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Adsorption of water on stepped Si(100) surface
Surface Science 416 (1998) 9–16
Adsorption of water on stepped Si(100) surface Saed A. Salman, S¸enay Katırcıog˘lu * Department of Physics, Middle East Technical University, 06531 Ankara, Turkey Received 24 October 1997; accepted for publication 12 May 1998
Abstract We have investigated possible water forms on stepped Si(100) surface. Calculations are performed by using the empirical tightbinding method. Two types of adsorption model of water on stepped Si(100) surface have been considered; one of them is the dissociative type (H, OH ) and the other is the molecular type (H O). The results of the density of states supported by total electronic 2 energy calculations indicate a dissociative type of water adsorption on the stepped Si(100) surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption kinetics; Semi-empirical models and model calculations; Silicon; Single crystal surfaces; Surface roughening; Water
1. Introduction In the past, the adsorption of water in either dissociative or molecular form on flat silicon surfaces has been investigated intensively [1–12] because of the important role of the interaction of water with silicon surfaces in semiconductor device technology (wet oxidation and cleaning). In recent years, owing to the existence of some step structures on Si surfaces, as observed in experiments especially in scanning tunnelling microscopy (STM ) images, water-stepped Si systems have been investigated to understand the form of adsorption of water [13–16 ]. In a recent study, Bergen and Ranke [13] investigated the adsorption kinetics on vicinal Si(100) surfaces using UV photoemission spectroscopy ( UPS); the adsorption of water was investigated for the steps * Corresponding author. Fax: +90-312-2101281; e-mail: [email protected]
along [11: 0] and [100] directions. They concluded that, during the water adsorption process, the abrupt saturation of the dangling bonds of the Si(100) surface occurs at half-monolayer coverage and the adsorbate dissociates into OH+H. In the same study [13], the initial sticking probabilities on Si(100) and on the stepped vicinal surfaces in [11: 0] and the [100] zone were found to be equal. In a second experimental study [14], the room temperature adsorption of water was studied on vicinal Si(100) (2×1) surfaces titled toward [01: 1]. Using STM, it was observed that water adsorbs in small islands via a molecular precursor state. According to the STM images [14], the water islands grow on terraces leaving edge dimers unsaturated at first; finally the edge dimers become saturated. In recent theoretical studies, the adsorption of water was investigated for stepped Si(110) [15] and stepped Si(111) [16 ] surfaces and the adsorption of the water on these surfaces was found to be dissociative.
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In the present work, to eliminate the disagreement between the two experimental results mentioned above [13,14], the water-stepped Si(100) systems were studied for dissociative and molecular forms by the empirical tight-binding ( ETB) method. The ETB results on water-stepped Si(100) systems will also indicate any parallelism between the water-flat Si(100) and the other waterstepped Si(110) and Si(111) surfaces, if it exists.
2. Stepped Si(100) surface There is increasing evidence that the surface is not simply a static substrate upon which the processes of growth and reaction occur, but often actively participates in these processes through rearrangement of the structure. These rearrangements can occur as local atomic parameters [17], as large scale constructions [18] or as changes in surface monostructure in which layers of atoms are displaced to new positions on the surface [19] (step structures on Si surfaces). Therefore surfaces are far more interesting than their simplest description as infinitely periodic two-dimensional structures would suggest. With the help of LEED [20], ion scattering spectroscopy [21] and UPS [22] studies, the Si(100) surface has been shown to reconstruct to the (2×1) structure upon cleaning and heat treatment. Reconstruction of the Si(100) surface occurs in a way in which the surface can eliminate the dangling bonds and the surface atoms can rearrange themselves into a new periodic configuration so as to reduce the surface energy. A relatively simple (2×1) reconstruction of the Si(100) surface was first proposed by Schlier and Farnswarth [23]. The basic features of their model and the formation of dimers on the surface were confirmed by subsequent experimental and theoretical work. However, there is considerable experimental evidence indicating that more complicated patterns of reconstruction with greater periodicities occur on this surface. Moreover, some theoretical studies have suggested that missing atom defects [24] and the atomic step [25] may actually be part of the ground state structure. The structure of the stepped Si(100) surface has
