THE JOURNAL OF CHEMICAL PHYSICS 139, 194709 (2013)

Adsorption and dissociation of oxygen molecules on Si(111)-(7×7) surface Chun-Yao Niu and Jian-Tao Wanga) Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

(Received 20 June 2013; accepted 5 November 2013; published online 21 November 2013) The adsorption and dissociation of O2 molecules on Si(111)-(7×7) surface have been studied by first-principles calculations. Our results show that all the O2 molecular species adsorbed on Si(111)(7×7) surface are unstable and dissociate into atomic species with a small energy barrier about 0.1 eV. The single O2 molecule adsorption tends to form an ins×2 or a new metastable ins×2* structure on the Si adatom sites and the further coming O2 molecules adsorb on those structures to produce an ad-ins×3 structure. The ad-ins×3 structure is indeed highly stable and kinetically limited for diving into the subsurface layer to form the ins×3-tri structure by a large barrier of 1.3 eV. Unlike the previous views, we find that all the ad-ins, ins×2, and ad-ins×3 structures show bright images, while the ins×2*, ins×3, and ins×3-tri structures show dark images. The proposed oxidation pathways and simulated scanning tunneling microscope images account well for the experimental results and resolve the long-standing confusion and issue about the adsorption and reaction of O2 molecules on Si(111) surface. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4832340] I. INTRODUCTION

The oxidation of silicon surfaces has been an important subject of surface physics with regard to the fabrication of semiconductor devices. The adsorbed states of O2 on Si(111)-(7×7) have been a long-standing issue of both experimental1–18 and theoretical studies.19–24 Several scanning tunneling microscope (STM) studies were conducted at various temperatures and found that the adsorption of O2 on Si(111)-(7×7) can show whether bright or dark images at the adatom positions depending on temperature, coverage, and bias voltage.1–9 Some molecular precursors have been proposed as candidates for the bright or dark images in some studies.19–21 However, Lee and Kang carried out firstprinciples calculations and concluded that O2 molecules dissociate directly upon adsorption on Si(111) surface.22, 23 They claimed that the dark images are due to the saturation of the adatom dangling bond with O atom on the top of Si adatom site, while the bright sites are caused by a configuration with insertion of O atoms in the backbond of the adatom leaving the dangling bond intact. Lately, the idea that molecular states stabilized against dissociation by coadsorbed oxygen atom inserted into the backbond was proposed by Matsui et al.11, 12 However, this model is only tentative and never reliably justified by the first-principles calculations. On the other hand, limited by the complexity of the (7×7) surface structure, most of the previous theoretical calculations employed a simple (2×2)19, 20 or (4×2)21–24 surface model instead of the dimer-adatom-stacking-fault (7×7) structure. However, it is well-known that the dimers, corner holes, and the stacking fault play a significant role in the oxidation of silicon surfaces. Moreover, a small unit cell will overestimate the longrange electrostatic couplings among negative O ions and compensating positive charges on Si adatom. Thus, the simplified a) E-mail: [email protected]

0021-9606/2013/139(19)/194709/5/$30.00

(2×2) or (4×2) structure model may lead to an inaccurate result. In this paper, we present a comprehensive first-principles study on the adsorption and dissociation of oxygen molecules on the Si(111) surface using the dimer-adatom-stacking-fault (7×7) structure model. We show that all the O2 molecular species adsorbed on Si(111) surface are unstable. The single O2 molecule adsorption tends to form an ins×2 or a new metastable ins×2* structure, and the further coming O2 molecules adsorb on those structures to produce an adins×3 structure. The ad-ins×3 structure is found highly stable and kinetically limited for diving into the subsurface layer to form the ins×3-tri structure by a large reaction barrier of 1.3 eV. Unlike the previous views, we find that all the ad-ins, ins×2, and ad-ins×3 structures show bright images, while the ins×2*, ins×3, and ins×3-tri structures show dark images. The proposed oxidation pathways and simulated STM images account well for the experimental results and resolve the long-standing confusion and issue about the adsorption and reaction of O2 molecules on Si(111) surface.

