- Email: [email protected]
Quantitative analysis of individual atom images in FIM of an ordered Ni4Mo alloy
===================================================================================
.:.:.:~:~................~...:.:.:.:.:.~......~....~....:.:~:.:.;....~...........~.*..:.:.~`
i~i~;i:~:~::::::::' ............................. : :":'~:~:~!~!~.~;~
applied surface science ELSEVIER
Applied SurfaceScience87/88 (1995) 291-297
Quantitative analysis of individual atom images in FIM of an ordered NiaMo alloy Masahiko Yamamoto *, Kouichi Nishikawa, Takeshi Nishiuchi Department of Materials Science and Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan
Received 10 July 1994; acceptedfor publication 1 August 1994
Abstract Quantitative measurements of the position and intensity of individual atom images in field ion microscopy of an ordered Ni4Mo alloy were made successfully. The accuracy of the atom positions in the image was investigated. Intensity distributions especially for low intensity regions around individual atoms were demonstrated successfully by both contour maps and a perspective view. The effects of the geometry and surrounding atoms were discussed.
1. Introduction A field-ion microscope (FIM) image can give an individual atom image. The information in such an individual atom image is very useful in terms of position and intensity. Positions of atom images show the atomistic structure of the tip material. In order to learn about the accuracy of atom images, quantitative analysis of individual atom images is necessary. On the other hand, the intensities of atom images reflect the local electric field about the atom and tunneling characteristics because the FIM image is formed by tunneling of an electron from an imaging atom. Here, also, it is required to quantitatively analyze the intensity of the individual atom images. Analyses of individual atom images in field-ion
* Corresponding author. Fax: +81 6 876 4729; E-mail: [email protected]
images have been made so far [1,2], although they are limited to qualitative or semi-quantitative analyses. For the present study we selected an ordered NiaMo alloy and quantitatively measured the position and intensity of individual atom images. Then we investigated the accuracy of the atom positions and intensity distributions around individual atoms.
2. Experimental procedure A 79,9at%Ni-20.1at%Mo alloy (chemically analyzed) was prepared by vacuum-melting electrolytic nickel of 99.9% purity and molybdenum powder of 99.9% purity in an alumina crucible. From the resulting ingot, wires of 0.16 mm in diameter were obtained by forging, swaging and drawing. The wire specimens were quenched from 1373 K into ice-water and subsequently annealed at 1073 K for 360 ks in order to attain the ordered state. Tips for FIM were made from the annealed wires by electropolishing.
0169-4332/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved SSDI 0169-4332(94)00500-1
292
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
FIM observation was made at 21 K by using He gas as an imaging gas. The obtained FIM image was recorded on Kodak 103AG film whose dynamic intensity range is from 103 to 1 0 4, which is enough for the present investigation. Here channel plate was not used in order to exclude artifact noise. Subsequently, the micrograph was quantitatively measured by a densitometer with a resolution of 256. The digitalized intensity was processed by computer and displayed in various forms. Each atom image position was determined from the maximum intensity position.
3. Experimental results and discussion 3.1. Position of individual atom images Fig. la is a field-ion image of an ordered Ni4Mo alloy and Fig. lb is the schematic representation corresponding to Fig. la. The crystal structure of ordered NiaMo phase is body-centered tetragonal (bct) and so the planes are indexed on the basis of the bct lattice. Determination of the planes was made by the method developed by Yamamoto et al. [3,4]. In a number of planes, individual atom images can be seen. Figs. 2a-2i are the measured results of the (222), (442) and (552) planes, respectively. The (222) and (442) planes are examples of unmixed planes, which are called superlattice planes, and the (552) plane is an example of a mixed plane, which is called a fundamental plane. All superlatfice planes in the ordered NiaMo structure consist of one Mo layer and four Ni layers, and every layer of all fundamental planes consists of one Mo and four Ni atoms. Figs. 2a, 2 d a n d 2g are FIM images digitalized by a densitometer. Figs. 2b, 2e and 2h show the topmost high intensity position obtained from the digitalized FIM image of Figs. 2a, 2d and 2g, respectively, and the angle between the principal atom rows. Figs. 2c, 2f and 2i show atom arrangements, angles between the principal atom rows and the distances between adjacent atoms in the principal atom rows which are predicted from X-ray structure analysis. Highly intense atom images in Figs. 2a, 2d and 2g are saturated in their intensity. However, this is made to show weak atom images clearly and the real picture
-~ 3i3 01~1 [3 TO, 1 1 .
