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Surface Orientation and Temperature Effects on Interaction of Silicon With Water: Molecular Dynamics Simulations Using ReaxFF Reactive Force Field Jialin Wen, Tianbao Ma, Weiwei Zhang, Adri C.T. van Duin, and Xinchun Lu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11310 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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The Journal of Physical Chemistry

Surface Orientation and Temperature Effects on Interaction of Silicon with Water: Molecular Dynamics Simulations Using ReaxFF Reactive Force Field Jialin Wen1, Tianbao Ma1, Weiwei Zhang2, Adri C.T. van Duin2, Xinchun Lu1* 1

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

2

Department of Mechanical and Nuclear Engineering, Pennsylvania State

University, University Park, Pennsylvania 16802, United States

ABSTRACT: In this work, we use ReaxFF molecular dynamics simulations to investigate the interaction between water molecules and silicon surfaces with different orientations under ambient temperatures of 300 K and 500 K. We studied the water adsorption and dissociation processes as well as the silicon oxidation process on the Si (100), (110) and (111) surfaces. The simulation results indicate that water can adsorb on the Si surfaces in the forms of molecular adsorption and dissociative adsorption, making the surfaces terminated by H2O, OH and H species. The molecular adsorption of H2O dominates the (100) and (110) surfaces, while the dissociative adsorption dominates the (111) surface. Besides, the adsorbed hydroxyl oxygen can insert into the Si-Si bond of the substrate to make the surface oxidized, forming the Si-O-Si bonds. Our simulation results also indicate that the (100) surface is mostly terminated by H while (111) is mostly terminated by OH. The higher temperature causes more H2O to dissociate and also make all these

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surfaces more oxidized. Our results are consistent with most experiments. This study sheds lights on the wet oxidation process of Si and Si surface structure evolution in microelectromechanical systems as well as Si chemical mechanical polishing process. 1. INTRODUCTION The interaction between water and silicon has attracted considerable attention in the semiconductor industry due to the significant roles of water in the wet oxidation process,1,2 friction and wear behaviors in the microelectromechanical systems (MEMS)3–5 as well as the Si chemical mechanical polishing (CMP) process.6–8 In the wet oxidation process, water plays a critical role in the growth of SiO2 films on the Si substrate.2 In MEMS, it is not easy to prohibit water from the interacting surfaces, and water can have both detrimental and beneficial effects on the performance of the MEMS systems because of the high adhesion and static friction force as well as lubrication role associated with its existence.4 The Si surface termination has a direct effect on the surface properties that hydrophilicity of silicon wafers is caused by adsorbed OH groups, while hydrophobicity is caused by adsorbed H.9 The hydrophilicity of the surface can affect the interfacial friction force3 as well as the wear behavior of Si surfaces5 and suggests the technology of surface treatment to minimize the potential wear failure of MEMS. The Si CMP process demonstrates its importance in semiconductor devices. It has been

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experimentally studied that after interaction with water molecules, the silicon surfaces are predominated by hydrogen termination, which is responsible for its strong hydrophobicity and chemical stability.7 Besides, hydrogen termination varies due to different surface orientations, for example, the Si (111) surface is characterized by an ideal monohydride termination, while the Si (100) surface has a variety of hydrides.8 Si orientations can significantly influence the material removal process as well as the Si surface state during the CMP process. Since the interaction between Si and H2O plays such an important role in these semiconductor engineering processes, characterizing the interaction process is of critical importance. Many experiments10–20 were conducted to understand the interaction process between Si and H2O by inspecting the Si surfaces termination after interaction with H2O using techniques such as electron energy loss spectroscopy and high-solution surface infrared spectroscopy. Besides, theoretical calculations2,21–29 were performed to uncover the behavior of H2O on the Si surfaces. Based on these studies, it is known that water molecules can adsorb on the silicon surface, and some of them dissociate into H atoms and OH groups (dissociative adsorption30) while some of the adsorbed water molecules do not dissociate (molecular adsorption30), making the silicon surface terminated by H2O, H and OH.10–19 In addition, the silicon surface can also be oxidized during this interaction process.1,15,20 Besides, the experiments show that the surface orientation

