Light-gated amorphous carbon memristors with indium-free transparent electrodes

Abstract

Amorphous carbon-based memristors with transparent conductive electrodes can be programmed both optically and electronically. We demonstrate neuromorphic logic and memory functions using light-gated amorphous carbon (a-C) memristors with indium-free transparent electrodes. The devices consist of insulating tetrahedral a-C layers with Pt top contacts supported on conductive zinc tin oxide electrodes with intentionally-oxygenated a-C interfaces. Resistive switching occurs by electron trapping/de-trapping, allowing low power, self-rectifying operation. Broadband light pulses evoke paired-pulse facilitation, emulating photonic coincidence detection. Coincidence detection was also achieved using heterogeneous optoelectronic inputs demonstrating dendritic computation at the single memory cell level.

Introduction

Bio-inspired memory and neuromorphic systems are the subject of an intensive research effort to alleviate the well-known bottleneck in von Neumann computing [[1], [2], [3]]. In 2008, the memristor was announced by HP labs as the key component in such systems and has since been touted as enabling low power, non-volatile and parallel computing [4,5]. Further ambitions to integrate memristors with high-speed photonic circuitry have also been proposed [6]. The key challenge is to be able to write electronic memory and perform computations by photonic stimulation.

On-chip optoelectronic memory devices offer extended functionality including heterogeneous-input logic operations [7], high-speed/bandwidth signal transmission [8,9], ‘non-contact’ data writing [10] and the implementation of neuromorphic learning/memory functions [[11], [12], [13]]. The advent of optogenetics is also enabling the next generation of neuroscience discoveries using photonic excitation of neurons in place of electronic stimulus [14] and optogenetic methods, in turn, have already been utilised in memristive devices to alter their emulated synaptic plasticity/memory [15].

The majority of memristor research to-date has focused on metal oxide-based devices, which operate either by the electric field-induced migration of itinerant dopants (vacancies or impurities) to form conducting pathways [16,17] or by the trapping/de-trapping of charge carriers in non-itinerant defect states [18,19]. However, several reports have emerged on carbon-based devices that can operate by similar mechanisms [[20], [21], [22], [23], [24], [25], [26]] and they have since been included in the International Technology Roadmap for Semiconductors (ITRS) [27]. Amorphous carbon, in particular, has attracted interest due to its elemental nature, reproducibility, biocompatibility and widely tunable optical/electronic properties [[28], [29], [30]]. While photonic transduction has been reported for several metal oxide-based memristors, optical-writing of amorphous carbon memory devices has yet to be achieved.

In this paper, we report on the fabrication and characterisation of light-gated amorphous carbon memristors. Several neuromorphic functions are demonstrated with the devices. An indium-free transparent electrode is used as a low-cost window layer. Amorphous zinc tin oxide (ZTO) was chosen due to its high transparency and tunable electronic properties [31,32]. Both the active carbon layers and the ZTO electrodes were energetically deposited by filtered cathodic vacuum arc (FCVA). Energetic deposition by FCVA allows dense, well adhered, high quality coatings to be formed at moderate growth temperatures and has previously been used to fabricate optical/electronic devices using the aforementioned materials [33].