbs free energy (ΔG, kcal/mol) profiles of the B4 section of the H2O reaction with the aminated surface.

Gibbs free energy (ΔG, kcal/mol) profiles of the B5 section of the H2O reaction with the aminated surface.

 3.2.3 Coupling reactions between surface amino and hydroxyl groups <italic>via</italic> the B6 and B7 reactions

From Figure 1, it can be seen that amino ligands of the intermediate products P B3 and P B4 can react not only with water but also with the adjacent hydroxyl groups on the surface to eliminate the amino ligands and form dimethylamine and bridged products, namely, the coupling reactions between surface amino hydroxyl groups. As shown in Figure 10, the Zr atom of the product P B3 (–Zr(NMe2)2OH) can attack the O atom on the adjacent hydroxyl group to form an intermediate Im1 B6 with lower energy. Then, the H atom of the adjacent hydroxyl group on the surface can be transferred to the N atom of the amino ligand to form dimethylamine. It can go through a lower-energy 4MR transition state TS B6 with the Gibbs activation energy of 3.2 kcal/mol and an intermediate Im2 B6 . Finally, HNMe2 is released and the intermediate product P B6 is generated. As a whole, the B6 reaction is exoergic by 34.0 kcal/mol. As listed in Supplementary Table S6, the lengths of the H–O and Zr–N bonds gradually increase and the lengths of the H–N and O–Zr bonds gradually decrease in the reaction center. The bond lengths of Zr–N and O–H increase from 2.127 and 0.976 Å in Im1 B6 to 2.465 and 2.898 Å in Im2 B6 , and the bond lengths of N–H and Zr–O decrease from 2.012 and 2.478 Å to 1.020 and 1.987 Å, respectively.

Gibbs free energy (ΔG, kcal/mol) profiles of the B6 section of the H2O reaction with the aminated surface.

Similar to the B6 reaction, the Zr atom of the intermediate product P B4 (–OZr(NMe2)(OH)2) can attack the O atom on the surrounding hydroxyl group during the B7 reaction. It can go through an intermediate Im1 B7 and a hydrogen-transfer transition state TS B7 , in which the H atom on the hydroxyl group can be transferred to the N atom of the dimethylamino group. Lastly, the HNMe2 molecule can be released and intermediate product P B7 (–OZr(OH)2O–) can be formed. The B7 reaction is exoergic by 24.8 kcal/mol and requires a very low Gibbs activation energy of 2.4 kcal/mol. From Figure 11, it can be seen that the Gibbs free energy of intermediate Im1 B7 is higher than that of TS B7 , but the electronic energy of Im1 B7 is lower than that of TS B7 , which results from the overestimation of the thermodynamic correction of Im1 B7 by the harmonic frequency. According to the data in Supplementary Table S7, the tendencies of the breakage of H–O and Zr–N bonds and the formation of H–N and O–Zr bonds are also shown.

Gibbs free energy (ΔG, kcal/mol) profiles of the B7 section of the H2O reaction with the aminated surface.

 3.2.4 Coupling reactions between surface hydroxyl groups <italic>via</italic> the B8 to B10 reactions

In addition, the hydroxyl groups on the intermediate products P B3 , P B4 and P B5 can also combine with the adjacent hydroxyl groups to form water molecules, namely the coupling reactions between surface hydroxyl groups. These coupling reactions correspond to B8, B9 and B10 reactions, shown in Figure 1. The structure of the Zr reaction center can change from a tetrahedral structure to a bridged structure.

In the B8 reaction, the Zr atom of P A3 can attack the O atom of the adjacent hydroxyl group on the surface to form the intermediate Im1 B8 , which is similar to the B6 reaction. Subsequently, the H atom of the hydroxyl group on the surface can react with the adjacent hydroxyl group to form a water molecule. Considering that the steric hindrance of the hydroxyl group is smaller than that of the dimethylamino group, the hydroxyl group on the Zr atom can react with the adjacent hydroxyl group not only from above but also from the side, as shown in Figure 12. The corresponding Gibbs activation energy is 4.6 or 3.3 kcal/mol, respectively. As a whole, the B8 reaction is exoergic by 13.5 kcal/mol. As listed in Supplementary Table S8, the changes in the bond lengths of the reaction center show the cleavage of old O–H and Zr–O bonds and the formation of new O–H and Zr–O bonds.

