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Hafnium (Hf) has been used as a cathode material for thermionic emission in high temperature environments for a long time. However, the effect of grain boundaries (GBs) on its work function has not been reported. In this work, by using first-principles calculations, we find that the introduction of GBs would reduce the work function of Hf surface as compared with that of the perfect crystal, and by increasing the distance between two grain boundaries, the work function converges gradually to the value of monocrystalline Hf. By analyzing the surface atomic structure and charge density distribution, we find that the reduced work function of GB-containing structures originates from the increase of atomic distance and the changes of atomic coordination environments at the GB region, which results in redistribution of electrons and enhances the electronic density of states at the Fermi level. [ABSTRACT FROM AUTHOR]

AlB2-type metal diborides represented by MgB2 become an important type of conventional superconductors. However, AlB2-type HfB2 is not superconductive. Pressure-induced HfH10 are predicted to be high-temperature superconductors. Here, first-principles swarm-intelligence structural search calculations identified two ternary compounds, HfBH2 and HfB2H, showing superconductivity with the predicted superconducting transition temperatures of 2.1–14.9 and 2.2–15.0 K for a wide range of Coulomb pseudopotential μ* = 0.15–0.05 at 300 GPa, respectively. The two compounds exhibit interesting structural features, such as an Hf–H layer, covalent B–H framework, and graphene-type boron layer, as observed in AlB2-type HfB2. Besides atomic hydrogen and covalent skeleton contributing to superconducting transition, H-activated hafnium also plays a certain role. [ABSTRACT FROM AUTHOR]

The discovery of hafnium oxide-based ferroelectrics has resulted in the pursuit of ferroelectric field-effect transistors with higher scalability, lower power consumption, and enhanced switching speed. An in-depth understanding of ferroelectric polarization switching kinetics is essential for both scientific and technological purposes. Polarization switching is analyzed based on the nucleation-limited switching model with a Lorentzian distribution of logarithmic switching times. The activation field governing the switching kinetics is explained by a dependence on the density of oxygen vacancies, which are caused by various annealing temperatures. This indicates that oxygen vacancies can be a dominant factor in the polarization switching of Si-doped HfO2 films. [ABSTRACT FROM AUTHOR]

While self-propagating reactions in metallic multilayers have been known since the early 1990s, the vast majority of research has been conducted on binary systems comprising either elemental or alloyed layers. The properties of reaction and ignition in these samples are, for a given composition, mainly determined by the bilayer thickness as main design parameter. Here, we present how the stacking sequence is established as an additional design parameter when going from binary to ternary layered systems. The basis for this study is reactive Ru/Al multilayers which are extended to ternary samples by introducing a third element (Ni, Pt, or Hf) as a third, individual component. It is shown that exothermic solid state reactions at the layer interfaces and grain boundaries control the ignition temperature. Furthermore, the stacking sequence determines the specific volume density of phase boundaries and, thus, the magnitude of contribution of these reactions to the process of ignition. While the addition of hafnium is found to have little to no effect on ignition, nickel and platinum lower the ignition temperatures by up to 150 °C (Ni) and 230 °C (Pt), respectively. This can be attributed to the exothermic formation of Ni2Al9, amorphous a-(Al,Pt), and Pt2Al3 which are formed at temperatures significantly lower than that of RuAl6 which is the driving reaction for ignition in binary Ru/Al multilayers. [ABSTRACT FROM AUTHOR]

While the main application of resistive switching structures currently targets non-volatile memories, increasing interest is being focused on their logic applications. Resistive switches are suitable for Boolean logic, neuromorphic computing and for implementation of Zadeh fuzzy logic. In this work, we analyzed implementation of the resistive switching structures for logic application based on Zadeh fuzzy logic. Resistive switching structures based on hafnium oxide and tantalum oxide were connected in an anti-serial configuration (complementary resistive switch). The complementary resistive switches integrated into logic circuit for Min-Max function implementation were analyzed using quasi-static voltage sweeps. We have shown that the accuracy of the Min/Max function determination depends on the ratio of the high and low resistivity states of the single switches. Determination of the Min/Max values is relevant only above the threshold voltage of the resistive structures. Reproducibility of the Min/Max function constructed from the resistive switching structures was evaluated. In addition, pulsed reconfiguration of complementary resistive switch using 100 ns long pulses was demonstrated. [ABSTRACT FROM AUTHOR]

The electron emission properties of hafnium nitride field emitter arrays with 10 000 emission tips in the triode configuration were investigated at room temperature, 100 °C, 200 °C, and 300 °C. After cooling the sample to room temperature, the same measurements were repeated again. In the first high temperature test, the current-voltage curve at room temperature and 100 °C did not show a significant difference, but it shifted to a lower voltage for temperatures higher than 200 °C. In the second high temperature test, the current-voltage characteristics were quite similar to those obtained in the first high temperature test, except for the fact that the current-voltage curves at room temperature and 100 °C were lower than those in the first high temperature test. A Seppen-Katamuki analysis revealed that the emitter surface at temperatures less than 200 °C had a work function of about 4.3 eV once they had been heated up higher than 200 °C. At 300 °C, both for the first and second high temperature tests, the work function of the emitter was estimated to be approximately 3.1-3.3 eV, which was lower than the value expected from our previous study. The validity of the estimated work functions was confirmed by the fact that the experimentally obtained enhancement of the current and transconductance agreed with the values calculated from the theory using the estimated work function. Arguments were made for different situations such as changes in the apex radius or in a number of active tips. [ABSTRACT FROM AUTHOR]

