Hafnium Zirconium Oxide Research Articles

High Memory Window, Dual‐Gate Amorphous InGaZnO Thin‐Film Transistor with Ferroelectric Gate Insulator

Ferroelectric (FE) hafnium zirconium oxide (HZO) thin‐film transistors (TFTs) are of increasing interest for next‐generation memory and computing applications. However, these devices face challenges in achieving a substantial memory window (MW). This report presents amorphous InGaZnO (a‐IGZO) ferroelectric–dielectric (FD), dual‐gate thin‐film transistors (DG‐TFTs) with FE‐HZO as a bottom gate insulator (GI) and SiO2 as a top GI. The ferroelectricity in HZO is confirmed through the grazing incidence X‐ray diffraction (GI‐XRD), capacitance, and polarization measurements. The FD‐DG TFT can increase the MW by tuning the threshold voltage (VTH) due to electrostatic coupling between the top gate (TG) and bottom gate (BG). The increase of MW at the TG driving is related to the coupling factor which is equal to the ratio of the equivalent capacitance of top to bottom gated transistors. During the bottom sweep, the FD‐DG‐TFT demonstrates an anticlockwise hysteresis with a MW of 4.98 V, a high ION/IOFF ratio of ≈106, and a steep subthreshold swing (SS) of 90 mV dec−1. On the contrary, MW of 12 V and SS of 140 mV dec−1 are observed for the top sweep operation. A thinner ferroelectric GI at BG TFT induces sufficient electrostatic coupling to cause a large VTH shift at TG driving, resulting in a boosted MW.

Stabilization of Tetragonal Phase in Hafnium Zirconium Oxide by Cation Doping for High-K Dielectric Insulators.

As electronic circuit integration intensifies, there is a rising demand for dielectric insulators that provide both superior insulation and high dielectric constants. This study focuses on developing high-k dielectric insulators by controlling the phase of the Hf0.5Zr0.5O2 (HZO) film with additional doping, utilizing yttrium (Y), tantalum (Ta), gallium (Ga), silicon (Si), and aluminum (Al) as dopants. Doping changes the ratio of tetragonal to monoclinic phases in doped HZO films, and Y-doped HZO (Y:HZO) films specifically exhibit a high tetragonal phase ratio and a dielectric constant of 40.9, indicating superior insulating properties compared to undoped HZO films. To clarify the fundamental mechanism driving the enhancement in dielectric properties, we have carried out various analyses combined with density functional theory (DFT) calculations. Through the optimization of the post-deposition annealing (PDA) process and the heterojunction structure with Al2O3, an Al2O3/Y:HZO heterojunction with a high dielectric constant and even lower leakage current compared to a single layer was developed. The thin-film transistor (TFT) with the Au/Ti/amorphous InGaZnO4 (a-IGZO)/Al2O3/YHZO/TiN heterojunction structure exhibits low subthreshold swing (SS) values within a narrow gate-source voltage (Vsg) range. This study advances knowledge on how the controlled-phase doped HZO films affect the dielectric constant and leakage current and will contribute to semiconductor technology advancements by overcoming the limitations of conventional high-k dielectric insulators.

Unveiling pyroelectricity in ferroelectric planar capacitors with area-selective wet etched hafnium zirconium oxide: from ab initio and multiphysics simulations to experiments

Abstract In this work, a systematic approach aimed at investigating and validating a novel way of realizing pyroelectric harvesting is presented. Generating a direct-current (dc) signal through a temperature gradient within a less than 7 nm-thick ferroelectric zirconium-doped hafnium oxide (HZO) nano-film, embedded in planar interdigitated capacitors on high-resistivity silicon, is a new, simple, effective, and reproducible solution. Temperature-related structural modifications in HZO are first simulated using advanced ab initio calculations. Then, rigorous multiphysics simulations of the final devices provide insight into the expected performance of the pyroelectric harvester, as a function of temperature, contact area, and crystal orientation, showing a maximum open-circuit voltage of up to 900 mV. The fabrication of the harvesters involves the area-selective wet etching of the HZO layer to retain it exclusively in between the fingers of each capacitor. This choice maximizes the pyroelectric effect (which strongly depends on the area) and represents a new paradigm in the development of HZO-based electronics, which are conventionally built on ferroelectric continuous films. Experimental validation at both low frequencies and microwaves confirms the pyroelectric effect, exhibiting a significant increase in the output current for higher temperature gradients, and a generated dc voltage of several hundred millivolts.

