IT-2-O-1555 Imaging of light elements by annular dark-field Cs-corrected STEM
Nowadays, many crystalline lattices can be imaged directly at atomic resolution in Cs-corrected STEM. Recently, it was shown that light and heavy elements in crystalline lattices can be detected with an ABF method1 or with a double-detector STEM method.2 However, imaging of atomic columns of light elements by ADF method remains challenging. Particularly, the observation of light element columns at the interface between two different materials is still a difficult issue. In this work, we were able to detect directly and simultaneously the N and C atomic columns at the GaN-SiC interface and within the GaN and SiC materials. Additionally, the O atomic columns in a SrTiO3 single crystal were also observed by our method. We have studied the influence of imaging conditions on the appearance of N and C atomic columns in the GaN and SiC materials. The obtained results are discussed and are supported by image simulations.
The GaN thin film for this study was grown on 6H-SiC(0001) substrate by ion-beam assisted molecular beam epitaxy. STEM experiments were performed on a probe Cs-corrected Titan3 G2 60-300 microscope operated at 300 kV. A probe forming aperture of 20 mrad was used. Cross-sectional samples for STEM work were prepared by FIB technique. To improve the surface quality of the TEM specimens and to reduce the samples thicknesses, a focused low-energy argon ion milling (NanoMill system) was applied.3 Ion energies from 900 eV down to 200 eV were applied to remove implanted Ga ions and amorphous regions caused by the FIB. Image simulations were performed with the xHREM/STEM software package.
Figures 1 and 2 show the results of our work.4 We found that by adjusting the settings of HAADF detector and defocus value in STEM, the light element columns at the GaN-SiC interface and within the w-GaN, 6H-SiC and SrTiO3 lattices can be imaged using only a single HAADF detector. We concluded that image simulations for interpretation of atomic-resolution STEM images are only necessary when the probe forming aperture angle overlaps the inner angle of an annular STEM detector or when a complex defect structure is observed in a studied TEM sample. Our method works well using either ADF or HAADF detector, because their angular ranges and defocus values can be easily adjusted on any Cs-corrected STEM. Thus, on TEM systems equipped with only one HAADF detector, the technique can be used without any doubt and upgrades to an ABF detector.
1. S.D. Findlay, N. Shibata, H. Sawada et al., Appl. Phys. Lett. 95, 191913 (2009).
2. Y. Kotaka, Appl. Phys. Lett. 101, 133107 (2012).
3. D. Poppitz, A. Lotnyk, J.W. Gerlach, B. Rauschenbach Acta Mater. 65, 98 (2014).
4. A. Lotnyk, D. Poppitz, J.W. Gerlach, B. Rauschenbach Appl. Phys. Lett. 104, 071908 (2014).
The financial support of the European Union and the Free State of Saxony (LenA project; Project No. 100074065) is greatly acknowledged.
Cs-corrected: aberration-corrected; STEM: scanning transmission electron microscopy; ABF: annular bright-field; ADF: annular dark-filed; HAADF: high-angle ADF; FIB: focused ion beam; w-GaN: wurtzite-type GaN; i: detector inner angle; o: detector outer angle.
Fig. 1: (a) Atomic-resolution STEM image of the GaN-SiC interface taken with a HAADF detector (i20.4-o124.6 mrad) and schematic representation of w-GaN and 6H-SiC lattices along the [2-1-10] zone axis. (b) and (c) Simulated images of w-GaN and 6H-SiC, respectively, at 5 nm underfocus. The TEM sample thickness is measured to be about 16 nm. |
Fig. 2: High-resolution STEM images of SrTiO3 acquired with (a) ADF (i19.1-o106.5 mrad) and (b) ABF (i10.1-o19.1 mrad) detectors. The insets in (a) and (b) show the SrTiO3 structure viewed along the [001] zone axis. The TEM sample thickness is measured to be about 60 nm. |