- Email: [email protected]
Structural study of SrTiO3(100) surfaces by low energy ion scattering
__ g Eill
L!s2& ELSEVTER
,.,:.. ..\ ::..,,.:.:..:. .:.~. .... .,. .’
:.:
.j:-..:
. .._..
applied surface science Applied Surface Science 82/83
(1994) 528-53
1
Structural study of SrTiO,ilOO) surfaces by low energy ion scattering Yasunori Tanaka ‘,*, Hideki Morishita a, Michio Watamori Itsuo Katayama ’
‘, Kenjiro Oura a,
“ Department of Electronic Engineering, Faculty ofEngineering, Osaka Uni[~ersify, Suila, Osaka 565, Japan h Department of Applied Physics, Faculty of General Education, Osaka Institute of Technology, Asahiku, Osaka 535, Japan Received 7 May 1994; accepted for publication
5 July 1994
Abstract We have studied the atomic structure of the SrTiOJlOO) surface annealed at 600°C in ultrahigh vacuum (UHV) by using time-of-flight impact collision ion scattering spectroscopy (TOF-ICISSI and low energy electron diffraction (LEED). Considering its perovskite structure, the SrTiO,(lOOI surface consists of a SrO-terminated surface, a TiO,-terminated surface or a mixture of these surfaces. From the results of TOF-ICISS and LEED experiments, we have found that the SrTiO,(lOO) surface annealed at 600°C is an ordered surface (1 X 1 structure) and consists of a mixture of SrO-terminated and TiO,-terminated surfaces.
1. Introduction In recent years, strontium titanate (SrTiO,) with a perovskite structure has attracted much attention due to its typical dielectric properties. Especially, SrTiO,(lOOI surfaces are well known to be used as substrates for epitaxial growth of cuprate superconductor films, because 0% their good lattice matchirrg (SrTiO, : ,a = b = 3.91 A, YBa2Cu30, : a = 3.89 A, b = 3.83 A) [l-.5]. In order to realize epitaxial growth from the surface of the substrate, it is very important to know the structure of the SrTiO,(lOO) surface on an atomic scale. For these reasons, many studies on the atomic structure of the SrTiO,(lOO) surface were carried out in recent years by using various tech-
* Corresponding
author. Fax: +X1 6 875 0506
0169.4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00268-l
niques. Using scanning tunneling microscopy @TM) Tanaka et al. observed high-resolution images of a (6 X 6jR26.6” superstructure indicating the existence of localized surface states arising from oxygen vacancies by annealing at 1200°C [6]. Andersen et al. observed p(2 X 2) surface reconstruction caused by the segregation of Ca to the surface after annealing at 800-900°C using Auger electron spectroscopy (AES) [7]. Liang et al. observed the formation of intergrowths of lamellae of Sr,, ,Ti,O,, + , using STM [8]. Watanabe et al. proposed a new cleaning method of the SrTiO,(lOO) surface by Bi adsorption/ desorption treatment [9]. Despite these many studies, there is still no consensus on the point of which species (SrO or TiO?) forms the outermost surface. Actually, Andersen et al. proposed that the p(2 X 2) surface reconstruction is formed by a SrOterminated surface and segregated Ca atoms [7], on
Y. Tanaka et al. /Applied Surface Science 82 /83
the other hand, Tanaka et al. proposed that the (6 X fi> surface reconstruction is formed by a TiO,-terminated surface and ordered 0 vacancies [6]. Although we consider that this difference comes from different surface cleaning methods (Ar sputter + annealing or only annealing) or the annealing temperature, a detailed mechanism has not been obvious yet. It is a very interesting subject to define which species form the outermost surface in the field of surface science. So in this study, using TOF-ICISS and LEED, we have studied the atomic structure of the SrTiO,(lOO) surface annealed at 600°C.
