Article pubs.acs.org/JPCC
Oxidation Mechanism of Si(111)‑7 × 7 by Water: A Theoretical Study Xinlan Wang, Sai Duan, and Xin Xu* Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai, 200433, China S Supporting Information *
ABSTRACT: Density functional theory at the level of (U)B3LYP has been used to investigate the complete oxidation of the Si(111)-7 × 7 surface by water. The results suggest that the initial water dissociation readily occurs across an adjacent adatom−rest atom (Sia−Sir) pair, resulting in either Sia−OH + Sir−H (i.e., the Sia route) or Sir−OH + Sia−H (i.e., the Sir route). Both routes are found to follow the precursormediated pathway, while the Sir atom is concluded to be more reactive than the Sia atom toward the initial decomposition of the H2O molecule due to its higher binding affinity to the incident water. With increasing water exposure and reaction temperature, deep oxidation can be accomplished by steady oxygen atom insertion into the Sia−Sis backbonds until the Si4+ oxidation state is eventually developed. Our calculations uncover that the most favorable pathway for deep oxidations begins with the direct water dissociation over the Sia−Sis backbond, followed by H2 liberation. This differs from the recent proposal via the OH insertion. Our deep oxidation mechanism can also be applied to the oxidation of an isolated adatom, which gives an explanation to the experimental observation where more than 50% of Sia are involved in water oxidation. The present work provides the detailed energetics that sheds light on the wet oxidation mechanism of silicon surfaces at the molecular level.
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a H2O molecule into the −OH and −H fragments. However, controversy still exists regarding on which site the dissociative adsorption of H2O initiates preferentially. On the basis of their valence and Si 2p soft X-ray photoemission spectra, Poncey et al.15 suggested that the Sia atoms are more reactive toward H2O adsorption than the Sir atoms. Theoretically, the early totalenergy calculations by Ezzehar et al.16 revealed that the interaction of H2O with Si(111)-7 × 7 initially occurs on the Sir site, whereas a most recent density functional theory (DFT) cluster model calculation, performed by Peng et al.,17 led to the conclusion that H2O prefers Sia over Sir to undergo initial dissociation. It was generally accepted that the adjacent Sia−Sir pair acts as the reactive site to participate in the Si(111)-7 × 7 surface reactions. However, many experimental findings suggested that such a simple reaction model involving only the Sia−Sir pair is not complete enough to describe the dissociative adsorption process of water on Si(111)-7 × 7. As the number of Sia is double that of Sir, the isolated Sia atoms, in addition to those Sia in forming direct pairs, were found to also exhibit some reactivity. An early report has already revealed that water adsorption on Si(111)-7 × 7 can be divided into two distinct stages that occur at different water coverages.18 Dissociative adsorption occurs readily across all the available Sia−Sir pairs
INTRODUCTION Silicon oxide (SiO2) films on silicon surfaces play an important role in electronic device manufacture, serving as insulator films or protective coatings against etching. Exposing a hot silicon surface to oxygen molecules (dry oxidation) and to gaseous water (wet oxidation) are two well-known procedures for the formation of a silicon oxide layer,1 and the efficiency of wet oxidation is much higher than that of dry oxidation.2 In more recent years, the development of large-capacity integrated circuits continuously demands the reduction of the thickness of the insulating silicon oxide layer, up to nearly 2 nm.3 Hence, the study of the oxidation mechanism of silicon surfaces by water at the molecular level shall provide a guideline to produce and control extremely thin silicon oxide films. Compared to other silicon surfaces, the Si(111)-7 × 7 surface exhibits a diversity of reactive sites, serving as a good platform for understanding silicon surface chemistry. Over the past decades, the interaction of H2O with the Si(111)-7 × 7 surface has continuously attracted considerable attention not only from fundamental interest but also from technological importance in the microelectronics industry.4−17 Earlier fundamental studies mainly focused on whether the adsorption of H2O on the Si(111)-7 × 7 surface is molecular or dissociative.4−13 There has now been a majority opinion among these results that H2O dissociates to form the Si−H and Si−OH species on Si(111)-7 × 7.4,5,8,9,13 Using a scanning tunneling microscope (STM), Avouris and Lyo14 first asserted that the adjacent adatom−rest atom (Sia−Sir) dangling bond pair is involved in the splitting of © 2013 American Chemical Society
Received: May 26, 2013 Revised: July 5, 2013 Published: July 8, 2013 15763
