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Convergent-beam electron diffraction in the high-voltage electron microscope with continuously variable reference voltage for lenses and alignments
Uhramicroscopy 39 (1991) 58-64 North-Holland
~d:~r,~m~~a
Convergent-beam electron diffraction in the high-voltage electron microscope with continuously variable reference voltage for lenses and alignments K. Kuroda
a, C. Morita b, S. Arai b, N. Y o k o i b and H. Saka ~
" Department of Materials Science and Engineering, Faculty of Engineering, Nagoya University, Nagoya 464-01, Japan
h Electron Optics Laboratory, Faculty of Engineering, Nagoya University, Nagoya 464-01, Japan Received 26 March 1991
A device which provides a continuously variable reference voltage for lenses and alignments is developed for the high-voltage scanning transmission electron microscope H-1250ST. When continuously varying the operating voltage, compensation for all lenses and alignments is easily performed by the adoption of the device. The device is especially useful for forming the convergent-beam electron diffraction (CBED) patterns at any accelerating voltage. The performance of the device is demonstrated by observing the CBED patterns. Large-angle CBED patterns are taken for Si- and Y-based hJgh-T~ superconductors at various voltages. Systematic critical voltages of Y-based high-T~ superconductors are measured for the 400 and 220 reflections.
1. Introduction
Convergent-beam electron diffraction (CBED) is now one of the more widely used techniques in materials science. CBED patterns are used to identify the high-symmetry zone axes and determine the diffraction group, point group and space group of crystals. Higher-order Laue zone (HOLZ) patterns in the bright-field disc of the zone axis CBED patterns are used to determine lattice parameters and local lattice distortions. The facility for continuous voltage variation between the usual stepped settings is an important accessory for CBED. Fine control of the electron wave vector is useful for direct measurement of small shifts in H O L Z deficiency line intersections caused by lattice parameter variations and also for exciting important H O L Z reflections that may affect the whole pattern symmetry. Furthermore, the direct control of the projected crystal potential via changing the accelerating voltage is essential for measurements of the systematic and zone axis critical voltages.
The systematic critical voltages were accurately determined in combination with the CBED in the high-voltage electron microscope (HVEM) [1]. For the accurate determination a technique changing the specimen temperature was also proposed in-
7 1500
-
6
5 1000
4
500
_ >
2 1
0
I 0
200
I
I
400
600 E
I
I
800
1000
I 1250
(kV)
Fig. 1. Reference voltage Vr for lenses and alignments and V@rr, where E r is the relativistic accelerating voltage, expressed as a function of the accelerating voltage E. V~ is directly proportional to 1/rE,. Reference voltages at stepped settings in the H-1250ST are indicated by open circles.
0304-3991/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
59
K. Kuroda et a L / CBED in H V E M
stead of varying the accelerating voltage, since the electron-optical parameters of the microscope are hardly kept constant at various voltages. An accurate determination of the zone axis critical voltage was made by observing dark-field CBED discs of the zero-order Laue zone reflections [2]. Since
zone axis critical voltages often occur at lower voltages than systematic critical voltages, the intermediate-voltage electron microscope is thought to be an effective tool. In the intermediate-voltage electron microscope, the operating voltage can be varied in steps of minimum 100 V and all lenses
I DC POWER
LENS COIL CIRCUIT LENS
I
SUPPLY
REFERENCE 1
REFERENCE
I
VOLTAGE [|SELECTOR TERMINAL ~MODE
VOLTAGE
STAB ILlZER AMP.
BANK CONTINUOUSLY (~ ---
VARIABLE REFERENCE
(~)
VOLTAGE
(~
t REFERENCE ] RES l STOR
:
[
ALIGNMENT COIL CIRCUIT
G ALIGNMENT REFERENCE VOLTAGE
Q o
(a) -l-15V
$÷15V
VARIABLE
~(~)FOR LENS
RES1STOR
"!"
CONTINUOUSLY VARIABLE REFERENCE VOLTAGE
FOR ALIGNMENT
-15V
(b) Fig. 2. (a) Schematic drawing of the experimental setup of the device which provides a continuously variable reference voltage for lenses and alignment; (b) electric circuit of the device.