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significantly affects the adsorption and dissociation behaviors of H2O on the Si surfaces,10 and the temperature can also influence the oxidation process based on the study of the annealing process of water induced oxidation of silicon.1,20 In this work, ReaxFF molecular dynamics simulations have been employed to study the dynamic interaction process between H2O and Si surfaces with different orientations. Three Si surfaces, namely (100), (110) and (111), are exposed to the H2O molecules under 300 K and 500 K, so as to investigate the surface orientation effects as well as the thermal evolution of water exposed Si surfaces. Since the ReaxFF reactive force field leads to proper description of the formation and dissociation of bonds and is able to simulate the reactive systems containing a large number of atoms (1000s of atoms).31 it is a proper means to study the adsorption and dissociation process of H2O on the Si surfaces. The next section describes the computational details, including the ReaxFF reactive force field method and the computational setup. The subsequent sections describe the molecular dynamics simulation results and discussions about the Si surface chemistry after reaction with H2O, adsorption and dissociation process of H2O, and the oxidation process of Si surfaces. Conclusions are given in the final section. 2. COMPUTATIONAL DETAILS

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2.1 ReaxFF Reactive Force Field Method. In this study, molecular dynamics (MD) simulations using the ReaxFF reactive force field31 were employed to study the reaction process between Si and H2O. Different from the nonreactive force field, ReaxFF uses a general relationship between bond order and bond distance as well as between bond energy and bond order that allows proper formation and dissociation of bonds during MD simulations. The total system energy is divided into variable partial energy contributions, including bond energies (‫ܧ‬௕௢௡ௗ ), lone pair energy (‫ܧ‬௟௣ ), the energy to penalize over coordination of atoms (‫ܧ‬௢௩௘௥ ), the energy to stabilize under coordination of atoms (‫ܧ‬௨௡ௗ௘௥ ), valence angle energies (‫ܧ‬௩௔௟ ), the additional energy penalty to reproduce the stability of systems with two double bonds sharing an atom in a valence angle (‫ܧ‬௣௘௡ ), three-body conjugation term (‫ܧ‬௖௢௔ ), C2 molecule correction term (‫ܧ‬஼ଶ ), triple bond energy correction term (‫ܧ‬௧௥௜௣௟௘ ), torsion angle energies (‫ܧ‬௧௢௥௦ ), conjugation energy (‫ܧ‬௖௢௡௝ ), Hydrogen bond interaction energy (‫ܧ‬ுି௕௢௡ௗ ), van der Waals interaction energy (‫ܧ‬௩ௗௐ௔௔௟௦ ) and Coulomb interaction energy (‫ܧ‬஼௢௨௟௢௠௕ ). ‫ܧ‬ୱ୷ୱ୲ୣ୫ = ‫ܧ‬௕௢௡ௗ + ‫ܧ‬௟௣ + ‫ܧ‬௢௩௘௥ + ‫ܧ‬௨௡ௗ௘௥ + ‫ܧ‬௩௔௟ + ‫ܧ‬௣௘௡ + ‫ܧ‬௖௢௔ + ‫ܧ‬஼ଶ + ‫ܧ‬௧௥௜௣௟௘ + ‫ܧ‬௧௢௥௦ + ‫ܧ‬௖௢௡௝ + ‫ܧ‬ுି௕௢௡ௗ + ‫ܧ‬௩ௗௐ௔௔௟௦ + ‫ܧ‬஼௢௨௟௢௠௕ A detailed description of these terms is given by Chenoweth et al.32 Since the parameters of this force field is trained against quantum chemical results, the 5



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simulations based on such force field can ensure the accuracy of chemical reaction and deal with a relatively larger chemical reaction system with a longer time scale. ReaxFF reactive MD simulation method has already been successfully applied to interaction between metal/metal oxide and water.33–36 In this work, we employ the ReaxFF reactive force field developed based on the combination of Si/Ge/H force field37 which deals with the reaction between Si and H atoms, with the water force field38 which can describe the properties of water molecules, so as to describe the reaction between Si and H2O properly. This reactive force field has been validated in our previous work.39 In order to account for the electrostatic interactions, we use the charge equilibration (QEq) model40,41 to equilibrate charge of the system at each time step. 2.2 Computational setup. The interaction between silicon and water are studied on three characteristic surfaces, the Si (100), (110) and (111). Since these surfaces were cleaved from a bulk crystal, they were equilibrated under 300 K using the Berendsen42 heat bath for 50 ps with a damping constant of 25 fs prior to reaction with water. Then these reconstructed Si surfaces were each combined with 10 Å thick water molecules to construct the final models, as shown in Figure 1. The details of these models are shown in Table 1.