II. COMPUTATIONAL METHOD

Our first-principles calculations are based on the density functional theory (DFT) with the projector augmented wave (PAW) method25 implemented in the Vienna ab initio simulation package (VASP) code.26 The generalized gradient approximation-Perdew-Burke-Ernzerhof (GGA-PBE) method27 is used to describe the exchange-correlation functions. The valence states 2s2 2p6 for O, and 3s2 3p2 for Si are used with an energy cutoff of 400 eV for the plane wave basis set. We used the slab model of the Si(111)-(7×7) dimeradatom-stacking-fault structure. The (7×7) surface cell has the XYZ = 26.8806 Å × 26.8806 Å × 27 Å with seven layers of silicon in the Z direction. One layer of hydrogen is set

139, 194709-1

© 2013 AIP Publishing LLC

194709-2

C.-Y. Niu and J.-T. Wang

J. Chem. Phys. 139, 194709 (2013)

TABLE I. Binding energies (BE in eV) of an O2 molecule calculated with the LDA, GGA, and Meta-GGA in comparison with the experimental result (Expt).31 Methods

LDA

GGA

TPSS

RTPSS

Expt31

BE (eV)

7.514

6.696

6.651

6.803

5.11

to passivate the back surface of the Si substrate with a vacuum layer of about 14 Å in the Z direction. Only the  point is employed to sample the Brillouin zone. The total energy is converged to 10−4 eV for the structural relaxations. All atoms except for H and Si atoms at the bottom are fully relaxed during the calculations to optimize the total energy of the system.28 To test the accuracy of our calculations, the binding energy of O2 molecule is calculated under LDA-CA,29 GGAPBE,27 Meta-GGA-TPSS, and Meta-GGA-RTPSS30 methods. As shown in Table I, the binding energy under LDA-CA is estimated to be 7.514 eV, which is about 2.40 eV larger than the experimental data of 5.11 eV;31 the binding energy under GGA-PBE is 6.696 eV, which is about 1.59 eV larger than the experimental value; meanwhile, both Meta-GGA-TPSS and Meta-GGA-RTPSS methods give the similar results as GGAPBE method. The similar results are also obtained for the adsorption energies of O on Si(111) under Meta-GGA-TPSS method as well as GGA-PBE method (see the supplementary material32 ). Therefore, the discussions throughout this paper are mainly based on the calculation by GGA-PBE method, except for specially notation. III. RESULTS AND DISCUSSIONS

Here we mainly focus on the Si adatom sites as the primary O2 reaction sites at low coverage based on experimental evidences.1–16 The adatoms show high activity due to the dangling bond. Several structural models for single and double O2 adsorption are shown in Figs. 1(a) and 1(b).32 Both faulted half unit cell (FHUC) and unfaulted half unit cell (UFHUC) are considered. The “ad” denotes an O atom adsorbed on top of the Si adatoms, “ins×n” means that O atoms are inserted into Si adatom backbonds, and “tri” denotes an O atom at the threefold-coordinated subsurface. For each cases, there are four adsorption sites: corner (Co) and center (Ce) adatom sites in both faulted (F) and unfaulted (U) half unit cell. Meanwhile, there are two inequivalent backbonds around the Si adatoms denoted with “i” (inside) and “o” (outside) as shown in Fig. 1(c). The adsorption energy Ead (N) is given by Ead = [EO/Si(111) − ESi(111) ]/N − 0.5EO2 ,

(1)

where EO/Si(111) , ESi(111) , and EO2 are the total energies of O/Si(111) system, the clean Si(111) surface, and O2 molecule, respectively, and N is the number of O atoms on the Si(111) surface. The calculated adsorption energies for each model are listed in Table II. For the case of single O2 adsorption, the grif structure on the adatom site is shown as a precursor state when the O2 molecules drop on Si(111)-(7×7) surface.33 After the adsorption, the O–O bond length shows an increase of about 0.365 Å from 1.234 Å for the isolate O2 molecule. Around the adatom

