Organic Ferroelectric Memory Devices

Hafnium oxide (HfO2 ) is one of the mature high-k dielectrics that has been standing strong in the memory arena over the last two decades. Its dielectric properties have been researched rigorously for the development of flash memory devices. In this review, the application of HfO2 in two main emerging nonvolatile memory technologies is surveyed, namely resistive random access memory and ferroelectric memory. How the properties of HfO2 equip the former to achieve superlative performance with high-speed reliable switching, excellent endurance, and retention is discussed. The parameters to control HfO2 domains are further discussed, which can unleash the ferroelectric properties in memory applications. Finally, the prospect of HfO2 materials in emerging applications, such as high-density memory and neuromorphic devices are examined, and the various challenges of HfO2 -based resistive random access memory and ferroelectric memory devices are addressed with a future outlook.

Ferroelectric memories have undergone a transformative evolution from conventional perovskite-based materials to modern fluorite-structured ferroelectrics, driven by the pursuit of scalable, low-power, and CMOS-compatible non-volatile memory solutions. The observation of ferroelectricity in nanoscale HfO2-based films has enabled integration with CMOS-compatible processes, providing advantages such as potential scalability, low power consumption, and non-volatility, while facilitating continued scaling and high-density integration. Leveraging established materials infrastructure in the semiconductor industry, hafnia–based ferroelectrics have been incorporated in various memory architectures, including ferroelectric random-access memory (FeRAM), ferroelectric tunnel junctions (FTJs), ferroelectric field-effect transistors (FeFETs), and ferroelectric memcapacitors (FeCAPs). Beyond conventional non-volatile storage, these devices have also emerged as promising building blocks for in-memory computing applications, including neuromorphic systems, hardware security primitives, and associative memory. In this review, we explore the historical development of ferroelectric memories from a materials–device co-design perspective, examine recent advances in device architectures and in-memory computing applications, and discuss the remaining challenges in endurance, retention, variability, and scaling. Finally, we propose future research directions that integrating material innovation, interface engineering, and circuit-level optimization to realize the full potential of ferroelectric memories in next-generation computing platforms.Graphical abstract

There is a considerable interest in ferroelectric random access memories (FeRAMs), because they combine non volatility with fast access time and low operation voltage. However, current FeRAMs still have a large cell size compared to DRAM or Flash. To develop a small and competitive FeRAM cell is the most challenging issue for a high density and stand-alone memory. In this work we demonstrate a novel Chain FeRAM cell using a new 3 dimensional Vertical Capacitor concept for the first time.

In recent years, organic resistive memory devices in which active organic materials possess at least two stable resistance states have been extensively investigated for their promising memory potential. From the perspective of device fabrication, their advantages include simple device structures, low fabrication costs, and printability. Furthermore, their exceptional electrical performances such as a nondestructive reading process, nonvolatility, a high ON/OFF ratio, and a fast switching speed meet the requirements for viable memory technologies. Full understanding of the underlying physics behind the interesting phenomena is still challenging. However, many studies have provided useful insights into scientific and technical issues surrounding organic resistive memory. This Feature Article begins with a summary on general characteristics of the materials, device structures, and switching mechanisms used in organic resistive devices. Strategies for performance enhancement, integration, and advanced architectures in these devices are also presented, which may open a way toward practically applicable organic memory devices.

Layered perovskite ferroelectric materials (also known as “Y-1”) allowed a quantum jump in Ferroelectric Random Access Memories (FeRAMs) due to their low operating voltages and excellent endurance properties. The ability to impose technological control in the material microstructure and overall device properties has made a major impact in the commercialization of FeRAMs. In this paper, we review our new material and integration technologies, and present the performance of “state-of-the-art” devices incorporating Y-1 FeRAMs, such as a microcontroller and an RF-ID tag. These data indicate not only the great potential of the Y-1 technology but also its maturity for the production of commercial products.

