- Email: [email protected]
Chemisorption of H2O on the Si(111) 7 × 7 surfaces
Surface Scknce 86 (1979) 700.- 705 0 North-Holland Publishing Company
CHEMISORPTION OF Hz0 ON THE Si(ll1)
7 X 7 SURFACES
K. FUJIWARA and H. OGATA Central Research Lahoratoq’,
Mitsubishi Electn’c Corporation, Amagasaki, H_vogo 601, Japarl
Manuscript received in final form 11 December
1978
C‘hemisorption of 1120 on the thermally clcancd Si( I 1 1) 7 X 7 surfaces has been studied by ultraviolet photoemission, energ loss and Auger electron spcctroscopics. We provide direct experimental cvidcncc that. at room temperature. the molecules non-dissociatively adsorb in ;I single state. It is also found that. with increasing annealing temperature up to -1500 K. tile reaction proccsses proceed in three steps: molecular adsorption. dissociation and hydrogen desnrption, and osypen desorption.
1. Introduction In this paper.
we have investigated
Si(l 1 1) 7 X 7 surface
at room
Hz0 chemisorption
temperature
and
on the thermally
on the surface
annealed
cleaned up to
-1500 K in order to understand the initial stage of the interaction of the molecule with the semiconductor surfaces: chemisorption processes and the geometrical and electronic structures of the adsorbates. Ultraviolet
photoemission
spectroscopy
(UPS)
and
energy
loss spectroscopy
(ELS) have been employed to study the surface electronic states. Auger electron spectroscopy (AES) has been applied for elemental analysis to identify a clean surface.
the amount
of adsorbates
and the residual
after annealing.
2. Experimental The ELS and UPS data were obtained
in two sets of independent
UHV apparatus.
For the ELS-AES measurements, the electron spectrometer composed of two 127” type cylindrical analyzers was used [ 11. The energy loss spectra were recorded in the second-derivative mode with a constant resolution of -0.8 eV. The UPS measurements were made at photon energy Izu = 2 1.2 eV. using He 1 radiation from a differentially pumped He dc resonance lamp. Kinetic energy distribution curves (EDC) of the photoelectrons were obtained with a spherical sector type analyzetand pulse counting techniques were referred to the vacuum
[?I. In this paper. level. Conventional
all energies in the UPS spectra AES measurements were also
K. Fujiwara, H. Ogata / Chemisorption
of Hz0 on Si
701
performed in situ with the same analyzers. A base pressure of the UHV chambers was below 3 X lo-” Torr (4 X lo-’ Pa). An atomically clean surface was prepared with heating the optically flat p-type (15 R cm, boron doped) Si( 111) sample (2.5 X 11 X 0.15 mm3) to about 1500 K in UHV. For the adsorption studies, deionized water was introduced into the UHV chamber through the bakeable leak valve. The purification of deionized water was followed by the same method described previously [I 1.
3. Results 3.1. Chemisorption
at room temperature
When a clean surface was exposed to gas molecules at room temperature, the amount of adsorbates was measured by Auger measurements. The coverage has been observed to saturate with the Hz0 exposures of -1 O* L (1 L = lO-‘j Torr set). With increasing the adsorbate coverage, the characteristic loss peaks due to the intrinsic surface state transitions of the Si(ll1) 7 X 7 surface [3] are diminished and new peaks appear at 3.7, 4.7 and 8.7 eV [ 1,4]. These loss peak amplitudes saturate with the exposure corresponding to the AES results. The peak positions of the characteristic loss peaks are independent of the coverage and the primary electron energies (Ep = SO-200 eV) within the experimental errors (50.3 eV) [ 1,4]. Fig. 1 shows the UPS results for the Hz0 adsorption on the Si(ll1) 7 X 7 surface. With increasing the exposures, the emission intensity near the top of the valence band decreases, while new peaks appear within the valence band. This means the compensation of the dangling-bond surface states and the development of the chemisorption state. The band bending effect is negligibly small in the present case because the flat-band condition is satisfied [S] The structural changes in the UPS spectra seem to saturate at the exposure of -lo2 L also in agreement with the AES and ELS results. At the saturation coverage, peaks are observed at 10.1, 12.0 and 13.6 eV relative to the vacuum level, while the work function change A@ = -0.4 eV is obtained from the displacement of the low-energy cutoff of the EDC. No coverage dependent shifts are observed of the peaks within the experimental error (+O.:! eV), when the spectra are aligned relative to the Fermi level -FF, assuming the work function @= 4.7 eV for the Si(ll1) 7 X 7 surface (61. 3.2. Annealing ejyects After revealed recovery peak at that for
annealing the H20-covered Si(ll1) surface, the chemisorpLlon state has a substantial change in the binding state. The ELS results show the gradual of the intrinsic surface state transitions, while the adsorbate-indui.: 1 loss 8.7 eV blurs [I]. At about 1100 K, the loss spectrum becomes sim ‘ar to the oxygen-covered surface [ I]. This spectrum is interpreted to correspond
_. mL 4;$ 5’
N (El
NORMALIZED
( ARBITRARY
AUGER
SCALE
INTENSITY
1
. .
