Simultaneous conductivity and electric field mapping with multi-modal STEM EBIC

Abstract text (incl. figure legends and references)

The transmission electron microscope (TEM) excels at physical characterization, with modern systems capable of precisely determining the type, number, and arrangement of atoms in a sample. TEM detection of electronic and thermal signals is also possible but often requires cutting-edge hardware, substantial computation, and an ideal sample. These signals are more elusive because standard TEM-based techniques are insensitive to electronic and thermal structure compared to physical and chemical structure. Often, especially when studying devices, the former are much more important than the latter.

Electron beam-induced current (EBIC) measures the current generated in a sample by an electron beam. In the "standard" EBIC mode, current is generated when electron-hole pairs are separated by local electric fields. In a recently demonstrated mode of EBIC, secondary electron emission EBIC (SEEBIC), the current consists of holes produced by secondary electron emission (1). SEEBIC is ~1000× smaller than std. EBIC, accessible at high resolution (2), and generates straightforward contrast related to electronic signals, including conductivity (3,4), and temperature (5). Here we demonstrate obvious electronic contrast in TEM via measurement of two EBIC modes simultaneously, mapping resistance, via SEEBIC, and electric field, via std. EBIC, within a single image.

Figs. 1 (A) and (B) show annular dark-field (ADF) STEM and STEM EBIC images, respectively, of a GeSbTe (GST) strip patterned between two TiN electrodes on a SiN membrane. At each TiN/GST interface, the Schottky barrier between the materials creates an electric field that produces std. EBIC contrast. The EBIC in these regions indicates both the direction (current polarity) and magnitude of the field. SEEBIC dominates in the rest of the device, where there is no electric field, and indicates the resistance at each pixel to the left, grounded electrode (3). For example, signal is darkest (resistance lowest) near the grounded left electrode because it provides a lower resistance path to ground; less holes produced there reach the EBIC amplifier via the right electrode (Fig. 1C).

STEM EBIC imaging generates obvious electronic contrast that would otherwise be either difficult (electric field) or impossible (conductivity) to visualize in the TEM. Complementing TEM's physical contrast, STEM EBIC provides a unique opportunity to study electronic dynamics in electronic devices at high resolution.

References:

(1) W. A. Hubbard, et al., Phys. Rev. Appl., 10 (2018), p. 044066.

(2) M. Mecklenburg, et al., Ultramicroscopy, 207 (2019), p. 112852.

(3) W. A. Hubbard, et al., Appl. Phys. Lett., 115 (2019), p. 133502.

(4) W. A. Hubbard, et al., Adv. Funct. Mater., 32 (2022), p. 2102313.

(5) W. A. Hubbard, et al., Microsc. Microanal., 26 S2 (2020), p. 3124.

Figure 1. STEM EBIC resistance and electric field mapping. The ADF STEM image (A) shows a TiN/GST/TiN device. The STEM EBIC image (B) has bright (dark) contrast at the left (right) GST/TiN interface due to std. EBIC at the Schottky barriers. Outside of these interfaces SEEBIC dominates. SEEBIC is largest in the TiN connected to the EBIC amplifier, decreases right-to-left along the GST, and is lowest on the left TiN electrode. The diagram (C) depicts SEEBIC current generation in the GST strip. Holes are more likely to reach ground via the left or right electrode depending on the relative resistance to each, generating resistance contrast in the image.