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Convergent-beam electron diffraction characterization of dislocations in GaS single crystals
Ultramicroscopy 33 (1990) 143-149 North-Holland
143
CONVERGENT-BEAM E L E C T R O N D I F F R A C T I O N C H A R A C T E R I Z A T I O N OF D I S L O C A T I O N S IN GaS S I N G L E CRYSTALS C. DE BLASI, D. M A N N O and A. RIZZO Dipartimento di Scienza dei Materiali, Universitgt di Lecce, GNSM/CISM, Unitd di Lecce, via Arnesano, 1-73100 Lecce, Italy Received 20 March 1990
The effects of dislocations in GaS single crystals both on the high symmetry zone-axis convergent-beam electron diffraction patterns and on the Tanaka patterns have been studied. The dislocations induce splitting in some reflections of the low camera length diffraction patterns. The modifications have been analyzed according to the kinematical theory, so the splitting and the unsplitting of the reflections correspond to the visibility and invisibility of the defect. The Burgers vectors of the dislocations have been determined by the analysis of a single zone-axis diffraction pattern. In addition, the deformations induced by isolated dislocations in Tanaka patterns have been analyzed and the distortions have been seen to depend on the character of the observed dislocation. A quick discrimination a m o n g edge, screw and mixed dislocations has been performed by the analysis of the Tanaka pattern distortion.
1. Introduction
It is well known that the properties of crystals depend strongly on the defects. Dislocations are the most widespread of crystal structure defects. They appreciably affect some properties of materials, of semiconductors particularly, which are very important in practice. The behaviour of the material is due both to the presence of a specific dislocation structure and to its interaction with other crystal imperfections (primarily stacking faults, point defects and impurities). In order to understand the physical properties of materials, it is necessary to have comprehensive information about the structure of the defects, primarily dislocations. Dislocations can be described in terms of the Burgers vectors, which are connected with the displacements of atoms from their regular positions in the lattice. They can be directly observed by different electron microscopy techniques, such as bright field, dark field, weak beam imaging, and diffraction techniques. The investigation always starts from the diffraction pattern and, usually, is carried out by the analysis of the image contrast, which arises from changes in the local
diffracting conditions in the zones affected by dislocations. In this way, a complete characterization of the defect needs both several images and related selected area diffraction (SAD) patterns [1,2]. However, the two-dimensional nature of SAD at high energies has generally been regarded as a major limitation of the method. On the contrary, convergent-beam electron diffraction (CBED) has a three-dimensional character; therefore in a single pattern several reflections of the zeroth- and higher-order Laue zones (HOLZ) are excited, together with Kikuchi lines [3]. Through the analysis both of the H O L Z reflections [4] and of the Kikuchi lines [5] a single zone-axis CBED pattern can be used to determine the Burgers vector of any dislocation, without restriction to the plane normal to the electron beam. In addition, dislocations induce distortions in the large-angle convergent-beam electron diffraction (LACBED) patterns which depend on the character of the given dislocation [6]. In this paper we report the analysis of different types of dislocations performed in melt-grown GaS single crystals by the CBED technique. The
0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
144
C. De Blasi et al. / CBED characterization of dislocations in GaS
Burgers vector of the defect has been determined by the analysis of a single zone-axis CBED pattern. Furthermore, the modifications induced by different dislocations in LACBED patterns have been studied.
2. Experiment GaS is one of the AIIIBVI semiconductor family that crystallizes into stacked hexagonal layers, each consisting of four close-packed monatomic sheets in the sequence B - A - A - B . The layers interact with each other through van der Waals forces, while within the layers the atoms are bound by valence forces [7]. The anisotropy of the bonds gives rise to large densities of extended defects [8]. It is worth stressing that the relatively larger electronegativity difference between gallium and sulphur, with respect to the one between anion and cation in the other A In Bvx compounds, produces a partially ionic bond between adjacent slabs in GaS crystals. For this reason it is the very common opinion that the stacking fault energy in GaS is very high and the occurrence of such a defect in this material is very unlikely. Several specimens have been obtained from different ingots of melt-grown GaS. Repeated cleavages supplied samples with many zones so thin as to be transparent to the beam of a Philips E M 4 0 0 T electron microscope operating at nominal 120 kV. Only a few patterns have been recorded at different voltages; their values are reported in the pertinent diffractions. High-symmetry zone-axis CBED patterns have been recorded on photographic plate in the TEM diffraction mode, the probe being focused onto the surface of the specimen by the second condenser lens. To focus" the probe on a dislocation, the shadow image of the defect has been observed in the transmitted disk of the diffraction pattern by a slight defocusing of the second condenser lens. The position of the probe on the defect has been maintained by adjustment of the beam deflection controls. A second condenser aperture of 100/~m diameter has been used, which produced a convergence angle of 10 mrad. The probe size at
focus has been estimated to be approximately 40 am.
