Monica Materano

Ferroelectric hafnium‐zirconium oxide is one of the most relevant CMOS‐compatible materials for next‐generation, non‐volatile memory devices. Nevertheless, performance reliability remains an issue. With TiN electrodes (the most reported electrode material), Hf‐Zr‐based ferroelectric capacitors struggle to provide reliable retention due to electrode‐ferroelectric interface interactions. Although Hf‐Zr‐based ferroelectric capacitors are fabricated with other electrodes, the focus is predominantly directed toward obtaining a large ferroelectric response. The impact of the electrodes on data retention for these ferroelectrics remains underreported and greater insight is needed to improve device reliability. Here, a comprehensive set of electrodes are evaluated with emphasis on the core ferroelectric memory reliability metrics of endurance, retention, and imprint. Metal‐ferroelectric‐metal capacitors comprised of a Hf_0.5Zr_0.5O₂ layer deposited between different combinations of nitride (TiN, TiAlN, and NbN), pure metal (W), and oxide (MoO₂, RuO₂, and IrO₂) top and bottom electrodes are fabricated for the investigation. From the electrical, physical, and structural analysis, the low reactivity of the electrode with the ferroelectric is found to be key for improved reliability of the ferroelectric capacitor. This understanding of interface properties provides necessary insight for the broad implementation of Hf‐Zr‐based ferroelectrics in memory technology and, overall, boosts the development of next‐generation memories.

The discovery of ferroelectric properties in the doped HfO₂ and mixed Hf_1−xZr_xO₂ systems made precise phase determination very important. However, due to the similarities of the diffraction peaks between the tetragonal and the orthorhombic phases, the discrimination of these two critical phases by x-ray diffraction remains challenging. This work introduces Raman spectroscopy as a structural characterization method to unambiguously identify phases by comparing experimental data with density functional simulation results for the mixed hafnia–zirconia system in the complete composition range. Raman modes for the non-polar monoclinic and tetragonal phases are presented in comparison to those of the polar orthorhombic phase. Changes in phonon mode frequencies in the hafnia–zirconia system with Hf/Zr composition are related to the appearance of ferroelectric properties.

Hafnia-zirconia (HfO₂-ZrO₂) solid solution thin films have emerged as viable candidates for electronic applications due to their compatibility with Si technology and demonstrated ferroelectricity at the nanoscale. The oxygen source in atomic layer deposition (ALD) plays a crucial role in determining the impurity concentration and phase composition of HfO₂-ZrO₂ within metal-ferroelectric-metal devices, notably at the Hf_0.5Zr_0.5O₂ /TiN interface. The interface characteristics of HZO/TiN are fabricated via sequential no-atmosphere processing (SNAP) with either H₂O or O₂-plasma to study the influence of oxygen source on buried interfaces. Time-of-flight secondary ion mass spectrometry reveals that HZO films grown via O₂-plasma promote the development of an interfacial TiO_x layer at the bottom HZO/TiN interface. The presence of the TiO_x layer leads to the development of 111-fiber texture in HZO as confirmed by two-dimensional X-ray diffraction (2D-XRD). Structural and chemical differences between HZO films grown via H₂O or O₂-plasma were found to strongly affect electrical characteristics such as permittivity, leakage current density, endurance, and switching kinetics. While HZO films grown via H₂O yielded a higher remanent polarization value of 25 μC/cm², HZO films grown via O₂-plasma exhibited a comparable P_r of 21 μC/cm² polarization and enhanced field cycling endurance limit by almost 2 orders of magnitude. Our study illustrates how oxygen sources (O₂-plasma or H₂O) in ALD can be a viable way to engineer the interface and properties in HZO thin films.

Si doped HfO2 based ferroelectric capacitors integrated into Back-End-Of-Line (BEOL) 130 nm CMOS technology were investigated in regard to critical reliability parameters for their implementation in non-volatile one-transistor one-capacitor ferroelectric random-access memory applications. The assessed reliability parameters are electric field, capacitor area, and temperature and are evaluated on single and parallel structured capacitors to understand their respective impact on wake-up, fatigue, imprint, and retention.

Inducing and detecting the polar orthorhombic phase are crucial for the establishment of ferroelectricity in HfO₂‐ and ZrO₂‐based thin films. Unfortunately, commonly used structural characterization techniques such as grazing incidence angle X‐ray diffraction (GIXRD) only partially allow an accurate detection of this crystalline phase, whose characteristic pattern almost coincides with the one of the tetragonal phase. As a consequence, phase determination is commonly based on peak deconvolution tracing the position of the main peak at 2θvalues of around 30°, which can be assigned both to the t(101) and the o(111) plane directions and additionally be influenced by mechanical stress in the layers. Alternatively, epitaxial layers are required to differentiate the phase. Herein, using an integrated experimental–computational approach, it is shown how Raman spectroscopy can distinguish between the monoclinic, the tetragonal, and the orthorhombic phase of ZrO₂. The Raman spectra calculated from first principles match the experimentally measured data and thus enable an unambiguous phase assignment. Therefore, Raman spectroscopy proves to be a powerful technique for discerning the three main crystalline phases in these materials. As demonstrated by the good agreement between structural and electrical data, it can therefore be used to predict ferroelectricity in the addressed layers.

