Featured publications

Ultra-Fast Non-Volatile Resistive Switching Devices with Over 512 Distinct and Stable Levels for Memory and Neuromorphic Computing

Ming Xiao, Markus Hellenbrand, Nives Strkalj, Babak Bakhit, Zhuotong Sun, Nikolaos Barmpatsalos, Dovydas Koksas, Hongyi Dou, Zedong Hu, Ping Lu, Samip Karki, Sundar Kunwar, Jonathan D. Major, Aiping Chem, Haiyan Wang, Quanxi Jia, Adnan Mehonic, Judith L. MacManus-Driscoll

Low-current multilevel programmability with inherent non-volatility and high stability of resistance states is required for both multi-bit memory storage and deep learning accelerators but is difficult to achieve. Here, in a resistive switching system, this work realizes >512 (>9 bits) distinct non-volatile conductance levels with stable retention for each state with current levels down to the nanoampere range, highly promising for potential integration with small processing nodes with ultra-low power consumption requirements. This is achieved by demonstrating a new thin film design concept that encompasses three key features: an ultra-thin epitaxial oxygen ionic switching layer that provides a tunable energy barrier at the bottom electrode, an overcoat amorphous layer that acts as an ion migration barrier for stable state retention, and a partial conductive filament as a localized electronic transport channel to the epitaxial switching layer. A large dynamic resistance range of up to seven orders of magnitude is achieved with reset-free transitions among intermediate states, and programmability is demonstrated with ultra-fast (20 ns) pulses. Artificial neural network (ANN) simulations, based on the experimental performance and its non-idealities, demonstrate close-to-ideal inference accuracies for various Modified National Institute of Standards and Technology (MNIST) data sets.

Progress of emerging non-volatile memory technologies in industry

Markus Hellenbrand, Isabella Teck, Judith L. MacManus-Driscoll

This prospective and performance summary provides a view on the state of the art of emerging non-volatile memory (eNVM) in the semiconductor industry. The overarching aim is to inform academic researchers of the status of these technologies in industry, so as to help direct the right academic research questions for future materials and device development. eNVM already have a strong foothold in the semiconductor industry with the main target of replacing embedded flash memory, and soon possibly DRAM and SRAM, i.e. replacing conventional memory. Magnetic and resistive memory are the current frontrunners among eNVM for embedded flash replacement and they are very advanced in this, which poses high demands on future academic research for eNVM for this purpose. Phase-change and ferroelectric memory are less available as commercially available products. The use of eNVM for new forms of artificial intelligence hardware is a much more open field for future academic research.

Multi-level resistive switching in hafnium-oxide-based devices for neuromorphic computing

Markus Hellenbrand, Judith L. MacManus-Driscoll

In the growing area of neuromorphic and in-memory computing, there are multiple reviews available. Most of them cover a broad range of topics, which naturally comes at the cost of details in specific areas. Here, we address the specific area of multi-level resistive switching in hafnium-oxide-based devices for neuromorphic applications and summarize the progress of the most recent years. While the general approach of resistive switching based on hafnium oxide thin films has been very busy over the last decade or so, the development of hafnium oxide with a continuous range of programmable states per device is still at a very early stage and demonstrations are mostly at the level of individual devices with limited data provided. On the other hand, it is positive that there are a few demonstrations of full network implementations. We summarize the general status of the field, point out open questions, and provide recommendations for future work.

