- Email: [email protected]
The process of oxygen chemisorption on the Si(111) surface
L543
Surface Science 219 (1989) L543-L550 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
THE PROCESS OF OXYGEN CHEMISORPTION ON THE Si(ll1) SURFACE X.M. ZHENG Department
of Physics, University of Newcastle,
NS W 2308, Australia
and P.L. CA0 Department
of Physics, Zhejiang
University, Hangzhoy
People’s Rep. of China
Received 16 February 1989; accepted for publication 16 May 1989
The chemisorption of oxygen on the Si(ll1) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO, film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.
1. Introduction Due to the technological importance of both the SiO, film and the SiO,-Si interface in the electronics industry, along with the scientific importance of understanding the oxidation mechanism on a semiconductor surface, the initial steps of oxygen chemisorption on silicon surfaces have received considerable attention in the last years [l-7]. In spite of these studies, the picture of the initial steps of the oxidation of silicon is still not clear. Disputed questions are focused on whether molecular or dissociated atomic adsorption occurs, on the geometry of bonds, and the actual nature of the oxygen states. Different and controversial results have been reported [l-7]. Ibach et al. [l] reported that both atomic and molecular species exist for exposures less than lo2 L 02, and only atomic species for exposures greater than lo2 L O2 at room temperature. Hollinger et al. [2] claimed that up to four different oxidation 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L544
X.M. Zheng, P.L. Cao / Oxygen chemisorption on Si(IlI)
states of silicon coexist even at monolayer oxygen coverage. Hofer et al. [3] reported that the initial adsorption of oxygen on the Si(ll1) surface proceeds via a molecular intermediate which is a precursor to the stable dissociated adsorption states, and proposed some adsorption bond models. Tabe et al. [4] studied the chemical shifts of the Si 2p core level as a function of temperature and 0, exposure and proposed the initial steps of silicon oxidation. Very recently, Silvestre et al. [5] reported the observation of a metastable precursor for the adsorption of oxygen on the Si(ll1) surface. In this paper, we use the ASED-MO (atomic superposition and electron delocalization - molecular orbital theory [8,9]) program to study the initial behavior of oxygen chemisorption on the Si(ll1) surface. From surface potential (total energy) calculations, we have obtained the chemisorption potential energy curves and shown clearly the precursor of molecular chemisorption. Most publications [1,3,4] have claimed that atomic oxygen chemisorption takes place at the short bridge site (between the first and second layer silicon atoms). We find, however, that this only occurs once the adjacent on-top position is occupied. The bond geometry and the binding energy (adsorption energy) are determined in each case by minimising the total energy. Our results are in good agreement with the experimental data and can explain the results of most existing publications.
2. The method and calculation model The ASED-MO method [8,9] has been applied to the study of chemisorption on both metal and semiconductor surfaces, and has proved to be quite 0
0
0
m
0
I 0
El 0
m
2 ‘I
Et 3
0
Ezl 0
I
5
El
El 0
m
m
0
0
0
Fig. 1. The cluster model of the Si(ll1) surface: (m) first layer silicon atoms; (0) second layer silicon atoms; (0) the hydrogen atoms for saturating the dangling bonds of silicon; (1) on-top; (2) short bridge; (3) long bridge; (4 and 5) two kinds hollow positions.
Table 1 Atomic parameters ionization Atom
potential;
X. M. Zheng, P. L. Cao / Oxygen chemisorption
on Si(I I I)
employed
quantum number; IP (ev),
in the calculations:
n, the principal
L545
the
[ (a.u.), the Slater orbital exponent
s
P
n
IP
5
n
IP
5
Si 0
3 2
14.687 28.48
1.634 2.246
3 2
8.082 13.62
1.428 2.227
H
1
13.60
1.20
successful in determining the binding energies, bond geometries, surface potentials, force constants, and electronic structures of these systems [8,9]. We use the molecular cluster H17Si1202 to simulate the molecular 0, chemisorption on the Si(ll1) surface, and the cluster H,,Si,,O to simulate the atomic 0 chemisorption on the Si(ll1) surface. The hydrogens are only used to saturate the dangling bonds of bulk silicon. The cluster is shown in fig. 1. Experimental work [4] has indicated that when oxygen is chemisorbed on the Si(ll1) surface a (1 X 1) LEED pattern can be seen. We thus employ the ideal Si(ll1) surface (silicon lattice distance 5.42 A) in all of our calculations. For molecular chemisorption, the gas 0, bond length of 1.2 A is used, and the atomic parameters employed in the calculations are listed in table 1. The zero potential (the total energy of the system) corresponds to the oxygen being very far from the surface. The binding energy (adsorption energy) and bond length (adsorption distance) are determined from the minimum total energy. The adsorption potential energy curves are obtained by bringing the oxygen in from infinity towards the surface.
