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In situ studies of fast atom bombardment and annealing processes by reflection electron microscopy
474
Nuclear
Instruments
IN SITU STUDIES OF FAST ATOM BOMBARDMENT BY REFLECTION ELECTRON MICROSCOPY S. OGAWA,
Y. TANISHIRO
Physics Department,
Tokyo Institute
and
and Methods
in Physics
AND ANNEALING
Research B33 (1988) 474-478 North-Holland, Amsterdam
PROCESSES
K. YAGI
of Technology,
Oh-Okayama,
Megwo,
Tokyo 152, Japan
The capability of reflection electron microscopy (REM) to characterize surface structures is applied to in situ studies of fast atom bombardment (FAB) and annealing process. A FAB gun (4-6 kV Ar) is attached to a UHV electron microscope. Heat cleaned Pt(ll1) and Si(ll1) surfaces are bombarded. Low temperature bombardment produces defects on surfaces and they mask images of surface atomic steps. Higher temperature annealing is required to get atomically flat surfaces for a longer time bombardment. In cases of high temperature bombardment, sputtering and annealing take place simultaneously and surface atomic steps move as a result of the sputtering.
1. Introduction Most of the studies on sputtering concern the analysis of the sputtered species and a few works involve studies of the sputtered surfaces [1,2]. Most of the latter studies used scanning electron microscopes or transmission electron microscopes (TEM) to characterize the topography of the sputtered surfaces, which showed characteristic hills or crystallographic faces [3-51. In-situ studies of sputtering by TEM were also carried out [6]. In that study a preferential sputtering at twins in vacuum deposited Au(001) films was reported but roughing of the Au(001) surfaces was not found. They also reported that sputtering at high temperatures is effective in removing defects produced by sputtering: the lowest temperature at which defects (except bubbles) are not formed during sputtering is lower than the lowest temperature at which defects formed during the sputtering and annealing are annealed out. However, changes of
Fig. 1. A cross section of the UHV microscope
the surface structures by sputtering at the atomic level were not revealed. Reflection electron microscopy (REM) in an ultrahigh vacuum (UHV) condition has been developed and has been shown to be sensitive to surface structures and processes at the monolayer level [7,8]. Surface atomic steps are clearly seen by REM. In the present study this capability of REM is used to study sputtering and annealing processes. Some preliminary results on Pt(ll1) surfaces were given in ref. [9] and here the details and studies on Si(ll1) surfaces are reported.
2. Experimental A fast atom bombardment gun (Ion Tech FAB 11N) was introduced to our UHV electron microscope as shown in fig. 1 [8]. A specimen (1) attached to the side entry heating holder (A) is bombarded by 5-6 keV
which shows the arrangement
0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
of the FAB gun and the specimen (see text).
S. Ogawa et al. / In situ studies of fast atom bombardment
argon atoms from the FAB gun (7) in the gun chamber (B) through a collimator (5) and holes of Liq He cooled (2) and Liq N, cooled (3, 4) cryogenic tip and shrouds. Argon gases controlied by a variable leak valve are fed through a tube (9) and the gun chamber is evacuated through an evacuation tube (12). During operation the pressure in the specimen chamber is 4-6 X f0-6 Torr. The samples studied were heat-cleaned Pt(ll1) and Si(111)7 X 7 crystalline surfaces. In the case of Pt, bombardment is carried out at room temperature and REM images are taken at room temperature after each annealing process. In the case of Si, surface are observed in situ during bombardment at high temperature, where sputtering and annealing processes take place simultaneously.
