Suzanne Lancaster

Metal thiophosphates are emerging as a multifunctional material platform for neuromorphic electronics due to their accessible polar phases and ion dynamics on biologically relevant timescales. While resistive switching in these materials is frequently attributed to ferroelectric or antiferroelectric polarization, the intrinsic role of ion dynamics remains underexplored. Here, we isolate and demonstrate interfacial resistive switching driven purely by ionic hopping in paraelectric CuCrP2S6. Robust and reproducible resistive switching is observed in the absence of measurable ferroelectricity. The conductance can be tuned through both voltage amplitude and sweep rate, revealing a rate dependence characteristic of ion dynamics. The resulting resistance states exhibit controllable volatility, where the switching rate determines the decay time constant of the readout current, attributed to ionic relaxation. Using either inert or reactive electrodes, we observe electrical evidence of solid-state redox activity associated with the interfacial reduction of native Cu+ ions, enabling the controlled formation of filamentary conduction pathways. Analysis of this process allows the extraction of the Cu+ diffusion coefficient, providing quantitative insight into the underlying transport kinetics. The understanding of ionic-redox-based resistive switching in CuCrP2S6 is crucial for unleashing its full potential as a material platform for dual- or multi-mode operation.

The excellent scalability and compatibility to current CMOS manufacturing processes make ferroelectric Hf_xZr_{1 - x}O₂ thin films a promising candidate for embedded nonvolatile memories, as well as for synaptic devices in neuro-inspired computing. In order to achieve precise control over the polarization states and to ensure reliable operation in these thin films, a thorough understanding of the film's domain switching kinetics and behavior under field cycling is necessary. The Hf composition in Hf_xZr_1 - xO₂ thin films plays a crucial role in determining the disorders, phase composition, and crystallographic texture within the film when integrated in a metal-ferroelectric-metal (MFM) device, all of which affect the evolution of its field cycling response and switching kinetics. In this work, the impact of Hf content on wake-up and domain switching kinetics in Hf_xZr_1 - xO₂ thin films is investigated, and the physical mechanisms behind these differences are explored. This study highlights that multiple wake-up mechanisms can coexist in the same film and that as the Hf composition is increased, the dominant physical mechanism of wake-up in the film changes from a field-induced phase transition to field-induced ferroelastic domain switching. Furthermore, since the inhomogeneity within the ferroelectric film depends on Hf composition, the speed of polarization switching and the available partial polarization states in the film can be precisely controlled as a function of Hf content due to the disorder-driven domain nucleation and nonlinear domain wall dynamics.

Ferroelectric tunnel junctions (FTJs) are a class of memristor which promise low-power, scalable, field-driven analog operation. In order to harness their full potential, operation with identical pulses is targeted. In this paper, several weight update schemes for FTJs are investigated, using either nonidentical or identical pulses, and with time delays between the pulses ranging from 1 μs to 10 s. Experimentally, a method for achieving nonlinear weight update with identical pulses at long programming delays is demonstrated by limiting the switching current via a series resistor. Simulations show that this concept can be expanded to achieve weight update in a 1T1C cell by limiting the switching current through a transistor operating in subthreshold or saturation mode. This leads to a maximum linearity in the weight update of 86% for a dynamic range (maximum switched polarization) of 30 μC/cm². It is further demonstrated via simulation that engineering the device to achieve a narrower switching peak increases the linearity in scaled devices to >93% for the same range.

Ferroelectric tunnel junctions (FTJs) are a class of memristor which promise low-power, scalable, field-driven analog operation. In order to harness their full potential, operation with identical pulses is targeted. In this paper, several weight update schemes for FTJs are investigated, using either non-identical or identical pulses, and with time delays between the pulses ranging from 1 us to 10 s. Experimentally, a method for achieving non-linear weight update with identical pulses at long programming delays is demonstrated by limiting the switching current via a series resistor. Simulations show that this concept can be expanded to achieve weight update in a 1T1C cell by limiting the switching current through a transistor operating in sub-threshold or saturation mode. This leads to a maximum linearity in the weight update of 87%. The scaling behaviour of this scheme as devices are reduced to the sub-micron range is investigated, and the linearity slightly reduces in scaled devices to a maximum of 81%. The origin of the remaining non-linearity is discussed, as well as how this can be overcome.

The discovery of ferroelectricity in hafnia has revolutionized the field and brought industry applications closer than ever. One of the most interesting aspects of hafnia compared to other ferroelectric materials is the possibility of scaling film thicknesses down to the 10 nm regime and even below. However, going significantly below 10 nm poses some challenges in terms of materials engineering. In this review paper, the topic of thickness scaling in ferroelectric hafnia will be discussed in terms of physical limits, current achievements and challenges, and potential applications in different device types.

Hyper Dimensional Computing is a new approach in artificial intelligence (AI). The specific requirements of massive parallel readout operation during HDC inference makes the high-impedance ferroelectric tunneling junction (FTJ) devices a very interesting candidate for the hardware realization of a HDC accelerator. Therefore, we propose to research both, the technological and the architectural constraints when bringing together this two distinct concepts from the architectural and the devices perspective.