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Figure 1. Side views of the Si and H2O systems, which contain the Si (yellow), O (red) and H (white) atoms.

Table 1. Initial configurations of H2O and Si surfaces. Cell dimensions of the configurations, thickness of Si substrate (t) and numbers of Si atoms as well as numbers of H2O molecules. t is the thickness of the Si substrate, N1 is the total number of Si atoms in the system, N2 is the number of 1 monolayer (ML) Si atoms and N3 is the number of H2O molecules in the system. Surface

(100)

(110)

(111)

x (Å)

38.40

38.01

38.40

y (Å)

38.40

38.40

39.91

z (Å)

36.45

36.57

36.17

t (Å)

19.01

19.20

18.81

N1

1500

1540

1560

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N2

100

140

120

N3

490

485

505

Periodic boundary conditions were applied along both x and y directions and fixed boundary conditions were applied along the z direction. In order to confine the water molecules as well as avoid the reaction between water and the silicon bottom surface, a reflecting wall was used at the top of the cell along the z direction, as used in the previous work.35 Lammps code43,44 was used to perform all of the simulations, using the canonical (NVT) ensemble with a time step of 0.25 fs. The temperature was controlled using the Berendsen42 thermostat with a damping constant of 25 fs and the Verlet algorithm was adopted to integrated the atom trajectories, the total simulation time is 500 ps. Ovito45 was used to produce the snapshot pictures of all of the simulations. In order to study the temperature effects, two kinds of temperature conditions were employed, 300 K and 500 K. 3. RESULTS AND DISCUSSION 3.1 Si surface chemical state after interaction with H2O. After 500 ps interaction between silicon surfaces and water molecules, the system potential energy has become stable and the water has fully reacted with the Si surfaces. The side views under both 300 K and 500 K as well as the top views under 300 K of the silicon surfaces at 500 ps are shown in Figure 2, for the views under 500 K, they

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present the similar results and are not shown here. From the top views of Si surfaces at 300 K, it is obvious that the surfaces are terminated by H, OH and H2O, which is consistent with most experiments.14,15 For the H termination on the Si surfaces, there exist two kinds of species on the (100) surface, the monohydride and dihydride species, while only the monohydride species were observed on (110) and (111) surfaces. Besides, oxygen atoms diffuse into the Si substrates to form SiO-Si bonds, making the Si surfaces oxidized, and the (100) surface seems to be the most oxidized while the least for (111) surface. It is obvious that with the increase of the temperature, oxygen atoms diffuse deeper into these three different kinds of surfaces, especially for the substrate with the (110) surface. In addition, after interacting with H2O, there are HSi-O-SiH and HSi-SiH dimers on the Si (100) surface, which is in agreement with the experimental results of H2O oxidized Si surface state detected by surface infrared adsorption spectroscopy.1

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Figure 2. Side and Top views of Si unit cell after 500 ps simulations at 300 K and 500 K.

3.2 Adsorption and dissociation of H2O on Si surfaces. To understand the adsorption and dissociation of H2O on the Si surfaces, we present the details in Figure 3. Figure 3a~d shows the interaction processes between H2O molecules and the Si surface, and only three H2O molecules are visualized so as to clearly display these processes. It is shown that when the H2O gets close to the Si surface (Figure 3a), it adsorbs on one Si atom (Figure 3b) and dissociates into OH and H, the OH 10


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adsorbs on the Si surface to form a Si-OH bond, while the H then combines with a nearby H2O to form a hydronium (H3O+) (Figure 3c). Once the H3O+ is formed, the proton can easily transfer in the water phase (i.e. from the second water molecule to the third one, as shown in Figure 3d), which has already been confirmed by the ab initio MD simulation study.46 In addition, H2O can also dissociate directly into H and OH when it gets close to the Si surface, and the H combines with a Si atom to form a Si-H bond as shown in Figure 3e~f. As demonstrated in Figure 3g~i, it is observed that the Si-OH formed in Figure 3c decomposes, and the hydroxyl oxygen inserts into a Si-Si bond to form the Si-O-Si bond while the hydroxyl hydrogen combines with a nearby Si atom to form the Si-H bond, which is also true for the experimental observations10,14 and the first-principles calculations.2

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Figure 3. Interaction between H2O and Si (100) substrate surface. (a) Three H2O molecules on the Si substrate surface. (b) Adsorption of a H2O on the Si substrate surface. (c) Dissociation process of a H2O as well as the formation of Si-OH and a H3O+. (d) H proton transfer process between two H2O molecules. (e) One H2O molecule on the Si substrate surface. (f) Dissociation of H2O and formation of Si-H. (g) Si-OH bond on the Si substrate surface. (h) Dissociation of OH on the Si substrate surface. (i) Formation of Si-O-Si and Si-H.