60
K. Kuroda et aZ / C BED in H V E M
and alignments including stigmators can be compensated in conjunction with the 1 kV steps of the operating voltage [3]. The object of this paper is to describe a simple device for compensation for all lenses and alignments at any accelerating voltage. Some CBED patterns observed by using the device are also presented.
2. Experimental setup A high-voltage scanning transmission electron microsope H-1250ST at Nagoya University has a capability to vary continuously the accelerating voltage. This microscope, however, did not have a function for the lenses and alignments to be compensated with changes in the accelerating voltage. In order to produce CBED patterns at any operating voltage between the stepped settings, which are 400, 600, 800, 1000 and 1250 kV in the H1250ST, a very complicated operation was required to adjust lenses and alignments at each operating voltage. Fig. 1 shows the reference voltage V~ for lenses and alignments as a function of the accelerating voltage E. As shown in fig. 2a, the reference voltage is essential for the electric circuit which provides the current for lenses and alignments. Reference voltages at the stepped settings are indicated as open circles in fig. 1. If the accelerating voltage is between the stepped settings, the reference voltage is kept at the value for the higher setting, e.g., 5 V reference voltage 700 kV accelerating voltage, and the image-focusing is performed by changing the objective lens current. Therefore, in order to obtain focused images at any operating voltage, the objective lens current must be able to vary over a wide range. With changes in the objective lens current, the electron-optical parameters are varied, so that the electron-optical condition for forming the CBED pattern cannot be held at any operating voltage. The performance of a magnetic lens depends on its pole-piece. From studies on the practical aspects of pole-piece design by Liebmann, it is well known that f(NI)2/Er should be constant for a particular pole-piece geometry [4], where f is the focal length, NI the lens excitation and E r the
"relativistic accelerating voltage", E r = E ( 1 + 0.97845 x 10-6E). Therefore, if the reference voltage for the lenses and alignments at the operating voltage E varies proportional to ~-~-~, the excitation of lenses can be controlled to keep the same focal length; in other words, the electron-optical parameters for forming the CBED pattern can be essentially maintained at any accelerating voltage. The variation of ~ r r and Vr with the accelerating voltage is shown in the solid line in fig. 1. In order to control the excitation of lenses and alignments when varying the operating voltage, a simple device has been developed. The experimental setup of the device which provides the continuously variable reference voltage is schematically shown in fig. 2a and the electric circuit of the device in fig. 2b. This device has a function that the reference voltage for all lenses and alignments can be continuously controlled with a stability of 1 x 10 6/min associated with the operating voltage variation between 1250 and 200 kV. High-resolution images were taken at a magnification of 400000 times using the device. The micrograph
Fig. 3. HREM micrograph of gold particle taken at 1000 kV with the devicewhich provides the continuouslyvariable reference voltage. Lattice image showing0.204 nm of (200) spacing.
K. Kuroda et a L / CBED in HVEM
shown in fig. 3 indicates that the stability of the reference voltage of the device is more or less the same as that originally created in the microscope. The image-focusing is done using the variable resistors for coarse and fine controls of the reference voltage in the device. The focused image is obtained in the image mode at a particular operating voltage and then the diffraction pattern is obtained by just switching the microscope to the diffraction mode. This means that compensation for the all lenses and alignments can be performed almost automatically at any accelerating voltage by adjusting the image-focusing. The reference
61
voltage, which is monitored by a digital voltmeter, obviously followed the curve in fig. 1.
3. Applications 3.1. L A C B E D f r o m semiconductors and superconductors
Large-angle convergent-beam electron diffraction (LACBED) patterns from Si- and Y-based high-T~ superconductors were observed using the device which produces the continuously variable
Fig. 4. (100) LACBED patterns of Si at various accelerating voltages: (a) 500 kV, (b) 700 kV, (c) 800 kV, (d) 1000 kV.
