AOS-based memristive devices towards TFT integration: Materials and challenges

Abstract

Von Neumann’s architecture, currently applied to electronic computing systems, is known to be at its optimization limits since CMOS technology cannot be further miniaturized without performance degradation. Neuromorphic computation is an effective alternative as it allows for power-efficient systems with high density, in memory computation and parallel data processing. A key component for this technology is the memristor, whose conductance can be altered upon application of an electric field. These devices have striking similarities to biological synapses, revealing many potential applications in neuromorphic engineering. To achieve a fully integrated system consisting of memristors and thin-film transistors (TFTs), both should share the same materials. Here, we study two amorphous oxide semiconductors (AOS) as resistive switching (RS) layers on memristors: indium-gallium-zinc oxide (IGZO) and zinctin oxide (ZTO) and the engineering of the top and bottom contacts. These two materials are widely applied in TFT driver circuits of active-matrix display technology, making them an ideal choice for memristors in neuromorphic system-on-panel solutions. In the memristive devices we present, the main intrinsic donors within the AOS, commonly referred to as oxygen vacancies, are responsible for the RS, generally following the valence change mechanism (VCM). To explore self-rectification, an asymmetry in the device structure was established. Pt was chosen for the bottom electrode as it is inert and forms a Schottky barrier when in contact with ZTO and IGZO. For the top electrode, Ti/Au was used since Ti has a high oxygen affinity which leads to the removal of oxygen from the AOS layer, forming a highly conductive region at the interface. We show these devices can be operated at two distinct modes. Device initialization in reverse polarity leads to an abrupt VCM-type counter-8-wise filamentary switching, whereas device initialization in the forward direction reveals an 8-wise area-dependent switching with resistance state ratios up to two orders of magnitude. The latter switching mode maintain the rectifying characteristic and allows analog control over resistance states which provides the means for subsequent high-density processing and storage in a neuromorphic system.[1] However, Pt and Ti/Au are expensive and, usually, patterned by lift-off, which limits device downsizing, reproducibility, and yield, due to irregularly shaped electrode edges, causing electrical shortcuts. A noble metal-free strategy with easy integration reduces the overall system cost: Mo was employed as bottom and top electrodes which, being patterned by dry etching, improves reproducibility and yield. Here, we present an IGZO-based memristor with Mo electrodes ready for TFT integration tests. With this memristor structure, the current rectification was achieved by oxidizing the bottom Mo at the interface. Devices down to 0.5 μm2 were fabricated using conventional photolithography, with an extraordinary yield of 100%. We successfully demonstrate the memristor’s synaptic behavior through potentiation and depression, short-term to long-term memory transition and simulation of the learning process.[2] [1] Casa, N., Deuermeier, J., Martins, J., Carlos, E., Pereira, M., Martins, R., Fortunato, E., Kiazadeh, A., 2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes. Adv. Electron. Mater. 2020, 6, 1900958. https://doi.org/10.1002/aelm.201900958 [2] Pereira, M., Deuermeier, J., Nogueira, R., Carvalho, P. A., Martins, R., Fortunato, E., Kiazadeh, A., Noble‐Metal‐Free Memristive Devices Based on IGZO for Neuromorphic Applications. Adv. Electron. Mater. 2020, 6, 2000242. https://doi.org/10.1002/aelm.202000242