Ferroelectric Orthorhombic Phase Research Articles

Comprehensive Study on the Kinetic Formation of the Orthorhombic Ferroelectric Phase in Epitaxial Y-Doped Ferroelectric HfO2 Thin Films

Comprehensive Study on the Kinetic Formation of the Orthorhombic Ferroelectric Phase in Epitaxial Y-Doped Ferroelectric HfO₂ Thin Films

Impact of the channel length on molybdenum disulfide field effect transistors with hafnia-based high-k dielectric gate

Field effect transistors (FETs) using two-dimensional molybdenum disulfide (MoS2) as the channel material has been considered one of the most potential candidates for future complementary metal-oxide-semiconductor technology with low power consumption. However, the understanding of the correlation between the device performance and material properties, particularly for devices with scaling-down channel lengths, is still insufficient. We report in this paper back-gate FETs with chemical-vapor-deposition grown and transferred MoS2 and Zr doped HfO2 ((Hf,Zr)O2, HZO) high-k dielectric gates with channel lengths ranging from 10 to 30 µm with a step of 5 µm. It has been demonstrated that channels with the length to width ratio of 0.2 lead to the most superior performance of the FETs. The MoS2/HZO hybrid FETs show a stable threshold voltage of ∼1.5 V, current on/off ratio of >104, and field effect mobility in excess of 0.38 cm2 V−1 s−1. The impact of the channel lengths on FET performance is analyzed and discussed in depth. A hysteresis loop has been observed in the Ids − Vgs characteristics of the hybrid FETs, which has been further studied and attributed to the charge effect at the interfaces. The HZO films show a relatively weak ferroelectric orthorhombic phase and thus serve mainly as the high-k dielectric gate. Charge trapping in the HZO layer that might induce hysteresis has been discussed. Our results show that MoS2/HZO hybrid FETs possess great potential in future low power and high-speed integrated circuits, and future work will focus on further improvement of the transistor performances using ferroelectric HZO films and the study of devices with even shorter MoS2 channels.

Enhancement of ferroelectricity and homogeneity of orthorhombic phase in Hf0.5Zr0.5O2 thin films

By adoption of a high permittivity ZrO2 capping layer (ZOCL), enhanced ferroelectric properties were achieved in the Hf0.5Zr0.5O2 (HZO) thin films. For HZO thin film with 10 Å ZOCL, the 2P r value can reach as high as ∼43.1 μC cm−2 under a sweep electric field of 3 MV cm−1. In addition, a reduced coercive field of 1.5 MV cm−1 was observed, which is comparable to that of HZO with metallic CL. Furthermore, the homogeneity of ferroelectric orthorhombic phase in HZO films was observed to be clearly increased, as evidenced by nanoscale piezoelectric force microscopy measurements. These results demonstrate that ZOCL is very favorable for high performance ferroelectric HZO films and their future device applications.

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O2 to AlN and Related Materials

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O₂ to AlN and Related Materials

Electric‐Field‐Induced Ferroelectricity in 5%Y‐doped Hf0.5Zr0.5O2: Transformation from the Paraelectric Tetragonal Phase to the Ferroelectric Orthorhombic Phase

Ferroelectricity facilitated by field-induced phase transition is demonstrated by Takao Shimizu, Hiroshi Funakubo, and co-workers in article number 2000589. Their X-ray diffraction and scanning transparent electron microscopy studies show the as-deposited paraelectric tetragonal phase transforms to the ferroelectric orthorhombic phase in the Y-doped Hf0.5Zr0.5O2 film by applying a strong electric field.

Impact of Wet Annealing on Ferroelectric Phase Formation and Phase Transition of HfO2–ZrO2 System

Wet annealing of the ZrO2 thin film can promote martensitic phase transition, leading to the formation of the ferroelectric orthorhombic (o-) phase. However, the present remanent polarization does ...

