Three
types of detectors are used to obtain STEM images:
1. BF
detector
It is
placed at the same site as the aperture in BF-TEM and
detects the intensity in the direct beam from a point on the specimen.
2.
ADF detector
The
annular
dark field (ADF) detector is a disk with a hole in its center where the BF detector
is installed. The ADF detector uses scattered electrons for image
formation, similar to the DF mode in
TEM. The
measured contrast mainly results from electrons diffracted in
crystalline areas but is superimposed by incoherent Rutherford
scattering.
3.
HAADF detector
The high-angle annular dark field detector is also a disk with a
hole, but the disk diameter and the hole are much larger than in
the ADF detector. Thus, it detects electrons that are scattered
to higher angles and almost only incoherent Rutherford scattering
contributes to the image. Thereby, Z contrast
is achieved.
In addition, there is the option to install a secondary electron detector above the sample like in a SEM and thereby to obtainn additional morphological information (examples). |