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with a high sticking probability at lower coverage, whereas water can still react with the remaining isolated Sia dangling bonds, albeit with a reduced probability. According to their experimental observation for the development of the oxidation state from Si(I) to Si(IV) with increasing water exposure at 90 K and at room temperature, Poncey et al.15 affirmed that the cleavage of the H2O molecule into −OH and −H fragments across the Sia−Sir pairs is not the only reaction mechanism, and they proposed that an additional channel is for oxygen insertion into the Sis−Sis backbond, where Sis stands for a subsurface silicon. Through their study using STM and TPD (temperature-programmed desorption), Self et al.19 observed that 69% of the Sia atoms are reacted at 300 K, which also suggested that the Sia atoms other than those in the Sia−Sir pairs are also involved in H2O adsorption on Si(111)-7 × 7. On the basis of their results from STM and EELS (electron energy loss spectroscopy) carried out at 80 K, Yamada et al.20 proposed a modified Sia−Sir pair reaction mechanism in which a dihydride species is formed at the Sir sites, enabling more than half of the Sia atoms to be involved in the reaction of water with Si(111)-7 × 7. Although much experimental effort has been devoted to trying to give a thorough description of the adsorption of water on the Si(111)-7 × 7 surface, the associated theoretical investigation is deficient. Recently, on the basis of the firstprinciples DFT calculations, Peng et al.17 first illuminated the reaction mechanism that water molecules completely dissociate to develop a SiO4 structure over the Si(111)-7 × 7 surface. Two reaction stages have been explored in their calculations, including the initial dissociation of H2O across the adjacent Sia−Sir pair, which is followed by the secondary oxidation process where the as-formed OH group inserts into the Sia−Sis backbond. These resemble our previous mechanisms for complete dissociations of NH3 and PH3, where the as-formed NHx or PHx (x = 2, 1) groups insert into the Sia−Sis backbond.21,22 In this paper, we will explore the reaction behavior of H2O on the Si(111)-7 × 7 surface. Despite that Peng et al.17 have suggested that H2O preferentially reacts with the Sia atom of the Si(111)-7 × 7 surface, such a proposal is based on a Si16H18 cluster model, which may display an unrealistic reactivity of the Sia−Sir pair site upon H2O initial attacks. We will first reexamine the initial site-selectivity problem of Si(111)-7 × 7 using an improved model that includes additional subsurface Si adatoms and bulk Si atoms,21−23 leading to a conclusion that the Sir site is preferred over the Sia site for initial H2O dissociation. Second, we will present a detailed oxidation mechanism of Si(111)-7 × 7, which uncovers a new reaction pathway where the oxygen atom insertion begins with an efficient cleavage of the Sia−Sis backbond by an incident water molecule rather than the OH group produced by initial H2O dissociation.
Figure 1. (a) Top view of the dimer−adatom stacking fault (DAS) model for the Si(111)-7 × 7 reconstructed surface. (b) Si22 cluster model. Unwanted dangling bonds are saturated by 22 hydrogen atoms. Sia represents an adatom, whereas Sir represents a rest atom.
of a Si(111)-7 × 7 surface unit cell (see Figure 1b). It distinguishes itself from the previously used Si16H18 model17 by adding a subsurface dimer and four bulk Si atoms in order to avoid the unrealistic distortion of the silicon framework during the reactions.21−23 Our calculations were based on the hybrid B3LYP25,26 density functional method, which consists of the Slater local exchange,27 the semilocal exchange of Becke 88,28 the exact exchange, the local correlation functional of Vosco−Wilk− Nusair,29 and the semilocal correlation functional of Lee− Yang−Parr.30 The 6-31++G(d,p)31−33 basis set was used for H2O, together with the 6-31G(d,p)31,32 for the Si22H22 cluster model. Full geometry optimizations were performed with no constrained degree of freedom. Vibrational frequencies were calculated analytically to ensure that each minimum was a true local minimum (only positive frequencies) and that each transition state had only one single imaginary frequency. Zeropoint energies (ZPEs) were evaluated at the same level to correct the energies of all species. The same methodology has been successfully applied to the study of the dissociations of CH3OH, NH3, and PH3 on Si(111)-7 × 7 in our previous work.21−23 All calculations were performed with the Gaussian 03 package.34
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COMPUTATIONAL DETAILS The reconstructed Si(111)-7 × 7 surface adopts the dimer− adatom stacking fault (DAS) structure, as shown in Figure 1a.24 In a 7 × 7 unit cell, there exist seven chemically distinguishable types of surface atoms, including three corner Sia atoms, three center Sia atoms, and three Sir atoms at either the faulted or the unfaulted half, as well as one rest atom at the bottom of the deep corner hole (Sih), making a total of 19 dangling bonds (DBs). In the present work, a Si22H22 cluster model was employed to represent an adjacent Sia−Sir pair on a faulted half
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RESULTS AND DISCUSSION Initial Dissociation of H2O. Previous calculations showed that the open-shell singlet, identified at the level of unrestricted B3LYP/6-31G(d,p), RO, is 8.7 kcal/mol lower in energy than the closed-shell singlet with restricted B3LYP, RC, for the Si22H22 cluster.22 Therefore, in the present case, all H2O 15764
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Figure 2. Potential energy diagrams for the initial dissociation of H2O on the Sia and Sir sites of Si(111)-7 × 7.
