A Theoretical Investigation - American Chemical Society

Apr 7, 2009 - Density functional theory at the level of (U)B3LYP has been used to explore the dissociation of PH3 on the adatom site (Sia) and rest at...
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J. Phys. Chem. C 2009, 113, 7843–7850

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Dissociative Adsorption of PH3 on the Si(111)-7 × 7 Surface: A Theoretical Investigation Xinlan Wang and Xin Xu* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College for Chemistry and Chemical Engineering, Xiamen UniVersity, Fujian, P.R. China ReceiVed: February 2, 2009; ReVised Manuscript ReceiVed: March 6, 2009

Density functional theory at the level of (U)B3LYP has been used to explore the dissociation of PH3 on the adatom site (Sia) and rest atom site (Sir) of the Si(111)-7 × 7 surface. A detailed comparison between PH3 and NH3 adsorption on Si(111)-7 × 7 is performed. Our results show that PH3 initial dissociation to adsorbed species, PH2(a) and H(a), is facile and preferentially occurs on the Sir site. The same trend was found for NH3, but PH3 shows a site selectivity higher than NH3. XH2(a) is thermally stable, and an elevated temperature is required for further X-H (X ) N or P) bond decomposition. The general mechanism for further X-H bond decomposition is XHn (n ) 2 or 1) insertion into Si-Si backbond, followed by H2 liberation, with the former usually being the rate-determining step. Full XH3 decomposition may lead to the formation of SidX or Si3X unit with the preference on the Sir site for N and that on the Sia site for P. Such a difference should be attributed to X-H bond energy difference, the atomic radius difference between P and N, and the release of the strain energy of the reconstructed surface. We anticipate that the detailed energetics obtained from this study can be used as the quantum-mechanical input for a chemical-kinetics model of chemical vapor deposition. Introduction Owing to its technological importance in device manufacture, the interaction of phosphine with silicon surfaces has been the focus of intensive studies over the past four decades.1-12 In the in situ doping technique of phosphorus into silicon films using PH3 by CVD (chemical vapor deposition), one of the problems related to the use of PH3 as a dopant is the resultant delay of the n-type silicon growth rate, which degrades the throughput of the ultralarge-scale-integrated (ULSI) device processing.1 Accordingly, a detailed, atom-by-atom understanding of the PH3 dissociation chemistry on the Si surfaces is required, not only from the point of view of practical application but also from that of fundamental interest. However, it should be noted that the literature work is more devoted to PH3 on the Si(100)-2 × 1 surface1-6 than on the Si(111)-7 × 7 surface.7-12 Various experimental techniques have been applied to the adsorption of PH3 on the Si(111)-7 × 7 surface. An elaborate exploration from Wallace et al.7 revealed that phosphine adsorbs on Si(111)-7 × 7 with an initial sticking coefficient of So ≈ 1 at 120 K up to a surface coverage of ∼0.20 ML. On the basis of their results by HREELS (high-resolution electron energy loss spectroscopy), Chen et al.8 established that the Si-H and Si-PH2 species are produced upon PH3 adsorption on Si(111)-7 × 7 at 80 K, with very little or no PH3(a) for PH3 coverages below 0.19 ML. Using AES (Auger electron spectroscopy) and LEED (low energy electron diffraction), Van Bommel and Crombeen9 noted very early that the 7 × 7 reconstruction pattern remains after room temperature exposure of PH3 on the Si(111)-7 × 7 surface. Hence, the general picture emerged from these experiments,7-9 in the initial uptake, is that PH3 dissociates on dangling bonds (DBs) of the Si(111)-7 × 7 surface without destroying the 7 × 7 pattern. In regard to the initial adsorption site of PH3 on the Si(111)-7 × 7 surface, according to UPS (ultraviolet photoemission spectroscopy) study of PH3 adsorption at 100 K, Bozso and * To whom correspondence should be addressed.