001 ]13 T11 " ~ ' ~ J 5 ,TO
Fig. 1. (a) Field-ion image of an ordered Ni4Mo alloy which was annealed for 360 ks at 1073 K after quenching from 1373 K. Image is taken at 21 K. (b) Schematic representation corresponding to (a). Planes and zone axes are indexed on the basis of a bct lattice.
does not saturate. In order to prove this, a perspective view of the intensity distribution is given in Fig. 3 for the case of the (222) plane. The tops of all peaks have a smooth shape. A number of planes were investigated and summ ~ z e d in the Tables 1 and 2, for the cases of domains I and II, respectively. Here, S and F stand for the superlattice and fundamental planes, respectively. [uvw] is the direction of the atom row. 0 is the angle between the atom rows. L1/L 2 is the ratio of the distance between adjacent atom images. From
293
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
[b)
•
•
(h)
(e)
•
[01~
[Toll 84.~'~
o
o
•
[0]-11
• •
•
•
•
•
•
•
•
•
• •
[110]
•
•
•
•
•
0"6746nm~_. q
•• •
•• •
•
•
•
8°
•
•
•
•
•
•
•
[1]-01
•
•
•
(i) 0.9146nm~
73.8"~
•
•
•
(f)
Co)
[11g]
•
126.2°
•
•
•
•
•
- - 1 £
~1
o~ O • • • 16.3 • • • , i i 0.8100nm • • •
0.81 OOnm
~
~
i TM
fl
~
•
0.9804nrn
0.6746nm
Fig. 2. Intensity distribution around individual atoms in (a) (222), (d) (442) and (g) (552) planes, respectively; (b), (e) and (h) show atom positions analyzed from (a), (d) and (g), respectively; (c), (f) and (i) show crystallographic atom arrangements for each plane.
Table 1 Comparison between observed values and calculated values (domain I) (hkl)
(141) (150) (251) ~61) (271) (163) (381) (192) a
S/F S S S S S S S S
a
[UlUIW1] , [U21)2W2] [1151, [101] [513], [001] [311], [113] [313], [101] [1151, [1021 [012], [311] [115], [311] [115], [313]
S: superlattice plane; F: fundamental plane.
AO/Ocalc.
L1/L 2
Obs.
0 (deg) Calc.
(%)
Obs.
Calc.
89.3 71.3 70.3 61.8 125.5 115.6 91.4 100.6
93.4 69.9 67.9 64.0 121.8 110,2 100.8 107,7
-4.4 2.0 3.5 -4.8 3.0 4.9 --9.3 - 6.6
1.33
1.01
31.7
1.33 1.17 1.08 1.10 1.01 1.01
1.37 1.55 1.05 1.00 0.94 1.07
- 2.9 -24.5 2.9 8.9 7.4 - 5.6
A (L1/Lz)/(L1/Lz)ealc.
(%)
M. Yamamoto et al. /Applied Surface Science 87/88 (1995) 291-297
294
the above analyses the following results are summarized:
(1) The values of the angles between the principal atom rows in the FIM images are comparable to the values of the angles predicted from the crystal structure. The values of A 0/0talc are within a 5% accuracy. (2) But the distances between adjacent atom images do not coincide when we consider the Value of
[10]-]
A(L1/L2)/(L1/L2)calc.
(3) Images of atoms located in the periphery of the plane are shifted toward the outer side. (4) The quantity of the shift increases with increasing the applied voltage. (5) In the superlattice planes the top Mo layer contributes to the images; in the fundamental plane only the Mo atoms contribute to the image.
Fig. 3. Perspective view of the intensity distribution in a (222) plane.
intensity peaks are isolated, they have a symmetrical shape. The intensity of the atom image was several times stronger than that of an inner atom. Figs. 5a and 5b are contour maps of intensity distributions in the (552) plane. The (552) plane is an example of a fundamental plane. Fig. 5a is focused at the overall intensity distribution and so the high
3:2. Intensity of individual atom images Fig. 4 is a contour map of an intensity distribution in the (732) plane. The (732) plane is an example of a superlattice plane and a high index plane. Since all
Table 2 Comparison between observed values and calculated values (domain II)
(hkl)
S/F
a
[UlU1W1] , [U2U2W2]
0
(deg)
a o~ ooa~o.