dissociative processes on Si(111)-7 × 7 and the relevant energetics are with respect to the open-shell singlet, RO. As depicted in Figure 2, the dissociative adsorption of H2O can be initiated by adsorbing either on the Sia atom or on the Sir atom. Two genuine molecular precursor states, LM1a and LM1r, are identified with an optimal Sia−OH2 and Sir−OH2 bond length of 2.016 and 1.942 Å, respectively. Our previous calculations suggested that the adjacent Sia−Sir pair in the free Si(111)-7 × 7 surface should be better regarded as a diradical.21−23 Such a picture is not only in agreement with the fact that RO is lower in energy than RC, but also naturally avoids the counterintuition that H2O can react with a “basic” Sir, if doubly occupied as in RC, to form a Sir←H2O dative bond in LM1r. In the diradical perspective, it can be visualized that the lone-pair electrons of H2O are able to simultaneously steer charge transfer from the originally singly occupied Sir in RO to Sia to empty the Sir DB when H2O attacks via Sir (i.e., the Sir route) or the other way around for charge transfer from Sia to Sir when H2O attacks via Sia (i.e., the Sia route).21−23 LM1a and LM1r are metastable and well prepared for further H2O dissociation. Following two corresponding transition states, TS1a and TS1r, they can be smoothly transformed into LM2a and LM2r with little reaction barrier and with a large exothermicity of −65.1 and −62.2 kcal/mol, respectively (Figure 2). Our computational results thus suggest that, once the incident H2O molecule is trapped onto the surface DBs, the dissociation of H2O into −OH plus −H is facile either via the Sia route or via the Sir route. We also found that the binding energy of H2O→Sia (3.1 kcal/mol) in LM1a is lower than that of H2O→Sir (8.2 kcal/mol) in LM1r. Previously, we have proposed that the strength of the binding energy in dative complexes holds the key that differentiates the reactivity of Sia and Sir toward CH3OH, NH3, and PH3.21−23 The larger formation energy of the H2O→Sir dative complex suggests that the initial adsorption of H2O on Si(111)-7 × 7 occurs on the Sir site with a higher sticking probability. Hence, there shall be more initial H2O dissociation via the Sir route. We note that experiments with various techniques agreed that water readily dissociates across the adjacent Sia−Sir pair at lower coverage, and an early total-energy calculation carried out by Ezzehar et al,16 suggested that the Sir site is the preferred one.