Sub-Boltzmann Switching, Hysteresis-Free Charge Modulated Negative Differential Resistance FinFET.

The ever-increasing power consumption in integrated circuits has raised concerns about the relentless doubling of transistor density in chips and cost drop per combinational/sequential circuits. To address the physical limit of thermionic emission carrier transport (i.e., subthreshold swing >60 mV/decade at 300 K), alternative charge-transport mechanisms or the implementation of functional substances have been attempted but without appreciable success. One such choice is to take advantage of negative differential resistance with the activation of localized electrons or migration of atom and oxygen vacancies to extend the capabilities of Si-transistors. However, inconsistency in current during forward/reverse bias sweep is confronted as a notable weak point. This work proposes an eye-catching solution to modulating potential distribution between a resistance switching layer and a transistor by employing charge trapping within a hafnium zirconium oxide layer. This approach introduces features advancing the potential of "More Moore" technologies.

First demonstration of 2T0C-FeDRAM: a-ITZO FET and double gate a-ITZO/a-IGZO FeFET with a record-long multibit retention time of >4-bit and >2000 s.

Conventional DRAM, consisting of one transistor and one capacitor (1T1C), requires periodic data refresh processes due to its limited retention time and data-destructive read operation. Here, we propose and demonstrate a novel 3D-DRAM memory scheme available with a single transistor and a single ferroelectric field-effect transistor (FeFET) DRAM (2T0C-FeDRAM), which offers extended retention time and non-destructive read operation. This architecture uses a back-end-of-line (BEOL)-compatible amorphous oxide semiconductor (AOS) that is suitable for increasing DRAM cell density. Notably, the device structures of a double gate a-ITZO/a-IGZO FeFET, used for data storage and reading, are engineered to achieve an enlarged memory window (MW) of 1.5 V and a prolonged retention time of 104 s. This is accomplished by a double gate and an a-ITZO/a-IGZO heterostructure channel to enable efficient polarization control in hafnium-zirconium oxide (HZO) layers. We present successful program/erase operations of the double gate a-ITZO/a-IGZO FeFET through incremental step pulse programming (ISPP), demonstrating multi-level states with remarkable retention characteristics. Most importantly, we perform 2T0C-FeDRAM operations by electrically connecting the double gate a-ITZO/a-IGZO FeFET and the a-ITZO FET. Leveraging the impressive performance of the double gate a-ITZO/a-IGZO FeFET technology, we have effectively showcased an exceptionally record-long retention time exceeding 2000 s and 4-bit multi-level states, positioning it as a robust contender among emerging memory solutions in the era of artificial intelligence.

Novel Two-Terminal Synapse/Neuron Based on an Antiferroelectric Hafnium Zirconium Oxide Device for Neuromorphic Computing.

Functionally diverse devices with artificial neuron and synapse properties are critical for neuromorphic systems. We present a two-terminal artificial leaky-integrate-fire (LIF) neuron based on 6 nm Hf0.1Zr0.9O2 (HZO) antiferroelectric (AFE) thin films and develop a synaptic device through work function (WF) engineering. LIF neuron characteristics, including integration, firing, and leakage, are achieved in W/HZO/W devices due to the accumulated polarization and spontaneous depolarization of AFE HZO films. By engineering the top electrode with asymmetric WFs, we found that Au/Ti/HZO/W devices exhibit synaptic weight plasticity, such as paired-pulse facilitation and long-term potentiation/depression, achieving >90% accuracy in digit recognition within constructed artificial neural network systems. These findings suggest that AFE HZO capacitor-based neurons and WF-engineered artificial synapses hold promise for constructing efficient spiking neuron networks and artificial neural networks, thereby advancing neuromorphic computing applications based on emerging AFE HZO devices.

