Hf Metal Research Articles

Effects of Hf buffer layer at the Y-doped HfO2/Si interface on ferroelectric characteristics of Y-doped HfO2 films formed by reactive sputtering

10 nm-thick ferroelectric (FE) HfO2 films with 1.5 mol% yttrium-doping were fabricated by mid-frequency reactive magnetron co-sputtering deposition on bare Si and underlying metal Hf. Yttrium was incorporated into the HfO2 layers by simultaneously sputtering from Y and Hf metal targets under oxygen atmosphere. The effects of interfacial layer on structural and electrical properties of Y:HfO2 films have been systemically studied using high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrical measurements. Based on TEM and XRD measurements, the presence of a non-centrosymmetric orthorhombic phase which is responsible for ferroelectricity in yttrium-doped HfO2 (Y:HfO2) films could be detected. As an initial protective layer, the ultra-thin metal Hf (~1 nm) layer covers the silicon surface and prevents the native oxide growth during the deposition and post-annealing process. Results reveal that the thickness of interfacial layer is significantly reduced by deposit a Hf buffer layer and a remanent polarization Pr of up to 14 μC/cm2 can be detected in P-V measurement. The strategy of using Hf buffer layer to improve electrical properties of HfO2 film is a feasible method for the fabrication of metal-insulator-semiconductor (MIS) capacitor based on FE-HfO2/Si gate stack.

A library of atomically thin metal chalcogenides.

Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional superconductivity 5 , suggesting potential applications for functional devices6-10. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization 18 of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature 19 and this approach 20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of thesynthesized materials inour library are useful, as supported by evidence of superconductivity in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.

Hafnium-Oxide 3-D Nanofilms via the Anodizing of Al/Hf Metal Layers

Hafnium-oxide films with self-organized nanostructured 3-D architectures and variable dimension (10 to 400 nm) are synthesized via the high-current anodizing of thin aluminum-on-hafnium layers in phosphoric, malonic, and oxalic acid electrolytes. In the approach, the self-organized growth of a porous anodic alumina (PAA) film is immediately followed by the fast PAA-assisted reanodizing of the hafnium underlayer. The PAA-dissolved films consist of arrays of upright-standing hafnium-oxide nanorods held on the substrate by the tiny needle-like “nanoroots” widespread over a continuous hafnium-oxide bottom layer. The roots are amorphous Hf2O3, while the rods are amorphous HfO2–Hf2O3–Al2O3 mixed oxides, the bottom layer being, however, highly textured nanocrystalline HfO2. The calculated transport numbers for O2– and Hf4+(3)+ ions are, respectively, ∼0.55 and ∼0.45, which is a unique situation for anodic hafnium oxide, which normally grows by O2– transport only. Annealing the films in air at 600 °C oxidizes the remaining Hf metal to polycrystalline HfO2, still leaving the roots and rods amorphous. The annealing in vacuum results in partial oxide reduction and crystallization of the roots and rods to stable orthorhombic and monoclinic HfO2 phases. A model of the anodic film growth and solid-state ionic transport is proposed and experimentally justified. Potential applications of the 3-D hafnium-oxide nanofilms are in advanced electronic, photonic, or magnetic micro- and nanodevices.

Formation of Superhard State of the TiZrHfNbTaYN Vacuum–Arc High-Entropy Coating

Complex studies of the formation of the superhard state in the TiZrHfNbTaYN vacuum-arc high-entropy coating were carried out. Based on the approach of the structural surface engineering, the regularities of the formation of the triads composition–structure–physico-mechanical properties depending on the supplied potential displacement are established. It is shown that the increase of U b at the formation of a coating leads to a decrease of the relative content of a light (Ti) and increase of a heavy (Ta, Hf) metal components, which is determined by radiationally stimulated processes in a near-surface region at the deposition. The formation of the single-phase state (based on the fcc of metal lattice) in the range U b from–50 to–250 V and revealed the formation of the preferred orientation of the crystallites with the axis [111], which is perpendicular to the growth plane. The increase of the perfection of the texture with the [111] axis with increasing U b is accompanied with an increase of the coatings hardness, which makes it possible to achieve the superhard state (H = 40.2 GPa) at U b =–250 V.

