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Ab-initio study of the adsorption of H2S onto the Si(001) surface
Surface Science 433–435 (1999) 420–424 www.elsevier.nl/locate/susc
Ab-initio study of the adsorption of H S onto the 2 Si(001) surface M. C ¸ akmak *, G.P. Srivastava School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
Abstract The adsorption of H S on to the Si(001)-(1×2) surface is studied, based upon ab-initio pseudopotential 2 calculations. We have used a dissociative adsorption model with adsorbed species (SH )− and H+. The sulphur atom is located along the dangling bond direction on one component of the Si dimer, the H atom in the (SH )− complex is bonded to the S atom, and H+ is bonded along the dangling bond direction of the other dimer component. The adsorption of the molecule removes the buckling of the Si dimer. Our calculated bond lengths for Si–S, Si–H and S– ˚ , which are very close to the sum of their corresponding covalent radii. We find that the H are 2.15, 1.53 and 1.41 A adsorption of H S passivates the Si(001) surface. The system is characterized by the presence of a single surface state 2 just below the bulk valence band maximum. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Density functional theory; Electronic states; Molecular adsorption; Pseudopotential; Si surface
1. Introduction The adsorption of certain molecules on semiconductor surfaces is of considerable importance due to their passivating nature. Little work has been carried out on the investigation of H S 2 adsorption on semiconductors. The adsorption of H S on III–V(110) surfaces is believed to be in a 2 dissociative form, with the presence of SH and H species [1,2]. Similarly, Schro¨der-Bergen and Ranke [3] have investigated the adsorption of H S on flat and stepped Si(001) surfaces by photo2 electron spectroscopy ( UPS). From a study of the photoelectron difference spectra, i.e. difference of spectra obtained from the covered and clean surfaces, these authors have concluded that at 150 K, * Corresponding author. Fax: +44 1392-264-111. E-mail address: [email protected] (M. C ¸ akmak)
the adsorption of the H S gas shows an abrupt 2 saturation at half a monolayer coverage. At saturation, three sharp peaks appear in the difference spectra, which are attributed to the formation of the SH species. Thus, the adsorbate is believed to be dissociated into SH and H. However, there are no reports of detailed geometry or electronic structure of this system. In the present work, we report on a detailed ab-initio theoretical investigation of the atomic geometry, electronic states, and bonding for the dissociative adsorption of H S on 2 Si(001).
2. Method We consider an artificially constructed periodic geometry along the surface normal. The unit cell included an atomic slab with eight layers of Si
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 01 2 4 -7
M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
substrates plus a vacuum region equivalent to about six substrate layers in thickness. The back surface was saturated with two pseudo-hydrogen atoms per Si atom. The two Si back layers were frozen into their bulk positions, and all the remaining atoms in the unit cell were allowed to relax into their minimum energy positions using a conjugate gradient method [4]. In our calculation, the electron–ion interaction was considered in the form of ab-initio normconserving pseudopotentials listed by Bachelet et al. [5]. The electron–electron interaction was considered within the local density approximation of the density functional theory, using the correlation scheme of Ceperley and Alder [6 ] as parametrized by Perdew and Zunger [7]. Single-particle wave functions were expanded using a plane wave basis up to a kinetic energy cut-off of 8 Ry. Selfconsistent solutions to the Kohn–Sham equations were obtained by employing a set of four special k-points in the irreducible segment of the surface Brillouin zone [8]. This cut-off was found to be adequate for the structural studies and the electronic structure.