Competition between target erosion and compound layer formation during pulse cycles in reactive HiPIMS opens up the possibility of tuning discharge conditions and the properties of deposited films by varying the duty cycle in situ without altering the reactive gas mixture. Three different reactive systems, hafnium in oxygen, tungsten in oxygen, and tungsten in oxygen/nitrogen, are studied in which amorphous films of hafnium oxide (HfO2), tungsten oxide (WO3), and tungsten oxynitride (WOxNy) are deposited. We show that the cyclic evolution of the target surface composition depends on the properties of the target including its affinity for the reactive gas mix and the compound layer melting point and volatility. We find that pulse length variations modulate the target compound layer and hence the discharge chemistry and properties of the films deposited. The refractive indices of HfO2 and WO3 were progressively reduced with the duty cycle, whereas that of WOxNy increased. These variations were found to be due to changes in the chemical composition and/or densification. We present and validate a phenomenological model that explains these findings in terms of a compound layer on the target surface that undergoes evolution during each pulse resulting in a cyclic equilibrium. The end points of the composition of the target surface depend on the duty cycle. Tuning the pulse characteristics holds great promise for the fabrication of multilayer films with through thickness graded properties. [ABSTRACT FROM AUTHOR]

First-principles evolutionary simulations are used to systematically predict stable compounds in the Zr-O system at pressures up to 120 GPa. Zr-O compounds and Hf-O compounds share many similarities, but four new phases Cmmm-Zr3O, R3c-Zr3O2, Pmma-ZrO, and Fe2P-type ZrO2 (P62m) appear in the Zr-O system: the latter two phases appear in the Hf-O system at higher pressure. OII ZrO2 (Pnma) transforms into Fe2P-type ZrO2 at 102 GPa. Meanwhile, Fe2P-type ZrO2 and P62m-ZrO have similar structures based on ω-Zr. However, the calculated Vickers hardness of Fe2P-type ZrO2 (5.6 GPa) is inferior to that of P62m-ZrO (14.1 GPa). The hardness of P62m-ZrO (14.1 GPa) is lower than that of P62m-HfO (16.1 GPa) and P62m-TiO (16.6 GPa). On the whole, Zr-O compounds exhibit lower hardnesses and bulk moduli than Hf-O compounds. [ABSTRACT FROM AUTHOR]

The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2, and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations. We find that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf1-xZrxO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films, we explore the Gibbs/Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability, we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model, we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm. Furthermore, we calculate the strength of an applied electric field necessary to cause the antiferroelectric phase transformation in ZrO2 from the P42/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation. [ABSTRACT FROM AUTHOR]

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500 μW K-2m-1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K-2m-1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl3. The obtained results are plotted on a semilogarithmic log (r) versus S graph to demonstrate that a very strong linear trend that limits the power factor around 500 μW K-2m-1 exists. Further improvement of Bi2S3 as thermoelectric material will require finding new doping modes that will break through the observed trend. The results of stability tests demonstrate that properties of optimally doped Bi2S3 are stable. [ABSTRACT FROM AUTHOR]

Electronic and transport properties of Bi2S3 with various dopants are studied using density functional theory and experimental characterizations. First, principle calculations of thermoelectric properties are used to evaluate the thermoelectric potential of the orthorhombic Bi2S3 structure. The computational screening of extrinsic defects is used to select the most favorable n-type dopants. Among all the dopants considered, hafnium and chlorine are identified as prospective dopants, whereas, e.g., germanium is found to be unfavorable. This is confirmed by experiment. Seebeck coefficient (S) and electrical conductivity (σ) measurements are performed at room temperature on pellets obtained by spark plasma sintering. An increase of power factors (S2·σ) from around 50 up to 500 μW K-2m-1 is observed for differently doped compounds. In several series of samples, we observed an optimum of power factor above 500 μW K-2m-1 at room temperature for a chlorine equivalence of 0.25 mol. % BiCl3. The obtained results are plotted on a semilogarithmic log (r) versus S graph to demonstrate that a very strong linear trend that limits the power factor around 500 μW K-2m-1 exists. Further improvement of Bi2S3 as thermoelectric material will require finding new doping modes that will break through the observed trend. The results of stability tests demonstrate that properties of optimally doped Bi2S3 are stable. [ABSTRACT FROM AUTHOR]

Hafnium dioxide or hafnia is a wide band gap dielectric used in a range of electronic applications from field effect transistors to resistive memory. In many of these applications, it is important to maintain control over oxygen stoichiometry, which can be realized in practice by using a metal layer, specifically hafnium, to getter oxygen from the adjacent dielectric. In this paper, we employ density functional theory to study the thermodynamic stability of an interface between (100)-oriented monoclinic hafnia and hafnium metal. The nudged elastic band method is used to calculate the energy barrier for migration of oxygen from the oxide to the metal. Our investigation shows that the presence of hafnium lowers the formation energy of oxygen vacancies in hafnia, but more importantly the oxidation of hafnium through the migration of oxygen from hafnia is favored energetically. [ABSTRACT FROM AUTHOR]

