High throughput optimisation of functional nanomaterials and composite structures for resistive switching memory

Abstract

The Semiconductor industry is investigating high speed, low power consumption, high-density memory devices that can retain their information without power supply. Resistive Random Access Memory (ReRAM) is one of the most attractive candidates as an alternative to conventional flash memory devices due to of its simple metal-insulator-metal (MIM) structures. A compositional gradient of thin film materials produced by the simultaneous combination of elements provides a powerful tool for the combinatorial synthesis of materials. It was applied here to control the composition, structure and morphology of materials in composite devices of ReRAM. This allows the systematic high throughput screening of the intrinsic properties of the materials, as well as the high throughput optimisations of composite thin films that mimic memory device structures. Therefore, the focus of this project is to develop a novel capacitor for ReRAM application. We present here details of the preparation technique and the screening methodologies of this approach by applying the synthesis to various phases of titania, for which there is an extensive literature, as a prelude to the screening of more complex systems. Inert Pt electrodes and active Cu electrodes were deposited on TiO2 as top electrodes using different mask sizes (50 micron and 250 micron). The bottom electrode is Si/ SiO2/ TiO2/ Pt (SSTOP) was constant throughout this project. TiO2 was prepared using evaporative physical vapour deposition (PVD) with a variation of thickness between 10 nm and 300 nm on SSTOP. The synthetic conditions were chosen to produce TiO2 oxygen stoichiometric and sub-stoichiometric amorphous, anatase and rutile materials. The oxides have been fully characterised by X-Ray Diffraction (XRD), X-ray Photo electron Spectroscopy (XPS), Raman Spectroscopy, Four Point Probe (4pp) and Atomic Force Microscopy (AFM). The electrical screening was carried out on capacitor-like structures produced using 250 micron diameter top electrodes deposited using a 14 x 14 array contact mask. Current-Voltage (I-V) measurements were conducted employing a variety of current compliances (IC). The typical I-V switching of the unipolar mode (both state in one polarity) was achieved on all titania phases, whereas the bipolar mode (each state in different polarity) was achieved only on the amorphous phase. The resistance differences between High Resistance State (HRS) and Low Resistance State (LRS) were clearly identified in each system. It was observed that for all the devices investigated, a lower forming field was required on the thicker layer of the active switching layers. Devices with copper electrodes, and composite devices with sub-stoichiometric titania adjacent to the stoichiometric titania could be formed at lower voltages and electric fields. The results obtained here confirm the feasibility of the high-throughput approach to optimise functional nanomaterials and composite device structures for resistive switching memory application