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There is evidence for both molecular adsorption and dissociative adsorption of H2O on the Si surface according to STM experiments.15 In order to study the effects of both surface orientation and temperature on the adsorption and dissociation behaviors of H2O on Si surfaces, Figure 4 presents the coverages of Si-H2O, Si-OH and Si-H during the interaction process, which are relative to the number of 1 ML Si atoms of the Si substrates, as shown in Table 1. According to Figure 4a, (100) has the highest H2O coverage while (111) presents the lowest H2O coverage at 500 ps. At the initial few picoseconds, there is a sharp increase followed by a sudden decrease of H2O coverages for all these three kinds of Si substrates, demonstrating that once the H2O molecules adsorb on the Si surfaces, they quickly decompose into OH and H, which is confirmed by the increase of coverages of OH and H on the Si surfaces as shown in Figure 4b and Figure 4c. The adsorption and dissociation of H2O molecules can also be proved from Figure 4d that the total numbers of H2O molecules that are not adsorbed on Si surfaces decrease to relatively stable values during the interaction process, besides, with the increase of temperature, the numbers of H2O also decrease, this may be because higher temperature can increase the possibility of collision between H2O molecules and Si surfaces, thus increasing the possibility of adsorption and dissociation of H2O on Si surfaces. For the adsorption of OH on the Si surfaces as shown in Figure 4b, (111) has the highest Si-OH coverage, which means that the Si (111) surface

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becomes the most hydrophilic after interaction with H2O. The coverages of Si-OH on all three kinds of surfaces also present a sharp increase followed by a sudden decrease, demonstrating the dissociation of the adsorbed OH during the interaction process. For the adsorption of H on the Si surfaces as shown in Figure 4c, (100) has the highest Si-H coverage while (111) has the lowest coverage. The Si-H coverages keep steady after a major increase due to the dissociation of H2O, demonstrating that the Si-H bond is relatively stable during the whole interaction process.

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Figure 4. Coverages of Si-H2O (a), Si-OH (b) and Si-H (c) on Si surfaces with different orientations and number of H2O in the systems (d) under 300 K and 500 K during the simulation time of 500 ps.

As shown in Figure 2, the adsorbed H atoms exist in two forms on the Si surfaces, namely the monohydride and dihydride species, which is consistent with the results of the infrated-absorption spectroscopy of Si surfaces after the CMP process conducted by Pietsch et. al.8 To further study their contribution, we show the coverages of such two kinds of species on different Si surfaces at different temperatures in Figure 5. Comparing Figure 5a and 5b, it is clear that the adsorbed H on the Si surfaces mainly exist in the form of monohydride Si-H for all these three kinds of surfaces while only a few dihydride Si-H species appear on the Si surface at the temperature of 300 K. For example, the monohydride species accounts for up to 60% (Figure 5a), while the dihydride species accounts for only about 2% (Figure 5b) for the Si (100) surface at 300 K. As temperature increases to 500 K, the coverage of dihydride Si-H species shows increase tendency on these three kinds of surfaces, this can also be ascribed to the stronger interaction between Si and H2O under the higher temperature.

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Figure 5. Coverages of monohydride (a) and dihydride (b) species on the three Si surfaces at 300 K and 500 K.

To evaluate which kind of adsorption (molecular or dissociative adsorption) dominates the interaction between Si substrate and H2O, the ratio (w) of the number of Si-H2O to the number of Si-OH is given in Figure 6a. For the (100) and (110) surfaces, molecular adsorption dominates the interaction process at either temperature, while dissociative adsorption is the main interaction for the (111) surface. With the increase of temperature, the ratio w will increase for all these 16


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three kinds of surfaces. To quantify the dissociation of water, we calculated the water dissociation percentage (WDP), which is the percentage of water molecules that have dissociated in the whole simulation box.36 Figure 6b gives the WDP over the Si surfaces under the temperature of 300 K and 500 K. As temperature increases, the WDP increase, demonstrating that more water molecules are likely to dissociate under a higher temperature condition.

