Regulating crystal structure and ferroelectricity in Sr doped HfO2 thin films fabricated by metallo-organic decomposition

HfO₂ based binary ferroelectric oxides are promising candidate for nonvolatile memory devices due to their compatibility with the current Si-based technology. In this work, Sr doped HfO₂ (Sr:HfO₂) ferroelectric thin films with Sr concentration from 0% to 10?mol% were prepared on the platinum electrodes by metallo-organic decomposition (MOD). It was demonstrated that uniform Sr:HfO₂ thin films with extremely low roughness can be achieved and crystallized by MOD under a 700?°C annealing process. A wake-up stage was believed more essential for the ferroelectricity of the MOD derived Sr:HfO₂ thin film, since the remnant polarization of 13.3 µC/cm² and high dielectric constant of 30 were obtained after 10⁵ cycling tests. The transformation from monoclinic phase to cubic phase was observed with increasing the Sr concentration and the thickness of the films. X-ray photoelectron spectroscopy analysis confirmed the bonding type of O-Hf-O and O-Sr-O bonds in the film. The microscopic crystal structure of ferroelectric orthorhombic phase was observed by high resolution transmission electronic microscope. The intrinsic ferroelectricity of Sr:HfO₂ film was demonstrated by the hysteresis polarization-voltage loops and distinct current peaks in the current-voltage curve. Stable domain structure and its switching dynamics were monitored by piezoresponse force microscopy, indicating the native polarization of Sr:HfO₂. This work will provide a controllable routine to fabricate ferroelectric HfO₂ based thin films using MOD method.     

Y-doped HfO2 deposited by atomic layer deposition using a cocktail precursor for DRAM capacitor dielectric application

A Y-doped HfO₂ thin film deposited using a cocktail precursor for a DRAM capacitor dielectric application was investigated. It has been difficult to adapt HfO₂, a potential high-dielectric-constant material, deposited by a typical thin-film deposition technique to actual devices owing to its low dielectric constant of approximately 20, resulting from its monoclinic-phase crystal structure. Although several methods have been investigated to increase the dielectric constant by crystal structure transformation to the tetragonal phase, which has a dielectric constant as high as approximately 40, the formation of the monoclinic phase was not successfully suppressed. In this study, the tetragonal-phase formation of HfO₂ thin films was investigated using a cocktail precursor consisting of Y and Hf precursors. The monoclinic formation suppression mechanism in the Y-doped HfO₂ thin film was determined from the physical and chemical analyses results. Moreover, the leakage current change caused by the introduced oxygen vacancy with respect to the Y dopant concentration was investigated. Improved electrical properties of the dielectric constant and leakage current were achieved with Y-doped HfO₂.     

Effect of annealing protection atmosphere on the ferroelectric yttrium doped hafnium oxide thin films

The 10 nm thick yttrium doped hafnium oxide (Y:HfO₂) thin films, prepared by chemical solution deposition which using all-inorganic aqueous salt reagents, were fabricated on Si (100) substrates. The crystalline structure, chemical composition and ferroelectric properties of thin films, annealed in protection atmosphere of Air, Ar and N₂, were examined. Result showed that the crystalline structure and ferroelectric properties of films exhibited a strong annealing protection atmosphere dependence. When compared to annealing protection atmosphere of Air and Ar, the films with the N₂ exhibited lowest m-phase fraction of 19.4%, and the highest oxygen vacancy percentage content of 3.06%, accompanied with the highest relative permittivity of 50.9 and the remanent polarization of 14.6 μC/cm². These excellent ferroelectric properties were correlated with asymmetric orthorhombic phase and the concentration of oxygen vacancy introduced from the nitrogen doping concentration.     