3. The results and discussion The molecular and atomic oxygen chemisorption processes are first considered separately. The on-top, long bridge (for molecular chemisorption, this site is often referred to as the peroxy bridge), short bridge, and two kinds of hollow positions (see fig. 1) have been studied for both molecular and atomic oxygen chemisorption. For molecular adsorption, we first consider the vertical model, where the axis of the oxygen molecule is perpendicular to the Si(ll1) surface. We calculated the potential energy curves for all of the sites as mentioned above. The only adsorption position which has a minimum in the potential energy curve is the on-top site (see fig. 2a). The determined Si-0 bond length is 1.59 A and the adsorption energy is 3.04 eV. Secondly, we consider the parallel model, where the axis of the oxygen molecule is parallel to the Si(ll1) surface. The minimum point now occurs in the potential energy curve when the oxygen
L546
X.M. Zheng, P. L. Cao / Oxygen chemisorption on Si(I II)
. . _--_-_
%
&ITIY!&-
---__S”rfwe
(a)
(b)
--__-_ P ------Surface
---
. p% ___’
ICI
__--__
tdf
Fig. 2. Calculated chemisorption models: (0) the oxygen atoms; {m) fist layer sihcon atom; (17) second layer silicon atom; (a) molecular 0, on-top (peroxy vertical model); (b) molecular 0s long bridge (peroxy bridge model); (c) atomic 0 on-top; (d) dissociated oxygen, one on-top, and the other at the short bridge site.
che~so~tion site is a long bridge position (see fig. 2b). The determined Si-0 bond length is 2.06 A and the adsorption energy is 1.04 eV. This adsorption model is called the peroxy bridge in most publications [3,6,7]. The calculated binding energies and adsorption distances (the height of the oxygen above the surface) are listed in table 2. From this table we see that although both the peroxy vertical and peroxy bridge models are possible, the vertical model is the more stable. Hijfer et al. [33 and,Bhandia et al. [7] prefer the peroxy bridge model, whilst Goddard et al. [9] prefer the peroxy radical model. In order to compare our peroxy vertical model with the Goddard peroxy radical model, we have calculated the total energy as a function of the orientation, 8, of the 0, molecule about the surface normal. We find the minimum energy to be that for the peroxy vertical model (8 = 0 o ) with the peroxy radical model (B = 60 o ) some 0.60 eV higher in energy. We therefore prefer the vertical model. For the atomic chemisorption process, we first considered the adsorption of a single, dissociated oxygen atom at each of the previously mentioned sites. Table 2 Calculated adsorption energies E, (ev), and adsorption heights above the surface h (au.) Short bridge
On-top
Molecular Oz Single atomic 0
J%
h
3.04 2.17
3.00 2.75
Long bridge
E,
h
E,
h
4.68
0.50
1.04 _
3.00 -
X.M. Zheng P.L. Cao / Oxygen chemisorption
on Si(ll1)
L541
The only stable adsorption site is the on-top site (see fig. 2~). The Si-0 bond length of 1.46 A is a little shorter than that for molecular chemisorption. According to experimental work [1,3,4], it seems that the short bridge site is the most widely accepted candidate for atomic chemisorption. We did not obtain this result when considering direct single oxygen chemisorption. However, when molecular oxygen is chemisorbed at the on-top position, we find that placing one oxygen atom on-top, bonded to the silicon atom, and moving the other oxygen atom to the short bridge site changes the surface potential dramatically. The optimization calculations show that the dittance between the on-top oxygen atom and its underlying silicon is now 1.59 A and the short bridge site has large adsorption energy. The short bridge position is a small amount (0.5 a.u.), above the surface, 2.43 a.u. from the first layer on-top silicon atom (atom 1 in fig. 1) and 2.75 a.u. to the second layer hollow position silicon atom (atom 4 in fig. 1). The bond angle is LSiOSi = 148.6 o as shown in fig. 2d. The