STEM Detectors
Three types of detectors are used to obtain STEM images:
- 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. - 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.
Arrangement of the various STEM detectors (schematic outline).
- 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 obtain additional morphological information (examples).