Ferroelectric Al0.85Sc0.15N and Hf0.5Zr0.5O2 Domain Switching Dynamics.

The capability to reliably program partial polarization states with nanosecond programming speed and femtojoule energies per bit in ferroelectrics makes them an ideal candidate to realize multibit memory elements for high-density crossbar arrays, which could enable neural network models with a large number of parameters at the edge. However, a thorough understanding of the domain switching dynamics involved in the polarization reversal is required to achieve full control of the multibit capability. Transient current integration measurements are adopted to investigate the domain dynamics in aluminum scandium nitride (Al0.85Sc0.15N) and hafnium zirconium oxide (Hf0.5Zr0.5O2). The switching dynamics are correlated to the crystal structure of the films. The contributions of domain nucleation and domain wall motion are decoupled by analyzing the rate of change of the time-dependent normalized switched polarization. Thermally activated creep domain wall motion characterizes the Al0.85Sc0.15N switching dynamics. The statistics of independently nucleating domains and the domain wall creep motion in Hf0.5Zr0.5O2 are associated with the spatially inhomogeneous distribution of local switching field due to polymorphism, absence of preferential crystallite orientation, as well as defects and charges that can be located at the grain boundaries. The c-axis texture, single-phase nature, and strong likelihood of less fabrication process-induced defects contribute to the homogeneity of the local switching field in Al0.85Sc0.15N. Nonetheless, defects generated and redistributed upon bipolar electric field switching cycling result in Al0.85Sc0.15N domain wall pinning. The wake-up effect in Hf0.5Zr0.5O2 is explained thorough the continuous addition of switchable regions associated with two independent distributions of characteristic switching times.

Improvement of the Ferroelectric Response of La-Doped Hafnium Zirconium Oxide Employing Tungsten Oxide Interfacial Layer with Back-End-of-Line Compatibility.

In this work, the impact of a tungsten oxide (WO3) seed and capping layer for ferroelectric La-doped (Hf, Zr)O2 (La:HZO) based capacitors, designed with back-end-of-line (BEOL) compatibility, is systematically investigated. The WO3 capping layer supplies oxygen to the La:HZO layer throughout the fabrication process and during device cycling. This facilitates the annihilation of oxygen vacancies (Vo) within the La:HZO layer, thereby stabilizing its ferroelectric orthorhombic phase and resulting in an increase of the remanent polarization (Pr) value in the capacitor. Moreover, the effectiveness of the WO3 capping layer depends on the seed layer of the HZO film, suggesting that proper combination of the seed and capping layers should be employed to maximize the ferroelectric response. Finally, a TiN/TiO2 seed layer/La:HZO/WO3 capping layer/TiN capacitor is successfully fabricated and optimized by a complete set of atomic layer deposition (ALD) processes, achieving a superior 2Pr value and endurance value of more than 109 cycles at an electric field of 2.5 MV/cm. The WO3 capping layer is anticipated to offer a viable solution for doped HZO capacitors with reduced thickness, addressing the challenge of elevated Vo levels that favor the tetragonal phase and result in low 2Pr values.

Mapping Ferroelectric Fields Reveals the Origins of the Coercivity Distribution.

Better techniques for imaging ferroelectric polarization would aid the development of new ferroelectrics and the refinement of old ones. Here we show how scanning transmission electron microscope (STEM) electron beam-induced current (EBIC) imaging reveals ferroelectric polarization with obvious, simply interpretable contrast. Planar imaging of an entire ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2, HZO) capacitor shows an EBIC response that is linearly related to the polarization determined in situ with the positive-up, negative-down (PUND) method. The contrast is easily calibrated in MV/cm. The underlying mechanism is magnification-independent, operating equally well on micrometer-sized devices and individual nanoscale domains. Coercive-field mapping reveals that individual domains are biased "positive" and "negative", as opposed to being "easy" and "hard" to switch. The remanent background E-fields generating this bias can be isolated and mapped. Coupled with STEM's native capabilities for structural identification, STEM EBIC imaging provides a revolutionary tool for characterizing ferroelectric materials and devices.