corresponding binding energy and adsorption distance are listed in table 2. The calculated binding energies for atomic chemisorption indicate that both on-top and the short bridge positions are possible. The short bridge site is more stable but can be occupied only after the prior occupation of the on-top position or following the saturation of the dangling bonds of the surface silicon with oxygen. From table 2 we notice that the binding energy for on-top molecular chemisorption is larger than that for single atomic adsorption at the on-top site. It requires 5.1 eV [lo] to break the bond of gaseous 0, and the chemisorption energy corresponding to two atomic oxygens occupying on-top chemisorption sites is not large enough for this purpose. Short bridge chemisorption requires the prior occupation of the on-top positions. Molecular chemisorption can therefore be rather stable under certain conditions such as low temperatures, and should thus be the first step in silicon oxidation. This is presumably why the molecular oxygen states can be seen in such experiments [l-5]. We have also calculated the potential energy curves for oxygen approaching the Si(ll1) surface. The potential-energy curves for the two cases of molecular chemisorption on-top and dissociated atomic chemisorption in which one oxygen is adsorbed on-top and the other one moved to the short bridge site, are shown in fig. 3. (The curves corresponding to the peroxy bridge site for molecular chemisorption and the on-top position for atomic chemisorption, are not included in this figure.) From fig. 3 we can clearly see the metastable precursor of the molecular chemisorption. To go from this state to dissociated atomic chemisorption (with one oxygen sited on-top and the other chemisorbed on the short bridge site), requires approximately 1.5 eV activation energy. The second step of 0, dissociation on the Si(ll1) surface follows from the larger binding energy of 6.83 eV (for one on-top oxygen atom along with one short bridge oxygen atom) and is more stable from the thermal point of
L548
X.M. Zheng, P.L. Cao / Oxygen chemisorption
-L
-3
-2
8 -*
I--
! !
6--
I
5--
i
4..
i
3.-
i
2..
i
-1
Fig. 3. Adsorption potential energy curves: (.- .- .) molecular (- - -) dissociated oxygen chemisorption; () from chemisorption.
on Si(lIl)
I
a
oxygen molecular
9
lo
chemisorption to
atomic
h (au1 t
on-top; oxygen
view. This is in good agreement with most experimental work [l-4] which shows that at elevated temperatures, dissociated oxygen atoms occupy short bridge sites. Hijfer et al. [3] rule out the atomic on-top chemisorption while Ibach [l] and Tabe et al. [4] proposed both on-top and short bridge atomic chemisorption. Our results, which indicate that short bridge chemisorption occurs only after the occupation of the on-top site with oxygen, are strongly supportive of the Tabe [4] models and confirm the thoughts of Ibach et al. [l]. To further consider the behavior of chemisorbed oxygen, we let the short bridge oxygen atom diffuse into the bulk of silicon and calculated the associated total energy. The resulting potential energy curve displays a 2.62 eV barrier and then reduces in magnitude as the oxygen moves below the surface (see fig. 3), until finally the oxygen becomes bound beneath the surface to the on-top silicon atom. The diffusion is limited to certain paths as shown in step 3 of fig. 4. At this stage the oxygen bonds to silicon forming an SiO, film in the manner proposed by Tabe et al. [4]. The ASED-MO approach has thus obtained a three step picture for the oxidation of silicon (as shown in fig. 4)
. ----k
X.M. Zheng, P.L. Cao / Oxygen chemisorption
------
on Si(ll I)
ls49
Surface
---
Fig. 4. Three steps of the silicon oxidation process: (1) molecular oxygen chemisorption on-top; (2) dissociated oxygen, one sited on-top, and the other at the short bridge site; (3) short bridge oxygen atom diffusion into the bulk of silicon; (- - - - - -) the diffusion paths.
