Phase contrast scanning transmission electron microscopy imaging of light and heavy atoms at the limit of contrast and resolution

Abstract

Using state of the art scanning transmission electron microscopy (STEM) it is nowadays possible to directly image single atomic columns at sub-Å resolution. In standard (high angle) annular dark field STEM ((HA)ADF-STEM), however, light elements are usually invisible when imaged together with heavier elements in one image. Here we demonstrate the capability of the recently introduced Integrated Differential Phase Contrast STEM (iDPC-STEM) technique to image both light and heavy atoms in a thin sample at sub-Å resolution. We use the technique to resolve both the Gallium and Nitrogen dumbbells in a GaN crystal in [[Formula: see text]] orientation, which each have a separation of only 63 pm. Reaching this ultimate resolution even for light elements is possible due to the fact that iDPC-STEM is a direct phase imaging technique that allows fine-tuning the microscope while imaging. Apart from this qualitative imaging result, we also demonstrate a quantitative match of ratios of the measured intensities with theoretical predictions based on simulations.

Figure 1

Schematic representation of the experimental setup. (a) Focused probe without sample. (b) Focused probe with a sample.

Figure 2

(a) ADF-STEM and (b) iDPC-STEM images of a GaN crystal in [$10\overline{1}1$] orientation. (c) ADF-STEM and (d) iDPC-STEM image of a GaN crystal in [$11\overline{2}0$] orientation. ADF- and iDPC-STEM images were recorded simultaneously in both cases. Beam current is 10 pA in all cases; the dwell time is 20 μs in (a,b), and 10 μs in (c,d); beam opening angle 29.4 mrad FOV is 6.2 nm. All images are shown as they appear live on the screen without any post-processing.

Figure 3

(a) Schematic representation of GaN crystal in [$11\overline{2}0$] direction. (b) Schematic representation of GaN crystal in [$10\overline{1}1$] direction. (c) iDPC-STEM image (CTF-corrected and high pass filtered) of GaN [$11\overline{2}0$] and (d) iDPC-STEM image (CTF-corrected and high pass filtered) of [$10\overline{1}1$] GaN. FOV is 2.0 nm. In (c) some features are visible that can be attributed to the fact that this sample for the [$11\overline{2}0$] orientation was slightly thicker than the other orientation. Based on simulations we estimate the thickness to be around 15 nm whereas it is around 7 nm for the[$10\overline{1}1$] orientation.

Figure 4

(a) iDPC- and (b) ADF-STEM images of GaN in [$10\overline{1}1$] orientation. Both images are CTF corrected using (1) and high pass filtered such that the large scale variatons are suppressed (same parameters as in Fig. 3). The FOV is 3.1 nm. (c) Normalized intensity profile plots of iDPC- (solid line) and ADF-STEM (dashed line) along the indicated red dashed lines. The green dots indicate the position and expected intensity (based on numerical simulations) of the Ga columns, the solid blue dots correspond to the N columns and expected intensity in the iDPC signal, the open blue dots correspond to the expected intensity of the ADF-STEM image.

Figure 5

Simulated (background images) vs. experimental (inserted and rotated images) iDPC-STEM images of GaN in [$10\overline{1}1$] orientation: (a) raw and (b) CTF corrected and Gaussian high pass filtered as in Fig. 3. (c) Normalized intensity profiles (dashed line - simulation, solid line - experiment) along the red dashed lines indicated in (b). The FOV is 3.1 nm for the experimental and 5 nm for the simulated images. The thickness of the sample in the simulated image is 7.46 nm.