Hf Atoms Research Articles

Atomic Layer Deposition of HfO2Films on Ge

We investigated the growth characteristics and interfacial properties of HfO₂ films deposited on Ge substrate through atomic layer deposited (ALD) by using an in-situ medium energyion scattering analysis. The growth kinetics of HfO₂ grown on a GeO₂/Ge substrate through ALD is similar to that grown on an SiO₂/Si substrate. However, the incubation period of HfO₂ deposition on Ge is shorter than that on Si. The HfO₂ grown on the GeO/Ge substrate shows a significant diffusion of Hf atoms into the substrate interface and GeO volatilization after annealing at 700℃. The presence of low-quality Ge oxide or suboxide may degrade the electrical performance of device.

Characterization of TiHfN ternary alloy films as a barrier between Cu plug and Si

A TiHfN ternary alloy film is examined as a candidate reliable barrier to replace W with Cu in contact plug with Si. A low-resistivity Ti12Hf32N56 film (∼120 µΩ cm) is successfully obtained by reactive sputtering. The phase stability of the compound with a N-rich composition is observed. This is because both TiN and HfN, the constituents of this compound, have stable phases with N-rich composition and the same crystalline structure, and the TiHfN compound is considered as an alloy of these constituents. The Cu/Si contact model with a 10-nm-thick TiHfN barrier of nanocrystalline texture showed excellent barrier properties with negligible structural changes owing to annealing for 1 h at 500 °C or higher. The excellent barrier properties and low resistivity obtained are attributed to the structure of the alloy film, in which the alloy has a NaCl structure with Ti and Hf atoms randomly substituted at cation sites.

Synchrotron radiation photoemission study of interfacial electronic structure of HfO2 on In0.53Ga0.47As(001)-4 × 2 from atomic layer deposition

The growth of a passivating layer on a In0.53Ga0.47As(001)-4 × 2 surface by atomic-layer deposition of tetrakis[ethylmethylamino]Hafnium (TEMAHf)) followed by the water pulse was investigated by synchrotron radiation photoemission. The Hf atoms maintain four-fold coordination, both after the initial TEMAHf deposition and the subsequent water pulse. The Hf atoms initially bond to the As dangling bonds of the surface As atom located on the edges of the raised ridges. One EMA ligand is removed in this process. Subsequent water exposure substitutes OH ligand for one or more remaining EMA ligands. These in turn react with TEMAHf to form Hf-O-Hf bonds allowing the hafnium oxides to grow. The surface In atoms on the terrace of the raised ridges were partially removed, but none bonded of the precursor atoms. Correlations between the interfacial electronic structure and the electric performance are discussed.

Ionic exchange and the local structure in the HfO2/Ho2O3 system studied by PAC spectroscopy

The ionic exchange of Hf and Ho atoms in the HfO2/Ho2O3 system was studied at the atomic level applying the nuclear solid-state Time-Differential γ–γ Perturbed-Angular-Correlation (PAC) spectroscopy. This exchange was promoted by a ball-milling-assisted solid-state reaction between Ho2O3 and m-HfO2 initial powders. In order to follow and to elucidate the effect of different variables (milling time, temperature, pressure) on the exchange process and the appearance of new phases, 181Hf(→181Ta) ions were used as local probes in the PAC experiments. The measured hyperfine interactions enabled the electric-field gradient tensor (EFG) characterization at Hf sites at each step of the process. At the final stages of the solid-state reaction, 75–90% Hf-doping at both substitutional defect-free cation sites of Ho2O3 was achieved, being the EFG measured at these sites in excellent agreement with those determined in 181Hf-implanted Ho2O3 samples and to those predicted by the EFG systematics established in rare-earth bixbyites doped by ion-implantation of 181Hf(→181Ta) ions. Ab initio electronic structure calculations of the EFG at Ta impurities localized at both cation sites in Ho2O3 also confirm the 181Hf cationic substitution in both PAC experiments. Additional ab initio calculations at Hf impurity sites in Ho2O3 and Tm2O3 were performed to study the relative Hf preference for the symmetric site of the structure. We showed that high-energy milling plus high temperature treatments are both necessary to achieve a high degree of Hf substitution in the cation sublattice of the Ho2O3 structure. Also, we found that the pressure effect on the crystal structure favors the impurity substitution at cationic sites closer to a homogenous distribution of the probes and with much less local and far disorder. Additional spurious hyperfine interactions that were always present in 181Hf-implanted Ho2O3 samples were not observed when using this solid-state reaction method. The appearance of a modified m-HfO2 phase produced after heavy Ho-doping was also discussed.

Electronic structure study of Ce1-xAxO2 (A = Zr & Hf) nanoparticles: NEXAFS and EXAFS investigations.