62
K. Kuroda et al. / C BED in H VEM
reference voltage. LACBED patterns were obtained by the method of Tanaka et al. [5]. Once the LACBED pattern is produced at 1000 kV, the electron-optical condition for forming the LACBED patterns can be held down to 500 kV by continuously varying the reference voltage. Comprehensive studies of the relationship between the projected crystal potential and the form of zone axis patterns in high-energy electron diffraction were carried out by the Bristol group, and a characteristic sequence of the zone axis patterns was observed in the first Brillouin zone as a function of the accelerating voltage [6]. Similar experiments have been done by observing LACBED patterns. Fig. 4 shows the ~100) LACBED patterns of Si taken between 500 and 1026 kV. In these patterns fringes cross the zone edges orthogonally and they progressively take over the first zone by advancing towards its center. At 500 kV a very dark center is observed at the center of the zone due to the high absorption. An increase of the accelerating voltage makes the dark center smaller. The fringe system is accomplished by a remarkable reduction of absorption at the zone center. These LACBED patterns are quite different from those observed around 100 kV, in which a ring system centered on the axis also appears ]7,8]. At lower voltages, however, the shape of outer rings approximates that of the zone boundary, square in the (100) zone, but the rings near the center of the zone are approximately circular. Fig. 5 shows the ~111) LACBED pattern of Si taken at 1026 kV. Four central rings overlap the radial fringe system. The shape of the outermost ring is hexagonal. These tings are different from those observed in the LACBED patterns taken around 100 kV [7,8]. The outermost ring is pulled out towards the middle of the zone edges so that its vertices are rotated by 30 o in comparison with the ring system at lower voltages, in which the shapes of outer rings approximate that of the zone boundary. The brightness of the zone center indicates that the critical voltage is not far from 1026 kV. The observation of the ~111) zone axis pattern with increasing voltage from 500 kV showed that the sequence approaches the critical voltage. Radial lines extended almost to the zone center at
Fig. 5. (111) LACBED of Si at 1026 kV.
600 kV. The central spot appeared at 800 kV and its brightness increased with an increase of the operating voltage. The critical voltage should be recognized by the maximum brightness of the central spot,: and the calculated critical voltage is 1320 kV for the S i ~ l l l ) zone axis [6]. Fig. 6 shows the C001) LACBED patterns of the Y-based high-Tc superconductor, YBazCu ~ AI,~Ov, x ~ 0.2, y ~ 6.7, taken at various voltages. Because the patterns are obtained from a relatively large area, an appreciable variation of thickness can be observed accross the pattern. Even though the patterns are not perfect, the sequence of the patterns indicates that the brightness of the central spots increases with decreasing accelerating voltage. The appearance of the bright spot at the center was observed in the Kossel patterns and explained in terms of the excitation of the Bloch wave with a particular quantum number [9]. As mentioned before, the maximum brightness of the central spot appears at critical voltage. One of the ~001) zone axis critical voltages of this material is expected to be near 200 kV from this observation and from voltage variation experiments below 200 kV using a 200 kV microscope.
K. Kuroda et al. / CBED in H V E M
3.2. Critical ooltage effect in Y-based high-TC superconductors
Critical voltages were measured in Y-based high-T~ superconductors for the 220 and the 400 reflections in the 110 and the 100 systematic rows. There is no critical voltage for the second-order reflection in the 100 systematic row, since the structure factor for the 100 reflection is smaller than that for the 200 reflection. Fig. 7 shows a series of the CBED patterns from the same specimen that was used for the observation of LACBED patterns. The intensity at the exact Bragg position in the 400 convergent beam diffraction disc is very weak in fig. 7b. Experimental values of the 400 and 220 critical voltages have been determined to be 950_+ 10 and 755 + 5 kV, respectively. The
63
critical voltages have been calculated using crystallographic data for tetragonal YBa 2Cu 2.78A10.2206.4 by Siegrist et al. [10] and scattering factors for ionized atoms [11]. The calculated values of 993 kV for the 400 reflection and 753 kV for the 220 reflection are in fairly good agreement with experimental values. Fig. 8 shows the critical voltages of YBa 2 Cu2.vsA10.e2Oy calculated as a function of oxygen content by introducing oxygen vacancies at O1 sites (0, ½, 0). The increase of 5% oxygen vacancies causes a decrease of the 220 critical voltage by 9 kV and an increase of the 400 critical voltage by 5 kV. It should be noted that the 220 critical voltage increases and the 400 critical voltage decreases when the oxygen content increases. The systematic critical voltage is highly sensitive to the oxygen
Fig. 6. (001) LACBED patterns of Y-based superconductor at various accelerating voltages: (a) 413 kV, (b) 350 kV, (c) 250 kV, (d) 200 kV.