With the Si16H18 cluster model, Peng et al.17 also revealed the diradical character of Si(111)-7 × 7. However, they were unable to locate a dative complex of Sia←H2O on the Sia site, but only found the corresponding complex on the Sir site. They interpreted this as an indication that Sia is more reactive than Sir, and thus proposed that water dissociation is initiated preferably by Sia. However, their calculations have suggested two different mechanisms for the dissociative chemisorption of water. One is the direct mechanism where the incident water molecule reacts with the surface directly at the Sia site of impact to form the dissociation products, whereas the other is the socalled precursor-mediated mechanism,35 where the incident water molecule is first trapped into a physisorbed precursor state on the Sir site. Because of their different nature based on Peng’s calculations,17 it is difficult to make direct comparison for the reaction probabilities of these two pathways without a detailed characterization of the initial states of the incident molecules and the surface. Furthermore, they calculated the binding energy of H2O to the Sir site to be 17.4 kcal/mol. This is much higher than ours for LM1r (8.2 kcal/mol). With their Si16H18 cluster model, the optimized Sia···Sir distance is only 4.387 Å, whereas that from our cluster model calculation is 4.537 Å. The latter is closer to the experimental value (∼4.6 Å36). We believe that their Sia···Sir distance is artificially too short, which should have led to biased results in regards to the initial activity and selectivity of Sia and Sir. We conclude that initial dissociation of H2O via either Sia or Sir proceeds through the precursor-mediated pathway. The latter possesses a high binding energy for the precursor state, displaying a higher sticking probability, and hence is preferred at low coverage and at low temperature. Both pathways are, however, found to be barrierless and thus feasible, where formation of LM2a is actually more thermodynamically favored (∼3 kcal/mol) than LM2r. We expect that increasing water coverage and surface temperature would diminish the preference of Sir for initial water dissociation and even the direct dissociation mechanism would come into play. Deep Oxidation of Si(111)-7 × 7 by H2O. Experimentally, it was found that, by heating around 700 K, the SiOH species were decomposed into the SiOSi and SiH species, while some of the SiH species were proposed to be backbonded to 15765
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Figure 3. Potential energy diagrams for the complete oxidation of LM2a by water direct dissociation over the Sia−Sis backbonds with the oxygen atom attacking the Sia site (H2O@Sia-LM2a).
one, two, or, possibly, three O atoms.37 It was further found that, by heating to around 900 K, the adsorbed H atoms are desorbed as H2.37 Indeed, many experiments found that the O atom accumulated around the Sia site upon deep oxidation of Si(111)-7 × 7 by H2O.14,15,19,36 This is in line with the bonding feature of the Sia atoms in the DAS model of the reconstructed surface.22 Figures 3−9 provide some plausible oxidation mechanisms, while some more can be found in the Supporting Information (Figures S1 and S2). We start by considering deep oxidation of LM2a, where O in H2O directly attacks the Sia atom. We label this route as H2O@ Sia-LM2a. As displayed in Figure 3, the cleavage of one of the three Sia−Sis backbonds in LM2a requires overcoming an energy barrier of 19.0 kcal/mol at the transition state TS2, giving rise to the intermediate LM3 with a high formation energy of −53.1 kcal/mol. In LM3, the dissociative OH and H adspecies are covalently bound to the Sia and Sis sites, respectively. Here, we found that the following step is H2(g) liberation via TS3, which completes the first oxygen atom insertion into the Sia−Sis backbond. The transition state, TS3, was calculated to possess an activation energy barrier of 43.6 kcal/mol, leading to the formation of LM4. The further development to the Si(IV) oxidation state follows the similar
route, that is, water dissociation and H2 liberation (see Figure 3). In this reaction route, the highest intrinsic barrier (TS5 in Figure 3) is 47.1 kcal/mol, which corresponds to a H2 release accompanied with the second oxygen atom insertion into the Sia−Sis backbond. Finally, LM8 is developed. In LM8, the surface Sia is being surrounded by four oxygen atoms and takes on a formal chemical valence of +IV. During these processes, it is required to overcome a barrier between 16.7 and 24.8 kcal/ mol for water dissociation, whereas the barriers to release H2 are within the range of 42.5 and 47.1 kcal/mol. Every oxidation step is progressively exothermic from 49.4 to 62.4 kcal/mol. LM8 eventually lies at −200.2 kcal/mol in the potential energy surface with respect to the entrance level. It was found that H2O molecules may approach the Sia−Sis backbond of LM2a in a different orientation, that is, with O in H2O directly attacking the subsurface Sis atom, as in Figure 4. We label this route as H2O@Sis-LM2a. The overall oxidation processes are similar, characterized by the oxygen atom insertion into the Sia−Sis backbond accompanied with H2 liberation. The corresponding transition states and intermediates are depicted in Figure 4. When compared to Figure 3, TS8 was found to lie 7.8 kcal/mol higher than TS2. In TS2 and TS8, the distances of Sia···Sis were calculated to be 2.958 and 15766
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Figure 4. Potential energy diagrams for the complete oxidation of LM2a by water direct dissociation over the Sia−Sis backbonds with the oxygen atom attacking the Sis site (H2O@Sis-LM2a).