Single phase nanoparticles (NPs) of CeO2, Ce0.5Zr0.5O2, Ce0.5Hf0.5O2 and Ce0.5Hf0.25Zr0.25O2 were successfully synthesized by co-precipitation method at constant pH and temperature. The X-ray diffraction results revealed that the additive atoms did not segregate to form secondary phases but led to grain size variation in the NPs. The 10 Dq values in the near edge X-ray absorption fine structure (NEXAFS) spectra at the O K-edge did not vary in the same way as the average grain size was changed for the doped CeO2 NPs. The deconvolution of Ce M5-edge and detailed analysis of O K pre-edge peak have shown the higher Ce(+3)/(Ce(+3) + Ce(+4)) ratio in the Zr- and Hf-doped samples. The local atomic structure around the Ce, Zr and Hf atoms was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at Ce K-edge, Zr K-edge and Hf L3-edge, respectively, and the EXAFS data were fitted with the theoretical calculations. The 4f occupancy, Ce(+3)/(Ce(+3) + Ce(+4)) ratio of Ce ions, coordination number of Ce and Ce-Ce/Ce-O bond distances were sensitive to the additive atoms but not explicitly changed according to the grain size variation in the NPs.

Catalyst‐Assisted Growth of Single‐Crystalline Hafnium Carbide Nanotubes by Chemical Vapor Deposition

Single‐crystalline hafnium carbide (HfC) nanotubes were synthesized by a one‐step catalyst‐assisted chemical vapor deposition (CVD) method. The typical nanotubes had uniform diameters of ~60 nm and wall thicknesses of ~15 nm and preferentially grew along [201]. From HRTEM/EELS analysis, the growth mechanism based on carbon nanotubes (CNT) tip growth and CNT‐templated reaction was proposed for explaining the formation of HfC nanotubes. According to the mechanism, CNTs were first formed by diffusion of C atoms on the surface of solid Ni catalyst particles. Then, gaseous Hf species reacted with C atoms from CNTs to form HfC nanotubes. During the entire growth process, Hf atoms did not participate in the catalytic reaction. Thus, this process was distinguished from the conventional vapor–liquid–solid process.

First-principles generalized gradient approximation (GGA) + Ud + Up studies of electronic structures and optical properties in cubic HfO2

The electronic structures and optical properties of cubic HfO2 are calculated by means of generalized gradient approximation (GGA)+U approach. Without on-site Coulomb interactions, the band gap of cubic HfO2 is 2.92eV, much lower than the experimental value (5.7eV). Introducing the Coulomb interactions of 5d orbitals on Hf atom (Ud) and of 2p orbitals on O atom (Up), we can reproduce the experimental value of the band gap. The resulting dielectric function of cubic HfO2 by the GGA+Ud+Up approach predicts the presence of a shoulder structure below the main peak of the absorption spectrum. These indicate that the GGA+Ud+UP approach is a convenient and powerful method to calculate and predict the electronic structures and the optical properties of wide-gap optical materials.

Two-Dimensional Transition Metal Honeycomb Realized: Hf on Ir(111)

Two-dimensional (2D) honeycomb systems made of elements with d electrons are rare. Here, we report the fabrication of a transition metal (TM) 2D layer, namely, hafnium crystalline layers on Ir(111). Experimental characterization reveals that the Hf layer has its own honeycomb lattice, morphologically identical to graphene. First-principles calculations provide evidence for directional bonding between adjacent Hf atoms, analogous to carbon atoms in graphene. Calculations further suggest that the freestanding Hf honeycomb could be ferromagnetic with magnetic moment μ/Hf = 1.46 μ(B). The realization and investigation of TM honeycomb layers extend the scope of 2D structures and could bring about novel properties for technological applications.

Theoretical Views on Activation of Methane Catalyzed by Hf2+and Oxidation of CO (x1Σ+) by N2O (x1Σ+) Catalyzed by HfO2+and TaO2+

The mechanisms of activation of CH4 catalyzed by (1/3)Hf(2+) and oxidation of CO by N2O catalyzed by (1/3)HfO(2+) or (2/4)TaO(2+) have been investigated using the B3LYP level of theory. For the activation of methane, the TSR (two-state reactivity) mechanism has been certified through the spin-orbit coupling (SOC) calculation and the Landau-Zener-type model. In the vicinity of the minimum energy crossing point (MECP), SOC equals 900.23 cm(-1) and the probability of intersystem crossing is approximately 0.62. Spin inversion makes the activation barrier decline from 1.63 to 0.57 eV. NBO analysis demonstrates that empty 6s and 5d orbitals of the Hf atom play the major role for the activation of C-H bonds. Finally, CH4 dehydrogenates to produce Hf-CH2(2+). For oxidation of CO by N2O catalyzed by HfO(2+) or TaO(2+), the covalent bonds between transition metal atoms and the oxygen atom restrict the freedom of valence electrons. Therefore, they are all SSR (single-state reactivity). The oxygen atom is directly extracted during the course of oxygen transfer, and its microscopic essence has been discussed. The detailed kinetic information of two catalytic cycles has been calculated by referencing the "energetic span (δE)" model. Finally, TOF(HfO(2+))/TOF(TaO(2+)) = 2.7 at 298.15 K, which has a good consistency with the experimental result.