Avouris10 first suggested that the rest atoms (Sir) are more reactive toward PH3 adsorption than the adatoms (Sia). Subsequently, a theoretical calculation, using the atom superposition and electron delocalization (ASED) molecular orbital (MO) and cluster models, for the initial adsorption of PH3 on Si adatoms and rest atoms, performed by Cao et al.,11 led to the conclusion that PH3 preferentially adsorbs on the rest atoms. More recently, through a STM (scanning tunneling spectroscopy) observation, Shen et al.12 asserted that PH3 prefers Sir to Sia to undergo adsorption at room temperature. It has been pointed out that both dissociative nature and site selectivity of PH310 on Si(111)-7 × 7 are analogous to the case of NH3 adsorption.13 Nevertheless, there exist hot debates on the initial adsorption site of NH3 on the Si(111)-7 × 7 surface, in which the adatom, rather than the rest atom, is also advocated to initiate the NH3 dissociation.14-16 At high temperatures, there are more reaction pathways accessible between PH3 and Si(111)-7 × 7. AES and ESD (electron stimulated desorption) studies showed that the decomposition of PHx(a) (3 g x g 1) takes place by the breaking of P-H bonds to form Si-H species on the surface over the range 120-700 K.7 In addition, there exists evidence for P penetrating into bulk Si(111) at 875 K.7 The thermal dissociation of the surface PH2 species was observed to occur between 450 and 500 K, accompanied by the capture of hydrogen and phosphorus on the surface.8 It was predicted that the 7 × 7 reconstruction is stable to a temperature of 770 K7 where H2(g) was seen to desorb at T ≈ 740 K based on TPD (temperature programmed desorption) data.8 LEED shows that the reconstruction converts from 7 × 7 to 1 × 1 after annealing to 800 K < T < 1000 K, and returns to a 7 × 7 structure after P2(g) liberation.10 TPD shows that P2(g) desorption occurs at T ≈ 1010 K. 8 Despite experimental examinations that have provided a great deal of knowledge on the reaction of PH3 with the Si(111)-7 × 7 surface, 7-12 the associated theoretical investigation is deficient. Except for an early ASED calculation,11 no calculations based on first principle have ever been reported. We carried out a

10.1021/jp9009367 CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Wang and Xu frequencies are calculated to ensure that each minimum is a true local minimum (only positive frequencies) and that each transition state has only a single imaginary frequency. The basis set used on all atoms for geometry optimizations and vibrational frequency calculations is the standard 6-31G(d,p).29,30 Zero-point energies (ZPE) evaluated at the same level are used to correct the energies of all species. In particular, the UB3LYP method is used for the open-shell singlet Si22H22 (Ro in Figure 2), whereas the closed-shell singlet Si22H22 (Rc in Figure 2) and other species are calculated using the RB3LYP method. Similar strategy has been successfully applied in our previous studies of CH3OH17 and NH318 dissociations on Si(111)-7 × 7. Being distinct from the ionic viewpoint that the 6 rest atoms and 1 corner hole are doubly occupied, whereas the adatoms are nearly empty-occupied such that the adatom and rest atom are positively and negatively charged, respectively,31-34 our calculations supported an electronic structure picture that the adatom-rest atom pair in the free Si(111)-7 × 7 surface should be best regarded as a diradical.17,18 Such a picture was identified by the experimental observations of di-σ bond formation in benzene and thiophene chemisorptions on Si(111)-7 × 7.35,36 All our calculations were performed with the Gaussian 98 package.37

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 while Sir represents a rest atom.

systematic DFT (density functional theory) study so as to have further comprehension of the site-selectivity problem of PH3 adsorption on Si(111)-7 × 7. We present here the full dissociation mechanism of PH3, which should be of significance to the technological development on achieving control of phosphorus dopant concentration and its spatial distribution in the silicon film. Previously, we reported the dissociative adsorption mechanisms of CH3OH17 and NH318 on the Si(111)-7 × 7 surface, as well as NH319-21 on the Si(100)-2 × 1 surface. In view of the fact that both NH3 and PH3 are the hydrides belonging to main group V, it is expected that their dissociation processes on Si(111)-7 × 7 should bear much similarity. Our calculations confirmed this speculation and also revealed the difference between them. Computational Details ThereconstructedSi(111)-7×7surfaceadoptsthedimer-adatom stacking fault (DAS22) structure, which reduces the number of dangling bonds within the surface unit cell from 49 to 19. Such a model contains 9 subsurface dimers, 12 adatoms (Sia), 6 rest atoms (Sir), 1 corner hole (Sih), and a stacking fault in a halfunit cell, as shown in Figure 1a. In the present work, a Si22H22 cluster model is employed to simulate the Si(111)-7 × 7 surface (see Figure 1b). It includes an adatom-rest atom pair and a subsurface dimer and thus can be considered as a local model for a center adatom-rest atom pair. Our calculations are based on the hybrid B3LYP23,24 density functional method, which consists of the Slater local exchange,25 the GGA (generalized gradient approximation) exchange of Becke 88,26 the exact exchange, the local correlation functional of Vosco-Wilk-Nusair,27 and the GGA correlation functional of Lee-Yang-Pair.28 Full geometry optimizations are performed with no constrained degree of freedom. Vibrational

