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Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces
Volume 113A, number 6
PHYSICS LETTERS
30 December 1985
T H E O R E T I C A L S T U D Y OF OXYGEN C H E M I S O R P T I O N O N (111) AND (100) S I L I C O N SURFACES Nino RUSSO, Mar±rosa T O S C A N O Dipartimento di Chimica, Universiti~ della Calabria, 1-87030 Arcavacata di Rende (CS), Italy
Vincenzo B A R O N E and Francesco LELJ Dipartimento di Chimica, Universit& di Napoli, 1-80134 Naples, Italy
Received 12 February 1985; revised manuscript received 23 August 1985; accepted for publication 8 October 1985
The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especiallyfor the (111) surface.
The initial step of oxidation of silicon surfaces has been the subject of extensive theoretical [1-8] and experimental [9-19] work, but several questions (e.g. preference for a dissociative or non-dissociative chemisorption of molecular oxygen and preferred chemisorption site) remain still open to speculation. Recent experimental [19] and theoretical [8] reviews conclude that a dissociative chemisorption is more likely to occur, but the evidences for this conclusion are quite indirect. The data reported so far essentially concern the atomic chemisorption on the (100) surface [1,2] and the non-dissociative molecular chemisorption on the (111) surface [3,4]. The results obtained for one surface are generally considered to apply directly to the other one, but explicit comparisons of the same process on both surfaces are rare [8]. In this letter we report a detailed investigation of the initial stage of oxidation of silicon for both the (111) and (100) surfaces, using the semiempirical MNDO method [20], which has been recently proven to be particularly effective in the analysis of bulk defects [21,22] and surface phenomena [23,24]. In the framework of the cluster approach we have studied the chemisorption of atomic oxygen on three characteristic sites of both silicon surfaces, namely the ontop, bridge and open sites (fig. 1). Following previous studies [ 1,23,24] embedding hydrogen atoms have 0.375-9601/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
been used to terminate the clusters (employing Sill bond distances of 1.50 A) in order to retain the correct number of nearest neighbours and a tetrahedral geometry for each silicon atom. When bare substrate clusters were considered, it was assumed that each surface atom has one or two dangling bonds for (111) and (100) surfaces, respectively. The ideal bulk geometry of silicon (i.e. d s i _ s i = 2.35 A and tetrahedral valence angles) have been used in all computations, so that our clusters represent unrelaxed and unreconstructed surfaces. In fact, it has been demonstrated that in the low-coverage limit the Si surfaces essentially retain their ideal structures [ 1,3,4]. The distance r±between the oxygen atom and Si surfaces has been optimized by the gradient procedure implemented in the MNDO package. This optimized distance is denoted by R± and the O binding energy (denoted by De) is taken as the difference between the energy of the composite system at r x = R± and the sum of the energies of the separate substrate cluster and oxygen atom. In the case of the (100) surface two non-equivalent bridge positions are actually present. Both situations were considered in the calculations, but we report in table 1 only the results obtained for the geometry where the dangling bonds of the surface atoms are directed toward the center of the site (fig. 1) in view of the much higher binding energies obtained for this situation. ]21
Volume 113A, number 6
PHYSICS LETTERS o
30 December 1985
o
I I
o
I~ t ~
/
/i I" ixs.
I
A a
c
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first layer second layer
a
on top
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third layer
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Q
fourth layer
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open
Fig. 1. Schematic drawing of the four layer clusters employed in the study of the chemisorption of atomic oxygen on silicon sur'faces. (A) Clusters used for the (111) surface: Silo(1,3,3,3)Hls and Si10(3,3,3,1)H13. (B) Clusters used for the (100) surface: Si9(2,4,2,1)H12 , Si9(1,2,4,2)H14 and Sils(4,6,3,2)H16.