The interfacial chemistry of thin Al (∼3 nm) and Hf (∼2 nm) metal films deposited by electron beam (e-beam) evaporation on native oxide InP (100) samples at room temperature and after annealing has been studied by in situ angle resolved X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. The In-oxides are completely scavenged forming In-In/In-(Al/Hf) bonding after Al and Hf metal deposition. The P-oxide concentration is significantly decreased, and the P-oxide chemical states have been changed to more P-rich oxides upon metal deposition. Indium diffusion through these metals before and after annealing at 250 °C has also been characterized. First principles calculation shows that In has lower surface formation energy compared with Al and Hf metals, which is consistent with the observed indium diffusion behavior. [ABSTRACT FROM AUTHOR]

We measured the volume of hafnium at several pressures up to 67 GPa and at temperatures between 300 to 780 K using a resistively heated diamond anvil cell with synchrotron x-ray diffraction at the Advanced Photon Source. The measured data allows us to determine the P-V-T equation of state of hafnium. The previously described [Xia et al., Phys. Rev. B 42, 6736-6738 (1990)] phase transition from hcp (α) to simple hexagonal (ω) phase at 38 GPa at room temperature was not observed even up to 51 GPa. The ω phase was only observed at elevated temperatures. Our measurements have also improved the experimental constraint on the high P-T phase boundary between the ω phase and high pressure bcc (β) phase of hafnium. Isothermal room temperature bulk modulus and its pressure derivative for the α-phase of hafnium were measured to be B0 = 112.9 ± 0.5 GPa and B0′ = 3.29 ± 0.05, respectively. P-V-T data for the α-phase of hafnium was used to obtain a fit to a thermodynamic P-V-T equation of state based on model by Brosh et al. [CALPHAD 31, 173-185 (2007)]. [ABSTRACT FROM AUTHOR]

We report a triggering voltage Vtrig for observing gate leakage current (Ig) random telegraph noise (RTN) in the post-breakdown regime of HfLaO dielectric metal gate stacks. Vtrig, the onset voltage to observe RTN in degraded dielectrics, is highly dependent on breakdown hardness, which is controlled by the leakage current compliance (Igl). This dependence is qualitatively analyzed using a percolation dilation model, where generation of additional traps either at different energy levels or spatial locations gives lower Vtrig at higher Igl. The magnitude of Vtrig can be used as a direct indication of the impact of RTN on the performance of the transistor at device level, when Vtrig is evaluated by comparing with the device operating voltage. [ABSTRACT FROM AUTHOR]

We present quantitative kinetic information regarding small, 1-10 μm in diameter, single particle combustion of Zr-rich metal alloy foils subjected to either mechanical impacts or laser-ablation. The lights from combustion of metal fragments were recorded on a high-speed camera. The particle size was determined by the motion analysis of individual particle trajectory based on an aerodynamic drag law and further verified by the microstructure and chemical composition analysis of recovered post-burn particles. The measured particle sizes show a log-normal distribution centered at around 3.1 μm in diameter, and the composition of recovered particles is that of fully oxidized ZrO2. The temperature evolution of each particle along the space/time-trajectory is determined based on the thermal emission from combustion using a single-color photographic spectro-pyrometry. The result indicates that the particle has reached the maximum combustion temperature of 4000 K, well beyond the melting temperature of ZrO2, and undergone the solidification of molten ZrO2 during the cooling stage. It also shows that the maximum combustion temperature decreases linearly with increasing the particle diameter, following the correlation t = aD1.5-1.8 between the burn time (t) and the particle diameter (D). Combining the particle size, the burn time, and the particle temperature, both temperature and mass burn rates are obtained as a function of particle size. As the particle size increases, the temperature burn rate decreases, whereas the mass burn rate goes in the opposite direction. [ABSTRACT FROM AUTHOR]

In this work, the impact of the local and remote Coulomb scattering mechanisms on electron and hole mobility are investigated. The effective mobilities in quasi-planar finFETs with TiN/Hf0.4Si0.6O/SiO2 gate stacks have been measured at 300 K and 4 K. At 300 K, electron mobility is degraded below that of bulk MOSFETs in the literature, whereas hole mobility is comparable. The 4 K electron and hole mobilities have been modeled in terms of ionized impurity, local Coulomb, remote Coulomb and local roughness scattering. An existing model for remote Coulomb scattering from a polycrystalline silicon gate has been adapted to model remote Coulomb scattering from a high-κ/SiO2 gate stack. Subsequently, remote charge densities of 8 × 1012 cm-2 at the Hf0.4Si0.6O/SiO2 interface were extracted and shown to be the dominant Coulomb scattering mechanism for both electron and hole mobilities at 4 K. Finally, a Monte Carlo simulation showed remote Coulomb scattering was responsible for the degraded 300 K electron mobility. [ABSTRACT FROM AUTHOR]

The effects of tetravalent hafnium doping on the structural, transport, and magnetic properties of polycrystalline La1-xHfxMnO3 (LHMO) (0.05 ≤ x ≤ 0.3) were investigated systematically. LHMO exhibited a typical colossal magnetoresistance effect via the double-exchange between Mn2+ and Mn3+ ions, instead of that between Mn3+ and Mn4+ ions in hole-doped manganites. A phase diagram was obtained for the first time through magnetization and resistance measurements in a broad temperature range. As the Hf concentration varied from x = 0.05 to 0.3, the Curie point and metal-to-insulator transition temperature increased significantly, whereas the magnetization and resistivity decreased remarkably. An abnormal enhancement of the magnetization was observed at about 42 K. It was further confirmed that a second magnetic phase MnO2 in LHMO gives rise to such a phenomenon. The possible causes are discussed in detail. The dynamic magnetic properties of LHMO, including relaxation and aging processes, were studied, demonstrating a spin-glass state at low temperature accompanied by a ferromagnetic phase. [ABSTRACT FROM AUTHOR]

