Phase Of HfO2 Research Articles

Structural modification of Pr3+-/Zr4+ doped HfO2:Eu3+ and the resulting luminescence and dielectric properties

Incorporating a minute amount of rare earth ions has been suggested to improve the functionality of electronic materials through the introduction of luminescence properties. HfO2-based materials have received significant attention owing to their high dielectric permittivity (k), excellent thermodynamic stability, and ferroelectricity. In this study, we fabricated luminescent high-k materials by co-doping Pr3+/Zr4+ and Eu3+ into HfO2, i.e., Hf1-xPrxO2-δ:Eu for x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.09, and into Hf1-yZryO2:Eu for y = 0.00, 0.10, 0.50, and 0.90 via the solid-state reaction method. The doping concentration of Eu3+ is 0.4 mol%. Rietveld refinement based on the X-ray diffraction pattern of Hf1-xPrxO2-δ:Eu shows that HfO2 undergoes structural transformation from monoclinic (P21/c) to cubic (Fm3̅m) as x increases, along with an increase in its dielectric constant. Although the structures of Hf1-yZryO2:Eu maintain the P21/c phase of HfO2 even with Zr4+ doping, their dielectric constants increase with y, which is associated closely with an increase in the grain size. The photoluminescence (PL) spectra of Hf1-xPrxO2-δ:Eu and Hf1-yZryO2:Eu under 396 nm excitation show the typical orange-red emission of Eu3+ ions. As x increases, the PL intensity of Hf1-xPrxO2-δ:Eu decreases because of its higher local structural symmetry. However, the PL intensity of Hf1-yZryO2:Eu increases with y because of its larger grain size. Our experiments suggest that Hf1-xPrxO2-δ:Eu and Hf1-yZryO2:Eu are promising candidates as multifunctional luminescent high-k materials.

Microstructure characterization of plasma electrolytic oxidation coating on Hf-Nb-Ta-Zr high entropy alloy

The morphology, composition, and microstructure of plasma electrolytic oxidation (PEO) coating on Hf-Nb-Ta-Zr high entropy alloy (HEA) were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, glow discharge optical emission spectroscopy (GDOES) and transmission electron microscopy (TEM). The formation mechanism of PEO coating was analyzed. It was found that a dense PEO coating of ∼4 μm thick on the HEA surface was formed, which consisted of tetragonal and monoclinic ZrO2 phases, tetragonal and monoclinic HfO2 phases, Nb2O5 and Ta2O5 phases. The PEO coating from the alloy substrate to the surface contained five distinctive layers: amorphous barrier layer, nanocrystalline layer, columnar grain layer, amorphous outer layer, and top porous layer. Their formation was ascribed to the different cooling rates of the melt in the different depths of the discharge channel across the coating. Meanwhile, the formation of an amorphous barrier layer near the HEA substrate was also related to the mutual migration and diffusion of Hf4+, Nb5+, Ta5+, Zr4+ and O2− besides the rapid cooling of melt in the bottom of the discharge channel. The columnar grains of ∼70 nm wide were mainly composed of the monoclinic ZrO2 and monoclinic HfO2 phases, but both the amorphous layers enrich the Ta and Nb elements. It was believed that the high-content Ta and Nb in the HEA enhanced the formation of the amorphous layers.

Thermal cycling property of the novel Hf6Ta2O17/YSZ TBCs prepared by atmospheric plasma spraying technology

Hf6Ta2O17, with low thermal conductivity, high thermal expansion coefficient, and excellent fracture toughness, was a promising candidate ceramic top coat material of thermal barrier coating (TBC). A novel Hf6Ta2O17/YSZ double ceramic top coat prepared by atmospheric plasma spraying (APS) was applied to study the role of the microstructure and mechanical property on the thermal cycling property at 1200 °C. Results show that the rapid decomposition of Hf6Ta2O17 occurred during the spraying process. As the increased spraying power, more HfO2 phases were observed in the Hf6Ta2O17/YSZ TBCs. Besides, the porosity of the Hf6Ta2O17 ceramic top coat decreased with the increased spraying power, resulting in the elastic modulus of the Hf6Ta2O17 ceramic top coat enhancement. The highest cycles at 1200 °C were obtained for the Hf6Ta2O17/YSZ TBCs with the lowest elastic modulus and least HfO2 phases, and were twice as long as the cycles of the single YSZ TBCs. The chemical reaction between HfO2 and Hf6Ta2O17 might have contributed to the cracking of the Hf6Ta2O17/YSZ TBCs. This work provides a new option for the preparation and development of the ternary oxides by APS.