As a first step we have made MNDO calculations on exactly the same clusters as used in ref. [1]. Table 1 shows that the MNDO m e t h o d gives results in general agreement with the M C - S C F ones concerning geometry, charge transfer and chemisorption energy for on-top and bridge sites. It is particularly gratifying 322
that the authors o f r e f . [11] "expect that a more accurate calculation will lead to even larger values of D e'', just as obtained at the MNDO level. F o r the open site the MNDO energy minimum occurs, contrary to the results o f r e f . [1] below the silicon surface, between the first and second silicon layer and this re-
Volume 113A, number 6
PHYSICS LETTERS
30 December 1985
Table 1 Computed adsorption properties (R± in A, De in eV) for atomic oxygen at the on-top, bridge and open sites of Si (100) and Si (111) surfaces. The table also contains values (in A) for the respective nearest neighbor Si-O distance d in the equilibrium position and the charge accumulation q(O) on the O atom. Positive values of q(O) refer to electron accumulation. Cluster
Method
Site
R±
d
De
q(O)
(100) surface
Si~(2,4)H12-O Si6(2,4)H12-O Si9(2,4,2,1)H 12-O Si3(1,2)H6-O Si3(1,2)H6-O Si9(2,4,2,1)H14-O Si7(4,2,1)Ha-O Si7(4,2,1)H 8-O Sils(4,6,3,2)H16 -O
MC-SCF a) MNDO MNDO MC-SCF a) MNDO MNDO MC-SCF a) MNDO MNDO
bridge bridge bridge on-top on-top on-top open open open
0.06 0.06 0.25 1.64 1.55 1.54 0.96 - 1.06 -0.90
1.92 1.92 1.94 1.64 1.55 1.54 2.88 2.92 2.86
4.28 4.85 5.31 3.81 4.11 4.05 0.27 1.07 0.98
1.25 0.90 0.84 0.74 0.58 0.59 0.19 0.20 0.20
(111) surface
Silo(3,3,3,1)H13 -O Silo(1,3,3,3)H1s -O Si10(3,3,3,1)H13-O
MNDO MNDO MNDO
bridge on-top open
0.40 1.58 -0.67
1.96 1.58 2.32
3.37 3.25 0.61
a) Ref. [1]. suit could be in agreement with the intermediate loss peak at 830 cm -1 observed in EELS spectra [19]. However the MNDO method is known to perform poorly for this kind of situations, the D e of this site is much lower than for other sites, and a significant anal. ysis of such situations would demand explicit consideration of silicon relaxation. Pending further analysis, open sites will not be further considered in this note. In view of the good performances of the MNDO method we have then investigated the four-layer model clusters shown in fig. 1. In the case of the (100) surface the results (see table 1) are practically unmodified for the on-top site, whereas the D e of the bridge site increases significantly (by about 0.5 eV), probably due to the peculiar characteristics of the Si6(2,4)H12 model (namely the silicon atoms of the second layer are not connected). The bridge site is always the most stable one and for the largest clusters considered the energy difference between bridge and on-top chemisorptions amounts to 1.26 eV. The behaviour of the (111) surface, even if generally similar to that of the (100) surface, shows some interesting new features (see table 1). In particular, although the lowest energy minimum is found again for the bridge site, the energy difference between bridge and on-top chemisorptions is reduced to only 0.12 eV. Also in this case the energy minimum for the open site occurs when the oxygen atom penetrates below the surface, between
the first and second silicon layer, and its binding energy is much lower (2.76 eV) than the absolute minimum. From our data the following conclusions can be drawn: (1) The MNDO method is able to give reliable resuits for the geometry, charge transfer and chemisorption energy of oxygen on silicon and its low computer costs allow the comparison of large clusters simulating different surfaces. (2) Oxygen chemisorption occurs preferentially on bridge sites and, to a lower extent, at on-top sites, irrespective of the particular silicon surface considered. The preference for the bridge situation is, however, much higher for the (100) surface than for the (111) surface, where it becomes almost negligible (about 0.1 eV). (3) From an al~alysis of the binding energies it is possible to predict that molecular oxygen chemisorbs preferentially in a dissociative manner. In fact half of the binding energy of the 0 2 molecule (about 2.6 eV) is considerably smaller than the binding energies of 5.31 and 3.37 eV found for the chemisorption of atomic oxygen at the bridge site on (100) and (111) surfaces, respectively. The exothermicity of the process is predicted to be greater for the (100) than for the (111) surface.
323
Volume 113A, number 6
PHYSICS LETTERS
References [1] I.P. Batra, P.S. Bagus and K. Hermann, Phy~ Rev. Lett. 52 (1984) 384; J. Vac. Sci. Technol. A2 (1984) 1075. [2] M. Chen, I. P. Batra and C.R. Brundle, J. Vac. Sci. TechnoL 16 (1979) 1216. [3] W.A. Goddard III, J.J. Burton, A Redondo and T.C. McGiU, J. Vac. Sci. Technol. 15 (1978) 1274. [4] A. Redondo, W.A. Goddard III, C.A. Swatts and T.C. McGilI, J. Vac. Sci. Technol. 19 (1981) 498. [5] A.S. Bhandia and J.A. Schwarz, Surf. SoL 108 (1981) 587. [6] T. Kunjunny and D.K. Ferry, Phys. Rev. B24 (1981) 4593. [7] S. Ellialtioglu and S. Ciraci, Solid State Commun. 42 (1982) 879. [8] S. Ciraci, S. Ellialtioglu and S. Erkoc, Phys. Rev. B26 (1982) 5716. [9] H. Ibach, K. Horn, R. Dorn and U. Luth, Surf. Sci. 38 (1973) 433. [10] H. Ibach and J.E. Rowe, Phys. Rev. B9 (1974) 1951; B10 (1974) 710. [11] F.M. Meyer and J.J. Vrakking, Surf. Sci. 38 (1973) 275 [12] R. Ludeke and A. Koma, Phys. Rev. Lett. 34 (1975) 110; 35 (1975) 107.
