Letter pubs.acs.org/NanoLett
Observing Oxygen Vacancy Driven Electroforming in Pt−TiO2−Pt Device via Strong Metal Support Interaction Moon Hyung Jang,† Rahul Agarwal,† Pavan Nukala,† Dooho Choi,† A. T. Charlie Johnson,‡ I-Wei Chen,† and Ritesh Agarwal*,† †
Department of Materials Science and Engineering, ‡Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Oxygen vacancy formation, migration, and subsequent agglomeration into conductive filaments in transition metal oxides under applied electric field is widely believed to be responsible for electroforming in resistive memory devices, although direct evidence of such a pathway is lacking. Here, by utilizing strong metal−support interaction (SMSI) between Pt and TiO2, we observe via transmission electron microscopy the electroforming event in lateral Pt/TiO2/Pt devices where the atomic Pt from the electrode itself acts as a tracer for the propagating oxygen vacancy front. SMSI, which originates from the d-orbital overlap between Pt atom and the reduced cation of the insulating oxide in the vicinity of oxygen vacancies, was optimized by fabricating nanoscale devices causing Pt atom migration tracking the moving oxygen vacancy front from the anode to cathode during electroforming. Experiments performed in different oxidizing and reducing conditions, which tune SMSI in the Pt-TiO2 system, further confirmed the role of oxygen vacancies during electroforming. These observations also demonstrate that the noble metal electrode may not be as inert as previously assumed. KEYWORDS: Resistive memory, strong metal−support interaction, electroforming, in situ TEM, oxygen vacancy, filament, TiO2
R
TMOs is the migration and accumulation of oxygen vacancies under electric field leading to formation of localized regions with an off-stoichiometric, oxygen-deficient conductive phase, such as the Magnéli phase (TinO2n−1).6−10 Kwon et al.10 observed the Magnéli phase (Ti4O7) in electroformed Pt/ TiO2/Pt stacked structure via ex situ TEM experiments, where the samples were prepared by cross-sectioning the device postelectroforming via focused ion-beam (FIB), which may alter the state of the device. Even though extensively studied, a direct observation of the electroforming process in TMO based memory devices is lacking thus far owing to challenges in device fabrication and observing the phenomenon in situ while in operation. It must be realized that directly tracking the dynamics of point defects such as oxygen vacancies in real-time during the electroforming process is extremely challenging using electron microscopy techniques, further complicated due to the polycrystalline or even amorphous nature of the films typical of TMO-based devices. Given these challenges, it would be helpful if the unique properties of the oxygen deficient phase can itself be utilized to observe oxygen vacancy motion under an electrical bias in a direct manner to verify oxygen vacancy’s role in resistive switching. It is well-known in catalysis that certain noble metals (e.g., Pt, Pd, or Au) display enhanced surface interaction with partially
esistive memory devices that can be switched between low and high resistance states (LRS and HRS) by the application of electrical pulses are being extensively investigated as promising candidates for next-generation nonvolatile memory systems and also as an artificial synapse material for applications in neuromorphic computing.1−15 A typical resistive memory device consists of an active material sandwiched between two metal electrodes, which can be configured in crossbar architecture for random access memory. A variety of active materials such as transition metal oxides (TMOs), solidstate electrolytes, perovskites, and organic materials that can be reversibly switched between different resistance states have been studied.1−6 Among these candidates, TMOs (e.g., TiO2, Ta2O5, NiO) have distinct advantages owing to their high temperature stability, ease of film deposition, and compatibility with conventional semiconductor fabrication processes with sub 10-nanometer scale feature size.5−12 For any resistive memory device, the preliminary step before practical operation is the process of electroforming which enables the as-deposited HRS to switch into a LRS upon the application of an electrical bias. Electroforming process is believed to be due to the formation of a low resistance filament in a high resistance active material matrix,15 which can then be reversibly disrupted or reformed with dc or pulsed electrical bias to access different resistance states. Therefore, given the significance of the electroforming process in device operation, it is critical that the underlying physical mechanism is properly understood. One broadly accepted hypothesis explaining the electroforming process in © 2016 American Chemical Society
Received: July 27, 2015 Revised: February 18, 2016 Published: March 16, 2016 2139
DOI: 10.1021/acs.nanolett.5b02951 Nano Lett. 2016, 16, 2139−2144
Letter
Nano Letters
Figure 1. Characterization of a lateral Pt/TiO2/Pt device before and after the electroforming process in ambient conditions (a) Schematic of a lateral nanogap Pt/TiO2/Pt device with Pt electrodes stacked between SiNx membrane and TiO2 film. (b) SEM image of a nanogap Pt/TiO2/Pt device prior to TiO2 deposition. (c) SEM image of the Pt/TiO2/Pt device after electroforming. The electrode polarity is indicated by “+” (anode) and “−” (cathode). Brighter region corresponds to higher atomic number material. (d) I−V characteristic curves during electroforming (red filled circles) and after electroforming (black filled circles) in Pt/TiO2/Pt device at the compliance current of 30 μA. (e) TEM image of Pt/TiO2/Pt device after forming. (f) Magnified TEM micrograph of the conductive filament. (g) Pt O-edge EFTEM map of the nanogap region showing a continuous Pt filament across the electrodes. (h) Nanobeam diffraction (NBD) pattern of conductive filament (region inside the circle in f) shows pure-Pt facecentered cubic phase.
create metal filament based resistive memory devices4,13 which can be reversibly switched between LRS and HRS. In order to optimize SMSI in our system and to observe the formation of conductive filament by high-resolution transmission electron microscopy (TEM), nanoscale lateral Pt/ TiO2/Pt devices were fabricated on thin (5 MV/cm) to move them over very small distances (∼5 nm), which they attributed to poor polarizability of Pt. In comparison, in our system, the Pt electrode can release Pt at lower electric field intensities (