Although 10 years have passed since the initial report of ferroelectricity in hafnia (HfO2), researchers are still intensely fascinated by this material system and the promise it holds for future applications. A wide variety of deposition methods have been deployed to create ferroelectric HfO2 thin films such as atomic layer deposition, chemical solution deposition, and physical vapor deposition methods such as sputtering and pulsed laser deposition. Process and design parameters such as deposition temperature, precursor choice, target source, vacuum level, reactive gases, substrate strain, and many others are often integral in stabilizing the polar orthorhombic phase and ferroelectricity. We examine processing parameters across four main different deposition methods and their effect on film microstructure, phase evolution, defect concentration, and resultant electrical properties. The goal of this review is to integrate the process knowledge collected over the past 10 years in the field of ferroelectric HfO2 into a single comprehensive guide for the design of future HfO2-based ferroelectric materials and devices.

A novel system-on-a-chip compatible one-transistor one-capacitor ferroelectric random-access memory array (1T1C FeRAM) based on ferroelectric Hf0.5Zr0.5O2 with a capacitor under bitline (CUB) structure was experimentally demonstrated. The CUB structure facilitates the application of post-metallization annealing on metal/ferroelectric/metal capacitors above 500∘C because they are fabricated before the back-end-of-line process. A large remanent polarization of 2Pr > 40 μC/cm2, projected endurance > 1011 cycles, and ten years of data retention at 85∘C were obtained at 500∘C, after metallization using a single large capacitor. Furthermore, a large memory window of the 64 kbit 1T1C FeRAM array with 500∘C post-metallization was comprehensively demonstrated without degradation of the underlying CMOS logic transistors. The operation voltage and speed dependence were extensively investigated using a dedicated sense amplifier for the 1T1C FeRAM. Furthermore, the perfect bit functionality at an operation voltage of 2.5 V and a read/write speed < 10 ns were obtained. Therefore, superior properties of CUB-structured 1T1C FeRAM can be achieved by flexible process engineering of crystallization annealing for metal/ferroelectric/metal fabrication.

The unique nonlinear dielectric properties of antiferroelectric (AFE) oxides are promising for advancements in solid state supercapacitor, actuator, and memory technologies. AFE behavior in high‐k ZrO₂ is of particular technological interest, but the origin of antiferroelectricity in ZrO₂ remains questionable. The theory of reversible electric field‐induced phase transitions between the nonpolar P4₂/nmc tetragonal phase and the polar Pca2₁ orthorhombic phase is experimentally tested with local structural and electromechanical characterization of AFE ZrO₂ thin films. Piezoresponse force microscopy identifies signature evidence of a field‐induced phase transition. A significant size effect in AFE ZrO₂ is experimentally observed as film thickness is scaled down from 14.7 to 4.3 nm. The size effect is explained by modifications to the phase transition energy barrier heights ranging from 0.6 to 7.6 meV f.u⁻¹ depending on crystallite size and in‐plane compressive strain with decreasing ZrO₂ film thickness. Using the size effect, it is possible to double the energy storage density in ZrO₂ from 20 J cm⁻³ to greater than 40 J cm⁻³, thus highlighting a feasible route for superior performance in AFE fluorite supercapacitors.

Ferroelectricity in fluorite-structured ferroelectrics such as HfO₂ and ZrO₂ has been attracting increasing interest since its first publication in 2011. Fluorite-structured ferroelectrics are considered to be promising for semiconductor devices because of their compatibility with the complementary metal–oxide–semiconductor technology and scalability for highly dense information storage. The research on fluorite-structured ferroelectrics during the first decade of their conceptualization has been mainly focused on elucidating the origin of their ferroelectricity and improving the performance of electronic devices based on such ferroelectrics. Furthermore, as is known, to achieve optimal performance, the emerging biomimicking electronic devices as well as conventional semiconductor devices based on the classical von Neumann architecture require high operating speed, sufficient reliability, and multilevel data storage. Nanoscale electronic devices with fluorite-structured ferroelectrics serve as candidates for these device systems and, thus, have been intensively studied primarily because in ferroelectric materials the switching speed, reliability, and multilevel polarizability are known to be strongly correlated with the domains and domain dynamics. Although there have been important theoretical and experimental studies related to domains and domain dynamics in fluorite-structured ferroelectrics, they are yet to be comprehensively reviewed. Therefore, to provide a strong foundation for research in this field, herein, domains, domain dynamics, and emerging applications, particularly in neuromorphic computing, of fluorite-structured ferroelectrics are comprehensively reviewed based on the existing literature.

A review on ferroelectric phase formation and reliability in HfO₂-based thin films and semiconductor devices.