Letter pubs.acs.org/NanoLett
Three-Dimensional Location of a Single Dopant with Atomic Precision by Aberration-Corrected Scanning Transmission Electron Microscopy Ryo Ishikawa,*,† Andrew R. Lupini,† Scott D. Findlay,‡ Takashi Taniguchi,§ and Stephen J. Pennycook∥ †
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States School of Physics, Monash University, Victoria 3800, Australia § Advanced Key Technologies Division, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan ∥ Department of Materials Science and Engineering, The University of Tennessee, 328 Ferris Hall, Knoxville, Tennessee 37996, United States ‡
S Supporting Information *
ABSTRACT: Materials properties, such as optical and electronic response, can be greatly enhanced by isolated single dopants. Determining the full three-dimensional single-dopant defect structure and spatial distribution is therefore critical to understanding and adequately tuning functional properties. Combining quantitative Z-contrast scanning transmission electron microscopy images with image simulations, we show the direct determination of the atomic-scale depth location of an optically active, single atom Ce dopant embedded within wurtzite-type AlN. The method represents a powerful new tool for reconstructing three-dimensional information from a single, two-dimensional image. KEYWORDS: Three-dimensional imaging, photoluminescence, single dopant, atomic-resolution ADF STEM
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three-dimensional reconstruction of the surface morphology of a free-standing gold nanoparticle with 2.4 Å resolution.18 However, this method requires a large tilt series of twodimensional projection images for the reconstruction and imposes restrictions on the shape of the sample (for example, thin films are extremely challenging). Discrete tomography reduces the number of images required19 but there is no guarantee that single dopant atoms will remain at the same positions during acquisition at different orientations. Recently, a new tomographic scheme using high-resolution TEM with focal-series reconstruction has determined the three-dimension position of every individual atom in bilayer graphene.20 However, in this approach the electron scattering should be kinematical (a weak-phase object specimen), whereas even single heavy atoms can produce large phase shifts21 and dynamical (or multiple) scattering will occur in bulk materials. Another potential approach is optical sectioning in depth by recording focal-series ADF STEM images.11,22,23 However, in a current electron microscope the width of the point spread function along the optical axis, the “depth of focus”, is larger than 3 nm, which is insufficient to achieve depth resolution at atomic-scale, although in some cases the precision might be
ollowing the establishment of high purification technology for silicon and other semiconductors, it has been shown that the dilute doping of functional single atoms in semiconductors can dramatically enhance a variety of physical properties such as electrical transport, magnetism, and optical response.1−6 These physical properties are highly sensitive to the valence state, atomic site, and location (surface, subsurface, or bulk) of the dopants. To fully understand and control such properties, it is therefore of critical importance to determine the three-dimensional point defect structure and the spatial distribution of individual dopants at the atomic-scale. Recent advances in electron optics have enabled scanning transmission electron microscopy to directly determine the atomic structure of materials with sub-angström spatial resolution7−10 but only in the lateral directions. The resolution along the remaining dimension, depth, is still usually several nanometers.11,12 However, a broad range of applications in materials science and device engineering fields necessitate the ability to directly characterize individual dopant atoms with atomic-resolution in all three dimensions. Owing to the “Z-contrast” nature13 of the images, annular dark-field scanning transmission electron microscopy (ADF STEM) is one of the most promising methods to directly image heavy single dopants within materials.6,14−16 Electron tomography incorporating ADF STEM is a promising method to determine three-dimensional structure,17 and it has achieved © 2014 American Chemical Society
Received: December 17, 2013 Published: March 19, 2014 1903
dx.doi.org/10.1021/nl500564b | Nano Lett. 2014, 14, 1903−1908
Nano Letters
Letter
Figure 1. (a) Atomic-resolution ADF STEM image of w-AlN viewed along the [112̅0] direction, where the intensity is normalized as a percentage of the total electron intensity incident upon the specimen.28,33 (b) Pearson’s χ2 test profile as a function of fwhm Gaussian source size. (c) Simulated ADF STEM image convolved with a 0.62 Å Gaussian profile.28 (d) Z-contrast profiles along X−X′ for both the experiment (circles) and the simulation (solid line). The scale bar in (a) is 2 Å.
better than this limit.16 Here, we show the direct determination of the depth location of an isolated single Ce dopant in photoluminescent wurtzite-type aluminum nitride (w-AlN). By combining atomic-resolution quantitative ADF STEM imaging with frozen phonon image simulations24 on an absolute intensity scale, we are able to determine the depth of a single Ce dopant in an Al column with an uncertainty corresponding to about one atomic-spacing. When a finely focused electron probe is placed on the entrance surface of the atomic column viewed along a zone axis, the incident electrons are strongly attracted to the atomic column and can dynamically scatter along the atomic column (the so-called channeling effect).25 Channeling causes a nonuniform redistribution of electron density along the depth direction compared to the free-space evolution of the electron probe, which is usually an obstacle for interpretation and analysis.11,26 However, here we use this dynamical effect to our advantage through the strong dependence exhibited by the ADF signal on the depth-location of the single dopant.14 Recent progress in the quantification of ADF STEM images using the area-averaged intensity has allowed the number of atoms per projected column to be determined with high accuracy (±1 atom precision for thin samples).27,28 The mean signal value over a suitably chosen cell is a very robust method for counting the number of atoms because it is independent of uniform coherent and incoherent aberrations, including defocus.29,30 This robustness is distinct from the usual Zcontrast profile, which depends on the “absolute” defocus value and effective source size (spatial incoherence). Thus having found the number of atoms in each column, we are able to use the Z-contrast intensity distribution to determine these other parameters. Details of this procedure are given in the Supporting Information.
Figure 1a shows an atomic-resolution ADF STEM image of w-AlN viewed along the [112̅0] direction, where the intensity is normalized as a percentage of the total electron intensity incident upon the specimen. The profile across one Al column is then used to extract the effective source size of the microscope, whereupon the simulated image and Z-contrast profile exhibits excellent agreement with the experiment (Figure 1c,d). The Ce doping level of our single crystal is ∼1018 atoms cm−3 and so we were able to observe a number of “isolated” single Ce dopants at thin regions (