In order to manufacture high-density ferroelectric random access memory (FRAM) device, it is required to develop one mask capacitor etching technology, because it can provide greatly reduced cell size. However, as the capacitor size shrinks further, the influence of etching damage on the ferroelectric properties becomes much serious due to the high ratio of perimeter/area for patterned capacitor. Since undesired polymeric etch byproducts were formed on sidewall of the edge cell capacitors in 32 Mb FRAM with 0.25 μm design rule, we developed novel post-etch curing technology using O2 plasma treatment with wet cleaning process. After the post-etch curing treatment, the hysteresis loops of block edge cells were almost identical to those of block center cells, which results in improving the relative 2Pr value as ratio of edge cells/center cells from 33% to 98%. In conclusion, novel curing technology was successfully developed for one mask etching damaged ferroelectric capacitor using O2 plasma treatment with wet cleaning process, resulting in high wafer yield.

We demonstrate multilevel data storage in organic ferroelectric resistive memory diodes consisting of a phase-separated blend of P(VDF-TrFE) and a semiconducting polymer. The dynamic behaviour of the organic ferroelectric memory diode can be described in terms of the inhomogeneous field mechanism (IFM) model where the ferroelectric components are regarded as an assembly of randomly distributed regions with independent polarisation kinetics governed by a time-dependent local field. This allows us to write and non-destructively read stable multilevel polarisation states in the organic memory diode using controlled programming pulses. The resulting 2-bit data storage per memory element doubles the storage density of the organic ferroelectric resistive memory diode without increasing its technological complexity, thus reducing the cost per bit.

Modeling and simulation of low power ferroelectric non-volatile memory tunnel field effect transistors using silicon-doped hafnium oxide as gate dielectric

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.

Intensive research into functional oxides has been triggered by the quest for a solid-state universal memory with high-storage density, non-volatility, high read/write speed, and random access. The ferroelectric random-access memory (FeRAM), in which the information is stored in the spontaneous ferroelectric polarization of the material, offers great promise as nonvolatile and multistate memory, but its destructive electrical reading step requires a rewrite step after each reading, increasing energy consumption. As an alternative, optical nondestructive readout is based on the ferroelectric polarization dependence of the photovoltaic response in materials and has been reported in two-states ferroelectric memories and multistate devices with limited photocurrent switchability due to asymmetric interfacial effects. In this work, we report a nonvolatile oxide memory device based on a symmetric heterostructure with eight stable and well-controlled remanent polarization (Pr) states, written electrically by voltage pulse and read optically through polarization-dependent short-circuit photocurrent Isc or open circuit photovoltage Voc. This symmetric capacitor demonstrates a clear proportionality between Isc (Voc) and Pr, allowing to achieve a 100% switchability of the photovoltaic response. The memory devices based on 3-bit data storage show good performance in terms of data retention, fatigue behavior, and repeatability of writing and reading cycles. Thanks to the very high sensitivity of the optical reading method, the number of states could largely exceed eight, being limited only by the electrical writing step precision. These results are particularly exciting for the development of next-generation ferroelectric memory devices with increased memory storage density and lower power consumption.

Strain investigation of PECVD passivation layer for ferroelectric domain wall random access memory

Polymer electrets for organic nonvolatile memory devices: Recent advances

The National Aeronautics and Space Administration (NASA) is developing high-tolerance, radiation-hardened electronics for missions in and beyond Low Earth orbit. Ferroelectric-based electronics are highly viable candidates for these electronics because of their inherent radiation-hardened property. Since standard memory devices are prone to damage caused by radiation, ferroelectric memory may provide the needed radiation-tolerance. To test the effectiveness of ferroelectric random access memory (FRAM) in Low Earth orbit, a 512 K Ramtron FRAM will be flown on a Low Earth orbit satellite that will be launched by NASA. This paper discusses the advantages of ferroelectric electronics and outlines the Low Earth orbit ferroelectric memory test experiment.

Since ferroelectric capacitors prepared by 1-mask etching are degraded after the etching, we systematically investigated the origin of the degradation. It was found that the major degradation originates from the formation of the nonstoichiometric and amorphorized Pb(ZrxTi1-x)O3 (PZT) layer on the sidewall of the PZT film during etching of the bottom electrode (BE). Therefore, to eliminate the undesired etch-damaged layer, we developed a novel etching technology using a ferroelectric (FE) sidewall spacer, which results in the enhancement of the remnant polarization after completing the capacitor etching process. Using the novel FE sidewall spacer, the sensing margin of bit-line-developed voltage was improved to 400 mV, which can guarantee highy reliable high-density ferroelectric random access memory (FRAM) devices.

Robust in-plane polarization switching in epitaxial BiFeO3 films