Results and Discussion 1. Initial Dissociation of PH3. The previous calculations, performed at the level of (U)B3LYP/6-31G(d,p), showed that the open-shell singlet (UB3LYP), RO, is 8.7 kcal/mol lower in energy than the closed-shell singlet (RB3LYP), RC.18 Hence, in the present case of PH3, all its dissociative pathways on Si(111)-7 × 7 and the relevant energetics are with respect to the open-shell singlet, RO. As is depicted in Figure 2, either Sia or Sir can initiate PH3 adsorption to form LM1a′ or LM1r′, respectively. LM1a′ and LM1r′ are both dative complexes, characterized by a long Si-P bond length of 2.399 and 2.334 Å, respectively. Such a picture is not in consistent to the ionic dimer pair 31-34 where Sir is negatively charged and Sia is positively charged. Instead, the dimer pair may be better regarded as a diradical35,36 (open-shell singlet), where the lone-pair electrons on PH3 steer charge transfer from Sir to Sia when PH3 approaches to Sir or that from Sia to Sir when PH3 approaches to Sia.17,18 Such a picture avoids the counterintuition that a ‘basic’ Sir can react with PH3 to form a stable SirrPH3 dative bond. Although there exist two genuine transition states, TS1a′ and TS1r′, through which LM1a′ and LM1r′ can be transformed into LM2a′ and LM2r′, respectively, the cleavage of the first P-H bond is actually barrierless. This can be anticipated as seen from Figure 2 that either TS1a′ or TS1r′ is an early transition state, bearing close resemblance to LM1a′ or LM1r′. Our calculations, thus, suggest that once the incident PH3 molecule is trapped onto the surface DBs, the dissociation of PH3(a) to PH2(a) and H(a) is facile. Such a prediction coincides with the experimental report that the initial decomposition of PH3 on Si(111)-7 × 7 readily occurs even at 80 K.8 The products of LM2a′ and LM2r′ were calculated to lie 54.1 and 53.0 kcal/mol, respectively, below the free reactants. This high exothermicity is in accord with the experimental findings that the dissociation of the adsorbed surface PH2 species takes place at a temperature up to 450-500 K.8 Furthermore, our calculations show that the reverse reaction from PH2(a) and H(a) fragments to PH3(a) has to surmount an energy barrier as high as 52.9 or 43.3 kcal/mol. This confirms Wallace’s conclusion that TPD-observed desorption of molecular PH3(g) at 120-180 K is not a surface recombination process.7

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Figure 2. Potential energy diagram for the initial dissociation of PH3 on the Sia and Sir sites of Si(111)-7 × 7.

Figure 3. Potential energy diagram for the decomposition of the second P-H bond on the Si rest atom site of Si(111)-7 × 7.

2. Full Dissociation of PH3 on the Sir Site. Figure 3 summarizes a possible reaction pathway to dissociate the second P-H bond from the Sir site, in which the adsorbed PH2 species inserts into the Si-Si backbond. It is found that the conversion from LM2r′ to LM3r′ has to surmount an energy barrier of

60.2 kcal/mol. Thus, PH2(a) is associated with high backward and forward barriers. This is in agreement with the experimental finding that the surface PH2 species is stable up to the temperature range of 450 to 500 K.7,8 The formation energy of LM3r′ is predicted to be -25.1 kcal/mol. In LM3r′, PH2 is

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Figure 4. Potential energy diagram for the decomposition of the third P-H bond on the Si rest atom site of Si(111)-7 × 7.

