Umberto Celano

Three-dimensional observation of the conductive filament in nanoscaled resistive memory devices.

The basic unit of information in filamentary-based resistive switching memories is physically stored in a conductive filament. Therefore, the overall performance of the device is indissolubly related to the properties of such filament. In this Letter, we report for the first time on the three-dimensional (3D) observation of the shape of the conductive filament. The observation of the filament is done in a nanoscale conductive-bridging device, which is programmed under real operative conditions. To obtain the 3D-information we developed a dedicated tomography technique based on conductive atomic force microscopy. The shape and size of the conductive filament are obtained in three-dimensions with nanometric resolution. The observed filament presents a conical shape with the narrow part close to the inert-electrode. On the basis of this shape, we conclude that the dynamic filament-growth is limited by the cation transport. In addition, we demonstrate the role of the programming current, which clearly influences the physical-volume of the induced conductive filaments.

Imaging the Three-Dimensional Conductive Channel in Filamentary-Based Oxide Resistive Switching Memory

Filamentary-based oxide resistive memory is considered as a disruptive technology for nonvolatile data storage and reconfigurable logic. Currently accepted models explain the resistive switching in these devices through the presence/absence of a conductive filament (CF) that is described as a reversible nanosized valence-change in an oxide material. During device operation, the CF cycles billion of times at subnanosecond speed, using few tens of microamperes as operating current and thus determines the whole devices performance. Despite its importance, the CF observation is hampered by the small filament size and its minimal compositional difference with the surrounding material. Here we show an experimental solution to this problem and provide the three-dimensional (3D) characterization of the CF in a scaled device. For this purpose we have recently developed a tomography technique which combines the high spatial resolution of scanning probe microscopy with subnanometer precision in material removal, leading to a true 3D-probing metrology concept. We locate and characterize in three-dimensions the nanometric volume of the conductive filament in state-of-the-art bipolar oxide-based devices. Our measurements demonstrate that the switching occurs through the formation of a single conductive filament. The filaments exhibit sizes below 10 nm and present a constriction near the oxygen-inert electrode. Finally, different atomic-size contacts are observed as a function of the programming current, providing evidence for the filaments nature as a defects modulated quantum contact.

Cellulose Nanofiber Paper as an Ultra Flexible Nonvolatile Memory

On the development of flexible electronics, a highly flexible nonvolatile memory, which is an important circuit component for the portability, is necessary. However, the flexibility of existing nonvolatile memory has been limited, e.g. the smallest radius into which can be bent has been millimeters range, due to the difficulty in maintaining memory properties while bending. Here we propose the ultra flexible resistive nonvolatile memory using Ag-decorated cellulose nanofiber paper (CNP). The Ag-decorated CNP devices showed the stable nonvolatile memory effects with 6 orders of ON/OFF resistance ratio and the small standard deviation of switching voltage distribution. The memory performance of CNP devices can be maintained without any degradation when being bent down to the radius of 350 μm, which is the smallest value compared to those of existing any flexible nonvolatile memories. Thus the present device using abundant and mechanically flexible CNP offers a highly flexible nonvolatile memory for portable flexible electronics.

Filament observation in metal-oxide resistive switching devices

Metal-oxide-based resistive random access memory (RRAM) is a predominant candidate for future non-volatile memories. In this Letter, we report on an innovative technique to observe conductive filaments in these oxide-based RRAM devices. We demonstrate the role of these conductive filaments as responsible for the different ON/OFF resistive states in memory devices by means of Conductive Atomic Force Microscopy (C-AFM). More specifically, C-AFM is used to cycle, de-process, and finally characterizes capacitor-like devices. Different conductive filaments are found for the different memory states. As we show, the ON/OFF state of the devices is associated to changes in morphological and electrical properties of the conductive filaments.

