In Situ Nanostructural Analysis of Volatile Threshold Switching and Non‐Volatile Bipolar Resistive Switching in Mixed‐Phased a‐VO_x Asymmetric Crossbars

Abstract

structure due to its ability to be designed in high capacity 3D crossbar arrays. [1] The major disadvantage of 3D crossbars currently is the high-leakage or sneak-path current. [2] Complex class of vanadium oxide spans a broad range of 20 phases. [3] Depending on the stoichiometry, crystal structure, and device structure it has demonstrated bipolar resistive switching (BRS) [4][5][6][7][8][9][10] as well as apolar threshold switching (TS). [11][12][13] Apolar TS property in the above papers is desirable as a selector element in crossbar arrays to reduce the sneak-path current (which disrupts stored data). Crystalline VO 2 (c-VO 2 ) as TS element shows significant OFF-current, [12,13] which defeats its primary purpose as a selector. For this reason, a few recent studies [10,[14][15][16] demonstrated TS based on amorphous vanadium oxide (a-VO x ). However, due to the amorphous nature of the film, devices based on same stoichiometry of vanadium oxide show multifunctional memory behavior, [17] which is not desirable for the practical application. This variability highlights that nanostructural investigations are needed to understand the origins of different memory behaviors, in particular the reasons for observing volatile and apolar TS in a-VO x films for its practical utilization as a selector element. Further, in-depth electrical characterization is required to ascertain the electrical conditions for the occurrence of volatile or non-volatile switching in the single device.Here, we present asymmetric cross-point device (Pt/Ti/a-VO x /Pt) with ≈100 nm a-VO x film as a functional switching layer which shows multifunctional behavior in a single device. We further introduce a fabrication procedure (Section S1, Supporting Information) to incorporate a-VO x in crossbar architecture, which has been found to dissolve in water. We choose amorphous films because crystalline films are porous due to grain boundaries and pin-holes, [11] which is unsuitable for stacked architectures. Through X-ray photoelectron spectroscopy (XPS) we analyze the stoichiometry of as-deposited film which revealed the mixed-phased oxidation, including V 3+ , V 4+ , and V 5+ (corresponding to V 2 O 3 , VO 2 , and V 2 O 5 , respectively).

Artificial Somatosensors: Feedback Receptors for Electronic Skins

“…[ 21 ] Memristors, which mimic characteristics of human nervous system, [ 22 ] can essentially resolve the bottleneck due to their exceptional switching performance in sub‐nanometer scale. [ 15,23 ] Therefore, it is of great scientific and technological importance to develop a somatosensory, which responds against real‐life stimuli in the form of pressure, temperature, and pain, exploiting memristor as the fundamental unit. The replication of somatosensor can pave new pathways in the development of skin‐like electronics and human‐like robots.…”

Amorphous Metal Oxide Bilayers to Avoid Sneak‐Path Currents for High‐Density Resistive Memory Arrays

“…Moreover, apolar TS in a ‐VO x ‐based devices has been reported due to the combination of filamentary breakdown and insulator‐to‐metal transition in local crystal islands of c‐VO 2 formed within or near the filament. [ 23 ] As proved in the aforementioned sections, the hybrid device retains electrical properties of both the individual components. It has nonvolatile nature like a ‐STO along with the TS behavior like a ‐VO x .…”

“…This contributing factor is due to different surface free energies of macrosized electrodes, nanosized filament in two different regions a ‐STO and a ‐VO x , and presence of localized VO 2 crystal islands within or near the filament. [ 23 ] Further investigations would be required to ascertain this suggestion and confirm the origin of leakage current using nanostructural analysis in future.…”

“…In a ‐STO region, the filament of oxygen vacancies and metallic ion would be present, [ 38 ] whereas it would be made up of vanadium and oxygen ions in the a ‐VO x region. [ 23 ] Local crystal islands of c‐VO 2 would form within or near the filament region of a ‐VO x as a part of the electroforming process, as shown in Figure 6b. After electroforming, c‐VO 2 crystal islands in a ‐VO x would undergo the insulator‐to‐metal transition, resulting in threshold behavior.…”

“…Polarity of the bias does not affect the insulator‐to‐metal transition in c‐VO 2 as the TS in a ‐VO x is volatile, apolar, and symmetric. [ 23 ] However, it affects the direction of oxygen‐ion migration at the interface of Ti/ a ‐STO, resulting in the nonvolatile nature of the hybrid device.…”