Obs.
Calc.
(%)
L1/L2 Obs.
Calc.
A (Zl/Z2)/(Zl/Z2)ealc. (%)
(442) (321) (222) (501)
S S S F
[1i0], [012], [011], [0101,
[012] [111] [101] [115]
126.2 81.6 84.1 74.6
116.3 84.8 73.8 73.0
8.5 -3.8 14.0 2.2
1.05
0.88
19.3
1.15 1.08
1.00
1.71
15.0 -36.8
(332) (521) (631)
S S F
[ii3], [110] ~131, [131] [113], [120]
93.2 117.0 123.5
90.0 112.1 124.9
3.6 4.4 -1.1
1.62 1.40 2.11
1.20 1.37 1.90
35.0 2.2 11.1
(433) (334) (552) (732)
S S F S
[3131, [1i0], [1151, [117],
[011] [133] [110] [151]
87.1 117.7 91.8 114.5
92.2 112.6 90.0 110.9
-5.5 4.5 2.0 3.2
1.83 1.24 1.13 1.09
1.55 1.29 0.82 0.94
18.1 -3.9 37.8 16.0
(534) (554)
S F
[313], [131] [1i0], [1371
60.2 113.0
65.9 108.6
-8.6 4.1
1.14 1.16
0.87 1.56
31.0 -25.6
S: superlattice plane; F: fundamental plane.
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
intensity peaks can be clearly seen. Fig. 5b is focused at the low intensity parts, so that the weak intensity part with the Ni atoms can be seen. In Fig: 5a intensity peak B is the strongest and A is the next strongest. Both are symmetrical although A is located at an edge site and B is located at a kink site, Intensity peaks D and E have the same tendency as intensity peaks A and B. Another case are the intensity peaks C and F, which are located in the middle of atom rows. C is not symmetrical, but its high intensity part runs slightly toward the inner part, as shown by the arrow (Fig. 5a). This modification from a circle suggests that the image of the Mo layer is modified by the surrounding Ni atoms. Weak intensity peaks in the inner part of the plane (peaks surrounded by peaks A to F) have a circular shape and they are not distorted much. In Fig. 5b, a weak intensity can be seen around the Ni position. However, intensity does not reflect
295
thecorrect atom shape. This suggests that the surface electronic cloud may change. In the high density plane, i.e.', the 10windex plane such as the (222) plane, the intensity of grooves between adjacent atoms is relatively high and no atom image is symmetrical. Suchorski et al. measured the energy distribution experimentally and derived the local electric field at and near the individual surface atoms [5-8] after the theoretical prediction of the local field enhancement above the single atom on the surface [9]: Suchorski and Block proposed the concept of the non-destructive atom probe, based on measurements of the energy distribution and the derived local electric field [10]. The results in the present study clearly show that intensity measurements can be made quantitatively and show that measurements of local electric fields are possible because the local intensity is related to the loc~il electric field. The present study is
Fig. 4. Intensitydistribution i~a (732) plane shownin a.contourmap~
296
M. Yamamoto et aL /Applied Surface Science 8 7 / 8 8 (1995) 291-297
Fig. 5. Intensity distribution in a (552) plane shown in a contour map; (a) overall view and (b) low intensity view.
M. Yamamoto et al. /Applied Surface Science 87/88 (1995) 291-297
very important in developing the non-destructive atom probe described above.
297
stated that intensity distribution measurements contribute to the development of a non-destructive atom probe.
