Operation analysis of resistive switching of CBRAM using in-situ TEM

Resistive random access memories (ReRAMs) have great potential as a candidate for next-generation nonvolatile memories for the high speed, high density storage per cost [1] and their ability to the neural network devices. In order to analyze the reliability of ReRAMs and to find out the origin of the failures, it indispensable to understand the resistive switching mechanism. Since the transmission electron microscopy (TEM) provides a high resolution images of the nanostructure, in-situ TEM should be a powerful tool for the analysis. In our in-situ TEM system [2, 3], repeatable switching characteristics, are achieved together with clear images of formation and rupture of conductive filaments corresponding to the low and high resistance states. In this work, we used several kinds of Cu-based ReRAM (CBRAM: Conductive Bridge RAM). TEM samples are fabricated by two methods. One is an ion-shadow method [4, 5], which is an ion milling technique with carbon mask particles. The other is FIB that is a conventional technique to make a sample observable in TEM. Almost the same characteristics as those measured at the outside of TEM by the use of real ReRAM cells are achieved in TEM by the both method. Fig. 1 shows an example of I-V switching characteristics of Cu-Te based ReRAM [6, 7] and the corresponding TEM images [2, 3]. It was clearly shown that a dark spot corresponding to a conductive filament appeared by SET and erased after RESET. These resistive switching characteristics by I-V sweep were reproducible at least 60 cycles in TEM. In addition, SET/RESET pulse operation more than 100k times are confirmed during TEM observation as shown in Fig. 2. These results clearly indicate that the in-situ TEM will be a powerful tool to guarantee the reliability of ReRAMs. Please click Additional Files below to see the full abstract

Nanoscale Switching and Degradation of Resistive Random Access Memory Studied by In Situ Electron Microscopy

The metal-filament-type resistive random access memories (ReRAMs) with copper were investigated from the point of view of dynamical microstructure evolution in the repetitive switching operations using in situ transmission electron microscopy (in situ TEM). Through a series of experiments for uncovered solid electrolyte films, stacked devices, and nanofabricated cells, formation and erasure of the copper filaments and deposits were confirmed. The behavior of the filament and deposit depended on the switching condition and history. Based on these in situ TEM results, the switching schematics and the degradation process were discussed

Multilevel recording in Bi-deficient Pt/BFO/SRO heterostructures based on ferroelectric resistive switching targeting high-density information storage in nonvolatile memories

We demonstrate the feasibility of multilevel recording in Pt/Bi1-δFeO3/SrRuO3 capacitors using the ferroelectric resistive switching phenomenon exhibited by the Pt/Bi1−δFeO3 interface. A tunable population of up and down ferroelectric domains able to modulate the Schottky barrier height at the Pt/Bi1−δFeO3 interface can be achieved by means of either a collection of SET/RESET voltages or current compliances. This programming scheme gives rise to well defined resistance states, which form the basis for a multilevel storage nonvolatile memory

Filamentary switching of ReRAM investigated by in-situ TEM

The filament operation of resistive random-access memory was studied via in-situ transmission electron microscopy, and the contribution of the conductive filament to the resistance switching was experimentally confirmed. In addition to the operation principles the device degradation mechanism was studied through repeated write/erase operations. The importance of controlling Cu movement in the switching layer was confirmed for stable CBRAM (conductive bridge random access memory) operations. A device structure with double switching layers and device miniaturization was effective in restricting over accumulation of Cu in the switching layer and localizing the filament. This may improve the robustness of the device against performance degradation. (C) 2020 The Japan Society of Applied Physic

EELS Analysis of Oxygen Scavenging Effect in a Resistive Switching Structure of Pt/Ta/SrTiO3/Pt

A complex mechanism of interfacial oxygen scavenging is revealed by electron energy-loss spectroscopy (EELS) for a resistive switching oxide of SrTiO3 with a scavenging layer of Ta. When Ta thin layer is inserted at one of the interfaces of Pt/SrTiO3/Pt structure, a large reduction of electrical resistance is induced for the structure, and oxygen defects are introduced at the interfacial part of SrTiO3. In the resistance decrease by voltage applications, simultaneous occurrence of oxidation and reduction of Ta scavenging layer is shown by EELS analyses from the low-loss spectra. The EELS and scanning transmission electron microscopy observations demonstrate that oxygen scavenging by Ta layer is an interfacial phenomenon where the redox reactions occur at the whole part of the interface. In addition, Pt electrode of the structure, which is chemically inert for oxidation, is revealed to have significant effects in the scavenging processes