Trichlorido{methyl[4-(methylimino)pent-2-en-2-yl]azanido-κ2N,N′}hafnium(IV)

The title complex, [Hf(C29H41N2)Cl3], was synthesized from HfCl4 and HC[C(Me)N(iPr2C6H3)]2Li(Et2O) in toluene. The structure is isotypic with those of TiIV and ZrIV complexes reported previously [Nikiforov et al. (2007 ▶). Dalton Trans. pp. 4149–4159; Kakaliou et al. (1999 ▶). Inorg. Chem. 38, 5964–5977]. There is a crystallographic mirror plane containing the Hf atom, one chloride ligand and the central diketiminate C atom. The HfIV ion has slightly distorted square-pyramidal geometry surrounded by two N atoms from the β-diketiminate ligand and by three Cl− anions. Coordination bond lengths are Hf—N = 2.181 (3) Å, and Hf—Cl = 2.3148 (15) and 2.3727 (12) Å.

Hf dopants in γ′–Ni3Al alloy

The Time Differential Perturbed Angular (TDPAC) measurements of nuclear quadrupole interactions (NQIs) at 181Ta ion probe in the polycrystalline intermetallic alloy γ′–Ni3Al doped with 0.2 at. % Hf were performed in the temperature range 78-1230 K, in order to determine the lattice location of Hf atoms in the ordered γ′–Ni3Al structure. The two NQIs obtained are discussed within the present L12 cubic structure and a tetragonal distortion of L12 to another two DO22 and L60 type structures. The first low frequency NQI at the site of the 181Ta ion-probe after substitution of aluminum for hafnium in DO22 at ambient temperature, is vQ1(300 K) = 39(1) MHz with η1 = 0. The corresponding high frequency value on the second crystallographic site in L60, is vQ2(300 K) = 204(14) MHz with η2 = 0.47(11). These two NQI's have different temperature behavior. The presence of both DO22 and L60 tetragonal distortions of the parent cubic L12 lattice, detected after adding 0.2 at. % Hf, are with modulations to the lattice con...

Optical properties of monoclinic HfO2 studied by first-principles local density approximation + U approach

The band structures and optical properties of monoclinic HfO2 are investigated by the local density approximation + U approach. With the on-site Coulomb interaction being introduced to 5d orbitals of Hf atom and 2p orbitals of O atom, the experimental band gap is reproduced. The imaginary part of the complex dielectric function shows a small shoulder at the edge of the band gap, coinciding with the experiments. This intrinsic property of crystallized monoclinic HfO2, which is absent in both the tetragonal phase and cubic phase, can be understood as a consequence of the reconstruction of the electronic states near the band edge following the adjustment of the crystal structure. The existence of a similar shoulder-like-structure in the monoclinic phase of ZrO2 is predicted.

Electronic Structure and Optical Properties of Monoclinic HfO2 with Oxygen Vacancy

Abstract The electronic structure and optical properties of monoclinic HfO 2 ( m -HfO 2 ) with the consideration of oxygen vacancy were calculated using the plane-wave ultrasoft pseudopotential method based on the first-principles density functional theory (DFT). The results reveal that a new defect level at the conduction band edge is introduced for the defective m -HfO 2 , and the 5d states of Hf atoms nearest to the oxygen vacancy make significant contribution to it. Furthermore, the dielectric functions and absorption coefficients of the perfect and defective m -HfO 2 were calculated and analyzed by means of the electronic structures, which were related to the defect levels. The calculated absorption coefficient shows that m -HfO 2 with oxygen vacancy exhibits an absorption band in the ultraviolet region.