FIG. 1. (a) Adsorption models considered in this study. The small (red) and large (gray) balls represent the O atoms and the Si atoms, respectively. (b) Side view of single O2 adsorb on the corner Si adatom site with grif structure. (c) Top view of the Si(111)-(7×7) dimer-adatom-stacking-fault structure. The FHUC and UFHUC indict the faulted and unfaulted half unit cell. The yellow and blue balls represent the Si adatoms and the Si rest atoms, respectively. There are two inequivalent backbonds around the Si adatoms which denoted with “i” (inside) and “o” (outside). TABLE II. Calculated oxygen adsorption energies (eV) for various structures depicted in Fig. 1(a). The adsorption and dissociation of oxygen will mainly occur at the F-Co site, as denoted by boldface below. Model grif ad-ins-i ad-ins-o ins×2-io ins×2*-io ins×2-oo ins×2*-oo ins×2-ii ins×2*-ii ins×2-grif ad-ins×3 ins×3-tri ins×3

F-Ce

F-Co

U-Ce

U-Co

−1.052 −3.315 −3.293 −3.772 −3.714 −3.763 −3.746 ... ... −2.545 −3.697 −3.787 −3.866

−1.097 −3.317 −3.303 −3.739 −3.675 ... ... −3.724 −3.688 −2.530 −3.689 −3.702 −3.804

−1.017 −3.274 −3.251 −3.771 −3.758 −3.760 −3.740 ... ... -2.519 −3.703 −3.802 −3.873

−1.057 −3.272 −3.256 −3.720 −3.674 ... ... −3.713 −3.689 −2.515 −3.690 −3.720 −3.813

194709-3

C.-Y. Niu and J.-T. Wang

site, a molecular bridge structure has been proposed as an intermediate state in the bright sites hopping progress,9 but it is indeed unstable and dissociate spontaneously into ad-rest structure (one O on top of Si adatom site, and the other one on top of rest atom site).33 The ad-ins structure is much stable than the molecular grif structure but metastable relative to the ins×2 structure. The vertical position of the Si adatom in the ad-ins structure is significantly shifted compared to the clean surface. The formation of the strong Si–O bond induces a large charge transfer from the Si adatom to the O atom, which in turn weakens the Si–Si backbond. The ins×2 structure is the most stable structure for the single O2 adsorption, and its Si adatom slightly higher than the O atoms. Meanwhile, the ins×2 structure has a metastable counterpart ins×2* structure with Si adatom little lower than the O atoms as shown in Fig. 1(a). For the case of double O2 adsorption, the ins×2-grif structure with the O2 molecule coadsorbed on the O(ins) structure, also has a much higher energy compared to the atomic ad-ins×3 and ins×3-tir structures. Meanwhile, the adins×3 and ins×3-tri structures show almost the same adsorption energy at the F-Co site, but larger than the ins×3 structure. The ins×3 structure which can be viewed as losing of the ad type O atom in the ad-ins×3 structure is the most stable structure in our calculations. We then discuss the preferable adsorption sites for each structure in detail. From Table II, we can see that both grif and ad-ins structures prefer to adsorb at the F-Co site. The ins×2 structure prefers both F-Ce and U-Ce sites with one O insert in the outside backbond and the other in the inside backbond (ins×2-io). For the ad-ins×3, ins× 3-tri, and ins×3 structures, the U-Ce site becomes the most stable site and the F-Co the most unstable site. One noticeable tendency can be seen that as more O atoms insert into the backbond, the strain effect becomes more important and the U-Ce site becomes more favorable than the F-Co site. However, due to the first coming O2 molecules prefer to adsorb at F-Co site, the successive reaction and dissociation will mainly occur at F-Co site, explaining well the experimental findings.2, 5 We next discuss the dissociation of O2 molecules on the Si(111) surface. For the case of single O2 adsorption, the O2 molecules initially form the grif structure on the Si adatom sites. However, the grif structure is unstable and dissociates into the ad-ins structure with a large energy gain of about 4.44 eV per O2 . The energy barrier for this transition is calculated less than 0.1 eV [see Fig. 2(a)], in agreement with the previous studies.22 Meanwhile, the ad-ins structure is metastable and can further decay into an ins× 2 structure with an energy gain of about 0.84 eV. The energy barrier for this process is estimated to be about 0.40 eV. Similar energy gain and energy barrier are also estimated for the structural change from ad-ins to ins×2* structure. Therefore, single adsorption of O2 tends to produce an ins×2 or a metastable ins×2* structure at room temperature with a small reaction barrier of 0.40 eV. As shown in Table II, the adsorption energy of the double adsorption ins×2-grif structure is smaller than the average adsorption energy of grif and ins×2 structures, showing that the ins×2 structure is more reactive compared to the bare