4. Conclusions Acknowledgements Quantitative measurements of the position and intensity of field ion microscopy individual atom images of an ordered Ni4Mo alloy were made successfully. Conceming the atom position in the FIM images, the values of the angles between the principal atom rows coincide with the values of theangles predicted from the crystal structure within a 5% accuracy. However, distance.s between adjacent atom images do not coincide well, as is shown in the value Of A ( L 1 / L 2 ) / ( L 1 / L 2 ) c a l c . Atom images located in the periphery of the plane are shifted toward the outer side. The quantity of the shift increases with increasing the applied voltage. Intensity distributions can be demonstrated successfully by both contour maps and by perspective view. All intensity peaks at the isolated atoms in the high index planes have a symmetrical shape. In the mixed planes, the peaks of the M o atoms are influenced by the adjacent Ni atoms and they have an intensity toward the Ni atom side. Although a weak intensity can be seen around the Ni position, the intensity does not reflect the correct atom shape, due to the change of the surface electronic cloud. In the low index plane, i.e. the high density plane, the intensity of grooves between adjacent atoms is relatively high and no atom image is symmetrical. It is
The authors express their appreciation for the help of Mr. H. Matsumoto (Osaka University). This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Japanese Ministry of Education, Science and Culture.
References [1] M.K. Miller and G.D.W. Smith, in: Atom-Probe Microanalysis (MRS, Pittsburgh, PA, 1989) p. 61. [2] T.T. Tsong, in: Atom-Probe Field Ion Microscopy (Cambridge University Press, Cambridge, 1990) ch. 2 and 4. [3] M. Yamamoto, M. Futamoto, S. Nenno and S. Nakamura, J. Phys. Soc. Jpn. 36 (1974) 1330. [4] M. Yamamoto, S. Nenno, M. Futamoto and S. Nakamura, Jpn. J. Appl. Phys. 11 (1972) 437. [5] Yu. Suchorski, W.A. Schmidt and J.H. Block, Appl. Surf. Sci. 67 (1993) 124. [6] W.A. Schmidt, N. Ernst and Yu. Suchorski, Appl. Surf. Sci. 67 (1993) 101. [7] N. Ernst, G. Bozdech, H. Schmidt, W.A. Schmidt and G.L. Larkins, Appl. Surf. Sci. 67 (1993) 111. [8] Yu. Suchorski, W.A. Schmidt, J.H. Block and H.J. Kreuzer, Vacuum 45 (1994) 259. [9] H.J. Kreuzer, L.C. Wang and N.D. Lang, Phys. Rev. B 45 (1992) 12050. [10] Yu. Suchorski and J.H. Block, commented in the 40th International Field Emission Symposium, Nagoya, Japan (1993).
.:.:.:~:~................~...:.:.:.:.:.~......~....~....:.:~:.:.;....~...........~.*..:.:.~`
i~i~;i:~:~::::::::' ............................. : :":'~:~:~!~!~.~;~
applied surface science ELSEVIER
Applied SurfaceScience87/88 (1995) 291-297
Quantitative analysis of individual atom images in FIM of an ordered NiaMo alloy Masahiko Yamamoto *, Kouichi Nishikawa, Takeshi Nishiuchi Department of Materials Science and Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan
Received 10 July 1994; acceptedfor publication 1 August 1994
Abstract Quantitative measurements of the position and intensity of individual atom images in field ion microscopy of an ordered Ni4Mo alloy were made successfully. The accuracy of the atom positions in the image was investigated. Intensity distributions especially for low intensity regions around individual atoms were demonstrated successfully by both contour maps and a perspective view. The effects of the geometry and surrounding atoms were discussed.
1. Introduction A field-ion microscope (FIM) image can give an individual atom image. The information in such an individual atom image is very useful in terms of position and intensity. Positions of atom images show the atomistic structure of the tip material. In order to learn about the accuracy of atom images, quantitative analysis of individual atom images is necessary. On the other hand, the intensities of atom images reflect the local electric field about the atom and tunneling characteristics because the FIM image is formed by tunneling of an electron from an imaging atom. Here, also, it is required to quantitatively analyze the intensity of the individual atom images. Analyses of individual atom images in field-ion
* Corresponding author. Fax: +81 6 876 4729; E-mail: [email protected]
images have been made so far [1,2], although they are limited to qualitative or semi-quantitative analyses. For the present study we selected an ordered NiaMo alloy and quantitatively measured the position and intensity of individual atom images. Then we investigated the accuracy of the atom positions and intensity distributions around individual atoms.
2. Experimental procedure A 79,9at%Ni-20.1at%Mo alloy (chemically analyzed) was prepared by vacuum-melting electrolytic nickel of 99.9% purity and molybdenum powder of 99.9% purity in an alumina crucible. From the resulting ingot, wires of 0.16 mm in diameter were obtained by forging, swaging and drawing. The wire specimens were quenched from 1373 K into ice-water and subsequently annealed at 1073 K for 360 ks in order to attain the ordered state. Tips for FIM were made from the annealed wires by electropolishing.