Table 1. Comparison of the Calculated Mulliken Charges in the Transition States Associated with the Water Attacking Processes from LM2a H2O@Sia-LM2a Sia···O TS2 TS4 TS6
0.558 1.279 1.675
H2O@Sis-LM2a Sis···H
−0.433 −0.645 −0.652
−0.097 −0.099 −0.385
Sis···O 0.265 0.270 0.280
TS8 TS10 TS12
3.072 Å, respectively, indicating that the Sia−Sis backbonds in both situations are completely broken. As listed in Table 1, the Mulliken charges on Sia, Sis, O, and H of the attacking O−H bond in TS2 are 0.558, −0.097, −0.433, and 0.265, respectively, showing that there exists a favorable electrostatic interaction between Sia and O and that between Sis and H, while the former is stronger than the latter. In TS8, however, the electrostatic interaction between Sis and O is still favorable, as verified by the Mulliken charges analysis, whereas Sia and the attacking H atom repel each other electrostatically (see Table 1), leading to a destabilization of TS8, as compared to TS2. The same viewpoint may be applied to reasoning the stability difference between TS4 and TS10, as well as that between TS6 and TS12, where the former is lower-lying than the latter for a more favorable electrostatic interaction. As compared with Figures 3 and 4, it can be seen that LM3, LM5, and LM7 are 7.3, 9.9, and 9.6 kcal/mol more stable than LM9, LM10, and LM11, respectively, whereas TS3, TS5, or
0.217 −0.011 0.037
Sia···H −0.346 −0.274 −0.293
0.305 0.587 0.977
0.162 0.067 0.017
TS7 is lower-lying than TS9, TS11, or TS13 by1.2, 4.0, or 5.6 kcal/mol, respectively. As a result, the barrier heights for H2 liberations in H2O@Sia-LM2a, as depicted in Figure 3, are ultimately higher than those in H2O@Sis-LM2a, as depicted in Figure 4, by 6.1, 5.9, and 4.0 kcal/mol, respectively. There seems to be a competition between two routes. On one hand, barriers for water dissociation processes are within 19.0 and 24.8 kcal/mol along the H2O@Sia-LM2a route (Figure 3), which are more favorable than those (26.8−39.7 kcal/mol) along the H2O@Sis-LM2a route (Figure 4). On the other hand, barriers for the followed H2 liberation processes are within 42.5 and 47.2 kcal/mol along the H2O@Sia-LM2a route (Figure 3), which are less favorable than those (37.5−41.2 kcal/ mol) along the H2O@Sis-LM2a route (Figure 4). Experimentally, it was found that high temperature (900 K37) is needed to achieve H2 liberation, which is in accordance with the present calculation results that H2 liberation is the ratedetermining step to complete every oxygen atom insertion 15767
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Figure 5. Potential energy diagrams for the complete oxidation of LM2r by water direct dissociation over the Sia−Sis backbonds with the oxygen atom attacking the Sia site (H2O@Sia-LM2r).
process. Detailed kinetic analysis is necessary to quantify the preferences of these two routes. Here, we tentatively favor the H2O@Sis-LM2a route for its lower H2 liberation barriers. Figures 5 and 6 summarize the reaction pathways where deep oxidations start from LM2r. The reaction barrier heights for the sequential O insertions are summarized in Table 2, along with those from LM2a. Hence, we may also have the H2O@SiaLM2r route (Figure 5) and the H2O@Sis-LM2r route (Figure 6), where H2O attacks the Sia−Sis bond in LM2r via different orientations. Similar to the situations in LM2a, the reaction barrier heights are smaller for the H2O dissociation processes via the H2O@Sia-LM2r route than those via the H2O@SisLM2r route. The former are within the range from 18.7 to 23.7 kcal/mol, whereas the latter are between 26.1 and 37.8 kcal/ mol. On the other hand, H2 liberation is more difficult via the H2O@Sia-LM2r route (barrier range from 40.4 to 49.2 kcal/ mol) than that via the H2O@Sis-LM2r route (barrier range from 40.9 to 43.6 kcal/mol). Hence, these two routes would be competitive. If assuming that H2 liberation is the ratedetermining step to complete the whole oxygen atom insertion processes at elevated temperature, one would again favor the H2O@Sis-LM2r route for its lower H2 liberation barriers. Figure 7 explores the reaction pathway where the as-formed OH group from the initial H2O dissociation reacts with the Sis
