Phase Of HfO2 Research Articles

A novel approach to low-temperature synthesis of cubic HfO2 nanostructures and their cytotoxicity

The development of a strategy to stabilise the cubic phase of HfO2 at lower temperatures is necessary for the emergence of unique properties that are not realised in the thermodynamically stable monoclinic phase. A very high temperature (>2600 °C) is required to produce the cubic phase of HfO2, whereas the monoclinic phase is stable at low temperature. Here, a novel rapid synthesis strategy was designed to develop highly crystalline, pure cubic-phase HfO2 nanoparticles (size <10 nm) using microwave irradiation. Furthermore, the as-prepared nanoparticles were converted to different morphologies (spherical nanoparticles and nanoplates) without compromising the cubic phase by employing a post-hydrothermal treatment in the presence of surface modifiers. The cytotoxicities and proliferative profiles of the synthesised cubic HfO2 nanostructures were investigated over the MCF-7 breast cancer cell line, along with caspase-3/7 activities. The low-temperature phase stabilisation was significantly attributed to surface imperfections (defects and deformations) induced in the crystal lattice by the desirable presence of Na2S·xH2O and NaOH. Our work provides unprecedented insight into the stabilisation of nanoscale cubic-phase HfO2 in ambient environments; the method could be extended to other challenging phases of nanomaterials.

The impact of charge compensated and uncompensated strontium defects on the stabilization of the ferroelectric phase in HfO2

Different dopants with their specific dopant concentration can be utilized to produce ferroelectric HfO2 thin films. In this work, it is explored for Sr in a comprehensive first-principles study. Density functional calculations reveal structure, formation energy, and total energy of the Sr related defects in HfO2. We found the charge compensated defect with an associated oxygen vacancy SrHfVO to strongly favour the non-ferroelectric, tetragonal P42/mnc phase energetically. In contrast, the uncompensated defect without oxygen vacancy SrHf favours the ferroelectric, orthorhombic Pca21 phase. According to the formation energy, the uncompensated defect can form easily under oxygen rich conditions in the production process. Low oxygen partial pressure existing over the lifetime promotes the loss of oxygen leading to VO, and thus, the destabilization of the ferroelectric, orthorhombic Pca21 phase is accompanied by an increase of the leakage current. This study attempts to fundamentally explain the stabilization of the ferroelectric, orthorhombic Pca21 phase by doping.

Effect of thermal annealing sequence on the crystal phase of HfO2 and charge trap property of Al2O3/HfO2/SiO2 stacks

In this study, we investigated the effect of a post annealing sequence on the HfO2 crystal phase and the memory window of charge trap devices with TiN-Al2O3-HfO2-SiO2-Si stacks. The charge trap dielectrics of HfO2 were deposited by atomic layer deposition and were annealed in an oxygen environment with or without Al2O3 blocking oxides. X-ray diffraction analysis showed that, after thermal annealing, the predominant crystal phase of HfO2 is divided into tetragonal and monoclinic phase depending on the presence or absence of Al2O3 blocking oxide. In addition, deconvolution of X-ray diffraction spectra showed that, with increasing annealing temperature, the fraction of the tetragonal phase in the HfO2 film was enhanced with the Al2O3 blocking oxide, while it was reduced without the Al2O3 blocking oxide. Finally, measurements of program/erase and increase-step-pulse programming showed that the charge trap efficiency and the memory window of the charge trap devices increased with decreasing fraction of tetragonal HfO2.

First-principles modeling of hafnia-based nanotubes.

Hybrid density functional theory calculations were performed for the first time on structure, stability, phonon frequencies, and thermodynamic functions of hafnia-based single-wall nanotubes. The nanotubes were rolled up from the thin free layers of cubic and tetragonal phases of HfO2 . It was shown that the most stable HfO2 single-wall nanotubes can be obtained from hexagonal (111) layer of the cubic phase. Phonon frequencies have been calculated for different HfO2 nanolayers and nanotubes to prove the local stability and to find the thermal contributions to their thermodynamic functions. The role of phonons in stability of nanotubes seems to be negligible for the internal energy and noticeable for the Helmholtz free energy. Zone folding approach has been applied to estimate the connection between phonon modes of the layer and nanotubes and to approximate the nanotube thermodynamic properties. It is found that the zone-folding approximation is sufficiently accurate for heat capacity, but less accurate for entropy. The comparison has been done between the properties of TiO2 , ZrO2 , and HfO2 . © 2017 Wiley Periodicals, Inc.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