Ferroelectric Phase Content in 7 nm Hf(1−x)ZrxO2 Thin Films Determined by X‐Ray‐Based Methods

The recent discovery of ferroelectric behavior in hafnia‐based dielectrics attributed to the formation of noncentrosymmetric orthorhombic phase has opened up potential uses in numerous applications ranging from ferroelectric field effect transistors to pyroelectric energy harvesters. Herein, the relative amounts of ferroelectric orthorhombic phase (space group Pca21) in 7 nm‐thick hafnia–zirconia (HZO) films of three compositions (Hf0.2Zr0.8O2, Hf0.5Zr0.5O2, and Hf0.8Zr0.2O2) are reported. The ferroelectric behavior in these ultrathin HZO films prepared by atomic layer deposition (ALD) in metal–insulator–metal (MIM) stacks is verified by polarization measurements. Using grazing incidence X‐ray diffraction (GIXRD) and grazing incidence‐extended X‐ray absorption fine structure spectroscopy (GI‐EXAFS), the relative phase percentages of three key phases (monoclinic—P21/c, orthorhombic—Pca21, and tetragonal—P42/nmc) are assessed. Among these compositions, Hf0.5Zr0.5O2 is determined to have the highest amount of the ferroelectric phase. The monoclinic phase is found to be dominant for Hf0.8Zr0.2O2, whereas the tetragonal phase is found to be dominant for Hf0.2Zr0.8O2. Independent GI‐EXAFS measurements of Hf and Zr suggest that there is no significant segregation of either oxide into a particular crystal phase. Strong agreement between the measurement methods reveals the structure–property function correlation in these ultrathin hafnia–zirconia films with greater certainty than previously achieved.

Ferroelectric Hf0.5Zr0.5O2 Thin Films Crystallized by Pulsed Laser Annealing

The crystallization of as‐grown amorphous Hf0.5Zr0.5O2 (HZO) thin films to the metastable ferroelectric phase by pulsed laser annealing (PLA) is investigated. PLA experiments are conducted using a Nd:YAG laser operating in two regimes: Q‐switched mode with a pulse duration of τ ≈ 16 ns and free‐running mode (τ ≈ 1 ms). The crystallization of a ferroelectric orthorhombic phase in the annealed films is confirmed by X‐ray diffraction, polarization versus electric field (P–E) measurements, and piezoresponse force microscopy (PFM) analyses. Remnant polarization up to 2P r ≈ 50 μC cm−2 is achieved in the TiN/HZO/W capacitor structures grown on the Si substrate and subjected to millisecond PLA. In contrast, the use of laser annealing in a 10 ns pulse duration range is found ineffective for the crystallization of any HZO phase in capacitor structures. Detailed PFM analysis across a capacitor device area reveals the effect of the local temperature on the sample surface during PLA on the resulting ferroelectric domain structure. The lower thermal impact on the substrate during PLA opens the possibility of creating local areas of the ferroelectric phase in HZO films using reflecting copper masks.

Metal Nitride Electrode Stress and Chemistry Effects on Phase and Polarization Response in Ferroelectric Hf0.5Zr0.5O2 Thin Films

AbstractFerroelectric phase stability in hafnium oxide is reported to be influenced by factors that include composition, biaxial stress, crystallite size, and oxygen vacancies. In the present work, the ferroelectric performance of atomic layer deposited Hf0.5Zr0.5O2 (HZO) prepared between TaN electrodes that are processed under conditions to induce variable biaxial stresses is evaluated. The post‐processing stress states of the HZO films reveal no dependence on the as‐deposited stress of the adjacent TaN electrodes. All HZO films maintain tensile biaxial stress following processing, the magnitude of which is not observed to strongly influence the polarization response. Subsequent composition measurements of stress‐varied TaN electrodes reveal changes in stoichiometry related to the different preparation conditions. HZO films in contact with Ta‐rich TaN electrodes exhibit higher remanent polarizations and increased ferroelectric phase fractions compared to those in contact with N‐rich TaN electrodes. HZO films in contact with Ta‐rich TaN electrodes also have higher oxygen vacancy concentrations, indicating that a chemical interaction between the TaN and HZO layers ultimately impacts the ferroelectric orthorhombic phase stability and polarization performance. The results of this work demonstrate a necessity to carefully consider the role of electrode processing and chemistry on performance of ferroelectric hafnia films.