2. Experimental In usual ICISS, ions scattered with a scattering angle in the range of 14”-170” are analyzed by electrostatic analyzers. Because of this specialization of the scattering angle close to 180”, ICISS allows one to analyze the atomic structure of solid surfaces straightforwardly. However, ICISS performed by electrostatic energy analyzers involves, more or less, difficulties arising from the ion neutralization effect of noble-gas and alkali-metal ions. Since this difficulty can be excluded by the use of TOF type energy analyzers which can detect both ions and neutrals scattered from the surface, several kinds of apparatus have recently been developed. Experiments were carried out in an ultrahigh vacuum (UHV) system, equipped with TOF-ICISS and low energy electron diffraction (LEED) apparatuses. Since the apparatus of TOF-ICISS is described in detail elsewhere [lo-121, only an outline is given in this paper. A beam of He+ produced by a differentially pumped ion source and mass-separated by an electromagnetic mass analyzer can be chopped by two pairs of electrostatic deflection plates with a chopping aperture of 2 mm diameter. After passing through the chopping aperture, the pulsed ion beam hits the sample in the UHV chamber. The beam energy is 2.5 keV. The beam current and diameter are lo-30 nA and 3 mm at the sample surface, respectively. Scattered ions and neutrals from the sample at a scattering angle 8 = 180” (actually 179”179.6”) can be detected by an annular type microchannel plate (MCP) placed coaxially along the
(1994) 528-531
529
primary ion beam at a flight path of 60 cm from the sample. Energy analysis of scattered particles is done by measuring the time of flight between the sample and the detector by the use of an &stop time-to-digital converter. The data are finally processed by a CAMAC system. We used Nb-doped SrTiO,(lOO) surfaces (0.05 wt%, 15 X 8 X 0.5 mm) as a sample substrate and annealing was carried out by direct current heating. The sample was mounted on a Ta holder and introduced into the UHV chamber. The base pressure is 2 X 10-‘” Torr and the experiment of TOF-ICISS was carried out under a base pressure of 2 x 10e9 Torr. Because this sample is not orientationally flat, we decided on the orientation by using both the LEED pattern and the result of the azimuth-scan of TOF-ICISS.
3. Results and discussion After annealing of the SrTiO,(lOO) surface at 600°C for 30 min, we observed a very clear 1 x 1 LEED pattern (Fig. 1). This result reveals that no reconstruction occurs on this surface and this pattern
Fig. 1. LEED pattern from the SrTiO,(lOO) surface annealed at 600°C for 30 min. This pattern indicates the 1 X 1 structure. E, = 60 eV.
530
Y. Tanaka et al. /Applied
Surface Science 82 /83
is also observed in many other experiments [69,13,14]. However, when the STM image reported by Tanaka et al. [6] is considered, it seems that the outermost surface is not regular. So, we carried out a TOF-ICISS experiment on this surface. Fig. 2a shows a TOF-ICISS spectrum taken from the SrTiO,(lOO) surface annealed at 600°C when the incident direction of the He+ (2.5 keV) ion is perpendicular to the surface. The observed peaks of scattered intensities at 4.9, 5.1 and 6.0 s correspond to the scattered signals from Sr, Ti and 0 atoms, respectively. By measuring many spectra at various impact angles, we can get information about the atomic structure of the SrTiO,(lOO) surface. Fig. 3 shows the variation of TOF-ICISS Sr signal intensities plotted against impact angles along the [OOl] direction. We can see several peaks of the Sr signal intensities originating from the so-called “focusing” effect at several specific incident directions of the ion beams. Among them, a very sharp peak at (Y= 14” originates from the focusing effect of the outermost nearest-neighbor Sr atoms along the [OOl] direction (Sr-Sr distance is 3.91 A> as shown in the inset of Fig. 3. This result, together with the 1 X 1 LEED pattern, shows that the
(1994) 528-531
He+(2.5keV)
_.
I
G
I
1
90 30 60 IMPACT ANGLE a [deg.]
0
Fig. 3. Impact-angle (a) dependence (along the [OOl) direction) of TOF-ICISS Sr signal intensities taken from the SrTiO,(lOO) surface annealed at 600°C for 30 min. The peak of the Sr signal intensity at LY= 14” originates from the focusing effect of the
outermost nearest-neighbor Sr atoms (Sr-Sr distance is 3.91 A).
outermost
surface
is SrO-terminated
and
that
the
more regular than that proposed by Tanaka et al. [6]. However, in the case of a impact angle of a = 90” (Fig. 2a), we can see both Sr and surface
is rather
0 Sr
after annealing at 600°C l
!s
I
Ti
(b) a =45”
-
(a)
4.5
6.5 FLI’GHT TlhiE [ ,tf se;]
Fig. 2. TOF-ICES spectra taken from the SrTiO,(lOO) surface annealed at 600°C for 30 min. The energy of the incident He + ion is 2.5 keV. (a) Incident angle w = 90” and (b) cy = 4.5”.