324
30 December 1985
[13] J.E. Rowe, G. Margaritondo, H. Ibach and H. Froitzheim, Solid State Commun. 20 (1976) 277. [14] G. Hollinger, J. Jugnet, P. Pertosa, L. Porte and T.M. Duc, Chem. Phys. Lett. 36 (1975) 441. [15] C.M. Garner, I. Lindau, J.N. Miller, P. Pianetta and W.E. Spicer, J. Vac. Sci. TechnoL 14 (1977) 372. [16] C.M. Garner, 1. Lindau, C.Y. Su, P. Pianetta and W.E. Spicer, Phys. Rev. Lett. 40 (1978) 403; Phys. Rev. B19 (1979) 3944. [17] C.Y. Su, P.R. Skeath, I. Lindau and W.E. Spicer, J. Vac, SoL Technol. 18 (1981) 843. [18] G. Hollinger and F.J. Himpsel, Phys. Rev. B28 (1983) 3651. [19] H. Ibach, H.D. Bruchmann and H. Wagner, Appl. Phys. A29 (1982) 113. [20] M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc. 99 (1977) 4899. [21] J.W. Corbett, S.N. Sahu, T.S. Shi and L.C. Snyder, Phys~ Lett. 93A (1983) 303. [22] G.G. de Leo, W.B. Fowler and G.D. Watkins, Phys. Rev. B29 (1984) 3193. [23] V. Barone, F. Lelj, N. Russo and G. Abbate, Solid State Commun. 49 (1984) 925. [24] G. Abbate, V. Barone, E. laconis, F. Lelj and N. Russo, SurL Sci. 152[153 (1985) 690.
PHYSICS LETTERS
30 December 1985
T H E O R E T I C A L S T U D Y OF OXYGEN C H E M I S O R P T I O N O N (111) AND (100) S I L I C O N SURFACES Nino RUSSO, Mar±rosa T O S C A N O Dipartimento di Chimica, Universiti~ della Calabria, 1-87030 Arcavacata di Rende (CS), Italy
Vincenzo B A R O N E and Francesco LELJ Dipartimento di Chimica, Universit& di Napoli, 1-80134 Naples, Italy
Received 12 February 1985; revised manuscript received 23 August 1985; accepted for publication 8 October 1985
The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especiallyfor the (111) surface.
The initial step of oxidation of silicon surfaces has been the subject of extensive theoretical [1-8] and experimental [9-19] work, but several questions (e.g. preference for a dissociative or non-dissociative chemisorption of molecular oxygen and preferred chemisorption site) remain still open to speculation. Recent experimental [19] and theoretical [8] reviews conclude that a dissociative chemisorption is more likely to occur, but the evidences for this conclusion are quite indirect. The data reported so far essentially concern the atomic chemisorption on the (100) surface [1,2] and the non-dissociative molecular chemisorption on the (111) surface [3,4]. The results obtained for one surface are generally considered to apply directly to the other one, but explicit comparisons of the same process on both surfaces are rare [8]. In this letter we report a detailed investigation of the initial stage of oxidation of silicon for both the (111) and (100) surfaces, using the semiempirical MNDO method [20], which has been recently proven to be particularly effective in the analysis of bulk defects [21,22] and surface phenomena [23,24]. In the framework of the cluster approach we have studied the chemisorption of atomic oxygen on three characteristic sites of both silicon surfaces, namely the ontop, bridge and open sites (fig. 1). Following previous studies [ 1,23,24] embedding hydrogen atoms have 0.375-9601/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
been used to terminate the clusters (employing Sill bond distances of 1.50 A) in order to retain the correct number of nearest neighbours and a tetrahedral geometry for each silicon atom. When bare substrate clusters were considered, it was assumed that each surface atom has one or two dangling bonds for (111) and (100) surfaces, respectively. The ideal bulk geometry of silicon (i.e. d s i _ s i = 2.35 A and tetrahedral valence angles) have been used in all computations, so that our clusters represent unrelaxed and unreconstructed surfaces. In fact, it has been demonstrated that in the low-coverage limit the Si surfaces essentially retain their ideal structures [ 1,3,4]. The distance r±between the oxygen atom and Si surfaces has been optimized by the gradient procedure implemented in the MNDO package. This optimized distance is denoted by R± and the O binding energy (denoted by De) is taken as the difference between the energy of the composite system at r x = R± and the sum of the energies of the separate substrate cluster and oxygen atom. In the case of the (100) surface two non-equivalent bridge positions are actually present. Both situations were considered in the calculations, but we report in table 1 only the results obtained for the geometry where the dangling bonds of the surface atoms are directed toward the center of the site (fig. 1) in view of the much higher binding energies obtained for this situation. ]21
Volume 113A, number 6
PHYSICS LETTERS o
30 December 1985
o
I I
o
I~ t ~
/
/i I" ixs.