Revealing the controlling mechanisms of atomic layer etching for high-k dielectrics in conventional inductively coupled plasma etching tool

The possibilities of optimization of the two-step atomic layer etching process for HfO2 in conventional plasma etching tools were studied. The surface modification step was realized in Ar/CF4/H2 plasma, and the reaction between the modified layer and the surface was activated by Ar ion bombardment from the plasma in the second step. Investigation of the effects of activation step duration, DC bias during activation, and Ar plasma density was carried out. The mechanism of the etching process has been shown to involve fluorination of oxide during the modification step and subsequent removal of fluorine-containing particles at the activation step. An increase in parasitic sputtering rate and lower process saturation with the growth of DC bias during activation was demonstrated. The advantage of the ALE process in lower surface roughness over the conventional etching process was shown. Similar etching characteristics of HfO2 and ZrO2 suggest a similarity in the etching process for the mixed hafnium-zirconium oxide material.

Volatile memory characteristics of CMOS-compatible HZO ferroelectric layer for reservoir computing

Recently, ferroelectric memory utilizing hafnium oxide has emerged as an attractive option compared to existing memory technologies, primarily due to its scalability and energy-efficient advantages. Among them, hafnium zirconium oxide (HZO) is examined for its short-term memory characteristics to achieve a reservoir computing system known to exhibit remarkable polarization properties, being able to switch between distinct polarization states under the influence of an electric field. These unique properties are of utmost importance in ferroelectric memory applications, where they play a pivotal role in the storage and retrieval of binary data. In this study, we identify and experiment with the electrical characteristics of a ferroelectric tunnel junction (FTJ) device with a metal-ferroelectric-semiconductor (MFS) structure using TiN as the top electrode and HZO as the ferroelectric layer. Moreover, we assess the performance of the device by evaluating its maximum 2Pr (remnant polarization) and tunneling electro resistance (TER) values in different conditions of cell area. Furthermore, we analyze and show short-term memory (STM) characteristics and synaptic properties with 5 cycles of potentiation and depression under conditions of stable dynamic range by coordinating identical and incremental pulses. In the case of incremental pulses (>95 %), the MNIST pattern recognition accuracy is higher than in the case of identical pulses (>94 %). Through a sequence of procedures, the synaptic characteristics of FTJs are confirmed to assess their suitability for use as an artificial synaptic device.

Optimization of ferroelectricity and endurance of hafnium zirconium oxide thin films by controlling element inhomogeneity

Optimization of ferroelectricity and endurance of hafnium zirconium oxide thin films by controlling element inhomogeneity

Phase Transformations Driving Biaxial Stress Reduction During Wake‐Up of Ferroelectric Hafnium Zirconium Oxide Thin Films

AbstractBiaxial stress is identified to play an important role in the polar orthorhombic phase stability in hafnium oxide‐based ferroelectric thin films. However, the stress state during various stages of wake‐up has not yet been quantified. In this work, the stress evolution with field cycling in hafnium zirconium oxide capacitors is evaluated. The remanent polarization of a 20 nm thick hafnium zirconium oxide thin film increases from 9.80 to 15.0 µC cm−2 following 106 field cycles. This increase in remanent polarization is accompanied by a decrease in relative permittivity that indicates that a phase transformation has occurred. The presence of a phase transformation is supported by nano‐Fourier transform infrared spectroscopy measurements and scanning transmission electron microscopy that show an increase in ferroelectric phase content following wake‐up. The stress of individual devices field cycled between pristine and 106 cycles is quantified using the sin2(ψ) technique, and the biaxial stress is observed to decrease from 4.3 ± 0.2 to 3.2 ± 0.3 GPa. The decrease in stress is attributed, in part, to a phase transformation from the antipolar Pbca phase to the ferroelectric Pca21 phase. This work provides new insight into the mechanisms controlling and/or accompanying polarization wake‐up in hafnium oxide‐based ferroelectrics.