Smooth Interfacial Scavenging for Resistive Switching Oxide via the Formation of Highly Uniform Layers of Amorphous TaOx

We demonstrate that the inclusion of a Ta interfacial layer is a remarkably effective strategy for forming interfacial oxygen defects at metal/oxide junctions. The insertion of an interfacial layer of a reactive metal, that is, a “scavenging” layer, has been recently proposed as a way to create a high concentration of oxygen defects at an interface in redox-based resistive switching devices, and growing interest has been given to the underlying mechanism. Through structural and chemical analyses of Pt/metal/SrTiO3/Pt structures, we reveal that the rate and amount of oxygen scavenging are not directly determined by the formation free energies in the oxidation reactions of the scavenging metal and unveil the important roles of oxygen diffusibility. Active oxygen scavenging and highly uniform oxidation via scavenging are revealed for a Ta interfacial layer with high oxygen diffusibility. In addition, the Ta scavenging layer is shown to exhibit a highly uniform structure and to form a very flat interface with SrTiO3, which are advantageous for the fabrication of a steep metal/oxide contact

Controlled Current Transport in Pt/Nb:SrTiO3 Junctions via Insertion of Uniform Thin Layers of TaOx

Systematic control of electronic transport is demonstrated for Pt/Nb-doped SrTiO3 (Nb:STO) junctions based on interface engineering with uniform thin layers of TaOx. By inserting TaOx layers fabricated via sputter deposition with different O-2-Ar ratios (r(O2)), the current-voltage characteristics and behavior of resistive switching can be well controlled in Pt/Nb:STO junctions. Reduction of the Schottky barrier is also demonstrated via the insertion, and formation of an ideal ohmic contact with a low contact resistance of r(O2) = 0%. Structural and chemical characterizations show that the resistivity of the TaOx layers depends significantly on r(O2) while maintaining a uniform structure independent of the resistivity. This indicates that the insertion of both insulating and metallic interface layers is possible by sputtering TaOx with no need for epitaxial growth, suggesting TaOx's potential as an interface-layer material. Even with very thin layers (1.0 nm) of TaOx the interfacial properties can be controlled to enhance both ohmic contact formation and resistive switching. These results demonstrate an easy and reliable way to control the characteristics of Pt/Nb:STO junctions and present new insights for their memory and semiconductor device applications

Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4

Owing to the recent discovery of the current-induced metal-insulator transition and unprecedented electronic properties of the concomitant phases of calcium ruthenate Ca2RuO4, it is emerging as an important material. To further explore the properties, the growth of epitaxial thin films of Ca2RuO4 is receiving more attention, as high current densities can be applied to thin-film samples and the amount can be precisely controlled in an experimental environment. However, it is difficult to grow high-quality thin films of Ca2RuO4 due to the easy formation of the crystal defects originating from the sublimation of RuO4; therefore, the metal-insulator transition of Ca2RuO4 is typically not observed in the thin films. Herein, a stable current-induced metal-insulator transition is achieved in the high-quality thin films of Ca2RuO4 grown by solid-phase epitaxy under high growth temperatures and pressures. In the Ca2RuO4 thin films grown by ex situ annealing at >1200 degrees C and 1.0 atm, continuous changes in the resistance of over 2 orders of magnitude are induced by currents with a precise dependence of the resistance on the current amplitude. A hysteretic, abrupt resistive transition is also observed in the thin films from the resistance-temperature measurements conducted under constant-voltage (variable-current) conditions with controllability of the transition temperature. A clear resistive switching by the current-induced transition is demonstrated in the current-electric-field characteristics, and the switching currents and fields are shown to be very stable. These results represent a significant step toward understanding the high-current-density properties of Ca2RuO4 and the future development of Mott-electronic devices based on electricity-driven transitions

Microstructural transitions in resistive random access memory composed of molybdenum oxide with copper during switching cycles

The switching operation of a Cu/MoOx/TiN resistive random access memory (ReRAM) device was investigated using in situ transmission electron microscopy (TEM), where the TiN surface was slightly oxidized (ox-TiN). The relationship between the switching properties and the dynamics of the ReRAM microstructure was confirmed experimentally. The growth and/or shrinkage of the conductive filament (CF) can be classified into two set modes and two reset modes. These switching modes depend on the device's switching history, factors such as the amount of Cu inclusions in the MoOx layer and the CF geometry. High currents are needed to produce an observable change in the CF. However, sharp and stable switching behaviour can be achieved without requiring such a major change. The local region around the CF is thought to contribute to the ReRAM switching process