Modulation of microstructure and interface properties of co-sputter derived Hf1−xTixO2 thin films with various Ti content

Hf1−xTixO2 dielectric thin films were deposited on Si (100) substrates by RF reactive co-sputtering with the variation in RF power of Ti target. The compositional, morphological, structural and optical properties of Hf1−xTixO2 films with various Ti concentration were systematically investigated by X-ray photoelectron spectroscopy (XPS), Field emmission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy techniques respectively. The electrical properties of the co-sputtered thin films were studied by capacitance–voltage and current density–voltage measurements. The XRD study has shown the enhancement in the the crystalline property of Hf1−xTixO2 film up to 60 W of Ti target power and amorphous like behaviour was observed for higher RF power. The Ti content in Hf1−xTixO2 was calculated from the XPS measurements, where the Ti content was found to be increased with rise in RF power. FESEM micrographs depict the increase in grain size upto the RF power 60 W. The Raman spectrum of the Hf1−xTixO2 film has shown that the major generated phase was titanium-substituted monoclinic phase of HfO2. The flatband voltage (Vfb) and oxide charge density (Qox) were extracted from the high frequency (1 MHz) C–V curve. The Dit has a minimum value for the film deposited at 60 W RF power of Ti target. The leakage current density of the Hf1−xTixO2 films was found to be minimum for the RF power 60 W.

Selected Success Stories from Twenty Years of High-k Gate Dielectric Research

There are many success stories from the high-k gate dielectric research, carried out during the last two decades or so (say, 1996 to 2016), only a few of which could be accommodated in this paper. We plan to highlight four successful enterprises, namely, (1) Atomic Layer Deposition (ALD) of HfO2 (including HfON, HfSiON), (2) Enhancement of the dielectric constant k through structural transformation and/or compound formation, (3) Role of the SiO2/high-k Interface Dipole and the Cap Layer at the high-k/metal interface in controlling the flat-band voltage, (4) Passivation of the Ge surface and Ge MOSFET with high channel mobility. These topics will be analyzed critically and the underlying physics will be presented. Atomic Layer Deposition scores over competitors (MOCVD, Magnetron Sputtering) in terms of better film uniformity and conformity (against uneven surfaces) with less pinhole. The alkyl-amide precursors: (TDMAH - Hf[N(CH3)2]4), (TEMAH - Hf[N(C2H5) (CH3)]4), (TDEAH - Hf[N(C2H5)2]4) were found to be suitable for low temperature deposition of smooth (RMS roughness ca. 1% of film thickness), uniform (thickness and composition wise), dense (ρ = 9.23 gcm-3, i.e. 95% of the density of the monoclinic phase), low contamination (< 1% C and < 0.25% N), and conformal (100% site coverage on holes with > 35 aspect ration) films of HfO2 with a deposition rate of ca. 0.093 nm per cycle.ALD is claimed to be a self-limiting process, but it was found that self limiting surface reaction occurred at a particular precursor temperature corresponding to a specific precursor dose; these precursor temperatures for the saturating dose were 75, 115, and 130 oC for TDMAH, TEMAH, and TDEAH, respectively. High deposition temperatures culminated in precursor decomposition followed by non-uniform films or no deposition on the substrate; moreover, HfO2 film surface roughness increased due to growth of crystallites. One main concern is the contamination of the deposited high-k layer by the chemical reaction by-products – generally C, Cl, N, and H. Post-deposition O3 treatment can reduce C, H contamination by producing H2O, CO or CO2 followed by their out-diffusion. Higher deposition temperature results in lower C contamination. High temperature annealing between ALD stages has been found to lower the C contamination; likewise has been NH3/Ar plasma treatment. Typically, the value of the dielectric constant k for a high-k alloy varies in a monotonous (somewhat linear) fashion as a function of the mixing ratio between those of the two components of a binary alloy. Departure from the above trend can happen, as experiments suggest, if a major structural transformation of the alloy matrix takes place. HfO2 can occur in three crystal structures – monoclinic, cubic, and tetragonal – with different values of permittivity. The cubic and the tetragonal HfO2 have higher dielectric constant k (from theory – 29 and 70 respectively) than the monoclinic polymorph. Permittivity is determined by the ratio αm/Vm, where αm is the molar polarization and Vm is the molar volume. Analysis suggests that a reduced molar volume may be mainly responsible for the larger k value of the cubic and tetragonal HfO2. Unfortunately, at room temperature and up to about 1750 oC, the stable phase of HfO2 is monoclinic - one with the lowest value of k among the polymorphs. The option therefore lies in finding phase modifiers, which can transform the low-k to the high-k phase and stabilize that phase in the temperature range the MOSFET is subjected to during processing. Experiments have revealed the ability of some dopants, e.g. Y, Si, La, Gd, Er, Dy, Sc, Ce, to be effective phase modifiers of HfO2. An interface dipole refers to an electrical bilayer of equal and opposite charges located within atomic distance on each side of an interface; as the electric field in the interface dipole will have a Dirac delta form, it will be represented by an interface dipole potential, which has no dependence on the physical thickness. There exists strong evidence to indicate that a dipole is present across the IL/high-k interface, when the IL is an SiO2 layer. Elements of the SiO2/high-k interface dipole model have been affirmed by the results of many investigations. Perhaps, its strongest support comes from the wide technological use of a capping layer in high-k MOSFET manufacturing to control and adjust the threshold voltage. The experimental data reported by many investigators appear to suggest that the SiO2/high-k interface dipole is the dominant entity controlling the flat-band voltage VFB.