64
K, Kuroda et al. / C BED in H V E M
c o n t e n t , b u t a b s o l u t e q u a n t i f i c a t i o n u s i n g t h e critical v o l t a g e s e e m s to b e d i f f i c u l t to achieve. Critical v o l t a g e a n a l y s i s p r o v i d e s a n a c c u r a t e q u a n t i t a tive m e a s u r e of r e l a t i v e o x y g e n c o n t e n t .
4. Conclusion
T h e c o n t i n u o u s v o l t a g e v a r i a t i o n b e t w e e n the s t e p p e d s e t t i n g s is a n i m p o r t a n t f u n c t i o n in the h i g h - v o l t a g e e l e c t r o n m i c r o s c o p e , since d i r e c t c o n trol of the p r o j e c t e d c r y s t a l p o t e n t i a l b y c h a n g i n g the a c c e l e r a t i n g v o l t a g e is e s s e n t i a l for o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n a b o u t the c r y s t a l l i n e m a t e r i a l . T h e d e v i c e w h i c h p r o v i d e s the c o n t i n u o u s l y v a r i a b l e r e f e r e n c e v o l t a g e for lenses a n d a l i g n m e n t s a c h i e v e s c o m p e n s a t i o n for lenses and a l i g n m e n t s w h e n v a r y i n g the a c c e l e r a t i n g voltage. T h e d e v i c e is e s p e c i a l l y useful for o b t a i n i n g C B E D p a t t e r n s at a n y o p e r a t i n g v o l t a g e .
References
Fig. 7. CBED patterns of Y-based superconductor near the (400) critical voltage: (a) 980 kV, (b) 950 kV, (c) 930 kV.
850
1050
800
lO00
l oJ *2
% >
%
r~ u
u
G
E
c,J
75O
950
I
.3
6.5
I
I
6.7 6.9 Y Fig. 8. Critical voltages of YBa2Cu2.78AIo.220v calculated as a function of oxygen content.
[1] D. Imeson, J.R. Sellar and C.J. Humphreys, in: Proc. 37th Annu. EMSA Meeting, San Antonio, TX, 1979, Ed. G.W. Bailey (Claitor's, Baton Rouge, 1979) p. 434. [2] H. Matsuhata and J.W. Steeds, Phil. Mag. B 55 (1987) 17. [3] Y. Ishida, T. Honda, S. Suzuki, S. Mori, H. Kitajima, M. Nakajima and T. Taoka, in: Proc. l l t h lnt, Congr. on Electron Microscopy, Kyoto, 1986, Eds. T. Imura, S. Maruse and T. Suzuki (Jpn. Soc. of Electron Microscopy, Tokyo, 1986) p. 907. [4] P.B. Hirsch, A. Howie, R. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977). [5] M. Tanaka, R. Saito, K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [6] M.D. Shannon and J.W. Steeds, Phil. Mag. 36 (1977) 279. [7] K.K. Fung, Ultramicroscopy 12 (1984) 243. [8] M. Tanaka and M. Terauchi, Convergent-Beam Electron Microscopy (JEOL, Tokyo, 1985). [9] Y. Uchida and F. Fujimoto, Phys. Status Solidi (a) 43 (1977) 269. [10] T. Siegrist, L.F, Schneemeyer, J.V. Waszczac, N.P. Singh, R.L. Opila, B. Batlogg, L.W. Rupp and D.W. Murphy, Phys. Rev. B 36 (1987) 8365. [11] D.T. Cromer and J.B. Mann, Acta Cryst. A 24 (1968) 321.