Gibbs free energy (ΔG, kcal/mol) profiles of the B8 section of the H2O reaction with the aminated surface.

As shown in Figure 13, the B9 reaction process is similar to the B8 section. The Zr atom of intermediate P B4 can attack the adjacent hydroxyl group to form intermediate Im B9 . Subsequently, the hydroxyl group on the Zr atom can react with the adjacent hydroxyl group through the transition state TS B9 and the intermediate Im2 B9 . Finally, the product P B8 (–OZr(NMe2)(OH)O–) can be obtained. The Gibbs free energy activation energy is about 5.0 kcal/mol. The desorption energy of H2O release is about 14.0 kcal/mol. The whole B9 process is exoergic by 7.0 kcal/mol. From Supplementary Table S9, it can be seen that with the elimination of the hydroxyl group, the lengths of Zr–O′ and O–H bonds increase from 2.004 and 0.970 Å to 2.284 and 2.882 Å, and the lengths of Zr–O and O′–H bonds decrease from 2.398 and 2.491 Å to 2.030 and 0.980 Å, respectively. All these indicate that the Zr–O bond is formed and the –OH group is eliminated.

Gibbs free energy (ΔG, kcal/mol) profiles of the B9 section of the H2O reaction with the aminated surface.

As shown in Figure 14, the intermediate product P B5 (–OZr(OH)3) of the B5 reaction can also eliminate a hydroxyl group on the Zr atom by bridged reaction B10. The hydroxyl group on P B5 can react with the H atom on the adjacent hydroxyl group via the intermediate Im B10 and the 4MR transition state TS B10 to release H2O molecules and form the final product P B10 (–OZr(OH)2O). The Gibbs activation energy in the B10 reaction is 7.9 kcal/mol and the desorption energy of the water molecule is 7.1 kcal/mol. Supplementary Table S10 lists the bond lengths at the reaction center, which indicates the breakage of old bonds and the formation of new bonds in the B10 section. In comparison with B6–B10, the Gibbs activation energies of the dimethylamino elimination reactions are lower than those of hydroxyl elimination reactions, which indicates that the elimination reactions of dimethylamine occur relatively easily and are kinetically more favorable.

Gibbs free energy (ΔG, kcal/mol) profiles of the B10 section of the H2O reaction with the aminated surface.

 4 Conclusion

Through DFT calculations, possible pathways for the ZrO2 ALD reaction of tetrakis(dimethylammonium)zirconium and water on the hydroxylated Si(100) surface were investigated in detail. The whole reaction mechanism includes two main reactions: TDMAZ reactions with the hydroxylated surface and water reactions with the aminated surface. In the TDMAZ reaction, the precursor can eliminate the dimethylamino group by a substitution reaction with the hydroxyl group on the surface. At the same time, the second dimethylamino group of the precursor can be eliminated with the help of other hydroxyl groups on the surface. Considering the configuration of the hydroxylated surface and the Zr–O bond length, only up to two dimethylamines can be eliminated on the Si surface, and the remaining dimethylamine needs to be eliminated by the H2O reaction. With increasing temperature, the release of a small molecule adsorbed on the surface takes place more readily. In the H2O reaction, the ligand exchange reactions and coupling reactions can alternately occur. In the ligand exchange reactions between the hydroxyl and amino groups, the Gibbs activation energies of the reaction are about 12 kcal/mol, which are the highest in the H2O reaction. In the coupling reactions, the hydroxyl or amino groups can react with the neighboring hydroxyl group with lower Gibbs activation energy. Moreover, the coupling reaction of the dimethylamino ligand with the hydroxyl group on the surface is easier than that between the hydroxyl groups on the surface. All these insights into ZrO2 ALD could guide the design of new precursors and ALD preparation of other oxides and zirconium compounds.

 Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

 Author contributions

RX: Investigation, Methodology, Writing. ZZ: Investigation. JL: Investigation. XZ: Investigation. YZ: Investigation. HX: Investigation, Resources, Writing. LX: Investigation, Resources, Writing. YD: Investigation. AL: Investigation. GF: Investigation, Resources, Writing.

This work was supported by National Natural Science Foundation of China (21873073 and 52073142), Zhejiang Provincial Natural Science Foundation (LY22B030007), Open Project (M30031 and M35047) of National Laboratory of Solid State Microstructures at Nanjing University and Hongzhiwei Technology.

 Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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