0169-4332/95/$09.50 © 1995 Elsevier ScienceB.V. All rights reserved SSDI 0169-4332(94)00500-1
292
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
FIM observation was made at 21 K by using He gas as an imaging gas. The obtained FIM image was recorded on Kodak 103AG film whose dynamic intensity range is from 103 to 1 0 4, which is enough for the present investigation. Here channel plate was not used in order to exclude artifact noise. Subsequently, the micrograph was quantitatively measured by a densitometer with a resolution of 256. The digitalized intensity was processed by computer and displayed in various forms. Each atom image position was determined from the maximum intensity position.
3. Experimental results and discussion 3.1. Position of individual atom images Fig. la is a field-ion image of an ordered Ni4Mo alloy and Fig. lb is the schematic representation corresponding to Fig. la. The crystal structure of ordered NiaMo phase is body-centered tetragonal (bct) and so the planes are indexed on the basis of the bct lattice. Determination of the planes was made by the method developed by Yamamoto et al. [3,4]. In a number of planes, individual atom images can be seen. Figs. 2a-2i are the measured results of the (222), (442) and (552) planes, respectively. The (222) and (442) planes are examples of unmixed planes, which are called superlattice planes, and the (552) plane is an example of a mixed plane, which is called a fundamental plane. All superlatfice planes in the ordered NiaMo structure consist of one Mo layer and four Ni layers, and every layer of all fundamental planes consists of one Mo and four Ni atoms. Figs. 2a, 2 d a n d 2g are FIM images digitalized by a densitometer. Figs. 2b, 2e and 2h show the topmost high intensity position obtained from the digitalized FIM image of Figs. 2a, 2d and 2g, respectively, and the angle between the principal atom rows. Figs. 2c, 2f and 2i show atom arrangements, angles between the principal atom rows and the distances between adjacent atoms in the principal atom rows which are predicted from X-ray structure analysis. Highly intense atom images in Figs. 2a, 2d and 2g are saturated in their intensity. However, this is made to show weak atom images clearly and the real picture
-~ 3i3 01~1 [3 TO, 1 1 .
001 ]13 T11 " ~ ' ~ J 5 ,TO
Fig. 1. (a) Field-ion image of an ordered Ni4Mo alloy which was annealed for 360 ks at 1073 K after quenching from 1373 K. Image is taken at 21 K. (b) Schematic representation corresponding to (a). Planes and zone axes are indexed on the basis of a bct lattice.
does not saturate. In order to prove this, a perspective view of the intensity distribution is given in Fig. 3 for the case of the (222) plane. The tops of all peaks have a smooth shape. A number of planes were investigated and summ ~ z e d in the Tables 1 and 2, for the cases of domains I and II, respectively. Here, S and F stand for the superlattice and fundamental planes, respectively. [uvw] is the direction of the atom row. 0 is the angle between the atom rows. L1/L 2 is the ratio of the distance between adjacent atom images. From
293
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
[b)
•
•
(h)
(e)
•
[01~
[Toll 84.~'~
o
o
•
[0]-11
• •
•
•
•
•
•
•
•
•
• •
[110]
•
•
•
•
•
0"6746nm~_. q
•• •
•• •
•
•
•
8°
•
•
•
•
•
•
•
[1]-01
•
•
•
(i) 0.9146nm~
73.8"~
•
•
•
(f)
Co)
[11g]
•
126.2°
•
•
•
•
•
- - 1 £
~1
o~ O • • • 16.3 • • • , i i 0.8100nm • • •
0.81 OOnm
~
~
i TM
fl
~
•
0.9804nrn
0.6746nm
Fig. 2. Intensity distribution around individual atoms in (a) (222), (d) (442) and (g) (552) planes, respectively; (b), (e) and (h) show atom positions analyzed from (a), (d) and (g), respectively; (c), (f) and (i) show crystallographic atom arrangements for each plane.
