Understanding the Effect of Top Electrode on Ferroelectricity in Atomic Layer Deposited Hf0.5Zr0.5O2 Thin Films

Probing Hf0.5Zr0.5O2 Ferroelectricity: Neutron Reflectivity Reveals Critical Interface Effects

Ferroelectric Hf0.5Zr0.5O2 (HZO) thin films emerge as promising candidates for next-generation memory devices; however, the device performance is strongly correlated to the interfacial structure. In this study, neutron reflectivity was used for the first time to conduct an in-depth analysis of the ferroelectric W/HZO/W devices, leveraging the high sensitivity of neutron scattering length density (SLD) to the buried interfaces. We explored the impact of different atomic layer deposition (ALD) techniques─thermal, plasma, and atomic layer annealing (ALA)─on the structural and ferroelectric characteristics of HZO thin films, with a particular focus on interfacial structures. Analyses using neutron reflectivity, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy revealed the critical role of the bottom WOx interfacial layer. The ALA treatment contributes to significant enhancements in structural and ferroelectric properties, including an increase in film density and crystallinity, as well as a maximum neutron SLD due to reduced oxygen vacancies. This work elucidates the crucial role of interface engineering via interfacial layer formation in stabilizing the ferroelectric phase, providing valuable insights for the development of advanced ferroelectric devices.

Direct growth of ferroelectric orthorhombic ZrO2 on Ru by atomic layer deposition at 300 °C

Fluorite-structured binary oxide ferroelectrics exhibit robust ferroelectricity at a thickness below 10 nm, making them highly scalable and applicable for high-end semiconductor devices. Despite this promising prospect, achieving highly reliable ferroelectrics still demands a significant thermal budget to form a ferroelectric phase, being a hurdle for their use in high-end complementary metal oxide semiconductor (CMOS) processing. Here, we report a robust ferroelectric behavior of an 8 nm-thick ZrO2 film deposited via plasma-enhanced atomic layer deposition at 300 °C on a (002)-oriented Ru without any post-annealing process, demonstrating high compatibility with CMOS processing. We propose that a plausible mechanism for this is the local domain matching epitaxy based on the high-resolution transmission electron microscopy and piezoelectric force microscopy results, where the templating effect between [101]-oriented grains of orthorhombic ZrO2 and [010]-oriented grains of Ru enables the direct growth of ferroelectric ZrO2. The 2Pr value is 20 μC cm-2, and it can be further improved by post-annealing at 400 °C to 23 μC cm-2 without showing the wake-up behavior. Ferroelectric switching shows stable endurance for up to 109 cycles, showcasing its high potential in CMOS-compatible applications and nanoelectronics with a low thermal budget.

Laser-Induced Phase Control of Morphotropic Phase Boundary Hafnium-Zirconium Oxide

A novel approach to delicately control the phase of a ferroelectric has been developed using a continuous-wave laser scanning annealing (CW-LSA) process. After proper process optimization, the equivalent oxide thickness (EOT) of 3.5 Å with a dielectric constant (κ) of 68 Å is achieved from HZO in a metal-ferroelectric-metal (MFM) capacitor structure. The leakage current density (Jg = 4.6 × 10-5 A/cm2 at +0.8 V) was four times lower compared to the reference group treated with an optimized RTA process. The outstanding electrical characteristics of the 6 nm HZO film were attributed to the stable formation of the morphotropic phase boundary (MPB) structure, which was enabled by the directional scanning nature of the CW-LSA process.