Figure 6. (a) Ratio (w) of the Si-H2O number to the Si-OH number for the Si surfaces with different surface orientations at 300 K and 500 K. The data are

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average of the last 100 ps. (b) WDP over (100), (110) and (111) Si surfaces at 300 K and 500 K. 3.3 Oxidation of Si surfaces. It is known that the incorporation of the oxygen atom into the Si surface can make the Si substrate oxidized during the interaction between H2O and Si substrates from the results of both surface infrared absorption spectroscopy and density function calculations.1 We also observed such phenomenon in our MD simulations as shown in Figure 2 and Figure 3i. In order to show the oxidation state of Si surfaces, Mulliken charge distributions are used to characterize the local oxidation state of Si atoms and the results are displayed in Figure 7. It is clear that surface Si atoms are oxidized after interaction with H2O, with the charge of some Si atoms to be close to 1.0 e. However, these three kinds of surfaces exhibit different degrees of oxidation that the (100) surface is the most oxidized in comparison with other surfaces at the same temperature.

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Figure 7. Top views of the Si atoms of Si surfaces after interaction with H2O at 500 ps at 300 K and 500 K. The atoms are colored by Mulliken charge.

Comparing the charge distributions at different temperatures, it is found that all three surfaces are more oxidized with the increase of temperature. To understand details of the temperature effect, we showed the number of oxidized Si atoms of the three substrates as a function of the simulation time in Figure 8. From Figure 8a, the oxidation process of Si can be divided into two stages, a first quick oxidation stage and a second relatively slow oxidation stage. These two stages are governed by surface orientations as well as the ambient temperature. As temperature increases, the number of oxidized Si atoms increases, especially for 19



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(100). At the same temperature, the surfaces present different initial oxidation rate behaviors. To compare the initial oxidation process, the oxidation rates during the initial 5 ps are calculated as the number of oxidized Si atoms in this time period. As shown in Figure 8b, under either temperature conditions, the (111) surface yields the highest initial oxidation rate while the (110) surface yields the lowest oxidation rate.

Figure 8. Oxidation of Si surfaces. (a) Number of oxidized Si atoms of different Si surfaces under 300 K and 500 K during the whole simulation time of 500 ps. (b) 20


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Initial surface oxidation rate for surfaces with (100), (110) and (111) orientations under 300 K and 500 K.

The oxidation of the Si surfaces is caused by the diffusion of O atoms into the substrates after the water dissociation, which is similar to the products during the oxidation process of silicon with oxygen.47,48 Therefore, we studied the oxygen distribution as well as the number of different oxidized Si components on all surfaces and different temperatures and the results are shown in Figure 9. The number of O atoms are summed every 0.4 Å sized bin. Under either temperatures, O atoms diffuse the deepest into the (110) surface with the depth of 6.3 Å and 13.7 Å below the initial surface at 300 K and 500 K, respectively (Figure 9b), while O atoms diffuse the shallowest into the (111) surface with the depth of 3.2 Å and 5.0 Å (Figure 9c). It also indicates that O atoms diffuse into the bulk with the increase of temperature. The oxidized Si atoms exist in the form of Si1+, Si2+, Si3+ and Si4+ components, which bind to one, two, three and four nearest-neighbor oxygen atoms, respectively. As Figure 9d shows, Si1+ accounts for the most among the oxidized Si atoms, while Si4+ accounts for the least under 300 K, which is also true for the surfaces under 500 K. With the increase of temperature, these components

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increase with the steady insertion of more oxygen atoms, which can also be ascribed to the stronger interaction between H2O and Si surfaces.

Figure 9. Oxygen and oxidized Si component distribution. (a) ~ (c) Oxygen atoms distributions on Si surfaces along the z direction at 500 ps under 300 K and 500 K. (d) Number of Si oxide components on Si surfaces with different orientations under different temperatures: Si1+, Si2+, Si3+ and Si4+ bind to one, two, three and four nearest neighbor oxygen atoms, respectively.