3. Results and discussion We used our theoretical equilibrium lattice con˚ ) in the surface calculastant for bulk Si (5.42 A tions. Geometry optimization is achieved by minimizing the total energy with the help of Hellmann–Feynmann forces. The resulting atomic geometries and band structures will be described and compared with existing experimental results as follows: It is well known that at room temperature, the clean Si(001) surface exhibits a 2×1 reconstruction with the formation of tilted Si dimers. The ˚ , and the bond length is approximately 2.25 A dimer tilt angle is approximately 16° [9]. The higher-lying dimer component is fully occupied with the associated electronic state close to the bulk valence band maximum, and the lower-lying dimer component is empty corresponding to the energy state in the upper half of the bulk silicon band gap [10]. In analogy with the adsorption of H S on III– 2
421
V(110) surfaces, the (SH )− species is attached to the lower-lying Si dimer component (whose dangling bond is empty), and the H+ ion is attached to the upper-lying Si dimer component (whose dangling bond is full ) on the clean Si(001)-(1×2) surface. As regards the orientation of the H atom in the (SH )− complex, we tried two models with all of H, S and Si being in the (001) plane: (i) S– H bond parallel to the Si–H bond and (ii) S–H bond in a direction opposite to that in (i) with respect to the (001) direction. Our energy minimization procedure suggests that the S atom would stay in the direction of dangling-bond direction of the lower-lying dimer atom, and the H atom in the (SH )− complex would stay along the direction given in (i) above and shown in Fig. 1a. The H+ ion is attached along the dangling bond direction of the upper-lying dimer component. Side and top views of this geometry are shown in Fig. 1. We find that upon adsorption of H S, the 2 Si-dimer becomes almost symmetric (with a small tilt angle approximately of 2°), and its bond length ˚ . The calculated Si–H bond is expanded to 2.41 A ˚ , close to the sum of their correlength is 1.53 A ˚ ) [11]. This sponding covalent radii (r =1.47 A Si–H ˚ calcubond length is similar to the value 1.53 A lated by Nardelli et al. [12] using both with pseudopotential and LMTO methods on the hydrogen-covered Si(111) surfaces. Our calculated ˚ , somewhat bigger than S–H bond length is 1.41 A the sum of their corresponding covalent radii ˚ ). It is interesting to note that the S– (r =1.34 A S–H ˚ for H bond length has been calculated as 1.38 A ˚ for H S–GaAs(110) and H S–InP(110), 1.40 A 2 ˚ 2 1.41 A for H S–GaP(110) [2]. It means that the 2 ˚ S–H bond length varies between 1.38 and 1.41 A upon adsorption of H S on different semi2 conductors. We have identified five occupied electronic states (S –S ) as shown in Fig. 2. The states S , 1 5 2 S and S can be compared with the three peaks 3 4 in the photoelectron difference spectra at −7.3, −5.0 and −3.9 eV obtained by Schro¨der-Bergen and Ranke [3]. The experiment, however, does not indicate any peak corresponding to the calculated state S . It should be remembered that the 5 three experimental peaks indicate peaks in the density of states rather than the energy location
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M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
Fig. 1. Schematic (a) side view and (b) top view for H S adsorption on Si(001). 2
Fig. 2. Electronic band structure of the H S–Si(001)-(1× 2) 2 interface. The projected bulk spectrum is shown by hatched regions. The calculated electronic states are shown by thick curves.
of a particular surface state at a particular point on the surface Brillouin zone. It would be useful to compare the calculated dispersion of the states with angle-resolved photoelectron spectra. The total valence charge density and the orbital : point are shown in nature of these states at the K Fig. 3. Examining the total charge density plot in Fig. 3a, it can be seen that the Si–Si dimer is purely covalent in character as expected, whereas Si–S is partly covalent and partly ionic. The Si–H
and S–H bonds are largely ionic in character with some degree of covalency. The Si–H bonding in this work is similar to that calculated for the SiH system [13]. 4 The calculated surface state S is due to the 1 s-bonding between S and H. Comparing with our earlier work, we can say that upon H S adsorption 2 on Si and III–V semiconductors, the (SH )− line appears in the binding energy 12.0–12.8 eV with the same orbital character [2]. The states S , S , 2 3 S are mostly related to orbitals localized between 4 the molecular species and the top-layer substrate. The state S is due to the H s-orbital in the 2 (SH )− complex and has a pronounced contribution from the dimer atoms with a s-antibonding. The state S is mostly due to the H s-orbital of 3 the (SH )− complex and has a small s-bonding contribution from the dimer back bond with the substrate. The surface state S has a complex 4 bonding pattern, with contribution from Si, H and the (SH )− complex. Another complex bonding picture, involving the Si–Si dimer s bond orbital and the (SH )− complex, gives rise to the localized : just below the valence band state S at K 5 maximum.
4. Conclusions From ab-initio density functional calculations, we have presented a detailed investigation of the atomic geometry, electronic states and bonding at the H S adsorbed Si(001)-(2×1) surface, using 2
M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
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Fig. 3. (a) Electronic total charge density and (b–f ) electronic charge density plots of the individual states for the : . The electron charge density is normalized to the number of electrons in the supercell (i.e. 76 H S–Si(001)-(1×2) interface at K 2 electrons per unit cell for the total charge density plot and two electrons per unit cell for the individual electronic states).
the partially dissociative adsorption model containing the species (SH )− and H+. The Si–Si dimer ˚ and its tilt is reduced to is expanded to 2.41 A approximately 2°. The calculated bond lengths Si– ˚ , respecS, Si–H and S–H of 2.15, 1.53 and 1.41 A tively, are very close to the sum of their corre-
sponding covalent radii. The Si–S bond is partly covalent and partly ionic. The bonds Si–H and S– H are largely ionic with some degree of covalency. The H S adsorption has passivated the dangling 2 bonds on the clean surface and removed the surface states from the fundamental band gap.
