- Email: [email protected]
Nanometer-scale passivation of Si(111) surfaces by using the √3 × √3-Ga reconstruction
applied surface science ELSEVIER
Applied Surface Science 107 (1996) 6.10
Nanometer-scale passivation of Si(111) surfaces by using the X --Ga reconstruction K. Fujita *, Y. Kusumi, M. Ichikawa Joint Research Center for Atom Technology, Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba-shi, Ibaraki 305, Japan Received 11 October 1995; accepted 17 November 1995
Abstract
Si(lll) surfaces with a nanometer-scale Si(lll)-v~-x f3-Ga area have been observed by scanning tunneling microscopy after exposure of molecular oxygen at room temperature, in order to compare the chemical activities on Si(lll)-7 X 7 and v~-X v~--Ga surfaces. After exposure to 10 L of oxygen, the 7 × 7 reconstruction around the v~- X v~--Ga area disappears, while the V~- × Vc3--Gareconstruction remains due to the effect of surface passivation. The X ,/3--Ga reconstruction is preserved after exposure to more than 20 L, though Ga atoms are oxidized from boundaries between the 7 X 7 surface and the ~ - × ~--Ga area and surface defects in the ~ × v~-Ga.
1. Introduction
The passivation of semiconductor surfaces by adsorbates is attractive to construct fine structures such as quantum wires and doping wires. Because the adsorption and dissociation of gas molecules supplied on a surface depend on the local chemical activity on the surface, selective adsorption or growth may be realized on the nanometer scale by removing the adsorbates with atomic precision. The S i ( l l l ) g3- X v~R30°-Ga reconstruction [1] is one of candidates for the surface passivation. We have recently found that nanometer-scale stripes of the Si(111)-7 X 7 structure, typically 9.3 nm in width and 140 nm in length, extend on S i ( l l l ) - f 3 - × v~--Ga surfaces by elevating substrate temperatures up to about 600°C [2]. The mechanism of extending of the Si(111)-7 X 7
* Corresponding author. Tel.: + 81-298-542598; fax: + 81-298542577; e-mail: [email protected].
structure is ascribed to the thermal desorption of Ga atoms from step edges [3]. Since the shape of Ga-desorbed areas is determined by the inherent property of the Si surface [2], the f 3 - × ~ - - G a surface is modified with atomic precision. This surface modification includes no lithography technique. It is, thereby, called self-organizing modification of surfaces. In order to apply the surface modification by self-organization to selective adsorption, passivated areas must be chemically less active than Si surfaces. In general, the S i ( l l l ) - f3- X ~ surface induced by group-III elements is expected to be chemically less active for molecular adsorption than the Si(111)-7 × 7 surface, because dangling bonds on the S i ( l l l ) surface are terminated by metal atoms [4]. The passivation effect of the v ~ × v/-3--Ga surface, however, is not confirmed on the atomic scale. In the present study, we have compared the chemical activities on S i ( l l l ) - 7 × 7 and v~- × v~'-Ga
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 5 0 2 - 8
K. Fujita et aL /Applied Surface Science 107 (1996) 6-10
7
surfaces by observing S i ( l l l ) surfaces with a nanometer-scale v~- × v~--Ga area after oxygen exposures using scanning tunneling microscopy (STM). We show that the v~- X v~--Ga area is less affected by exposure to 10 L of oxygen than the Si(111)-7 × 7 surface. The Ga-terminated area is oxidized after exposure to more than 20 L of exposure. The origin of oxidation in the Ga-terminated area is discussed.
2. Experimental The experiments were performed with a high-temperature STM (JEOL 4500XT) with base pressures below 3 X 10 -8 Pa. All samples used were cut from a n-type Si(111) wafer with a resistivity of 5 - 8 12. cm. Clean Si(111)-7 × 7 surfaces were produced by heating samples up to 1100°C three times for 10 s below 1 X 10 - 7 Pa. After cooling the sample to room temperature, Ga was deposited from an effusion cell below 5 X 10 -8 Pa. The Si(lll)-v/3 - X f 3 Ga reconstruction was produced on the surface by annealing at 500°C. The STM was operated at 0.1 nA tunneling current. The sample bias was from 1.0 to 1.5 V. Molecular oxygen with a impurity of 99.999% was introduced to the STM chamber while the STM tip was retracted from the surface. Pressures during exposures were from 1 X 10 . 6 to 1 X 10 - 4 P a . The variation of surface structures was observed between oxygen exposures.