Study on Brewster angle thin film polarizer using hafnia–silica mixture as high-refractive-index material

Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.

Accelerating exploitation of Co-Al-based superalloys from theoretical study

The discovery of γ′-Co3(Al,W) opens up a pathway to exploit novel Co-Al-based alloys as candidates of the next generation of superalloys. Inspired by the unexpected finding of γ′-Co3(Al,W), we present a comprehensive investigation concerning the effect of alloying elements on phase stabilities and mechanical properties of Co3Al. Up to 22 transition metal elements, e.g. Sc, Ti, V, Cr, Mn, Fe, Ni, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir, and Pt, are considered. Results show that W, Ti, Re, V, and Ta favor to occupy the Al-sublattice. Pd, Pt, Rh, Ni, Ir, Fe, and Ru favor to occupy the Co-sublattice. The others can occupy both the Co- and Al-sublattices. W, Ta, V, Ti, Nb, Re, Mo, Pt, and Cr are shown to stabilize Co3Al. It verifies that elastic properties are roughly in inverse proportional to the volume change induced by alloying elements within each group of 3d, 4d, or 5d. Based on the analysis of electronic configurations, the influence mechanism of alloying elements is traceable from the chemical bonding and the geometry of charge density. The present calculations are in favorable accord with available experimental data in the literature.

Synthesis, structure and characterization of two solvatochromic metal–organic frameworks for chemical-sensing applications

Two isostructural Zr and Hf metal–organic frameworks with doubly interpenetrated fcu-c topology, exhibits obvious solvatochromic behaviour for solvent sensing application.

Characterization of the interfacial structure and perpendicular magnetic anisotropy in CoFeB‐MgO structures with different buffer layers

In this study, we describe the deposition of Hf and Mo metal layers individually on Ta to compose new buffer layers, ie, Ta/Hf and Ta/Mo, where CoFeB/MgO stacks are deposited using magnetron sputtering. The synthesised Ta/Hf buffer has higher surface roughness, while the Ta/Mo buffer has lower surface roughness as compared with the Ta buffer. The surface roughness of the buffer appears to influence the interface of the subsequently deposited layers, resulting in rougher or smoother CoFeB/MgO interfaces. Additionally, we present a report on the magnetic properties of Ta, Ta/Hf, and Ta/Mo buffer samples. As the annealing temperature is below 200 °C, the saturation magnetisation (Ms) values for all buffer layers increase at similar rates, whereas the effective magnetic anisotropy energy (Keff) values increase at varying rates. After annealing at 350 °C, Keff reaches its maximum value for Ta/Hf and Ta/Mo buffer layers, whereas the CoFeB/MgO interface width decreases to a minimum value. The width increases as the annealing temperature is increased over 350 °C, and Keff gradually decreases with increase in the annealing temperature. The CoFeB/MgO interface width is primarily dependent on the buffer/CoFeB interface width, which is a critical parameter to obtain high perpendicular magnetic anisotropy (PMA) and high‐quality films. This work provides perspectives for understanding and controlling PMA from the viewpoint of interfacial structure.

Quasi-High-Pressure Effects in Transition-Metal-Rich Dichalcogenide, Hf3Te2

High pressure offers an intriguing avenue for the change in physical and chemical properties of condensed matters such as superconductivity, structural phase transition, and catalytic activity. However, it is hard to achieve the high-pressure phases of elements and compounds in an ambient condition by chemical pressure due to the limitation in bonding length variation. Here, we report the quasi-high-pressure state of Hf cuboid confined between two-dimensional chalcogen layers in transition-metal-rich dichalcogenide of Hf3Te2, exhibiting the enhanced degree of localization and chemical potential for valence electrons compared to ambient Hf elements. The structural analysis reveals that Hf metals in Hf3Te2 form a local cuboid structure, in which the coordination for central Hf is identical to that of high-pressure body-centered-cubic Hf phase. Density functional theory calculations verify that the compressed cuboid in Hf3Te2 has a key role for an abnormal heat capacity beyond Dulong–Petit limit and reduced ...