Dielectric relaxation is studied in 10 nm HfO2 thin films which are deposited by atomic layer deposition on TiN and Pt electrodes. Transient currents are recorded from 10-3 s to 10 s, as a function of bias (0.1 V to 1 V) and temperature (20 °C to 180 °C). A Curie-von Schweidler law is observed, I = Q0/tα. The power law exponent α is constant with bias and strongly depends on the temperature (varying in the 0.65-1.05 range, with a peak at 75 °C). The amplitude Q0 is described by a relation of the form Q0 = C0Vβ, where the factor C0 is weakly activated and the exponent β varies with temperature (in the 0.9-1.5 range as T varies). Transient currents are discussed along with tunneling based models from the literature. To complement transient current experiments, admittance spectroscopy (conductance G and capacitance C) is performed at low frequencies, from 0.01 Hz to 10 kHz. The dispersion law of the conductance is of the form G ∼ ωs. The capacitance is the sum of two terms, a non-dispersive term (C∞) and a low-frequency dispersive term, CLF ∼ ω-n. The critical exponents s and n verify s ≈ α and n ≈ 1-α. At room temperature, the dielectric constant is expressed as &eh;′ = Δ&eh;′ f-n + &eh;′∞, where &eh;′∞ = 11.1, n ≈ 0.2/0.3 (Pt/TiN), and Δ&eh;′ ≈ 1.5/0.7 (Pt/TiN). [ABSTRACT FROM AUTHOR]

The electrical leakage current of thin rutile structured TiO2 films deposited by atomic layer deposition on a Ru electrode was enormously reduced by depositing an extremely thin HfO2 (< 1 nm) on top. The sacrifice of the capacitance density by the HfO2 was minimized. The leakage mechanism analysis on the Pt/TiO2/Ru and Pt/HfO2/TiO2/Ru structures revealed that the improvement in leakage current was attributed to the reduction of defect (trap) density in the TiO2 film. The interfacial potential barrier height for electron transport in thinner (∼ 10 nm) TiO2 films was lower than that of thicker (∼ 20 nm) TiO2 films, which resulted in a higher leakage current in these films. The capping of ultra-thin (∼ 0.7 nm) HfO2 films effectively increased the potential barrier height, and the leakage current was decreased accordingly. The leakage current behavior was systematically analyzed from quantum mechanical transport simulations. [ABSTRACT FROM AUTHOR]

The effect of nitrogen concentration on the interfacial and optical properties, as well as band offsets of HfTiO thin films by rf sputtering HfTi alloy target has been systematically investigated. The results indicate that an interfacial layer is unavoidably formed between HfTiON thin films and Si substrate, and the main content of the interfacial layer is silicate. No silicide is formed in the interfacial layer which is partly responsible for the poor electrical properties of high-k gate dielectrics. The optical properties of HfTiON films change, such as the refractive index decreases, while the extinction coefficient increases with the increase of N content, due to the defects increase in the films. The results also indicate that the bandgap and VB offset reduce with the introduction of N into HfTiO thin films. The CB offset of the HfTiON thin films is almost unchanged indicating that the N concentration has little effect on CB offset. However, the bandgap and band offsets are all higher than 1 eV, the sufficient band offsets still makes sputtering-derived HfTiON films by HfTi alloy target a promising high-k gate dielectric for future complementary metal oxide semiconductor technology. [ABSTRACT FROM AUTHOR]

We report on the study of hafnium oxide thin films grown by pulsed laser deposition at various partial oxygen pressures by Time Differential Perturbed Angular Correlations using the nuclear probe 181Hf(β-)181Ta to determine the nuclear quadrupole interaction (NQI), and by x-ray diffraction. The samples were neutron activated and measured at room temperature as received as well as after annealing in air. All spectra exhibited two to three inequivalent probe sites, even after annealing. At 0.3 mbar oxygen partial pressure and annealing for 5 hs at 1073 K the majority (88%) of the sites exhibited NQI parameters as reported for the bulk monoclinic phase [ωQ = 125.4(2) Mrad/s, η = 0.335(5)]. We can exclude amorphous as well as cubic and tetragonal hafnium oxide phases in the annealed samples. There was no indication of room-temperature ferromagnetism. [ABSTRACT FROM AUTHOR]

In the present work we have reported the results of investigations on local structures of e-beam evaporated (HfO2–SiO2) composite thin films by synchrotron based extended x-ray absorption fine structure measurements. It has been observed that for the composite film with 10% SiO2 content, both Hf–O and Hf–Hf bond lengths are less than their values in pure HfO2 film. However the bond lengths subsequently increase to higher values as the SiO2 content in the composite films is increased further. It has also been observed that at the same composition of 10% SiO2 content, the films have smallest grain sizes (as obtained from atomic force microscopy measurements) and highest refractive index (as obtained from spectroscopic ellipsometry (SE) measurements) which suggests that the e-beam evaporated HfO2–SiO2 composite films with 10% SiO2 content leads to the most compact amorphous thin film structure. [ABSTRACT FROM AUTHOR]