been intensively studied recently, because of its importance in the heteroepitaxy of III–V semiconductors, particularly GaAs on Si [26,27]. Steps on clean Si(100) surfaces have been found to be either single-layer (S) or double-layer (D) atomic height. The terraces between single-layer steps belong alternately to the different face-centered cubic sublattices of the diamond lattice, having dangling bond directions rotated by 90°. The dimerizaton of the surface atoms creates a double domain surface with (1×2) and (2×1) reconstructed terraces. In accordance with the commonly adopted convention for these surfaces, steps for which the dimerization on the upper terrace is perpendicular to the step edge are labelled S or D (for singleA A or double-layer steps), while those for which the dimerization direction on the upper terrace is parallel to the step edge are labelled S or D (for B B single- or double-layer steps). These most probable types of steps appearing on Si(100) were labelled by Chadi [28]. It was found [29] that the formation of different steps appearing on the Si(100) surface (S or D type) depended on both the method of preparation of the sample and the degree of misorientation of the (001) axis towards the [110] or [11: 0] direction. The results of LEED [30–34], RHEED [35–38] and STM [39–42] on the step structure of Si(100) obtained by misorientation of the (001) axis towards [110] consistently showed that on a well-annealed surface double-layer steps were dominant for misorientation above a critical angle, whereas for smaller angles, the single-layer steps were dominant. This behavior is consistent with theoretical calculations of step formation energies, which predict a reduction in surface energy at high misorientation angles through a combination of S and S steps. Because of their A B high formation energies, it was found that D A steps are extremely unlikely [28]. In another study [29], by manipulating the step kinetics under appropriate growth conditions, it was found that, even for small misorientations, the double-layer steps are not thermodynamically stable. In that study [29], in a narrow substrate temperature range around 500 °C, almost all of the incoming Si can be incorporated into S steps on the clean B Si(100) surface. Giling et al. [43,44] explained the influence of surface reconstruction on the lateral
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orientations of growth steps appearing on {100} and {111} planes of diamond-like crystals. They observed that the growth steps on Si{100} were parallel to 110 whereas the step direction on {111} planes was [1: 1: 2]. Therefore the step parallel to 110 yielded a lower total free energy than that parallel to 100 did, and consequently the
110 step on the Si(100) surface was more stable [43,44]. By considering the results of LEED [30–34], RHEED [35–38], STM [39–42] and the calculated formation energies given in Ref. [28], one can easily conclude that the S model is the most stable B step model on Si(100). Therefore, in the present
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work, the stepped structure on Si(100) surface is taken to be an S -type step model and is given in B Fig. 1. As shown in the figure, two different periodic structures lie on both sides of the steps. In the step model considered the dimer bonds of the upper step are parallel to the step layer whereas the dimer bonds of the lower step layer are perpendicular to the step direction. The dimerization is accompanied by significant subsurface distortion extending to the fourth layer into the bulk for both sides of the step. The S -type step model on the Si(100) surface B is generated from a slab having a working cell of 417 Si atoms. The working cell consists of five
Fig. 1. (a) Side view of the S -type stepped Si(100) working cell; the dimer bonds and the dangling bonds are illustrated. (b) Top B view of S -type step on Si(100). B
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(1×2) unit cells on the left side of the step and five (2×1) unit cells on the right side of the step. The first upper and the first lower step layer Si atoms have only one dangling bond. The atoms at the step also have one dangling bond. The dangling bonds of the upper and the lower step layer Si atoms are extended outward from the surface, making a zenith angle 14.76°, and the direction of the dangling bonds in the upper layer is rotated by 90° with respect to the dangling bonds in the lower step layer. The open bonds belonging to the step atoms have the original tetrahedral direction of the diamond structure.
