Impact of electrode materials on the performance of amorphous IGZO thin-film transistors

This study reports on the fabrication and characterization of thin-film transistors (TFTs) based on indium–gallium–zinc–oxide (IGZO) with various source- and drain-region metals (Pt, W and Ti). The performance of the IGZO transistors is compared to TFTs based on hydrogenated amorphous silicon (a-Si:H) with Pt source- and drain-regions. From the output characteristics maximum saturation mobilities of µ = 0.45 cm2/Vs for a-Si:H, and µ = 24 to 50 cm2/Vs for IGZO TFTs are extracted, which are competitive to high-performance thin-film transistors. The study reveals a general influence of the source- and drain-electrode material on the maximum saturation mobility and inverse sub-threshold slope

Programmable mixed-signal circuits

A novel concept for programmable mixed-signal circuits is presented based on programmable transmission gates. For implementation, memristively switching devices are suggested as the most promising candidates for realization of fast and small-footprint signal routing switches with small resistance and capacity. As a proof-of-concept, LT Spice simulations of digital and analogue example circuits implemented by the new concept are demonstrated. It is discussed how important design parameters can be tuned in the circuity. Compared to competing technologies such as Field Programmable Analogue Arrays or Application-Specific Integrated Circuits, the presented concept allows for development of ultra-flexible, reconfigurable, and cheap embedded mixed-signal circuits for applications where only limited space is available or high bandwidth is required

Nanoscale Plasmon-Enhanced Spectroscopy in Memristive Switches.

Resistive switching memories are nonvolatile memory cells based on nano-ionic redox processes and offer prospects for high scalability, ultrafast write and read access, and low power consumption. In two-terminal cation based devices a nanoscale filament is formed in a switching material by metal ion migration from the anode to the cathode. However, the filament growth and dissolution mechanisms and the dynamics involved are still open questions, restricting device optimization. Here, a spectroscopic technique to optically characterize in situ the resistive switching effect is presented. Resistive switches arranged in a nanoparticle-on-mirror geometry are developed, exploiting the high sensitivity to morphological changes occurring in the tightly confined plasmonic hotspot within the switching material. The focus is on electrochemical metallization and the optical signatures detected over many cycles indicate incomplete removal of metal particles from the filament upon RESET and suggest that the filament can nucleate from different positions from cycle to cycle. The technique here is nondestructive and the measurements can be easily performed in tunable ambient conditions and with realistic cell geometries.G.D.M. and S.T. contributed equally to this work. The authors acknowledge Alan Sanders for developing the data collection software and Richard Bowman for providing part of the experimental equipment. The authors acknowledge funding from ERC grant LINASS 320503, EPSRC grant EP/L027151/1, and ERC grant InsituNANO 279342.This is the final version of the article. It first appeared from Wiley via https://doi.org/10.1002/smll.20150316

Transfer-free graphene passivation of sub 100 nm thin Pt and Pt–Cu electrodes for memristive devices

Memristive switches are among the most promising building blocks for future neuromorphic computing. These devices are based on a complex interplay of redox reactions on the nanoscale. Nanoionic phenomena enable non-linear and low-power resistance transition in ultra-short programming times. However, when not controlled, the same electrochemical reactions can result in device degradation and instability over time. Two-dimensional barriers have been suggested to precisely manipulate the nanoionic processes. But fabrication-friendly integration of these materials in memristive devices is challenging.Here we report on a novel process for graphene passivation of thin platinum and platinum/copper electrodes. We also studied the level of defects of graphene after deposition of selected oxides that are relevant for memristive switching

Self-assembled oxide films with tailored nanoscale ionic and electronic channels for controlled resistive switching.

Resistive switches are non-volatile memory cells based on nano-ionic redox processes that offer energy efficient device architectures and open pathways to neuromorphics and cognitive computing. However, channel formation typically requires an irreversible, not well controlled electroforming process, giving difficulty to independently control ionic and electronic properties. The device performance is also limited by the incomplete understanding of the underlying mechanisms. Here, we report a novel memristive model material system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow the separate tailoring of nanoscale ionic and electronic channels at high density (∼10(12) inch(-2)). We systematically show that these devices allow precise engineering of the resistance states, thus enabling large on-off ratios and high reproducibility. The tunable structure presents an ideal platform to explore ionic and electronic mechanisms and we expect a wide potential impact also on other nascent technologies, ranging from ionic gating to micro-solid oxide fuel cells and neuromorphics.This work was supported by the European Research Council (ERC) (Advanced Investigator grant ERC-2009-AdG-247276-NOVOX) and the Cambridge Commonwealth, European & International Trust. We further acknowledge funding from ERC grant InsituNANO, 279342, (S.T. and S.H.) and the Engineering and Physical Sciences Research Council (EPSRC), EP/P005152/1 (S.H.). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The work at Los Alamos was supported by the U.S. Department of Energy through the LDRD program and performed, in part, at the Center for Integrated Nanotechnologies (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility.This is the final version of the article. It first appeared from Nature Publishing Group via https://www.nature.com/articles/ncomms1237