This paper presents properties and nanocrystallization characteristics of HITPERM based Fe68.8Co17.2Hf7Cu1B6 and Fe69.6Co17.4Hf7B6 alloys. Both alloys differing only in copper content were subjected to series of characterizations in order to study the effect of copper. Evolution of magnetic properties as a function time has been investigated by isothermal annealing experiments and optimum annealing conditions of amorphous precursors have been reported. Resulting magnetic properties (ac and dc) measured at room temperature are presented. Nanocrystallization kinetics studied by using time dependant magnetization measurements was discussed in the framework of Johnson-Mehl-Avrami model. Results of thermal stability measurements conducted in air at 500 K for up to 1000 h are reported. [ABSTRACT FROM AUTHOR]

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater. [ABSTRACT FROM AUTHOR]

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater. [ABSTRACT FROM AUTHOR]

The controlled addition of La to HfxSi1-xO2/SiO2/Si dielectric stacks has been shown to enable the engineering of the work function to appropriate levels when TaN or TiN is employed as the capping metal gate. Work function tuning has been suggested to be controlled by La diffusion into the Hf-based dielectric as a result of further thermal treatments. In this paper, we performed high resolution angle resolved x-ray photoelectron spectroscopy (ARXPS) studies to investigate the chemical depth profile distribution of TaN/La2O3/HfO2/SiO2/Si dielectric stacks exposed to a nitridation treatment by NH3 at 700 °C. The stoichiometry and distribution of the HfO2 and SiO2 layers was examined using a self-consistent ARXPS analysis. This study shows that La diffuses to the SiO2/HfO2 interface, and that subsequent rapid thermal annealing at 1000 °C for 5 s does not significantly change the La distribution. [ABSTRACT FROM AUTHOR]

We introduce a scheme for the calculation of band offsets and defect energy levels at semiconductor-oxide interfaces. Our scheme is based on the use of realistic atomistic models of the interface structure and of hybrid functionals for the evaluation of the electronic structure. This scheme is herein applied to the technologically relevant Si–SiO2–HfO2 stack. Calculated band offsets show a very good agreement with experimental values. In particular, we focus on the energy levels of the oxygen vacancy defect and the interstitial hydrogen impurity. The defect levels are aligned with respect to the interface band structure and determined for varying location in the dielectric stack. The most stable charge states are identified as the Fermi level sweeps through the silicon band gap. [ABSTRACT FROM AUTHOR]

A face-centered-cubic (fcc) phase was obtained in high-purity hafnium (Hf) metal powders subjected to mechanical milling in a high-energy SPEX shaker mill. X-ray diffraction and electron microscopy techniques were employed to evaluate the structural changes in the milled powder as a function of milling time. The effects of mechanical milling included a reduction in grain size, an increase in lattice strain, and formation of an fcc phase instead of an equilibrium hexagonal-close-packed (hcp) phase. During milling, the grain size of Hf decreased to below about 7 nm. Additionally, there was approximately 6% increase in atomic volume during the formation of the fcc phase. Chemical analysis of the milled powder indicated the presence of significant amounts of interstitial impurities. Even though any or all of the above factors could contribute to the formation of the fcc phase in the milled powder, it appears that the high level of interstitial impurities is at least partially responsible for the formation of the fcc phase. [ABSTRACT FROM AUTHOR]

We systematically studied what effect Al diffusion from high-k dielectrics had on the flatband voltage (Vfb) of Al-incorporated high-k gate stacks. An anomalous positive shift fin Vfb with the decreasing equivalent oxide thickness (EOT) of high-k gate stacks is reported. As the SiO2 interfacial layer is aggressively thinned in Al-incorporated HfxAl1-xOy gate stacks with a metal-gate electrode, the Vfb first lies on the well known linear Vfb-EOT plot and deviates toward the positive-voltage direction (Vfb roll-up), followed by shifting toward negative voltage (Vfb roll-off). We demonstrated that the Vfb roll-up behavior remarkably decreases the threshold voltage (Vth) of p-type metal-oxide-semiconductor field-effect transistors (p-MOSFETs), and does not cause severe degradation in the characteristics of hole mobility. The Vfb roll-up behavior, which is independent of gate materials but strongly dependent on high-k dielectrics, was ascribed to variations in fixed charges near the SiO2/Si interface, which are caused by Al diffusion from HfxAl1-xOy through SiO2 to the SiO2/Si interface. These results indicate that anomalous positive shift in Vfb, i.e., Vfb roll-up, should be taken into consideration in quantitatively adjusting Vfb in thin EOT regions and that it could be used to further tune Vth in p-MOSFETs. [ABSTRACT FROM AUTHOR]

Hf-based dielectric stack is replacing SiON as gate dielectric even though our understanding of it is incomplete. It has been reported that a thermal exposure above 450 °C can lead to positive charging in both unoptimized SiO2 layer and Hf-based dielectric stack. At present, there is little information on how this process-induced positive charging (PIPC) occurs in the Hf-based stack and how to suppress it. The objective of the current work is to improve our understanding by addressing three key issues. First, the activation of PIPC precursors after device fabrication is investigated and it will be shown that the loss of certain species from the gate edge through lateral diffusion is responsible for it. Second, the passivation of the precursor is studied and the relevant species are explored. It is found that both water- and chlorine-related species play a role. Finally, the reactivation of the passivated precursor is examined and the results show that it is not thermally accelerated. [ABSTRACT FROM AUTHOR]