Effect of Y2O3 interlayer on the electric properties of Y-doped HfO2 film deposited by chemical solution deposition

Ferroelectric oxide becomes the focus of memory industry again after the discovery of ferroelectricity in doped-HfO₂ polycrystalline films. Thermal stress is an important factor for the variation of ferroelectric phase content. In this paper, the effect of film stress induced by Y₂O₃ interlayer in the ferroelectric properties of Y-doped HfO₂ (Y: HfO₂) ferroelectric films, which is deposited by chemical solution deposition (CSD), is investigated systematically by polarization-voltage measurement. Compared with Y-doped HfO₂ film without interlayer, 1 nm Y₂O₃ interlayer enhances the remanent polarization of Y: HfO₂ film due to the effects of film stress and surface energy. And thick Y₂O₃ interlayer benefits to reduce leakage current density. But polarization switching of HfO₂ film is restrained due to the capacitor voltage divider caused by thick Y₂O₃ interlayer. The obvious enhance effect of Y₂O₃ interlayer still exists in Y: HfO₂ film at high voltage due to the breakdown of Y₂O₃ interlayer, realizing a huge remanent polarization (P_r) of 22.8 μC/cm² in Y: HfO₂ film (doping content: 4 at %). It is 3.6 times than that of ferroelectric doped-HfO₂ film without Y₂O₃ interlayer.     

Synthesis and characterization of nanometric yttrium-doped hafnia solid solutions

Nanometric-sized yttrium doped HfO₂ powders were obtained by applying metathesis and combustion reactions. The tailored composition of solid solutions was: Hf_1?xY_xO_2?δ with concentration “x” ranging from 0 to 0.2. HfCl₄ was used as a source of hafnium whereas Y(NO₃)₃·6H₂O was used as a source of yttrium. The obtained powders were annealed at different temperatures in order to induce crystallization of HfO₂. The influence of dopant concentration, annealing temperature and annealing time on powder properties was examined. The XRD analysis revealed that the crystal structure of HfO₂ depends on the dopant concentration. The samples doped with 20 mol% of yttrium and annealed at 1500 °C had high-temperature, cubic structure even after cooling to room temperature. The presence of relatively large amount of dopant was beneficial in stabilizing highly desirable cubic phase of HfO₂. It was found that the crystallite size lies in the nanometric range (<10 nm).     

Effect of Y concentration and film thickness on microstructure and electrical properties of HfO2 based thin films

In this work, we introduced a simple solution processing method to prepare yttrium (Y) doped hafnium oxide (HfO₂) based dielectric films. The films had high densities, low surface roughness, maximum permittivity of about 32, leakage current < 1.0 × 10^?7 A/cm² at 2 MV/cm, and breakdown field >5.0 MV/cm. In addition to dielectric performance, we investigated the influence of YO_1.5 fraction on the electronic structure between Y doped HfO₂ thin films and silicon (Si) substrates. The valence band electronic structure, energy gap and conduction band structure changed linearly with YO_1.5 fraction. Given this cost-effective deposition technique and excellent dielectric performance, solution-processed Y doped HfO₂ based thin films have the potential for insulator applications.     

Influence of yttrium doping on the structural,morphological and optical properties of nanostructured ZnO thin films grown by spray pyrolysis

This study reports on the deposition of highly transparent, n-type ZnO thin films on glass substrate at 450?°C using spray pyrolysis processing, with the simultaneous insertion of yttrium (Y) at different percentages (0, 2, 5, 7?at%) as a dopant. The effect of Y doping on the structure, morphology and optical properties of Y doped ZnO (ZnO:Y) was investigated for optoelectronic applications. The obtained thin films were characterized by means of X-ray diffraction, field-emission scanning electron microscopy (FESEM), UV–visible absorbance measurements, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. The as-prepared films exhibit well-defined hexagonal wurtzite structure grown along [002]. Field emission scanning electron microscope micrographs of the pure ZnO and ZnO:Y showed that the films acquired a dominance of hexagonal-like grains, the morphology was influenced by Y incorporation. All the films showed high transparency in the visible domain with an average transmittance of 83%. The band gap energy, E_g, increased from 3.12?eV to 3.18?eV by increasing the Y doping concentration up to 5?at% and then decreased to 3.15?eV for 7?at% Y content. The PL and CL measurements reveal a strong ultraviolet (UV) emission, suggesting that the as-prepared ZnO:Y thin films can potentially be used in optoelectronic devices.     