3. Calculations In the present work, the adsorption density of states and the total electronic energy of the molecularly and dissociatively adsorbed water-stepped Si(100) systems are calculated using the ETB method. In the first stage, the stepped Si(100) is simulated by a working cell of a slab which was defined in the previous section. In the second stage, a molecular (non-dissociative) water-stepped Si(100) system is formed by attaching the water molecules to all dangling bonds of the stepped Si(100) surface (full coverage) through the oxygen atoms. Because the water adsorption does not change the bond structure of the stepped Si(100) surface [14], the original directions of the dangling bonds on the upper and lower stepped layers and at the step are retained. The angle between the OH bonds of the water molecule remains unchanged (105°). The most plausible Si–O internuclear distance is taken from bulk SiO to ˚. be 1.55 A In ETB calculations two molecular water adsorption models are considered; in the first adsorption model, the angle between the Si–O bond and the plane of the water is 180° and in the second model this angle is 150°. Since the rotation of the water molecule around its symmetry axis does not change the adsorption density of states [11–15], these models chosen are sufficient to express adsorption states of the water (molecular) -stepped Si(100) system. In the third stage the water (dissociative)
-stepped Si(100) system OH and H species are simultaneously attached to alternate open bonds of the stepped Si(100) surface. Therefore, when an OH molecule is attached to one Si atom, another surface Si atom which is at the nearest neighbor distance from the first one will be bonded to an H atom. This alternating attachment is also used for the adsorption of OH and H to the dangling bonds at the step. For the reason mentioned above [14], the original directions of the surface dangling bonds on stepped Si(100) are unchanged. The Si–O, O–H, and Si–H bond ˚ lengths are taken to be 1.55, 0.96 and 1.48 A respectively. The ETB calculations are carried out for two dissociative adsorption models of water; in the first model the angle between the Si–O bond and O–H bonds is taken to be 180° and in the second model this angle is considered to be 150°. These two models are sufficient to study the dissociative adsorption of water on stepped Si(100), because only the variation of the angle of the OH bond with respect to the open bond of the Si can change the adsorption states. The total and local density of states ( TDOS, LDOS ) for the models considered above were calculated using the ETB method at the gamma point to determine the adsorption tendency of water to the stepped Si(100) surface atoms. The ETB calculations usually reduce the quantum mechanical treatment to the valence electrons, the electrons that are involved in bonding. The technique, which is discussed in more detail elsewhere [45], uses parametric representation of interactions to obtain approximate solutions to the electronic bonding behavior. Typically, these parameterization are based on ab initio or experimental data. In the present calculations, s and p atomic orbitals are chosen to form the orthogonal basis functions of ETB and only the first- and the second-nearest neighbor interactions are considered. The necessary interaction energy parameters between Si, O and H are taken from the previous studies of oxygen and water adsorption on clean Si surfaces [11]. In ETB calculations of both water (molecular) -stepped Si(100) systems, the Si–H interaction is not included to determine the exact contribution of the molecular states of water to the adsorption
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states. Similarly, for both water (dissociative) -stepped Si(100) systems, the interaction between the H and O which are attached to different Si atoms is not included to determine the exact contribution of OH and H states to the adsorption states. Therefore when the density of states of the chosen adsorption models (either molecular or dissociative) is compared with the experimental features, the origin of the states can be clearly explained. In the present calculations, the electronic band structure is first calculated for both water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems. The density of states calculations for the above systems are carried out after every energy band calculations and the results are plotted using a Gaussian broadening technique. In the present work, the ground state total electronic energy has also been calculated for both water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems. In the ETB method, the total electronic energy calculation was based on the charge transfer between the interaction atoms. Considering the same Si–Si, Si–O and O–H interactions as in the density of states calculations, the total electronic energy is calculated for the molecular adsorption model of water: the contribution of the charge transfer due to the Si–H interaction is included in the above interactions for the dissociative water adsorption model. The total electronic energies of water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) are found to be −23 595.5 and −23 716.8 eV respectively.
4. Results Fig. 2a displays the TDOS of the water (molecular) -stepped Si(100) system. As mentioned in the previous section, this system was studied for two tilt angles, 45.7 and 76.23°. Since the difference between the tilt angles considered is not great, the density of states for both systems was found to be the same. The only difference is that in the in TDOS of the water (molecular) -stepped Si(100) system with a tilt angle of 45.7° a peak appears at about −32.2 eV which results from the interaction
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Fig. 2. (a) TDOS of water (molecular) -stepped Si(100); the small peak at −32.2 eV is obtained when zenith angle of the H O molecule is 76.23°. (b) LDOS of H O on stepped Si(100). 2 2 (c) TDOS of stepped Si(100).
of oxygen and silicon atoms. When the TDOS of the water (molecular) -stepped Si(100) system ( Fig. 2a) is analyzed together with the LDOS of water ( Fig. 2b), the peaks at about −35, −18.4, −13 and −10.6 eV can be considered as 2a , 1 1b , 3a and 1b molecular states of water, respec2 1 1 tively. The peak at about −19 eV originates from the chemisorption bond states of oxygen and silicon atoms. The remainder of the states can be associated with the Si and Si dimer bond states, s p as they are observed in the LDOS of the stepped Si(100) system (Fig. 2c). It is found that the TDOS of the water (dissociative) -stepped Si(100) systems with two different tilt angles, 45.7° and 76.23°, is the same, for the reason given above, i.e. the difference between the
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Fig. 3a results from the p-type dimer bond interactions of surface silicon atoms.
