Electron microscopy
ADF(HAADF)-STEM Contrast at Interfaces
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HAADF-STEM Contrast at Interface between Two Crystals

In most cases, the interface between two crystals is not ideal. It is very common that the atomic number of the interface is lower than those of the crystalline bulks and there is chemical intermixing between them. For instance, nickel oxide (NiO) nanocrystals epitaxially grown on (001) strontium titanate (SrTiO3) single crystal substrates [1] had been characterized by aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The rough and irregular interface between the NiO and SrTiO3 crystals had a lower average atomic number than the two crystalline bulks as indicated by the dark layer at the interface shown in Figure 1352 (a) and (d). Figures 1352 (b) and (c) show EDS (for Ni and Sr) and EELS (for O and Ti) data, respectively, simultaneously acquired from a line scan across the NiO-SrTiO3 interface (marked as a yellow arrow in Figure 1352 (a)). Figure 1352 (c) suggests lack of a sharp interface in the O K profile and presence of oxygen vacancies at the NiO-SrTiO3 interface. Figures 1352 (b) and (c) show that there is intermixing between NiO and SrTiO3 near the interface. Figure 1352 (c) shows that the real interface, where the oxygen minimum locates, is not the same as the interface, represented by the minimum of HAADF contrast.


Figure 1352. (a) HAADF STEM image at low magnification, (b) EDS data along the yellow arrow in (a), (c) EELS data along the yellow arrow in (a), and (d) HAADF STEM image at atomic resolution. Adapted from [1]

HAADF-STEM Contrast at Interface between Amorphous and Crystalline Layers

Table 1352a lists the possible contrast mechanisms of bright-band in the Si (silicon) crystal at an a-SiO2/c-Si interface or an a-Si/c-Si interface in an ADF-STEM image.

Table 1352a. Possible contrast mechanisms of bright-band of c-Si at a-Si or a-SiO2/c-Si interfaces.

Possible contrast mechanism of bright-band in c-Si
a-SiO2/c-Si interface
a-Si/c-Si interface
Roughness of the interface Yes [2] No [5]
Associated with oxygen segregation Yes [3] --
Dopant with annealing temperature   Yes [6]
With a strain field Yes [4] Yes [5]

Other factors such as sample thickness effect in zone-axis crystals [7] and detector inner angle, as shown in Table 1352b, may also affect the appearance of strain contrast in ADF images. In the existing of strain fields, the scattering of electrons results in different angular distributions and thus the intensity collected by the ADF detector from the strained region can be different than that from the strain-free region, resulting in strain contrast. The thickness dependence of strain contrast originates from the propagation process of the electron beam with the channeling effect [8-11] inside a zone-axis crystal.

Table 1352b. Strain fields in crystalline silicon layers at a-Si/c-Si interface. [5] If the ADF intensity from the strained region is higher than that from the strain free region, this contrast is then defined as a positive contrast and negative contrast if the strained region appears darker than the strain-free region.
TEM sample thickness
Strain contrast
LAADF <10 nm Negative
>15 nm Positive
HAADF <10 nm Negative
>15 nm Negative

As shown in Figure 1352, when the strain amplitude increases, the channeling effect decreases, which is also called dechanneling. Therefore, the strain fields cause dechanneling.

(a) HAADF image along (110) zone axis of silicon (Si), (b) LAADF images

Figure 1352. (a) HAADF image along (110) zone axis of silicon (Si), (b) LAADF images. The strained region is marked by the red arrow. Adapted from [5]




















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[10] S. Hillyard, R. F. Loane, and J. Silcox, Ultramicroscopy 49, 14 (1993).
[11] R. R. Vanfleet, M. Robertson, M. McKay, and J. Silcox, Characterization and Metrology for ULSI Technology (American Institute of Physics, New York, 1998), p. 901.