Table 1 Comparison between observed values and calculated values (domain I) (hkl)
(141) (150) (251) ~61) (271) (163) (381) (192) a
S/F S S S S S S S S
a
[UlUIW1] , [U21)2W2] [1151, [101] [513], [001] [311], [113] [313], [101] [1151, [1021 [012], [311] [115], [311] [115], [313]
S: superlattice plane; F: fundamental plane.
AO/Ocalc.
L1/L 2
Obs.
0 (deg) Calc.
(%)
Obs.
Calc.
89.3 71.3 70.3 61.8 125.5 115.6 91.4 100.6
93.4 69.9 67.9 64.0 121.8 110,2 100.8 107,7
-4.4 2.0 3.5 -4.8 3.0 4.9 --9.3 - 6.6
1.33
1.01
31.7
1.33 1.17 1.08 1.10 1.01 1.01
1.37 1.55 1.05 1.00 0.94 1.07
- 2.9 -24.5 2.9 8.9 7.4 - 5.6
A (L1/Lz)/(L1/Lz)ealc.
(%)
M. Yamamoto et al. /Applied Surface Science 87/88 (1995) 291-297
294
the above analyses the following results are summarized:
(1) The values of the angles between the principal atom rows in the FIM images are comparable to the values of the angles predicted from the crystal structure. The values of A 0/0talc are within a 5% accuracy. (2) But the distances between adjacent atom images do not coincide when we consider the Value of
[10]-]
A(L1/L2)/(L1/L2)calc.
(3) Images of atoms located in the periphery of the plane are shifted toward the outer side. (4) The quantity of the shift increases with increasing the applied voltage. (5) In the superlattice planes the top Mo layer contributes to the images; in the fundamental plane only the Mo atoms contribute to the image.
Fig. 3. Perspective view of the intensity distribution in a (222) plane.
intensity peaks are isolated, they have a symmetrical shape. The intensity of the atom image was several times stronger than that of an inner atom. Figs. 5a and 5b are contour maps of intensity distributions in the (552) plane. The (552) plane is an example of a fundamental plane. Fig. 5a is focused at the overall intensity distribution and so the high
3:2. Intensity of individual atom images Fig. 4 is a contour map of an intensity distribution in the (732) plane. The (732) plane is an example of a superlattice plane and a high index plane. Since all
Table 2 Comparison between observed values and calculated values (domain II)
(hkl)
S/F
a
[UlU1W1] , [U2U2W2]
0
(deg)
a o~ ooa~o.
Obs.
Calc.
(%)
L1/L2 Obs.
Calc.
A (Zl/Z2)/(Zl/Z2)ealc. (%)
(442) (321) (222) (501)
S S S F
[1i0], [012], [011], [0101,
[012] [111] [101] [115]
126.2 81.6 84.1 74.6
116.3 84.8 73.8 73.0
8.5 -3.8 14.0 2.2
1.05
0.88
19.3
1.15 1.08
1.00
1.71
15.0 -36.8
(332) (521) (631)
S S F
[ii3], [110] ~131, [131] [113], [120]
93.2 117.0 123.5
90.0 112.1 124.9
3.6 4.4 -1.1
1.62 1.40 2.11
1.20 1.37 1.90
35.0 2.2 11.1
(433) (334) (552) (732)
S S F S
[3131, [1i0], [1151, [117],
[011] [133] [110] [151]
87.1 117.7 91.8 114.5
92.2 112.6 90.0 110.9
-5.5 4.5 2.0 3.2
1.83 1.24 1.13 1.09
1.55 1.29 0.82 0.94
18.1 -3.9 37.8 16.0
(534) (554)
S F
[313], [131] [1i0], [1371
60.2 113.0
65.9 108.6
-8.6 4.1
1.14 1.16
0.87 1.56
31.0 -25.6
S: superlattice plane; F: fundamental plane.