5. Discussion and conclusions The TDOS of water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems are given again in Fig. 4a,b together with the UPS results [13] on the water-stepped Si(100) system ( Fig. 4c) for comparison. The main peaks in the TDOS of the water (molecular) -stepped Si(100) system ( Fig. 4a) are the molecular states of water (2a , 1b , 3a , 1b ). On the other hand, 1 2 1 1 the main features in the TDOS of the water
Fig. 3. (a) TDOS of water (dissociative) -stepped Si(100). (b) LDOS of OH molecule on stepped Si(100). (c) LDOS of Si –H on water (dissociative) -stepped Si(100). (d) Si –Si p s p p dimer bonds.
tilt angles is small. When the TDOS of the water (dissociative) -stepped Si(100) system (Fig. 3a) is analyzed together with molecular states of OH (Fig. 3b), it can be concluded that the sharp peak appearing at about −32.2 eV and the small peak at about −14.8 eV are the molecular states of OH molecule. The peak at about −19 eV is again associated with the chemisorption bond states of Si and O orbitals. The peak at about −10.2 eV s p is the Si–H peak which results from the interaction of silicon p states with hydrogen s states. The Si–H interaction states are given in Fig. 3c for comparison. According to LDOS calculations of dimer Si atoms on the water (dissociative) -stepped Si(100) system (Fig. 3d), the peak at −7.8 eV seen in
Fig. 4. TDOS of (a) water (molecular) -stepped Si(100), and (b) water (dissociative) -stepped Si(100). (c) Photoelectron difference spectra for saturation coverage of H O at 350 K for 2 vicinal orientation in the [110] zone [13].
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(dissociative) -stepped Si(100) system (Fig. 4b) are the molecular states of OH (2s, 3s) and the Si–H interaction peak (at about −10.2 eV ). Since the Si–H interaction is not taken into account in the ETB calculations of the molecular adsorption model of water on stepped Si(100), the Si–H feature is naturally absent in the water (molecular) -stepped Si(100) system. The UPS results obtained for the water-stepped Si(100) system by Schroder et al. [13] are in good agreement with our TDOS results obtained by the adsorption of OH and H on the stepped Si(100) surface ( Fig. 4b,c). The reproduced He (hf= 1 −21.2 eV ) UPS spectrum [13] obtained from water adsorbed on a stepped Si(10) surface revealed features at −15.5 and −10.2 eV with a shoulder at −11.2 eV with respect to the top of the valance band. The adsorbed species were identified as OH+H by vibration spectroscopy [4,46 ]; therefore the adsorption of water on stepped Si(001) was dissociative. The two main features mentioned above were also identified as OH and H adsorption states of stepped Si(110) and Si(111) surfaces [15,16 ]. As may be seen, there is a good agreement between the adsorption difference peaks (Fig. 4c) [13] of the water-stepped Si(001) system and the OH (3s) and Si–H peaks of the present water (dissociative) -stepped Si(100) systems (Fig. 4b). As has been mentioned, these peaks (3s, Si–H ) are the two main peaks that define the dissociative adsorption form of water on stepped Si(100) and do not exist in the TDOS of the water (molecular) -stepped Si(100) model. The coincidence of the UPS [13] features (at approximately −15 and −10 eV ) with the present OH (−14.8 eV ) and Si–H (−10.2 eV ) peaks strongly supports the conclusion, i.e. that the adsorption of water on stepped Si(100) is in dissociative form. On the other hand, the total electronic energies for the water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems were calculated and the results are given in Section 3. In the present total energy calculations only the electronic energy contribution was included; the core–core repulsive interactions were not considered. Therefore the calculated total energy values should not be taken into account numerically, but
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they do quantitatively demonstrate the relative variation if the total electronic energy with respect to the adsorption geometries of water on stepped Si(100) structure. It is found that the total electronic energy of the water (dissociative) -stepped Si(100) system is more negative than the energy of the water (molecular) -stepped Si(100) system. Therefore the calculated total electronic energy supports the conclusion that the water adsorption on stepped Si(100) surface is in dissociative form. Finally, in light of density of states results supported by total electronic energy calculations, we conclude that the adsorption of water on the stepped Si(100) surface is in the dissociative form.
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[24] K.C. Pandey, in: D.J. Chadi, W.A. Harrison (Eds.), Proc. 7th Int. Conf. on the Physics of Semiconductors, Springer, New York, 1985, p. 55. [25] O.L. Alerhand, D. Vanderbilt, R.D. Meade, J.D. Joanopoulos, Phys. Rev. Lett. 57 (1973) 3045. [26 ] D.E. Aspnes, J. Ihm, Phys. Rev. Lett. 57 (1986) 3054. [27] H. Kroemer, in: J.C.C. Fan ( Ed.), Heteroepitaxy on Silicon, vol. 67, Materials Research Society, Pittsburgh, PA, 1986, p. 3. [28] D.J. Chadi, Phys. Rev. Lett. 59 (1987) 1691. [29] L. Doewtitz, G.E. Crook, K. Ploog, Phys. Rev. B 44 (1991) 3083. [30] M. Henzler, J. Clabes, Jpn. J. Appl. Phys. Suppl. 2 (1987) 389. [31] B.Z. Olshanetsky, A.A. Shklyaev, Surf. Sci. 82 (1979) 445. [32] R. Kaplan, Surf. Sci. 93 (1980) 145. [33] R.D. Bringans, R.I.G. Urberg, M.A. Olmstead, R.Z. Bachrach, Phys. Rev. B 34 (1986) 7447. [34] C.E. Aumann, D.E. Savage, R. Kariotis, M.G. Lagally, J. Vac. Sci. Technol. 6 (1988) 1963.