bound to both surface Sir and subsurface Sis, with the optimal Sir-PH2 and Sis-PH2 bond lengths being 2.377 and 2.322 Å, respectively. Similar surface complex was also found in the case of NH2(a),18 where the difference between the optimal Sir · · · NH2 and Sis-NH2 bond lengths is as large as 0.26 Å, instead of 0.06 Å for PH2. Hence, Sir · · · NH2 may be regarded as a dative bond, and Sis-NH2 a covalent bond; whereas both Sir-PH2 and Sis-PH2 can be viewed as covalent bonds, forming a tetrahedral hypervalent species. After climbing the transition state, TS3r′, which is uphill by 29.0 kcal/mol, LM4r′ was thus formed, in which the hypervalency is removed so that its thermal stability is increased by 26.5 kcal/mol relative to LM3r′. The breaking of the last P-H bond in LM4r′ may be accomplished by direct H2 liberation to give rise to the product of LM5r′ (See Figure 4), which requires a large activation energy of 61.6 kcal/mol. Experimentally, it was observed that H2 desorption occurs at a temperature of around 740 K.8 In LM5r′, the bond length of the Sir-P bond is 0.153 Å shorter than that of the Sis-P bond, indicating that the former takes on the character of a SidP double bond. It can be seen that the whole dissociation process of all three P-H bonds in this way possesses an effective activation energy barrier of 10.0 kcal/ mol, with respect to the entrance level. The overall reaction energy to LM5r′ is -19.1 kcal/mol (see Figures 2-4). We expect that the dissociation of the last P-H bond can be facilitated by the insertion of the PH group into another Si-Si backbond (See Figure 4). Indeed, we locate a transition state, TS5r′, for this step, which leads to the formation of LM6r′. The energy barrier for TS5r′ and the formation energy for LM6r′ are calculated to be 47.9 and -27.2 kcal/mol, respectively. Afterward, a direct H2 release process occurs in LM6r′ via TS8r′, leading to LM8r′, where the P atom is covalently bound to the surface Sir and the two subsurface Si atoms. The required activation energy is calculated to be 34.2 kcal/mol.

On the other hand, LM6r′ may transfer the hydrogen atom from the PH group to the surface Sir through TS6r′ with an energy barrier of 27.0 kcal/mol. LM7r′ thus formed has the largest exothermicity of -54.8 kcal/mol among all the species of PH3 decomposition. In LM8r′, the original DBs are now saturated by H atoms and P penetrates into the Si sublayer. 3. Full Dissociation of PH3 on the Sia Site. For the dissociations of the last two P-H bonds on the Sia site, pathways analogous to those on the Sir site can be located, which are shown in Figures 5 and 6. The results show that the insertion of PH2(a) into the Sia-Sis backbond is much easier than that into the Sir-Sis backbond. We find that TS2r′ is lying 7.2 kcal/ mol above the entrance level, whereas TS2a′ is 8.7 kcal/mol below the entrance level. The insertion barrier for the latter (45.4 kcal/mol) is 14.8 kcal/mol lower than that for the former (60.2 kcal/mol). This is in line with the general concept that the Sia-Sis backbond is weaker than the Sir-Sis backbond.38 As for the formation of the insertion product, the endothermicity for LM3a′ is 14.2 kcal/mol, whereas that for LM3r′ is 27.9 kcal/mol. There is actually no bonding between Sia and Sis in LM3a′. The same is true for that between Sir and Sis in LM3r′. Hence, this endothermicity difference once again reveals that the bond strength of the Sia-Sis backbond is weaker than that of the Sir-Sis backbond. The followed H-transfer process makes the splitting of the second P-H bond come true. Relative to the entrance level, TS3a′ is downhill by 17.7 kcal/mol, whereas TS3r′ is uphill by 3.9 kcal/mol. The barriers for the H-transfer are both much lower than those for the PH2(a) insertion. This demonstrates that the PH2 insertion is a rate-determining step for the dissociation of the second P-H bond. In analogy to that on the Sir site, the SidP bond can also be formed on the Sia site by a direct H2 liberation process from LM4a′ as shown in Figure 6. Despite that LM5a′(-19.6 kcal/ mol) is found to possess nearly the same thermodynamic

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Figure 5. Potential energy diagram for the decomposition of the second P-H bond on the Si adatom site of Si(111)-7 × 7.

Figure 6. Potential energy diagram for the decomposition of the third P-H bond on the Si adatom site of Si(111)-7 × 7.

stability as LM5r′(-19.1 kcal/mol), the apparent activation energy of the overall route for the latter is 6.3 kcal/mol higher than that of the former. We can thus come to a conclusion that the formation of the SiadP bond is more favorable than the formation of the SirdP bond.