Thin film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity

Markus Hellenbrand, Babak Bakhit, Hongyi Duo, Ming Xiao, Megan O. Hill, Zhuotong Sun, Adnan Mehonic, Aiping Chen, Quanxi Jia, Haiyan Wang, Judith L. MacManus-Driscoll

A design concept of phase-separated amorphous nanocomposite thin films is presented that realizes interfacial resistive switching (RS) in hafnium-oxide-based devices. The films are formed by incorporating an average of 7% Ba into hafnium oxide during pulsed laser deposition at temperatures ≤400°C. The added Ba prevents the films from crystallizing and leads to ∼20-nm-thin films consisting of an amorphous HfO_x host matrix interspersed with ∼2-nm-wide, ∼5-to-10-nm-pitch Ba-rich amorphous nanocolumns penetrating approximately two-thirds through the films. This restricts the RS to an interfacial Schottky-like energy barrier whose magnitude is tuned by ionic migration under an applied electric field. Resulting devices achieve stable cycle-to-cycle, device-to-device, and sample-to-sample reproducibility with a measured switching endurance of ≥10⁴ cycles for a memory window ≥10 at switching voltages of ±2 V. Each device can be set to multiple intermediate resistance states, which enables synaptic spike-timing–dependent plasticity. The presented concept unlocks additional design variables for RS devices.

Ionic Conductivity Increased by Two Orders of Magnitude in Micrometer-Thick Vertical Yttria-Stabilized ZrO 2 Nanocomposite Films

Shinbuhm Lee, Wenrui Zhang, Fauzia Khatkhatay, Haiyan Wang, Quanxi Jia, Judith L. MacManus-Driscoll

We design and create a unique cell geometry of templated micrometer-thick epitaxial nanocomposite films which contain ∼20 nm diameter yttria-stabilized ZrO2 (YSZ) nanocolumns, strain coupled to a SrTiO3 matrix. The ionic conductivity of these nanocolumns is enhanced by over 2 orders of magnitude compared to plain YSZ films. Concomitant with the higher ionic conduction is the finding that the YSZ nanocolumns in the films have much higher crystallinity and orientation, compared to plain YSZ films. Hence, “oxygen migration highways” are formed in the desired out-of-plane direction. This improved structure is shown to originate from the epitaxial coupling of the YSZ nanocolumns to the SrTiO3 film matrix and from nucleation of the YSZ nanocolumns on an intermediate nanocomposite base layer of highly aligned Sm-doped CeO2 nanocolumns within the SrTiO3 matrix. This intermediate layer reduces the lattice mismatch between the YSZ nanocolumns and the substrate. Vertical ionic conduction values as high as 10–2 Ω–1 cm–1 were demonstrated at 360 °C (300 °C lower than plain YSZ films), showing the strong practical potential of these nanostructured films for use in much lower operation temperature ionic devices.

Strongly enhanced oxygen ion transport through samarium-doped CeO2 nanopillars in nanocomposite films

Sang Mo Yang, Shinbuhm Lee, Jie Jian, Wenrui Zhang, Ping Lu, Quanxi Jia, Haiyan Wang, Tae Won Noh, Sergei V. Kalinin, Judith L. MacManus‐Driscoll

Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.

Research Update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors, doping, and devices

Robert L. Z. Hoye, David Muñoz-Rojas, Shelby F. Nelson, Andrea Illiberi, Paul Poodt, Fred Roozeboom, Judith L. MacManus-Driscoll

Atmospheric pressure spatial atomic layer deposition (AP-SALD) has recently emerged as an appealing technique for rapidly producing high quality oxides. Here, AP-SALD was used to deposit functional, doped ZnO thin films. We highlight how these films are advantageous for the performance of solar cells, organometal halide perovskite light emitting diodes, and thin-film transistors. Future AP-SALD technology will enable the commercial processing of thin films over large areas on a sheet-to-sheet and roll-to-roll basis, with new reactor designs emerging for flexible plastic and paper electronics.

Enhanced Performance in Fluorene-Free Organometal Halide Perovskite Light-Emitting Diodes using Tunable, Low Electron Affinity Oxide Electron Injectors

Robert L. Z. Hoye, Matthew R. Chua, Kevin P. Musselman, Guangru Li, May-Ling Lai, Zhi-Kuang Tan, Neil C. Greenham, Judith L. MacManus-Driscoll, Richard H. Friend, Dan Credgington

Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence were obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH₃NH₃PbBr₃ perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier was reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.