I
A a
c
0|
A
I
a
_b c
B
,0..
o I
b
•
I
/gO.. x
I!
¢
first layer second layer
a
on top
@
third layer
b
bridge
Q
fourth layer
•
embedding H atoms
open
Fig. 1. Schematic drawing of the four layer clusters employed in the study of the chemisorption of atomic oxygen on silicon sur'faces. (A) Clusters used for the (111) surface: Silo(1,3,3,3)Hls and Si10(3,3,3,1)H13. (B) Clusters used for the (100) surface: Si9(2,4,2,1)H12 , Si9(1,2,4,2)H14 and Sils(4,6,3,2)H16.
As a first step we have made MNDO calculations on exactly the same clusters as used in ref. [1]. Table 1 shows that the MNDO m e t h o d gives results in general agreement with the M C - S C F ones concerning geometry, charge transfer and chemisorption energy for on-top and bridge sites. It is particularly gratifying 322
that the authors o f r e f . [11] "expect that a more accurate calculation will lead to even larger values of D e'', just as obtained at the MNDO level. F o r the open site the MNDO energy minimum occurs, contrary to the results o f r e f . [1] below the silicon surface, between the first and second silicon layer and this re-
Volume 113A, number 6
PHYSICS LETTERS
30 December 1985
Table 1 Computed adsorption properties (R± in A, De in eV) for atomic oxygen at the on-top, bridge and open sites of Si (100) and Si (111) surfaces. The table also contains values (in A) for the respective nearest neighbor Si-O distance d in the equilibrium position and the charge accumulation q(O) on the O atom. Positive values of q(O) refer to electron accumulation. Cluster
Method
Site
R±
d
De
q(O)
(100) surface
Si~(2,4)H12-O Si6(2,4)H12-O Si9(2,4,2,1)H 12-O Si3(1,2)H6-O Si3(1,2)H6-O Si9(2,4,2,1)H14-O Si7(4,2,1)Ha-O Si7(4,2,1)H 8-O Sils(4,6,3,2)H16 -O
MC-SCF a) MNDO MNDO MC-SCF a) MNDO MNDO MC-SCF a) MNDO MNDO
bridge bridge bridge on-top on-top on-top open open open
0.06 0.06 0.25 1.64 1.55 1.54 0.96 - 1.06 -0.90
1.92 1.92 1.94 1.64 1.55 1.54 2.88 2.92 2.86
4.28 4.85 5.31 3.81 4.11 4.05 0.27 1.07 0.98
1.25 0.90 0.84 0.74 0.58 0.59 0.19 0.20 0.20
(111) surface
Silo(3,3,3,1)H13 -O Silo(1,3,3,3)H1s -O Si10(3,3,3,1)H13-O
MNDO MNDO MNDO
bridge on-top open
0.40 1.58 -0.67
1.96 1.58 2.32
3.37 3.25 0.61
a) Ref. [1]. suit could be in agreement with the intermediate loss peak at 830 cm -1 observed in EELS spectra [19]. However the MNDO method is known to perform poorly for this kind of situations, the D e of this site is much lower than for other sites, and a significant anal. ysis of such situations would demand explicit consideration of silicon relaxation. Pending further analysis, open sites will not be further considered in this note. In view of the good performances of the MNDO method we have then investigated the four-layer model clusters shown in fig. 1. In the case of the (100) surface the results (see table 1) are practically unmodified for the on-top site, whereas the D e of the bridge site increases significantly (by about 0.5 eV), probably due to the peculiar characteristics of the Si6(2,4)H12 model (namely the silicon atoms of the second layer are not connected). The bridge site is always the most stable one and for the largest clusters considered the energy difference between bridge and on-top chemisorptions amounts to 1.26 eV. The behaviour of the (111) surface, even if generally similar to that of the (100) surface, shows some interesting new features (see table 1). In particular, although the lowest energy minimum is found again for the bridge site, the energy difference between bridge and on-top chemisorptions is reduced to only 0.12 eV. Also in this case the energy minimum for the open site occurs when the oxygen atom penetrates below the surface, between
the first and second silicon layer, and its binding energy is much lower (2.76 eV) than the absolute minimum. From our data the following conclusions can be drawn: (1) The MNDO method is able to give reliable resuits for the geometry, charge transfer and chemisorption energy of oxygen on silicon and its low computer costs allow the comparison of large clusters simulating different surfaces. (2) Oxygen chemisorption occurs preferentially on bridge sites and, to a lower extent, at on-top sites, irrespective of the particular silicon surface considered. The preference for the bridge situation is, however, much higher for the (100) surface than for the (111) surface, where it becomes almost negligible (about 0.1 eV). (3) From an al~alysis of the binding energies it is possible to predict that molecular oxygen chemisorbs preferentially in a dissociative manner. In fact half of the binding energy of the 0 2 molecule (about 2.6 eV) is considerably smaller than the binding energies of 5.31 and 3.37 eV found for the chemisorption of atomic oxygen at the bridge site on (100) and (111) surfaces, respectively. The exothermicity of the process is predicted to be greater for the (100) than for the (111) surface.