According to the Tanaka method, in the LACBED technique a small incident electron probe has been brought to a focus below the specimen. Several focused spots have been thus seen in the image mode, each spot corresponding to a single diffracted beam. By this "diffraction pattern" the transmitted spot has been selected by using a 15 /~m aperture so that, on passing to diffraction mode, the enlarged image of the single transmitted disk has been observed [9].
3. Results and discussion Fig. l a reports the weak-beam dark field image of an isolated straight dislocation. The [00.1] zone-axis CBED pattern, shown in fig. lb, has been taken with the probe positioned approximately over this defect. For comparison, a similar [00.1] zone-axis CBED pattern from a defect-free region is reported in fig. lc. Indexing of both patterns is reported in fig. ld. It is evident that the Kikuchi lines and the F O L Z reflections in fig. l c are sharp and unsplit, while in fig. l b only the (20.0) lines and the (61.1), (71.1) F O L Z reflections are unmodified. The splitting and the unsplitting of the reflections can be interpreted in terms of the kinematical theory of diffraction contrast [1]. Splitting and unsplitting correspond to the visibility and invisibility of the crystal defect, respectively. The g reflections corresponding to unsplit Kikuchi lines satisfy the relation g- b = 0 as well as the H O L Z unsplit reflections. The condition g - b = 0 is not strictly a sufficient condition for the vanishing of the modification induced by the dislocations, other than pure screws; nevertheless, it is generally very useful as a criterion for determining the direction of the Burgers vector. In order to calculate the amplitude of the Burgers vector we have used the Thompson construction that, in addition, discriminates perfect or partial dislocations [10]. The analysis of the diffraction pattern in fig. l b has shown that the observed dislocation is a perfect one with b = (12.3)/3. After drawing this vector on the image (fig. la), b has turned out to
C. De Blasi et al.
/ CBED
145
characterization of dislocations in GaS
°
j
Fig. 1. Observation of an edge dislocation. (a) Dark field weak-beam image of the dislocation. (b) [00.1] zone-axis CBED pattern from the defect at 120 kV. Arrows indicate the Kikuchi lines (K) and the FOLZ reflections (F) unmodified by the dislocation. (c) [00.1] zone-axis CBED pattern from a defect-free region. (d) Indexing of the [00.11 zone-axis patterns reported in (b) and (c). (e) [00.1] Tanaka pattern recorded from the defect zone. The presence of the dislocation is evident in the picture. The dislocation line direction u and the Burgers vector b are marked.
146
C. De Blasi et al. / CBED characterization of dislocations in GaS Table 1 g . b values for the FOLZ reflections in fig. lb; b = (12.3)/3 is the Burgers vector of the edge dislocation
Fig.
1 (continued).
g
g.b
g
g.b
g
g.b
61.1 71.1 ¢~0.1 72.1 60.1 7i.1 61.1 73.1 43.1 72.1 52.1 74.1 34.1 7].1
0 0 1 - 1 1 2 2 - 2 -2 3 3 -3 - 3 4
43.1 75.1 25.1 7,~.1 34.1 76.1 (~6.1 06.1 16.1 75.1 25.1 67.1 57.1 47.1
4 - 4 -4 5 5 - 5 - 5 - 5 -5 6 6 -6 - 6 - 6
37.1 27.1 i7.1 76.1 66.1 0¢~.1 16_1 67.1 57.1 47.1 37.1 27.1 17.1
- 6 - 6 -6 7 7 7 7 8 8 8 8 8 8
be perpendicular to the defect; therefore this is an edge dislocation [11]. The values of n = g . b for the FOLZ split reflections are listed in table 1. There is a correla-
Fig. 2. Observation of a screw dislocation. (a) CBED and (b) Tanaka patterns from the dislocation. The unmodified Kikuchi lines (K) and FOLZ reflections (F) are arrowed (a); the dislocation line direction u and the Burgers vector b are marked (b).