The dissociation of the last P-H bond begins with the insertion of the PH group into the second Si-Si backbond of Si(111)-7 × 7 (see Figure 6). It is found that the activation barrier for the PH insertion into the Sia-Sis backbond (i.e., 45.5 kcal/mol at TS5a′) is comparable to that of PH2 (i.e., 45.4 kcal/

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TABLE 1: The Energetics of the Dissociation of the First X-H Bond (X ) N or P) on Si(111)-7 × 7 (kcal/mol) NH3/Si(111)-7 × 7 species LM1a (XH3fSia) LM1r (XH3fSir)

PH3/Si(111)-7 × 7

adsorption energy

Ea

adsorption energy

Ea

-16.6 -24.6

2.7 8.1

-1.2 -9.7

0.0 0.0

mol at TS2a′), whereas the PH insertion (47.9 kcal/mol at TS5r′) is much lower than the PH2 insertion (61.6 kcal/mol at TS2r′), when they occur on the Sir-Sis backbond. With respect to the entrance level, the PH insertion product, LM6r′, lies 16.3 kcal/ mol higher than its counterpoint, LM6a′. This may be caused by the larger intrinsic strain existing in the Sir-Sis backbonds. The cleavage of the third P-H bond may be finally realized by direct H2 release, which has a net barrier of 7.0 kcal/mol on the Sir site, whereas the corresponding TS (TS8a′) is 13.2 kcal/ mol lower than the entrance level. It is noted that the thermodynamic stability of LM8a′ is significantly higher than that of LM8r′, being 13.1 kcal/mol. This should be related to the geometry difference of the Si3P unit, as well as the difference in release of the intrinsic strain of Si-Si backbond between LM8a′ and LM8r′. The Si3P unit may also be formed by a H-transfer process followed by a subsequent H2 desorption. Nearly equivalent activation barriers are found for TS6a′ and TS6r’, and a similar situation exists for TS7a′ and TS7r′. 4. Comparison between PH3 and NH3 Dissociations on Si(111)-7 × 7. A detailed study on the mechanism of full dissociation of NH3 on Si(111)-7 × 7 has been carried out in our previous work.18 Considering the similarities and differences of the properties between ammonia and phosphine, it will be interesting to compare their dissociations on the Si(111)-7 × 7 surface. The corresponding energetics are listed in Tables 1-4. In fact, Figures 2-6 in the present work are organized in a similar way to facilitate the comparison with our previous work for NH3,18 but we are adding prime (′) here for the labeling of PH3. The breaking of the first X-H bond is initiated on the Sia or Sir site of Si(111)-7 × 7, by forming a dative bond of XH3fSia or XH3fSir (X ) N or P). Significantly, the binding energies of LM1a′ (1.2 kcal/mol) and LM1r′ (9.7 kcal/mol) are considerably smaller than those of the corresponding dative complexes of NH3fSia (16.6 kcal/mol18) and NH3fSir (24.6 kcal/mol18). Such a difference has to be attributed to the intrinsic difference in the Lewis basicity of NH3 and PH3, in that PH3 is a worse electron-donor than NH3. This can be seen from the facts that the calculated dipole moments are 1.84 (NH3) and 0.96 (PH3) Debye while the calculated ionization potentials are 10.18 (NH3) and 9.84 (PH3) eV. The common feature is that both NH3 and PH3 bind to Sir more strongly than to Sia. Previously,13,18 it was shown that electron density of DB on Sia is delocalized over the Si atoms below it to form a partial Sia-Si bond. Thus, forming the dative bond of XH3fSia has to pay the penalty for breaking this partial Sia-Si bond, leading to a weak XH3fSia, as compared to XH3fSir. On the Sia or Sir site, the activation barrier for the dissociation of the first N-H bond is 2.7 or 8.1 kcal/mol, respectively, whereas no barrier is demanded for the splitting of the first P-H bond (see Table 1). Such a difference can be understood by the observation that the N-H bond is shorter and stronger than the P-H bond (1.1 vs 1.4 Å and 109 vs 84 kcal/mol). In a way similar to the adsorption of CH3OH17 and NH318 on Si(111)-7 × 7, we estimate the reactivity ratio between Sia and Sir by associating the initial reaction probability with the zero coverage reactive sticking coefficient, Si, which, in turn, is

