Ho Leung Chan (Los Angeles, CA / US), Shelby Fields (Charlottesville, VA / US), Yueyun Chen (Los Angeles, CA / US), Tristan O'Neill (Los Angeles, CA / US), William Hubbard (Los Angeles, CA / US), Jon Ihlefeld (Charlottesville, VA / US), B. Chris Regan (Los Angeles, CA / US)
Abstract text (incl. figure legends and references)
In its primary imaging mode, a scanning transmission electron microscope (STEM) can locate individual atoms. With various spectroscopic attachments, a STEM can even determine the chemical identity of those atoms. However, while it excels at determining a sample"s physical and chemical structure, traditional STEM struggles to map electronic structure. Inter-atom electric fields are small compared intra-atom electric fields, so visualizing the former generally requires advanced techniques such as holography or differential phase contrast imaging. These techniques are analysis intensive and can produce complicated images that are difficult to interpret.
In electron beam induced current (EBIC) imaging, a transimpedance amplifier electrically connected to the sample collects the current induced by a scanning electron beam. Associating that current with the beam position produces an image. In samples with intrinsic electric fields, a raw EBIC image can sometimes be quickly and quantitatively interpreted to give the local electric field magnitude and direction.
To illustrate STEM EBIC imaging"s capabilities for high-resolution, in situ electric field mapping, we fabricate electron-transparent TaN/HZO/TaN capacitors (30/20/30 nm) on 20-nm thick silicon nitride membranes. The tantalum nitride is deposited via DC sputtering and the zirconium-doped hafnia (Hf0.5Zr0.5O2, HZO) is deposited via plasma-enhanced atomic layer deposition [1]. Using traditional transport measurements, we determine the dielectric's polarization as a function of the applied electric field in situ over various ranges of the electric field. Interleaved with the P(E) measurements, we perform STEM EBIC imaging to map the remanent polarization as a function of position in the capacitor.
Collecting the EBIC separately from the two electrodes of the capacitor allows us to separate the standard EBIC, which depends on the electric fields internal to the sample, from the EBIC caused by the ejection of secondary electrons from the sample [2]. The former appears in the difference EBIC images (Fig. 1), while the latter appears in the sum (not shown). Subtracting a polarization "up" differential EBIC image from a polarization "down" EBIC image beautifully reveals the spatial variation of the switching fraction (Fig. 2). The switching fraction further varies with the switching history.
References
1.Fields, S. S. et al. Compositional and phase dependence of elastic modulus of crystalline and amorphous Hf1-xZrxO2 thin films. Appl. Phys. Lett. 118, 102901 (2021).
2.Hubbard, W. A., Mecklenburg, M., Chan, H. L. & Regan, B. C. STEM Imaging with Beam-Induced Hole and Secondary Electron Currents. Phys. Rev. Applied 10, 044066 (2018).
This work was supported by National Science Foundation (NSF) award DMR-2004897 and by NSF Science and Technology Center (STC) award DMR-1548924 (STROBE).
Figure 1. (left to right) Bright field, annular dark field, differential EBIC with polarization "up", and differential EBIC with polarization "down" images. The field of view is 11 μm on a side.
Figure 2. (left) The difference between the fourth and the third images from Fig. 1. Areas with a switchable remanent polarization appear black. (right) A higher magnification view (acquired separately, but analyzed in the same way) of the region indicated by the red box reveals ferroelectric domains. Here the field of view is 1.9 μm on a side.