Formation mechanism of Hf-based ceramic particles in tungsten alloys

Abstract To meet the requirements of high strength of tungsten (W) based alloys at high temperature applied in the aerospace field, the preparation and formation of Hf-related ceramic particles in W were studied. It is found that the HfO2 phase with a size of about 500 nm was observed at the grain boundary in both W-0.26 wt.% HfC and W-0.25 wt.% HfH2-0.125 wt.% C systems. Further study of phase stability and thermodynamic properties of Hf(W)-O(C) compounds reveals that the HfO2 phase possesses the most stability of all the Hf-related compounds, and the oxidation of Hf is preferred to the carbonization. It is predicted that more stability of W base metal, higher temperature, and higher carbon concentration can reduce the formation of HfO2 while promoting the formation of the HfC particle phase.

Effects of raw materials on microstructure and mechanical properties of second phase particles reinforced W[sbnd]Re composites

Combined strengthening of micron and nanoscale second phase particles is of great significance for improving the comprehensive properties of tungsten (W) based refractory alloys. In this work, with the addition of raw HfC, HfH2 and carbon powders, three types of W-3 wt%Re-0.3 wt%HfC (WRH) composites were fabricated by powder metallurgy and rotary swaging. The microstructure and mechanical properties of three WRH samples were investigated comparatively. Results show that three WRH samples are constituted by W matrix, HfO2 and HfC second phase particles. When HfH2 powder is served as the raw material, the obtained two kinds of composites show the microstructure configuration with a micro-nano combined second phase particles. That is, some larger HfO2 particles are located at the matrix grain boundaries and fine nanoscale HfC particles are inside W matrix grains. Based on it, both composites exhibit the better overall performance. When the carbon powder ratio reaches 0.03 wt%, WRH composite shows the smallest average matrix grain size and a smaller size of second phase particles inside the matrix grains, and thereby gaining the better ultimate tensile strength and hardness. As carbon powder ratio rises to 0.07 wt%, WRH sample presents an excellent elongation. Besides, the formation mechanisms of HfO2 and HfC particles, as well as the correlation between microstructure and mechanical properties, were discussed particularly.

Ablation behavior and mechanisms of Cf/(CrZrHfNbTa)C‒SiC high‐entropy composite at temperatures up to 2450°C

AbstractThe oxide layer formed by ultra‐high melt point oxides (ZrO2, HfO2) and SiO2 glassy melt is the key to the application of traditional thermal structural materials in extremely high‐temperature environment. However, the negative effect of ZrO2 and HfO2 phase transitions on the stability of oxide layer and rapid volatilization of low viscosity SiO2 melt limit its application in aerospace. In this study, the ablation behavior of Cf/(CrZrHfNbTa)C‒SiC high‐entropy composite was explored systematically via an air plasma ablation test, under a heat flux of 5 MW/m2 at temperatures up to 2450°C. The composite presents an outstanding ablation resistance, with linear and mass ablation rates of 0.9 µm/s and 1.82 mg/s, respectively. This impressive ablation resistance is attributed to the highly stable oxide protective layer formed in situ on the ablation surface, which comprises a solid skeleton of (Zr, Hf)6(Nb, Ta)2O17 combined with spherical particles and SiO2 glassy melt. The irregular particles provide a solid skeleton in the oxides protective layer, which increased stability of the oxide layer. Moreover, the spherical particles have a crystal structure similar to that of Ta2O5 and are uniformly distributed in SiO2 glassy melt, which hinder the flow of SiO2 glassy melt and enhance its viscosity to a certain degree. And it reduces the volatilization of SiO2. In summary, the stable oxide layer was formed by irregular particles oxide and the SiO2 glassy melt with certain viscosity, thereby resulting in the impressive ablation resistance of the composite. This study fills a gap in ablation research on the (CrZrHfNbTa)C system.

First-principles predictions of HfO2-based ferroelectric superlattices

The metastable nature of the ferroelectric phase of HfO2 is a significant impediment to its industrial application as a functional ferroelectric material. In fact, no polar phases exist in the bulk phase diagram of HfO2, which shows a dominant non-polar monoclinic ground state. As a consequence, ferroelectric orthorhombic HfO2 is stabilized either kinetically or via epitaxial strain. Here, we propose an alternative approach, demonstrating the feasibility of thermodynamically stabilizing polar HfO2 in superlattices with other simple oxides. Using the composition and stacking direction of the superlattice as design parameters, we obtain heterostructures that can be fully polar, fully antipolar or mixed, with improved thermodynamic stability compared to the orthorhombic polar HfO2 in bulk form. Our results suggest that combining HfO2 with an oxide that does not have a monoclinic ground state generally drives the superlattice away from this non-polar phase, favoring the stability of the ferroelectric structures that minimize the elastic and electrostatic penalties. As such, these diverse and tunable superlattices hold promise for various applications in thin-film ferroelectric devices