Ferroelectric ZrO2 ultrathin films on silicon for metal-ferroelectric-semiconductor capacitors and transistors

Enhanced ferroelectric properties of nanoscale ZrO₂ thin films by an HfO₂ seed layer are demonstrated in metal-ferroelectric-semiconductor (Si) capacitors and transistors prepared with a low thermal budget of 400 °C. The seeding effect of the HfO₂ layer leads to the enhancement of crystallization into the orthorhombic phase and the increase of remnant polarization of the sub-10 nm ZrO₂/HfO₂ bilayer structure. The ferroelectric field-effect transistor with the ZrO₂/HfO₂ bilayer gate stack reveals a large memory window of ~1.2 V and a steep subthreshold swing below 60 mV/decade. As compared with the Hf_0.5Zr_0.5O₂ thin film, superior ferroelectric properties of the ZrO₂/HfO₂ bilayer structure show great potential for ferroelectric memory devices fabricated on Si substrates.     

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature,excellent thermochromic properties,and superior durability

In the last few decades, smart windows made from VO₂-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (T_lum), low solar modulation ability (ΔT_sol), high phase transition temperature (T_c), and poor durability. In this study, glass/HfO₂/VO₂/HfO₂ tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO₂ buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (?111) crystal planes, respectively. HfO₂ buffer layer not only reduced T_c of VO₂ film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO₂ films. More importantly, by covering films with HfO₂ ARL, T_lum and ΔT_sol of VO₂ film were greatly improved. In particular, when the thicknesses of HfO₂ buffer layer and ARL were 80 nm and 120 nm, the obtained HfO₂/VO₂/HfO₂ tri-layer film reached a balance between high T_lum (～47.2%), high ΔT_sol (～9.1%) and low T_c (～49.1 °C). In addition, after 216 h of boiling water treatment, T_lum and ΔT_sol of HfO₂/VO₂/HfO₂ film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO₂-based smart windows with high performance and good durability.     

Modulation of ferroelectricity and antiferroelectricity of nanoscale ZrO2 thin films using ultrathin interfacial layers

In this study, tailoring the microstructures and ferroelectric(FE)/antiferroelectric(AFE) properties of nanoscale ZrO₂ thin films is demonstrated with an intentional introduction of sub-nanometre interfacial layers. The ferroelectricity of ZrO₂ thin films is significantly enhanced by the HfO₂ interfacial layers, while the TiO₂ interfacial layers lead to a dramatic transformation of ZrO₂ from ferroelectricity into antiferroelectricity. The HfO₂ and TiO₂ interfacial layers boost the formation of the polar orthorhombic phase with (111)-texture and the non-polar tetragonal phase with (110)-texture in the FE/AFE ZrO₂ thin films, respectively, as evidenced by grazing incidence, out-of-plane, and in-plane X-ray diffraction measurements. Furthermore, the modulation of ferroelectricity and antiferroelectricity of nanoscale ZrO₂ thin films by the HfO₂/TiO₂ interfacial layers can be achieved without high-temperature annealing, which is highly advantageous to process integration. The findings demonstrate the important role of the interfaces in the effective tuning of FE/AFE properties of nanoscale thin films.     

Reactive crystallization in HfO2 exposed to molten silicates

Hafnia is of interest in thermal and environmental barrier coatings, but little is known about its response to molten silicate attack. This article investigates that response using two model silicate melts, compares it with pure ZrO₂ and examines the effect of YO_1.5 additions. HfO₂ was found to form HfSiO₄ with acidic melts but undergoes grain boundary penetration in basic melts, which do not exhibit reactive crystallization. The latter can be exacerbated by microcracking resulting from the thermal expansion anisotropy of monoclinic HfO₂. Y additions generally degrade the ability to form hafnon (and zircon), and exacerbate grain boundary penetration, especially in HfO₂ where Y is present as a fluorite second phase. The fluorite controls grain growth in monoclinic HfO₂ and suppresses microcracking, but dissolves faster, especially in basic melts. The results are presented in the context of the relevant thermodynamics and kinetics. The implications for coating applications are discussed.     

