Tech Briefs

Study could lead to more efficient electrically switchable resistive random access memory devices.

Resistive Random Access Memory (RRAM) devices have drawn much interest in the last decade, particularly the concept of a memristor. In this case, the so-called memristance, which provides the relationship between the change in charge (time integral of the current) and flux (time integral of the voltage), is not a constant as in linear elements, but a function of the charge, resulting in a nonlinear circuit element. Applications of such two-terminal electrical devices that provide high densities and low-power operation include, for instance, neuromorphic-type computing elements.

Optimized MIM structural models for Ag/NiO/Ag with a) O on-top b) Ni on-top; Pt/NiO/Pt with a tensilestrained lattice with c) O on-top and d) Ni on-top; Pt/NiO/Pt with the Pt lattice constant with e) O on-top and f) Ni on-top. The balls represented by gray, red, yellow, and blue correspond to nickel, oxygen, silver, and platinum atoms, respectively. The optimized (final) configurations of the interfaces for the O on-top initial configuration are shown in a), c), and e), while those in b), d), and f) correspond to the final configurations of the interfaces with the Ni on-top initial configuration.

This area of research led to a study on the effects of ionizing radiation on such devices. Significant focus on filamentary-type resistive switching (RS) mechanisms emerged, where formation/rupture of a conductive filament (CF) ensures successive switching in the non-volatile metal-insulator-metal (MIM) memristors, dependent on the switching material. In such a RRAM device, binary oxide MIM structures are constructed using an insulating layer stacked between two electrodes, which can be built either symmetrically or asymmetrically using the same or different top or bottom electrodes, respectively.

In the filamentary RS mechanism, following the CF forming stage, where a compliance current is used for controlling its size, operation depends on the migration of ions across the metal oxide in the SET (RESET) stages upon application of positive (negative) voltage in a bipolar RRAM, or of the same polarity voltage in a unipolar system. The rupture of the CF causes a High Resistance State (HRS), and its re-formation results in a Low Resistance State (LRS). Proposed conductance mechanisms by metal cations or positively charged oxygen vacancies, namely electrochemical metallization memory or conductive-bridge memory, and valence change memory mechanisms, respectively, are most common.

A thermochemical mechanism was also proposed. However, despite much promise and progress in this field, many issues remain, requiring further understanding of the memristive mechanism for specific MIMs. Indeed, a consensus on materials selection has still not been reached because properties such as reliability, switching speed, or the range of resistance states, depend on the materials used.

Among a multitude of metal oxide selections for RRAM devices, the behavior of p-type NiO, one of the earliest studied, still raises questions on the mechanism of operation. Following early work, RS characteristics of NiO were determined in a number of examples demonstrating high stability and reliable memory characteristics, high speed, low voltage, fast programming, and compatibility with the CMOS process. Moreover, inclusion of an oxygen exchange layer (Ta) in a Pt/NiO/Cu device enabled reaching a very high resistance ON/OFF ratio of 106.

Characterization of NiO filaments by magnetoresistance demonstrated their structural evolution, and also that multi-filaments are involved in the LRS, rupturing separately during RESET. Unipolar memristive behavior for NiO-based MIMs was demonstrated, indicating that reliable RS depends on the oxygen partial pressure during growth, so that the initial oxygen vacancy defect configuration affected the reliability of RS. Experiments for NiO/Pt films using time-of-flight secondary ion mass spectroscopy and conductive atomic force microscopy (C-AFM) measurements showed that oxygen atoms move to the anode, changing the surface composition, and therefore the resistance.

Oxygen vacancy migration, such as in a valence change memory mechanism-type system, has also been postulated. Although Ni vacancies in p-type NiO can be present in films, when the growth conditions are modified, appreciable concentration of oxygen vacancies is achieved. Indeed, investigation of diffusion of oxygen vacancies in epitaxial NiO by local multimodal scanning probe microscopy was reported, consistent with earlier work. Nevertheless, the NiO-based MIM system for RRAM application, which demonstrates encouraging characteristics, still poses questions on the role of oxygen vacancies in RS for this MIM system.

This work was done by Handan Yildirim and Ruth Pachter for the Air Force Research Laboratory. AFRL-0260