which is both consistent with the three steps proposed by Ibach [l], and in good agreement with the proposal of Tabe et al. [4]. 4. Conclusion The chemisorption of oxygen on the Si(ll1) surface can be divided into three stages. The first step involves the adsorption of molecular oxygen as a peroxy vertical structure at the on-top positions. The second step involves oxygen dissociation with one atom occupying the on-top position and the other atom being adsorbed at the short bridge site. The third step consists of oxygen diffusion into the bulk of silicon to form an SiO, film. At certain temperatures, different oxygen states can coexist.
Acknowledgments The authors wish to thank Professor R.J. MacDonald, Professor P.V. Smith and Dr. B.I. Craig for critical reading of the manuscript and helpful discussions. One of us (X.M. Zheng) is grateful for the award of a University of Newcastle Postgraduate Research Scholarship. References [l] H. Ibach, H.D. Rruchmann and H. Wager, Appl. Phys. A 29 (1982) 113. [2] G. Hollinger and F.J. Himpsel, Phys. Rev. B 28 (1983) 3651.
L550 [3] [4] [5] [6]
[7] [8]
[9]
[lo]
X.M. Zheng,
P.L. Cao / Oxygen chemisorption on Si(l I I)
U. Hofer, P. Morgen, W. Wurth and E. Umbach, Phys. Rev. Letters 55 (1985) 2979. M. Tabe, T.T. Chiang, I. Lindau and W.E. Spicer, Phys. Rev. B 34 (1986) 2706. C. Silvestre and M. Shayegan, Phys. Rev. B 37 (1988) 10432. W.A. Goddard III, A Redondo and T. McGill, Solid State Comrnun. 8 (1976) 981; A. Redondo, W.A. Goddard III, C.A. Swarts and T.C. McGill, J. Vacuum Sci. Technol. 19 (1981) 498. A.S. Bhandia and J.A. Schwarz, Surface Sci. 108 (1981) 587. A.B. Anderson and R. Hoffmann, J. Chem. Phys. 60 (1974) 4271; A.B. Anderson, Surface Sci. 105 (1981) 159; S.Y. Chu and A.B. Anderson, Surface Sci. 194 (1988) 55. P.L. Cao and D.H. Shi, Acta Phys. Sinica 34 (1985) 1291; P.L. Cao, Chinese Phys. 7 (1987) 835; D.H. Shi and P.L. Cao, Chinese Phys. 8 (1988) 47. A.A. Radzig and B.M. Smimov, in: Reference Data on Atoms, Molecules, and Ions (Springer, Berlin, 1985) p. 381.
Surface Science 219 (1989) L543-L550 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
THE PROCESS OF OXYGEN CHEMISORPTION ON THE Si(ll1) SURFACE X.M. ZHENG Department
of Physics, University of Newcastle,
NS W 2308, Australia
and P.L. CA0 Department
of Physics, Zhejiang
University, Hangzhoy
People’s Rep. of China
Received 16 February 1989; accepted for publication 16 May 1989
The chemisorption of oxygen on the Si(ll1) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO, film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.