C. De Blasi et aL / CBED characterization of dislocations in GaS
tion between the splitting of the reflections and the related values of n: twofold splitting has been obtained in the case n < 6 and fourfold splitting in the case n > 6, according to the results obtained by Fung in silicon [4]. Fig. le shows the [00.1] Tanaka pattern recorded by placing the incident b e a m on the core of the edge dislocation line. The right-hand and left-hand sides of the Tanaka pattern are shifted towards each other along the direction perpendicular to the dislocation line. Summarizing, the shift is parallel to the Burgers vector direction. The Tanaka pattern seems to have undergone a compression. Fig. 2 reports the [00.1] zone-axis C B E D (fig. 2a) and the Tanaka (fig. 2b) patterns taken near another type of dislocation. The (16.1) and (17.1) F O L Z reflections and the (02.0) Kikuchi lines in panel (a) are unmodified, thus the dislocation is a perfect one and b = (21.3)/3. This Burgers vector turned out to be parallel to the dislocation line, hence the observed defect is a screw dislocation [11].
147
The T a n a k a pattern (fig. 2b) shows clearly that the fight-hand part is shifted with respect to the left-hand part of the picture along the shadow image of the screw dislocation. Also in this case the shift direction of each half of the Tanaka pattern is parallel to the Burgers vector. Fig. 3 shows the [00.1] zone-axis C B E D (a). and the Tanaka (b) patterns recorded from a third ty_p_e of dislocation. In fig. 2a it is evident that__the (34.1) and (43.1) F O L Z reflections and the (11.0) Kikuchi lines are unmodified, hence the observed dislocation is a perfect one and b = (11.1). This Burgers vector makes an angle of 60 o with the dislocation hne; therefore, the observed defect is a mixed dislocation [11]. The T a n a k a pattern (fig. 3b) at the right-hand and left-hand sides of the dislocation line are both slightly shifted along the shadow image of the defect and elongated in the perpendicular direction. The modification induced by a mixed dislocation in the T a n a k a pattern can be regarded as the superposition of the shift caused by a screw dislocation with Burgers vector bs = b cos ~ and
Fig. 3. Observation of a mixed dislocation. (a) CBED and (b) Tanaka patterns from the dislocation. The unmodified Kikuchi lines (K) and FOLZ reflections (F) are arrowed (a); the dislocation line direction u and the Burgers vector b are marked (b).
148
c. De Blasi et al. / CBED characterization of dislocations in GaS
an edge one with a vector be = b sin ~ (where ~ is the angle between the dislocation line and the Burgers vector). It is worth recalling that the actually observed H O L Z reflections are those near the Ewald sphere. Their observation depends on the experimental conditions, primarily the accelerating voltage. Sometimes, the unsplit H O L Z reflection can occur far enough from the Ewald sphere that they cannot be observed under the experimental conditions used. Usually, changing the accelerating voltage is sufficient to bring the Ewald sphere near such reflections and, thus, to observe them. A typical example is reported in fig. 4. Panel (a) shows the [00.1] zone axis CBED pattern taken at 200 kV near a dislocation. It is evident that the (20.0) Kikuchi lines are unsplit, while all the H O L Z reflections are split. The latter condition does not allow the Burgers vector to be determined. Fig. 4b shows the [00.1] zone-axis CBED pattern taken at 150 kV near the same dislocation. In this case both the unsplit Kikuchi lines (20.0) and the unsplit F O L Z reflections (71.1), (6]-.1) have been observed, so the Burgers vector can be de-
termined. The interpretation indicates that the observed dislocation is a perfect one with b = (12.3)/3. This value verifies the invisibility conditions in the unmodified Kikuchi lines of fig. 4a. 4. Conclusions CBED patterns, taken near different types of dislocations in GaS single crystal, have shown modifications in some reflections and no variation in others, according to the visibility and invisibility criterion in conventional diffraction contrast electron microscopy. These effects provide a powerful means to determine the Burgers vectors of dislocations. The Burgers vector of different types of dislocations has been determined from a single zoneaxis CBED pattern. The results obtained have shown that the three-dimensional character of the CBED patterns gives the dislocation vectors, including those with a component in the electron beam direction. When the experiments have not shown the unsplit H O L Z reflections because they occurred too
Fig. 4. CBED patterns from a dislocation recorded at 200 kV (a) and 150 kV (b). In (a) some Kikuchi lines (K) are unmodified (arrowed); all the FOLZ reflections are split. In (b) both the Kikuchi lines (K) and the FOLZ reflections (F) unmodified are evident.