~d:~r,~m~~a
Convergent-beam electron diffraction in the high-voltage electron microscope with continuously variable reference voltage for lenses and alignments K. Kuroda
a, C. Morita b, S. Arai b, N. Y o k o i b and H. Saka ~
" Department of Materials Science and Engineering, Faculty of Engineering, Nagoya University, Nagoya 464-01, Japan
h Electron Optics Laboratory, Faculty of Engineering, Nagoya University, Nagoya 464-01, Japan Received 26 March 1991
A device which provides a continuously variable reference voltage for lenses and alignments is developed for the high-voltage scanning transmission electron microscope H-1250ST. When continuously varying the operating voltage, compensation for all lenses and alignments is easily performed by the adoption of the device. The device is especially useful for forming the convergent-beam electron diffraction (CBED) patterns at any accelerating voltage. The performance of the device is demonstrated by observing the CBED patterns. Large-angle CBED patterns are taken for Si- and Y-based hJgh-T~ superconductors at various voltages. Systematic critical voltages of Y-based high-T~ superconductors are measured for the 400 and 220 reflections.
1. Introduction
Convergent-beam electron diffraction (CBED) is now one of the more widely used techniques in materials science. CBED patterns are used to identify the high-symmetry zone axes and determine the diffraction group, point group and space group of crystals. Higher-order Laue zone (HOLZ) patterns in the bright-field disc of the zone axis CBED patterns are used to determine lattice parameters and local lattice distortions. The facility for continuous voltage variation between the usual stepped settings is an important accessory for CBED. Fine control of the electron wave vector is useful for direct measurement of small shifts in H O L Z deficiency line intersections caused by lattice parameter variations and also for exciting important H O L Z reflections that may affect the whole pattern symmetry. Furthermore, the direct control of the projected crystal potential via changing the accelerating voltage is essential for measurements of the systematic and zone axis critical voltages.
The systematic critical voltages were accurately determined in combination with the CBED in the high-voltage electron microscope (HVEM) [1]. For the accurate determination a technique changing the specimen temperature was also proposed in-
7 1500
-
6
5 1000
4
500
_ >
2 1
0
I 0
200
I
I
400
600 E
I
I
800
1000
I 1250
(kV)
Fig. 1. Reference voltage Vr for lenses and alignments and V@rr, where E r is the relativistic accelerating voltage, expressed as a function of the accelerating voltage E. V~ is directly proportional to 1/rE,. Reference voltages at stepped settings in the H-1250ST are indicated by open circles.
0304-3991/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
59
K. Kuroda et a L / CBED in H V E M
stead of varying the accelerating voltage, since the electron-optical parameters of the microscope are hardly kept constant at various voltages. An accurate determination of the zone axis critical voltage was made by observing dark-field CBED discs of the zero-order Laue zone reflections [2]. Since
zone axis critical voltages often occur at lower voltages than systematic critical voltages, the intermediate-voltage electron microscope is thought to be an effective tool. In the intermediate-voltage electron microscope, the operating voltage can be varied in steps of minimum 100 V and all lenses
I DC POWER
LENS COIL CIRCUIT LENS
I
SUPPLY
REFERENCE 1
REFERENCE
I
VOLTAGE [|SELECTOR TERMINAL ~MODE
VOLTAGE
STAB ILlZER AMP.
BANK CONTINUOUSLY (~ ---
VARIABLE REFERENCE
(~)
VOLTAGE
(~
t REFERENCE ] RES l STOR
:
[
ALIGNMENT COIL CIRCUIT
G ALIGNMENT REFERENCE VOLTAGE
Q o
(a) -l-15V
$÷15V
VARIABLE
~(~)FOR LENS
RES1STOR
"!"
CONTINUOUSLY VARIABLE REFERENCE VOLTAGE
FOR ALIGNMENT
-15V
(b) Fig. 2. (a) Schematic drawing of the experimental setup of the device which provides a continuously variable reference voltage for lenses and alignment; (b) electric circuit of the device.