Leakage current control of Y-HfO2 for dynamic random access memory applications via ZrO2 stacking

Pristine HfO2 tends to have a monoclinic phase with a low dielectric constant. Reportedly, yttrium(Y) doping has been used to induce the tetragonal phase in HfO2, with Y contributing to the formation of the tetragonal phase and improving the crystallinity. However, Y doping in HfO2 forms oxygen vacancies, resulting in a relatively high leakage current. Owing to the difficulty in suppressing the oxygen vacancies crucial for inducing the tetragonal phase, we focused on another significant leakage current path: the grain boundary. Introducing a heterostructure is expected to reduce the grain size and mitigate the leakage current by increasing the length of the leakage path. This study compares the dielectric constant and leakage current based on the stacking sequence of ZrO2 and Y-doped HfO2 (Y-HfO2) and elucidate the underlying differences through electrical analysis. Additionally, crystallographic analysis reveals the reasons for the structural order-dependent variations in the electrical properties. Finally, the structure with a Y-HfO2 layer at the bottom demonstrates advantages in achieving a lower leakage current.

Fabrication of high speed steel electrodes with MoSi2–MoB–HfB2 ceramic additives for electrospark deposition on die steel

The electrodes for electrospark deposition (ESD) were fabricated from hot-pressed blanks composed of a mechanically alloyed powder mixture of R6M5K5 high speed steel. This mixture was enriched with a 40 % addition of heat-resistant MoSi2–MoB–HfB2 ceramics, produces through the self-propagating high-temperature synthesis method (resulting in the R6M5K5-K electrode), as well as variant without any ceramic addition (resulting in the R6M5K5 electrode). We examined both the composition and structure of the electrode materials and the coatings derived from them, identifying the characteristics of mass transfer from hot-pressed electrodes to substrates of 5KhNM die steel under various frequencies and energy conditions during processing. The R6M5K5 electrode consists of an α-Fe-based matrix incorporating dissolved alloying elements and contains discrete particles of ferrovanadium, tungsten carbide, and molybdenum. The R6M5K5-K electrode, in addition to the α-Fe-based matrix, includes borides and carbides, as well as hafnium oxide. The use of the R6M5K5 electrode resulted in a consistent weight increase in the cathode throughout the entire 10-minute processing period. In contrast, the application of the ceramicenhanced electrode led to weight gain only during the initial 3 min of processing. Subsequently, ESD produced coatings of 22 and 50 μm thickness on the surface of 5KhNM steel using R6M5K5 and R6M5K5-K electrodes, respectively. The introduction of SHS ceramics escalated the roughness (Ra) of the surface layers from 6 to 13 μm and the hardness from 9.1 to 15.8 GPa. The coating from the R6M5K5 electrode was composed of austenite (γ-Fe) and exhibited high uniformity. Conversely, the coating from the R6M5K5-K electrode consisted of a diverse matrix with both crystalline and amorphous iron, an amorphous phase rooted in the Fe–B alloy, and scattered phases of HfO2, HfSiO4, Fe3Si, and Fe3B. High-temperature tribological testing at 500 °C in an air atmosphere showed that the coatings possess a friction coefficient of 0.55–0.57 when coupled with a counterbody of AISI 440C steel. The integration of heat-resistant ceramics notably enhanced the coating's wear resistance, increasing it by a factor of 13.5.

Thermal shock resistance of carbon-based composites reinforced by HfC nanowires under extreme conditions

Lightweight composites with excellent thermal shock performance exhibit significant potential for thermal structural components. Herein, HfC nanowires modified carbon/carbon (HfCNWs-C/C) composites are fabricated, establishing a three-dimensional network within the matrix. The microstructure of the composites before and after thermal shock cycles were characterized by SEM, XRD and TEM. Additionally, the flexural strength with varied thermal shock were analyzed, which were initially increasing before subsequently decreasing with the number of thermal shock cycles under Ar atmosphere, exhibiting thermal shock strengthening phenomena. While within oxy-acetylene system, the strength of HfCNWs-C/C exhibit an increasement of 75 % compared with C/C after ablation for 6 cycles, which can be attributed to the sintering and the generation of molten film of partial HfO2 phases that can mitigates oxidation of the matrix, but still lead to a declining trend in the strength following ablative thermal cycles.