3. Results and discussion 3.1. Preparation of S i ( l l l ) surfaces with a nanometer-scale f 3 × vrff -Ga area In order to compare the chemical activities on Si(111)-7 X 7 and v~- × v~--Ga surfaces, it is necessary to observe a f 3 × v/-3--Ga area neighboring a 7 × 7 surface before and after exposure of oxygen. In this work, nanometer-scale 7~- X vr3--Ga areas are produced by depositing 0.1 monolayer (ML) 1 of Ga atoms on the surface. Since the saturation coverage
1 One monolayer defined by the top layer of bulk 1 × 1 cell on the S i ( l l l ) surface.
(b) Faulted~
[111] /,~ [1101 [i1"21 J'3x~/-3-Ga
Fig. 1. (a) STM image of a Si(111) surface with a Ga-terminated area and (b) schematic figure to show crystallographic orientations of the surface. Area shown is 60 × 55 nm 2. The sample bias is 1.0 V and the tunneling current is 0.1 hA.
of Ga on Si(111) surface is ~1 ML [1], the deposited Ga leaves the Si(111)-7 X 7 surface. After annealing to create the v~- X f 3 - G a structure, nanometer-scale f 3 X v~--Ga domains are formed on the Si(111)-7 X 7 surface. Fig. 1 shows a Si(111) surface with f3" X v~--Ga domains. The V/J- X V/3--Ga domains predominantly neighbor faulted halves of the 7 × 7 structure [5]. This is because Si atoms in faulted halves are connected to Si atoms beneath Ga atoms in the v/-3× v/-3--Ga domains through dimer arrays which is essential to reduce the energy of the 7 X 7 structure. Similar boundary formation has been reported for the Si(111)- f3- X ~ structures induced by other groupIII elements [6,7]. Because of restriction on the boundary between v~-X v ~ - G a and Si(111)-7 × 7 surfaces, a saw tooth pattern is formed near the terrace edge in Fig. 1. In this image, boundaries are observed between the E/3- × f3--Ga domains, which
8
K. Fujita et al./Applied Surface Science 107 (1996) 6-10
implies that each domain extended from two-dimensional nuclei during the annealing. When terrace sizes are narrow, stripes of the f 3 X v/3--Ga structure are formed as shown below, because the extending of domains is restricted by step edges during annealing. When terrace sizes are so wide that some v ~ - × vr3--Ga domains do not reach terrace edges, triangles of the f 3 X 7~--Ga structure are obtained.
3.2. Oxygen exposure to S i ( l l l ) surfaces with a !/3 × v ~ - G a area
Fig. 2(a) shows a stripe of the v ~ × v/'J--Ga structure which was produced on a stepped SKI 11) surface tilting toward the [112] direction. Such stripes were frequently observed on narrow terraces. In this image, the terrace of the top side is higher than that of the bottom side. The both sides of the stripe is bounded by step edges. The widest width of the stripe is 7.0 nm. The length was more than 100 nm. One edge of the f 3 × f3--Ga stripe was connected to a S i ( l l l ) - 7 × 7 terrace with three times width of a 7 X 7 unit cell. Two arrows indicate surface vacancies in the f 3 X v/'3--Ga area which are used as a marker to observe the same region after oxygen exposure. Fig. 2(b) and (c) show STM images of the same region after exposure to 12 L and 27 L of oxygen, respectively. In Fig. 2(b), the S i ( l l l ) - 7 X 7 surface exhibits dark and bright sites. The sites affected by oxygen exposure indicate that the Si(111) surface is oxidized, because the origin of dark and bright sites has been ascribed to dissociative adsorption of oxygen to Si adatoms [8]. In this image, the ~ X v ~ - G a structure is clearly observed, which suggests that the v/3- X f3--Ga area is not so affected by oxygen as the 7 X 7 surface. The fact that the V~ × v~--Ga reconstruction is preserved has been confirmed by observing a low energy diffraction pattern corresponding to the f3- X Vr3 structure which was separately formed on a whole Si(111) surface, after equivalent exposure of oxygen. In Fig. 2(c), no 7 N 7 structure is observed due to oxidation. Although the f 3 × v~--Ga structure is preserved, some Ga atoms in the area seem to be oxidized from step edges and surface vacancies. The Ga atoms at the step edge neighboring the lower terrace are more affected by oxygen
Fig. 2. STM images of a Si(lll) surface with a V/3×v~-Ga stripe before (a) and after exposure to 12 L (b) and 27 L (c) of oxygen. Area shown is 30 x 25 nm2. The sample bias is 1.5 V and the tunneling current is 0.1 nA.