60
K. Kuroda et aZ / C BED in H V E M
and alignments including stigmators can be compensated in conjunction with the 1 kV steps of the operating voltage [3]. The object of this paper is to describe a simple device for compensation for all lenses and alignments at any accelerating voltage. Some CBED patterns observed by using the device are also presented.
2. Experimental setup A high-voltage scanning transmission electron microsope H-1250ST at Nagoya University has a capability to vary continuously the accelerating voltage. This microscope, however, did not have a function for the lenses and alignments to be compensated with changes in the accelerating voltage. In order to produce CBED patterns at any operating voltage between the stepped settings, which are 400, 600, 800, 1000 and 1250 kV in the H1250ST, a very complicated operation was required to adjust lenses and alignments at each operating voltage. Fig. 1 shows the reference voltage V~ for lenses and alignments as a function of the accelerating voltage E. As shown in fig. 2a, the reference voltage is essential for the electric circuit which provides the current for lenses and alignments. Reference voltages at the stepped settings are indicated as open circles in fig. 1. If the accelerating voltage is between the stepped settings, the reference voltage is kept at the value for the higher setting, e.g., 5 V reference voltage 700 kV accelerating voltage, and the image-focusing is performed by changing the objective lens current. Therefore, in order to obtain focused images at any operating voltage, the objective lens current must be able to vary over a wide range. With changes in the objective lens current, the electron-optical parameters are varied, so that the electron-optical condition for forming the CBED pattern cannot be held at any operating voltage. The performance of a magnetic lens depends on its pole-piece. From studies on the practical aspects of pole-piece design by Liebmann, it is well known that f(NI)2/Er should be constant for a particular pole-piece geometry [4], where f is the focal length, NI the lens excitation and E r the
"relativistic accelerating voltage", E r = E ( 1 + 0.97845 x 10-6E). Therefore, if the reference voltage for the lenses and alignments at the operating voltage E varies proportional to ~-~-~, the excitation of lenses can be controlled to keep the same focal length; in other words, the electron-optical parameters for forming the CBED pattern can be essentially maintained at any accelerating voltage. The variation of ~ r r and Vr with the accelerating voltage is shown in the solid line in fig. 1. In order to control the excitation of lenses and alignments when varying the operating voltage, a simple device has been developed. The experimental setup of the device which provides the continuously variable reference voltage is schematically shown in fig. 2a and the electric circuit of the device in fig. 2b. This device has a function that the reference voltage for all lenses and alignments can be continuously controlled with a stability of 1 x 10 6/min associated with the operating voltage variation between 1250 and 200 kV. High-resolution images were taken at a magnification of 400000 times using the device. The micrograph
Fig. 3. HREM micrograph of gold particle taken at 1000 kV with the devicewhich provides the continuouslyvariable reference voltage. Lattice image showing0.204 nm of (200) spacing.
K. Kuroda et a L / CBED in HVEM
shown in fig. 3 indicates that the stability of the reference voltage of the device is more or less the same as that originally created in the microscope. The image-focusing is done using the variable resistors for coarse and fine controls of the reference voltage in the device. The focused image is obtained in the image mode at a particular operating voltage and then the diffraction pattern is obtained by just switching the microscope to the diffraction mode. This means that compensation for the all lenses and alignments can be performed almost automatically at any accelerating voltage by adjusting the image-focusing. The reference
61
voltage, which is monitored by a digital voltmeter, obviously followed the curve in fig. 1.
3. Applications 3.1. L A C B E D f r o m semiconductors and superconductors
Large-angle convergent-beam electron diffraction (LACBED) patterns from Si- and Y-based high-T~ superconductors were observed using the device which produces the continuously variable
Fig. 4. (100) LACBED patterns of Si at various accelerating voltages: (a) 500 kV, (b) 700 kV, (c) 800 kV, (d) 1000 kV.