J. Chem. Phys. 139, 194709 (2013)

FIG. 2. The decay pathway and corresponding energies for single (a) and double (b) O2 molecules drop, adsorption, and dissociation on the Si adatom. Oxygen atoms are in red and silicon atoms are in gray.

Si adatom. Thus, the further coming O2 molecules prefer to adsorb on the ins×2 structure and form an ins×2-grif structure. This structure is proposed by Matsui et al. to explain the bright images.11, 12 However, our calculations show that the ins×2-grif structure is also unstable, and easily decays into the ad-ins×3 structure with a large energy gain of about 4.64 eV and a small energy barrier about 0.1 eV [see Fig. 2(b)]. Like the ad-ins structure, the ad-ins×3 is also a metastable structure compared to the ins×3-tri structure. However, the energy barrier from ad-ins×3 to ins×3-tri structure is as large as 1.3 eV, indicating that the ad-ins×3 structure should be stable at room temperature. As mentioned above, another possible case is that the further coming O2 molecules adsorb at the place between the ins×2 and the Si rest atom as the bridge structure. In this case the O2 molecules will dissociate spontaneously with one O on top of the Si rest atom and the other O inserting into the remaining one Si backbond to form an ins×3 structure. Based on the above results, we can see that all the O2 molecular species, whether single molecule or coadsorbed structures are unstable, only the ins×2, ins×2*, ins×3, adins×3, and ins×3-tri can be obtained. Meanwhile, the O-ad configuration as an intermediate state should occur during the adsorption process, although it is less stable than the O-ins configuration.

194709-4

C.-Y. Niu and J.-T. Wang

J. Chem. Phys. 139, 194709 (2013)

The metastable dark imaged precursor state can be assigned only to the ins×2* structure, since the other metastable ad-ins and ad-ins×3 structures exhibit bright images. At very low temperature, when the thermal vibrations are much reduced, the metastable ins×2* structure can be identified; meanwhile, this ins×2* structure can change into the bright imaged ins×2 structure by STM tips, as found by Konishi et al.5 Several experiments9, 34 reported that there is a mobile precursor state for molecular oxygen on the Si(111) surface. This means that the O2 dissociation barriers reported here may be underestimated under conventional first-principles quantum mechanics (density functional theory), and a highly accurate embedded correlated wave function method36 should be used in the further work. It is well-known that the ground state of an isolated O2 molecule has a paramagnetic spin-triplet state. However, after the chemical adsorption and dissociation, the ground states of all the structures become spin-singlet state in our calculation. There is a spin transition from triplet to singlet state when an O2 molecule moves toward the Si surface, which still remains elusive. IV. CONCLUSION FIG. 3. Simulated STM images for the O with various structures on the F-Co site. Here U = −1.5 eV and the contours are drawn at 0.04 e/Å3 .