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M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
Acknowledgement M. C ¸ akmak is grateful to the University of Gazi, Arts and Science Faculty, Turkey, for research leave and financial support.
References [1] E. Dudzik, A. Leslie, E. O’Toole, I.T. McGovern, A. Patchett, D.R.T. Zahn, J. Ludecke, D.P. Woodruff, B.C.C. Cowie, J. Phys.: Condens. Matter 8 (1996) 15. [2] M. C ¸ akmak, G.P. Srivastava, Phys. Rev. B 57 (1998) 4486. [3] E. Schro¨der-Bergen, W. Ranke, Surf. Sci. 236 (1990) 103.
[4] A. Umerski, G.P. Srivastava, Phys. Rev. B 51 (1995) 2334. [5] G.B. Bachelet, D.R. Hamann, M. Schlu¨ter, Phys. Rev. B 26 (1982) 4199. [6 ] D.M. Ceperley, B.I. Alder, Phys. Rev. Lett. 45 (1980) 566. [7] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048. [8] R.A. Evarestov, V.P. Smirnov, Phys. Stat. Sol. B 119 (1983) 9. [9] S.J. Jenkins, G.P. Srivastava, J. Phys. Condens. Matter 8 (1996) 6641. [10] H.M. Tu¨tu¨ncu¨, S.J. Jenkins, G.P. Srivastava, Phys. Rev. B 56 (1997) 4056. [11] G. Burns, Solid State Physics, International Edition, Academic Press, London, 1990. [12] M.B. Nardelli, F. Finocchi, M. Palummo, R. Difelice, C.M. Bertoni, F. Bernardini, S. Ossicini, Surf. Sci. 269/270 (1992) 879. [13] M. C ¸ akmak, G.P. Srivastava, unpublished.
Ab-initio study of the adsorption of H S onto the 2 Si(001) surface M. C ¸ akmak *, G.P. Srivastava School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
Abstract The adsorption of H S on to the Si(001)-(1×2) surface is studied, based upon ab-initio pseudopotential 2 calculations. We have used a dissociative adsorption model with adsorbed species (SH )− and H+. The sulphur atom is located along the dangling bond direction on one component of the Si dimer, the H atom in the (SH )− complex is bonded to the S atom, and H+ is bonded along the dangling bond direction of the other dimer component. The adsorption of the molecule removes the buckling of the Si dimer. Our calculated bond lengths for Si–S, Si–H and S– ˚ , which are very close to the sum of their corresponding covalent radii. We find that the H are 2.15, 1.53 and 1.41 A adsorption of H S passivates the Si(001) surface. The system is characterized by the presence of a single surface state 2 just below the bulk valence band maximum. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Density functional theory; Electronic states; Molecular adsorption; Pseudopotential; Si surface
1. Introduction The adsorption of certain molecules on semiconductor surfaces is of considerable importance due to their passivating nature. Little work has been carried out on the investigation of H S 2 adsorption on semiconductors. The adsorption of H S on III–V(110) surfaces is believed to be in a 2 dissociative form, with the presence of SH and H species [1,2]. Similarly, Schro¨der-Bergen and Ranke [3] have investigated the adsorption of H S on flat and stepped Si(001) surfaces by photo2 electron spectroscopy ( UPS). From a study of the photoelectron difference spectra, i.e. difference of spectra obtained from the covered and clean surfaces, these authors have concluded that at 150 K, * Corresponding author. Fax: +44 1392-264-111. E-mail address: [email protected] (M. C ¸ akmak)
the adsorption of the H S gas shows an abrupt 2 saturation at half a monolayer coverage. At saturation, three sharp peaks appear in the difference spectra, which are attributed to the formation of the SH species. Thus, the adsorbate is believed to be dissociated into SH and H. However, there are no reports of detailed geometry or electronic structure of this system. In the present work, we report on a detailed ab-initio theoretical investigation of the atomic geometry, electronic states, and bonding for the dissociative adsorption of H S on 2 Si(001).
