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

Abstract

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/HfO and Ta/TaO. 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.

The ultimate switching speed limit of redox-based resistive switching devices

“…This value is in agreement with different spectro-microscopic observations of laments in resistive switching devices. 24,35,45,46 To calculate the temperature distribution in this structure, the transient heat equation,…”

On the Limits of Scalpel AFM for the 3D Electrical Characterization of Nanomaterials

“…Then the same location was scanned again applying one/few scans using FC > FETCH and V = 0 V in order to produce etching (namely etching scans). All etching scans in this work were collected without applying any bias in order to produce the lowest degree of damage possible to the sample during the etching; however, previous reports on scalpel AFM used small voltages <100 mV, [21][22][23][24][25][26][27][28][29][30][31] which may be also acceptable. It is also important to highlight that the scan line density during the etching scans was always ≥ 256 lines µm −1 , which is a recommended value that reduces the distance between the etching lines, and should reduce the surface roughness of the etched region.…”

“…This observation seems to be in contradiction to that reported in refs. [21,[23][24][25][27][28][29][30][31]; however, in those works the CNFs were formed at the device level, and the top electrode was etched with the CAFM tip too. One possibility may be that the high local temperatures during the etching of the top electrode might have disrupted the CNF even before the CAFM tip reaches the surface of the oxide, and that the conductive spots displayed in the subsequent slices may not correspond to the shape of the CNF formed across the oxide during the electrical stress.…”

“…In particular, scalpel AFM has been mostly used to characterize the size and currents driven by conductive nanofilaments (CNFs) across thin insulators in RS devices under an electrical field. [21][22][23][24][25][26][27][28][29][30][31][32][33] It should be noted that the diameter (Ø) of these CNFs in realistic RS devices is typically ≈10 nm, [34] and that one of the most important problems is their stability, i.e., they can self-disrupt over the time [35] or when exposed to high temperatures. [36] The RS cells studied by the IMEC group using scalpel AFM have been Cu/Al2O3/TiN, [21,23,24] Au/Cu/Ti/Al2O3/TiN, [25] Al2O3/Cu, [26] Ta/TaO2/Pt, [27] Ru/Hf/HfO2/TiN, [28,29] and TiN/HfO2/Hf/TiN, [30] and the experiments have been mainly carried out in air atmosphere, and sporadically in a vacuum of 10 −5 mbar.…”

“…[21][22][23][24][25][26][27][28][29][30][31][32][33] It should be noted that the diameter (Ø) of these CNFs in realistic RS devices is typically ≈10 nm, [34] and that one of the most important problems is their stability, i.e., they can self-disrupt over the time [35] or when exposed to high temperatures. [36] The RS cells studied by the IMEC group using scalpel AFM have been Cu/Al2O3/TiN, [21,23,24] Au/Cu/Ti/Al2O3/TiN, [25] Al2O3/Cu, [26] Ta/TaO2/Pt, [27] Ru/Hf/HfO2/TiN, [28,29] and TiN/HfO2/Hf/TiN, [30] and the experiments have been mainly carried out in air atmosphere, and sporadically in a vacuum of 10 −5 mbar. It should be noted that, in all cases: i) the oxides etched were very thin (<10 nm) and amorphous, ii) the diameter of the CNFs detected was always <30 nm, and iii) the experiments were carried out using solid B-doped diamond CAFM tips in order to avoid premature tip wearing-so far this type of CAFM tips have been fabricated and commercialized only by IMEC, and their price is up to four times higher than that of standard B-doped diamond coated Si tips from other manufacturers, such as Nanoworld.…”