First-Principles Study in New Hf2CoZ (Z = B, C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) Heusler Alloys

The full-potential linearized augmented plane wave method within density functional theory was used to investigate the electronic structures, magnetic properties, and half-metallicity in Hf2CoZ (Z = B,C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) alloys with AlCu2Mn- and CuHg2Ti-type structures. The AlCu2Mn-type structure is energetically favorable than the CuHg2Ti-type structure for all compounds. Hf2CoGa, Hf2CoAl, Hf2CoIn, Hf2CoTl, and Hf2CoSn compounds in the CuHg2Ti-type structure were half-metallic ferromagnets. The d-d and covalent hybridizations between transition metals Hf and Co were effective in the formation of minority band gap. The total magnetic moments of half-metallic Hf2CoZ (Z = Al, Ga, In, Sn, and Tl) compounds in the CuHg2Ti-type structure obeyed the Slater–Pauling rule (M tot = Z tot − 18). Hf2CoIn and Hf2CoTl compounds were half-metals in relatively wide ranges of lattice constants which indicates that they can be promising and interesting candidates for spintronic devices.

Synthesis, characterization and molecular modeling of some transition metal complexes of Schiff base derived from 5-aminouracil and 2-benzoyl pyridine

Multidentate Schiff base (H2L) ligand results from condensation of 5-aminouracil and 2-benzoyl pyridine and its metal chloride (Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Fe(III), Cr(III), Ru(III), Zr(IV) and Hf(IV)) complexes were prepared. The structural features of the ligand and its metal complexes were confirmed by elemental analyses, spectroscopic methods (IR, UV–Vis, 1H NMR, mass), magnetic moment measurements and thermal studies. The data refer to the ligand coordinates with metal ions in a neutral form and shows different modes of chelation toward the metal atom. All complexes have octahedral skeleton structure, tetrahedrally Mn(II), Ni(II), trigonalbipyramidal Co(II) and square planner Pd(II). Thermal decomposition of complexes as well as the interaction of different types of solvent of crystallization are assigned by thermogravimetric analysis. Molecular modeling of prepared complexes were investigated to study the expected anticancer activities of the prepared complexes. All metal complexes have no interaction except the complexes of Pd(II), Fe(III) and Mn(II).

Atomic layer deposition of aluminum fluoride using Al(CH3)3 and SF6 plasma

Metal fluorides typically have a low refractive index and a very high transparency and find many applications in optical and optoelectronic devices. Nearly stoichiometric, high-purity AlF3 films were deposited by atomic layer deposition (ALD) using trimethylaluminum [Al(CH3)3] and SF6 plasma. Self-limiting growth was confirmed and the growth per cycle was determined to range from 1.50 Å to 0.55 Å for deposition temperatures between 50 °C and 300 °C. In addition, the film density of ∼2.8 g cm−3 was found to be relatively close to the bulk value of 3.1 g cm−3. Vacuum ultraviolet spectroscopic ellipsometry measurements over the wavelength range of 140–2275 nm showed a refractive index n of 1.35 at 633 nm, and an extinction coefficient k of <10−4 above 300 nm, for all deposition temperatures. Optical emission spectroscopy during the SF6 plasma exposure step of the ALD cycle revealed the formation of C2H2 and CF2 species, resulting from the interaction of the plasma with the surface after Al(CH3)3 exposure. On the basis of these results, a reaction mechanism is proposed in which F radicals from the SF6 plasma participate in the surface reactions. Overall, this work demonstrates that SF6 plasma is a promising co-reactant for ALD of metal fluorides, providing an alternative to co-reactants such as metal fluorides, HF, or HF-pyridine.

Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Oxidation of metal layer surfaces due to atomic layer deposition (ALD) of thin metal oxide films is studied. For this, dense and smooth metal films of Pt, Ta and Hf were grown by off-axis sputtering and were in-situ transferred into the ALD system. Oxide films like HfO2, TiO2 and Al2O3 with a thickness of about 2 to 3 nm were deposited between 250 and 300°C by means of plasma assisted ALD utilizing a remote oxygen plasma as oxygen source. The ‘built-in’ oxygen gradients formed at different metal/ALD metal oxide interfaces were analysed by angle resolved X-ray photoelectron spectroscopy. The Pt metal films show no evidence of oxidation due to the PE-ALD process, independent of the grown metal oxide. In contrast, the thickness of the metal oxide layers formed at the surfaces of the Hf and Ta metal films depends on the ALD surface reactions as well as on the differences in oxidation enthalpy of the substrate metal and the metal oxide. The results are important for the design of high-density memristor arrays based on OxRAM devices.