atom. This is the pathway similar to the XH2(a) or XH(a) (X = N, P) insertion into the Sia−Sis backbond for the complete dissociation of NH3 and PH3.22,23 As is shown in Figure 7, it has to surmount an energy barrier of 58.4 kcal/mol (TS28) to reach LM22, which is 17.3 kcal/mol higher-lying than LM2a. Such an unfavorable energetics for this −OH insertion process can be used to explain the experimentally observed stability of the OH species at temperatures up to 560 K,36 which also suggests that such a process is unfavorable to develop deep oxidation of LM2a. Previously, Peng et al. has reported a mechanism for deep oxidations that went through this −OH insertion process.17 They have obtained a much higher barrier of 70.0 kcal/mol, as compared to ours (58.4 kcal/mol) for the same process, that has to be related to the large unrealistic deformation of their Si16H18 cluster model due to the lack of the subsurface dimer and bulk Si atoms. For completeness, we present our calculation results via this −OH insertion process in Figure S1 (Supporting Information). As compared to either H2O@Sia-LM2a or H2O@Sis-LM2a, we find that this −OH insertion route is much more unfavorable to carry on the deep oxidations, where the intrinsic barrier can be as high as 79.2 kcal/mol, occurring at TS34, as depicted in Figure S1 (Supporting Information). Hence, such a mechanism17 has to be discarded. 15768
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Figure 6. Potential energy diagrams for the complete oxidation of LM2r by water direct dissociation over the Sia−Sis backbonds with the oxygen atom attacking the Sis site (H2O@Sis-LM2r).
Table 2. Comparison of the Calculated Reaction Barrier Heights (in kcal/mol) Associated with the First, Second, and Third O Insertions into the Sia−Sis Backbondsa,b,c H2O@Sia-LM2a
H2O@Sis-LM2a
H2O@Sia-LM2r
H2O@Sis-LM2r
no.
H2O diss.
H2 lib.
H2O diss.
H2 lib.
H2O diss.
H2 lib.
H2O diss.
H2 lib.
(1) (2) (3)
19.0 16.7 24.8
43.6 47.2 42.5
26.8 32.1 39.7
37.5 41.2 38.5
23.7 18.7 20.9
40.4 49.2 43.2
26.1 26.7 37.8
43.6 40.6 40.9
a
An O insertion process involves a water dissociation, followed by a H2 liberation step. bH2O@Sia-LM2a stands for an O insertion process where H2O attacks the Sia atom of a Sia−Sis backbond in LM2a. Other routes are named in the same way. cThe highest barrier for each route is shown in bold face.
58.4 for LM2a → TS28 in Figure 7) due to favorable hydrogen bond interaction in the former. More results along the −OH insertion process are summarized in Figure S2 (Supporting Information), which are in accordance with those proposed by Peng et al.17 for the deep oxidation of subsurface Si from LM2r. These pathways would have to surmount a barrier height of 76.1 kcal/mol, which, we believe, is unfeasible. We propose that, in addition to the mechanisms shown in Figures 5 and 6, LM2r can be first transferred into LM28, as shown in Figure 8, which will then undergo deep oxidations following the mechanisms shown in Figures 3 and 4. We expect that the highest barrier might not
A key difference between LM2a and LM2r is that Sia is bounded to an OH in the former, whereas it is bounded to an H in the latter. Figure 8 shows a plausible pathway where an H terminal is transferred into an OH terminal via a direct H2O attack with a simultaneous H2 liberation. From LM2r to LM28, it is necessary to surmount a reasonable barrier of 37.1 kcal/ mol, which should be feasible at elevated temperature. We found that LM28 indeed acts just as LM2a. For example, Figure 8 displays the pathway for the first O insertion from LM28 to LM13, which closely resembles the −OH insertion process shown in Figure 7. Note that the energetics is found to be more favorable from LM28 to LM13 than those from LM2a to LM23 (e.g., 56.6 kcal/mol for LM28 → TS38 in Figure 8 vs 15769
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steady growth of the oxidation states from Si(I) to Si(IV). Our present calculations lend support to such an observation. On the other hand, Self et al.19 observed that 69% of Sia atoms were reacted at 300 K, which suggests that, in addition to the Sia atoms that are involved in the Sia−Sir pairs, some isolated Sia atoms should have also participated in the reactions with water. To explore the reactivity of an isolated Sia atom, a Si22H23 cluster has been adopted (see Figure 9). Attempts to locate a pathway for H2O dissociated over such an isolated-reacted Sia− Sir pair (e.g., Sir is saturated by H) were not successful. Hence, the proposal by Yamada et al.20 to form a dihydride species on Sir was disproved by our calculations, and an isolated Sia is indeed less reactive toward H2O dissociation. We will then explore the reaction routes for O insertion processes upon an isolated Sia. First, it shall be noted that species such as LM2a and LM28 provide the sites to form hydrogen-bonded complexes (e.g., H2O···HO−Sia) at increasing water exposure. This shall enhance the possibility