3. Results 3. I. Pt(l I I) surface Fig. 1 shows a series of REM images of a Pt(l11) surface taken during a bombardment. REM images are foreshortened along the beam direction (vertical direction in the micrographs) by a factor of 30-50. In (a), before the bombardment, surface atomic steps are clearly seen as wavey lines. U~own surface defects are contrasted as dark regions. In (b) after 15 s of the bombardment, the image contrast of the steps and defects decreases and in (c) their images disappear. The shapes of the steps with faint contrast in (b) are quite similar to those in (a) and this means that the steps are not destroyed at this stage and their disappearance in (c) is simply due to the formation of defects (primary defects} whose contrast masks the step contrast. This is the reason why the steps reappear at the same positions after slight annealing as seen in fig. 2. It is noted that a RHEED pattern corresponding to a surface similar to fig. l(c) shows diffuse rods 191, which indicates that surface layers are crystalline. Figs. 3(a) and (b) are REM images before and after bombardment, respectively. In (c) steps with shapes similar to those in (a) reappear after annealing at 250 o C. Defects (secondary defects} are formed uniformly on the terraces between the steps. By annealing at 450’ C these defects coagulate to form larger defects on the terraces, as seen in (d). They may be monolayer deep hollows because they interact with the steps. Finally by annealing at 800 o C defects are eliminated and a smooth surface with the steps of different configuration is obtained (e). These results are for cases when the bombardment time is 30 or so. When the bombardment time is longer, annealing at higher temperature is necessary to obtain flat and defect-free surfaces. Fig. 4 shows an example. A REM image shown in (a) was taken after 30 s bombardment
Fig. 2. A sequence of REM images of a Pt(ll1) bomb~dment
surface during
at room temperature.
at room temperature and 60 s annealing at 900* C. Then the surface was bombarded for 300 s (b). Images of bundles of steps as well as single steps are completely masked by images of the primary defects. A REM image (c) was taken after 60 s annealing at 900°C. Secondary defects still remain. A notable fact is that the atomic steps seen in (a) are not identified in (c) in strong contrast to the case of fig. 3, although the bundles of steps reappear again at similar positions. A REM image (d) was taken after 480 s annealing at 900 o C. Secondary defects grow from (c) to (d) and this means that the secondary defects formed after heavy bombardment cannot be annealed out at 900 o C even after a long annealing time, whereas the secondary defects formed after slight bombardment are easily annealed out at this temperature (fig. 2). To get a defect free surface, annealing at 1100 *C is needed as shown in (e). In (e) a step configuration is quite different from that in (a). Changes in the bundles of steps are also noted. VII. SPUTTERING/SIMS
476 3.2. Si(l11)
S. Ogawa et al. / In situ studies of fast atom bombardment surface
Fig. 5 shows REM images and corresponding RHEED patterns before and after the bombardment of the Si(ll1) surface for 15 s at room temperature. In (a) surface atomic steps are clearly seen together with bundles of steps seen as dark bands. The corresponding RHEED pattern shows sharp spots and Kikuchi lines. These images and patterns completely disappear in (b) and (d). In (d) only diffuse scattering is seen. This fact indicates that the surface layers are amorphous, as was reported previously [2,6]. These amorphous layers re-
Fig. 4. A sequence of REM images of a Pt(ll1)
surface which show a dose dependence of the annealing processes. (a) 60 s at 900 ’ C after 30 s bombardment. (b) 300 s bombardment. (c) 60 sat900°C.(d)480sat9000C.(e)60sat11000C.
Fig. 3. A sequence of REM images of a Pt(ll1) surface which shows a bombardment process ((a) and (b)) at room temperature and annealing processes ((c) 250 OC, (d) 450 o C, (e) 800 o C).
crystallize expitaxially after annealing [6]. In the RHEED pattern diffuse rods appear at about 400°C and Kikuchi lines are clear at 800°C. However, the surfaces contain a lot of secondary defects and sharp horizontal Kikuchi lines and flat surface images appear by annealing above 1000°C. The nature of the secondary defects depends on the thickness of the amorphous layer. Thus, the annealing process depends on the dose of Ar atoms. It was reported that annealing at 500 o C was enough to reproduce surface atomic steps in the case of a low dose of ions [lo]. When the surfaces are bombarded at elevated temperatures, sputtering, defect formation and annealing take place simultaneously. Fig. 6 shows a case of bombardment at 700” C. In (a), steps are seen. They move between images (a) and (b), which was taken after 15 s bombardment. The motions are due to the migration of vacancies (annealing effect) formed on the surface (sputtering) to the steps. At the same time, defects which show fine speckle images appear all over the surface. These defects also act as sinks for vacancies and their formation and growth mask the step images in the later stages, as seen in (c). A notable fact is that the diffraction spots remain in the corresponding RHEED
S. Ogawa et al. / In situ studies of fast atom bombardment
Fig. 5. REM images and corresponding RHEED patterns of a Si(ll1) surface before and after bombardment An amorphous surface is formed in (b).
pattern, although they are broader than those from perfect surfaces. The annealing during bombardment is enhanced at higher temperatures (900” C), as seen in fig. 7. At 900 o C, steps move spontaneously due to sublimation
at room temperature.