Oxygen diffusion coefficients in ferroelectric hafnium zirconium oxide thin films

Oxygen diffusion coefficients in the metastable ferroelectric phase of polycrystalline hafnium zirconium oxide (HZO) thin films have been quantified using 18O tracers and time-of-flight secondary ion mass spectrometry. 11.5 nm thick HZO films containing 16O were deposited by plasma-enhanced atomic layer deposition followed by post-metallization annealing to crystallize into the ferroelectric phase. A 1.2 nm thick HZO layer containing 18O was then deposited using thermal atomic layer deposition with H218O as a reactant. Thermal anneals were conducted at 300, 350, and 400 °C and the ferroelectric phase confirmed after the anneals by x-ray diffraction, infrared spectroscopy, and electrical property measurements. 18O depth profiles were measured and fit with a thin film diffusion equation to determine the oxygen diffusion coefficients. Oxygen diffusion coefficients ranged from approximately 2 × 10−18 cm2/s at 300 °C to 5 × 10−17 cm2/s at 400 °C with an activation energy of 1.02 ± 0.24 eV.

Nanoscale Phase and Orientation Mapping in Multiphase Polycrystalline Hafnium Zirconium Oxide Thin Films Using 4D-STEM and Automated Diffraction Indexing.

Ferroelectric hafnium zirconium oxide (HZO) holds promise for nextgeneration memory and transistors due to its superior scalability and seamless integration with complementary metal-oxide-semiconductor processing. A major challenge in developing this emerging ferroelectric material is the metastable nature of the non-centrosymmetric polar phase responsible for ferroelectricity, resulting in a coexistence of both polar and non-polar phases with uneven grain sizes and random orientations. Due to the structural similarity between the multiple phases and the nanoscale dimensions of the thin film devices, accurate measurement of phase-specific information remains challenging. Here, the application of 4D scanning transmission electron microscopy is demonstrated with automated electron diffraction pattern indexing to analyze multiphase polycrystalline HZO thin films, enabling the characterization of crystallographic phase and orientation across large working areas on the order of hundreds of nanometers. This approach offers a powerful characterization framework to produce a quantitative and statistically robust analysis of the intricate structure of HZO films by uncovering phase composition, polarization axis alignment, and unique phase distribution within the HZO film. This study introduces a novel approach for analyzing ferroelectric HZO, facilitating reliable characterization of process-structure-property relationships imperative to accelerating the growth optimization, performance, and successful implementation of ferroelectric HZO in devices.

Towards wake-up free ferroelectrics and scaling: Al-doped HZO and its crystallographic texture

Ferroelectric (FE) hafnium zirconium oxide (HZO) is an excellent candidate for data storage applications. However, it has some reliability limitations such as imprint and retention. Herein, we explore Al doping of HZO to overcome these limitations. FE behavior is tuned by the aluminum (Al) concentrations in the films and by annealing temperature. A correlation is done between electrical behavior, crystallographic texture, and FE phases determined by grazing-incidence X-ray diffraction (GIXRD) measurements. Reduced coercive field (2Ec) values and wake-up free HZO-based ferroelectrics are explored. We show the tunability of remanent polarization (2Pr) and 2Ec with respect to Al-doping concentration and anneal temperature, hence crystallographic texture.

Non-centrosymmetric crystallization in ferroelectric hafnium zirconium oxide via photon-assisted defect modulation

Ferroelectricity in Hf1-xZrxO2 (HZO) thin films has garnered significant attention for advanced memory devices. However, the challenge in understanding nanoscale polymorphism and the absence of non-centrosymmetric crystallization techniques compatible with back-end-of-line processes have restricted its broader application to various types of information storage systems. In this study, we report a novel method to generate the ferroelectric orthorhombic phase (o-phase) in HZO films via photon-assisted non-centrosymmetric crystallization. As-prepared HZO films (8 nm) prepare by atomic layer deposition underwent thermal annealing and subsequent deep ultraviolet (DUV) irradiation. The DUV treatment successfully triggered ferroelectricity in HZO films annealed at 300 °C. Moreover, the same post-treatment applied to HZO films annealed at 400 °C led to a further enhanced polarization up to 29.2 μC cm−2 under high bipolar triangular pulses and outstanding reliability for up to 106 bias stress cycles. Finally, based on in-depth microscopic and structural analyses, we proposed the mechanism on the symmetry-breaking phase transformation to the o-phase HZO with advanced ferroelectricity via oxygen vacancy-driven lattice rearrangement.