1. Introduction Due to the technological importance of both the SiO, film and the SiO,-Si interface in the electronics industry, along with the scientific importance of understanding the oxidation mechanism on a semiconductor surface, the initial steps of oxygen chemisorption on silicon surfaces have received considerable attention in the last years [l-7]. In spite of these studies, the picture of the initial steps of the oxidation of silicon is still not clear. Disputed questions are focused on whether molecular or dissociated atomic adsorption occurs, on the geometry of bonds, and the actual nature of the oxygen states. Different and controversial results have been reported [l-7]. Ibach et al. [l] reported that both atomic and molecular species exist for exposures less than lo2 L 02, and only atomic species for exposures greater than lo2 L O2 at room temperature. Hollinger et al. [2] claimed that up to four different oxidation 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L544
X.M. Zheng, P.L. Cao / Oxygen chemisorption on Si(IlI)
states of silicon coexist even at monolayer oxygen coverage. Hofer et al. [3] reported that the initial adsorption of oxygen on the Si(ll1) surface proceeds via a molecular intermediate which is a precursor to the stable dissociated adsorption states, and proposed some adsorption bond models. Tabe et al. [4] studied the chemical shifts of the Si 2p core level as a function of temperature and 0, exposure and proposed the initial steps of silicon oxidation. Very recently, Silvestre et al. [5] reported the observation of a metastable precursor for the adsorption of oxygen on the Si(ll1) surface. In this paper, we use the ASED-MO (atomic superposition and electron delocalization - molecular orbital theory [8,9]) program to study the initial behavior of oxygen chemisorption on the Si(ll1) surface. From surface potential (total energy) calculations, we have obtained the chemisorption potential energy curves and shown clearly the precursor of molecular chemisorption. Most publications [1,3,4] have claimed that atomic oxygen chemisorption takes place at the short bridge site (between the first and second layer silicon atoms). We find, however, that this only occurs once the adjacent on-top position is occupied. The bond geometry and the binding energy (adsorption energy) are determined in each case by minimising the total energy. Our results are in good agreement with the experimental data and can explain the results of most existing publications.
2. The method and calculation model The ASED-MO method [8,9] has been applied to the study of chemisorption on both metal and semiconductor surfaces, and has proved to be quite 0
0
0
m
0
I 0
El 0
m
2 ‘I
Et 3
0
Ezl 0
I
5
El
El 0
m
m
0
0
0
Fig. 1. The cluster model of the Si(ll1) surface: (m) first layer silicon atoms; (0) second layer silicon atoms; (0) the hydrogen atoms for saturating the dangling bonds of silicon; (1) on-top; (2) short bridge; (3) long bridge; (4 and 5) two kinds hollow positions.
Table 1 Atomic parameters ionization Atom
potential;
X. M. Zheng, P. L. Cao / Oxygen chemisorption
on Si(I I I)
employed
quantum number; IP (ev),
in the calculations:
n, the principal
L545
the
[ (a.u.), the Slater orbital exponent
s
P
n
IP
5
n
IP
5
Si 0
3 2
14.687 28.48
1.634 2.246
3 2
8.082 13.62
1.428 2.227
H
1
13.60
1.20
successful in determining the binding energies, bond geometries, surface potentials, force constants, and electronic structures of these systems [8,9]. We use the molecular cluster H17Si1202 to simulate the molecular 0, chemisorption on the Si(ll1) surface, and the cluster H,,Si,,O to simulate the atomic 0 chemisorption on the Si(ll1) surface. The hydrogens are only used to saturate the dangling bonds of bulk silicon. The cluster is shown in fig. 1. Experimental work [4] has indicated that when oxygen is chemisorbed on the Si(ll1) surface a (1 X 1) LEED pattern can be seen. We thus employ the ideal Si(ll1) surface (silicon lattice distance 5.42 A) in all of our calculations. For molecular chemisorption, the gas 0, bond length of 1.2 A is used, and the atomic parameters employed in the calculations are listed in table 1. The zero potential (the total energy of the system) corresponds to the oxygen being very far from the surface. The binding energy (adsorption energy) and bond length (adsorption distance) are determined from the minimum total energy. The adsorption potential energy curves are obtained by bringing the oxygen in from infinity towards the surface.