Creating Ferroelectricity in Monoclinic (HfO_{2})_{1}/(CeO_{2})_{1} Superlattices.

Ferroelectricity in CMOS-compatible hafnia (HfO_{2}) is crucial for the fabrication of high-integration nonvolatile memory devices. However, the capture of ferroelectricity in HfO_{2} requires the stabilization of thermodynamically metastable orthorhombic or rhombohedral phases, which entails the introduction of defects (e.g., dopants and vacancies) and pays the price of crystal imperfections, causing unpleasant wake-up and fatigue effects. Here, we report a theoretical strategy on the realization of robust ferroelectricity in HfO_{2}-based ferroelectrics by designing a series of epitaxial (HfO_{2})_{1}/(CeO_{2})_{1} superlattices. The designed ferroelectric superlattices are defects free, and most importantly, on the base of the thermodynamically stable monoclinic phase of HfO_{2}. Consequently, this allows the creation of superior ferroelectric properties with an electric polarization >25 μC/cm^{2} and an ultralow polarization-switching energy barrier at ∼2.5 meV/atom. Our work may open an avenue toward the fabrication of high-performance HfO_{2}-based ferroelectric devices.

Nanoscale Investigation of the Effect of Annealing Temperature on the Polarization Switching Dynamics of Hf0.5Zr0.5O2 Thin Films

AbstractRecently, HfO2‐based ferroelectric thin films have attracted widespread interest in developing next‐generation nonvolatile memories. To form a metastable ferroelectric orthorhombic phase in HfO2, a post‐annealing process is typically necessary. However, the microscopic mechanism underlying the effect of annealing temperature on ferroelectric domain nucleation and growth is still obscure, despite its importance in optimizing the operation speed of HfO2‐based devices. In this study, the ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at different temperatures (550–700 °C) are systematically investigated. Evidently, the crystal structure, remnant polarization, and dielectric constant monotonically change with annealing temperature. However, microscopic piezoresponse force microscopy images as well as macroscopic switching current measurements reveal non‐monotonic changes in the polarization switching speed with annealing temperature. This intriguing behavior is ascribed to the difference in the ferroelectric‐domain nucleation process induced by the amount of oxygen vacancies in the Hf0.5Zr0.5O2 thin films annealed at different temperatures. This work demonstrates that controlling the defect concentration of ferroelectric HfO2 by tuning the post‐annealing process is critical for optimizing device performance, particularly polarization switching speed.

Deterministic Orientation Control of Ferroelectric HfO2 Thin Film Growth by a Topotactic Phase Transition of an Oxide Electrode.

The scale-free ferroelectricity with superior Si compatibility of HfO2 has reawakened the feasibility of scaled-down nonvolatile devices and beyond the complementary metal-oxide-semiconductor (CMOS) architecture based on ferroelectric materials. However, despite the rapid development, fundamental understanding, and control of the metastable ferroelectric phase in terms of oxygen ion movement of HfO2 remain ambiguous. In this study, we have deterministically controlled the orientation of a single-crystalline ferroelectric phase HfO2 thin film via oxygen ion movement. We induced a topotactic phase transition of the metal electrode accompanied by the stabilization of the differently oriented ferroelectric phase HfO2 through the migration of oxygen ions between the oxygen-reactive metal electrode and the HfO2 layer. By stabilizing different polarization directions of HfO2 through oxygen ion migration, we can gain a profound understanding of the oxygen ion-relevant unclear phenomena of ferroelectric HfO2.

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping

In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

Interpretable structure-property correlation in X-ray diffraction patterns of HfZrO thin films via machine learning

The X-ray diffraction (XRD) patterns of materials contain important and rich information in terms of structure, strain state, grain size, etc. The XRD can become a powerful fingerprint for material characterizations when it is combined with machine learning techniques. Attempts utilizing machine-learning-based methods mainly focus on phase identification for mixture compounds. Herein, we applied a machine-learning-based method linking XRD patterns of HfZrO thin films directly to their electronic properties in experiments. In accordance with conventional understanding, the machine learning model suggests that non-monoclinic (NM) phases of HfO2 and ZrO2 are among the main contributors to higher relative permittivity and lower leakage current. Furthermore, some minor interfacial phases like TiO x and ZrN x are also proposed to be even more important contributors to our target electronic properties. Our research demonstrates that machine learning has the potential to reveal minor XRD signals from sub-1 nm interfacial layers that have long been considered undetectable and thus ignored by human interpretation.