C De Blasi et al. / CBED characterization of dislocations in GaS
far away from the Ewald sphere, a change of the accelerating voltage has been sufficient to bring the Ewald sphere near the unsplit reflections, and the Burgers vector has been determined. The analysis of the distortions induced by different dislocations has shown the Tanaka pattern to be compressed or elongated by an edge dislocation, shifted along the shadow image of a screw dislocation and affected by both modifications for a mixed dislocation. In any case the shift direction of each half of the Tanaka pattern with respect to the dislocation line is parallel to the Burgers vector. This phenomenon can be used to recognize quickly the character of a dislocation. In conclusion the CBED technique has been confirmed to be a powerful method for a complete and quick characterization of dislocations.
Acknowledgement The authors are very much indebted to Professor P.G. Merli (LAMEL Laboratory of C N R -
149
Bologna) for valuable suggestions, and to Mr. G. D'Elia for his technical support.
References [1] P. Hirsh, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977). [2] G. Thomas and M.J. Goringe, Transmission Electron Microscopy of Materials (Wiley, New York, 1979). [3] P.M. Jones, G.M. Rackham and J.W. Steeds, Proc. Roy. Soc. (London) A 354 (1977) 197. [4] K.K. Fung, Ultramicroscopy 17 (1985) 81. [5] R.W. Carpenter and J.C.H. Spence, Acta Cryst. A 38 (1982) 55. [6] J. Wen, R. Wang and G. Lu, Acta Cryst. A 45 (1989) 422. [7] R.M.A. Lieth, in: Crystal Growth of Material with Layered Structures, Ed. R.M.A. Lieth (Reidel, Dordrecht, 1976). [8] A. Rizzo, C. De Blasi, M. Catalano and P. Cavaliere, Phys. Status Solidi (a) 105 (1988) 101. [9] M. Tanaka, R. Saito, K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [10] J. Friedel, Dislocations (Pergamon Press, Oxford, 1967). [11] S. Amelinckx, The Direct Observation of Dislocations (Academic Press, New York, 1964).
143
CONVERGENT-BEAM E L E C T R O N D I F F R A C T I O N C H A R A C T E R I Z A T I O N OF D I S L O C A T I O N S IN GaS S I N G L E CRYSTALS C. DE BLASI, D. M A N N O and A. RIZZO Dipartimento di Scienza dei Materiali, Universitgt di Lecce, GNSM/CISM, Unitd di Lecce, via Arnesano, 1-73100 Lecce, Italy Received 20 March 1990
The effects of dislocations in GaS single crystals both on the high symmetry zone-axis convergent-beam electron diffraction patterns and on the Tanaka patterns have been studied. The dislocations induce splitting in some reflections of the low camera length diffraction patterns. The modifications have been analyzed according to the kinematical theory, so the splitting and the unsplitting of the reflections correspond to the visibility and invisibility of the defect. The Burgers vectors of the dislocations have been determined by the analysis of a single zone-axis diffraction pattern. In addition, the deformations induced by isolated dislocations in Tanaka patterns have been analyzed and the distortions have been seen to depend on the character of the observed dislocation. A quick discrimination a m o n g edge, screw and mixed dislocations has been performed by the analysis of the Tanaka pattern distortion.
