- Email: [email protected]
Initial oxidation of H-terminated Si(111) surfaces studied by HREELS
applied
surface science ELSEVIER
AppliedSurfaceScience
117/118(1997) 109-113
Initial oxidation of H-terminated Si( 111) surfaces studied by HREELS H. Ikeda *, Y. Nakagawa, M. Toshima, S. Furuta, S. Zaima, Y. Yasuda Department of Crystalline Materials Science, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Abstract We have investigated the initial oxidation process and the local bonding structure of Si-0-Si bonds of H-terminated Si(1 11)-l X 1 surfaces using high-resolution electron energy loss spectroscopy (HREELS) below an oxygen coverage of 2.5 ML. Oxygen atoms randomly adsorb on the sites between surface and subsurface Si atoms at room temperature in this oxidation coverage. The vibrational energy of the Si-0-Si asymmetric stretching mode increases monotonously with increasing the number of adsorbed 0 atoms in contrast with the case of Si( 100)~(1 X 1)H. The relaxation of Si-0-Si structures is promoted by the existence of Si-H bonds. Keywords:
SiO,/Si
interface;
Initial oxidation;
H-terminated
Si surface; HREELS;
1. Introduction With a reduction of device dimensions in ultralarge scale integrated circuits (ULSI’s), deviations from an ideal SiOJSi interface, such as defects and roughness, become a serious factor governing device characteristics. In order to realize a well-defined SiOJSi interface, it is essential to control the oxidation of Si surfaces on an atomic scale. The Htermination of Si surfaces has been expected to control the formation of native oxide [ 1,2] and the atomistic structures at the SiOJSi interface. Since an atomically flat Si(ll1) surface has been realized by a H-termination treatment in an NH,F solution [3], surface reactions with oxygen on the
* Corresponding author. [email protected].
Fax:
0169-4332/97/$17.00 Copyright PII SO169-4332(97)00021-4
+ 81-52-7893818;
e-mail:
Local bonding
structure of SiO,
Si(ll1) surface have been actively studied. It was reported that the oxidation of H-terminated Si(ll1) surfaces proceeds non-uniformly [4]. Moreover, layer-by-layer oxidation locally occurs [4]. However, the oxidation process and the SiO, bonding structure have not been sufficiently clarified yet. We have investigated the initial oxidation process of H-terminated Si(100) surfaces [5-S] and found the two-step oxidation process, which are closely related with the structural relaxation of Si-0-Si bonds [7,8]. In the present work, we report the initial oxidation process and the local bonding structure of Si-0-Si bonds on H-terminated Sic11 l)- 1 X 1 surfaces at room temperature using high-resolution electron energy loss spectroscopy (HREELS). The effect of H atoms on the oxidation process is discussed by comparing with the case of H-terminated Si(100) surfaces.
0 1997 Elsevier Science B.V. All rights reserved.
110
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Surface Science 117/118
2. Experiments Experiments in the present study were carried out in an ultra-high vacuum (UHV) chamber with a base pressure of 3 X lo- ” Torr, equipped with HREELS, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) apparatus. H-terminated Si(ll1) surfaces were obtained by treating p-type Si(ll1) substrates with dilute NH,F (40%). The contamination of C atoms on the Hterminated Si(ll1) surface was confirmed by AES measurements to be below the detection limit. Atomic oxygen was produced by thermal cracking of 0, using a tungsten filament heated at 1500°C. The amount of 0 exposure is indicated by using the partial pressure of O,, since the exact fraction of 0 atoms dissociated by this method could not be determined. The 0 coverage is defined as the ratio of the number of adsorbed 0 atoms to that of surface Si atoms by AES. HREELS measurements were performed at room temperature under a specular reflection condition, in which the ingoing and outgoing angles of electron beams were 55” with respect to the surface normal. The incident energy of electron beams was 7.8 eV.