Evaluation of the electrical contact area in contact-mode scanning probe microscopy

The tunneling current through an atomic force microscopy (AFM) tip is used to evaluate the effective electrical contact area, which exists between tip and sample in contact-AFM electrical measurements. A simple procedure for the evaluation of the effective electrical contact area is described using conductive atomic force microscopy (C-AFM) in combination with a thin dielectric. We characterize the electrical contact area for coated metal and doped-diamond tips operated at low force (<200 nN) in contact mode. In both cases, we observe that only a small fraction (<10 nm2) of the physical contact (∼100 nm2) is effectively contributing to the transport phenomena. Assuming this reduced area is confined to the central area of the physical contact, these results explain the sub-10 nm electrical resolution observed in C-AFM measurements.

Switching mechanism and reverse engineering of low-power Cu-based resistive switching devices

In the recent past, filamentary-based resistive switching devices have emerged as predominant candidates for future non-volatile memory storage. Most of the striking characteristics of these devices are still limited by the high power consumption and poor understanding of the intimate resistive switching mechanism. In this study, we present an atomic scale study of the filament formation in CuTe-Al2O3 by using a conductive scanning probe tip to analyse the shape and dimensions of the filament. Filaments studied were either created within a normal device or locally formed while using the tip as the top electrode. We demonstrate that it is possible to create with C-AFM a filament with a signature identical to a device (i.e. two orders of magnitude resistance window, 10(4) s retention operating at 5 μA). This is obtained by a dedicated material and resistance selection for the conductive tip. The filamentary mechanism of fully processed devices is demonstrated and observed by C-AFM. Filaments created with C-AFM can be repeatedly cycled and the ON state presents a 20 nm highly conductive spot which can be repeatedly turned into a poorly conductive path in the OFF state.

Conductive-AFM tomography for 3D filament observation in resistive switching devices

In this paper we demonstrate a novel characterization technique for the observation of the conductive filament in conductive bridging memory devices (CBRAM). The conductive filament is observed for a scaled memory element programmed under 10μA operative current. After the electrical programing, the C-AFM tomography enables the 3D analysis of the conductive filament within the switching layer.

Operating-Current Dependence of the Cu-Mobility Requirements in Oxide-Based Conductive-Bridge RAM

In this letter, we compare the switching performances of Cu-based CBRAM cells having either Al2O3 or SiO2 dielectric layer. Both electrical and physical characterizations revealed different Cu mobility in the two dielectrics, impacting forming/switching speed and variability as well as functionality at low current. The modeling of the conduction also indicated different filament shapes in the two dielectrics. Based on the results, dielectrics allowing high Cu mobility are required when filament temperature is low, i.e., for low-current application (<;10 μA), while dielectrics allowing moderate Cu mobility are more appropriate for higher current ranges (≥10 μA), whereby Cu mobility is highly assisted by temperature.

Progressive vs. abrupt reset behavior in conductive bridging devices: A C-AFM tomography study

We investigated the physical origin of progressive and abrupt reset in conductive bridging memories. The conductive filaments for both types of reset are observed in 3D using C-AFM tomography, enabling the observation of broken and non-broken filaments respectively for abrupt and progressive reset.

Direct Probing of the Dielectric Scavenging-Layer Interface in Oxide Filamentary-Based Valence Change Memory

A great improvement in valence change memory performance has been recently achieved by adding another metallic layer to the simple metal-insulator-metal (MIM) structure. This metal layer is often referred to as oxygen exchange layer (OEL) and is introduced between one of the electrodes and the oxide. The OEL is believed to induce a distributed reservoir of defects at the metal-insulator interface thus providing an unlimited availability of building blocks for the conductive filament (CF). However, its role remains elusive and controversial owing to the difficulties to probe the interface between the OEL and the CF. Here, using Scalpel SPM we probe multiple functions of the OEL which have not yet been directly measured, for two popular VCMs material systems: Hf/HfO2 and Ta/Ta2O5. We locate and characterize in three-dimensions the volume containing the oxygen exchange layer and the CF with nanometer lateral resolution. We demonstrate that the OEL induces a thermodynamic barrier for the CF and estimate the minimum thickness of the OEL/oxide interface to guarantee the proper switching operations is ca. 3 nm. Our experimental observations are combined to first-principles thermodynamics and defect kinetics to elucidate the role of the OEL for device optimization.