Sr O-terminated
(b)
Ti 4 -terminated
Fig. 4. Two structural models of SrTiO,(lOO) surfaces; (a) Srterminated surface, (b) Ti-terminated surface. In the case of (Y= 90”; if the SrTiO,(lOO) surface is terminated by SrO, we cannot observe the Ti signal because the Ti atom (B) is shadowed by the Sr atom (A) just above it. On the other hand, if the surface is terminated by TiO?, we can observe all signals (Sr, Ti and 0 atoms). In the case of cy = 45’; if the surface is terminated by SrO, we cannot observe the Sr signal because the Sr atom (D) is shadowed by the Ti atom (C).
Y. Tanaka et al. /Applied
Surface Science 82 /83
Ti signals which cannot be observed if the surface is regularly terminated by SrO as shown in Fig. 4a because of the shadowing effect of the Sr atom (A) to the Ti atom (B) just beneath it. This result indicates that this surface not only consists of a SrOterminated surface but also a TiO,-terminated surface as shown in Fig. 4b. In the case of an impact angle of (Y= 45” (Fig. 2b), we can see both signals which cannot be observed if the surface is regularly terminated by TiO, as shown in Fig. 4b because of the shadowing effect of the 0 atom (C) to the Sr atom (D). This result supports the structural model of the mixture of Sr-terminated and Ti-terminated surfaces as said above. Here, we must consider the influence of the surface relaxation on the TOF-ICISS spectra as shown in Fig. 2. Bickel et al. reported that the oxygen atom is pulled out of the surface by 0.08 A to the Ti atom (in the case of the TiO,-terminated surface) and 0.16 A to the Sr atom (in the case of the SrO-terminated surface) [15]. Considering the size of the shadow-cone in the energy range of keV and the value of the surface relaxation (< lo%), we suppose that there is no influence of surface relaxation on the TOF-ICISS spectra on the channeling direction when (Y= 45” ([Oil] direction) and (Y= 90” ([OOl] direction).
4. Summary By using TOF-ICISS and LEED apparatuses, we studied the atomic structure of the SrTiO,(lOO) surface annealed at 600°C in UHV. From the results of these experiments, we found that the SrTiO,(lOO) surface annealed at 600°C is an ordered surface (1 X 1 structure) and consists of a mixture of SrOterminated and TiO,-terminated surfaces.
(1994) 528-531
531
Acknowledgements Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. This work was carried out at “The Ion Beam Surface Analysis Facility” founded by the Ministry of Education, Science and Culture, Japan.
References [I] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [2] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Len. 58 (1987) 2684. [3] C. Webb, S.L. Weng, J.N. Eckstein, N. Missert, K. Char, D.G. Schlom, ES. Hellman, M.R. Beasley, A. Kapitulnik and J.S. Harris, Jr., Appl. Phys. Lett. 51 (1987) 1191. [4] D.G. Shlom, A.F. Marshall, J.T. Sizemore, Z.J. Chen, J.N. E&stein, I. Bozovic, K.E. von Dessonneck, J.S. Harris, Jr. and J.C. Bravman, J. Cryst. Growth 102 (1990) 361. [5] M. Kawai, S. Watanabe and T. Hanada, J. Cryst. Growth 112 (1991) 745. [6] H. Tanaka, T. Matsumoto, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 32 (1993) 1405. 171 J.E.T. Andersen and P.J. Moller, Appl. Phys. Lett. 56 (1990) 1847. 181 Y. Liang and D.A. Bonnell, Surf. Sci. 285 (1993) L 510. [9] S. Watanabe, T. Hikita and M. Kawai, J. Vat. Sci. Technol. A 9 (1991) 2394. [lo] K. Sumitomo, K. Oura, I. Katayama, F. Shoji and T. Hanawa, Nucl. Instrum. Methods B 33 (1988) 871. [ll] K. Sumitomo, K. Tanaka, Y. Izawa, I. Katayama, F. Shoji, K. Oura and T. Hanawa, Appl. Surf. Sci. 41/42 (1989) 112. [12] K. Sumitomo, PhD Thesis, Osaka University (1991). [13] T. Hikita, T. Hanada, M. Kudo and M. Kawai, Surf. Sci. 287/288 (1993) 377. [14] B. Cord and R. Courths, Surf. Sci. 162 (1985) 34. [15] N. Bickel, G. Schmidt K. Heinz and K. Muller, Phys. Rev. Lett. 62 (1989) 2009.