TABLE 2: Relative Energies of Transition States and Local Minima for the Dissociations of the Second and the Third X-H Bonds (X ) N or P) on the Siaand Sir Sites of Si(111)-7 × 7 (kcal/mol) NH3/Si(111)-7 × 7 species LM2 TS2 LM3 TS3 LM4 TS4 LM5 TS5 LM6 TS6 LM7 TS7 TS8 LM8

PH3/Si(111)-7 × 7

Sia

Sir

Sia

Sir

-59.4 -4.7 -45.5 -7.0 -68.5 6.7 -9.5 -30.0 -40.9 -2.8 -61.6 -7.8 8.2 -19.9

-56.6 3.3 -28.5 -1.3 -62.5 6.7 -22.0 -17.2 -25.5 -4.9 -67.2 -13.4 4.3 -26.0

-54.1 -8.7 -39.9 -17.7 -63.7 3.7 -19.6 -18.2 -43.5 -15.6 -72.3 -19.2 -13.2 -28.5

-53.0 7.2 -25.1 3.9 -51.6 10.0 -19.1 -3.7 -27.2 -0.2 -54.8 -1.2 7.0 -15.4

TABLE 3: The Energy Barriers for the Dissociations of the Second and the Third X-H Bonds (X ) N or P) on the Sir Site of Si(111)-7 × 7 (kcal/mol) NH3/Si(111)-7 × 7 similar reaction XH2 insertion H transfer H2 liberation XH insertion H transfer H2 liberation H2 liberation

PH3/Si(111)-7 × 7

process

Ea

process

Ea

LM2rfTS2r LM3rfTS3r LM4rfTS4r LM4rfTS5r LM6rfTS6r LM6rfTS8r LM7rfTS7r

59.9 27.2 69.2 45.3 20.6 29.8 53.8

LM2r′fTS2r′ LM3r′fTS3r′ LM4r′fTS4r′ LM4r′fTS5r′ LM6r′fTS6r′ LM6r′fTS8r′ LM7r′fTS7r′

60.2 29.0 61.6 47.9 27.0 34.2 53.6

proportional to the probability (ξ) of trapping the incident molecule PH3(g) into the precursor state PH3(a). We emphasize that the formation of molecular precursor states (LM1a′ and LM1r′) is a key step to differentiate the reactivity between Sia and Sir. Hence, the ratio for the initial reaction probability between Sia and Sir may be predicted according to Sa/Sr ) [1.2/ 9.7]1/2 ) 0.35, based on the assumption that ξ is proportional to the square root of the binding energy of the precursor state.17,18 In the case of NH3,18 we had Sa/Sr ) [16.6/24.6]1/2 ) 0.82 and concluded that the Sia site approximately possesses an 82% activity as compared to the Sir site. Therefore, in the case of PH3, the Sia site approximately possesses a 35% activity as compared to the Sir site, and the dissociation preference for PH3 on the Sir site is much higher than that for NH3. This is in line with the early experimental finding from Bozso and Avouris.10 Our calculations also provide the theoretical support to Shen’s assertion that the rest atoms are more reactive toward the initial adsorption of PH3 than the adatoms.12 The route, LM2 f TS2 f LM3 f TS3 f LM4, is the overall dissociation reaction of the second X-H bond, where the XH2 insertion into the Si-Si backbond of Si(111)-7 × 7 is the rate-determining step, due to its higher activation barrier than that of the H-transfer process (see Table 2 ). For the insertion of XH2 into the Sia-Sis backbond or the Sir-Sis backbond, the latter has to surmount a higher energy barrier (Table 3 vs Table 4 ). This reflects the fact that the Sia-Sis backbond is indeed weaker than the Sir-Sis backbond.38 We find that both NH2 and PH2 insertions into the Sir-Sis backbond possess nearly the same energy barrier (see Table 3), whereas the barrier difference for NH2 and PH2 insertions into the Sia-Sis backbond amounts to 9.3 kcal/mol (see Table 4). TS2r and TS2r′ are calculated to be 3.3 and 7.2 kcal/mol above their