4. CONCLUSIONS 22


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The interaction between water molecules and silicon surfaces with (100), (110) and (111) orientations at 300 K and 500 K ambient temperatures have been investigated using ReaxFF reactive molecular dynamics simulation. We found that H2O is likely to adsorb on the Si surfaces by both molecular and dissociative adsorptions, making the surfaces terminated by H2O, OH and H species. The molecular adsorption of H2O dominates the (100) and (110) surfaces, while the dissociative adsorption dominates the (111) surface. (100) is mostly adsorbed by and H, while (111) is mostly adsorbed by OH. There exist two kinds of Si-H species namely the monohydride and the dihydride species on the Si surfaces. At the low temperature such as 300 K, all three kinds of surfaces are covered by monohydride species, while only (100) is covered by the dihydride species. The increased temperature can enhance both adsorption types on the Si surfaces. Furthermore, the OH group that adsorbs on the Si surfaces can dissociate and break the Si-Si bonds to form the Si-O-Si bonds, making the surfaces oxidized. It is observed that (100) is the most oxidized while (111) is the least oxidized at the same temperature. Besides, higher temperatures can make the surfaces more oxidized, and the oxide thickness increases with increasing temperature due to the enhanced diffusivity of O atoms. Our simulation results may be helpful to illustrate the microstructures of Si surfaces during interaction with H2O accounting for the surface orientation and

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temperature effects. More importantly, they uncover the dynamic evolution of the Si surface structures during the interaction between Si and H2O at the atomic level, which may be useful for controlling this process more accurately so as to achieve the required Si surface structures. AUTHOR INFORMATION Corresponding Author *Phone: +861062797362. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (Grants 91323302, 51335005, 51375010). Simulations were carried out on the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology.

References (1)

Weldon, M.; Stefanov, B.; Raghavachari, K.; Chabal, Y. Initial H2O-Induced Oxidation of Si(100)– (2×1). Phys. Rev. Lett. 1997, 79 (15), 2851–2854.

(2)

Stefanov, B. B.; Raghavachari, K. Pathways for Initial Water-Induced Oxidation of Si(100). Appl. Phys. Lett. 1998, 73 (6), 824–826.

24


Page 24 of 32

Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3)

Scherge, M.; Li, X.; Schaefer, J. A. The Effect of Water on Friction of MEMS. Tribol. Lett. 1999, 6 (3/4), 215–220.

(4)

Patton, S. T.; Cowan, W. D.; Eapen, K. C.; Zabinski, J. S. Effect of Surface Chemistry on the Tribological Performance of a MEMS Electrostatic Lateral Output Motor. Tribol. Lett. 2001, 9 (3–4), 199–209.

(5)

Yu, J.; Qian, L.; Yu, B.; Zhou, Z. Effect of Surface Hydrophilicity on the Nanofretting Behavior of Si(100) in Atmosphere and Vacuum. J. Appl. Phys. 2010, 108, 1–10.

(6)

Pietsch, G. J.; Higashi, G. S.; Chabal, Y. J. Chemomechanical Polishing of Silicon: Surface Termination and Mechanism of Removal. Appl. Phys. Lett. 1994, 64 (23), 3115–3117.

(7)

Pietsch, G. J.; Chabal, Y. J.; Higashi, G. S. The Atomic-Scale Removal Mechanism during Chemo-Mechanical Polishing of Si(100) and Si(111). Surf. Sci. 1995, 331–333 (Part A), 395–401.

(8)

Pietsch, G. J.; Chabal, Y. J.; Higashi, G. S. Infrared-Absorption Spectroscopy of Si(100) and Si(111) Surfaces after Chemomechanical Polishing. J. Appl. Phys. 1995, 78 (100), 1650–1658.

(9)

Grundner, M.; Jacob, H. Investigations on Hydrophilic and Hydrophobic Silicon (100) Wafer Surfaces by X-Ray Photoelectron and High-Resolution

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Electron Energy Loss-Spectroscopy. Appl. Phys. A Solids Surfaces 1986, 39 (2), 73–82. (10) Ibach, H.; Wagner, H.; Bruchmann, D. Dissociative Chemisorption of H2O on Si(100) and Si(111) - a Vibrational Study. Solid State Commun. 1982, 42 (6), 457–459. (11) Schmeisser, D.; Himpsel, F. J.; Hollinger, G. Chemisorption of H2O on Si(100). Phys. Rev. B 1983, 27 (12), 7813–7816. (12) Schmeisser, D. A Comparative Study of O2, H2 and H2O Adsorption on Si(100). Surf. Sci. 1984, 137 (1), 197–210. (13) Chabal, Y. J. Hydride Formation on the Si(100):H2O Surface. Phys. Rev. B 1984, 29 (6), 3677–3680. (14) Zhou, X. L.; Flores, C. R.; White, J. M. Adsorption and Decomposition of Water on Si(100): A TPD and SSIMS Study. Appl. Surf. Sci. 1992, 62 (4), 223–237. (15) Chander, M.; Li, Y.; Patrin, J.; Weaver, J. Si (100)-(2× 1) Surface Defects and Dissociative and Nondissociative Adsorption of H2O Studied with Scanning Tunneling Microscopy. Phys. Rev. B 1993, 48 (4), 2493–2499. (16) Andersohn, L.; Köhler, U. In Situ Observation of Water Adsorption on Si(100) with Scanning Tunneling Microscopy. Surf. Sci. 1993, 284 (1–2), 77–90.