3. Results and discussion Fig. 1 shows the HREELS spectra of H-terminated Si(ll1) surfaces before and after exposing to 0 atoms at room temperature. Energy loss peaks of Si-H bending (vu, ) and stretching (vuz) modes are observed at 78 and 256 meV in the spectrum of the as-terminated Si(ll1) surface, respectively [9,10]. No rocking and scissors modes of Si-2H or Si-3H species can be detected in this experiment, whose loss energies are reported to be about 61 and 112 meV, respectively. It was confirmed by a LEED observation that this surface has a 1 X 1 structure. It can be concluded from these facts that a monohydride structure is formed on the H-terminated Sic1 11) surface. The Si-H bending mode is not observable according to the dipole selection rule if Si-H bonds are perpendicular to the surface [I 11. However, we confirmed by HREELS measurements under the off-specular condition that the peak at 78 meV arises from dipole scattering. A possible explanation of the
(1997) 109-113 I
I
I
I
I
Room temperature
x20
vo2
VOl
x200
0 20000L I'
0
100
200
300
Energy loss (meV) Fig. 1. HREELS spectra of H-terminated Si(ll1) surfaces before and after exposing to atomic oxygen at room temperature. The 0 exposure shown here is that for 0,.
existence of the Si-H bending mode is a tilt of Si-H bonds from the direction perpendicular to the surface due to repulsion between Si-H bonds. In addition, it is found from Fig. 1 that the loss peaks of the stretching mode (vOH > of Si-OH [ 121 and the symmetric bending (vol > and asymmetric stretching (vo2) mode of Si-0-Si [13] appear after exposing the surface to atomic oxygen. These findings indicate that 0 atoms adsorb on the Si-Si and Si-H bond sites at room temperature, as well as the case of H-terminated Si(100) surfaces [5,7,8]. The coverage of Si-OH species was less than 0.1, which was determined from the peak intensity of HREELS spectra. Therefore, most of the 0 atoms initially adsorb on back bonds of surface Si atoms. The energy loss peak of the Si-H stretching mode ( vn2 > splits into three peaks with increasing 0 exposure. The vibrational energy of the Si-H stretching mode changes with the number of adjacently bound 0 atoms [14]. The experimental vibrational energies of Si-H, 0-Si-H, 20-Si-H and 30-Si-H have been reported to be 255-260, 262, 279 and 283
H. Ikeda et al/Applied
Surface Science 117/118
meV, respectively [14-161. The loss peak of the Si-H stretching mode observed in Fig. 1 splits into three peaks with loss energies of 255-256, 263-268 and 277-278 meV. Therefore, it is considered that these peaks correspond to the Si-H vibrations of Si-H, 0-Si-H, and 20-Si-H and 30-Si-H bonding structures, respectively. A further peak splitting could not be achieved because of the resolution limit of our HREELS measurements. Fig. 2 shows the loss peak intensities of the Si-H stretching mode for n = 0, 1, and 2 and 3 of noSi-H at room temperature as a function of oxygen coverage. In this figure, the peak intensities are normalized by the total peak intensity of Si-H stretching modes, which is confirmed not to change after exposing the surface to atomic oxygen. Three curves shown in Fig. 2 indicate the calculated ratios of the no-Si-H species under the condition of random adsorption of 0 atoms. The experimental data are in good agreement with the calculated results, which indicates that the 0 atoms adsorb randomly on Si-Si bonds, in contrast with the case of H-terminated Si(100) surfaces [7,8]. The coverage of 0 atoms bonding with surface Si atoms can be obtained from the experimental results in Fig. 2. The 0 coverage obtained from HREELS spectra is shown in Fig. 3 as a function of the 0 exposure, in which the minimum and maximum values of the error bars correspond to n = 2 and n = 3, respectively. The 0 coverage evaluated from AES measurements is also shown in Fig. 3. It should I
100
I ,I.
__... __-.
no-Si-H stretching mode . n=o
/ 0
__..
0
__,. ^
.I’ I
1 2 Oxygen coverage (ML)
-_
3
Fig. 2. Loss peak intensities of the Si-H stretching mode for Si-H (n = O), 0-Si-H (n = 1). and 20-Si-H and 30-Si-H (n = 2, 3) bonding structures at room temperature as a function of oxygen coverage. The peak intensities are normalized by the total peak intensity of the Si-H bonds.