Negative bias temperature instability in HfSiON gate dielectric based p-channel transistors has been studied at 293, 363, 398, and 433 K and the “slow trap” creation and relaxation has been investigated. Analysis of transistor characteristics suggests that slow trapping in this case is almost entirely related to positive charge trapping in the dielectric as opposed to the dielectric/semiconductor interface. A linear threshold voltage shift versus fractional mobility variation relationship is established as expected. A simplified approach is used to crudely estimate the importance of fast relaxing trapped charges and its magnitude estimated for the four temperatures studied in this work. [ABSTRACT FROM AUTHOR]

We report on the stability under rapid thermal annealing (RTA) of the nitrogen depth profile in nitrided HfO2/SiO2/Si[001] and Hf0.8Si0.2O2/SiO2/Si[001] dielectric stacks. High resolution x-ray photoelectron spectroscopy (XPS) data indicate that the chemical component of nitrogen associated with the hafnium layer (binding energy of 396.7 eV) decreases considerably upon RTA. This was accompanied by a corresponding increase in the nitrogen component associated with silicon oxynitride (binding energy of 397.5 eV). A self-consistent analysis of the angle resolved XPS data indicated that the total amount of nitrogen in the film remains constant, suggesting that RTA causes an exchange of nitrogen between the hafnium and silicon layers. Transmission electron microscopy images show crystallization of the hafnium layer upon RTA, which is consistent with the loss of nitrogen. Data from time of flight secondary ion mass spectroscopy were consistent with the change in the nitrogen profile caused by RTA. [ABSTRACT FROM AUTHOR]

Structural stability of HfW2O8 is investigated at high pressure using Raman spectroscopy. Irreversible amorphization is found to occur when pressurized to above 3 GPa under hydrostatic conditions. The Raman spectrum of the pressure-amorphized sample closely resembles that of amorphous WO3. On the other hand, spectrum of the recovered sample subjected to uniaxial compression of about 5 GPa showed bands characteristic of crystalline HfO2 besides the parent cubic phase and amorphous phase. Ex situ x-ray diffraction measurements on the pressure-cycled samples also indicated the presence of monoclinic HfO2. These results suggest that the observed pressure-induced amorphization (PIA) under hydrostatic compression is indeed due to hindered decomposition, which is facilitated under uniaxial compression. Thus, the behavior of HfW2O8 appears to be different from that of ZrW2O8, which exhibited only PIA and not pressure-induced decomposition at ambient temperature. [ABSTRACT FROM AUTHOR]

We studied the development of crystallographic texture in titanium nitride (TiN) and hafnium nitride (HfN) films deposited by off-normal incidence reactive magnetron sputtering at room temperature. Texture measurements were performed by x-ray pole figure analysis of the (111) and (200) diffraction peaks. For a deposition angle of 40° from substrate normal, we obtained TiN biaxial textures for a range of deposition conditions using radio frequency (rf) sputtering. Typically, we find that the <111> orientation is close to the substrate normal and the <100> orientation is close to the direction of the deposition source, showing substantial in-plane alignment. We also introduced a 150 eV ion beam at 55° with respect to substrate normal during rf sputtering of TiN. Ion beam enhancement caused TiN to align its out-of-plane texture along <100> orientation. In this case, (200) planes are slightly tilted with respect to the substrate normal away from the ion beam source, and (111) planes are tilted 50° toward the ion beam source. For comparison, we found that HfN deposited at 40° without ion bombardment has a strong <100> orientation parallel to the substrate normal. These results are consistent with momentum transfer among adatoms and ions followed by an increase in surface diffusion of the adatoms on (200) surfaces. The type of fiber texture results from a competition among texture mechanisms related to surface mobilities of adatoms, geometrical, and directional effects. [ABSTRACT FROM AUTHOR]

In this work, we examine the valence band offset of hafnium aluminate (HfAlO), a material of interest for use as a high dielectric constant (high-k) gate oxide, following postdeposition annealing. It is observed that annealing leads to a change in the band offset between the high-k oxide and the semiconductor. Our results conclusively show that the change is due to the existence of an electrostatic dipole field at the interface between HfAlO and the silicon substrate. This provides evidence to partly account for the observed flatband voltage shifts in high-k dielectric stack structures in the literature. The origin of the dipole field is also discussed in terms of the interfacial permittivity. [ABSTRACT FROM AUTHOR]

The structural and energetic properties of A2Hf2O7 (A=Dy, Ho, Er) compounds have been investigated by means of ab initio total energy calculations. For Er2Hf2O7 the defect fluorite structure is predicted to be more stable than pyrochlore structure, agreeing well with previous experimental [J. Inorg. Nucl. Chem. 31, 2367 (1969); J. Less Common Met. 14, 435 (1968)] and theoretical [J. Am. Ceram. Soc. 85, 2139 (2002)] investigations. In contrast to Er2Hf2O7 pyrochlore, Dy2Hf2O7 and Ho2Hf2O7 prefer pyrochlore structures, which is consistent with the recent experimental observations of Dy2Hf2O7 compound [J. Solid State Chem. 179, 1990 (2006)]. [ABSTRACT FROM AUTHOR]

