Enabling In situ TEM biasing of Two-terminal Oxide-based Nanodevices using MEMS-based chips and gas cells

Abstract text (incl. figure legends and references)

Introduction: Materials characterization of two-terminal oxide-based electronic devices at the nano and sub-nanometer scale in operative conditions is undoubtedly reliant on existing in situ transmission electron microscopy (TEM) methods. However, electrical biasing experiments using MEMS-based platforms have been highly challenging up-to-date due to the complex Focused Ion Beam (FIB)-based methodologies involved, in which stray leakage current paths formed during TEM lamellae preparation, might lead to over-estimated electric responses and therefore, ambiguous in situ observations. Moreover, the electrical properties of oxide-based stack devices in actual conditions are strongly influenced by, for instance, gas flow and/or temperature [1]. Hence, a systematic in situ TEM investigation of leakage current in operando conditions is crucial to understanding device behavior at the micro and nanoscale.

Objectives: To reproduce the electrical responses of micrometer-sized oxide-based stack devices in the corresponding TEM lamella devices using a short-circuit-free FIB-based preparation approach. To establish a structure (chemical)-property correlations in operando conditions (i.e., gas and/or temperature) with the use of in situ (S)TEM techniques.

Materials and Methods: Micrometer-sized epitaxially grown SrTiO3 (STO)-based memristors and BaSrTiO3 (BST)-based high-performance capacitors have been grown by Pulsed Laser Deposition (PLD). TEM lamellae of both types of heterostructures were prepared using a Focused Ion Beam (FIB). The TEM lamellas were mounted on DENSsolutions MEMS-based chips for biasing, heating, and gas experiments. High-angle annular dark-field (HAADF)-STEM, bright field (BF)-STEM, and electron energy loss spectroscopy (EELS) data were obtained while operating TEM lamella devices under gas and biasing stimuli.

Results: Our short-circuit-free FIB methodology yielded current measurements of TEM lamellae devices as low as 10-12 A, mimicking the expected micrometer-sized electrical response. Operative TEM lamellae-based memristor devices could be measured inside the microscope. Additionally, high-performance capacitors (varactors) with high leakage currents have been exposed to an O2-rich environment and heated by the effect of electron beam irradiation while simultaneously tracking the suppression of the device leakage current inside a TEM (Fig. 1).

Conclusions: Our methodology enables current measurements e.g., STO-based memristors of at least six orders of magnitude in leakage current in comparison with conventional-FIB approaches and reported literature values [2], allowing for realistic and reproducible biasing operations of various oxide-based TEM lamella devices inside a TEM under heating or gas stimuli.

References:[1]L. Zeinar et al., "Matching conflicting oxidation conditions and strain accommodation in perovskite epitaxial thin-film ferroelectric varactors," Journal of Applied Physics, vol. 128, no. 21, p. 214104, Dec. 2020, doi: 10.1063/5.0021097. [2] D. Cooper and M. Bryan, "Reproducible in-situ electrical biasing of resistive memory materials using piezo-controlled electrical contacts and chip-based systems.," Microsc Microanal, vol. 27, no. S1, pp. 164–166, Aug. 2021, doi: 10.1017/S1431927621001197.

Fig. 1. Suppression of the leakage current of a TEM-lamella extracted from a BST-varactor device exposed to an O2 environment inside a TEM. A systematic electron beam irradiation (heating) study was performed.