111
(1997) 109-113
3.0 2.5 2 5. g
2.0
e 8 P
1.5
5 CR 1.0 E 0 0.5 0 0
1
2
3
4
5
6
Oxygen exposure (~10~ L)
Fig. 3. Oxygen coverage obtained from HREELS and AES measurements as a function of the 0 exposure at room temperature. The coverage from HREELS is calculated from data in Fig. 2. The 0, exposure is used instead of that of 0 atoms.
be noted that the coverage from HREELS is very close to that estimated from the AES results at 0 coverages below 2.5 ML. The 0 coverage obtained from Si-H signals in HREELS corresponds to the 0 atoms adsorbed on the site between surface and subsurface Si atoms. Consequently, the oxidation of H-terminated Sic1 11) surfaces hardly proceeds into the substrate at room temperature below 2.5 ML. Hattori et al. [4] reported that the oxidation progresses also into deeper layers even at 0.5 ML on H-terminated Sic1 11) surfaces at temperatures above 300°C in dry 0, at a pressure of 1 Torr. Therefore, it is considered that the uptake of 0 atoms into the Si substrate depends on the oxidation temperature and the gas pressure. It should be also noted in Fig. 1 that the energy loss peak of the Si-0-Si asymmetric stretching mode (uo2) shifts to higher energies with increasing atomic oxygen exposure. Fig. 4 shows the changes in the loss energy of the vo2 mode as a function of the ratio of the number of adsorbed 0 atoms to that of Si back bonds for H-terminated Si(l l l)-1 X 1 surfaces at room temperature, in comparison with Hterminated Si(lOO)-1 X 1 surfaces and Si(lOO)-2 X 1 clean surfaces [7,8]. It is found that the vibrational energy on the H-terminated Sic11 1) surface monotonously shifts to higher energies with increasing 0 atom adsorption. This phenomenon is similar to the oxidation process on the Si(100) clean surface rather than that on the H-terminated Si(100) surface. As we reported previously [7,8], a higher vibrational
H. Ikeda et al/Applied
112
115
0
0.2
0.4
0.6
0.6
1.0
1.2
Surface Science 117/118
1.4
Ratio of the number of adsorbed 0 atoms to that oi Si back bonds
Fig. 4. Changes in the loss energy of the Si-0-Si asymmetric stretching mode as a function of the ratio of the number of adsorbed 0 atoms to that of Si back bonds for H-terminated Si(lll)-1 X 1 surfaces at room temperature. The results for Hterminated Si(lOO)-1 X 1 surfaces and Si(lOO)-2 X 1 clean surfaces are also shown [7,8].
energy of the vo2 peak corresponds to a more relaxed Si-0-Si structure and a monotonous increase in the Si-0-Si vibrational energy observed on a Si(100) clean surface is due to the random adsorption of 0 atoms. Therefore, it is suggested that the 0 adsorption accompanied with the relaxation of the Si-0-Si structure occurs randomly on the Hterminated Si(ll1) surface. This conclusion is consistent with the result obtained from Fig. 2. The energy losses of the vo2 mode on both Sic1 11) and Si(100) surfaces are almost the same at 0 coverages on Si back bonds above 0.4. The degree of the Si-0-Si relaxation seems to be governed by the adsorption ratio of 0 atoms to back bond sites rather than the number of adsorbed 0 atoms, since the (111) surface density of Si atoms is 1.15 times larger than (100) surface density. Moreover, the Si-0-Si structure is more relaxed on H-terminated Si(100) covered with Si-2H species than Hterminated Si(ll1) with Si-H species, which indicates that the Si-0-Si relaxation is promoted by the existence of Si-H bonds [7,8].
4. Conclusions The Si(ll1)
initial oxidation surfaces has
process of H-terminated been investigated using
11997) 109-113
HREELS. Oxygen atoms adsorb randomly on Si-Si bonds on the H-terminated Si(ll1) surface at room temperature, in contrast with the case of H-terminated Si(100) surfaces which have a two-step process in the initial stage. The coverage of 0 atoms between surface and subsurface Si atoms is very close to that of total adsorbed 0 atoms below an 0 coverage of 2.5 ML, which indicates that the initial oxidation occurs mostly on the Si-Si bonds nearest the surface on H-terminated Si(ll1) surfaces at room temperature. The change in the vibrational energy of the Si-0-Si asymmetric stretching mode is a monotonous increase with the 0 adsorption. This tendency is also observed in the oxidation of a Si(100) clean surface. It is concluded that the random adsorption of 0 atoms takes place with the structural relaxation of Si-0-Si bonds on Hterminated Si(ll1) surfaces. The relaxation of Si0-Si structures seems to be dominated by the adsorption ratio of 0 atoms to back bond sites. Si-H bonds have an effect of promoting the Si-0-Si relaxation.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 08455019) from the Ministry of Education, Science and Culture of Japan.
