Emergent Chemical Mapping at Atomic-Column Resolution by Energy-Dispersive X-Ray Spectroscopy in an Aberration-Corrected Electron Microscope

Chemical mapping at atomic-column resolution by energy-dispersive x-ray spectroscopy in a spherical aberration-corrected scanning transmission electron microscope (STEM) has been demonstrated for the 1.47-Å dumbbell structure in InGaAs. The structural imaging and the chemical information in the two-dimensional map are directly correlated. Comparisons with the other existing mapping techniques of STEM in conjunction with electron energy-loss spectroscopy were discussed from aspects of ionization interactions.

Figure 1

(a) The [110] cross-sectional HAADF image of the $\mathrm{InGaAs}/\mathrm{InAlAs}$ superlattices with background-removed using the method indicated in Ref. 20. (b) The HAADF-intensity line profile of the solid white line (10 pixels in width, not shown for simplicity) in (a), revealing the dumbbell atomic columns of InGaAs with a spacing of 1.47 Å and the intensity variations due to local chemical inhomogeneities. The black filled curve represents the calculated HAADF intensity profile for an InGaAs dumbbell with nominal stoichiometry (see text for details).

Figure 2

The $[1\overline{1}0]$-projected intensities of the [110]-incident electron wave function in InGaAs with the electron probe focused on an InGa column (a), an As column (b), and an open site in InGaAs (c) and propagating through a thickness of 50 unit cells ($\sim 29\mathrm{nm}$). The gray level is in linearly normalized scale, black for 0 and white for 1. (d) The elastic (black) and TDS (gray) potentials are characteristic to the STEM calculations in (a)–(c) with the potentials normalized to the respective maxima on the InGa column.

Figure 3

(a) The STEM-EDX chemical mapping results for In, Ga, As, and the overlay of them. Open white circles, the atomic-column positions derived from the corresponding HAADF structural imaging in the inset of (b). Note that the missing In chemical contrast in the right-bottom InGa column is replenished by the Ga chemical contrast in the bottom-left panel. (b) The integrated EDX spectrum and the associated spectral ranges exploited for the mapping in (a). Details of mapping are given in Ref. 23.