Dislocation contrast in high-angle hollow-cone dark-field TEM

Dislocations were imaged using the high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM) technique. Experiments were performed using a Philips CM30 TEM at 300 kV Dislocations and grain boundaries show bright contrast in HADF-TEM images and there are no contrast reversals with thickness or defocus. The dislocation contrast shows no dramatic variation when the average semi-conical angle θ was increased from 40 to 110 mrad, but does show strong dependence on the diffracting conditions that are set up for the corresponding on-axis bright-field (BF) TEM image (Fig. 1). Under the "one-beam" (random orientation without strong diffraction) reflection condition (Figs, 1a and 1a'), the visibility of the dislocations is poor in either the BF-TEM or the HADF-TEM image. Under the two-beam diffracting condition (Figs, 1b and 1b') both BF-TEM and HADF-TEM images show optimized contrast. The features appearing in the HADF-TEM images have a good one-to-one correspondence with the features shown in the corresponding BF-TEM images; the dislocation contrast disappears in the HADF-TEM images if the condition g·b = 0 is satisfied in the BF-TEM images, where b is the Burgers vector (Fig 2).