References [l] T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, Y. Nagasawa, .I. Appl. Phys. 64 (1988) 3516. [2] N. Hirashita, M. Kinoshita, I. Aikawa, T. Ajioka, Appl. Phys. Lett. 56 (1990) 451. [3] G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, Appl. Phys. Lett. 56 (1990) 656. [4] T. Hattori, T. Aiba, E. Iijima, H. Nohira, N. Tate, M. Katayama, Ext. Abst. of 5th Int. Conf. on the Formation of Semiconductor Interfaces, 1995, p. 40. [5] H. Ikeda, K. Hotta, T. Yamada, S. Zaima, H. Iwano, Y. Yasuda, J. Appl. Phys. 77 (1995) 5125. [6] H. Ikeda, K. Hotta, T. Yamada, S. Zaima, Y. Yasuda, Jpn. J. Appl. Phys. 34 (1995) 2191. [7] H. Ikeda, K. Hotta, S. Furuta, S. Zaima, Y. Yasuda, Appl. Surf. Sci., in press. [8] H. Ikeda, K. Hotta, S. Furuta, S. Zaima, Y. Yasuda, Jpn. J. Appl. Phys. 35 (1996) 1069.
H. Ikeda et al./Applied
Su$ace Science 117/118
[9] W.B. Pollard, G. Lucovsky, Phys. Rev. B 26 (1982) 3172. [lo] H. Kobayashi, K. Edamoto, M. Onchi, M. Nishijima, J. Chem. Phys. 78 (1983) 7429. 1111 H. Jbach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic, New York, 1982, ch. 4. 1121 H. Ibach, H. Wagner, D. Bruchmann, Solid State Commun. 42 (1982) 457.
(1997) 109-113
113
[13] J.A. Schaefer, W. Gopel, Surf. Sci. 155 (1985) 535. 1141 J.A. Schaefer, D. Frankel, F. Stucki, W. GGpeJ, G. Lapeyre, Surf. Sci. 139 (1984) L209. 1151 F. Stucki, J.A. Schaefer, J.R. Anderson, G.J. Lapeyre, W. Giipel, Solid State Commun. 47 (1983) 795. [16] H. Kobayashi, K. Edamoto, M. Onchi, M. Nishijima, J. Chem. Phys. 78 (1983) 7429.
surface science ELSEVIER
AppliedSurfaceScience
117/118(1997) 109-113
Initial oxidation of H-terminated Si( 111) surfaces studied by HREELS H. Ikeda *, Y. Nakagawa, M. Toshima, S. Furuta, S. Zaima, Y. Yasuda Department of Crystalline Materials Science, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Abstract We have investigated the initial oxidation process and the local bonding structure of Si-0-Si bonds of H-terminated Si(1 11)-l X 1 surfaces using high-resolution electron energy loss spectroscopy (HREELS) below an oxygen coverage of 2.5 ML. Oxygen atoms randomly adsorb on the sites between surface and subsurface Si atoms at room temperature in this oxidation coverage. The vibrational energy of the Si-0-Si asymmetric stretching mode increases monotonously with increasing the number of adsorbed 0 atoms in contrast with the case of Si( 100)~(1 X 1)H. The relaxation of Si-0-Si structures is promoted by the existence of Si-H bonds. Keywords:
SiO,/Si
interface;
Initial oxidation;
H-terminated
Si surface; HREELS;
1. Introduction With a reduction of device dimensions in ultralarge scale integrated circuits (ULSI’s), deviations from an ideal SiOJSi interface, such as defects and roughness, become a serious factor governing device characteristics. In order to realize a well-defined SiOJSi interface, it is essential to control the oxidation of Si surfaces on an atomic scale. The Htermination of Si surfaces has been expected to control the formation of native oxide [ 1,2] and the atomistic structures at the SiOJSi interface. Since an atomically flat Si(ll1) surface has been realized by a H-termination treatment in an NH,F solution [3], surface reactions with oxygen on the
* Corresponding author. [email protected].