Enhanced ferroelectric switching speed of Si-doped HfO2 thin film tailored by oxygen deficiency

Investigations concerning oxygen deficiency will increase our understanding of those factors that govern the overall material properties. Various studies have examined the relationship between oxygen deficiency and the phase transformation from a nonpolar phase to a polar phase in HfO2 thin films. However, there are few reports on the effects of oxygen deficiencies on the switching dynamics of the ferroelectric phase itself. Herein, we report the oxygen- deficiency induced enhancement of ferroelectric switching properties of Si-doped HfO2 thin films. By controlling the annealing conditions, we controlled the oxygen deficiency concentration in the ferroelectric orthorhombic HfO2 phase. Rapid high-temperature (800 °C) annealing of the HfO2 film accelerated the characteristic switching speed compared to low-temperature (600 °C) annealing. Scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS) revealed that thermal annealing increased oxygen deficiencies, and first-principles calculations demonstrated a reduction of the energy barrier of the polarization flip with increased oxygen deficiency. A Monte Carlo simulation for the variation in the energy barrier of the polarization flipping confirmed the increase of characteristic switching speed.

Large thermal hysteresis of ferroelectric transition in HfO2-based ferroelectric films

The transition between the dielectric tetragonal and ferroelectric orthorhombic phases in 7%Y doped HfO2 and Hf0.5Zr0.5O2 films with various orientations and film thicknesses was investigated by high-temperature x-ray diffraction. All films demonstrate a different phase transition temperature on heating and cooling with thermal hysteresis with a gap of ΔT. This result clearly shows that the phase transition of the ferroelectric HfO2-based film is first order. The ΔT value of 40–210 °C in HfO2-based films is larger than that of other ferroelectric materials but similar to that of martensitic materials with large lattice deformation. This implies that the ferroelectric phase transition of HfO2-based films involves large lattice deformation. Moreover, we show that ΔT is changed by the size and composition effects. Our results are a step toward elucidating the mechanism of phase transition in ferroelectric HfO2-based films.

Impact of Iridium Oxide Electrodes on the Ferroelectric Phase of Thin Hf0.5Zr0.5O2 Films

Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O2, and N2 annealing atmospheres significantly change the ferroelectric performance. The use of oxygen‐deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake‐up effect. It is found that oxygen‐rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.

Influence of High-Pressure Annealing Conditions on Ferroelectric and Interfacial Properties of Zr-Rich HfₓZr₁₋ₓO₂Capacitors

In this study, we have carried out an in-depth analysis on the role of high-pressure annealing (HPA) conditions on ferroelectricity as well as the interfacial property of Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) capacitors in metal-ferroelectric-metal (MFM) structure. Unlike conventional 1:1 HZO films, HZO demonstrates the highest ferroelectricity at 1:3 Hf:Zr ratio in HPA and a significantly higher maximum remanent polarization ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ) of 31 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved as compared to 19 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> obtained in rapid thermal annealing (RTA) (HZO [1:1]). To understand the influence of HPA conditions, HZO [1:3] capacitors are annealed at various temperatures (300 °C, 400 °C, 500 °C, and 600 °C) and pressures (1, 50, and 200 atm). Ferroelectricity in HZO is found to enhance with increasing HPA temperature or pressure due to higher ferroelectric phase formation as revealed by grazing incidence X-ray diffractometer (GIXRD) analysis. Transient pulse switching measurement suggests the reduction in the effective thickness of the interfacial nonferroelectric film at a high HPA temperature or pressure which was also validated by impedance analysis. Further, impedance analysis reveals a reduction in the number of oxygen vacancy defects with increasing annealing temperature or pressure implying a phase transition from the tetragonal phase of nonferroelectric layer to the ferroelectric orthorhombic phase. In addition, excellent endurance property till 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cycles with reduced wake-up effect was observed in HZO [1:3] capacitors with increasing the HPA temperature, while higher annealing pressure is found to increase P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> without affecting its endurance property. The results obtained herein can be of significant scientific importance, especially to achieve enhanced ferroelectric property with reduced wake-up effect in Zr-rich HZO ferroelectrics.

