Elemental mapping with elastically scattered electrons

Abstract

We describe a technique for efficient, quantitative, standardless elemental mapping using a high-angle annular detector in a scanning transmission electron microscope (STEM) to collect elastically scattered electrons. With a single crystal specimen, contrast due to thickness variations, diffraction, and channelling effects can be avoided, so that the resulting image contrast quantitatively reflects variations in impurity concentration. We compare a number of simple analytical approximations to the elastic scattering cross sections and show that a standardless analysis is possible over a wide range of atomic number and inner detector angle to an absolute accuracy of better than 20%.

Visibility of single heavy atoms on thin crystalline silicon in simulated annular dark-field STEM images

“…The contrast does not fall off steadily for thicker specimens, and remains high up to at least 235 A. This result is consistent with the prediction (Pennycook et al 1986) that a single bismuth atom is visible on 270 ~ of silicon, even though the experimental conditions are quite different. These calculations become less accurate with thicker specimens due to numerical error, intensity lost from the wavefunction, and neglected effects such as inelastic scattering.…”

“…On crystalline substrates, diffraction can be a sig-0108-7673/88/060912-16503.00 nificant factor in the ADF signal (Cowley, 1973) and experimental images may be difficult to interpret (Donald & Craven, 1979). This difficulty has led to efforts to minimize dynamical diffraction effects by increasing the inner angle of the detector (Howie, 1979;Treacy, 1981) or by orienting the crystal away from the strong Bragg reflections found at low-order zone axes (Pennycook, Berger & Culbertson, 1986). As yet no experimental observations of single heavy atoms on crystalline substrates with ADF STEM have been reported, though the sensitivities produced to date suggest that one bismuth atom might have a 50% contrast in 270 A thick silicon (110) crystals (Pennycook et al, 1986).…”

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

“…of the constituent atoms of the imaged material and is fairly insensitive to short and long range strain fields [17][18] . By comparing Z-contrast images that had been recorded at different inner angles of the annular dark field detector (also called Howie detector), we could safely exclude strain field influences 17 from the interpretation of our data.…”

Local Chemistry at Interfaces and Boundaries: Ceramic and Electronic Composite Materials

“…When the inner collection angle of the annular dark field detector is increased to large angles (> 70 mrad) these images can be interpreted as two dimensional chemical maps for high atomic number constituents, provided that local thickness variations are avoided and channeling effects are suppressed by proper selection of diffracting conditions [11,12]. Some of these results are shown in Figure 7.…”