Fax:
0169-4332/97/$17.00 Copyright PII SO169-4332(97)00021-4
+ 81-52-7893818;
e-mail:
Local bonding
structure of SiO,
Si(ll1) surface have been actively studied. It was reported that the oxidation of H-terminated Si(ll1) surfaces proceeds non-uniformly [4]. Moreover, layer-by-layer oxidation locally occurs [4]. However, the oxidation process and the SiO, bonding structure have not been sufficiently clarified yet. We have investigated the initial oxidation process of H-terminated Si(100) surfaces [5-S] and found the two-step oxidation process, which are closely related with the structural relaxation of Si-0-Si bonds [7,8]. In the present work, we report the initial oxidation process and the local bonding structure of Si-0-Si bonds on H-terminated Sic11 l)- 1 X 1 surfaces at room temperature using high-resolution electron energy loss spectroscopy (HREELS). The effect of H atoms on the oxidation process is discussed by comparing with the case of H-terminated Si(100) surfaces.
0 1997 Elsevier Science B.V. All rights reserved.
110
H. Ikeda et al./Applied
Surface Science 117/118
2. Experiments Experiments in the present study were carried out in an ultra-high vacuum (UHV) chamber with a base pressure of 3 X lo- ” Torr, equipped with HREELS, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) apparatus. H-terminated Si(ll1) surfaces were obtained by treating p-type Si(ll1) substrates with dilute NH,F (40%). The contamination of C atoms on the Hterminated Si(ll1) surface was confirmed by AES measurements to be below the detection limit. Atomic oxygen was produced by thermal cracking of 0, using a tungsten filament heated at 1500°C. The amount of 0 exposure is indicated by using the partial pressure of O,, since the exact fraction of 0 atoms dissociated by this method could not be determined. The 0 coverage is defined as the ratio of the number of adsorbed 0 atoms to that of surface Si atoms by AES. HREELS measurements were performed at room temperature under a specular reflection condition, in which the ingoing and outgoing angles of electron beams were 55” with respect to the surface normal. The incident energy of electron beams was 7.8 eV.
3. Results and discussion Fig. 1 shows the HREELS spectra of H-terminated Si(ll1) surfaces before and after exposing to 0 atoms at room temperature. Energy loss peaks of Si-H bending (vu, ) and stretching (vuz) modes are observed at 78 and 256 meV in the spectrum of the as-terminated Si(ll1) surface, respectively [9,10]. No rocking and scissors modes of Si-2H or Si-3H species can be detected in this experiment, whose loss energies are reported to be about 61 and 112 meV, respectively. It was confirmed by a LEED observation that this surface has a 1 X 1 structure. It can be concluded from these facts that a monohydride structure is formed on the H-terminated Sic1 11) surface. The Si-H bending mode is not observable according to the dipole selection rule if Si-H bonds are perpendicular to the surface [I 11. However, we confirmed by HREELS measurements under the off-specular condition that the peak at 78 meV arises from dipole scattering. A possible explanation of the
(1997) 109-113 I
I
I
I
I
Room temperature
x20
vo2
VOl
x200
0 20000L I'
0
100
200
300
Energy loss (meV) Fig. 1. HREELS spectra of H-terminated Si(ll1) surfaces before and after exposing to atomic oxygen at room temperature. The 0 exposure shown here is that for 0,.