[7] as seen for step A in fig. 7(b) which was taken 165 s after (a). Micrographs (c), (d), (e) and (f) were taken at 0 s, 35 s, 105 s and 180 s after the start of the bombardment. Step B is seen to move from (c) to (e). The distance moved between images (c) and (d) is much
a, Osec
Fig. 6. A sequence of REM images of the Si(ll1) surface during bomb~dment
at 700 o C and a corresponding RHEED pattern. VII. SPUTTERING/SIMS
478
S. Ogawa et al. / In situ studies off&
atom bombardment
contrast of defects, changes in the surface structure during bombardment could not be studied in detail. Annealing processes are found to depend on the dose of Ar. For a low dose steps play an important role. Higher temperature annealing is necessary for higher doses. For high temperature bombardment, the annealing processes take place together with sputtering and defect formation. Ar atom fluxes on the surfaces during bombardment are hard to measure because of the narrow space and a complex geometry around the specimen. It should be noted that about 0.3-0.4 of a monolayer of the Pt(ll1) surface is removed by the bombardment of 20 s at 100 o C (compare fig. 3(f) with fig. 3(a) of ref. [8]) and 0.3-0.4 double layers of Si(ll1) are removed by the bombardment of 35 s at 900’ C (see figs. 7(c) and (d)). If we assume a sputtering yield of 5, the flux is estimated to be about 2-4 x 1014 atoms/cm* s. Claverie et al. [lo] observed that steps reappeared at the same positions when a Si(ll1) surface was annealed at 500 o C after bombardment of 5.5 x lOi ions/cm* (Ar 10 kV) at room temperature. This observation is reasonable because the dose is not high enough to produce sufficient vacancies to cause an observable shift in the steps. It should be noted that surface vacancies are mobile at this temperature [8]. In the in situ REM studies of surfaces, techniques have been developed which produce various modifications of surfaces. Bombardment of such surfaces will continue to be interesting.
,/-._
I
e
Fig. 7. REM images of the Si(ll1) surface at 900” C taken during sublimation ((a) 0 s, (b) 165 s) and bombardment ((c) 0 s, (d) 35 s, (e) 105 s, (f) 300 s). A motion of step A due to sublimation and that of step B due to sputtering are seen. longer than that moved by the step A between images (a) and (b), which means that the former motion is mainly due to the sputtering. The defect density in (d) is much smaller than that in fig. 6(b) for a lower dose of Ar. A velocity of the step motion during the exposure of the images (d) and (e) is smaller than that during (c) (d). This is due to an increase of vacancy capture by the defects. In (f) the step images are almost completely masked by the defects. It is clear that the annealing processes at 900 o C are not rapid enough for a simultaneous and complete elimination of the defects formed by the bombardment.
4. Summary and discussion Fast atom bombardment and annealing processes are observed in situ by REM. Due to the strong image
This work was supported by a Grant in aid for Scientific Research (Nos. 60220007 and 62102006) from the Ministry of Education of Japan.
References [l] M. Kaminsky, Atomic and Ion Impact Phenomena on Metal Surfaces (Springer-Verlag, Berlin, 1965). [2] G.K. Wehner and G.S. Auderson, in: Handbook of Thin Film Technology, eds. L.I. Maissel and R. Glang (McGraw Hill, New York, 1970) Chap. 3. [3] G.K. Wehner, Phys. Rev. 102 (1956) 690. [4] R. Smith and J.M. Walls, Surf. Sci. 80 (1979) 557. [5] J.A. Kubby and B.M. Siegel, Nucl. Instr. and Meth. B13 (1986) 319. [6] E. Morita, K. Takayanagi, K. Kobayashi, K. Yagi and G. Honjo, Jpn J. Appl. Phys. 19 (1980) 1981. [7] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surf. Sci. 97 (1980) 393. [8] K. Yagi, J. Appl. Cryst. 20 (1987) 147. [9] S. Ogawa, K. Kobayashi, Y. Tanishiro, K. Takayanagi and K. Yagi, Proc. 11th Int. Cong. Electron Microscopy 2 (1986) 1349. [lo] A. Claverie, J. Faure, C. Vieu, J. Beauvillain and B. Jouffrey, Proc. 11th Int. Cong. Electron Microscopy 2 (1986) 1357.