Group-IV HfNx transition-metal nitride layers, with 0.85≤x≤1.50, are grown on MgO(001) and analyzed by Raman spectroscopy. Samples with 0.85≤x≤1.20 are single-phase epitaxial NaCl-structure HfNx(001), while layers with higher N concentrations are two-phase mixtures containing N-rich inclusions. All samples exhibit both first- and second-order Raman scattering. The presence of first-order scattering indicates that Oh symmetry, a characteristic of the NaCl crystal structure, is broken even in stoichiometric HfN. As x decreases in understoichiometric HfNx samples, corresponding to a decrease in the number of valence electrons per unit cell from nine (x=1) to eight (x=0.75), the positions of the acoustic lines shift to higher frequencies. This provides an indirect probe of phonon anomalies in acoustic mode dispersion curves and signifies an increase in N vacancy concentration. The persistence of strong first-order acoustic scattering from overstoichiometric (x>1) samples indicates that adding more N does not substantially reduce the average defect density around Hf sites. The absence of a frequency shift in the acoustic modes of overstoichiometric HfNx, with 1.17≤x≤1.27, also shows that the local order around Hf sites does not change significantly, while the observed shift of the first-order optical modes to higher frequency with increasing x is characteristic of increasing lattice disorder adjacent to N sites. The Raman results, together with the nature of the variation in the HfNx lattice parameter, the electron carrier concentration, and the superconducting transition temperature as a function of x, can be explained by the presence of N vacancies for x<1, both N and Hf vacancies in stoichiometric HfN(001), and the combination of N vacancies and NHf antisite defects for x>1. [ABSTRACT FROM AUTHOR]

This study reports on the first experimental observations of electrically biased paramagnetic defects at 800 °C N2 annealed HfxSi1-xO2 (x=0.4, and 0.6)/(100)Si and HfO2/(100)Si interfaces in metal oxide silicon structures. These defects are examined by electrical-field controlled electron spin resonance (ESR) and correlated with capacitance-voltage (C-V) analysis. Distributions of ESR measured density of interface traps (ESR-Dit), Pb0 and Pb1, exhibit distinct charge humps and peaks in the Si bandgap with maximum defect density of 0.9–1.9×1012 cm-2 eV-1 in the Hf0.4Si0.6O2/Si interface. Three Pb0 and one Pb1 charged ESR-Dit peaks with density of 1.7–2.8×1012 cm-2 eV-1 are observed in the Hf0.6Si0.4O2/Si interface. Cross-sectional transmission electron microscopic images show decreasing interfacial layer (IL) thickness with increasing hafnium composition (x) at the HfxSi1-xO2/Si interface. The roughest IL observed at the HfO2/Si interface may have contributed to an ESR-Dit of Pb0 greater than 2×1013 cm-2 eV-1 and a pinned Fermi level near the midgap. It appears that the energy distributions of interface defects in HfxSi1-xO2/Si and HfO2/Si have different signatures compared to those at SiO2/Si interface, especially the charged peak near the midgap. [ABSTRACT FROM AUTHOR]

Amorphous hafnia (HfO2-y), zirconia (ZrO2-y), and hafnia-zirconia (xHfO2-y–(1-x)ZrO2-y)) nanoparticles were prepared by combining aqueous solutions of hexamethylenetetramine (HMT) with hafnium dichloride oxide (HfOCl2·8H2O), zirconium dichloride oxide (ZrOCl2·8H2O), or a mixture of these two salts at room temperature. For pure hafnia, transmission electron microscopy showed that the lower cation concentration (0.01M) resulted in the precipitation of smaller amorphous nanoparticles relative to higher concentrations (0.015M–0.04M). Consequently, the lower concentration preparation route coupled with a reducing environment (H2:N2=9:91) during annealing at temperatures between 650 and 850 °C allowed for nanoparticles with a cubic structure to be prepared as determined by x-ray diffraction. The structurally cubic hafnia nanoparticles were 6 nm or less in diameter and equiaxed. Using the same method (0.01M total metal cation concentration and reducing environment during annealing), nanoparticles of cubic structure were prepared across the entire hafnia-zirconia compositional spectrum, with a critical particle size for the cubic structure of about 6 nm. Nanoparticles of tetragonal and monoclinic structure were prepared by increasing the annealing temperature and/or using a less reducing environment. The unique role of HMT in sample preparation is discussed as well. [ABSTRACT FROM AUTHOR]

Hafnium tetrachloride is one of the most commonly used precursors for atomic layer deposition of hafnium based gate dielectrics. According to the previously reported experimental result, chlorine residue is almost unavoidably incorporated and piled up near the interface. We performed first-principles calculations to study the effect of chlorine residue in HfSiO4, which explained the experimental observations. The chlorine at interstitial site serves as a source of negative fixed charge, while the chlorine at oxygen substitutional site changes its charge state depending on the position of electron chemical potential within the HfSiO4 band gap, which possibly enlarges the hysteresis of the gate dielectrics. Moreover, chlorine incorporation also reduces the band gap of HfSiO4 by inducing lattice strain. [ABSTRACT FROM AUTHOR]

The x-ray irradiation sensitivity of hafnium silicon oxynitride/silicon oxide dielectric stacks deposited on Si has been measured. Clear evidence for trapped negative charge buildup is found for samples biased positively during irradiation and also for negative bias with or without radiation. A two phase buildup of trapped negative charge is observed in the case of irradiation in the presence of positive bias suggestive of different rates of electron trapping in the two layers. [ABSTRACT FROM AUTHOR]