j i
,I O
704
K. Fujiwara, H. 0gata / Chemisorption -I P (a)
71
1
1 T
I
Hz0
SI ( Ill i 7x7 hv=212eV
I
He0
EF
A
(b)
i
7-r
of 1120 on Si
I
A$
= -0.4e‘J
P- Rshlft
=4
8e‘J
II
E - EVAC
fe!J)
Fig. 3. Photoemission spectrum N(C‘) at hu = ?I .Z eV for clean and 1120 chcmisorbed surface (a) and difference spectrum (b) as determined from direct subtraction of the ,$‘(A’) spectra of the clean and Hz0 chemisorbed surface. I-or comparison, the gaseous ionization potentials ot 1120 are also shown in (c).
the difference spectrum for the Hz0 chemisorption in order to clarify the structural changes in the UT’S spectrum. The assignment of the three peaks is also shown in fig. 3. The energy shifts of the ionization potentials arc consistently explained, as discussed following, with the structural model that the molecule is oriented with the oxygen atom towards the surface. When the molecule sits on the Si( 1I I ) surface, the lb2 orbital is least overlapped with the dangling sp” orbital. This is because the 1b2 (a,) molecular orbital consists of oxygen P,~ and hydrogen antibonding orbitals and the wave function of the orbital is farthest from the surface in
K. Fujiwarn, II. Ogata / Chemisorption
of Hz0 on Si
705
the above structural model. Therefore, we align the lowest peak at 13.6 eV to the least perturbed 1b2 orbital, assuming a uniform polarization-relaxation (P-R) shift of 4.8 eV. Then, remaining differences for two peaks are attributed to the specific bonding effects (shift of -2.5 eV). The most prominent peak at 10.1 eV is assigned to the oxygen-lone pair lb, (n,) orbital. because the peak intensity of the lb1 orbital energy level in the gas phase spectrum is strongest for a photon energy of 31.3 eV. The peak at 12.0 eV is attributed to the ?a1 (uZ) orbital. The assignment of the two peaks is consistent with the structural model, because the wave-function overlap with the dangling sp3 orbital is better for the 2a, and 1b 1 orbitals.
5. Conclusion From ELS-AES and UPS-AES measurements, we have investigated the chemisorption of HZ0 on the Si(ll1) 7 X 7 surface. We provide experimental evidence that the molecules are nondissociatively adsorbed on the Si( 111) 7 X 7 surface in a single state at room temperature. The adsorbate-induced UPS peaks associated with the molecular orbitals (1 br , 2a, and 1b2) are interpreted with the structural model that the Hz0 molecule binds to the dangling sp3 orbital of the Si( 11 I) surface via its lb1 and 2a, orbitals as a result of rehybridization. With increasing annealing temperature up to -1500 K in UHV, the chemisorption processes proceed in three steps: molecular adsorption, dissociation and hydrogen desorption, and oxygen desorption.
References K. Fujiwara and H. Ogata, J. Appl. Phys. 48 (1977) 4360. T. Murotani, K. Fujiwara and M. Nishijima, Phys. Rev. 812 (1975) 2424. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. K. I-ujiwara, H. Ogata and M. Nishijima, Solid State Commun. 21 (1977) 895. The bulk Fermi level position (Ep - EJ)b = -0.26 eV for p-type Si (doped with boron atoms/cma) almost coincides with the surface potential pinned at (EP -0.24 eV for the Si( 111) 7 X 7 surface which is determined, for example. by P.P. W. Month, in: Proc. 2nd Intern. Conf. on Solid Surfaces, Kyoto, 1974. 161 J.E. Rowe and H. Ibach, Phys. Rev. Letters 32 (1974) 421. [7] H. Ibach and J.E. Rowe, Phys. Rev. RIO (1974) 710. [S] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron scopy (Wiley-Interscience, New York, 1970).