62
K. Kuroda et al. / C BED in H VEM
reference voltage. LACBED patterns were obtained by the method of Tanaka et al. [5]. Once the LACBED pattern is produced at 1000 kV, the electron-optical condition for forming the LACBED patterns can be held down to 500 kV by continuously varying the reference voltage. Comprehensive studies of the relationship between the projected crystal potential and the form of zone axis patterns in high-energy electron diffraction were carried out by the Bristol group, and a characteristic sequence of the zone axis patterns was observed in the first Brillouin zone as a function of the accelerating voltage [6]. Similar experiments have been done by observing LACBED patterns. Fig. 4 shows the ~100) LACBED patterns of Si taken between 500 and 1026 kV. In these patterns fringes cross the zone edges orthogonally and they progressively take over the first zone by advancing towards its center. At 500 kV a very dark center is observed at the center of the zone due to the high absorption. An increase of the accelerating voltage makes the dark center smaller. The fringe system is accomplished by a remarkable reduction of absorption at the zone center. These LACBED patterns are quite different from those observed around 100 kV, in which a ring system centered on the axis also appears ]7,8]. At lower voltages, however, the shape of outer rings approximates that of the zone boundary, square in the (100) zone, but the rings near the center of the zone are approximately circular. Fig. 5 shows the ~111) LACBED pattern of Si taken at 1026 kV. Four central rings overlap the radial fringe system. The shape of the outermost ring is hexagonal. These tings are different from those observed in the LACBED patterns taken around 100 kV [7,8]. The outermost ring is pulled out towards the middle of the zone edges so that its vertices are rotated by 30 o in comparison with the ring system at lower voltages, in which the shapes of outer rings approximate that of the zone boundary. The brightness of the zone center indicates that the critical voltage is not far from 1026 kV. The observation of the ~111) zone axis pattern with increasing voltage from 500 kV showed that the sequence approaches the critical voltage. Radial lines extended almost to the zone center at
Fig. 5. (111) LACBED of Si at 1026 kV.
600 kV. The central spot appeared at 800 kV and its brightness increased with an increase of the operating voltage. The critical voltage should be recognized by the maximum brightness of the central spot,: and the calculated critical voltage is 1320 kV for the S i ~ l l l ) zone axis [6]. Fig. 6 shows the C001) LACBED patterns of the Y-based high-Tc superconductor, YBazCu ~ AI,~Ov, x ~ 0.2, y ~ 6.7, taken at various voltages. Because the patterns are obtained from a relatively large area, an appreciable variation of thickness can be observed accross the pattern. Even though the patterns are not perfect, the sequence of the patterns indicates that the brightness of the central spots increases with decreasing accelerating voltage. The appearance of the bright spot at the center was observed in the Kossel patterns and explained in terms of the excitation of the Bloch wave with a particular quantum number [9]. As mentioned before, the maximum brightness of the central spot appears at critical voltage. One of the ~001) zone axis critical voltages of this material is expected to be near 200 kV from this observation and from voltage variation experiments below 200 kV using a 200 kV microscope.
K. Kuroda et al. / CBED in H V E M
3.2. Critical ooltage effect in Y-based high-TC superconductors
Critical voltages were measured in Y-based high-T~ superconductors for the 220 and the 400 reflections in the 110 and the 100 systematic rows. There is no critical voltage for the second-order reflection in the 100 systematic row, since the structure factor for the 100 reflection is smaller than that for the 200 reflection. Fig. 7 shows a series of the CBED patterns from the same specimen that was used for the observation of LACBED patterns. The intensity at the exact Bragg position in the 400 convergent beam diffraction disc is very weak in fig. 7b. Experimental values of the 400 and 220 critical voltages have been determined to be 950_+ 10 and 755 + 5 kV, respectively. The
63
critical voltages have been calculated using crystallographic data for tetragonal YBa 2Cu 2.78A10.2206.4 by Siegrist et al. [10] and scattering factors for ionized atoms [11]. The calculated values of 993 kV for the 400 reflection and 753 kV for the 220 reflection are in fairly good agreement with experimental values. Fig. 8 shows the critical voltages of YBa 2 Cu2.vsA10.e2Oy calculated as a function of oxygen content by introducing oxygen vacancies at O1 sites (0, ½, 0). The increase of 5% oxygen vacancies causes a decrease of the 220 critical voltage by 9 kV and an increase of the 400 critical voltage by 5 kV. It should be noted that the 220 critical voltage increases and the 400 critical voltage decreases when the oxygen content increases. The systematic critical voltage is highly sensitive to the oxygen
Fig. 6. (001) LACBED patterns of Y-based superconductor at various accelerating voltages: (a) 413 kV, (b) 350 kV, (c) 250 kV, (d) 200 kV.