Annealing effects on microstructure and laser-induced damage threshold of HfO2/SiO2 multilayer mirrors.

HfO2/SiO2 periodic multilayer high reflection mirrors have been prepared by a reactive electron-beam evaporation technique. The deposited mirrors were annealed in the temperature range from 300°C to 500°C. The effects of annealing on optical, microstructural, and laser-induced damage characteristics of the mirrors have been investigated. The high reflection band of the mirror shifts toward a shorter wavelength with increasing annealing temperature. As-deposited and annealed mirrors show polycrystalline structure with a monoclinic phase of HfO2. Crystalinity and grain size increase upon annealing. The laser-induced damage threshold (LIDT) has been assessed using a 532 nm pulsed laser at a pulse width of 7 ns. The LIDT value of the multilayer mirror increases from 44.1 J/cm2 to 77.6 J/cm2 with annealing up to 400°C. The improvement of LIDT with annealing is explained through oxygen vacancy defects as well as grain-size-dependent thermal conductivity. Finally, the observed laser damage morphology, such as circular scalds and ablated multilayer stacks with terrace structure, are analyzed.

Mesostructured HfxAlyO2 Thin Films as Reliable and Robust Gate Dielectrics with Tunable Dielectric Constants for High-Performance Graphene-Based Transistors.

We introduce a reliable and robust gate dielectric material with tunable dielectric constants based on a mesostructured HfxAlyO2 film. The ultrathin mesostructured HfxAlyO2 film is deposited on graphene via a physisorbed-precursor-assisted atomic layer deposition process and consists of an intermediate state with small crystallized parts in an amorphous matrix. Crystal phase engineering using Al dopant is employed to achieve HfO2 phase transitions, which produce the crystallized part of the mesostructured HfxAlyO2 film. The effects of various Al doping concentrations are examined, and an enhanced dielectric constant of ∼25 is obtained. Further, the leakage current is suppressed (∼10(-8) A/cm(2)) and the dielectric breakdown properties are enhanced (breakdown field: ∼7 MV/cm) by the partially remaining amorphous matrix. We believe that this contribution is theoretically and practically relevant because excellent gate dielectric performance is obtained. In addition, an array of top-gated metal-insulator-graphene field-effect transistors is fabricated on a 6 in. wafer, yielding a capacitance equivalent oxide thickness of less than 1 nm (0.78 nm). This low capacitance equivalent oxide thickness has important implications for the incorporation of graphene into high-performance silicon-based nanoelectronics.

Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders

This study reports on the synthesis and consolidation of HfB2-HfO2 ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl4, B2O3 and Mg starting powders were mechanically activated for 5min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500°C for 6 or 12h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl2 by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190nm comprised the HfB2 phase with an amount of 79.6wt% in addition to the HfO2 phase and a very small amount of Mg2Hf5O12 phase after mechanical activation for 5min and autoclave processing for 12h. They were consolidated at 1700°C both by PS for 6h and SPS for 15min. The Mg2Hf5O12 phase decomposed during sintering and bulk samples only had the HfB2 and HfO2 phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB2-HfO2 ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (~91%), microhardness (~16GPa) and relative wear resistance (2.5) values of the HfB2-HfO2 ceramics consolidated by SPS.