Nuclear
Instruments
IN SITU STUDIES OF FAST ATOM BOMBARDMENT BY REFLECTION ELECTRON MICROSCOPY S. OGAWA,
Y. TANISHIRO
Physics Department,
Tokyo Institute
and
and Methods
in Physics
AND ANNEALING
Research B33 (1988) 474-478 North-Holland, Amsterdam
PROCESSES
K. YAGI
of Technology,
Oh-Okayama,
Megwo,
Tokyo 152, Japan
The capability of reflection electron microscopy (REM) to characterize surface structures is applied to in situ studies of fast atom bombardment (FAB) and annealing process. A FAB gun (4-6 kV Ar) is attached to a UHV electron microscope. Heat cleaned Pt(ll1) and Si(ll1) surfaces are bombarded. Low temperature bombardment produces defects on surfaces and they mask images of surface atomic steps. Higher temperature annealing is required to get atomically flat surfaces for a longer time bombardment. In cases of high temperature bombardment, sputtering and annealing take place simultaneously and surface atomic steps move as a result of the sputtering.
1. Introduction Most of the studies on sputtering concern the analysis of the sputtered species and a few works involve studies of the sputtered surfaces [1,2]. Most of the latter studies used scanning electron microscopes or transmission electron microscopes (TEM) to characterize the topography of the sputtered surfaces, which showed characteristic hills or crystallographic faces [3-51. In-situ studies of sputtering by TEM were also carried out [6]. In that study a preferential sputtering at twins in vacuum deposited Au(001) films was reported but roughing of the Au(001) surfaces was not found. They also reported that sputtering at high temperatures is effective in removing defects produced by sputtering: the lowest temperature at which defects (except bubbles) are not formed during sputtering is lower than the lowest temperature at which defects formed during the sputtering and annealing are annealed out. However, changes of
Fig. 1. A cross section of the UHV microscope
the surface structures by sputtering at the atomic level were not revealed. Reflection electron microscopy (REM) in an ultrahigh vacuum (UHV) condition has been developed and has been shown to be sensitive to surface structures and processes at the monolayer level [7,8]. Surface atomic steps are clearly seen by REM. In the present study this capability of REM is used to study sputtering and annealing processes. Some preliminary results on Pt(ll1) surfaces were given in ref. [9] and here the details and studies on Si(ll1) surfaces are reported.
2. Experimental A fast atom bombardment gun (Ion Tech FAB 11N) was introduced to our UHV electron microscope as shown in fig. 1 [8]. A specimen (1) attached to the side entry heating holder (A) is bombarded by 5-6 keV
which shows the arrangement
0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
of the FAB gun and the specimen (see text).
S. Ogawa et al. / In situ studies of fast atom bombardment
argon atoms from the FAB gun (7) in the gun chamber (B) through a collimator (5) and holes of Liq He cooled (2) and Liq N, cooled (3, 4) cryogenic tip and shrouds. Argon gases controlied by a variable leak valve are fed through a tube (9) and the gun chamber is evacuated through an evacuation tube (12). During operation the pressure in the specimen chamber is 4-6 X f0-6 Torr. The samples studied were heat-cleaned Pt(ll1) and Si(111)7 X 7 crystalline surfaces. In the case of Pt, bombardment is carried out at room temperature and REM images are taken at room temperature after each annealing process. In the case of Si, surface are observed in situ during bombardment at high temperature, where sputtering and annealing processes take place simultaneously.