of deep oxidation by water. Such a mechanism, however, is not operative for an isolated Sia. Nonetheless, at elevated tempeture, we expect that direct water dissocation occurs, which is indeed the case, as shown in Figure 9. In general, the reaction route was found to be similar to those shown in Figures 3−6. For the first O insertion process, the reaction barriers for H2O dissociation and H2 liberation were calculated to be 25.8 and 39.4 kcal/mol, respectively. Here, O insertion was found to facilitate further H2O dissociations, for which the second and third reaction barriers were calculated to be 20.3 and 8.8 kcal/mol. However, the corresponding barriers for H2 liberations are consistently 44.3 and 44.1 kcal/mol, which remain to be the rate-determining steps for the full oxidation of the isolated adatom. In comparison, these are ∼3 kcal/mol higher than the lowest H2 liberation barriers (41.1 kcal/mol,
Figure 7. Potential energy diagrams for the oxygen atom insertion into the Sia−Sis backbond of LM2a via the −OH insertion mechanism.
exceed 41 kcal/mol as in TS11 due to favorable hydrogen bond interactions, and hence is plausible at elevated temperature. Using X-ray photoemission spectroscopy, Poncey et al.15 observed that, as the exposure to water increases, there exists a
Figure 8. Energetics for the transformation of Sia−H to Sia−OH and the following −OH insertion into the Sia−Sis backbond. 15770
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Figure 9. Potential energy diagrams for the complete oxidation of an isolated adatom by water direct dissociation over the Sia−Sis backbonds.
the Sia−Sis backbond with its O atom, and the mechanism is effective no matter how the Sia atom is decorated (i.e., by −H, −OH, or a DB as in an isolated Sia). H2 liberation is found to possess the highest intrinsic reaction barrier and is thus assumed to be the rate-determining process during the course of oxidation. In this paper, we report the detailed dissociative chemisorption mechanisms of H2O on the Si(111)-7 × 7 surface. It is foreseen that the microscopic picture for the oxidation of the silicon surfaces by “wet oxidation” can be deeply understood if these data are combined with future dynamic simulation and kinetic analysis. This should provide helpful information in understanding and controlling the growth of silicon oxide films in the microelectronics industry.
TS11) found for the H2O@Sis-LM2a (see Figure 4). On the basis of these results, we conclude that, at elevated temperature, the isolated Sia atoms can efficiently compete with those of the Sia atoms decorated by OH or H to undergo similar mechanisms for deep oxidations.
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CONCLUSION
The oxidation mechanisms of the Si(111)-7 × 7 surface by water have been studied by using density functional theory at the level of (U)B3LYP. Our computation results suggest that water molecules can easily dissociate into OH(a) and H(a) adspecies across an adatom−rest atom pair via a barrierless transition state. The molecular adsorption energy of Sir←H2O (LM1r) is found to be 5.1 kcal/mol larger than that of Sia← H2O (LM1a), indicating a higher probability of trapping an incident water molecule on a Sir site. This result suggests that the initial water dissociation follows the precursor-mediated mechanism where the rest atoms are more reactive than the adatoms. Such a mechanism is not operative over an isolatedreacted Sia−Sir pair. With increasing water exposure or at elevated temperature, deep oxidations occur. Our calculations reveal that the OH insertion process as proposed in the literature17 is unfavorable. Instead, the oxidation is achieved by the direct water dissociation over the Sia−Sis backbond, followed by H2 liberation. It is found that water can approach either end of
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ASSOCIATED CONTENT
S Supporting Information *
Potential energy curves for the complete oxidation by −OH insertion into the Sia−Sis backbonds. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*X. Xu; E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 15771
dx.doi.org/10.1021/jp4051879 | J. Phys. Chem. C 2013, 117, 15763−15772
The Journal of Physical Chemistry C
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Article
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ACKNOWLEDGMENTS This work was supported by NSFC (Grant Nos. 21133004, 91027044) and the Ministry of Science and Technology (Grant Nos. 2013CB834606, 2011CB808505)
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