existence of the Si-H bending mode is a tilt of Si-H bonds from the direction perpendicular to the surface due to repulsion between Si-H bonds. In addition, it is found from Fig. 1 that the loss peaks of the stretching mode (vOH > of Si-OH [ 121 and the symmetric bending (vol > and asymmetric stretching (vo2) mode of Si-0-Si [13] appear after exposing the surface to atomic oxygen. These findings indicate that 0 atoms adsorb on the Si-Si and Si-H bond sites at room temperature, as well as the case of H-terminated Si(100) surfaces [5,7,8]. The coverage of Si-OH species was less than 0.1, which was determined from the peak intensity of HREELS spectra. Therefore, most of the 0 atoms initially adsorb on back bonds of surface Si atoms. The energy loss peak of the Si-H stretching mode ( vn2 > splits into three peaks with increasing 0 exposure. The vibrational energy of the Si-H stretching mode changes with the number of adjacently bound 0 atoms [14]. The experimental vibrational energies of Si-H, 0-Si-H, 20-Si-H and 30-Si-H have been reported to be 255-260, 262, 279 and 283
H. Ikeda et al/Applied
Surface Science 117/118
meV, respectively [14-161. The loss peak of the Si-H stretching mode observed in Fig. 1 splits into three peaks with loss energies of 255-256, 263-268 and 277-278 meV. Therefore, it is considered that these peaks correspond to the Si-H vibrations of Si-H, 0-Si-H, and 20-Si-H and 30-Si-H bonding structures, respectively. A further peak splitting could not be achieved because of the resolution limit of our HREELS measurements. Fig. 2 shows the loss peak intensities of the Si-H stretching mode for n = 0, 1, and 2 and 3 of noSi-H at room temperature as a function of oxygen coverage. In this figure, the peak intensities are normalized by the total peak intensity of Si-H stretching modes, which is confirmed not to change after exposing the surface to atomic oxygen. Three curves shown in Fig. 2 indicate the calculated ratios of the no-Si-H species under the condition of random adsorption of 0 atoms. The experimental data are in good agreement with the calculated results, which indicates that the 0 atoms adsorb randomly on Si-Si bonds, in contrast with the case of H-terminated Si(100) surfaces [7,8]. The coverage of 0 atoms bonding with surface Si atoms can be obtained from the experimental results in Fig. 2. The 0 coverage obtained from HREELS spectra is shown in Fig. 3 as a function of the 0 exposure, in which the minimum and maximum values of the error bars correspond to n = 2 and n = 3, respectively. The 0 coverage evaluated from AES measurements is also shown in Fig. 3. It should I
100
I ,I.
__... __-.
no-Si-H stretching mode . n=o
/ 0
__..
0
__,. ^
.I’ I
1 2 Oxygen coverage (ML)
-_
3
Fig. 2. Loss peak intensities of the Si-H stretching mode for Si-H (n = O), 0-Si-H (n = 1). and 20-Si-H and 30-Si-H (n = 2, 3) bonding structures at room temperature as a function of oxygen coverage. The peak intensities are normalized by the total peak intensity of the Si-H bonds.
111
(1997) 109-113
3.0 2.5 2 5. g
2.0
e 8 P
1.5
5 CR 1.0 E 0 0.5 0 0
1
2
3
4
5
6
Oxygen exposure (~10~ L)
Fig. 3. Oxygen coverage obtained from HREELS and AES measurements as a function of the 0 exposure at room temperature. The coverage from HREELS is calculated from data in Fig. 2. The 0, exposure is used instead of that of 0 atoms.
be noted that the coverage from HREELS is very close to that estimated from the AES results at 0 coverages below 2.5 ML. The 0 coverage obtained from Si-H signals in HREELS corresponds to the 0 atoms adsorbed on the site between surface and subsurface Si atoms. Consequently, the oxidation of H-terminated Sic1 11) surfaces hardly proceeds into the substrate at room temperature below 2.5 ML. Hattori et al. [4] reported that the oxidation progresses also into deeper layers even at 0.5 ML on H-terminated Sic1 11) surfaces at temperatures above 300°C in dry 0, at a pressure of 1 Torr. Therefore, it is considered that the uptake of 0 atoms into the Si substrate depends on the oxidation temperature and the gas pressure. It should be also noted in Fig. 1 that the energy loss peak of the Si-0-Si asymmetric stretching mode (uo2) shifts to higher energies with increasing atomic oxygen exposure. Fig. 4 shows the changes in the loss energy of the vo2 mode as a function of the ratio of the number of adsorbed 0 atoms to that of Si back bonds for H-terminated Si(l l l)-1 X 1 surfaces at room temperature, in comparison with Hterminated Si(lOO)-1 X 1 surfaces and Si(lOO)-2 X 1 clean surfaces [7,8]. It is found that the vibrational energy on the H-terminated Sic11 1) surface monotonously shifts to higher energies with increasing 0 atom adsorption. This phenomenon is similar to the oxidation process on the Si(100) clean surface rather than that on the H-terminated Si(100) surface. As we reported previously [7,8], a higher vibrational
H. Ikeda et al/Applied
112
115
0
0.2
0.4
0.6
0.6
1.0
1.2
Surface Science 117/118
1.4
Ratio of the number of adsorbed 0 atoms to that oi Si back bonds
Fig. 4. Changes in the loss energy of the Si-0-Si asymmetric stretching mode as a function of the ratio of the number of adsorbed 0 atoms to that of Si back bonds for H-terminated Si(lll)-1 X 1 surfaces at room temperature. The results for Hterminated Si(lOO)-1 X 1 surfaces and Si(lOO)-2 X 1 clean surfaces are also shown [7,8].