2. Method We consider an artificially constructed periodic geometry along the surface normal. The unit cell included an atomic slab with eight layers of Si
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 01 2 4 -7
M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
substrates plus a vacuum region equivalent to about six substrate layers in thickness. The back surface was saturated with two pseudo-hydrogen atoms per Si atom. The two Si back layers were frozen into their bulk positions, and all the remaining atoms in the unit cell were allowed to relax into their minimum energy positions using a conjugate gradient method [4]. In our calculation, the electron–ion interaction was considered in the form of ab-initio normconserving pseudopotentials listed by Bachelet et al. [5]. The electron–electron interaction was considered within the local density approximation of the density functional theory, using the correlation scheme of Ceperley and Alder [6 ] as parametrized by Perdew and Zunger [7]. Single-particle wave functions were expanded using a plane wave basis up to a kinetic energy cut-off of 8 Ry. Selfconsistent solutions to the Kohn–Sham equations were obtained by employing a set of four special k-points in the irreducible segment of the surface Brillouin zone [8]. This cut-off was found to be adequate for the structural studies and the electronic structure.
3. Results and discussion We used our theoretical equilibrium lattice con˚ ) in the surface calculastant for bulk Si (5.42 A tions. Geometry optimization is achieved by minimizing the total energy with the help of Hellmann–Feynmann forces. The resulting atomic geometries and band structures will be described and compared with existing experimental results as follows: It is well known that at room temperature, the clean Si(001) surface exhibits a 2×1 reconstruction with the formation of tilted Si dimers. The ˚ , and the bond length is approximately 2.25 A dimer tilt angle is approximately 16° [9]. The higher-lying dimer component is fully occupied with the associated electronic state close to the bulk valence band maximum, and the lower-lying dimer component is empty corresponding to the energy state in the upper half of the bulk silicon band gap [10]. In analogy with the adsorption of H S on III– 2
421
V(110) surfaces, the (SH )− species is attached to the lower-lying Si dimer component (whose dangling bond is empty), and the H+ ion is attached to the upper-lying Si dimer component (whose dangling bond is full ) on the clean Si(001)-(1×2) surface. As regards the orientation of the H atom in the (SH )− complex, we tried two models with all of H, S and Si being in the (001) plane: (i) S– H bond parallel to the Si–H bond and (ii) S–H bond in a direction opposite to that in (i) with respect to the (001) direction. Our energy minimization procedure suggests that the S atom would stay in the direction of dangling-bond direction of the lower-lying dimer atom, and the H atom in the (SH )− complex would stay along the direction given in (i) above and shown in Fig. 1a. The H+ ion is attached along the dangling bond direction of the upper-lying dimer component. Side and top views of this geometry are shown in Fig. 1. We find that upon adsorption of H S, the 2 Si-dimer becomes almost symmetric (with a small tilt angle approximately of 2°), and its bond length ˚ . The calculated Si–H bond is expanded to 2.41 A ˚ , close to the sum of their correlength is 1.53 A ˚ ) [11]. This sponding covalent radii (r =1.47 A Si–H ˚ calcubond length is similar to the value 1.53 A lated by Nardelli et al. [12] using both with pseudopotential and LMTO methods on the hydrogen-covered Si(111) surfaces. Our calculated ˚ , somewhat bigger than S–H bond length is 1.41 A the sum of their corresponding covalent radii ˚ ). It is interesting to note that the S– (r =1.34 A S–H ˚ for H bond length has been calculated as 1.38 A ˚ for H S–GaAs(110) and H S–InP(110), 1.40 A 2 ˚ 2 1.41 A for H S–GaP(110) [2]. It means that the 2 ˚ S–H bond length varies between 1.38 and 1.41 A upon adsorption of H S on different semi2 conductors. We have identified five occupied electronic states (S –S ) as shown in Fig. 2. The states S , 1 5 2 S and S can be compared with the three peaks 3 4 in the photoelectron difference spectra at −7.3, −5.0 and −3.9 eV obtained by Schro¨der-Bergen and Ranke [3]. The experiment, however, does not indicate any peak corresponding to the calculated state S . It should be remembered that the 5 three experimental peaks indicate peaks in the density of states rather than the energy location
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M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
Fig. 1. Schematic (a) side view and (b) top view for H S adsorption on Si(001). 2
Fig. 2. Electronic band structure of the H S–Si(001)-(1× 2) 2 interface. The projected bulk spectrum is shown by hatched regions. The calculated electronic states are shown by thick curves.