Effect of HfO2 addition as intergranular grains on the energy storage behavior of Ca0.6Sr0.4TiO3 ceramics

Pore free Ca0.6Sr0.4TiO3 (CST) ceramics with HfO2 additives (0wt.%, 2.0wt.%, 4.0wt.% and 6.0wt.%) were fabricated via solid state reaction method at 1400°C. Structure characterization was conducted by XRD and SEM, indicating the existence of HfO2 at grain boundaries as the intergranular phase. The dielectric breakdown strength and energy storage efficiency were found to be greatly enhanced for CST with moderate HfO2 additives, the sample with 4.0wt.% HfO2 exhibited optimized microstructure and energy storage properties, with maximum average breakdown strength (28.9kV/mm) and high energy storage efficiency (nearly 95% at 24kV/mm). In addition, effect of Hf-rich intergranular phase on the charge transportation and interfacial polarization behavior was discussed. With good dispersion, the intergranular HfO2 phase was considered effective to modify the grain boundaries properties and consequently improve the energy storage performance for CST ceramics.

Low temperature formation of higher-k cubic phase HfO2 by atomic layer deposition on GeOx/Ge structures fabricated by in-situ thermal oxidation

We have demonstrated a low temperature formation (300 °C) of higher-k HfO2 using atomic layer deposition (ALD) on an in-situ thermal oxidation GeOx interfacial layer. It is found that the cubic phase is dominant in the HfO2 film with an epitaxial-like growth behavior. The maximum permittivity of 42 is obtained for an ALD HfO2 film on a 1-nm-thick GeOx form by the in-situ thermal oxidation. It is suggested from physical analyses that the crystallization of cubic phase HfO2 can be induced by the formation of six-fold crystalline GeOx structures in the underlying GeOx interfacial layer.

Stabilizing metastable tetragonal HfO2 using a non-hydrolytic solution-phase route: ligand exchange as a means of controlling particle size.

There has been intense interest in stabilizing the tetragonal phase of HfO2 since it is predicted to outperform the thermodynamically stable lower-symmetry monoclinic phase for almost every application where HfO2 has found use by dint of its higher dielectric constant, bandgap, and hardness. However, the monoclinic phase is much more thermodynamically stable and the tetragonal phase of HfO2 is generally accessible only at temperatures above 1720 °C. Classical models comparing the competing influences of bulk free energy and specific surface energy predict that the tetragonal phase of HfO2 ought to be stable at ultra-small dimensions below 4 nm; however, these size regimes have been difficult to access in the absence of synthetic methods that yield well-defined and monodisperse nanocrystals with precise control over size. In this work, we have developed a modified non-hydrolytic condensation method to precisely control the size of HfO2 nanocrystals with low concentrations of dopants by suppressing the kinetics of particle growth by cross-condensation with less-reactive precursors. This synthetic method enables us to stabilize tetragonal HfO2 while evaluating ideas for critical size at which surface energy considerations surpass the bulk free energy stabilization. The phase assignment has been verified by atomic resolution high angle annular dark field images acquired for individual nanocrystals.

Consideration of the formation mechanism of an Al2O3-HfO2 eutectic film on a SiC substrate

An Al2O3-HfO2 eutectic EBC film was prepared on a SiC substrate by using the electric furnace heating and the optical zone melting methods. All of Al2O3 phase disappeared during the heating step at a temperature below the melting point, and all of the HfO2 phase reacted with the carbon and boron, which are included in SiC bulk as sintering agents, during the heating step at a temperature below the melting point. The thermal decomposition of the SiC phase, the reduction reaction of Al2O3 phase, the vaporization of the Al2O3 component, the reduction reaction of HfO2 and the formation of the HfC phase occurred at a temperature below the melting point. However, a highly dense HfC phase was formed on the SiC substrate. A rapid heating process becomes possible by using the optical zone melting method. A solidified film that was composed of a highly dense HfC layer as the intermediate layer and the Al2O3-HfO2 eutectic structure layer as the top coat was obtained by using the optical zone melting method.