exposure than those neighboring the higher terrace. After prolonged exposure, the V~ × vr3--Ga reconstruction became ambiguous because of oxidation from step edges. To eliminate the oxidation from step edges, we formed a nanometer-scale ~ × Vr3--Ga area on a S i ( l l l ) terrace as shown in Fig. 3(a). As described in Section 3.1, the long sides of the triangle neighbor faulted halves of the 7 X 7 reconstruction. The trian-
K. Fujita et aL /Applied Surface Science 107 (1996) 6-10
9
also suggests that the f 3 X v~--Ga reconstruction effectively passivates the Si(111) surface. For this surface, further oxygen was exposed to investigate oxidation of the f 3 X v/3--Ga area. Fig. 3(c) shows a STM image of the surface after exposure to 135 L of oxygen. Although the v ~ X f3--Ga reconstruction is observed, Ga atoms are also oxidized. Ga atoms are predominantly oxidized from outlines of the f3- X f3--Ga area and surface defects including surface vacancies and the domain boundary. In addition, Ga atoms in the V~ X v~--Ga area are oxidized from the Ga atoms affected by exposure to 13 L. Avouris and his coworkers reported that the Si(111)-7 X 7 structure is affected by exposure to 5 L of ammonium whereas the Si(111)- f3- X !/-3--B surface is not affected by exposure to 400 L [4]. In the case of oxygen exposure on the f 3 X v~--Ga surface, the surface is affected by exposure to more than 20 L of exposure. This difference may be attributed to the fact that oxygen, which is a open shell system, is more reactive than ammonium. It is inferred that selective dissociation and adsorption of molecules take place on S i ( l l l ) surfaces with Gaterminated areas more effectively when t h e gas molecules are not so reactive as oxygen.
3.3. Effect of surface defects f o r initial oxidation of the vrff × f f f - G a area
Fig. 3. STM images of a Si(lll) surface with a a/3Xf3-Ga triangle before (a) and after exposure to 13 L (b) and t35 L (c) of oxygen. Area shown is 30× 27 nm2. The sample bias is 1.2 V and the tunneling current is 0.1 V.
gle of the v ~ - X v~--Ga structure is divided by a domain boundary indicated by a solid line A - X . The arrow indicates surface vacancies in the f3- X v~--Ga area. In the area, dark and bright sites are observed. The origin of the contrast is discussed later. Fig. 3(b) shows a STM image of the same region after exposure to 13 L of oxygen. Although several Ga atoms in the V~-X V~--Ga area are affected by oxygen, almost all Ga atoms preserve the v/J- X v/3--Ga reconstruction. On the other hand, the 7 × 7 structure around the Ga-terminated area disappears. This result
The initial stage of oxidation in the v~- × f 3 - G a area may depend on surface defects in the area to some extent. In Fig. 3(a), about 620 adatoms, except for surface vacancies, are included in the f3- × f3-Ga area in all and 61 adatoms are observed as dark sites. Fig. 4 shows the distribution of the dark sites in the area by open circles. Here, the filled circles denote the surface vacancies and the dash lines schematically show the outline of the triangle. In Fig. 4, we show the sites where adatoms were affected by exposure to 5 L of oxygen by crosses. The number of the affected sites is 52. In Fig. 4, 11 crosses are located above the open circles and 35 crosses are adjacent to the open circles. That is, 88% of the adatoms affected by exposure to 5 L are located at the dark sites in Fig. 3(a) or at their neighboring sites. This implies that the dark sites give rise to the oxidation of the V~- × V~--Ga area at
10
K. Fujita et al. /Applied Surface Science 107 (1996) 6-10
/0
0
"---..~ E, Xx
~
\
o
o
0 0
o Xo
o
o x@
'~x o
~
X~x O°0
Xox
oeee2i
~'---"---..~
"~""~X'""---.