Low oxidation conditions in pulsed laser deposition enhance polarization without degradation of endurance and retention in Hf0.5Zr0.5O2 films

Interplay between oxygen vacancies and the stabilization of the ferroelectric orthorhombic phase in doped HfO2, as well as the resulting impact on endurance and retention, is far from being well understood. In Hf0.5Zr0.5O2 (HZO) thin films, it is commonly found that high polarization occurs usually at the the expense of robustness upon cycling due to the polarization–endurance dilemma. It has been reported that HZO thin films grown by pulsed laser deposition under the mixed Ar and O2 atmosphere exhibit a high polarization. Here, we show that this strategy enables functional properties tuning, allowing to obtain HZO films with high polarization at low oxidation conditions without degradation of endurance and retention.

Ultra-high performance hafnium-based capacitors: Synergistic achievement of high dielectric constant and low leakage current

The relationship between the dielectric constant and leakage current of materials often exhibits a negative correlation. However, to sustain Moore's Law, the semiconductor industry necessitates novel materials that simultaneously possess a high dielectric constant and low leakage current, meeting the shrinking demands of electronic components. In this study, a high-performance hafnium-based thin film capacitor achieved through the collaborative effects of bottom electrode engineering and ion doping. Tungsten (W) as the bottom electrode, with its chemically inert nature and low thermal expansion coefficient, proves advantageous in improving interface quality and promoting the generation of the high-k phase in HfO2. Concurrently, yttrium (Y) ion doping contributes to stabilizing the high-k phase of HfO2, reducing non-lattice oxygen, and enhancing the disorderliness of the dielectric thin film. By synergistically integrating these factors, the hafnium-based thin film capacitor achieves a high dielectric constant of 36.6 while maintaining minimal leakage current (Electric field ∼ 6 MV/cm, J ∼ 10−8 A/cm2) and a high charge storage capacity (Urec ∼ 104.4 J/cm³). These research findings open a promising pathway for the development of novel hafnium-based materials with broad applicability and exceptional dielectric performance.

Improvement of ferroelectric phase fraction in HfO2 via La-containing co-doping method

In this work, the effect of co-doping lanthanide and VB group elements on the phase fraction of HfO2 is studied by first-principles calculations. A significant increase in the ferroelectric orthorhombic phase fraction can be achieved by doping La with Ta or Nb, which would enhance the ferroelectricity of HfO2. Furthermore, during the screening process, it is observed that oxygen vacancies coupled with dopants can promote the formation of the ferroelectric phase in HfO2. These studies and results provide valuable methods for improving the ferroelectric properties of HfO2.

Phase transitions in HfO2 probed by first-principles computations

Ever since ferroelectricity was discovered in HfO2, the question of its origin remains controversial. Here, we probe this question using a combination of Landau theory of phase transitions and first-principles computations. In such an approach, the energy landscape associated with the phase transition between cubic and different experimentally demonstrated phases of HfO2 (tetragonal, monoclinic, orthorhombic Pbca, orthorhombic Pnma, and orthorhombic Pca21) is explored using density functional theory calculations. Computations revealed that stabilization of all but orthorhombic Pbca phase is driven by a single unstable zone-boundary antipolar mode X2−. When coupled with zone-center modes (Γ1+ and Γ3+), it stabilizes the tetragonal phase. Coupling with four additional modes (Γ5+, X3−, X5−, X5+) results in the monoclinic phase, which is the ground state of the material. If, however, Γ5+ mode is replaced with Γ4− mode, orthorhombic polar phase Pca21 is stabilized. The application of this framework to examine the effect of electric field on the ferroelectric phase of hafnia reveals that the field of 5 MV/cm is capable of stabilizing ferroelectric phase over the monoclinic one at 0 K.

Effect of number of sol-layer on structural, optical, morphological, and compositional properties of HfO2 films

Using sol-gel based spin coating HfO2 thin films were prepared on quartz substrates. The effect of sol-layer thickness on structural, optical, morphological and compositional properties was investigated. The structural studies done using X-Ray Diffraction (XRD) and Raman confirm the presence of a polycrystalline monoclinic phase of HfO2. Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscope (FESEM) studies reveal a uniform distribution of grains on the substrates. An increment in Hf is confirmed from Energy Dispersive Spectroscopy (EDS) with respect to the number of sol-layers. From XPS results existence of oxygen vacancies and the formation of HfO2 was noted. These results suggest that tailoring the number of sol-layers plays a crucial role in modifying the properties of HfO2 based thin films.