L!s2& ELSEVTER
,.,:.. ..\ ::..,,.:.:..:. .:.~. .... .,. .’
:.:
.j:-..:
. .._..
applied surface science Applied Surface Science 82/83
(1994) 528-53
1
Structural study of SrTiO,ilOO) surfaces by low energy ion scattering Yasunori Tanaka ‘,*, Hideki Morishita a, Michio Watamori Itsuo Katayama ’
‘, Kenjiro Oura a,
“ Department of Electronic Engineering, Faculty ofEngineering, Osaka Uni[~ersify, Suila, Osaka 565, Japan h Department of Applied Physics, Faculty of General Education, Osaka Institute of Technology, Asahiku, Osaka 535, Japan Received 7 May 1994; accepted for publication
5 July 1994
Abstract We have studied the atomic structure of the SrTiOJlOO) surface annealed at 600°C in ultrahigh vacuum (UHV) by using time-of-flight impact collision ion scattering spectroscopy (TOF-ICISSI and low energy electron diffraction (LEED). Considering its perovskite structure, the SrTiO,(lOOI surface consists of a SrO-terminated surface, a TiO,-terminated surface or a mixture of these surfaces. From the results of TOF-ICISS and LEED experiments, we have found that the SrTiO,(lOO) surface annealed at 600°C is an ordered surface (1 X 1 structure) and consists of a mixture of SrO-terminated and TiO,-terminated surfaces.
1. Introduction In recent years, strontium titanate (SrTiO,) with a perovskite structure has attracted much attention due to its typical dielectric properties. Especially, SrTiO,(lOOI surfaces are well known to be used as substrates for epitaxial growth of cuprate superconductor films, because 0% their good lattice matchirrg (SrTiO, : ,a = b = 3.91 A, YBa2Cu30, : a = 3.89 A, b = 3.83 A) [l-.5]. In order to realize epitaxial growth from the surface of the substrate, it is very important to know the structure of the SrTiO,(lOO) surface on an atomic scale. For these reasons, many studies on the atomic structure of the SrTiO,(lOO) surface were carried out in recent years by using various tech-
* Corresponding
author. Fax: +X1 6 875 0506
0169.4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00268-l
niques. Using scanning tunneling microscopy @TM) Tanaka et al. observed high-resolution images of a (6 X 6jR26.6” superstructure indicating the existence of localized surface states arising from oxygen vacancies by annealing at 1200°C [6]. Andersen et al. observed p(2 X 2) surface reconstruction caused by the segregation of Ca to the surface after annealing at 800-900°C using Auger electron spectroscopy (AES) [7]. Liang et al. observed the formation of intergrowths of lamellae of Sr,, ,Ti,O,, + , using STM [8]. Watanabe et al. proposed a new cleaning method of the SrTiO,(lOO) surface by Bi adsorption/ desorption treatment [9]. Despite these many studies, there is still no consensus on the point of which species (SrO or TiO?) forms the outermost surface. Actually, Andersen et al. proposed that the p(2 X 2) surface reconstruction is formed by a SrOterminated surface and segregated Ca atoms [7], on
Y. Tanaka et al. /Applied Surface Science 82 /83
the other hand, Tanaka et al. proposed that the (6 X fi> surface reconstruction is formed by a TiO,-terminated surface and ordered 0 vacancies [6]. Although we consider that this difference comes from different surface cleaning methods (Ar sputter + annealing or only annealing) or the annealing temperature, a detailed mechanism has not been obvious yet. It is a very interesting subject to define which species form the outermost surface in the field of surface science. So in this study, using TOF-ICISS and LEED, we have studied the atomic structure of the SrTiO,(lOO) surface annealed at 600°C.