In-situ determination of the HfO2–Ta2O5-temperature phase diagram up to 3000°C

The previously unknown experimental HfO₂–Ta₂O₅-temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO₂ laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO₂–Ta₂O₅-temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.     

Ferroelectric enhancement of Al-doped HfO2 thin films by rapid electron beam annealing in a low thermal budget

In past few years, there was a great amount of research on ferroelectric Al-doped HfO₂ (HAO) thin films which suffer from the need of high annealing temperatures to achieve significant ferroelectricity. In this work, we realize pronounced remnant polarization 2P_r~29μC/cm² of HAO using rapid electron beam annealing (EBA) with a large area. The simulation of electron beam trajectories reveals that the effect of EBA concentrates on the region ~20 nm below the sample surface, which highly benefits the process integration where a low thermal budget is required. The energy-dispersive X-ray and high-angle annular dark-field analyses reveal the interdiffusion between Al and Hf in the HAO layer treated by EBA. The pronounced ferroelectricity of HAO can be accounted for by the lattice strain, which facilities the formation of the orthorhombic phase, due to the substitution of Al for Hf as supported by the fast Fourier transformation diffraction pattern.     

Preparation and high temperature oxidation of SiC compositionally graded graphite coated with HfO2

SiC compositionally graded (SCG) graphite was coated with sol-gel-derived HfO₂ films and oxidized at 1500 °C in air. SCGed graphite was produced by reaction of graphite with molten Si at 1450 °C for 10 h. The sol-gel HfO₂ precursor solution was prepared by dissolving HfCl₄ in ethanol and refluxing with diethanol-amine and HNO₃, and was coated on SCGed graphite by dipping. The HfO₂-coated SCGed graphite was produced by decomposition of the precursor under conditions determined from the results of TG, DTA, and MS analysis. Oxidation of HfO₂-coated SCGed graphite was performed at 1500 °C in air, revealing a small weight loss (0.6 mg cm⁻²) after 15 h. It was found that HfO₂-coated SCGed graphite exhibits extremely high oxidation resistance, which may be due to the formation of HfSiO₄ acting to heal pores or cracks.     

Phase transformation mechanism of the APS yttria partially stabilized hafnia coating undergoing thermal exposure

8 mol.% Y₂O₃ partially stabilized HfO₂ (YSH8) free-standing coating was prepared by air plasma spraying (APS) method, and the crystal structure and phase transformation under 1300 °C were studied using Rietveld method of XRD, SEM and TEM. Results show that the as-sprayed YSH8 coating is mainly composed of tetragonal structure (T phase). The phase transformation of YSH8 coating is controlled by the diffusion of Y element. As the thermal exposure time prolongs, the weight fraction of monoclinic phase (M phase) quickly increases and reaches 68.1 % after 24 h. The Y content at the interface between the M phase and the original microstructure increases to 9 %–17 % after 24 h, and is in cubic phase. After exposure for 24, the T phase completely transforms to C + M phase.     

Deposition and characterization of epitaxial Ta-doped TiO2 films for ultraviolet photoelectric detectors

Undoped and tantalum-doped titania (TiO₂:Ta) films were synthesized via metalorganic chemical vapor deposition (MOCVD). The crystallization qualities, surface morphologies and optical properties of the deposited films were systematically characterized. The results indicated that the films having low doping levels were epitaxial anatase titania along [001] orientation with high transparency in visible region. The optical band gap could be modulated from 3.38 to 3.52?eV by controlling Ta doping levels. Ultraviolet (UV) photoelectric detectors with metal-semiconductor-metal (MSM) structure were designed and fabricated based on the undoped and Ta-doped films. The maximum spectral response of 32.3?A/W was detected at about 315?nm for the 1% Ta-doped TiO₂ film-based detector. The detectors based on the undoped and 1% Ta-doped TiO₂ films also presented good temporal responses and visible-blind characteristics, showing excellent UV light detection performances.     