Effect of component substitution on the atomic dynamics in glass-forming binary metallic melts

We investigate the substitution of early transition metals (Zr, Hf, and Nb) in Ni-based binary glass-forming metallic melts and the impact on structural and dynamical properties by using a combination of neutron scattering, electrostatic levitation (ESL), and isotopic substitution. The self-diffusion coefficients measured by quasielastic neutron scattering (QENS) identify a sluggish diffusion as well as an increased activation energy by almost a factor of 2 for Hf35Ni65 compared to Zr36Ni64. This finding can be explained by the locally higher packing density of Hf atoms in Hf35Ni65 compared to Zr atoms in Zr36Ni64, which has been derived from interatomic distances by analyzing the measured partial structure factors. Furthermore, QENS measurements of liquid Hf35Ni65 prepared with 60Ni, which has a vanishing incoherent scattering cross section, have demonstrated that self-diffusion of Hf is slowed down compared to the concentration weighted self-diffusion of Hf and Ni. This implies a dynamical decoupling between larger Hf and smaller Ni atoms, which can be related to a saturation effect of unequal atomic nearest-neighbor pairs, that was observed recently for Ni-rich compositions in Zr-Ni metallic melts. In order to establish a structure-dynamics relation, measured partial structure factors have been used as an input for mode-coupling theory (MCT) of the glass transition to calculate self-diffusion coefficients for the different atomic components. Remarkably,MCT can reproduce the increased activation energy for Hf35Ni65 as well as the dynamical decoupling between Hf and Ni atoms.

Photophysical Features of Phthalocyanines Metallocomplexes with Out-of-Plane Ligands

The objective of this work was synthesizing of Ti-, Zr-, Hf-phthalocyanines, containing chloride ions or dibenzoylmethanate fragments coordinated in out-of-plane positions to macrocycle. Spectroscopic and photophysical parameters of phthalocyanines metallocomplexes with metal atoms Ti(IV), Zr(IV) and Hf(IV) and spectroscopic features of Mg- and Zn-phthalocyanines have been detected and characterized at ambient and liquid nitrogen temperatures. The efficient intersystem crossing observed for Hf-phthalocyanine leads to very weak fluorescence and suppresses the fluorescence lifetimes to less than 1 ns. Changing of nature of central metals from heavy Hf(IV) atom to lighter atoms: Zr(IV), Zn(II), Ti(IV), Mg(II) lead to noticeable increasing of fluorescence lifetimes up to maximum value of 6,6 ns for Mg-phthalocyanine. At the same time the values of fluorescence quantum yield rises from 1 % for Hf-phthalocyanine to 56 % for Mg-phthalocyanine. The heavy atom effect has a substantially greater impact on photophysical parameters metallocomplexes of phthalocyanines than influence of attachment out-of-plane ligands.

Epitaxial growth of HfS2 on sapphire by chemical vapor deposition and application for photodetectors

Group IVB transition metal (Zr and Hf) dichalcogenides (TMDs) have been attracting intensive attention as promising candidates in the modern electronic and/or optoelectronic fields. However, the controllable growth of HfS2 monolayers or few layers still remains a great challenge, thus hindering their further applications so far. Here, for the first time we demonstrate the epitaxial growth of high-quality HfS2 with a controlled number of layers on c-plane sapphire substrates by chemical vapor deposition (CVD). The HfS2 layers exhibit an atomically sharp interface with the sapphire substrate, followed by flat, 2D layers with octahedral coordination. The epitaxial relationship between HfS2 and substrate was determined by x-ray diffraction and transmission electron microscopy measurements to be: HfS2 (0 0 0 1) [10-10]||sapphire (0 0 0 1)[1-100]. Moreover, a high-performance photodetector with a high on/off ratio of more than 103 and an ultrafast response rate of 130 µs for the rise and 155 µs for the decay times were fabricated based on the CVD-grown HfS2 layers on sapphire substrates. This simple and controllable approach opens up a new way to produce highly crystalline HfS2 atomic layers, which are promising materials for nanoelectronics.