26


Page 26 of 32

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Yu, S. Y.; Kim, H.; Koo, J. Y. Extrinsic Nature of Point Defects on the Si(001) Surface: Dissociated Water Molecules. Phys. Rev. Lett. 2008, 100 (3), 1–4. (18) Yu, S. Y.; Kim, Y. S.; Kim, H.; Koo, J. Y. Influence of Flipping Si Dimers on the Dissociation Pathways of Water Molecules on Si(001). J. Phys. Chem. C 2011, 115 (50), 24800–24803. (19) Liu, D.; Li, L.; Gao, Y.; Wang, C.; Jiang, J.; Xiong, Y. The Nature of Photocatalytic “water Splitting” on Silicon Nanowires. Angew. Chemie - Int. Ed. 2015, 54 (10), 2980–2985. (20) Weldon, M. K.; Queeney, K. T.; Gurevich, A. B.; Stefanov, B. B.; Chabal, Y. J.; Raghavachari, K. Si-H Bending Modes as a Probe of Local Chemical Structure: Thermal and Chemical Routes to Decomposition of H2O on Si(100)-(2×1). J. Chem. Phys. 2000, 113 (6), 2440–2446. (21) Ciraci, S.; Erkoç, Ş.; Ellialioglu, Ş. States of Water Molecule Adsorbed on Si(111) Surface. Solid State Commun. 1983, 45 (1), 35–38. (22) Ciraci, S.; Wagner, H. Dissociation of Water Molecules on Si Surfaces. Phys. Rev. B 1983, 27 (8), 5180–5183. (23) Russo, N.; Toscano, M.; Barone, V.; Lelj, F. On the Chemisorption of Water on the (100) Surface of Silicon. Surf. Sci. 1987, 180 (2–3), 599–604.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(24) Barone, V. The Cluster Approach in the Study of Atomic and Molecular Chemisorption on Silicon. Surf. Sci. 1987, 189–190 (C), 106–113. (25) Ong, C. K. Dissociative Adsorption of Water on the Si (1 0 0) 2 × 1 Reconstructed Surface. Solid State Commun. 1989, 72 (11), 1141–1143. (26) Engler, C. Adsorption of H2O (Molecular and Dissociative) on Si(100) and Si(111) Surfaces Considered by an Empirical Pair Potential Method. Phys. Status Solidi 1990, 159 (2), 721–730. (27) Struck, L. M.; Eng, J.; Bent, B. E.; Flynn, G. W.; Chabal, Y. J.; Christman, S. B.; Chaban, E. E.; Raghavachari, K.; Williams, G. P.; Radermacher, K.; et al. Vibrational Study of Silicon Oxidation: H2O on Si(100). Surf. Sci. 1997, 380 (2–3), 444–454. (28) Konečný, R.; Doren, D. Adsorption of Water on Si (100)-(2× 1): A Study with Density Functional Theory. J. Chem. Phys. 1997, 106 (6), 2426–2435. (29) Wang, X.; Duan, S.; Xu, X. Oxidation Mechanism of Si(111)-7 × 7 by Water: A Theoretical Study. J. Phys. Chem. C 2013, 117 (30), 15763–15772. (30) Thiel, P. A. The Interaction of Water With Solid Surfaces : Fundamental Aspects. Surf. Sci. Rep. 1990, 7 (1987), 211–385. (31) Van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105 (41), 9396–9409.

