large-angle convergent-beam electron diffraction, LACBED
large-angle convergent-beam electron diffraction
In conventional CBED, if the angular diameter of a diffraction disk exceeds the Bragg angle, the disk overlaps with an adjacent diffraction disk. Thus, the angular diameter is limited to an angle smaller than the Bragg angle. The use of the large-angle convergent-beam electron diffraction (LACBED) technique breaks through the angular limitation. Using a condenser-objective (CO) lens, when a specimen is placed on the focus position of the CO lens, the bright-field and dark-field images are formed on the image plane (on the selected-area aperture) but these images overlap with each other. When the specimen is shifted upper or lower from the focused position of the incident beam, the bright-field and dark-field images on the selected-area aperture are separated. If only the bright-field image is chosen with the selected-area aperture and the intermediate lens is set to take a diffraction pattern (the diffraction mode), the bright-field diffraction disk covers an angle of three to four times larger than that for the conventional CBED disk. Since the LACBED pattern contains information on both the image and diffraction pattern, it is effectively utilized for identification of lattice defects and analysis of strains at interfaces.
(a) In the case of convergence semi-angle to be equal to the Bragg angle or smaller.
In conventional CBED, the convergence semi-angle a is limited at the maximum to the Bragg angle θ to avoid the overlap of adjacent diffraction disks.
(b) In the case of the convergence semi-angle to be larger than the Bragg angle.
When the convergence semi-angle a exceeds the Bragg angle θ, the adjacent diffraction disks overlap, thus making it impossible to extract information on each diffraction disk.
(c) Ray diagram of conventional CBED where the convergence semi-angle is set to an angle larger than the Bragg angle.
In conventional CBED, the incident beam is focused on a specimen and the objective lens is focused on the specimen. A diffraction pattern is formed on the back focal plane of the objective lens and an image of the specimen (a spot image in this case) is formed on the selected-area aperture. When the convergence semi-angle of the incident electron beam is set to an angle larger than the Bragg angle, a transmitted wave disk and diffracted wave disks overlap with each other on the back focal plane of the objective lens. The image formed by the transmitted wave and images formed by diffracted waves are superposed on the selected-area aperture to form a spot image.
(d) Ray diagram of LACBED.
①In LACBED, the specimen position is shifted to a higher (lower) position from the focused position of the incident beam without changing the excitation of the objective lens.
②The diffraction pattern is formed on the back focal plane of the objective lens as it is in (c). The image of the specimen, which is formed on the selected-area aperture in (c), is shifted to an upper (lower) position, and on the selected-area aperture the (spot) image of the transmitted wave and (spot) images of the diffracted waves are separated.
③If only the transmitted spot (beam) is selected using the selected-area aperture in the image observation mode of the intermediate lens system, and then the intermediate lens system is switched to the diffraction mode, a large-angle convergent-beam electron diffraction (LACBED) pattern formed only by the transmitted wave is obtained. The LACBED pattern removes the overlap due to the diffracted wave disks and extends its angular diameter beyond the limitation of the diffraction (Bragg) angle.
LACBED pattern of Si [111] taken at an accelerating voltage of 200 kV.
(a) In the case of convergence semi-angle to be equal to the Bragg angle or smaller.
In conventional CBED, the convergence semi-angle a is limited at the maximum to the Bragg angle θ to avoid the overlap of adjacent diffraction disks.
(b) In the case of the convergence semi-angle to be larger than the Bragg angle.
When the convergence semi-angle a exceeds the Bragg angle θ, the adjacent diffraction disks overlap, thus making it impossible to extract information on each diffraction disk.
(c) Ray diagram of conventional CBED where the convergence semi-angle is set to an angle larger than the Bragg angle.
In conventional CBED, the incident beam is focused on a specimen and the objective lens is focused on the specimen. A diffraction pattern is formed on the back focal plane of the objective lens and an image of the specimen (a spot image in this case) is formed on the selected-area aperture. When the convergence semi-angle of the incident electron beam is set to an angle larger than the Bragg angle, a transmitted wave disk and diffracted wave disks overlap with each other on the back focal plane of the objective lens. The image formed by the transmitted wave and images formed by diffracted waves are superposed on the selected-area aperture to form a spot image.
(d) Ray diagram of LACBED.
①In LACBED, the specimen position is shifted to a higher (lower) position from the focused position of the incident beam without changing the excitation of the objective lens.
②The diffraction pattern is formed on the back focal plane of the objective lens as it is in (c). The image of the specimen, which is formed on the selected-area aperture in (c), is shifted to an upper (lower) position, and on the selected-area aperture the (spot) image of the transmitted wave and (spot) images of the diffracted waves are separated.
③If only the transmitted spot (beam) is selected using the selected-area aperture in the image observation mode of the intermediate lens system, and then the intermediate lens system is switched to the diffraction mode, a large-angle convergent-beam electron diffraction (LACBED) pattern formed only by the transmitted wave is obtained. The LACBED pattern removes the overlap due to the diffracted wave disks and extends its angular diameter beyond the limitation of the diffraction (Bragg) angle.
LACBED pattern of Si [111] taken at an accelerating voltage of 200 kV.
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