of a particular surface state at a particular point on the surface Brillouin zone. It would be useful to compare the calculated dispersion of the states with angle-resolved photoelectron spectra. The total valence charge density and the orbital : point are shown in nature of these states at the K Fig. 3. Examining the total charge density plot in Fig. 3a, it can be seen that the Si–Si dimer is purely covalent in character as expected, whereas Si–S is partly covalent and partly ionic. The Si–H
and S–H bonds are largely ionic in character with some degree of covalency. The Si–H bonding in this work is similar to that calculated for the SiH system [13]. 4 The calculated surface state S is due to the 1 s-bonding between S and H. Comparing with our earlier work, we can say that upon H S adsorption 2 on Si and III–V semiconductors, the (SH )− line appears in the binding energy 12.0–12.8 eV with the same orbital character [2]. The states S , S , 2 3 S are mostly related to orbitals localized between 4 the molecular species and the top-layer substrate. The state S is due to the H s-orbital in the 2 (SH )− complex and has a pronounced contribution from the dimer atoms with a s-antibonding. The state S is mostly due to the H s-orbital of 3 the (SH )− complex and has a small s-bonding contribution from the dimer back bond with the substrate. The surface state S has a complex 4 bonding pattern, with contribution from Si, H and the (SH )− complex. Another complex bonding picture, involving the Si–Si dimer s bond orbital and the (SH )− complex, gives rise to the localized : just below the valence band state S at K 5 maximum.
4. Conclusions From ab-initio density functional calculations, we have presented a detailed investigation of the atomic geometry, electronic states and bonding at the H S adsorbed Si(001)-(2×1) surface, using 2
M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
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Fig. 3. (a) Electronic total charge density and (b–f ) electronic charge density plots of the individual states for the : . The electron charge density is normalized to the number of electrons in the supercell (i.e. 76 H S–Si(001)-(1×2) interface at K 2 electrons per unit cell for the total charge density plot and two electrons per unit cell for the individual electronic states).
the partially dissociative adsorption model containing the species (SH )− and H+. The Si–Si dimer ˚ and its tilt is reduced to is expanded to 2.41 A approximately 2°. The calculated bond lengths Si– ˚ , respecS, Si–H and S–H of 2.15, 1.53 and 1.41 A tively, are very close to the sum of their corre-
sponding covalent radii. The Si–S bond is partly covalent and partly ionic. The bonds Si–H and S– H are largely ionic with some degree of covalency. The H S adsorption has passivated the dangling 2 bonds on the clean surface and removed the surface states from the fundamental band gap.
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M. C ¸ akmak, G.P. Srivastava / Surface Science 433–435 (1999) 420–424
Acknowledgement M. C ¸ akmak is grateful to the University of Gazi, Arts and Science Faculty, Turkey, for research leave and financial support.
References [1] E. Dudzik, A. Leslie, E. O’Toole, I.T. McGovern, A. Patchett, D.R.T. Zahn, J. Ludecke, D.P. Woodruff, B.C.C. Cowie, J. Phys.: Condens. Matter 8 (1996) 15. [2] M. C ¸ akmak, G.P. Srivastava, Phys. Rev. B 57 (1998) 4486. [3] E. Schro¨der-Bergen, W. Ranke, Surf. Sci. 236 (1990) 103.
[4] A. Umerski, G.P. Srivastava, Phys. Rev. B 51 (1995) 2334. [5] G.B. Bachelet, D.R. Hamann, M. Schlu¨ter, Phys. Rev. B 26 (1982) 4199. [6 ] D.M. Ceperley, B.I. Alder, Phys. Rev. Lett. 45 (1980) 566. [7] J.P. Perdew, A. Zunger, Phys. Rev. B 23 (1981) 5048. [8] R.A. Evarestov, V.P. Smirnov, Phys. Stat. Sol. B 119 (1983) 9. [9] S.J. Jenkins, G.P. Srivastava, J. Phys. Condens. Matter 8 (1996) 6641. [10] H.M. Tu¨tu¨ncu¨, S.J. Jenkins, G.P. Srivastava, Phys. Rev. B 56 (1997) 4056. [11] G. Burns, Solid State Physics, International Edition, Academic Press, London, 1990. [12] M.B. Nardelli, F. Finocchi, M. Palummo, R. Difelice, C.M. Bertoni, F. Bernardini, S. Ossicini, Surf. Sci. 269/270 (1992) 879. [13] M. C ¸ akmak, G.P. Srivastava, unpublished.