ZnO/(Hf,Zr)O2/ZnO-trilayered nanowire capacitor structure fabricated solely by metalorganic chemical vapor deposition

HfO2-based thin films are one of the key dielectric and ferroelectric materials in Si-CMOS LSIs as well as in oxide electronic nanodevices. In this study, we demonstrated the fabrication of a ZnO/(Hf,Zr)O2/ZnO-trilayered nanowire (NW) capacitor structure solely by metalorganic chemical vapor deposition (MOCVD). 15-nm-thick dielectric (Hf,Zr)O2 and 40-nm-thick top ZnO electrode layers were uniformly grown by MOCVD on a ZnO NW template with average diameter, length, and aspect ratio of 110 nm, 10 µm, and ∼90, respectively. The diameter and aspect ratio of the resultant trilayerd NWs are 200–300 nm and above 30, respectively. The crystalline phase of HfO2 and stacked the structure are also discussed.

Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4

HfB2 powders were synthesized via a borothermal reduction route from mechanically activated HfCl4 and B powder blends. Mechanical activation of the powder blends was carried out for 1h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100°C for 1h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B2O3) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO2 phase. HfB2 powders without any detectable HfO2 were prepared by adding 20wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM).

SiO&lt;sub&gt;2&lt;/sub&gt; Free HfO&lt;sub&gt;2&lt;/sub&gt; Gate Dielectrics by Physical Vapor Deposition

HfO2 layers, 25-A thick, were grown by cyclic Hf sputter deposition and room temperature oxidation steps on chemically oxidized Si(001). Subsequent in situ annealing and TiN deposition yield a high- $\kappa$ gate-stack for which the original 8-A-thick SiO2 layer is eliminated, as confirmed by transmission electron microscopy. Transistors fabricated with this gate-stack achieve an equivalent oxide thickness in inversion $T_{\rm inv} =9.7$ A, with a gate leakage $J_{g} =0.8$ A/cm2. Devices fabricated without in situ annealing of the HfO2 layer yield a $T_{\rm inv}$ which increases from 10.8 to 11.2 A as the oxidation time during each HfO2 growth cycle increases from 10 to 120 s, also causing a decrease in $J_{g}$ from 0.95 to 0.60 A/cm2, and an increase in the transistor threshold voltage from 272 to 294 mV. The annealing step reduces $T_{\rm inv}$ by 1.5 A (10%) but also increases the gate leakage by 0.1 A/cm2 (30%), and causes a 61 mV reduction in $V_{t}$ . These effects are primarily attributed to the oxygen-deficiency of the as-deposited HfO2, which facilitates both the reduction of an interfacial SiO2 layer and a partial phase transition to a high- $\kappa $ cubic or tetragonal HfO2 phase.

Stabilizing the ferroelectric phase in doped hafnium oxide

The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a Pr of up to 35 μC/cm2. This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO2 films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO2.