[ l] [2] [3] [4] [5]
1 X 10’ 5 -- Et)S = Auer and
Spectro-
CHEMISORPTION OF Hz0 ON THE Si(ll1)
7 X 7 SURFACES
K. FUJIWARA and H. OGATA Central Research Lahoratoq’,
Mitsubishi Electn’c Corporation, Amagasaki, H_vogo 601, Japarl
Manuscript received in final form 11 December
1978
C‘hemisorption of 1120 on the thermally clcancd Si( I 1 1) 7 X 7 surfaces has been studied by ultraviolet photoemission, energ loss and Auger electron spcctroscopics. We provide direct experimental cvidcncc that. at room temperature. the molecules non-dissociatively adsorb in ;I single state. It is also found that. with increasing annealing temperature up to -1500 K. tile reaction proccsses proceed in three steps: molecular adsorption. dissociation and hydrogen desnrption, and osypen desorption.
1. Introduction In this paper.
we have investigated
Si(l 1 1) 7 X 7 surface
at room
Hz0 chemisorption
temperature
and
on the thermally
on the surface
annealed
cleaned up to
-1500 K in order to understand the initial stage of the interaction of the molecule with the semiconductor surfaces: chemisorption processes and the geometrical and electronic structures of the adsorbates. Ultraviolet
photoemission
spectroscopy
(UPS)
and
energy
loss spectroscopy
(ELS) have been employed to study the surface electronic states. Auger electron spectroscopy (AES) has been applied for elemental analysis to identify a clean surface.
the amount
of adsorbates
and the residual
after annealing.
2. Experimental The ELS and UPS data were obtained
in two sets of independent
UHV apparatus.
For the ELS-AES measurements, the electron spectrometer composed of two 127” type cylindrical analyzers was used [ 11. The energy loss spectra were recorded in the second-derivative mode with a constant resolution of -0.8 eV. The UPS measurements were made at photon energy Izu = 2 1.2 eV. using He 1 radiation from a differentially pumped He dc resonance lamp. Kinetic energy distribution curves (EDC) of the photoelectrons were obtained with a spherical sector type analyzetand pulse counting techniques were referred to the vacuum
[?I. In this paper. level. Conventional
all energies in the UPS spectra AES measurements were also
K. Fujiwara, H. Ogata / Chemisorption
of Hz0 on Si
701
performed in situ with the same analyzers. A base pressure of the UHV chambers was below 3 X lo-” Torr (4 X lo-’ Pa). An atomically clean surface was prepared with heating the optically flat p-type (15 R cm, boron doped) Si( 111) sample (2.5 X 11 X 0.15 mm3) to about 1500 K in UHV. For the adsorption studies, deionized water was introduced into the UHV chamber through the bakeable leak valve. The purification of deionized water was followed by the same method described previously [I 1.
3. Results 3.1. Chemisorption
at room temperature
When a clean surface was exposed to gas molecules at room temperature, the amount of adsorbates was measured by Auger measurements. The coverage has been observed to saturate with the Hz0 exposures of -1 O* L (1 L = lO-‘j Torr set). With increasing the adsorbate coverage, the characteristic loss peaks due to the intrinsic surface state transitions of the Si(ll1) 7 X 7 surface [3] are diminished and new peaks appear at 3.7, 4.7 and 8.7 eV [ 1,4]. These loss peak amplitudes saturate with the exposure corresponding to the AES results. The peak positions of the characteristic loss peaks are independent of the coverage and the primary electron energies (Ep = SO-200 eV) within the experimental errors (50.3 eV) [ 1,4]. Fig. 1 shows the UPS results for the Hz0 adsorption on the Si(ll1) 7 X 7 surface. With increasing the exposures, the emission intensity near the top of the valence band decreases, while new peaks appear within the valence band. This means the compensation of the dangling-bond surface states and the development of the chemisorption state. The band bending effect is negligibly small in the present case because the flat-band condition is satisfied [S] The structural changes in the UPS spectra seem to saturate at the exposure of -lo2 L also in agreement with the AES and ELS results. At the saturation coverage, peaks are observed at 10.1, 12.0 and 13.6 eV relative to the vacuum level, while the work function change A@ = -0.4 eV is obtained from the displacement of the low-energy cutoff of the EDC. No coverage dependent shifts are observed of the peaks within the experimental error (+O.:! eV), when the spectra are aligned relative to the Fermi level -FF, assuming the work function @= 4.7 eV for the Si(ll1) 7 X 7 surface (61. 3.2. Annealing ejyects After revealed recovery peak at that for
annealing the H20-covered Si(ll1) surface, the chemisorpLlon state has a substantial change in the binding state. The ELS results show the gradual of the intrinsic surface state transitions, while the adsorbate-indui.: 1 loss 8.7 eV blurs [I]. At about 1100 K, the loss spectrum becomes sim ‘ar to the oxygen-covered surface [ I]. This spectrum is interpreted to correspond
_. mL 4;$ 5’
N (El
NORMALIZED
( ARBITRARY
AUGER
SCALE
INTENSITY
1
. .