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TABLE 4: The Energy Barriers for the Dissociations of the Second and the Third X-H Bonds (X ) N or P) on the Sia Site of Si(111)-7 × 7 (kcal/mol) NH3/Si(111)-7 × 7 similar reaction XH2 insertion H transfer H2 liberation XH insertion H transfer H2 liberation H2 liberation

PH3/Si(111)-7 × 7

process

Ea

process

Ea

LM2afTS2a LM3afTS3a LM4afTS4a LM4afTS5a LM6afTS6a LM6afTS8a LM7afTS7a

54.7 38.5 75.2 38.5 38.1 49.1 53.8

LM2a′fTS2a′ LM3a′fTS3a′ LM4a′fTS4a′ LM4a′fTS5a′ LM6a′fTS6a′ LM6a′fTS8a′ LM7a′fTS7a′

45.4 22.2 67.4 45.5 27.9 30.3 53.1

respective entrance level; nevertheless, TS2a and TS2a′ are lying -4.7 and -8.7 kcal/mol below their own entrance level, respectively (see Table 2). This shows that the insertion of PH2 into the Sia-Sis backbond is the easiest process to proceed at elevated temperatures. By direct H2 desorption, the formation of the SidX unit is possible (i.e., LM4 f TS4 f LM5). Specifically, in order to form SiadN or SirdN, the same apparent activation energy, namely, 6.7 kcal/mol, has to be overcome, whereas the former is thermodynamically less feasible than the latter, because of its lower exothermicity of -9.5 kcal/mol. On the contrary, the overall formation route of the SiadP bond is kinetically more favorable than that of the SirdP bond with equal exothermicity (see Table 2). With the aim to accomplish the cleavage of the final X-H bond, the bridged XH group can undergo the insertion into the second Si-Si backbond of Si(111)-7 × 7. The activation energy barriers for the XH insertion follow the order of TS5a (38.5) < TS5r (45.3) = TS5a′ (45.5) < TS5r′ (47.9). This not only reconfirms the fact that the Sia-Sis backbond possesses a weaker bond strength38 but also suggests that NH insertion is the easiest way to break the second Sia-Sis backbond. The Si3X unit can be formed by two different pathways (i.e., stepwise: LM6 f TS6 f LM7 fTS7 f LM8; or directly: LM6 f TS8 f LM8). For the direct H2 liberation process, the activation energy barrier from LM6r to TS8r (29.8 kcal/mol) is comparable to that from LM6a′ to TS8a′(30.3 kcal/mol), and TS8a′ is below the entrance level by -13.2 kcal/mol. For the stepwise pathway, LM6 is converted to LM7 by a H-transfer process, where X atom is covalently bound to one surface Si and two subsurface Si atoms. LM7 is the most stable species during the dissociations of the three X-H bonds. It can be noted from Table 2 that LM7r′ is 17.5 kcal/mol higher in energy than LM7a′, whereas LM7r is 5.8 kcal/mol more stable than LM7a. This should be attributed to the atomic radius difference between P and N, which leads to the release of a different amount of strain energy of the reconstructed surface. Despite that TS7a, TS7r, TS7a′, and TS7r′ are below the entrance level, the intrinsic barriers for LM7 f TS7 are calculated to be around 53.0 kcal/mol, which is higher than that from LM6 to TS8. Hence, divesting LM7 of H2 can only be realized at elevated temperatures. 5. Conclusion We present here the first density functional theory study at the level of (U)B3LYP to explore the dissociation chemistry of PH3 on the Si(111)-7 × 7 surface. On the basis of our calculations, a detailed comparison and contrast between PH3 and NH3 dissociations on Si(111)-7 × 7 is obtained. The calculations show that there is no activation barrier for the breaking of the first P-H bond. The dissociation of PH3 to PH2(a) and H(a) preferentially occurs on the rest atom. This is consistent with the observations from an early UPS experiment10