//
o~g
•°
g
xO ° o
°°
~o
®x
o
x \
o~
0%
x o
o
~
o o o
° _
x^//
x
o o(/
x Xo
~ •
~o
/
× vr3-Ga structure is produced on the whole surface 1 after depositing more than g ML of Ga [3]. This is because excess Ga atoms eliminate surface defects during the anneal to create the ~ × f3--Ga reconstrnction. Adsorption of oxygen may be reduced by refining the Ga termination.
/
x 1
.x// /
/° / /
Xo \ ' ~ /
\ xX×% • / ~ ' \ \\ /f Fig. 4. Adatom sites in the f 3 × f 3 - G a area shown in Fig. 3. The open circles denote the surface defect on the ~/3 × ~ - - G a area observed before oxygen exposure. The filled circles denote surface vacancies. The crosses denote Ga sites affected by exposure t o 5 L.
the initial stage, because the ratio of dark sites and their neighboring sites to total adatom sites is 49%. It is pointed out that for Si(111)-vr3 × v~- structures induced by metals [4,7,9], the contrast among adatom sites on the v/3- × v~- surface can be ascribed to surface defects where Si atoms reside instead of metal atoms. Although detailed investigation is necessary, the dark sites are likely to be attributed to Si atoms adsorbing at adatom sites instead of Ga atoms in the case of the f3- × v~--Ga surface. At the initial stage of oxygen exposure, surface dangling bonds of Si atoms left in a Ga-terminated area may promote dissociation and adsorption of oxygen. In this case, adsorption of oxygen on the Ga-terminated surface is expected to be reduced by preparing v ~ - × v~--Ga surfaces with few surface defects. With respect to S i ( 1 1 1 ) - f 3 × v/3--Ga surfaces, clear diffraction patterns with sharp spots are observed, when the v~-
4. Conclusions We have compared the chemical activities on S i ( l l l ) - 7 × 7 and v ~ - × V~--Ga surfaces by exposing molecular oxygen. It has been found that the Ga-terminated area effectively passivates the Si(ll 1) surface against exposure to 10 L of oxygen. It is inferred that the selective chemical-adsorption is possible on the Ga-terminated S i ( l l l ) surface by using a source gas which is not so reactive as oxygen.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization.
References [1] A. Kawazu and H. Sakama, Phys. Rev. B 37 (1988) 2704. [2] K. Fujita, Y. Kusumi and M. Ichikawa, Surf. Sci. 357-358 (1996) 490. [3] H. Nakahara and M. Ichikawa, AppL Phys. Lett. 61 (1992) 1531. [4] Ph. Avouris, I.-W. Lyo, F. Bozso and E. Kaxiras, J. Vac. Sci. Technol. A 8 (1990) 3405; Ph. Avouris, J. Phys. Chem. 94 (1990) 2246. [5] K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502; Surf. Sci. 164 (1985) 367. [6] K. Takaoka, M. Yoshimura and T. Yao, Phys. Rev. B 48 (1993) 5657. [7] T.-C. Shen, C. Wang, J.W. Lyding and J.R. Tucker, Phys. Rev. B 50 (1994) 7453. [8] Ph. Avouris, I.-W. Lyo and F. Bozso, J. Vac. Sci. Technol. B 9 (1991) 424. [9] H. Hibino and T. Ogino, Surf. Sci. 328 (1995) L547.
