IMAGING WITH HIGH-ANGLE SCATTERED ELECTRONS AND SECONDARY ELECTRONS IN THE STEM

High-resolution images can be obtained by collecting high-angle scattered electrons with an annular detector in a scanning transmission electron microscope (STEM). For a crystal, with incident electron beam along a principle axis, the high-angle annular dark-field (HAADF) signal may be mainly generated by quasi-elastic (thermal diffuse) scattering (TDS). The angular distribution of TDS is determined by the Fourier transform of the differential of the atomic potential. The collected signal is thus dependent on the atomic number Z, the shape of the atom, the amplitude of the thermal vibration and the total number of atoms encountered by the electron probe. This imaging mode reveals high Z sensitivity with atomic resolution. Angular filtered ADF imaging is also possible by selecting the desired angular window to extract information about thick specimens in transmission geometry or information about bulk specimens in reflection geometry. Subnanometer-resolution secondary electron (SE) images can be obtained simultaneously with HAADF images, providing topographic and material contrast of surfaces of the materials studied. In the UHV STEM MIDAS (microscope for imaging, diffraction and analysis of surfaces) both the entrance and the exit surfaces of the sample can be imaged with secondary electrons. Surface steps can be observed with high contrast. Applications of HAADF and high-resolution SE imaging to the study of several kinds of materials are discussed.