1. Introduction
It is well known that the properties of crystals depend strongly on the defects. Dislocations are the most widespread of crystal structure defects. They appreciably affect some properties of materials, of semiconductors particularly, which are very important in practice. The behaviour of the material is due both to the presence of a specific dislocation structure and to its interaction with other crystal imperfections (primarily stacking faults, point defects and impurities). In order to understand the physical properties of materials, it is necessary to have comprehensive information about the structure of the defects, primarily dislocations. Dislocations can be described in terms of the Burgers vectors, which are connected with the displacements of atoms from their regular positions in the lattice. They can be directly observed by different electron microscopy techniques, such as bright field, dark field, weak beam imaging, and diffraction techniques. The investigation always starts from the diffraction pattern and, usually, is carried out by the analysis of the image contrast, which arises from changes in the local
diffracting conditions in the zones affected by dislocations. In this way, a complete characterization of the defect needs both several images and related selected area diffraction (SAD) patterns [1,2]. However, the two-dimensional nature of SAD at high energies has generally been regarded as a major limitation of the method. On the contrary, convergent-beam electron diffraction (CBED) has a three-dimensional character; therefore in a single pattern several reflections of the zeroth- and higher-order Laue zones (HOLZ) are excited, together with Kikuchi lines [3]. Through the analysis both of the H O L Z reflections [4] and of the Kikuchi lines [5] a single zone-axis CBED pattern can be used to determine the Burgers vector of any dislocation, without restriction to the plane normal to the electron beam. In addition, dislocations induce distortions in the large-angle convergent-beam electron diffraction (LACBED) patterns which depend on the character of the given dislocation [6]. In this paper we report the analysis of different types of dislocations performed in melt-grown GaS single crystals by the CBED technique. The
0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
144
C. De Blasi et al. / CBED characterization of dislocations in GaS
Burgers vector of the defect has been determined by the analysis of a single zone-axis CBED pattern. Furthermore, the modifications induced by different dislocations in LACBED patterns have been studied.
2. Experiment GaS is one of the AIIIBVI semiconductor family that crystallizes into stacked hexagonal layers, each consisting of four close-packed monatomic sheets in the sequence B - A - A - B . The layers interact with each other through van der Waals forces, while within the layers the atoms are bound by valence forces [7]. The anisotropy of the bonds gives rise to large densities of extended defects [8]. It is worth stressing that the relatively larger electronegativity difference between gallium and sulphur, with respect to the one between anion and cation in the other A In Bvx compounds, produces a partially ionic bond between adjacent slabs in GaS crystals. For this reason it is the very common opinion that the stacking fault energy in GaS is very high and the occurrence of such a defect in this material is very unlikely. Several specimens have been obtained from different ingots of melt-grown GaS. Repeated cleavages supplied samples with many zones so thin as to be transparent to the beam of a Philips E M 4 0 0 T electron microscope operating at nominal 120 kV. Only a few patterns have been recorded at different voltages; their values are reported in the pertinent diffractions. High-symmetry zone-axis CBED patterns have been recorded on photographic plate in the TEM diffraction mode, the probe being focused onto the surface of the specimen by the second condenser lens. To focus" the probe on a dislocation, the shadow image of the defect has been observed in the transmitted disk of the diffraction pattern by a slight defocusing of the second condenser lens. The position of the probe on the defect has been maintained by adjustment of the beam deflection controls. A second condenser aperture of 100/~m diameter has been used, which produced a convergence angle of 10 mrad. The probe size at
focus has been estimated to be approximately 40 am.
According to the Tanaka method, in the LACBED technique a small incident electron probe has been brought to a focus below the specimen. Several focused spots have been thus seen in the image mode, each spot corresponding to a single diffracted beam. By this "diffraction pattern" the transmitted spot has been selected by using a 15 /~m aperture so that, on passing to diffraction mode, the enlarged image of the single transmitted disk has been observed [9].
3. Results and discussion Fig. l a reports the weak-beam dark field image of an isolated straight dislocation. The [00.1] zone-axis CBED pattern, shown in fig. lb, has been taken with the probe positioned approximately over this defect. For comparison, a similar [00.1] zone-axis CBED pattern from a defect-free region is reported in fig. lc. Indexing of both patterns is reported in fig. ld. It is evident that the Kikuchi lines and the F O L Z reflections in fig. l c are sharp and unsplit, while in fig. l b only the (20.0) lines and the (61.1), (71.1) F O L Z reflections are unmodified. The splitting and the unsplitting of the reflections can be interpreted in terms of the kinematical theory of diffraction contrast [1]. Splitting and unsplitting correspond to the visibility and invisibility of the crystal defect, respectively. The g reflections corresponding to unsplit Kikuchi lines satisfy the relation g- b = 0 as well as the H O L Z unsplit reflections. The condition g - b = 0 is not strictly a sufficient condition for the vanishing of the modification induced by the dislocations, other than pure screws; nevertheless, it is generally very useful as a criterion for determining the direction of the Burgers vector. In order to calculate the amplitude of the Burgers vector we have used the Thompson construction that, in addition, discriminates perfect or partial dislocations [10]. The analysis of the diffraction pattern in fig. l b has shown that the observed dislocation is a perfect one with b = (12.3)/3. After drawing this vector on the image (fig. la), b has turned out to