Applied Surface Science 107 (1996) 6.10
Nanometer-scale passivation of Si(111) surfaces by using the X --Ga reconstruction K. Fujita *, Y. Kusumi, M. Ichikawa Joint Research Center for Atom Technology, Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba-shi, Ibaraki 305, Japan Received 11 October 1995; accepted 17 November 1995
Abstract
Si(lll) surfaces with a nanometer-scale Si(lll)-v~-x f3-Ga area have been observed by scanning tunneling microscopy after exposure of molecular oxygen at room temperature, in order to compare the chemical activities on Si(lll)-7 X 7 and v~-X v~--Ga surfaces. After exposure to 10 L of oxygen, the 7 × 7 reconstruction around the v~- X v~--Ga area disappears, while the V~- × Vc3--Gareconstruction remains due to the effect of surface passivation. The X ,/3--Ga reconstruction is preserved after exposure to more than 20 L, though Ga atoms are oxidized from boundaries between the 7 X 7 surface and the ~ - × ~--Ga area and surface defects in the ~ × v~-Ga.
1. Introduction
The passivation of semiconductor surfaces by adsorbates is attractive to construct fine structures such as quantum wires and doping wires. Because the adsorption and dissociation of gas molecules supplied on a surface depend on the local chemical activity on the surface, selective adsorption or growth may be realized on the nanometer scale by removing the adsorbates with atomic precision. The S i ( l l l ) g3- X v~R30°-Ga reconstruction [1] is one of candidates for the surface passivation. We have recently found that nanometer-scale stripes of the Si(111)-7 X 7 structure, typically 9.3 nm in width and 140 nm in length, extend on S i ( l l l ) - f 3 - × v~--Ga surfaces by elevating substrate temperatures up to about 600°C [2]. The mechanism of extending of the Si(111)-7 X 7
* Corresponding author. Tel.: + 81-298-542598; fax: + 81-298542577; e-mail: [email protected].
structure is ascribed to the thermal desorption of Ga atoms from step edges [3]. Since the shape of Ga-desorbed areas is determined by the inherent property of the Si surface [2], the f 3 - × ~ - - G a surface is modified with atomic precision. This surface modification includes no lithography technique. It is, thereby, called self-organizing modification of surfaces. In order to apply the surface modification by self-organization to selective adsorption, passivated areas must be chemically less active than Si surfaces. In general, the S i ( l l l ) - f3- X ~ surface induced by group-III elements is expected to be chemically less active for molecular adsorption than the Si(111)-7 × 7 surface, because dangling bonds on the S i ( l l l ) surface are terminated by metal atoms [4]. The passivation effect of the v ~ × v/-3--Ga surface, however, is not confirmed on the atomic scale. In the present study, we have compared the chemical activities on S i ( l l l ) - 7 × 7 and v~- × v~'-Ga
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 5 0 2 - 8
K. Fujita et aL /Applied Surface Science 107 (1996) 6-10
7
surfaces by observing S i ( l l l ) surfaces with a nanometer-scale v~- × v~--Ga area after oxygen exposures using scanning tunneling microscopy (STM). We show that the v~- X v~--Ga area is less affected by exposure to 10 L of oxygen than the Si(111)-7 × 7 surface. The Ga-terminated area is oxidized after exposure to more than 20 L of exposure. The origin of oxidation in the Ga-terminated area is discussed.
2. Experimental The experiments were performed with a high-temperature STM (JEOL 4500XT) with base pressures below 3 X 10 -8 Pa. All samples used were cut from a n-type Si(111) wafer with a resistivity of 5 - 8 12. cm. Clean Si(111)-7 × 7 surfaces were produced by heating samples up to 1100°C three times for 10 s below 1 X 10 - 7 Pa. After cooling the sample to room temperature, Ga was deposited from an effusion cell below 5 X 10 -8 Pa. The Si(lll)-v/3 - X f 3 Ga reconstruction was produced on the surface by annealing at 500°C. The STM was operated at 0.1 nA tunneling current. The sample bias was from 1.0 to 1.5 V. Molecular oxygen with a impurity of 99.999% was introduced to the STM chamber while the STM tip was retracted from the surface. Pressures during exposures were from 1 X 10 . 6 to 1 X 10 - 4 P a . The variation of surface structures was observed between oxygen exposures.