Novel Electroforming-Free Nanoscaffold Memristor with Very High Uniformity, Tunability, and Density

Shinbuhm Lee, Abhijeet Sangle, Ping Lu, Aiping Chen, Wenrui Zhang, Jae Sung Lee, Haiyan Wang, Quanxi Jia, Judith L. MacManus-Driscoll

Dynamical tuning of the concentration of defects in oxides provides a route to controlling new functionalities. The chemical potential to capture the functionalities driven by mobile ions and defects can be one of key control parameters (as well as electric field, magnetic field, and stress) for tuning the functionality of complex oxides. Interesting signatures related to oxygen vacancies have been explicitly observed in widespread physical applications, including solid oxide fuel cells, catalysts, optoelectronics, and electronics. Here, in very simple, self-assembled nanoscaffold films containing nanocolumns with ~10-nm-radius and ~10-nm-intercolumnar-spacing, we demonstrate electroforming-free reversible electroresistance at room temperature. The nanoscaffold films are very easy-to-grow, since they self-assemble to give vertical heterointerfaces with V_o¨ channels along the interfaces. The structure has a clear advantage over conventional multilayers in multifunctional device nanoengineering

Schematic diagrams of device structures to generate Vo¨. (a) Irreversible electroforming with application of a high electrical stimulus to single-phase oxides (b) Conventional single-phase oxide film fractionally substituted with dopants. (c) Conventional multilayer film causing oxygen disorder at the lateral interfaces of dissimilar crystal structures. (d) Nanoscaffold film causing oxygen defect formation at the vertical interfaces of dissimilar crystal structures. Bottom figure shows strong retention of resistive switching behaviour of composite films. In comparison the plain films do not show switching because nanoionic channels are not present.

Caloric materials near ferroic phase transitions

Xavier Moya, Sohini Kar-Narayan, Neil D. Mathur

This Review Article presents the first thorough comparison of research into materials that get hot and cold due to changes of magnetic field near ferromagnetic phase transitions, with the growing bodies of analogous research into materials that get hot and cold due to changes of electric field near ferroelectric phase transitions, and materials that get hot and cold due to changes of stress field near structural phase transitions. We cross-fertilize the resulting magnetocaloric (MC), electrocaloric (EC) and mechanocaloric (mC) effects by contrasting experimental methods and performance, and we present a historical perspective that dates back to the Battle of Trafalgar and includes secret developments from opposing sides in World War II.

Prospective cooling applications are discussed critically based on industrial input. The relevant materials parameters of adiabatic temperature change |ΔT| and isothermal heat |Q| are plotted here for the three different types of caloric material [mC effects are sub-divided into elastocaloric (eC) and barocaloric (BC) effects, which arise due to uniaxial and isotropic stress fields, respectively].

A Scalable Nanogenerator Based on Self-Poled Piezoelectric Polymer Nanowires with High Energy Conversion Efficiency

Richard A. Whiter, Vijay Narayan, Sohini Kar-Narayan

Nanogenerators based on piezoelectric materials can convert ever-present mechanical vibrations into electrical power for energetically autonomous wireless and electronic devices. Nanowires of piezoelectric polymers are particularly attractive for harvesting mechanical energy in this way, as they are flexible, lightweight and sensitive to small vibrations. Previous studies have focused exclusively on nanowires grown by electrospinning, but this involves complex equipment, and high voltages of ~10 kV that electrically pole the nanowires and thus render them piezoelectric. In the present paper, nanowires of poly(vinylidene fluoride‑trifluoroethylene) [P(VDF‑TrFE)], grown using a simple, scalable, and cost-effective template-wetting technique, are shown to be successfully exploited in high-output nanogenerators without the need for electrical poling. The “self-poled” nanowires reported here exhibit high mechanical-to-electrical conversion efficiency comparable to the best previously reported values. This work therefore offers a scalable means of achieving high‑performance nanogenerators for the next generation of self-powered electronics, and also for the development of low-cost strain sensors in applications ranging from biomedicine to robotics.