j i
,I O
704
K. Fujiwara, H. 0gata / Chemisorption -I P (a)
71
1
1 T
I
Hz0
SI ( Ill i 7x7 hv=212eV
I
He0
EF
A
(b)
i
7-r
of 1120 on Si
I
A$
= -0.4e‘J
P- Rshlft
=4
8e‘J
II
E - EVAC
fe!J)
Fig. 3. Photoemission spectrum N(C‘) at hu = ?I .Z eV for clean and 1120 chcmisorbed surface (a) and difference spectrum (b) as determined from direct subtraction of the ,$‘(A’) spectra of the clean and Hz0 chemisorbed surface. I-or comparison, the gaseous ionization potentials ot 1120 are also shown in (c).
the difference spectrum for the Hz0 chemisorption in order to clarify the structural changes in the UT’S spectrum. The assignment of the three peaks is also shown in fig. 3. The energy shifts of the ionization potentials arc consistently explained, as discussed following, with the structural model that the molecule is oriented with the oxygen atom towards the surface. When the molecule sits on the Si( 1I I ) surface, the lb2 orbital is least overlapped with the dangling sp” orbital. This is because the 1b2 (a,) molecular orbital consists of oxygen P,~ and hydrogen antibonding orbitals and the wave function of the orbital is farthest from the surface in
K. Fujiwarn, II. Ogata / Chemisorption
of Hz0 on Si
705
the above structural model. Therefore, we align the lowest peak at 13.6 eV to the least perturbed 1b2 orbital, assuming a uniform polarization-relaxation (P-R) shift of 4.8 eV. Then, remaining differences for two peaks are attributed to the specific bonding effects (shift of -2.5 eV). The most prominent peak at 10.1 eV is assigned to the oxygen-lone pair lb, (n,) orbital. because the peak intensity of the lb1 orbital energy level in the gas phase spectrum is strongest for a photon energy of 31.3 eV. The peak at 12.0 eV is attributed to the ?a1 (uZ) orbital. The assignment of the two peaks is consistent with the structural model, because the wave-function overlap with the dangling sp3 orbital is better for the 2a, and 1b 1 orbitals.
5. Conclusion From ELS-AES and UPS-AES measurements, we have investigated the chemisorption of HZ0 on the Si(ll1) 7 X 7 surface. We provide experimental evidence that the molecules are nondissociatively adsorbed on the Si( 111) 7 X 7 surface in a single state at room temperature. The adsorbate-induced UPS peaks associated with the molecular orbitals (1 br , 2a, and 1b2) are interpreted with the structural model that the Hz0 molecule binds to the dangling sp3 orbital of the Si( 11 I) surface via its lb1 and 2a, orbitals as a result of rehybridization. With increasing annealing temperature up to -1500 K in UHV, the chemisorption processes proceed in three steps: molecular adsorption, dissociation and hydrogen desorption, and oxygen desorption.
References K. Fujiwara and H. Ogata, J. Appl. Phys. 48 (1977) 4360. T. Murotani, K. Fujiwara and M. Nishijima, Phys. Rev. 812 (1975) 2424. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. K. I-ujiwara, H. Ogata and M. Nishijima, Solid State Commun. 21 (1977) 895. The bulk Fermi level position (Ep - EJ)b = -0.26 eV for p-type Si (doped with boron atoms/cma) almost coincides with the surface potential pinned at (EP -0.24 eV for the Si( 111) 7 X 7 surface which is determined, for example. by P.P. W. Month, in: Proc. 2nd Intern. Conf. on Solid Surfaces, Kyoto, 1974. 161 J.E. Rowe and H. Ibach, Phys. Rev. Letters 32 (1974) 421. [7] H. Ibach and J.E. Rowe, Phys. Rev. RIO (1974) 710. [S] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron scopy (Wiley-Interscience, New York, 1970).
[ l] [2] [3] [4] [5]
1 X 10’ 5 -- Et)S = Auer and
Spectro-