3. Results and discussion 3.1. Preparation of S i ( l l l ) surfaces with a nanometer-scale f 3 × vrff -Ga area In order to compare the chemical activities on Si(111)-7 X 7 and v~- × v~--Ga surfaces, it is necessary to observe a f 3 × v/-3--Ga area neighboring a 7 × 7 surface before and after exposure of oxygen. In this work, nanometer-scale 7~- X vr3--Ga areas are produced by depositing 0.1 monolayer (ML) 1 of Ga atoms on the surface. Since the saturation coverage
1 One monolayer defined by the top layer of bulk 1 × 1 cell on the S i ( l l l ) surface.
(b) Faulted~
[111] /,~ [1101 [i1"21 J'3x~/-3-Ga
Fig. 1. (a) STM image of a Si(111) surface with a Ga-terminated area and (b) schematic figure to show crystallographic orientations of the surface. Area shown is 60 × 55 nm 2. The sample bias is 1.0 V and the tunneling current is 0.1 hA.
of Ga on Si(111) surface is ~1 ML [1], the deposited Ga leaves the Si(111)-7 X 7 surface. After annealing to create the v~- X f 3 - G a structure, nanometer-scale f 3 X v~--Ga domains are formed on the Si(111)-7 X 7 surface. Fig. 1 shows a Si(111) surface with f3" X v~--Ga domains. The V/J- X V/3--Ga domains predominantly neighbor faulted halves of the 7 × 7 structure [5]. This is because Si atoms in faulted halves are connected to Si atoms beneath Ga atoms in the v/-3× v/-3--Ga domains through dimer arrays which is essential to reduce the energy of the 7 X 7 structure. Similar boundary formation has been reported for the Si(111)- f3- X ~ structures induced by other groupIII elements [6,7]. Because of restriction on the boundary between v~-X v ~ - G a and Si(111)-7 × 7 surfaces, a saw tooth pattern is formed near the terrace edge in Fig. 1. In this image, boundaries are observed between the E/3- × f3--Ga domains, which
8
K. Fujita et al./Applied Surface Science 107 (1996) 6-10
implies that each domain extended from two-dimensional nuclei during the annealing. When terrace sizes are narrow, stripes of the f 3 X v/3--Ga structure are formed as shown below, because the extending of domains is restricted by step edges during annealing. When terrace sizes are so wide that some v ~ - × vr3--Ga domains do not reach terrace edges, triangles of the f 3 X 7~--Ga structure are obtained.
3.2. Oxygen exposure to S i ( l l l ) surfaces with a !/3 × v ~ - G a area
Fig. 2(a) shows a stripe of the v ~ × v/'J--Ga structure which was produced on a stepped SKI 11) surface tilting toward the [112] direction. Such stripes were frequently observed on narrow terraces. In this image, the terrace of the top side is higher than that of the bottom side. The both sides of the stripe is bounded by step edges. The widest width of the stripe is 7.0 nm. The length was more than 100 nm. One edge of the f 3 × f3--Ga stripe was connected to a S i ( l l l ) - 7 × 7 terrace with three times width of a 7 X 7 unit cell. Two arrows indicate surface vacancies in the f 3 X v/'3--Ga area which are used as a marker to observe the same region after oxygen exposure. Fig. 2(b) and (c) show STM images of the same region after exposure to 12 L and 27 L of oxygen, respectively. In Fig. 2(b), the S i ( l l l ) - 7 X 7 surface exhibits dark and bright sites. The sites affected by oxygen exposure indicate that the Si(111) surface is oxidized, because the origin of dark and bright sites has been ascribed to dissociative adsorption of oxygen to Si adatoms [8]. In this image, the ~ X v ~ - G a structure is clearly observed, which suggests that the v/3- X f3--Ga area is not so affected by oxygen as the 7 X 7 surface. The fact that the V~ × v~--Ga reconstruction is preserved has been confirmed by observing a low energy diffraction pattern corresponding to the f3- X Vr3 structure which was separately formed on a whole Si(111) surface, after equivalent exposure of oxygen. In Fig. 2(c), no 7 N 7 structure is observed due to oxidation. Although the f 3 × v~--Ga structure is preserved, some Ga atoms in the area seem to be oxidized from step edges and surface vacancies. The Ga atoms at the step edge neighboring the lower terrace are more affected by oxygen
Fig. 2. STM images of a Si(lll) surface with a V/3×v~-Ga stripe before (a) and after exposure to 12 L (b) and 27 L (c) of oxygen. Area shown is 30 x 25 nm2. The sample bias is 1.5 V and the tunneling current is 0.1 nA.