The simulated STM images for the O with various structures on Si(111)-(7×7) surface are represented in Fig. 3 with applied voltage U = −1.5 eV. These simulations are carried out according to the Tersoff-Hamann approximation.35 It represents the local density of states integrated from the Fermi level to the corresponding applied voltage level. We find that both the ad-ins and ad-ins×3 structures show bright images, while the ins×3 and ins×3-tri structures show dark images. Moreover, it is interesting to find that the two energies near degenerate structures ins×2 and ins×2* display completely different images: the ins×2 structures show bright image while the ins ×2* exhibit dark image. Our results correct the longstanding wrong views that all the ad-ins×n structures show dark images, while the ins×n structures show as bright images.19, 20, 22, 24 The inconsistency may arise from the small unit cell (2 × 2) or (4 × 2) previously used, which cannot fully describe the (7×7) surface. Our results about the structural stability and the simulated STM images account well for the STM observations2–6 that (1) adsorption of O2 at room temperature always results in bright images that prefer at F-Co site at low coverage; (2) the bright sites are much more reactive than the unreacted adatom sites; (3) increase of the oxygen coverage leads to limited formation of dark images; and (4) some metastable dark imaged precursor state can be found at very low temperature. The O2 prefers first to adsorb at F-Co site with the grif structure, thus successive products are also mainly at this site. The bright image at low O2 coverage is ascribed to the ins×2 structure, which is high reactivity. The ad-ins×3 structure is much stable and attributes the main bright image at high O2 coverage. The ins×3 or ins×3-tri structures contribute to the small amount of the dark images at higher coverage.

In conclusion, we have systematically studied the adsorption and dissociation of the O2 molecules on Si(111)-(7×7) surface. Our results show that all the O2 molecular species, whether single molecule or atomic coadsorbed, are unstable and dissociate into atomic species with a small energy barrier about 0.1 eV. The single O2 molecule adsorption tends to form the ins×2 or ins×2* structure, while the double adsorption of O2 molecules gives rise to the ad-ins×3, ins×3, and ins×3-tri structures. The ad-ins×3 structure is high stable and kinetically limited for diving into the subsurface layer to form the dark ins×3-tri structure by a large barrier of 1.3 eV. At lower O2 coverage, the ins×2 and ins×2* structures contribute to the bright and dark images. However, for higher O2 coverage, the ad-ins×3 gives the main bright images while the ins×3 and ins×3-tri structures contribute to the small amount of dark images. Our results resolve the long-standing confusion and issue about the adsorption and dissociation of O2 molecules on the Si(111) surface. ACKNOWLEDGMENTS

This study was supported by the NSFC of China (Grant Nos. 10974230 and 11274356) and the Ministry of Environmental Protection of China (Grant Nos. 200909086 and 201109037). 1 R.

Martel, Ph. Avouris, and I. W. Lyo, Science 272, 385 (1996). Dujardin, A. Mayne, G. Comtet, L. Hellner, M. Jamet, E. Le Goff, and P. Millet, Phys. Rev. Lett. 76, 3782 (1996). 3 A. J. Mayne, F. Rose, G. Comtet, L. Hellner, and G. Dujardin, Surf. Sci. 528, 132 (2003). 4 H. Okuyama, T. Miki, T. Aruga, and M. Nishijima, Jpn. J. Appl. Phys. 41, L1419 (2002). 5 Y. Konishi, S. Yoshida, Y. Sainoo, O. Takeuchi, and H. Shigekawa, Phys. Rev. B 70, 165302 (2004). 6 H. Okuyama, T. Aruga, and M. Nishijima, Phys. Rev. Lett. 91, 256102 (2003). 2 G.

194709-5 7 K.