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Adsorption of water on stepped Si(100) surface Saed A. Salman, S¸enay Katırcıog˘lu * Department of Physics, Middle East Technical University, 06531 Ankara, Turkey Received 24 October 1997; accepted for publication 12 May 1998
Abstract We have investigated possible water forms on stepped Si(100) surface. Calculations are performed by using the empirical tightbinding method. Two types of adsorption model of water on stepped Si(100) surface have been considered; one of them is the dissociative type (H, OH ) and the other is the molecular type (H O). The results of the density of states supported by total electronic 2 energy calculations indicate a dissociative type of water adsorption on the stepped Si(100) surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Adsorption kinetics; Semi-empirical models and model calculations; Silicon; Single crystal surfaces; Surface roughening; Water
1. Introduction In the past, the adsorption of water in either dissociative or molecular form on flat silicon surfaces has been investigated intensively [1–12] because of the important role of the interaction of water with silicon surfaces in semiconductor device technology (wet oxidation and cleaning). In recent years, owing to the existence of some step structures on Si surfaces, as observed in experiments especially in scanning tunnelling microscopy (STM ) images, water-stepped Si systems have been investigated to understand the form of adsorption of water [13–16 ]. In a recent study, Bergen and Ranke [13] investigated the adsorption kinetics on vicinal Si(100) surfaces using UV photoemission spectroscopy ( UPS); the adsorption of water was investigated for the steps * Corresponding author. Fax: +90-312-2101281; e-mail: [email protected]
along [11: 0] and [100] directions. They concluded that, during the water adsorption process, the abrupt saturation of the dangling bonds of the Si(100) surface occurs at half-monolayer coverage and the adsorbate dissociates into OH+H. In the same study [13], the initial sticking probabilities on Si(100) and on the stepped vicinal surfaces in [11: 0] and the [100] zone were found to be equal. In a second experimental study [14], the room temperature adsorption of water was studied on vicinal Si(100) (2×1) surfaces titled toward [01: 1]. Using STM, it was observed that water adsorbs in small islands via a molecular precursor state. According to the STM images [14], the water islands grow on terraces leaving edge dimers unsaturated at first; finally the edge dimers become saturated. In recent theoretical studies, the adsorption of water was investigated for stepped Si(110) [15] and stepped Si(111) [16 ] surfaces and the adsorption of the water on these surfaces was found to be dissociative.
0039-6028/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 04 4 0 -3
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In the present work, to eliminate the disagreement between the two experimental results mentioned above [13,14], the water-stepped Si(100) systems were studied for dissociative and molecular forms by the empirical tight-binding ( ETB) method. The ETB results on water-stepped Si(100) systems will also indicate any parallelism between the water-flat Si(100) and the other waterstepped Si(110) and Si(111) surfaces, if it exists.