Formation and energetics of head-to-head and tail-to-tail domain walls in hafnium zirconium oxide

180∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document} domains walls (DWs) of head-to-head/tail-to-tail (H–H/T–T) type in ferroelectric (FE) materials are of immense interest for a comprehensive understanding of the FE attributes as well as harnessing them for new applications. Our first principles calculation suggests that such DW formation in hafnium zirconium oxide (HZO) based FEs depends on the unique attributes of the HZO unit cell, such as polar-spacer segmentation. Cross pattern of the polar and spacer segments in two neighboring domains along the polarization direction (where polar segment of one domain aligns with the spacer segment of another) boosts the stability of such DWs. We further show that low density of oxygen vacancies at the metal-HZO interface and high work function of metal electrodes are conducive for T–T DW formation. On the other hand, high density of oxygen vacancy and low work function of metal electrode favor H–H DW formation. Polarization bound charges at the DW get screened when band bending from depolarization field accumulates holes (electrons) in T–T (H–H) DW. For a comprehensive understanding, we also investigate their FE nature and domain growth mechanism. Our analysis suggests that a minimum thickness criterion of domains has to be satisfied for the stability of H–H/T–T DW and switching of the domains through such DW formation.

Performance improvements in complementary metal oxide semiconductor devices and circuits based on fin field-effect transistors using 3-nm ferroelectric Hf0.5Zr0.5O2

In this work, a conventional HfO2 gate dielectric layer is replaced with a 3-nm ferroelectric (Fe) HZO layer in the gate stacks of advanced fin field-effect transistors (FinFETs). Fe-induced characteristics, e.g., negative drain induced barrier lowering (N-DIBL) and negative differential resistance (NDR), are clearly observed for both p- and n-type HZO-based FinFETs. These characteristics are attributed to the enhanced ferroelectricity of the 3-nm hafnium zirconium oxide (HZO) film, caused by Al doping from the TiAlC capping layer. This mechanism is verified for capacitors with structures similar to the FinFETs. Owing to the enhanced ferroelectricity and N-DIBL phenomenon, the drain current (IDS) of the HZO-FinFETs is greater than that of HfO2-FinFETs and obtained at a lower operating voltage. Accordingly, circuits based on HZO-FinFET achieve higher performance than those based on HfO2-FinFET at a low voltage drain (VDD), which indicates the application feasibility of the HZO-FinFETs in the ultra-low power integrated circuits.Graphical abstract

Polarization fatigue mechanism of laminated hafnium zirconium oxide ferroelectric thin films

Laminated HfO2-based ferroelectric thin films (FE-HfO2) have offered unexpected opportunities to implement the high-density ferroelectric memory and on chip piezoelectric or pyroelectric devices. But the polarization fatigue performance is poor and the underlying mechanism is unknown. In this work, we comprehensively investigate the polarization fatigue behaviors of Hf0.5Zr0.5O2(HZO)/Al2O3/HZO laminated thin films. Several possible mechanisms for the fatigue performances are discussed in detail. On basis of the analysis of micro-structures and domain switching dynamics before and after fatigue, we confirm that domain wall pinning phenomenon exists in the fatigued laminated HZO, without field-induced phase transition as well as thickness and chemical properties changes of interfacial layers. Moreover, we discover by a pulse measurement method that domain wall pinning induced by the increase of Al2O3/HZO interface trap charge is responsible for the fatigue performances of the laminated films. This work will provide useful guidance for designing high-reliability laminated FE-HfO2, and contribute to understand the physical mechanisms of FE-HfO2.