exposure than those neighboring the higher terrace. After prolonged exposure, the V~ × vr3--Ga reconstruction became ambiguous because of oxidation from step edges. To eliminate the oxidation from step edges, we formed a nanometer-scale ~ × Vr3--Ga area on a S i ( l l l ) terrace as shown in Fig. 3(a). As described in Section 3.1, the long sides of the triangle neighbor faulted halves of the 7 X 7 reconstruction. The trian-
K. Fujita et aL /Applied Surface Science 107 (1996) 6-10
9
also suggests that the f 3 X v~--Ga reconstruction effectively passivates the Si(111) surface. For this surface, further oxygen was exposed to investigate oxidation of the f 3 X v/3--Ga area. Fig. 3(c) shows a STM image of the surface after exposure to 135 L of oxygen. Although the v ~ X f3--Ga reconstruction is observed, Ga atoms are also oxidized. Ga atoms are predominantly oxidized from outlines of the f3- X f3--Ga area and surface defects including surface vacancies and the domain boundary. In addition, Ga atoms in the V~ X v~--Ga area are oxidized from the Ga atoms affected by exposure to 13 L. Avouris and his coworkers reported that the Si(111)-7 X 7 structure is affected by exposure to 5 L of ammonium whereas the Si(111)- f3- X !/-3--B surface is not affected by exposure to 400 L [4]. In the case of oxygen exposure on the f 3 X v~--Ga surface, the surface is affected by exposure to more than 20 L of exposure. This difference may be attributed to the fact that oxygen, which is a open shell system, is more reactive than ammonium. It is inferred that selective dissociation and adsorption of molecules take place on S i ( l l l ) surfaces with Gaterminated areas more effectively when t h e gas molecules are not so reactive as oxygen.
3.3. Effect of surface defects f o r initial oxidation of the vrff × f f f - G a area
Fig. 3. STM images of a Si(lll) surface with a a/3Xf3-Ga triangle before (a) and after exposure to 13 L (b) and t35 L (c) of oxygen. Area shown is 30× 27 nm2. The sample bias is 1.2 V and the tunneling current is 0.1 V.
gle of the v ~ - X v~--Ga structure is divided by a domain boundary indicated by a solid line A - X . The arrow indicates surface vacancies in the f3- X v~--Ga area. In the area, dark and bright sites are observed. The origin of the contrast is discussed later. Fig. 3(b) shows a STM image of the same region after exposure to 13 L of oxygen. Although several Ga atoms in the V~-X V~--Ga area are affected by oxygen, almost all Ga atoms preserve the v/J- X v/3--Ga reconstruction. On the other hand, the 7 × 7 structure around the Ga-terminated area disappears. This result
The initial stage of oxidation in the v~- × f 3 - G a area may depend on surface defects in the area to some extent. In Fig. 3(a), about 620 adatoms, except for surface vacancies, are included in the f3- × f3-Ga area in all and 61 adatoms are observed as dark sites. Fig. 4 shows the distribution of the dark sites in the area by open circles. Here, the filled circles denote the surface vacancies and the dash lines schematically show the outline of the triangle. In Fig. 4, we show the sites where adatoms were affected by exposure to 5 L of oxygen by crosses. The number of the affected sites is 52. In Fig. 4, 11 crosses are located above the open circles and 35 crosses are adjacent to the open circles. That is, 88% of the adatoms affected by exposure to 5 L are located at the dark sites in Fig. 3(a) or at their neighboring sites. This implies that the dark sites give rise to the oxidation of the V~- × V~--Ga area at
10
K. Fujita et al. /Applied Surface Science 107 (1996) 6-10
/0
0
"---..~ E, Xx
~
\
o
o
0 0
o Xo
o
o x@
'~x o
~
X~x O°0
Xox
oeee2i
~'---"---..~
"~""~X'""---.
