Research ArticleMATERIALS SCIENCE

In operando x-ray imaging of nanoscale devices: Composition, valence, and internal electrical fields

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Science Advances  08 Dec 2017:
Vol. 3, no. 12, eaao4044
DOI: 10.1126/sciadv.aao4044
  • Fig. 1 Composition of the nanowire heterostructure.

    Composition of the nanowire heterostructure. (A) Schematic illustration of the device and measurement approach. (B) Scanning electron microscopy (SEM) image with a superimposed color plot of the Kα-XRF intensities of the heavy elements in the Si-Ga-GaAs-Si nanowire heterostructure device with Ni contacts. (C) Normalized XRF intensities plotted as a function of position along the wire. The Ga/(Ga + As) ratio is 0.5 at the GaAs segment (highlighted in green) and approaches 1 at one end of the GaAs segment (highlighted in red). The numbered arrows in (C) indicate the position on the nanowire axis, where the XRF spectra of single pixels shown in (D) were measured.

  • Fig. 2 Simultaneous XRF and XBIC recording.

    (A) 2D mapping of the Ga/(Ga + As) ratio, normalized XRF signal of (Ga + As), and XBIC at −0.7-V bias; each pixel is 20 × 20 nm2. In the line scan (B) extracted from the maps, the colored background indicates the Si (white), Ga (red), and GaAs (green) sections of the heterostructure as identified by XRF. (C) I-V characteristics of this device in logarithmic (top) and linear scale (bottom). The black hollow circles show the current without x-ray nanobeam (that is, dark conditions), whereas the red filled circles represent the maximum current found when the x-ray nanobeam was focused on the active area of the device.

  • Fig. 3 Position-resolved Ga-edge XANES spectra measured on a nanoscale along the nanowire heterostructure.

    (A) The lower group of spectra (1 blue→10 red) shows the normalized, averaged data offset according to their position along the nanowire. The top four graphs (green, red, blue, and dark blue) show the spectra of reference materials, as indicated in the legend (6, 27). The inset SEM image is overlaid with a color plot indicating the points from which the respective XANES spectra (1 to 10) were extracted. The green vertical line indicates the absorption edge of Ga in GaAs. Relative to this line, the black horizontal lines and the bar graph in (B) indicate the shift of the measured absorption edge as a function of the position along the nanowire. (C) Normalized fractions of a linear combination analysis of the measured XANES spectra using the reference materials.

  • Fig. 4 Energy-dependent XBIC analysis.

    (A) Simultaneously recorded Ga XRF (red) and XBIC (black, standardized; blue, as measured) signals as a function of x-ray energy. The insets show an SEM image overlaid with the map of the sum of the respective signal for all energies. The black square in the inset maps indicated the four pixels averaged to give the plotted spectra. (B) Band diagram showing the principal excitation mechanisms of absorption and inelastic scattering. (C) Main charge amplification process of impact ionization. Note that the de-excitation cascade of a single absorption event leads to a large number of charge carriers.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/12/eaao4044/DC1

    Detailed description of XANES mapping

    Standard XANES data treatment

    Discussion of energy calibration

    Discussion of x-ray–matter interaction with respect to XBIC yield

    Discussion of the spatial resolution

    fig. S1. Ga in Ga2O3 K-edge XANES references.

    fig. S2. Ga in GaAs K-edge XANES.

    fig. S3. Linear combination analysis of XRF data.

    fig. S4. Linear combination analysis of XBIC data.

    fig. S5. Determination of the spatial resolution of the setup.

    table S1. XANES energy steps of the x-ray beam for the XANES measurements.

    table S2. Cross sections and Auger yields for relevant elements and x-ray energies.

    References (3542)

  • Supplementary Materials

    This PDF file includes:

    • Detailed description of XANES mapping
    • Standard XANES data treatment
    • Discussion of energy calibration
    • Discussion of x-ray–matter interaction with respect to XBIC yield
    • Discussion of the spatial resolution
    • fig. S1. Ga in Ga2O3 K-edge XANES references.
    • fig. S2. Ga in GaAs K-edge XANES.
    • fig. S3. Linear combination analysis of XRF data.
    • fig. S4. Linear combination analysis of XBIC data.
    • fig. S5. Determination of the spatial resolution of the setup.
    • table S1. XANES energy steps of the x-ray beam for the XANES measurements.
    • table S2. Cross sections and Auger yields for relevant elements and x-ray energies.
    • References (35–42)

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