3. The results and discussion The molecular and atomic oxygen chemisorption processes are first considered separately. The on-top, long bridge (for molecular chemisorption, this site is often referred to as the peroxy bridge), short bridge, and two kinds of hollow positions (see fig. 1) have been studied for both molecular and atomic oxygen chemisorption. For molecular adsorption, we first consider the vertical model, where the axis of the oxygen molecule is perpendicular to the Si(ll1) surface. We calculated the potential energy curves for all of the sites as mentioned above. The only adsorption position which has a minimum in the potential energy curve is the on-top site (see fig. 2a). The determined Si-0 bond length is 1.59 A and the adsorption energy is 3.04 eV. Secondly, we consider the parallel model, where the axis of the oxygen molecule is parallel to the Si(ll1) surface. The minimum point now occurs in the potential energy curve when the oxygen
L546
X.M. Zheng, P. L. Cao / Oxygen chemisorption on Si(I II)
. . _--_-_
%
&ITIY!&-
---__S”rfwe
(a)
(b)
--__-_ P ------Surface
---
. p% ___’
ICI
__--__
tdf
Fig. 2. Calculated chemisorption models: (0) the oxygen atoms; {m) fist layer sihcon atom; (17) second layer silicon atom; (a) molecular 0, on-top (peroxy vertical model); (b) molecular 0s long bridge (peroxy bridge model); (c) atomic 0 on-top; (d) dissociated oxygen, one on-top, and the other at the short bridge site.
che~so~tion site is a long bridge position (see fig. 2b). The determined Si-0 bond length is 2.06 A and the adsorption energy is 1.04 eV. This adsorption model is called the peroxy bridge in most publications [3,6,7]. The calculated binding energies and adsorption distances (the height of the oxygen above the surface) are listed in table 2. From this table we see that although both the peroxy vertical and peroxy bridge models are possible, the vertical model is the more stable. Hijfer et al. [33 and,Bhandia et al. [7] prefer the peroxy bridge model, whilst Goddard et al. [9] prefer the peroxy radical model. In order to compare our peroxy vertical model with the Goddard peroxy radical model, we have calculated the total energy as a function of the orientation, 8, of the 0, molecule about the surface normal. We find the minimum energy to be that for the peroxy vertical model (8 = 0 o ) with the peroxy radical model (B = 60 o ) some 0.60 eV higher in energy. We therefore prefer the vertical model. For the atomic chemisorption process, we first considered the adsorption of a single, dissociated oxygen atom at each of the previously mentioned sites. Table 2 Calculated adsorption energies E, (ev), and adsorption heights above the surface h (au.) Short bridge
On-top
Molecular Oz Single atomic 0
J%
h
3.04 2.17
3.00 2.75
Long bridge
E,
h
E,
h
4.68
0.50
1.04 _
3.00 -
X.M. Zheng P.L. Cao / Oxygen chemisorption
on Si(ll1)
L541
The only stable adsorption site is the on-top site (see fig. 2~). The Si-0 bond length of 1.46 A is a little shorter than that for molecular chemisorption. According to experimental work [1,3,4], it seems that the short bridge site is the most widely accepted candidate for atomic chemisorption. We did not obtain this result when considering direct single oxygen chemisorption. However, when molecular oxygen is chemisorbed at the on-top position, we find that placing one oxygen atom on-top, bonded to the silicon atom, and moving the other oxygen atom to the short bridge site changes the surface potential dramatically. The optimization calculations show that the dittance between the on-top oxygen atom and its underlying silicon is now 1.59 A and the short bridge site has large adsorption energy. The short bridge position is a small amount (0.5 a.u.), above the surface, 2.43 a.u. from the first layer on-top silicon atom (atom 1 in fig. 1) and 2.75 a.u. to the second layer hollow position silicon atom (atom 4 in fig. 1). The bond angle is LSiOSi = 148.6 o as shown in fig. 2d. The corresponding binding energy and adsorption distance are listed in table 2. The calculated binding energies for atomic chemisorption indicate that both on-top and the short bridge positions are possible. The short bridge site is more stable but can be occupied only after the prior occupation of the on-top position or following the saturation of the dangling bonds of the surface silicon with oxygen. From table 2 we notice that the binding energy for on-top molecular chemisorption is larger than that for single atomic adsorption at the on-top site. It requires 5.1 eV [lo] to break the bond of gaseous 0, and the chemisorption energy corresponding to two atomic oxygens occupying on-top chemisorption sites is not large enough for this purpose. Short bridge chemisorption requires the prior occupation of the on-top positions. Molecular chemisorption can therefore be rather stable under certain conditions such as low temperatures, and should thus be the first step in silicon oxidation. This is presumably why the molecular oxygen states can be seen in such experiments [l-5]. We have also calculated the potential energy curves for oxygen approaching the Si(ll1) surface. The potential-energy curves for the two cases of molecular chemisorption on-top and dissociated atomic chemisorption in which one oxygen is adsorbed on-top and the other one moved to the short bridge site, are shown in fig. 3. (The curves corresponding to the peroxy bridge site for molecular chemisorption and the on-top position for atomic chemisorption, are not included in this figure.) From fig. 3 we can clearly see the metastable precursor of the molecular chemisorption. To go from this state to dissociated atomic chemisorption (with one oxygen sited on-top and the other chemisorbed on the short bridge site), requires approximately 1.5 eV activation energy. The second step of 0, dissociation on the Si(ll1) surface follows from the larger binding energy of 6.83 eV (for one on-top oxygen atom along with one short bridge oxygen atom) and is more stable from the thermal point of
L548
X.M. Zheng, P.L. Cao / Oxygen chemisorption
-L
-3
-2
8 -*
I--
! !
