Amorphous ZrO2 Research Articles

Dielectric property and conduction mechanism of ultrathin zirconium oxide films

Stoichiometric, uniform, amorphous ZrO2 films with an equivalent oxide thickness of ∼1.5 nm and a dielectric constant of ∼18 were deposited by an atomic layer controlled deposition process on silicon for potential applications in metal–oxide–semiconductor (MOS) devices. The conduction mechanism is identified as Schottky emission at low electric fields and as Poole–Frenkel emission at high electric fields. The MOS devices showed low leakage current, small hysteresis (<50 mV), and low interface state density (∼2×1011 cm−2 eV−1). Microdiffraction and high-resolution transmission electron microscopy showed a localized monoclinic phase of α-ZrO2 and an amorphous interfacial ZrSixOy layer which has a corresponding dielectric constant of 11.

Highly conformal ZrO2 deposition for dynamic random access memory application

ZrO 2 is investigated in this work to replace SiO2 as the dielectric material in metal–oxide–metal capacitors in dynamic random access memory (DRAM) devices for its high dielectric constant, good thermal stability, excellent conformality, and large band gap. ZrO2 films were deposited on planar Si (100) wafers and patterned amorphous silicon cylinders by rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. At substrate temperature below 300 °C, no significant deposition was observed. At temperatures between 300 and 400 °C, the reaction is thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. In this regime, one atomic layer of ZrO2 can be deposited after each alternating exposure to the precursor and oxygen, ideal for achieving conformal coverage of ZrO2 over high aspect ratio features. At temperatures above 400 °C, the deposition rate is less sensitive to temperature with an apparent activation energy of 4 kcal/mol due to decomposition and desorption of the precursors. Stoichiometric, uniform, and amorphous ZrO2 was obtained at all temperatures investigated. Highly conformal step coverage of the deposited ZrO2 was achieved on 300 nm×600 nm cylindrical features with an aspect ratio of 4. The dielectric constant of ZrO2 achieved in this work ranges from 15 to 18 depending upon process conditions and small capacitance–voltage hysteresis is observed, ideal for DRAM application.

In Situ Studies of Nitridation of Zirconia (ZrO2)

The nitridation of an amorphous ZrO2 ·H2O precursor with gaseous ammonia has been studied by in situ X-ray powder diffraction (temperature range 25−1100 °C), electron diffraction (temperature range...

Rapid thermal chemical vapor deposition of zirconium oxide for metal-oxide-semiconductor field effect transistor application

ZrO 2 is investigated in this work to replace SiO2 as the gate dielectric material in metal-oxide-semiconductor devices for its high dielectric constant, good thermal stability on silicon, and large band gap. ZrO2 films were deposited on p-Si(100) wafers by a rapid thermal chemical vapor deposition process using a zirconium (IV) t-butoxide Zr(OC4H9)4 precursor and oxygen. At temperatures between 300 and 400 °C, the reaction was thermally activated with an activation energy of 29 kcal/mol, consistent with a β-hydride elimination mechanism leading to ZrO2 deposition. In this regime at substrate temperatures below 350 °C, one atomic layer of ZrO2 can be deposited after each alternating exposure to the precursor and oxygen, ideal for achieving conformal coverage of ZrO2 over high aspect ratio features. Stoichiometric, uniform, and amorphous ZrO2 was obtained, and highly conformal step coverage of the deposited ZrO2 was observed on 300 nm features with an aspect ratio of 4. The dielectric constant of ZrO2 achieved in this work ranged from 15 to 18 depending upon process conditions and small C–V hysteresis and low interfacial state density were observed, ideal for metal-oxide-semiconductor field effect transistor application.