2. Stepped Si(100) surface There is increasing evidence that the surface is not simply a static substrate upon which the processes of growth and reaction occur, but often actively participates in these processes through rearrangement of the structure. These rearrangements can occur as local atomic parameters [17], as large scale constructions [18] or as changes in surface monostructure in which layers of atoms are displaced to new positions on the surface [19] (step structures on Si surfaces). Therefore surfaces are far more interesting than their simplest description as infinitely periodic two-dimensional structures would suggest. With the help of LEED [20], ion scattering spectroscopy [21] and UPS [22] studies, the Si(100) surface has been shown to reconstruct to the (2×1) structure upon cleaning and heat treatment. Reconstruction of the Si(100) surface occurs in a way in which the surface can eliminate the dangling bonds and the surface atoms can rearrange themselves into a new periodic configuration so as to reduce the surface energy. A relatively simple (2×1) reconstruction of the Si(100) surface was first proposed by Schlier and Farnswarth [23]. The basic features of their model and the formation of dimers on the surface were confirmed by subsequent experimental and theoretical work. However, there is considerable experimental evidence indicating that more complicated patterns of reconstruction with greater periodicities occur on this surface. Moreover, some theoretical studies have suggested that missing atom defects [24] and the atomic step [25] may actually be part of the ground state structure. The structure of the stepped Si(100) surface has
been intensively studied recently, because of its importance in the heteroepitaxy of III–V semiconductors, particularly GaAs on Si [26,27]. Steps on clean Si(100) surfaces have been found to be either single-layer (S) or double-layer (D) atomic height. The terraces between single-layer steps belong alternately to the different face-centered cubic sublattices of the diamond lattice, having dangling bond directions rotated by 90°. The dimerizaton of the surface atoms creates a double domain surface with (1×2) and (2×1) reconstructed terraces. In accordance with the commonly adopted convention for these surfaces, steps for which the dimerization on the upper terrace is perpendicular to the step edge are labelled S or D (for singleA A or double-layer steps), while those for which the dimerization direction on the upper terrace is parallel to the step edge are labelled S or D (for B B single- or double-layer steps). These most probable types of steps appearing on Si(100) were labelled by Chadi [28]. It was found [29] that the formation of different steps appearing on the Si(100) surface (S or D type) depended on both the method of preparation of the sample and the degree of misorientation of the (001) axis towards the [110] or [11: 0] direction. The results of LEED [30–34], RHEED [35–38] and STM [39–42] on the step structure of Si(100) obtained by misorientation of the (001) axis towards [110] consistently showed that on a well-annealed surface double-layer steps were dominant for misorientation above a critical angle, whereas for smaller angles, the single-layer steps were dominant. This behavior is consistent with theoretical calculations of step formation energies, which predict a reduction in surface energy at high misorientation angles through a combination of S and S steps. Because of their A B high formation energies, it was found that D A steps are extremely unlikely [28]. In another study [29], by manipulating the step kinetics under appropriate growth conditions, it was found that, even for small misorientations, the double-layer steps are not thermodynamically stable. In that study [29], in a narrow substrate temperature range around 500 °C, almost all of the incoming Si can be incorporated into S steps on the clean B Si(100) surface. Giling et al. [43,44] explained the influence of surface reconstruction on the lateral
S.A. Salman, S. Katırcıog˘lu / Surface Science 416 (1998) 9–16
orientations of growth steps appearing on {100} and {111} planes of diamond-like crystals. They observed that the growth steps on Si{100} were parallel to 110 whereas the step direction on {111} planes was [1: 1: 2]. Therefore the step parallel to 110 yielded a lower total free energy than that parallel to 100 did, and consequently the
110 step on the Si(100) surface was more stable [43,44]. By considering the results of LEED [30–34], RHEED [35–38], STM [39–42] and the calculated formation energies given in Ref. [28], one can easily conclude that the S model is the most stable B step model on Si(100). Therefore, in the present
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work, the stepped structure on Si(100) surface is taken to be an S -type step model and is given in B Fig. 1. As shown in the figure, two different periodic structures lie on both sides of the steps. In the step model considered the dimer bonds of the upper step are parallel to the step layer whereas the dimer bonds of the lower step layer are perpendicular to the step direction. The dimerization is accompanied by significant subsurface distortion extending to the fourth layer into the bulk for both sides of the step. The S -type step model on the Si(100) surface B is generated from a slab having a working cell of 417 Si atoms. The working cell consists of five
Fig. 1. (a) Side view of the S -type stepped Si(100) working cell; the dimer bonds and the dangling bonds are illustrated. (b) Top B view of S -type step on Si(100). B
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(1×2) unit cells on the left side of the step and five (2×1) unit cells on the right side of the step. The first upper and the first lower step layer Si atoms have only one dangling bond. The atoms at the step also have one dangling bond. The dangling bonds of the upper and the lower step layer Si atoms are extended outward from the surface, making a zenith angle 14.76°, and the direction of the dangling bonds in the upper layer is rotated by 90° with respect to the dangling bonds in the lower step layer. The open bonds belonging to the step atoms have the original tetrahedral direction of the diamond structure.