64
K, Kuroda et al. / C BED in H V E M
c o n t e n t , b u t a b s o l u t e q u a n t i f i c a t i o n u s i n g t h e critical v o l t a g e s e e m s to b e d i f f i c u l t to achieve. Critical v o l t a g e a n a l y s i s p r o v i d e s a n a c c u r a t e q u a n t i t a tive m e a s u r e of r e l a t i v e o x y g e n c o n t e n t .
4. Conclusion
T h e c o n t i n u o u s v o l t a g e v a r i a t i o n b e t w e e n the s t e p p e d s e t t i n g s is a n i m p o r t a n t f u n c t i o n in the h i g h - v o l t a g e e l e c t r o n m i c r o s c o p e , since d i r e c t c o n trol of the p r o j e c t e d c r y s t a l p o t e n t i a l b y c h a n g i n g the a c c e l e r a t i n g v o l t a g e is e s s e n t i a l for o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n a b o u t the c r y s t a l l i n e m a t e r i a l . T h e d e v i c e w h i c h p r o v i d e s the c o n t i n u o u s l y v a r i a b l e r e f e r e n c e v o l t a g e for lenses a n d a l i g n m e n t s a c h i e v e s c o m p e n s a t i o n for lenses and a l i g n m e n t s w h e n v a r y i n g the a c c e l e r a t i n g voltage. T h e d e v i c e is e s p e c i a l l y useful for o b t a i n i n g C B E D p a t t e r n s at a n y o p e r a t i n g v o l t a g e .
References
Fig. 7. CBED patterns of Y-based superconductor near the (400) critical voltage: (a) 980 kV, (b) 950 kV, (c) 930 kV.
850
1050
800
lO00
l oJ *2
% >
%
r~ u
u
G
E
c,J
75O
950
I
.3
6.5
I
I
6.7 6.9 Y Fig. 8. Critical voltages of YBa2Cu2.78AIo.220v calculated as a function of oxygen content.
[1] D. Imeson, J.R. Sellar and C.J. Humphreys, in: Proc. 37th Annu. EMSA Meeting, San Antonio, TX, 1979, Ed. G.W. Bailey (Claitor's, Baton Rouge, 1979) p. 434. [2] H. Matsuhata and J.W. Steeds, Phil. Mag. B 55 (1987) 17. [3] Y. Ishida, T. Honda, S. Suzuki, S. Mori, H. Kitajima, M. Nakajima and T. Taoka, in: Proc. l l t h lnt, Congr. on Electron Microscopy, Kyoto, 1986, Eds. T. Imura, S. Maruse and T. Suzuki (Jpn. Soc. of Electron Microscopy, Tokyo, 1986) p. 907. [4] P.B. Hirsch, A. Howie, R. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977). [5] M. Tanaka, R. Saito, K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [6] M.D. Shannon and J.W. Steeds, Phil. Mag. 36 (1977) 279. [7] K.K. Fung, Ultramicroscopy 12 (1984) 243. [8] M. Tanaka and M. Terauchi, Convergent-Beam Electron Microscopy (JEOL, Tokyo, 1985). [9] Y. Uchida and F. Fujimoto, Phys. Status Solidi (a) 43 (1977) 269. [10] T. Siegrist, L.F, Schneemeyer, J.V. Waszczac, N.P. Singh, R.L. Opila, B. Batlogg, L.W. Rupp and D.W. Murphy, Phys. Rev. B 36 (1987) 8365. [11] D.T. Cromer and J.B. Mann, Acta Cryst. A 24 (1968) 321.