6--
I
5--
i
4..
i
3.-
i
2..
i
-1
Fig. 3. Adsorption potential energy curves: (.- .- .) molecular (- - -) dissociated oxygen chemisorption; () from chemisorption.
on Si(lIl)
I
a
oxygen molecular
9
lo
chemisorption to
atomic
h (au1 t
on-top; oxygen
view. This is in good agreement with most experimental work [l-4] which shows that at elevated temperatures, dissociated oxygen atoms occupy short bridge sites. Hijfer et al. [3] rule out the atomic on-top chemisorption while Ibach [l] and Tabe et al. [4] proposed both on-top and short bridge atomic chemisorption. Our results, which indicate that short bridge chemisorption occurs only after the occupation of the on-top site with oxygen, are strongly supportive of the Tabe [4] models and confirm the thoughts of Ibach et al. [l]. To further consider the behavior of chemisorbed oxygen, we let the short bridge oxygen atom diffuse into the bulk of silicon and calculated the associated total energy. The resulting potential energy curve displays a 2.62 eV barrier and then reduces in magnitude as the oxygen moves below the surface (see fig. 3), until finally the oxygen becomes bound beneath the surface to the on-top silicon atom. The diffusion is limited to certain paths as shown in step 3 of fig. 4. At this stage the oxygen bonds to silicon forming an SiO, film in the manner proposed by Tabe et al. [4]. The ASED-MO approach has thus obtained a three step picture for the oxidation of silicon (as shown in fig. 4)
. ----k
X.M. Zheng, P.L. Cao / Oxygen chemisorption
------
on Si(ll I)
ls49
Surface
---
Fig. 4. Three steps of the silicon oxidation process: (1) molecular oxygen chemisorption on-top; (2) dissociated oxygen, one sited on-top, and the other at the short bridge site; (3) short bridge oxygen atom diffusion into the bulk of silicon; (- - - - - -) the diffusion paths.
which is both consistent with the three steps proposed by Ibach [l], and in good agreement with the proposal of Tabe et al. [4]. 4. Conclusion The chemisorption of oxygen on the Si(ll1) surface can be divided into three stages. The first step involves the adsorption of molecular oxygen as a peroxy vertical structure at the on-top positions. The second step involves oxygen dissociation with one atom occupying the on-top position and the other atom being adsorbed at the short bridge site. The third step consists of oxygen diffusion into the bulk of silicon to form an SiO, film. At certain temperatures, different oxygen states can coexist.
Acknowledgments The authors wish to thank Professor R.J. MacDonald, Professor P.V. Smith and Dr. B.I. Craig for critical reading of the manuscript and helpful discussions. One of us (X.M. Zheng) is grateful for the award of a University of Newcastle Postgraduate Research Scholarship. References [l] H. Ibach, H.D. Rruchmann and H. Wager, Appl. Phys. A 29 (1982) 113. [2] G. Hollinger and F.J. Himpsel, Phys. Rev. B 28 (1983) 3651.
L550 [3] [4] [5] [6]
[7] [8]
[9]
[lo]
X.M. Zheng,
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