3. Calculations In the present work, the adsorption density of states and the total electronic energy of the molecularly and dissociatively adsorbed water-stepped Si(100) systems are calculated using the ETB method. In the first stage, the stepped Si(100) is simulated by a working cell of a slab which was defined in the previous section. In the second stage, a molecular (non-dissociative) water-stepped Si(100) system is formed by attaching the water molecules to all dangling bonds of the stepped Si(100) surface (full coverage) through the oxygen atoms. Because the water adsorption does not change the bond structure of the stepped Si(100) surface [14], the original directions of the dangling bonds on the upper and lower stepped layers and at the step are retained. The angle between the OH bonds of the water molecule remains unchanged (105°). The most plausible Si–O internuclear distance is taken from bulk SiO to ˚. be 1.55 A In ETB calculations two molecular water adsorption models are considered; in the first adsorption model, the angle between the Si–O bond and the plane of the water is 180° and in the second model this angle is 150°. Since the rotation of the water molecule around its symmetry axis does not change the adsorption density of states [11–15], these models chosen are sufficient to express adsorption states of the water (molecular) -stepped Si(100) system. In the third stage the water (dissociative)
-stepped Si(100) system OH and H species are simultaneously attached to alternate open bonds of the stepped Si(100) surface. Therefore, when an OH molecule is attached to one Si atom, another surface Si atom which is at the nearest neighbor distance from the first one will be bonded to an H atom. This alternating attachment is also used for the adsorption of OH and H to the dangling bonds at the step. For the reason mentioned above [14], the original directions of the surface dangling bonds on stepped Si(100) are unchanged. The Si–O, O–H, and Si–H bond ˚ lengths are taken to be 1.55, 0.96 and 1.48 A respectively. The ETB calculations are carried out for two dissociative adsorption models of water; in the first model the angle between the Si–O bond and O–H bonds is taken to be 180° and in the second model this angle is considered to be 150°. These two models are sufficient to study the dissociative adsorption of water on stepped Si(100), because only the variation of the angle of the OH bond with respect to the open bond of the Si can change the adsorption states. The total and local density of states ( TDOS, LDOS ) for the models considered above were calculated using the ETB method at the gamma point to determine the adsorption tendency of water to the stepped Si(100) surface atoms. The ETB calculations usually reduce the quantum mechanical treatment to the valence electrons, the electrons that are involved in bonding. The technique, which is discussed in more detail elsewhere [45], uses parametric representation of interactions to obtain approximate solutions to the electronic bonding behavior. Typically, these parameterization are based on ab initio or experimental data. In the present calculations, s and p atomic orbitals are chosen to form the orthogonal basis functions of ETB and only the first- and the second-nearest neighbor interactions are considered. The necessary interaction energy parameters between Si, O and H are taken from the previous studies of oxygen and water adsorption on clean Si surfaces [11]. In ETB calculations of both water (molecular) -stepped Si(100) systems, the Si–H interaction is not included to determine the exact contribution of the molecular states of water to the adsorption
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states. Similarly, for both water (dissociative) -stepped Si(100) systems, the interaction between the H and O which are attached to different Si atoms is not included to determine the exact contribution of OH and H states to the adsorption states. Therefore when the density of states of the chosen adsorption models (either molecular or dissociative) is compared with the experimental features, the origin of the states can be clearly explained. In the present calculations, the electronic band structure is first calculated for both water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems. The density of states calculations for the above systems are carried out after every energy band calculations and the results are plotted using a Gaussian broadening technique. In the present work, the ground state total electronic energy has also been calculated for both water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems. In the ETB method, the total electronic energy calculation was based on the charge transfer between the interaction atoms. Considering the same Si–Si, Si–O and O–H interactions as in the density of states calculations, the total electronic energy is calculated for the molecular adsorption model of water: the contribution of the charge transfer due to the Si–H interaction is included in the above interactions for the dissociative water adsorption model. The total electronic energies of water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) are found to be −23 595.5 and −23 716.8 eV respectively.
4. Results Fig. 2a displays the TDOS of the water (molecular) -stepped Si(100) system. As mentioned in the previous section, this system was studied for two tilt angles, 45.7 and 76.23°. Since the difference between the tilt angles considered is not great, the density of states for both systems was found to be the same. The only difference is that in the in TDOS of the water (molecular) -stepped Si(100) system with a tilt angle of 45.7° a peak appears at about −32.2 eV which results from the interaction
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Fig. 2. (a) TDOS of water (molecular) -stepped Si(100); the small peak at −32.2 eV is obtained when zenith angle of the H O molecule is 76.23°. (b) LDOS of H O on stepped Si(100). 2 2 (c) TDOS of stepped Si(100).
of oxygen and silicon atoms. When the TDOS of the water (molecular) -stepped Si(100) system ( Fig. 2a) is analyzed together with the LDOS of water ( Fig. 2b), the peaks at about −35, −18.4, −13 and −10.6 eV can be considered as 2a , 1 1b , 3a and 1b molecular states of water, respec2 1 1 tively. The peak at about −19 eV originates from the chemisorption bond states of oxygen and silicon atoms. The remainder of the states can be associated with the Si and Si dimer bond states, s p as they are observed in the LDOS of the stepped Si(100) system (Fig. 2c). It is found that the TDOS of the water (dissociative) -stepped Si(100) systems with two different tilt angles, 45.7° and 76.23°, is the same, for the reason given above, i.e. the difference between the
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Fig. 3a results from the p-type dimer bond interactions of surface silicon atoms.