and a recent STM experiment.12 Our calculations demonstrate that the initial site selectivity for PH3 is higher than that for NH3. Despite that the first X-H (X ) N, P) bond decomposition is facile, especially for the first P-H bond, elevated temperature is required for further X-H bond decomposition. The general mechanism for further X-H bond cleavage is the XHn (n ) 2 and 1) insertion, followed by H2 libration. Generally, we find that the XHn insertion into Si-Si backbond is a rate-determining step, and XH2 insertion demands more activation energy than XH insertion. For XH2, the insertion of PH2 into the Sia-Sis backbond is the easiest process to occur. After H2 liberation at high temperature, the surface SidX unit may be formed. Our calculations show that formation of the SidN bond is more feasible on the Sir site than that on the Sia site because of thermodynamics, whereas the production of the SiadP bond is more favorable on the Sia site than that on the Sir site because of kinetics. By direct or indirect H2 release, the Si3X unit can be formed. The formation of the Si3P unit on the Sia site is more facile than on the Sir site whereas the formation of the Si3N unit on the Sir site is more facile than on the Sia site. Such a difference should be attributed to the atomic radius difference between P and N as well as the release of the strain energy of the reconstructed surface. These results can be used as the quantum-mechanical input for a chemical-kinetics model of CVD and should be of significance in the microelectronic industry. Acknowledgment. This work was supported by NSFC (20525311, 20533030, 20423002, 10774126) and the Ministry of Science and Technology (2007CB815206, 2004CB719902). References and Notes (1) Tsukidate, Y.; Suemitsu, M. Appl. Surf. Sci. 1999, 151, 148. (2) Jacobson, M. L.; Chiu, M. C.; Crowell, J. E. Langmuir 1998, 14, 1428. (3) Miotto, R.; Srivastava, G. P.; Miwa, R. H.; Ferraz, A. C. J. Chem. Phys. 2001, 114, 9549. (4) Schofield, S. R.; Curson, N. J.; Simmons, M. Y.; Ruess, F. J.; Hallam, T.; Oberbeck, L.; Clark, R. G. Phys. ReV. Lett. 2003, 91, 136104. (5) Wilson, H. F.; Warschkow, O.; Marks, N. A.; Schofield, S. R.; Curson, N. J.; Smith, P. V.; Radny, M. W.; McKenzie, D. R.; Simmons, M. Y. Phys. ReV. Lett. 2003, 93, 226102. (6) Sen, P.; Gupta, B. C.; Batra, I. P. Phys. ReV. B 2006, 73, 085319. (7) (a) Wallace, R. M.; Taylor, P. A.; Choyke, W. J.; Yates, J. T., Jr. J. Appl. Phys. 1990, 68, 3669. (b) Taylor, P. A.; Wallace, R. M.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1990, 238, 1. (8) Chen, P. J.; Colaianni, M. L.; Wallace, R. M.; Yates, J. T., Jr. Surf. Sci. 1991, 244, 177. (9) Van Bommel, A. J.; Crombeen, J. E. Surf. Sci. 1973, 36, 773. (10) Bozso, F.; Avouris, P. Phys. ReV. B 1991, 43, 1847. (11) Cao, P. L.; Lee, L. Q.; Dai, J. J.; Zhou, R. H. J. Phys.: Condens.Matter 1994, 6, 6103. (12) Young, J. J.; Shen, J. C. Surf. Sci. 2007, 601, 1768. (13) (a) Wolkow, R.; Avouris, P. Phys. ReV. Lett. 1988, 60, 1049. (b) Avouris, P.; Wolkow, R. Phys. ReV. B 1989, 39, 5091. (14) Chen, P. J.; Colaianni, M. L.; Yates, J. T. Surf. Sci. 1992, 274, L605. (15) Colaianni, M. L.; Chen, P. J.; Yates, J. T. J. Chem. Phys. 1992, 96, 7826. (16) Ezzehar, H.; Sonnet, P.; Minot, C.; Stauffer, L. Surf. Sci. 2000, 454-456, 358. (17) Xu, X.; Wang, C. J.; Xie, Z. X.; Lu, X.; Chen, M. S.; Tanaka, K. Chem. Phys. Lett. 2004, 388, 190. (18) Wang, X. L.; Xu, X. J. Phys. Chem. C 2007, 111, 16974. (19) Xu, X.; Kang, S.-Y.; Yamabe, T. Chem.sEur. J. 2002, 8, 5351. (20) Xu, X.; Kang, S.-Y.; Yamabe, T. Phys. ReV. Lett. 2002, 88, 076106. (21) Xu, X.; Kang, S.-Y.; Yamabe, T. Bull. Chem. Soc. Jpn. 2001, 74, 817. (22) Takayanagi, K.; Tanishiro, Y.; Takahashi, M.; Takahashi, S. J. Vac. Sci. Technol. A 1985, 3, 1502. (23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

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