2. Experimental In usual ICISS, ions scattered with a scattering angle in the range of 14”-170” are analyzed by electrostatic analyzers. Because of this specialization of the scattering angle close to 180”, ICISS allows one to analyze the atomic structure of solid surfaces straightforwardly. However, ICISS performed by electrostatic energy analyzers involves, more or less, difficulties arising from the ion neutralization effect of noble-gas and alkali-metal ions. Since this difficulty can be excluded by the use of TOF type energy analyzers which can detect both ions and neutrals scattered from the surface, several kinds of apparatus have recently been developed. Experiments were carried out in an ultrahigh vacuum (UHV) system, equipped with TOF-ICISS and low energy electron diffraction (LEED) apparatuses. Since the apparatus of TOF-ICISS is described in detail elsewhere [lo-121, only an outline is given in this paper. A beam of He+ produced by a differentially pumped ion source and mass-separated by an electromagnetic mass analyzer can be chopped by two pairs of electrostatic deflection plates with a chopping aperture of 2 mm diameter. After passing through the chopping aperture, the pulsed ion beam hits the sample in the UHV chamber. The beam energy is 2.5 keV. The beam current and diameter are lo-30 nA and 3 mm at the sample surface, respectively. Scattered ions and neutrals from the sample at a scattering angle 8 = 180” (actually 179”179.6”) can be detected by an annular type microchannel plate (MCP) placed coaxially along the
(1994) 528-531
529
primary ion beam at a flight path of 60 cm from the sample. Energy analysis of scattered particles is done by measuring the time of flight between the sample and the detector by the use of an &stop time-to-digital converter. The data are finally processed by a CAMAC system. We used Nb-doped SrTiO,(lOO) surfaces (0.05 wt%, 15 X 8 X 0.5 mm) as a sample substrate and annealing was carried out by direct current heating. The sample was mounted on a Ta holder and introduced into the UHV chamber. The base pressure is 2 X 10-‘” Torr and the experiment of TOF-ICISS was carried out under a base pressure of 2 x 10e9 Torr. Because this sample is not orientationally flat, we decided on the orientation by using both the LEED pattern and the result of the azimuth-scan of TOF-ICISS.
3. Results and discussion After annealing of the SrTiO,(lOO) surface at 600°C for 30 min, we observed a very clear 1 x 1 LEED pattern (Fig. 1). This result reveals that no reconstruction occurs on this surface and this pattern
Fig. 1. LEED pattern from the SrTiO,(lOO) surface annealed at 600°C for 30 min. This pattern indicates the 1 X 1 structure. E, = 60 eV.
530
Y. Tanaka et al. /Applied
Surface Science 82 /83
is also observed in many other experiments [69,13,14]. However, when the STM image reported by Tanaka et al. [6] is considered, it seems that the outermost surface is not regular. So, we carried out a TOF-ICISS experiment on this surface. Fig. 2a shows a TOF-ICISS spectrum taken from the SrTiO,(lOO) surface annealed at 600°C when the incident direction of the He+ (2.5 keV) ion is perpendicular to the surface. The observed peaks of scattered intensities at 4.9, 5.1 and 6.0 s correspond to the scattered signals from Sr, Ti and 0 atoms, respectively. By measuring many spectra at various impact angles, we can get information about the atomic structure of the SrTiO,(lOO) surface. Fig. 3 shows the variation of TOF-ICISS Sr signal intensities plotted against impact angles along the [OOl] direction. We can see several peaks of the Sr signal intensities originating from the so-called “focusing” effect at several specific incident directions of the ion beams. Among them, a very sharp peak at (Y= 14” originates from the focusing effect of the outermost nearest-neighbor Sr atoms along the [OOl] direction (Sr-Sr distance is 3.91 A> as shown in the inset of Fig. 3. This result, together with the 1 X 1 LEED pattern, shows that the
(1994) 528-531
He+(2.5keV)
_.
I
G
I
1
90 30 60 IMPACT ANGLE a [deg.]
0
Fig. 3. Impact-angle (a) dependence (along the [OOl) direction) of TOF-ICISS Sr signal intensities taken from the SrTiO,(lOO) surface annealed at 600°C for 30 min. The peak of the Sr signal intensity at LY= 14” originates from the focusing effect of the
outermost nearest-neighbor Sr atoms (Sr-Sr distance is 3.91 A).
outermost
surface
is SrO-terminated
and
that
the
more regular than that proposed by Tanaka et al. [6]. However, in the case of a impact angle of a = 90” (Fig. 2a), we can see both Sr and surface
is rather
0 Sr
after annealing at 600°C l
!s
I
Ti
(b) a =45”
-
(a)
4.5
6.5 FLI’GHT TlhiE [ ,tf se;]
Fig. 2. TOF-ICES spectra taken from the SrTiO,(lOO) surface annealed at 600°C for 30 min. The energy of the incident He + ion is 2.5 keV. (a) Incident angle w = 90” and (b) cy = 4.5”.