Hafnium aluminate (HfAlO) and nitrided hafnium aluminate (HfAlON) dielectrics were sputter deposited on Si (100) substrate. The N2/(N2+Ar) flow ratio was varied between 0 and 66.6% in order to vary the nitrogen concentration. The Hf/Al ratio was varied from 0.5 to 1. In situ x-ray photoelectron spectroscopy and electrical measurements were utilized to characterize the as-deposited HfAlO and HfAlON films. The thermal stability studies of the HfAlO and HfAlON thin films after a 1000 °C, 10 s argon rapid thermal anneal were performed using grazing incidence x-ray diffraction and backside secondary ion mass spectrometry. Suppression of crystallization and no detectible outdiffusion of hafnium and aluminum into the silicon substrate were seen for HfAlO and HfAlON thin films with a Hf/Al ratio of 0.5. [ABSTRACT FROM AUTHOR]

We studied the oxide charges and traps within nitrided Hf-silicate (HfSiON)/SiO2 gate stacks processed with high-temperature annealing with a spectroscopic technique by using high spatial resolution scanning capacitance microscopy. Spectroscopy was performed by detecting the static capacitance (dC/dZ) between a conductive probe and the sample while sweeping the sample bias. The dC/dZ image and spatially resolved dC/dZ-V spectrum revealed the existence of positive fixed charges within HfSiON and interface trap charges between the SiO2 underlayer and Si substrate. We also observed a transient electron trap process from the conductive probe to the HfSiON film as abrupt discontinuities in the dC/dZ-V spectrum and with bias-induced topography change of the HfSiON surface. These oxide charges and trap sites distribute inhomogeneously within HfSiON/SiO2 gate stacks, and the origin of these charged defects is ascribable to phase separation induced by high-temperature postdeposition annealing. [ABSTRACT FROM AUTHOR]

High permittivity hafnium titanate thin films deposited by chemical vapor deposition have been studied systematically as gate insulators. The electrical and physical characteristics of the films were analyzed using a variety of techniques. It was observed that the films with higher Ti content are more resistant to crystallization when subjected to high temperature annealing. The capacitance-voltage curves for devices with hafnium titanate displayed relatively low (∼50 mV) hysteresis. When the concentration of Hf is comparable to the concentration of Ti, both the interfacial layer equivalent oxide thickness and permittivity of HfxTi1-xO2 increase linearly with increasing Ti content. The sample with higher Hf content showed weaker temperature dependence of the current. In metal-oxide-semiconductor field-effect transistor devices with hafnium titanate films, normal transistor characteristics were observed. The devices exhibited electron mobility degradation. The thermodynamic stability of stoichiometric films in contact with Si was also studied. [ABSTRACT FROM AUTHOR]

Hafnium silicate has a high dielectric constant and is a leading candidate to act as a gate dielectric. The defect energy levels have been calculated. The oxygen vacancy is found to give rise to Si-like levels which lie within the band gap of Si. The vacancy states are very localized and are localized on the neighboring Si sites. A second defect level high in the oxide gap is localized on the Hf sites. The behavior of ZrSiO4 is similar. [ABSTRACT FROM AUTHOR]

Hafnium silicate has a high dielectric constant and is a leading candidate to act as a gate dielectric. The defect energy levels have been calculated. The oxygen vacancy is found to give rise to Si-like levels which lie within the band gap of Si. The vacancy states are very localized and are localized on the neighboring Si sites. A second defect level high in the oxide gap is localized on the Hf sites. The behavior of ZrSiO4 is similar. [ABSTRACT FROM AUTHOR]

The influence of Hf-based dielectrics on the underlying SiO2 interfacial layer (IL) in high-k gate stacks is investigated. An increase in the IL dielectric constant, which correlates to an increase of the positive fixed charge density in the IL, is found to depend on the starting, pre-high-k deposition thickness of the IL. Electron energy-loss spectroscopy and electron spin resonance spectra exhibit signatures of the high-k-induced oxygen deficiency in the IL consistent with the electrical data. It is concluded that high temperature processing generates oxygen vacancies in the IL responsible for the observed trend in transistor performance. [ABSTRACT FROM AUTHOR]

This work examines the structural and electrical properties of HfSixOy film based metal-insulator-semiconductor capacitors by means of x-ray diffraction, x-ray photoelectron spectroscopy, capacitance-voltage (C-V), deep level transient spectroscopy, and conductance transient (G-t) techniques. Hafnium-rich silicate films were atomic layer deposited onto HF-etched or SiO2 covered silicon. Although as-deposited samples exhibit high interfacial state and disorder-induced gap state densities, a postdeposition thermal annealing in vacuum under N2 flow for 1 min at temperatures between 600 and 730 °C clearly improves the interface quality. Marked crystallization and phase separation occurred at 800 °C, increasing the structural heterogeneity and defect density in the dielectric oxide layers. [ABSTRACT FROM AUTHOR]

A high-k Hf-silicate interfacial layer grown by the solid phase reaction between sputtered metallic Hf films and an underlying SiO2/Si substrate through in situ vacuum annealing and subsequent thermal oxidation has been investigated. By means of the chemical shifts of Si 2p, Hf 4f, and O 1s core-level spectra determined by x-ray photoelectron spectroscopy (XPS), the formation of an interfacial Si-O-Si bond as the dominant reaction during in situ thermal oxidation of the Hf/SiO2/Si gate stack has been confirmed. In situ vacuum anneals without air exposure at 700 °C accelerates the interface reaction and results in the formation of Si-rich Hf-silicate interfacial layer. Analysis by Fourier transform infrared spectroscopy (FTIR) has indicated that vacuum annealed samples containing the silicate interface layer effectively suppresses the growth of the interfacial SiOx layer compared to unannealed samples during postdeposition annealing. [ABSTRACT FROM AUTHOR]