Ferroelectric hafnium and zirconium oxides have undergone rapid scientific development over the last decade, pushing them to the forefront of ultralow-power electronic systems. Maximizing the potential application in memory devices or supercapacitors of these materials requires a combined effort by the scientific community to address technical limitations, which still hinder their application. Besides their favorable intrinsic material properties, HfO2–ZrO2 materials face challenges regarding their endurance, retention, wake-up effect, and high switching voltages. In this Roadmap, we intend to combine the expertise of chemistry, physics, material, and device engineers from leading experts in the ferroelectrics research community to set the direction of travel for these binary ferroelectric oxides. Here, we present a comprehensive overview of the current state of the art and offer readers an informed perspective of where this field is heading, what challenges need to be addressed, and possible applications and prospects for further development.

While technologically challenging, the integration of ferroelectric thin films with graphene spintronics potentially allows the realization of highly efficient, electrically tunable, nonvolatile memories through control of the interfacial spin-orbit driven interaction occurring at graphene/Co interfaces deposited on heavy metal supports. Here, the integration of ferroelectric Hf_0.5Zr_0.5O₂ on graphene/Co/heavy metal epitaxial stacks is investigated via the implementation of several nucleation methods in atomic layer deposition. By employing in situ Al₂O₃ as a nucleation layer sandwiched between Hf_0.5Zr_0.5O₂ and graphene, the Hf_0.5Zr_0.5O₂ demonstrates a remanent polarization (2Pr) of 19.2 μC/cm². Using an ex situ, naturally oxidized sputtered Ta layer for nucleation, we could control 2Pr via the interlayer thickness, reaching maximum values of 28 μC/cm² with low coercive fields. Magnetic hysteresis measurements taken before and after atomic layer deposition show strong perpendicular magnetic anisotropy, with minimal deviations in the magnetization reversal pathways due to the Hf_0.5Zr_0.5O₂ deposition process, thus pointing to a good preservation of the magnetic stack including single-layer graphene. X-ray diffraction measurements further confirm that the high-quality interfaces demonstrated in the stack remain unperturbed by the ferroelectric deposition and anneal. The proposed graphene-based ferroelectric/magnetic structures offer the strong advantages of ferroelectricity and ferromagnetism at room temperature, enabling the development of novel magneto-electric and nonvolatile in-memory spin-orbit logic architectures with low power switching.

Computing systems are becoming more and more power-constrained due to unconventional computing requirements like computing on the edge, in-sensor, or simply an insufficient battery. Emerging Non-Volatile Memories are explored to build low-power computing circuits, and adders are one among them. In this work, we propose a low-power adder using a Ferroelectric Tunnel Junction (FTJ). FTJs are two-terminal devices where the data is stored in the polarization state of the device. An FTJ-based majority gate is proposed, which uses a current-mode sensing technique to evaluate the majority of the inputs. By conditionally selecting between the majority and its complement, an XOR operation is implemented, thereby achieving full-adder functionality. Since FTJ-based majority operation is slow, a ternary adder architecture is used to compensate for the speed loss. The ternary adder proposed by us has two stages of full adder and requires O(1) time for n-bit addition. The proposed adder is verified using a simulation in CMOS 130 nm technology. A 32-bit addition can be achieved in 100 μs and consumes 0.78 pJ, which is very power efficient (7.8 nW). The proposed adder can be used in applications where power consumption is crucial, and speed is not a strict requirement.

Hf_0.5Zr_0.5O₂ (HZO) thin films are promising candidates for non-volatile memory and other related applications due to their demonstrated ferroelectricity at the nanoscale and compatibility with Si processing. However, one reason that HZO has not been fully scaled into industrial applications is due to its deleterious wake-up and fatigue behavior which leads to an inconsistent remanent polarization during cycling. In this study, we explore an interfacial engineering strategy in which we insert 1 nm Al₂O₃ interlayers at either the top or bottom HZO/TiN interface of sequentially deposited metal-ferroelectric-metal capacitors. By inserting an interfacial layer while limiting exposure to the ambient environment, we successfully introduce a protective passivating layer of Al₂O₃ that provides excess oxygen to mitigate vacancy formation at the interface. We report that TiN/HZO/TiN capacitors with a 1 nm Al₂O₃ at the top interface demonstrate a higher remanent polarization (2P_r ∼ 42 μC cm⁻²) and endurance limit beyond 10⁸ cycles at a cycling field amplitude of 3.5 MV cm⁻¹. We use time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, and grazing incidence x-ray diffraction to elucidate the origin of enhanced endurance and leakage properties in capacitors with an inserted 1 nm Al₂O₃ layer. We demonstrate that the use of Al₂O₃ as a passivating dielectric, coupled with sequential ALD fabrication, is an effective means of interfacial engineering and enhances the performance of ferroelectric HZO devices.