323
Volume 113A, number 6
PHYSICS LETTERS
References [1] I.P. Batra, P.S. Bagus and K. Hermann, Phy~ Rev. Lett. 52 (1984) 384; J. Vac. Sci. Technol. A2 (1984) 1075. [2] M. Chen, I. P. Batra and C.R. Brundle, J. Vac. Sci. TechnoL 16 (1979) 1216. [3] W.A. Goddard III, J.J. Burton, A Redondo and T.C. McGiU, J. Vac. Sci. Technol. 15 (1978) 1274. [4] A. Redondo, W.A. Goddard III, C.A. Swatts and T.C. McGilI, J. Vac. Sci. Technol. 19 (1981) 498. [5] A.S. Bhandia and J.A. Schwarz, Surf. SoL 108 (1981) 587. [6] T. Kunjunny and D.K. Ferry, Phys. Rev. B24 (1981) 4593. [7] S. Ellialtioglu and S. Ciraci, Solid State Commun. 42 (1982) 879. [8] S. Ciraci, S. Ellialtioglu and S. Erkoc, Phys. Rev. B26 (1982) 5716. [9] H. Ibach, K. Horn, R. Dorn and U. Luth, Surf. Sci. 38 (1973) 433. [10] H. Ibach and J.E. Rowe, Phys. Rev. B9 (1974) 1951; B10 (1974) 710. [11] F.M. Meyer and J.J. Vrakking, Surf. Sci. 38 (1973) 275 [12] R. Ludeke and A. Koma, Phys. Rev. Lett. 34 (1975) 110; 35 (1975) 107.
324
30 December 1985
[13] J.E. Rowe, G. Margaritondo, H. Ibach and H. Froitzheim, Solid State Commun. 20 (1976) 277. [14] G. Hollinger, J. Jugnet, P. Pertosa, L. Porte and T.M. Duc, Chem. Phys. Lett. 36 (1975) 441. [15] C.M. Garner, I. Lindau, J.N. Miller, P. Pianetta and W.E. Spicer, J. Vac. Sci. TechnoL 14 (1977) 372. [16] C.M. Garner, 1. Lindau, C.Y. Su, P. Pianetta and W.E. Spicer, Phys. Rev. Lett. 40 (1978) 403; Phys. Rev. B19 (1979) 3944. [17] C.Y. Su, P.R. Skeath, I. Lindau and W.E. Spicer, J. Vac, SoL Technol. 18 (1981) 843. [18] G. Hollinger and F.J. Himpsel, Phys. Rev. B28 (1983) 3651. [19] H. Ibach, H.D. Bruchmann and H. Wagner, Appl. Phys. A29 (1982) 113. [20] M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc. 99 (1977) 4899. [21] J.W. Corbett, S.N. Sahu, T.S. Shi and L.C. Snyder, Phys~ Lett. 93A (1983) 303. [22] G.G. de Leo, W.B. Fowler and G.D. Watkins, Phys. Rev. B29 (1984) 3193. [23] V. Barone, F. Lelj, N. Russo and G. Abbate, Solid State Commun. 49 (1984) 925. [24] G. Abbate, V. Barone, E. laconis, F. Lelj and N. Russo, SurL Sci. 152[153 (1985) 690.