C.-Y. Niu and J.-T. Wang

Sakamoto, S. T. Jemander, G. V. Hansson, and R. I. G. Uhrberg, Phys. Rev. B 65, 155305 (2002). 8 H. Okuyama, T. Yamada, and T. Aruga, Jpn. J. Appl. Phys. 44, 5362 (2005). 9 I. S. Hwang, R. L. Lo, and T. T. Tsong, Phys. Rev. Lett. 78, 4797 (1997). 10 K. Y. Kim, T. H. Shin, S. J. Han, and H. Kang, Phys. Rev. Lett. 82, 1329 (1999). 11 F. Matsui, H. W. Yeom, K. Amemiya, K. Tono, and T. Ohta, Phys. Rev. Lett. 85, 630 (2000). 12 K. Sakamoto, S. Doi, Y. Ushimi, K. Ohno, H. W. Yeom, T. Ohta, S. Suto, and W. Uchida, Phys. Rev. B 60, R8465 (1999). 13 G. Comtet, L. Hellner, G. Dujardin, and K. Bobrov, Phys. Rev. B 65, 035315 (2001). 14 K. Sakamoto, H. M. Zhang, and R. I. G. Uhrberg, Phys. Rev. B 68, 075302 (2003). 15 K. Sakamoto, H. M. Zhang, and R. I. G. Uhrberg, Phys. Rev. B 72, 075346 (2005). 16 H. Okuyama, Y. Ohtsuka, and T. Aruga, J. Chem. Phys. 122, 234709 (2005). 17 N. T. Kinahan, D. E. Meehan, T. Narushima, S. Sachert, J. J. Boland, and K. Miki, Phys. Rev. Lett. 104, 146101 (2010). 18 J. W. Lee, J. H. Park, W. J. Jung, and I. W. Lyo, Surf. Interface Anal. 44, 666 (2012). 19 B. Schubert, P. Avouris, and R. Hoffmann, J. Chem. Phys. 98, 7593 (1993); 98, 7606 (1993). 20 T. Hoshino and Y. Nishioka, Phys. Rev. B 61, 4705 (1999). 21 M. H. Tsai, Y. H. Tang, I. S. Hwang, and T. T. Tsong, Phys. Rev. B 66, 241304(R) (2002).

J. Chem. Phys. 139, 194709 (2013) 22 S.

H. Lee and M. H. Kang, Phys. Rev. Lett. 82, 968 (1999). H. Lee and M. H. Kang, Phys. Rev. B 61, 8250 (2000). 24 S. H. Lee and M. H. Kang, Phys. Rev. Lett. 84, 1724 (2000). 25 P. E. Blöchl, Phys. Rev. B 50, 17953 (1994); G. Kresse and D. Joubert, ibid. 59, 1758 (1999). 26 G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996); G. Kresse and J. Hafner, ibid. 47, 558 (1993). 27 J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996). 28 F. A. Soria, E. M. Patrito, and P. O. Patricia, J. Phys. Chem. C 116, 24607 (2012). 29 J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. B 23, 5048 (1981). 30 J. W. Sun, M. Marsman, G. I. Csonka, A. Ruzsinszky, P. Hao, Y. S. Kim, G. Kresse, and J. P. Perdew, Phys. Rev. B 84, 035117 (2011). 31 K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure (Van Nostrand Reinhold, New York, 1979). 32 See supplementary material at http://dx.doi.org/10.1063/1.4832340 for the O adsorption energies that were calculated using the Meta-GGA method with TPSS function and the atomic positions for some typical structures. 33 The Si adatoms have the highest activity due to the dangling bond, and can capture almost all coming O2 molecules to form the grif structure initially. When an O2 molecule drops on the sites away from the adatoms, it should move toward the adatom sites to form the grif structure or dissociate spontaneously into an ad-rest structure at the bridge adsorption sites. 34 I. S. Hwang, R. L. Lo, and T. T. Tsong, Surf. Sci. 399, 173 (1998). 35 J. Tersoff and D. R. Hamann, Phys. Rev. B 31, 805 (1985). 36 F. Libisch, C. Huang, P. Liao, M. Pavone, and E. A. Carter, Phys. Rev. Lett. 109, 198303 (2012). 23 S.

The Journal of Chemical Physics is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/jcpo/jcpcr/jsp

Adsorption and dissociation of oxygen molecules on Si(111)-(7×7) surface.

The adsorption and dissociation of O2 molecules on Si(111)-(7×7) surface have been studied by first-principles calculations. Our results show that all...
989KB Sizes 0 Downloads 0 Views