Room Temperature Corrosion Behavior of ZrB2-HfB2 Solid Solutions in Acidic and Basic Aqueous Environments

Several solid solutions mixtures of ZrB2 and HfB2 with and without additives were investigated for corrosion behavior in aqueous solutions in a wide range of pH. The samples were prepared through ball milling followed by spark plasma sintering (SPS). In addition to the single component diborides and mixtures thereof, additives of Hf, Zr, and Ta pure metals as well as LaB6 and Gd2O3 were mixed in to test for corrosion improvements. Several electrochemical methods were employed including potentiostatic polarization, potentiodynamic polarization and electrical impedance spectroscopy (EIS). The results showed that mixed diboride solid solutions had better corrosion resistance than their unmixed counterparts. ZrB2-rich compositions performed better in the high pH solutions; whereas HfB2-rich compositions showed better corrosion resistance under acidic conditions. Metal-rich compositions and Ta additions did not show improvements in the corrosion resistance, the addition of LaB6 to the Hf0.5Zr0.5B2 solid solution, however, exhibited enhanced corrosion resistance in non-acidic solutions.

Relationships between the distribution of O atoms on partially oxidized metal (Al, Ag, Cu, Ti, Zr, Hf) surfaces and the adsorption energy: A density-functional theory study

We investigate the oxidation of selected metal (Al, Ag, Cu, Ti, Zr, and Hf) surfaces by the density functional theory. We go through a wide range of (233 per metal) distributions of O atoms on a partially oxidized metal surface. First, we focus on the qualitative information whether the preferred distribution of O atoms is heterogeneous (stoichiometric oxide + metal) or homogeneous (substoichiometric oxide). We find that the former is energetically preferred, e.g., for Al, while the latter is energetically preferred, e.g., for Ti, Zr, and Hf. Second, we provide the quantitative values of adsorption energies corresponding to the energetically preferred O atom distributions for various partial coverages of various metals by O. Third, we discuss and show an example of implications of the aforementioned findings for the understanding and simulations of sputtering.

Metal–Organic Frameworks from Group 4 Metals and 2,5-Dihydroxyterephthalic Acid: Reinvestigation, New Structure, and Challenges Toward Gas Storage and Separation

The reactions of group 4 metals (Ti, Zr, and Hf) and 2,5-dihydroxyterephthalic acid (H4dobdc) under solvothermal conditions have been systematically explored, and their major crystalline phases have been investigated by single-crystal diffractions. Ti(IV) forms a layered framework [Ti2(Hdobdc)3] where honeycomb-type sheets are interconnected through strong hydrogen bonding. Various gases are reversibly adsorbed within the straight one-dimensional channels decorated with polar O atoms, and H2 and CO2 show relatively high isosteric heats of adsorption at 6.6 and 29.4 kJ/mol, respectively. Zr(IV)- and Hf(IV)-based MOFs have also been synthesized using the same ligand and are isostructural with the formula (H3O)x[M(dobdc)(bz)x] (M = Zr or Hf). They have a unique, nonoxo-trinuclear building block that forms a polyhedral network of 6-connected topology. Unlike the two-dimensional net of Ti, the Zr and Hf metal–organic frameworks are hydrothermally stable as unambiguously shown by variable-temperature X-ray diff...

Field Emission Characteristics of Metal-doped Nano-diamond Cathode on Titanium Substrate

Field emission characteristics of metal-doped nano-diamond on titanium substrate were studied, wherein metals—Ti and Hf were mixed with nano-diamond separately, and cathode samples were prepared by electrophoresis. Then the structures and morphologies of cathode samples were characterized by SEM and XRD; finally, field emission tests were conducted. Results show that the morphology of 5 mg Ti-doped sample is more uniform and dense, but the morphologies of Hf-doped samples become worse. Meanwhile, compared with the undoped sample, field emission characteristics of Ti-doped samples improve, while those of Hf-doped samples decrease. From the comparison between Ti-doped samples, it is found that the amount of Ti mixed with nano-diamond should not be too much; a moderate amount of Ti can improve field emission characteristics of nano-diamond on Ti substrate. It is determined by the nature of metal Ti and Hf, which influences the bonding reaction between nano-diamond and substrate Ti, thus affecting, field emission characteristics of doped nano-diamond on Ti substrate.