M. Yamamoto et al. /Applied Surface Science 8 7 / 8 8 (1995) 291-297
intensity peaks can be clearly seen. Fig. 5b is focused at the low intensity parts, so that the weak intensity part with the Ni atoms can be seen. In Fig: 5a intensity peak B is the strongest and A is the next strongest. Both are symmetrical although A is located at an edge site and B is located at a kink site, Intensity peaks D and E have the same tendency as intensity peaks A and B. Another case are the intensity peaks C and F, which are located in the middle of atom rows. C is not symmetrical, but its high intensity part runs slightly toward the inner part, as shown by the arrow (Fig. 5a). This modification from a circle suggests that the image of the Mo layer is modified by the surrounding Ni atoms. Weak intensity peaks in the inner part of the plane (peaks surrounded by peaks A to F) have a circular shape and they are not distorted much. In Fig. 5b, a weak intensity can be seen around the Ni position. However, intensity does not reflect
295
thecorrect atom shape. This suggests that the surface electronic cloud may change. In the high density plane, i.e.', the 10windex plane such as the (222) plane, the intensity of grooves between adjacent atoms is relatively high and no atom image is symmetrical. Suchorski et al. measured the energy distribution experimentally and derived the local electric field at and near the individual surface atoms [5-8] after the theoretical prediction of the local field enhancement above the single atom on the surface [9]: Suchorski and Block proposed the concept of the non-destructive atom probe, based on measurements of the energy distribution and the derived local electric field [10]. The results in the present study clearly show that intensity measurements can be made quantitatively and show that measurements of local electric fields are possible because the local intensity is related to the loc~il electric field. The present study is
Fig. 4. Intensitydistribution i~a (732) plane shownin a.contourmap~
296
M. Yamamoto et aL /Applied Surface Science 8 7 / 8 8 (1995) 291-297
Fig. 5. Intensity distribution in a (552) plane shown in a contour map; (a) overall view and (b) low intensity view.
M. Yamamoto et al. /Applied Surface Science 87/88 (1995) 291-297
very important in developing the non-destructive atom probe described above.
297
stated that intensity distribution measurements contribute to the development of a non-destructive atom probe.
4. Conclusions Acknowledgements Quantitative measurements of the position and intensity of field ion microscopy individual atom images of an ordered Ni4Mo alloy were made successfully. Conceming the atom position in the FIM images, the values of the angles between the principal atom rows coincide with the values of theangles predicted from the crystal structure within a 5% accuracy. However, distance.s between adjacent atom images do not coincide well, as is shown in the value Of A ( L 1 / L 2 ) / ( L 1 / L 2 ) c a l c . Atom images located in the periphery of the plane are shifted toward the outer side. The quantity of the shift increases with increasing the applied voltage. Intensity distributions can be demonstrated successfully by both contour maps and by perspective view. All intensity peaks at the isolated atoms in the high index planes have a symmetrical shape. In the mixed planes, the peaks of the M o atoms are influenced by the adjacent Ni atoms and they have an intensity toward the Ni atom side. Although a weak intensity can be seen around the Ni position, the intensity does not reflect the correct atom shape, due to the change of the surface electronic cloud. In the low index plane, i.e. the high density plane, the intensity of grooves between adjacent atoms is relatively high and no atom image is symmetrical. It is
The authors express their appreciation for the help of Mr. H. Matsumoto (Osaka University). This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Japanese Ministry of Education, Science and Culture.
References [1] M.K. Miller and G.D.W. Smith, in: Atom-Probe Microanalysis (MRS, Pittsburgh, PA, 1989) p. 61. [2] T.T. Tsong, in: Atom-Probe Field Ion Microscopy (Cambridge University Press, Cambridge, 1990) ch. 2 and 4. [3] M. Yamamoto, M. Futamoto, S. Nenno and S. Nakamura, J. Phys. Soc. Jpn. 36 (1974) 1330. [4] M. Yamamoto, S. Nenno, M. Futamoto and S. Nakamura, Jpn. J. Appl. Phys. 11 (1972) 437. [5] Yu. Suchorski, W.A. Schmidt and J.H. Block, Appl. Surf. Sci. 67 (1993) 124. [6] W.A. Schmidt, N. Ernst and Yu. Suchorski, Appl. Surf. Sci. 67 (1993) 101. [7] N. Ernst, G. Bozdech, H. Schmidt, W.A. Schmidt and G.L. Larkins, Appl. Surf. Sci. 67 (1993) 111. [8] Yu. Suchorski, W.A. Schmidt, J.H. Block and H.J. Kreuzer, Vacuum 45 (1994) 259. [9] H.J. Kreuzer, L.C. Wang and N.D. Lang, Phys. Rev. B 45 (1992) 12050. [10] Yu. Suchorski and J.H. Block, commented in the 40th International Field Emission Symposium, Nagoya, Japan (1993).