C. De Blasi et al.
/ CBED
145
characterization of dislocations in GaS
°
j
Fig. 1. Observation of an edge dislocation. (a) Dark field weak-beam image of the dislocation. (b) [00.1] zone-axis CBED pattern from the defect at 120 kV. Arrows indicate the Kikuchi lines (K) and the FOLZ reflections (F) unmodified by the dislocation. (c) [00.1] zone-axis CBED pattern from a defect-free region. (d) Indexing of the [00.11 zone-axis patterns reported in (b) and (c). (e) [00.1] Tanaka pattern recorded from the defect zone. The presence of the dislocation is evident in the picture. The dislocation line direction u and the Burgers vector b are marked.
146
C. De Blasi et al. / CBED characterization of dislocations in GaS Table 1 g . b values for the FOLZ reflections in fig. lb; b = (12.3)/3 is the Burgers vector of the edge dislocation
Fig.
1 (continued).
g
g.b
g
g.b
g
g.b
61.1 71.1 ¢~0.1 72.1 60.1 7i.1 61.1 73.1 43.1 72.1 52.1 74.1 34.1 7].1
0 0 1 - 1 1 2 2 - 2 -2 3 3 -3 - 3 4
43.1 75.1 25.1 7,~.1 34.1 76.1 (~6.1 06.1 16.1 75.1 25.1 67.1 57.1 47.1
4 - 4 -4 5 5 - 5 - 5 - 5 -5 6 6 -6 - 6 - 6
37.1 27.1 i7.1 76.1 66.1 0¢~.1 16_1 67.1 57.1 47.1 37.1 27.1 17.1
- 6 - 6 -6 7 7 7 7 8 8 8 8 8 8
be perpendicular to the defect; therefore this is an edge dislocation [11]. The values of n = g . b for the FOLZ split reflections are listed in table 1. There is a correla-
Fig. 2. Observation of a screw dislocation. (a) CBED and (b) Tanaka patterns from the dislocation. The unmodified Kikuchi lines (K) and FOLZ reflections (F) are arrowed (a); the dislocation line direction u and the Burgers vector b are marked (b).
C. De Blasi et aL / CBED characterization of dislocations in GaS
tion between the splitting of the reflections and the related values of n: twofold splitting has been obtained in the case n < 6 and fourfold splitting in the case n > 6, according to the results obtained by Fung in silicon [4]. Fig. le shows the [00.1] Tanaka pattern recorded by placing the incident b e a m on the core of the edge dislocation line. The right-hand and left-hand sides of the Tanaka pattern are shifted towards each other along the direction perpendicular to the dislocation line. Summarizing, the shift is parallel to the Burgers vector direction. The Tanaka pattern seems to have undergone a compression. Fig. 2 reports the [00.1] zone-axis C B E D (fig. 2a) and the Tanaka (fig. 2b) patterns taken near another type of dislocation. The (16.1) and (17.1) F O L Z reflections and the (02.0) Kikuchi lines in panel (a) are unmodified, thus the dislocation is a perfect one and b = (21.3)/3. This Burgers vector turned out to be parallel to the dislocation line, hence the observed defect is a screw dislocation [11].
147
The T a n a k a pattern (fig. 2b) shows clearly that the fight-hand part is shifted with respect to the left-hand part of the picture along the shadow image of the screw dislocation. Also in this case the shift direction of each half of the Tanaka pattern is parallel to the Burgers vector. Fig. 3 shows the [00.1] zone-axis C B E D (a). and the Tanaka (b) patterns recorded from a third ty_p_e of dislocation. In fig. 2a it is evident that__the (34.1) and (43.1) F O L Z reflections and the (11.0) Kikuchi lines are unmodified, hence the observed dislocation is a perfect one and b = (11.1). This Burgers vector makes an angle of 60 o with the dislocation hne; therefore, the observed defect is a mixed dislocation [11]. The T a n a k a pattern (fig. 3b) at the right-hand and left-hand sides of the dislocation line are both slightly shifted along the shadow image of the defect and elongated in the perpendicular direction. The modification induced by a mixed dislocation in the T a n a k a pattern can be regarded as the superposition of the shift caused by a screw dislocation with Burgers vector bs = b cos ~ and
Fig. 3. Observation of a mixed dislocation. (a) CBED and (b) Tanaka patterns from the dislocation. The unmodified Kikuchi lines (K) and FOLZ reflections (F) are arrowed (a); the dislocation line direction u and the Burgers vector b are marked (b).