Strain induced decomposition and precipitation of carbides in a molybdenum–hafnium–carbon alloy

Abstract In the present study of a sintered and thermo-mechanically processed Mo–Hf–C (MHC) alloy the strain induced decomposition of Mo–carbides and subsequent precipitation of Hf-carbides was revealed. The microstructural characterization was performed with atom probe tomography (APT), transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical light microscopy (OLM). During annealing of deformed conditions it can be shown by TEM analysis that Hf-carbides were preferentially formed on dislocations and occasionally on (sub)grain boundaries. XRD and OLM investigations revealed that the required C for the Hf-carbides was delivered by the decomposition of Mo–carbides which are originally located at former grain boundaries in the as-sintered condition. It has been shown that for this decomposition mechanical strain prior to annealing was required. In the early stages of precipitation APT analyses revealed a significant segregation of C and Hf atoms to dislocations and boundaries. Finally, the authors deduced that due to the dislocation–precipitation interaction the misfit strain was effectively reduced and therefore, the Hf-carbides are located on dislocations and (sub)grain boundaries.

Crystal structure of monoclinic modifications of zirconium and hafnium tetrafluoride trihydrates

The monoclinic modification of ZrF4·3H2O, isostructural to HfF4·3H2O, is synthesized and structurally studied for the first time. Unlike the triclinic modification of ZrF4·3H2O with a dimeric structure, the synthesized compound has a polymer structure formed from infinite chains composed of ZrF6(H2O)2 groups sharing F…F edges. The crystal structure of HfF4·3H2O, previously determined by the photo method, is refined. The refined data on the geometric characteristics of the coordination polyhedron of the Hf atom and the system of hydrogen bonds in the structure are obtained.

Structure and dielectric properties of LaxHf(1−x)Oy thin films: The dependence of components

LaxHf(1−x)Oy (x=0, 0.1, 0.3, 0.5, 0.7, y=2−(1/2)x) thin films were grown by pulsed laser deposition (PLD) method. The component dependence of the structure and vibration properties of these thin films is studied by combining X-ray diffraction, X-ray absorption fine structure (XAFS) and infrared spectroscopy. The thin film with 10% La/(La+Hf) atom ratio forms a cubic HfO2 phase and it has the largest static dielectric constant. More La atoms introduced cause amorphous phase formed and the static dielectric constants increase with the La content. Although XAFS indicates that these amorphous thin films have almost same local structures, the infrared phonon modes with most contribution to the static dielectric constant move to lower frequency, which results in the component dependence of the dielectric constant.

In Situ Study of ALD Processes Using Synchrotron-based X-ray Fluorescence and Scattering Techniques

In this paper, we present an x-ray based approach for in situ characterization during ALD processes. In a first case study the growth of HfO2 on H-terminated and oxidized Si and Ge surfaces is studied. X-ray fluorescence (XRF) is used to monitor the amount of deposited Hf atoms, while grazing incidence small angle x-ray scattering (GISAXS) provides information on the evolution of surface roughness. A second case study concerns the growth of TiO2 in a porous titania film containing ink-bottle mesopores. XRF and GISAXS measurements are used to monitor the Ti uptake and the density of the porous film, respectively.

Hafnium intercalation between epitaxial graphene and Ir(111) substrate

We report on the change of structural and electronic properties while depositing Hf atoms onto the graphene epitaxially grown on Ir(111) substrate. We find that the Hf atoms intercalate between the graphene and its iridium host. This intercalation induces a new interface superstructure, as confirmed by scanning tunneling microscopy and low energy electron diffraction. Raman spectra reveal that the Hf-intercalated graphene shows the prominent features of intrinsic graphene. Our study suggests that the Hf intercalation acts as a buffer layer between the graphene and the Ir(111) substrate, restoring the graphene's intrinsic electronic properties.

Dislocation densities and internal stress in Ni3Al

Dislocation densities have been measured in Ni3Al(Hf) single crystals after deformation at various temperatures and strains. In addition, effective and athermal stresses have been determined by strain dip tests. A model is proposed for the strength anomaly domain which correlates the athermal part of the stress with the operation of dislocation sources, to compensate for the strong dislocation exhaustion. Dislocation density data, which evidence an anomaly in the density with temperature over the stress anomaly domain, are shown to support this model predictions between 373 K and 683 K for plastic strains larger than 3 × 10−2 but not at yield. In addition, the presence of Hf atoms (3.3 at. %) induces a friction stress close to 63 MPa.

Interaction of metal impurities with native oxygen defects in GeO2

Injection of metal atoms from a high-k oxide cap to a passivating GeO2 layer could ultimately affect the quality of Ge-based gate stacks. Here we investigate with first-principles calculations the incorporation of different metal atoms and their effect on the electronic properties of GeO2. We find that tetravalent Hf and Zr substitutional atoms have no beneficial influence on pre-existing defect states in GeO2 whereas trivalent Al species can passivate existing oxygen vacancies. The results are consistent with the improved channel mobility of devices employing Al2O3/GeO2 gate oxides.