Robust semiconductor-on-ferroelectric structures with hafnia–zirconia–alumina UTBOX stacks compatible with CMOS technology

Silicon wafers with an ultrathin buried high-k oxide were fabricated by the atomic layer deposition of high-k layers on sapphire and silicon substrates with subsequent silicon layer transfer onto their surfaces by bonding and rapid thermal annealing (RTA). An extremely high thermal stability of hafnia orthorhombic Pca21 ferroelectric phase of up to 1100 °C was observed in the silicon-on-ferroelectric structure on the sapphire substrate. Silicon–ferroelectric–silicon structures with hafnia BOX and alumina inclusions also demonstrated increased thermal stability for hafnia or hafnia–zirconia alloys during the RTA treatment up to 900 °C, which makes them fully compatible with current complementary metal oxide semiconductor technology, promising integrated circuits for neuromorphic computation and optoelectronic switching devices.

Compositional and phase dependence of elastic modulus of crystalline and amorphous Hf1-xZrxO2 thin films

The elastic moduli of amorphous and crystalline atomic layer-deposited Hf1-xZrxO2 (HZO, x = 0, 0.31, 0.46, 0.79, 1) films prepared with TaN electrodes on silicon substrates were investigated using picosecond acoustic measurements. The moduli of the amorphous films were observed to increase between 211 ± 6 GPa for pure HfO2 and 302 ± 9 GPa for pure ZrO2. In the crystalline films, it was found that the moduli increased upon increasing the zirconium composition from 248 ± 6 GPa for monoclinic HfO2 to 267 ± 9 GPa for tetragonal ZrO2. Positive deviations from this increase were observed for the Hf0.69Zr0.31O2 and Hf0.54Zr0.46O2 compositions, which were measured to have moduli of 264 ± 8 GPa and 274 ± 8 GPa, respectively. These two compositions contained the largest fractions of the ferroelectric orthorhombic phase, as assessed from polarization and diffraction data. The biaxial stress states of the crystalline films were characterized through sin2(ψ) x-ray diffraction analysis. The in-plane stresses were all found to be tensile and observed to increase with the increasing zirconium composition, between 2.54 ± 0.6 GPa for pure HfO2 and 5.22 ± 0.5 GPa for pure ZrO2. The stresses are consistent with large thermal expansion mismatches between the HZO films and silicon substrates. These results demonstrate a device-scale means to quantify biaxial stress for investigation on its effect on the ferroelectric properties of hafnia-based materials.

Electric‐Field‐Induced Ferroelectricity in 5%Y‐doped Hf0.5Zr0.5O2: Transformation from the Paraelectric Tetragonal Phase to the Ferroelectric Orthorhombic Phase