148
c. De Blasi et al. / CBED characterization of dislocations in GaS
an edge one with a vector be = b sin ~ (where ~ is the angle between the dislocation line and the Burgers vector). It is worth recalling that the actually observed H O L Z reflections are those near the Ewald sphere. Their observation depends on the experimental conditions, primarily the accelerating voltage. Sometimes, the unsplit H O L Z reflection can occur far enough from the Ewald sphere that they cannot be observed under the experimental conditions used. Usually, changing the accelerating voltage is sufficient to bring the Ewald sphere near such reflections and, thus, to observe them. A typical example is reported in fig. 4. Panel (a) shows the [00.1] zone axis CBED pattern taken at 200 kV near a dislocation. It is evident that the (20.0) Kikuchi lines are unsplit, while all the H O L Z reflections are split. The latter condition does not allow the Burgers vector to be determined. Fig. 4b shows the [00.1] zone-axis CBED pattern taken at 150 kV near the same dislocation. In this case both the unsplit Kikuchi lines (20.0) and the unsplit F O L Z reflections (71.1), (6]-.1) have been observed, so the Burgers vector can be de-
termined. The interpretation indicates that the observed dislocation is a perfect one with b = (12.3)/3. This value verifies the invisibility conditions in the unmodified Kikuchi lines of fig. 4a. 4. Conclusions CBED patterns, taken near different types of dislocations in GaS single crystal, have shown modifications in some reflections and no variation in others, according to the visibility and invisibility criterion in conventional diffraction contrast electron microscopy. These effects provide a powerful means to determine the Burgers vectors of dislocations. The Burgers vector of different types of dislocations has been determined from a single zoneaxis CBED pattern. The results obtained have shown that the three-dimensional character of the CBED patterns gives the dislocation vectors, including those with a component in the electron beam direction. When the experiments have not shown the unsplit H O L Z reflections because they occurred too
Fig. 4. CBED patterns from a dislocation recorded at 200 kV (a) and 150 kV (b). In (a) some Kikuchi lines (K) are unmodified (arrowed); all the FOLZ reflections are split. In (b) both the Kikuchi lines (K) and the FOLZ reflections (F) unmodified are evident.
C De Blasi et al. / CBED characterization of dislocations in GaS
far away from the Ewald sphere, a change of the accelerating voltage has been sufficient to bring the Ewald sphere near the unsplit reflections, and the Burgers vector has been determined. The analysis of the distortions induced by different dislocations has shown the Tanaka pattern to be compressed or elongated by an edge dislocation, shifted along the shadow image of a screw dislocation and affected by both modifications for a mixed dislocation. In any case the shift direction of each half of the Tanaka pattern with respect to the dislocation line is parallel to the Burgers vector. This phenomenon can be used to recognize quickly the character of a dislocation. In conclusion the CBED technique has been confirmed to be a powerful method for a complete and quick characterization of dislocations.
Acknowledgement The authors are very much indebted to Professor P.G. Merli (LAMEL Laboratory of C N R -
149
Bologna) for valuable suggestions, and to Mr. G. D'Elia for his technical support.
References [1] P. Hirsh, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977). [2] G. Thomas and M.J. Goringe, Transmission Electron Microscopy of Materials (Wiley, New York, 1979). [3] P.M. Jones, G.M. Rackham and J.W. Steeds, Proc. Roy. Soc. (London) A 354 (1977) 197. [4] K.K. Fung, Ultramicroscopy 17 (1985) 81. [5] R.W. Carpenter and J.C.H. Spence, Acta Cryst. A 38 (1982) 55. [6] J. Wen, R. Wang and G. Lu, Acta Cryst. A 45 (1989) 422. [7] R.M.A. Lieth, in: Crystal Growth of Material with Layered Structures, Ed. R.M.A. Lieth (Reidel, Dordrecht, 1976). [8] A. Rizzo, C. De Blasi, M. Catalano and P. Cavaliere, Phys. Status Solidi (a) 105 (1988) 101. [9] M. Tanaka, R. Saito, K. Ueno and Y. Harada, J. Electron Microsc. 29 (1980) 408. [10] J. Friedel, Dislocations (Pergamon Press, Oxford, 1967). [11] S. Amelinckx, The Direct Observation of Dislocations (Academic Press, New York, 1964).