3. Results 3. I. Pt(l I I) surface Fig. 1 shows a series of REM images of a Pt(l11) surface taken during a bombardment. REM images are foreshortened along the beam direction (vertical direction in the micrographs) by a factor of 30-50. In (a), before the bombardment, surface atomic steps are clearly seen as wavey lines. U~own surface defects are contrasted as dark regions. In (b) after 15 s of the bombardment, the image contrast of the steps and defects decreases and in (c) their images disappear. The shapes of the steps with faint contrast in (b) are quite similar to those in (a) and this means that the steps are not destroyed at this stage and their disappearance in (c) is simply due to the formation of defects (primary defects} whose contrast masks the step contrast. This is the reason why the steps reappear at the same positions after slight annealing as seen in fig. 2. It is noted that a RHEED pattern corresponding to a surface similar to fig. l(c) shows diffuse rods 191, which indicates that surface layers are crystalline. Figs. 3(a) and (b) are REM images before and after bombardment, respectively. In (c) steps with shapes similar to those in (a) reappear after annealing at 250 o C. Defects (secondary defects} are formed uniformly on the terraces between the steps. By annealing at 450’ C these defects coagulate to form larger defects on the terraces, as seen in (d). They may be monolayer deep hollows because they interact with the steps. Finally by annealing at 800 o C defects are eliminated and a smooth surface with the steps of different configuration is obtained (e). These results are for cases when the bombardment time is 30 or so. When the bombardment time is longer, annealing at higher temperature is necessary to obtain flat and defect-free surfaces. Fig. 4 shows an example. A REM image shown in (a) was taken after 30 s bombardment
Fig. 2. A sequence of REM images of a Pt(ll1) bomb~dment
surface during
at room temperature.
at room temperature and 60 s annealing at 900* C. Then the surface was bombarded for 300 s (b). Images of bundles of steps as well as single steps are completely masked by images of the primary defects. A REM image (c) was taken after 60 s annealing at 900°C. Secondary defects still remain. A notable fact is that the atomic steps seen in (a) are not identified in (c) in strong contrast to the case of fig. 3, although the bundles of steps reappear again at similar positions. A REM image (d) was taken after 480 s annealing at 900 o C. Secondary defects grow from (c) to (d) and this means that the secondary defects formed after heavy bombardment cannot be annealed out at 900 o C even after a long annealing time, whereas the secondary defects formed after slight bombardment are easily annealed out at this temperature (fig. 2). To get a defect free surface, annealing at 1100 *C is needed as shown in (e). In (e) a step configuration is quite different from that in (a). Changes in the bundles of steps are also noted. VII. SPUTTERING/SIMS
476 3.2. Si(l11)
S. Ogawa et al. / In situ studies of fast atom bombardment surface
Fig. 5 shows REM images and corresponding RHEED patterns before and after the bombardment of the Si(ll1) surface for 15 s at room temperature. In (a) surface atomic steps are clearly seen together with bundles of steps seen as dark bands. The corresponding RHEED pattern shows sharp spots and Kikuchi lines. These images and patterns completely disappear in (b) and (d). In (d) only diffuse scattering is seen. This fact indicates that the surface layers are amorphous, as was reported previously [2,6]. These amorphous layers re-
Fig. 4. A sequence of REM images of a Pt(ll1)
surface which show a dose dependence of the annealing processes. (a) 60 s at 900 ’ C after 30 s bombardment. (b) 300 s bombardment. (c) 60 sat900°C.(d)480sat9000C.(e)60sat11000C.
Fig. 3. A sequence of REM images of a Pt(ll1) surface which shows a bombardment process ((a) and (b)) at room temperature and annealing processes ((c) 250 OC, (d) 450 o C, (e) 800 o C).
crystallize expitaxially after annealing [6]. In the RHEED pattern diffuse rods appear at about 400°C and Kikuchi lines are clear at 800°C. However, the surfaces contain a lot of secondary defects and sharp horizontal Kikuchi lines and flat surface images appear by annealing above 1000°C. The nature of the secondary defects depends on the thickness of the amorphous layer. Thus, the annealing process depends on the dose of Ar atoms. It was reported that annealing at 500 o C was enough to reproduce surface atomic steps in the case of a low dose of ions [lo]. When the surfaces are bombarded at elevated temperatures, sputtering, defect formation and annealing take place simultaneously. Fig. 6 shows a case of bombardment at 700” C. In (a), steps are seen. They move between images (a) and (b), which was taken after 15 s bombardment. The motions are due to the migration of vacancies (annealing effect) formed on the surface (sputtering) to the steps. At the same time, defects which show fine speckle images appear all over the surface. These defects also act as sinks for vacancies and their formation and growth mask the step images in the later stages, as seen in (c). A notable fact is that the diffraction spots remain in the corresponding RHEED
S. Ogawa et al. / In situ studies of fast atom bombardment
Fig. 5. REM images and corresponding RHEED patterns of a Si(ll1) surface before and after bombardment An amorphous surface is formed in (b).
pattern, although they are broader than those from perfect surfaces. The annealing during bombardment is enhanced at higher temperatures (900” C), as seen in fig. 7. At 900 o C, steps move spontaneously due to sublimation
at room temperature.