The ferroelectric phase transformation from the tetragonal phase to the orthorhombic phase, induced by an electric field, is demonstrated in a 5%YO1.5‐doped Hf0.5Zr0.5O2 epitaxial film which is grown on Sn‐doped In2O3‐covered (111) yttria‐stabilized zirconia by the pulsed laser deposition method at room temperature and subsequent heat treatment. Although X‐ray diffraction shows the film to consist of a paraelectric tetragonal phase after the heat treatment, polarization–electric field (P–E) measurements reveal a hysteresis loop attributed to the ferroelectricity. To clarify the discrepancy between the crystal structure and electric characteristics, the crystal structure after electric field loading is determined by scanning transmission electron microscopy and synchrotron X‐ray diffraction measurements. Both structural characterizations clearly reveal that the application of an electric field promotes the phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase. This ferroelectric transition is irreversible, as the ferroelectric phase remains after the removal of the electric field. These results facilitate the elucidation of the mechanism by which ferroelectricity is displayed in HfO2‐based fluorite ferroelectric materials and imply unimportance of the orthorhombic phase for as‐prepared films.

Correlation between ferroelectricity and ferroelectric orthorhombic phase of HfxZr1−xO2 thin films using synchrotron x-ray analysis

The change in the interplanar spacing (d-spacing) including the ferroelectric orthorhombic (O) phase in the low-temperature fabricated HfxZr1−xO2 (HZO) films was studied using synchrotron grazing-incidence wide-angle x-ray scattering analysis. The 10-nm-thick HZO films were fabricated by thermal and plasma-enhanced atomic layer deposition (TH- and PE-ALD) methods using H2O gas and O2 plasma as oxidants, respectively, and a post-metallization annealing (PMA) was performed at 300–400 °C. The d-spacing of the mixture of (111)-, (101)-, and (111)-planes of O, tetragonal (T), and cubic (C) phases, respectively, for the TH- and PE-ALD HZO films increased up to 2.99 Å with an increase in PMA temperature, while the d-spacing estimated by conventional x-ray diffraction was 2.92 Å regardless of the PMA temperature. The remanent polarization (2Pr = Pr+ − Pr−) of the HZO films increased as the PMA temperature increased. It is clear that the 2Pr value satisfied a linear relationship as a function of the d-spacing of O(111)/T(101)/C(111) phases. Furthermore, the wake-up effect was found to depend on the ferroelectric O phase formation. The wake-up effect was significantly reduced in both the TH- and PE-ALD HZO films after the PMA at 400 °C due to the increase in the ferroelectric O phase formation. The leakage current density (J)–electric field properties of the PE-ALD HZO film with the lowest d-spacing were divided into three steps, such as low, middle, and large J values, in the wake-up (103 cycles), pristine (100 cycle), and fatigue (107 cycles) states, respectively. Therefore, an analysis of the ferroelectric O phase is very important for understanding the ferroelectricity including endurance.

Elucidating possible crystallographic origins of wake-up mechanisms in ferroelectric hafnia

The wake-up in doped hafnia ferroelectric devices is an extremely important process to understand in order to integrate these materials successfully into working ferroelectric memory devices. The crystallographic origins of this process are clarified with three main mechanisms. Strain relaxation in the ferroelectric orthorhombic phase led to an adjustment of the unit cell volume toward a “bulk-like” value. The undistorted cell allowed for easier polarizability within the unit cell, allowing higher polarization. Reversible phase transformations between the tetragonal and orthorhombic phases depend on the nature of the strain. Finally, a model is developed describing grain reorientation, inducing a 90° rotation of the orthorhombic unit cell and allowing the phase to respond to the E-field more readily under cycling.

A nanoscale continuous transition from the monoclinic to ferroelectric orthorhombic phase inside HfO2 nanocrystals stabilized by HfO2 capping and self-controlled Ge doping

A transition from the monoclinic to ferroelectric orthorhombic phase, takes place within a region of few atomic layers of single HfO2 crystals. Atomic calculations show high stability of the ferroelectric orthorhombic phase.

Critical effect of the bottom electrode on the ferroelectricity of epitaxial Hf0.5Zr0.5O2thin films

Comparison of a set of perovskite electrodes shows that La1−x(Ca,Sr)xMnO3is critical to stabilize the ferroelectric orthorhombic phase in epitaxial films. The stabilization is favored if the La content in the manganite is high.