Nanoscale Potential Fluctuation in Non-Stoichiometric Hafnium Suboxides

We study the structure of nonstoichiometric HfO x films with variable composition using methods of X-ray photoelectron spectroscopy and spectroscopic ellipsometry. HfO x , to a first approximation, is a mixture of HfO2 and Hf metal with a small amount (~10–15%) of hafnium sub-oxide HfO y (y<2). In modern silicon devices SiO2 is superseded by high-κ dielectrics, such as HfO2, ZrO2, Ta2O5 etc. Hafnia (HfO2) permittivity depends on the modification, varies in the range of 12–40. HfO2 is the promising material for CMOS devices, DRAM capacitors, and memory insulator in SONOS-type flash memory cells [High Permittivity Gate Dielectric Materials, S. Kar (Ed.), Springer Series in Advanced Microelectronics 43 (2013)]. Of great interest is the use of nonstoichiometric hafnium suboxides HfO x (x<2). Variation of HfO x chemical composition (stoichiometry) leads to changes in its electronic structure, which opens up the possibility of controlling the physical (optical and electrical) properties. Comparing with tetrahedral compounds of silicon, coordination numbers of Hf and O atoms in HfO2 are high. Thus, is not clear which model describes the structure of nonstoichiometric HfO x . Purpose of the present work is to study the atomic and electronic structure of variable composition HfO x . HfO x films were produced using Hf target ion beam sputtering deposition in oxygen. The composition (x-parameter) of HfO x was defined by partial oxygen pressure. For our experiments we grew three sets of HfO x samples at the partial oxygen pressures of 4.4×10-4Pa, 1.0×10-3Pa, and 3.6×10-3Pa. In these conditions, we produced two sets of the samples of nonstoichiometric (x<2) films and one set of almost stoichiometric composition (x≈2), respectively. The core-level Hf4f 7/2-Hf4f 5/2 and valence band spectra of HfO x films were obtained using XPS machine with monochromatic Al Kα radiation. The dispersive refractive index and absorption coefficient of HfO x films were determined by means of spectroscopic ellipsometry. The higher-level electronic-structure calculations were performed using the density functional theory with the QUANTUM ESPRESSO software package. XPS spectra of Hf4f 7/2-Hf4f 5/2 level in HfO x variable composition are shown in Fig.1. For HfO x , grown at high pressures of oxygen, a peak is observed at an energy corresponding to the stoichiometric HfO2. Decreasing the oxygen pressure during HfO x synthesis accompanied by the appearance of XPS peaks corresponding to metal Hf. With decreasing of oxygen pressure, the intensity of the peaks corresponding to metal hafnium increases. The decomposition of the spectra indicates that in addition to HfO2 and Hf the films have a nonstoichiometric phase of HfO y . Corresponding to this phase peaks are located approximately in the middle between the Hf- and HfO2-related peaks (Fig.1). So, according to XPS HfO x is a mixture of stoichiometric HfO2, metal Hf, and nonstoichiometric phase of HfO y . Experimental valence band spectra of HfO2 is in good agreement with simulated spectra for m-HfO2 as shown in Fig.2. A proof of the existence of nonstoichiometric hafnia in our films can be obtained from the comparison of the experimental valence-band XPS with the corresponding one calculated from the first principles for m-HfO2 with neutral oxygen vacancy and polyvacancy (Fig.2). The calculation satisfactorily describes the experiment, with the fact that the XPS spectra are compared for HfO2 in amorphous and monoclinic phases. The presence of the neutral oxygen vacancy in m-HfO2leads to the defect levels at 3.2eV. Fig.3 shows the spectra of absorption coefficient α for HfOx. Bandgap of amorphous HfO2 is E g=5.6eV [V.V. Afanas’ev et al., J. Appl. Phys. 102, 081301 (2007)]. HfO x absorption coefficient increases monotonically with increasing of photon energy E in the range of 1.1-4.5eV. At E>4.5eV a sharp increase of α is observed. In the range of 1.1-4.5eV absorption is low due Hf metallic clusters. Absorption at E>4.5eV is caused by nonstoichiometric HfO y presence. According to XPS and optical absorption experiment data HfO x consists of metal Hf and nonstoichiometric HfO y . HfO y can be placed between HfO2 and Hf, inside HfO2, inside Hf. Fig.4(a) shows planar model of HfO x in terms of intermediate structure model (IM). According to this model HfO x consists of three phases: HfO2, HfO y и Hf. Fig.4(b) shows energy diagram of HfO x . Electron affinity оf HfO2 is 2.0eV [W. Zheng et al., J. Phys. Chem. A 109, 11521 (2005)]. According to IM space fluctuations of chemical composition cause space fluctuations of bandgap in HfO x (Fig.4(b)). Bandgap HfO x varies in the range of 0-5.6eV. The bottom of the conduction band E c against the energy of an electron in a vacuum E 0 varies in the range of 2.0-4.0eV, which leads to E c fluctuation scope of 2.0eV. The valence band ceiling E v against E 0 varies in the range of 3.9-7.7eV. Figure 1

Processing and crystallographic structure of non-equilibrium Si-doped HfO2

Si-doped HfO2 was confirmed to exist as a non-equilibrium state. The crystallographic structures of Si-doped HfO2 were studied using high-resolution synchrotron X-ray diffraction and the Rietveld refinement method. Incorporation of Si into HfO2 and diffusion of Si out of (Hf,Si)O2 were determined as a function of calcination temperature. Higher thermal energy input at elevated calcination temperatures resulted in the formation of HfSiO4, which is the expected major secondary phase in Si-doped HfO2. The effect of SiO2 particle size (nano- and micron-sized) on the formation of Si-doped HfO2 was also determined. Nano-crystalline SiO2 was found to incorporate into HfO2 more readily.