Abstract
We present a study of the structure and chemical composition of the Cr-doped 3D topological insulator Bi2Se3. Single-crystalline thin films were grown by molecular beam epitaxy on Al2O3 (0001), and their structural and chemical properties determined on an atomic level by aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. A regular quintuple layer stacking of the Bi2Se3 film is found, with the exception of the first several atomic layers in the initial growth. The spectroscopy data gives direct evidence that Cr is preferentially substituting for Bi in the Bi2Se3 host. We also show that Cr has a tendency to segregate at internal grain boundaries of the Bi2Se3 film.
Highlights
The structural ordering of the film is shown in high-angle annular dark field (HAADF) images acquired along the [1120] zone axis (Fig. 2)
Once the quintuple layers (QLs) growth is established, the subsequent film is continuously ordered as confirmed by x-ray diffraction (XRD) and the atomic force microcopy (AFM) imaging (Supplementary Figs S2 and S3); along this crystallographic orientation Bi and Se atomic columns do not overlap, Bi and Se atomic columns are distinguishable due to the much higher atomic number of Bi compared to Se
Chemical maps where created by integrating at each point of these spectrum images the spectrum intensity over a ∼20 eV window above the energy loss spectroscopy (EELS) edge onsets, while HAADF intensity signal was simultaneously acquired allowing for unambiguous correlation of the chemical information to the structural image
Summary
The Cr:Bi2Se3 thin film samples were prepared by MBE on c-plane sapphire substrates, following the recipe described in ref. The AFM image (Supplementary Fig. S2) illustrates the spiral islands, common for c-axis oriented Bi2Se3 films, with QL-high steps (~1 nm). STEM imaging and EELS measurements were performed in a Nion UltraSTEM100TM equipped with a Gatan Enfina spectrometer. The microscope was operated at 100 kV, with a convergence angle of 30 mrad; at these optical conditions the electron probe size is determined to be 0.9 Å. The native energy spread of the electron beam for EELS measurements was 0.3 eV; with the spectrometer dispersion set at 03 eV/channel & 1 eV/channel, yielding effective an energy resolution of 0.9 eV and 3 eV, respectively.
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