5. Discussion and conclusions The TDOS of water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems are given again in Fig. 4a,b together with the UPS results [13] on the water-stepped Si(100) system ( Fig. 4c) for comparison. The main peaks in the TDOS of the water (molecular) -stepped Si(100) system ( Fig. 4a) are the molecular states of water (2a , 1b , 3a , 1b ). On the other hand, 1 2 1 1 the main features in the TDOS of the water
Fig. 3. (a) TDOS of water (dissociative) -stepped Si(100). (b) LDOS of OH molecule on stepped Si(100). (c) LDOS of Si –H on water (dissociative) -stepped Si(100). (d) Si –Si p s p p dimer bonds.
tilt angles is small. When the TDOS of the water (dissociative) -stepped Si(100) system (Fig. 3a) is analyzed together with molecular states of OH (Fig. 3b), it can be concluded that the sharp peak appearing at about −32.2 eV and the small peak at about −14.8 eV are the molecular states of OH molecule. The peak at about −19 eV is again associated with the chemisorption bond states of Si and O orbitals. The peak at about −10.2 eV s p is the Si–H peak which results from the interaction of silicon p states with hydrogen s states. The Si–H interaction states are given in Fig. 3c for comparison. According to LDOS calculations of dimer Si atoms on the water (dissociative) -stepped Si(100) system (Fig. 3d), the peak at −7.8 eV seen in
Fig. 4. TDOS of (a) water (molecular) -stepped Si(100), and (b) water (dissociative) -stepped Si(100). (c) Photoelectron difference spectra for saturation coverage of H O at 350 K for 2 vicinal orientation in the [110] zone [13].
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(dissociative) -stepped Si(100) system (Fig. 4b) are the molecular states of OH (2s, 3s) and the Si–H interaction peak (at about −10.2 eV ). Since the Si–H interaction is not taken into account in the ETB calculations of the molecular adsorption model of water on stepped Si(100), the Si–H feature is naturally absent in the water (molecular) -stepped Si(100) system. The UPS results obtained for the water-stepped Si(100) system by Schroder et al. [13] are in good agreement with our TDOS results obtained by the adsorption of OH and H on the stepped Si(100) surface ( Fig. 4b,c). The reproduced He (hf= 1 −21.2 eV ) UPS spectrum [13] obtained from water adsorbed on a stepped Si(10) surface revealed features at −15.5 and −10.2 eV with a shoulder at −11.2 eV with respect to the top of the valance band. The adsorbed species were identified as OH+H by vibration spectroscopy [4,46 ]; therefore the adsorption of water on stepped Si(001) was dissociative. The two main features mentioned above were also identified as OH and H adsorption states of stepped Si(110) and Si(111) surfaces [15,16 ]. As may be seen, there is a good agreement between the adsorption difference peaks (Fig. 4c) [13] of the water-stepped Si(001) system and the OH (3s) and Si–H peaks of the present water (dissociative) -stepped Si(100) systems (Fig. 4b). As has been mentioned, these peaks (3s, Si–H ) are the two main peaks that define the dissociative adsorption form of water on stepped Si(100) and do not exist in the TDOS of the water (molecular) -stepped Si(100) model. The coincidence of the UPS [13] features (at approximately −15 and −10 eV ) with the present OH (−14.8 eV ) and Si–H (−10.2 eV ) peaks strongly supports the conclusion, i.e. that the adsorption of water on stepped Si(100) is in dissociative form. On the other hand, the total electronic energies for the water (molecular) -stepped Si(100) and water (dissociative) -stepped Si(100) systems were calculated and the results are given in Section 3. In the present total energy calculations only the electronic energy contribution was included; the core–core repulsive interactions were not considered. Therefore the calculated total energy values should not be taken into account numerically, but
15
they do quantitatively demonstrate the relative variation if the total electronic energy with respect to the adsorption geometries of water on stepped Si(100) structure. It is found that the total electronic energy of the water (dissociative) -stepped Si(100) system is more negative than the energy of the water (molecular) -stepped Si(100) system. Therefore the calculated total electronic energy supports the conclusion that the water adsorption on stepped Si(100) surface is in dissociative form. Finally, in light of density of states results supported by total electronic energy calculations, we conclude that the adsorption of water on the stepped Si(100) surface is in the dissociative form.
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