Sr O-terminated
(b)
Ti 4 -terminated
Fig. 4. Two structural models of SrTiO,(lOO) surfaces; (a) Srterminated surface, (b) Ti-terminated surface. In the case of (Y= 90”; if the SrTiO,(lOO) surface is terminated by SrO, we cannot observe the Ti signal because the Ti atom (B) is shadowed by the Sr atom (A) just above it. On the other hand, if the surface is terminated by TiO?, we can observe all signals (Sr, Ti and 0 atoms). In the case of cy = 45’; if the surface is terminated by SrO, we cannot observe the Sr signal because the Sr atom (D) is shadowed by the Ti atom (C).
Y. Tanaka et al. /Applied
Surface Science 82 /83
Ti signals which cannot be observed if the surface is regularly terminated by SrO as shown in Fig. 4a because of the shadowing effect of the Sr atom (A) to the Ti atom (B) just beneath it. This result indicates that this surface not only consists of a SrOterminated surface but also a TiO,-terminated surface as shown in Fig. 4b. In the case of an impact angle of (Y= 45” (Fig. 2b), we can see both signals which cannot be observed if the surface is regularly terminated by TiO, as shown in Fig. 4b because of the shadowing effect of the 0 atom (C) to the Sr atom (D). This result supports the structural model of the mixture of Sr-terminated and Ti-terminated surfaces as said above. Here, we must consider the influence of the surface relaxation on the TOF-ICISS spectra as shown in Fig. 2. Bickel et al. reported that the oxygen atom is pulled out of the surface by 0.08 A to the Ti atom (in the case of the TiO,-terminated surface) and 0.16 A to the Sr atom (in the case of the SrO-terminated surface) [15]. Considering the size of the shadow-cone in the energy range of keV and the value of the surface relaxation (< lo%), we suppose that there is no influence of surface relaxation on the TOF-ICISS spectra on the channeling direction when (Y= 45” ([Oil] direction) and (Y= 90” ([OOl] direction).
4. Summary By using TOF-ICISS and LEED apparatuses, we studied the atomic structure of the SrTiO,(lOO) surface annealed at 600°C in UHV. From the results of these experiments, we found that the SrTiO,(lOO) surface annealed at 600°C is an ordered surface (1 X 1 structure) and consists of a mixture of SrOterminated and TiO,-terminated surfaces.
(1994) 528-531
531
Acknowledgements Part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. This work was carried out at “The Ion Beam Surface Analysis Facility” founded by the Ministry of Education, Science and Culture, Japan.
References [I] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki and H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [2] P. Chaudhari, R.H. Koch, R.B. Laibowitz, T.R. McGuire and R.J. Gambino, Phys. Rev. Len. 58 (1987) 2684. [3] C. Webb, S.L. Weng, J.N. Eckstein, N. Missert, K. Char, D.G. Schlom, ES. Hellman, M.R. Beasley, A. Kapitulnik and J.S. Harris, Jr., Appl. Phys. Lett. 51 (1987) 1191. [4] D.G. Shlom, A.F. Marshall, J.T. Sizemore, Z.J. Chen, J.N. E&stein, I. Bozovic, K.E. von Dessonneck, J.S. Harris, Jr. and J.C. Bravman, J. Cryst. Growth 102 (1990) 361. [5] M. Kawai, S. Watanabe and T. Hanada, J. Cryst. Growth 112 (1991) 745. [6] H. Tanaka, T. Matsumoto, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 32 (1993) 1405. 171 J.E.T. Andersen and P.J. Moller, Appl. Phys. Lett. 56 (1990) 1847. 181 Y. Liang and D.A. Bonnell, Surf. Sci. 285 (1993) L 510. [9] S. Watanabe, T. Hikita and M. Kawai, J. Vat. Sci. Technol. A 9 (1991) 2394. [lo] K. Sumitomo, K. Oura, I. Katayama, F. Shoji and T. Hanawa, Nucl. Instrum. Methods B 33 (1988) 871. [ll] K. Sumitomo, K. Tanaka, Y. Izawa, I. Katayama, F. Shoji, K. Oura and T. Hanawa, Appl. Surf. Sci. 41/42 (1989) 112. [12] K. Sumitomo, PhD Thesis, Osaka University (1991). [13] T. Hikita, T. Hanada, M. Kudo and M. Kawai, Surf. Sci. 287/288 (1993) 377. [14] B. Cord and R. Courths, Surf. Sci. 162 (1985) 34. [15] N. Bickel, G. Schmidt K. Heinz and K. Muller, Phys. Rev. Lett. 62 (1989) 2009.