Operando STEM and EELS Study of Oxide Memristor Devices

The resistive-switching (RS) phenomenon observed in a variety of transitional metal oxides (TMO) is of great research interest since it opens enormous opportunities for the next-generation electronic devices such as nonvolatile memory and neuromorphic computing units, etc.1 Conventional filamentary-type RS devices rely on the formation of conductive filaments via a destructive forming process.2,3 Recent advances have reported another interface-type RS devices with lower operation current and more reliable switching repeatability.4,5 Different from conductive filamentary channels formation, the change of oxygen vacancies densities at the RS interface lead to a wide fluctuation of the oxygen stoichiometry and transitional metal valence states. However, the RS mechanism for many TMO memristor devices are still unclear, not alone the distinction between filament-type and interface-type devices. In this project, we use in situ transmission electron microscopy (TEM) to investigate the RS mechanisms of different types of memristor devices. Here, we use Au/VO2/Nb:TiO2 vertical device as a typical example for this experiment. The TEM lamellas were lifted out by focused ion beam (FIB) then mounted on a 4-point electrical MEMS E-chip (Protochips FIB optimized E-chip). Two big trenches were cut perpendicularly through the film and substrate respectively (Figure 1). Then the lamella was milled by Ga+ FIB at 30kev followed by 2kev cleaning step. The final thickness of the TEM sample is below 50nm. The Protochips Fusion Selected TEM holder with heating and electrical functions were used for the in situ STEM and EELS experiment. In order to probe the oxygen stoichiometry and V valence state, EEL spectroscopy was collected for the pristine VO2 film. Figure 2 shows the V-L2,3, O-K and Ti-L2,3 edge spectra of the pristine VO2 film. Next, during the in situ biasing experiment, different magnitudes of electrical bias were applied on the sample and high resolution and EELS data were acquired simultaneously during biasing. Meanwhile, the resistivity of the two terminal TEM sample device was captured all the way during the experiment. Hence, any resistivity change of the devices which is an indicator of the metal-insulator transition of VO2 film has been captured together with the STEM images and EEL spectra. This in situ STEM and EELS experiment will capture the direct proof of the phase transition and oxygen stoichiometry and vanadium valence states during the RS process, which can be applied to various memristor devices to clarify the RS mechanisms and working principles of different types of memristor devices [6]. (a) SEM image showing the two-terminal Au/VO2/Nb:TiO2 device TEM lamella after cutting two trenches before final milling. Current directions in the device is labeled out under a proposed electric field. (b) FIB image showing the TEM sample after final cleaning. (a) Cross-sectional STEM-HAADF image showing the configuration of the device. The EELS scanned area was denoted by yellow dots “1” to “6”. (b) Ti-L2,3, V-L2,3, and O-K edge spectra of the pristine VO2 film at different scanned locations in (a).