Catalyst Interface Engineering for Improved 2D Film Lift-Off and Transfer

The mechanisms by which chemical vapor deposited (CVD) graphene and hexagonal boron nitride (h-BN) films can be released from a growth catalyst, such as widely used copper (Cu) foil, are systematically explored as a basis for an improved lift-off transfer. We show how intercalation processes allow the local Cu oxidation at the interface followed by selective oxide dissolution, which gently releases the 2D material (2DM) film. Interfacial composition change and selective dissolution can thereby be achieved in a single step or split into two individual process steps. We demonstrate that this method is not only highly versatile but also yields graphene and h-BN films of high quality regarding surface contamination, layer coherence, defects, and electronic properties, without requiring additional post-transfer annealing. We highlight how such transfers rely on targeted corrosion at the catalyst interface and discuss this in context of the wider CVD growth and 2DM transfer literature, thereby fostering an improved general understanding of widely used transfer processes, which is essential to numerous other applications.We acknowledge funding from the ERC (InsituNANO, grant 279342). R.W. acknowledges an EPSRC Doctoral Training Award (EP/M506485/1). During this work, S.T. was supported in parts by a DFG research fellowship under grant TA 1122/1-1:1. J.A.A.-W. acknowledges a Research Fellowship from Churchill College, Cambridge. Z.A.V.V. acknowledges funding from ESPRC grant EP/L016087/1. P.B. and B.S.J. thank the Danish National Research Foundation Centre for Nanostructured graphene, DNRF103, and EU Horizon 2020 “Graphene Flagship” 696656. T.J.B. and P.R.W. acknowledge financial support from EU FP7-6040007 “GLADIATOR” and Innovation Fund Denmark Da-Gate 0603-005668B. P.R.K. acknowledges a Lindemann Trust Fellowship

Redox and mass transport phenomena in resistively switching thin films

In the last decades, modern information and communication technology have become part of daily life, which is unequivocally linked with the progressive scaling of electronic components and particularly with the ongoing miniaturization of transistors and memory devices such as Flash. As the scaling limit of conventional memory technology is approaching, new concepts are part of current research. In this context, nonvolatile redox based resistive switches (Redox based Resistive Switching Random Access Memories, ReRAMs) are considered as a highly promising alternative to Flash memories. These two-terminal memory cells are based on a simple layer structure and the information is stored as different resistance levels by applying appropriate voltage pulses. In comparison to Flash, ReRAMs offer a high potential of scalability, low power consumption and fast write access. The strongly local resistive switching effect is observed in various material systems and often based on a valance change mechanism or electrochemical metallization effect. However, the processes involved during the resistance transition are not yet studied in detail, which is disadvantageous for device optimization. In this thesis, ReRAM cells based on the electrochemical metallization effect are analyzed in respect to electrochemical and physical processes, which are contributing to the resistive switching effect. Since resistive switching is observed in various materials, two different materials, i.e. silicon dioxide (SiO2) and silver iodide (AgI), representing the material class of insulators and ion conductors, respectively, were selected. In particular, nanoscale SiO2 is characterized by an unexpected high cation mobility, which is essential for the resistive switching effect but not reported in bulk SiO2. This work can be divided into two parts. In the first part, electrochemical processes prior to the switching event are analyzed by potentiodynamic and spectroscopic measurement methods. It has been observed that in SiO2 OH--ions act as counter charges, which are required for resistive switching. In case of silver iodide the Ag/AgI-interface was found to be chemically inactive, but silver can penetrate as small metal-crystallites in AgI. Moreover, in redox based resistive switches nonequilibrium states are inherently induced, which have been neglected in device models reported in literature. These effects are directly affecting the resistive switching process itself, which is studied in the second part of this thesis. Quantized conductance values have been observed both in silicon dioxide and silver iodide giving the prospect of the ultimate atomic scaling potential of ReRAMs. Additionally, the strongly nonlinear, and from an application point of view beneficial, switching kinetic is analyzed experimentally and is theoretically discussed

Research data supporting "Programmable Mixed-Signal Circuits"

Research data supporting "Programmable Mixed-Signal Circuits". This file contains all raw LT Spice simulation files and the measurement of the resisitive switching curve

Redox and mass transport phenomena in resistively switching thin films

In the last decades, modern information and communication technology have become part of daily life, which is unequivocally linked with the progressive scaling of electronic components and particularly with the ongoing miniaturization of transistors and memory devices such as Flash. As the scaling limit of conventional memory technology is approaching, new concepts are part of current research. In this context, nonvolatile redox based resistive switches (Redox based Resistive Switching Random Access Memories, ReRAMs) are considered as a highly promising alternative to Flash memories. These two-terminal memory cells are based on a simple layer structure and the information is stored as different resistance levels by applying appropriate voltage pulses. In comparison to Flash, ReRAMs offer a high potential of scalability, low power consumption and fast write access. The strongly local resistive switching effect is observed in various material systems and often based on a valance change mechanism or electrochemical metallization effect. However, the processes involved during the resistance transition are not yet studied in detail, which is disadvantageous for device optimization. In this thesis, ReRAM cells based on the electrochemical metallization effect are analyzed in respect to electrochemical and physical processes, which are contributing to the resistive switching effect. Since resistive switching is observed in various materials, two different materials, i.e. silicon dioxide (SiO2) and silver iodide (AgI), representing the material class of insulators and ion conductors, respectively, were selected. In particular, nanoscale SiO2 is characterized by an unexpected high cation mobility, which is essential for the resistive switching effect but not reported in bulk SiO2. This work can be divided into two parts. In the first part, electrochemical processes prior to the switching event are analyzed by potentiodynamic and spectroscopic measurement methods. It has been observed that in SiO2 OH--ions act as counter charges, which are required for resistive switching. In case of silver iodide the Ag/AgI-interface was found to be chemically inactive, but silver can penetrate as small metal-crystallites in AgI. Moreover, in redox based resistive switches nonequilibrium states are inherently induced, which have been neglected in device models reported in literature. These effects are directly affecting the resistive switching process itself, which is studied in the second part of this thesis. Quantized conductance values have been observed both in silicon dioxide and silver iodide giving the prospect of the ultimate atomic scaling potential of ReRAMs. Additionally, the strongly nonlinear, and from an application point of view beneficial, switching kinetic is analyzed experimentally and is theoretically discussed