[7] as seen for step A in fig. 7(b) which was taken 165 s after (a). Micrographs (c), (d), (e) and (f) were taken at 0 s, 35 s, 105 s and 180 s after the start of the bombardment. Step B is seen to move from (c) to (e). The distance moved between images (c) and (d) is much
a, Osec
Fig. 6. A sequence of REM images of the Si(ll1) surface during bomb~dment
at 700 o C and a corresponding RHEED pattern. VII. SPUTTERING/SIMS
478
S. Ogawa et al. / In situ studies off&
atom bombardment
contrast of defects, changes in the surface structure during bombardment could not be studied in detail. Annealing processes are found to depend on the dose of Ar. For a low dose steps play an important role. Higher temperature annealing is necessary for higher doses. For high temperature bombardment, the annealing processes take place together with sputtering and defect formation. Ar atom fluxes on the surfaces during bombardment are hard to measure because of the narrow space and a complex geometry around the specimen. It should be noted that about 0.3-0.4 of a monolayer of the Pt(ll1) surface is removed by the bombardment of 20 s at 100 o C (compare fig. 3(f) with fig. 3(a) of ref. [8]) and 0.3-0.4 double layers of Si(ll1) are removed by the bombardment of 35 s at 900’ C (see figs. 7(c) and (d)). If we assume a sputtering yield of 5, the flux is estimated to be about 2-4 x 1014 atoms/cm* s. Claverie et al. [lo] observed that steps reappeared at the same positions when a Si(ll1) surface was annealed at 500 o C after bombardment of 5.5 x lOi ions/cm* (Ar 10 kV) at room temperature. This observation is reasonable because the dose is not high enough to produce sufficient vacancies to cause an observable shift in the steps. It should be noted that surface vacancies are mobile at this temperature [8]. In the in situ REM studies of surfaces, techniques have been developed which produce various modifications of surfaces. Bombardment of such surfaces will continue to be interesting.
,/-._
I
e
Fig. 7. REM images of the Si(ll1) surface at 900” C taken during sublimation ((a) 0 s, (b) 165 s) and bombardment ((c) 0 s, (d) 35 s, (e) 105 s, (f) 300 s). A motion of step A due to sublimation and that of step B due to sputtering are seen. longer than that moved by the step A between images (a) and (b), which means that the former motion is mainly due to the sputtering. The defect density in (d) is much smaller than that in fig. 6(b) for a lower dose of Ar. A velocity of the step motion during the exposure of the images (d) and (e) is smaller than that during (c) (d). This is due to an increase of vacancy capture by the defects. In (f) the step images are almost completely masked by the defects. It is clear that the annealing processes at 900 o C are not rapid enough for a simultaneous and complete elimination of the defects formed by the bombardment.
4. Summary and discussion Fast atom bombardment and annealing processes are observed in situ by REM. Due to the strong image
This work was supported by a Grant in aid for Scientific Research (Nos. 60220007 and 62102006) from the Ministry of Education of Japan.
References [l] M. Kaminsky, Atomic and Ion Impact Phenomena on Metal Surfaces (Springer-Verlag, Berlin, 1965). [2] G.K. Wehner and G.S. Auderson, in: Handbook of Thin Film Technology, eds. L.I. Maissel and R. Glang (McGraw Hill, New York, 1970) Chap. 3. [3] G.K. Wehner, Phys. Rev. 102 (1956) 690. [4] R. Smith and J.M. Walls, Surf. Sci. 80 (1979) 557. [5] J.A. Kubby and B.M. Siegel, Nucl. Instr. and Meth. B13 (1986) 319. [6] E. Morita, K. Takayanagi, K. Kobayashi, K. Yagi and G. Honjo, Jpn J. Appl. Phys. 19 (1980) 1981. [7] N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surf. Sci. 97 (1980) 393. [8] K. Yagi, J. Appl. Cryst. 20 (1987) 147. [9] S. Ogawa, K. Kobayashi, Y. Tanishiro, K. Takayanagi and K. Yagi, Proc. 11th Int. Cong. Electron Microscopy 2 (1986) 1349. [lo] A. Claverie, J. Faure, C. Vieu, J. Beauvillain and B. Jouffrey, Proc. 11th Int. Cong. Electron Microscopy 2 (1986) 1357.