Tomography of crystal defects in the electron microscope was rst attempted
in 2005 by the author and colleagues. This thesis further develops the
technique, using a variety of samples and methods. Use of a more optimised,
commercial tomographic reconstruction program on the original GaN weakbeam
dark- eld (WBDF) tilt series gave a ner reconstruction with lower
background, line width 10-20 nm. Four WBDF tilt series were obtained
of a microcrack surrounded by dislocations in a sample of indented silicon,
tilt axes parallel to g = 220, 220, 400 and 040. Moir e fringes in the defect
impaired alignment and reconstruction. The e ect on reconstruction of moir e
fringe motion with tilt was simulated, resulting in an array of rods, not a
at
plane. Dislocations in a TiAl alloy were reconstructed from WBDF images
with no thickness contours, giving an exceptionally clear reconstruction.
The e ect of misalignment of the tilt axis with systematic row g(ng) was
assessed by simulating tilt series with di raction condition variation across
the tilt range of n = 0, 1 and 2. Misalignment changed the inclination
of the reconstructed dislocation with the foil surfaces, and elongated the
reconstruction in the foil normal direction; this may explain elongation
additional to the missing wedge e ect in experiments.
Tomography from annular dark- eld (ADF) STEM dislocation images
was also attempted. A tilt series was obtained from the GaN sample; the
reconstructed dislocations had a core of bright intensity of comparable width
to WBDF reconstructions, with a surrounding region of low intensity to
60 nm width. An ADF STEM reconstruction was obtained from the Si
sample at the same microcrack as for WBDF; here automatic specimen
drift correction in tomography acquisition software succeeded, a signi cant
improvement. The microcrack surfaces in Si reconstructed as faint planes and
dislocations were recovered as less fragmented lines than from the WBDF
reconstruction. ADF STEM tomography was also carried out on the TiAl
sample, using an detector inner angle ( in) that included the rst order Bragg
spots (in other series in had been 4-6 B). Extinctions occurred which were
dependent on tilt; this produced only weak lines in the reconstruction. Bragg
scattering in the ADF STEM image was estimated by summing simulated
dark- eld dislocation images from all Bragg beams at a zone axis; a double
line was produced. It was hypothised that choosing the inner detector angle
to omit these rst Bragg peaks may preclude most dynamical image features.
Additional thermal di use scattering (TDS) intensity due to dilatation
around an edge dislocation was estimated and found to be insigni cant. The
Huang scattering cross section was estimated and found to be 9 A, ten times
thinner than experimental ADF STEM dislocation images. The remaining
intensity may be from changes to TDS from Bloch wave transitions at the
dislocation; assessing this as a function of tilt is for further work. On simple
assessment, only three possible axial channeling orientations were found over
the tilt range for GaN; if this is typical, dechanneling contrast probably does
not apply to defect tomography.This work was supported by an EPSRC Doctoral Training Account