//
o~g
•°
g
xO ° o
°°
~o
®x
o
x \
o~
0%
x o
o
~
o o o
° _
x^//
x
o o(/
x Xo
~ •
~o
/
× vr3-Ga structure is produced on the whole surface 1 after depositing more than g ML of Ga [3]. This is because excess Ga atoms eliminate surface defects during the anneal to create the ~ × f3--Ga reconstrnction. Adsorption of oxygen may be reduced by refining the Ga termination.
/
x 1
.x// /
/° / /
Xo \ ' ~ /
\ xX×% • / ~ ' \ \\ /f Fig. 4. Adatom sites in the f 3 × f 3 - G a area shown in Fig. 3. The open circles denote the surface defect on the ~/3 × ~ - - G a area observed before oxygen exposure. The filled circles denote surface vacancies. The crosses denote Ga sites affected by exposure t o 5 L.
the initial stage, because the ratio of dark sites and their neighboring sites to total adatom sites is 49%. It is pointed out that for Si(111)-vr3 × v~- structures induced by metals [4,7,9], the contrast among adatom sites on the v/3- × v~- surface can be ascribed to surface defects where Si atoms reside instead of metal atoms. Although detailed investigation is necessary, the dark sites are likely to be attributed to Si atoms adsorbing at adatom sites instead of Ga atoms in the case of the f3- × v~--Ga surface. At the initial stage of oxygen exposure, surface dangling bonds of Si atoms left in a Ga-terminated area may promote dissociation and adsorption of oxygen. In this case, adsorption of oxygen on the Ga-terminated surface is expected to be reduced by preparing v ~ - × v~--Ga surfaces with few surface defects. With respect to S i ( 1 1 1 ) - f 3 × v/3--Ga surfaces, clear diffraction patterns with sharp spots are observed, when the v~-
4. Conclusions We have compared the chemical activities on S i ( l l l ) - 7 × 7 and v ~ - × V~--Ga surfaces by exposing molecular oxygen. It has been found that the Ga-terminated area effectively passivates the Si(ll 1) surface against exposure to 10 L of oxygen. It is inferred that the selective chemical-adsorption is possible on the Ga-terminated S i ( l l l ) surface by using a source gas which is not so reactive as oxygen.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization.
References [1] A. Kawazu and H. Sakama, Phys. Rev. B 37 (1988) 2704. [2] K. Fujita, Y. Kusumi and M. Ichikawa, Surf. Sci. 357-358 (1996) 490. [3] H. Nakahara and M. Ichikawa, AppL Phys. Lett. 61 (1992) 1531. [4] Ph. Avouris, I.-W. Lyo, F. Bozso and E. Kaxiras, J. Vac. Sci. Technol. A 8 (1990) 3405; Ph. Avouris, J. Phys. Chem. 94 (1990) 2246. [5] K. Takayanagi, Y. Tanishiro, M. Takahashi and S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502; Surf. Sci. 164 (1985) 367. [6] K. Takaoka, M. Yoshimura and T. Yao, Phys. Rev. B 48 (1993) 5657. [7] T.-C. Shen, C. Wang, J.W. Lyding and J.R. Tucker, Phys. Rev. B 50 (1994) 7453. [8] Ph. Avouris, I.-W. Lyo and F. Bozso, J. Vac. Sci. Technol. B 9 (1991) 424. [9] H. Hibino and T. Ogino, Surf. Sci. 328 (1995) L547.