energy of the vo2 peak corresponds to a more relaxed Si-0-Si structure and a monotonous increase in the Si-0-Si vibrational energy observed on a Si(100) clean surface is due to the random adsorption of 0 atoms. Therefore, it is suggested that the 0 adsorption accompanied with the relaxation of the Si-0-Si structure occurs randomly on the Hterminated Si(ll1) surface. This conclusion is consistent with the result obtained from Fig. 2. The energy losses of the vo2 mode on both Sic1 11) and Si(100) surfaces are almost the same at 0 coverages on Si back bonds above 0.4. The degree of the Si-0-Si relaxation seems to be governed by the adsorption ratio of 0 atoms to back bond sites rather than the number of adsorbed 0 atoms, since the (111) surface density of Si atoms is 1.15 times larger than (100) surface density. Moreover, the Si-0-Si structure is more relaxed on H-terminated Si(100) covered with Si-2H species than Hterminated Si(ll1) with Si-H species, which indicates that the Si-0-Si relaxation is promoted by the existence of Si-H bonds [7,8].
4. Conclusions The Si(ll1)
initial oxidation surfaces has
process of H-terminated been investigated using
11997) 109-113
HREELS. Oxygen atoms adsorb randomly on Si-Si bonds on the H-terminated Si(ll1) surface at room temperature, in contrast with the case of H-terminated Si(100) surfaces which have a two-step process in the initial stage. The coverage of 0 atoms between surface and subsurface Si atoms is very close to that of total adsorbed 0 atoms below an 0 coverage of 2.5 ML, which indicates that the initial oxidation occurs mostly on the Si-Si bonds nearest the surface on H-terminated Si(ll1) surfaces at room temperature. The change in the vibrational energy of the Si-0-Si asymmetric stretching mode is a monotonous increase with the 0 adsorption. This tendency is also observed in the oxidation of a Si(100) clean surface. It is concluded that the random adsorption of 0 atoms takes place with the structural relaxation of Si-0-Si bonds on Hterminated Si(ll1) surfaces. The relaxation of Si0-Si structures seems to be dominated by the adsorption ratio of 0 atoms to back bond sites. Si-H bonds have an effect of promoting the Si-0-Si relaxation.
Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 08455019) from the Ministry of Education, Science and Culture of Japan.
References [l] T. Takahagi, I. Nagai, A. Ishitani, H. Kuroda, Y. Nagasawa, .I. Appl. Phys. 64 (1988) 3516. [2] N. Hirashita, M. Kinoshita, I. Aikawa, T. Ajioka, Appl. Phys. Lett. 56 (1990) 451. [3] G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, Appl. Phys. Lett. 56 (1990) 656. [4] T. Hattori, T. Aiba, E. Iijima, H. Nohira, N. Tate, M. Katayama, Ext. Abst. of 5th Int. Conf. on the Formation of Semiconductor Interfaces, 1995, p. 40. [5] H. Ikeda, K. Hotta, T. Yamada, S. Zaima, H. Iwano, Y. Yasuda, J. Appl. Phys. 77 (1995) 5125. [6] H. Ikeda, K. Hotta, T. Yamada, S. Zaima, Y. Yasuda, Jpn. J. Appl. Phys. 34 (1995) 2191. [7] H. Ikeda, K. Hotta, S. Furuta, S. Zaima, Y. Yasuda, Appl. Surf. Sci., in press. [8] H. Ikeda, K. Hotta, S. Furuta, S. Zaima, Y. Yasuda, Jpn. J. Appl. Phys. 35 (1996) 1069.
H. Ikeda et al./Applied
Su$ace Science 117/118
[9] W.B. Pollard, G. Lucovsky, Phys. Rev. B 26 (1982) 3172. [lo] H. Kobayashi, K. Edamoto, M. Onchi, M. Nishijima, J. Chem. Phys. 78 (1983) 7429. 1111 H. Jbach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic, New York, 1982, ch. 4. 1121 H. Ibach, H. Wagner, D. Bruchmann, Solid State Commun. 42 (1982) 457.
(1997) 109-113
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