Structural,magnetic and electronic structure properties of pure and Ti doped Mg0.95Mn0.05Fe2O4 nanocrystalline thin films

Thin films of pure and Ti doped Mg_0.95Mn_0.05Fe₂O₄ deposited using pulsed laser deposition technique, have been characterized using X-ray diffraction, Raman spectroscopy, dc magnetization, atomic force microscopy, magnetic force microscopy and near edge X-ray absorption fine structure spectroscopy measurements. X-ray diffraction and Raman spectroscopy measurements indicate that both the films have single phase and the polycrystalline behavior with FCC structure. The grain size calculated using XRD data was 18 and 27 nm for pure and Ti doped films, respectively. Magnetic measurements reflect that pure film has superparamagnetic behavior while Ti doped film has soft ferrimagnetic behavior at room temperature. Atomic force microscopy measurements indicate that both the films are nanocrystalline in nature. Near edge X-ray absorption fine structure spectroscopy measurements clearly infer that Fe ions are in mixed valence state.     

Constraint Annealing of HfO2 Films on Silicon Substrate: Suppression of Si Outward Emission

Flexural tensile and compressive constraints were applied mechanically to the 7.5 nm thick HfO₂ films on Si substrates to investigate the influences of stress on the Si outward emission behavior in Si/HfO₂ during annealing. The constraint stress inhibited further growth of the interfacial layer (IL) between HfO₂ and Si, suppressing the IL‐growth‐induced Si outward emission. This fact was associated with atomic rearrangement that was induced during constrained annealing, resulting in the formation of a robust HfO₂ layer with low oxygen vacancy. Such an HfO₂ layer effectively suppressed the inward diffusion of oxygen, the IL growth and the Si out‐diffusion.     

Structure and properties of Hf-O-N films prepared by high-rate reactive HiPIMS with smoothly controlled composition

Hafnium oxynitride ceramics were prepared in the form of thin films by high-power impulse magnetron sputtering of Hf in various Ar+O₂+N₂ gas mixtures. Smooth composition control was achieved by maximizing the degree of dissociation in plasma, suppressing the importance of the difference between reactivities of undissociated O₂ and N₂. The application potential of the films was further enhanced by extremely high deposition rates (e.g. 230 nm/min for stoichiometric HfO₂; achieved by feedback pulsed reactive gas flow control), low deposition temperatures (<140 °C) and not using any substrate bias. We focus on the relationships between elemental composition, phase structure, and optical, electrical, mechanical and hydrophobic properties of the materials. We quantify the evolution of smoothly controlled film properties along the transition from an oxide to a nitride, such as increasing extinction coefficient, decreasing electrical resistivity, increasing hardness or increasing water droplet contact angle.     

Absorption boost of TiO2 nanotubes by doping with N and sensitization with CdS quantum dots

A process of obtaining N-doped TiO₂ nanotubes sensitized by CdS nanoparticles is presented, including detailed characterizations performed along the synthesis. Transparent TiO₂ films consisting of nanotubes, 2.5 µm long and of ~60 nm inner diameter, were obtained after anodization of a titanium film deposited onto FTO glass substrate. N-doping was achieved by annealing of TiO₂ film in ammonia. X-ray Photoelectron Spectroscopy measurements showed that nitrogen was substitutionally incorporated in the TiO₂ matrix, with the N:Ti concentration ratio of 1:100. The doping changed the optical properties of the material in such a way that the absorption edge was shifted from 380 nm to 507 nm, as observed from diffuse reflectance spectra. The influence of the microwave (MW) irradiation on the synthesized CdS quantum dots and their optical properties was investigated. It was shown that the diameter of CdS nanoparticles was increased due to releasing of S^2- ions from dimethyl sulfoxide (DMSO) as a consequence of the MW treatment. The (N)TiO₂ films were then used as substrates for matrix assisted pulsed laser deposition of the CdS quantum dots with DMSO as a matrix. The laser parameters for the deposition were optimized in order to preserve the nanotubular structure open, the latter being an important feature of this type of photoanode. The structure obtained under optimized conditions has an additional absorption edge shift, reaching 603 nm.