28


Page 28 of 32

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Chenoweth, K.; van Duin, A. C. T.; Goddard, W. a. ReaxFF Reactive Force Field for Molecular Dynamics Simulations of Hydrocarbon Oxidation. J. Phys. Chem. A 2008, 112 (5), 1040–1053. (33) Fogarty, J. C.; Aktulga, H. M.; Grama, A. Y.; van Duin, A. C. T.; Pandit, S. a. A Reactive Molecular Dynamics Simulation of the Silica-Water Interface. J. Chem. Phys. 2010, 132 (17), 174704. (34) Russo, M. F.; Li, R.; Mench, M.; van Duin, A. C. T. Molecular Dynamic Simulation of Aluminum–water Reactions Using the ReaxFF Reactive Force Field. Int. J. Hydrogen Energy 2011, 36 (10), 5828–5835. (35) Assowe, O.; Politano, O.; Vignal, V.; Arnoux, P.; Diawara, B.; Verners, O.; van Duin, A. C. T. Reactive Molecular Dynamics of the Initial Oxidation Stages of Ni111 in Pure Water: Effect of an Applied Electric Field. J. Phys. Chem. A 2012, 116 (48), 11796–11805. (36) Raju, M.; Kim, S. Y.; Van Duin, A. C. T.; Fichthorn, K. A. ReaxFF Reactive Force Field Study of the Dissociation of Water on Titania Surfaces. J. Phys. Chem. C 2013, 117 (20), 10558–10572. (37) Psofogiannakis, G.; van Duin, A. C. T. Development of a ReaxFF Reactive Force Field for Si/Ge/H Systems and Application to Atomic Hydrogen Bombardment of Si, Ge, and SiGe (100) Surfaces. Surf. Sci. 2015, 646, 253– 260.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(38) van Duin, A. C. T.; Zou, C.; Joshi, K.; Bryantsev, V.; Goddard, W. a. A Reaxff Reactive Force-Field for Proton Transfer Reactions in Bulk Water and Its Applications to Heterogeneous Catalysis. Comput. Catal. 2014, No. 14, 223–243. (39) Wen, J.; Ma, T.; Zhang, W.; Psofogiannakis, G.; van Duin, A. C. T.; Chen, L.; Qian, L.; Hu, Y.; Lu, X. Atomic Insight into Tribochemical Wear Mechanism of Silicon at the Si/SiO2 Interface in Aqueous Environment: Molecular Dynamics Simulations Using ReaxFF Reactive Force Field. Appl. Surf. Sci. 2016, 390, 216–223. (40) Rappé, A. K.; Goddard III, W. a. Charge Equilibration for Molecular Dynamics Simulations. J. Phys. Chem. 1991, 95 (8340), 3358–3363. (41) Nakano, A. Parallel Multilevel Preconditioned Conjugate-Gradient Approach to Variable-Charge Molecular Dynamics. Comput. Phys. Commun. 1997, 104 (1–3), 59–69. (42) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, a; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81 (8), 3684–3690. (43) Plimpton, S. Fast Parallel Algorithms for Short – Range Molecular Dynamics. J. Comput. Phys. 1995, 117 (1), 1–19.

30


Page 30 of 32

Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Aktulga, H. M.; Fogarty, J. C.; Pandit, S. A.; Grama, A. Y. Parallel Reactive Molecular Dynamics: Numerical Methods and Algorithmic Techniques. Parallel Comput. 2012, 38 (4–5), 245–259. (45) Stukowski, A. Visualization and Analysis of Atomistic Simulation Data with OVITO–the Open Visualization Tool. Model. Simul. Mater. Sci. Eng. 2009, 18 (1), 15012. (46) Tuckerman, M.; Laasonen, K.; Sprik, M.; Parrinello, M. Ab Initio Molecular Dynamics Simulation of the Solvation and Transport of Hydronium and Hydroxyl Ions in Water. J. Chem. Phys. 1995, 103 (1), 150–161. (47) Khalilov, U.; Pourtois, G.; van Duin, A. C. T.; Neyts, E. C. Hyperthermal Oxidation of Si(100)2x1 Surfaces: Effect of Growth Temperature. J. Phys. Chem. C 2012, 116 (15), 8649–8656. (48) Dumpala, S.; Broderick, S. R.; Khalilov, U.; Neyts, E. C.; van Duin, A. C. T.; Provine, J.; Howe, R. T.; Rajan, K. Integrated Atomistic Chemical Imaging and Reactive Force Field Molecular Dynamic Simulations on Silicon Oxidation. Appl. Phys. Lett. 2015, 106 (1), 11602.

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TOC

Si surface chemical state after interaction with water

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Surface Orientation and Temperature Effects on the Interaction of

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