Influence of the TiN diffusion barrier on the leakage current and ferroelectricity in an Al-doped HfOx ferroelectric memristor and its application to neuromorphic computing

The HfOx-based ferroelectric memristor is in the spotlight due to its complementary metal-oxide-semiconductor compatibility and scaling compared to existing perovskite-based ferroelectric memory. However, ferroelectric properties vary depending on the coefficient of thermal expansion of the top electrode, which is caused by strain engineering. When tungsten (W) with a small coefficient of thermal expansion is used as an electrode, the ferroelectric properties are improved, although the reliability is poor due to the diffusion of W atoms. Here, TiN can be used to prevent the diffusion of W. This metal nitride successfully suppresses the leakage current and induces a larger remanent polarization of 19.7 μC cm-2, a smaller coercive voltage of 9.26 V, and a faster switching speed. W/TiN/HAO/n+ Si can also exhibit multi-level characteristics and achieve a 10% read margin in 320 × 320 arrays. Ferroelectrics can also be applied to neuromorphic computing by imitating synaptic properties such as potentiation, depression, paired-pulse facilitation, and excitatory postsynaptic current. Using short-term plasticity, successful implementation in reservoir computing is also realized, achieving 95% classification accuracy. This paper shows promise for the use of memristors in artificial neural networks.

Ferroelectric freestanding hafnia membranes with metastable rhombohedral structure down to 1-nm-thick

Two-dimensional freestanding membranes of materials, which can be transferred onto and make interfaces with any material, have attracted attention in the search for functional properties that can be utilized for next-generation nanoscale devices. We fabricated stable 1-nm-thick hafnia membranes exhibiting the metastable rhombohedral structure and out-of-plane ferroelectric polarizations as large as 13 μC/cm2. We also found that the rhombohedral phase transforms into another metastable orthorhombic phase without the ferroelectricity deteriorating as the thickness increases. Our results reveal the key role of the rhombohedral phase in the scale-free ferroelectricity in hafnia and also provide critical insights into the formation mechanism and phase stability of the metastable hafnia. Moreover, ultrathin hafnia membranes enable heterointerfaces and devices to be fabricated from structurally dissimilar materials beyond structural constrictions in conventional film-growth techniques.

Tunable defect engineering of Mo/TiON electrode in angstrom-laminated HfO2/ZrO2ferroelectric capacitors towards long endurance and high temperature retention

A novel defect control approach based on laminated HfO2/ZrO2with multifunctional TiN/Mo/TiOxNyelectrode is proposed to significantly improve the endurance and data retention in HZO-based ferroelectric capacitor. The O-rich interface reduces leakage current and prolong the endurance up to 1011cycles while retaining a 2Pr value of 34 (μC cm-2) at 3.4 MV cm-1. Using first-principles calculations and experiments, we demonstrate that the enhancement of endurance is ascribed to the higher migration barrier of oxygen vacancies within the laminated HZO film and higher work function of MoOx/TiOxNybetween top electrode and the insulating oxide. This 2.5 nm thick TiOxNybarrier further increase the grain size of HZO, lowering the activation field and thus improving polarization reversal speed. This interfacial layer further decreases the overall capacitance, increases the depolarization field, thereby enhancing the data retention. By fitting the data using the Arrhenius equation, we demonstrate a 10 years data retention is achieved at 109.6 °C, surpassing traditional SS-HZO of 78.2 °C with a 450 °C rapid thermal annealing (required by backend-of-the-line). This work elucidates that interfacial engineering serves as a crucial technology capable of resolving the endurance, storage capability, and high-temperature data retention issues for ferroelectric memory.

Electrochemistry of Thin Films and Nanostructured Materials

In the last few decades, the development and use of thin films and nanostructured materials to enhance physical and chemical properties of materials has been common practice in the field of materials science and engineering. The progress which has recently been made in tailoring the unique properties of thin films and nanostructured materials, such as a high surface area to volume ratio, surface charge, structure, anisotropic nature, and tunable functionalities, allow expanding the range of their possible applications from mechanical, structural, and protective coatings to electronics, energy storage systems, sensing, optoelectronics, catalysis, and biomedicine. Recent advances have also focused on the importance of electrochemistry in the fabrication and characterization of functional thin films and nanostructured materials, as well as various systems and devices based on these materials. Both cathodic and anodic processes are being extensively developed in order to elaborate new procedures and possibilities for the synthesis and characterization of thin films and nanostructured materials.