Atomic scale structure of alcohol-based zirconia xerogels by X-ray diffraction

The structural evolution of zirconia thin films and gel powders obtained from an alkoxide precursor has been evaluated by X-ray diffraction (XRD). The results indicate that the topological short-range structure (<5 Å) of amorphous alcohol-based zirconia materials is independent of the product form (gel powders or thin films). However, the short-range structure depends on the processing temperature. For temperatures below ∼340°C, local atomic ordering of both ZrO 2 gel powders and films resembles that of crystalline monoclinic ZrO 2. An alcoxolation mechanism appears to control thin films polymerization processes, while an oxolation mechanism appears responsible for ZrO 2 gel powders polymerization. At higher temperatures, the local atomic structure of both amorphous ZrO 2 gel powders and films resembles that of crystalline tetragonal ZrO 2 polymorph. Tetragonal ZrO 2 crystallites were also observed by XRD analysis in all ZNP-based zirconia samples processed above 340°C.

Kinetic analysis of solid-state processes

A simple method for kinetic analysis of solid-state processes has been developed. A criteria capable of classifying different processes is explored here with a view toward visualizing the complexity of solid-state kinetics. They provide a useful tool for the determination of the most suitable kinetic model. The method has been applied to the analysis of crystallization processes in amorphous ZrO2 and RuO2. It is found that the crystallization kinetics of as-prepared sample exhibits a complex behavior under nonisothermal conditions. This is probably due to an overlapping of the nucleation- and crystal-growth processes at the beginning of crystallization. As a consequence, the Johnson–Mehl–Avrami nucleation-growth model cannot be applied. A two-parameter autocatalytic model provides a good description of the crystallization process under isothermal and nonisothermal conditions.

Low temperature densification behaviour of metastable phases in ZrO 2–Al 2O 3 powders produced by spray pyrolysis

It has been established that powder compacts of amorphous ZrO 2–Al 2O 3, prepared by spray pyrolysis, densify up to nearly full density at temperatures below 923 K and above 573 K, when a moderately high pressure of 750 MPa is applied. The samples remain amorphous after hot pressing. Densification behaviour has been studied in the composition ZrO 2–40 mol% Al 2O 3 with particle sizes in the ranges 15–35, 3–15 and below 3 μm. Temperatures of 573, 723 and 873 K were used along with applied pressures of 250, 500 and 750 MPa. The fine particle size led to a maximum final relative density of 96.5%. Pressure is the dominant variable in densification once the temperature is above a minimum of 573 K. Time dependent densification is relatively insignificant. Particle rearrangement at low pressures and deformation and fracture at high pressures are likely to be responsible for the high relative densities between 84 and 92 obtained during pressurising the various particle sizes up to 750 MPa. Hot pressing of composition with 62 mol% Al 2O 3 at 873 K, 750 MPa led to a final density of 99.6%, indicating that higher Al 2O 3 content imparts greater sinterability to the amorphous phase.

AMORPHOUS AND NANOCRYSTALLINE ZrO2·Y2O3(15%) STUDIED BY EXTENDED X-RAY ABSORPTION FINE STRUCTURE

Chemical coprecipitated amorphous and nanocrystalline ZrO2·Y2O3(15%) was studied by Powder X-ray diffraction(XRD) and extended X-ray absorption fine structure(EXAFS). XRD result shows that the sample is an amorphous phase annealed below 300℃; and it crystallizes into cubic nanocrystalline zirconia at 500℃. EXAFS analysis indicates that the coordination number and bond length of Zr-O shell has not changed when transforming from amorphous into nanocrystalline. This suggests that at 300℃,it has formed the same nearest-neighbor local structure as at 900℃. For Zr-Zr(Y) shell, with the reducing of grain size, the coordination number decreases obviously: the bond length is shorter and the disorder becomes larger. This result suggests that the grain size effect is much stronger for more distant coordination shell (Zr-Zr ) than for the first nearest-neighbor shell(Zr-O).

Low temperature hydrothermal synthesis of ZrO2-CaO solid solutions

Nanosized powders of ZrO2-CaO solid solutions have been synthesized by hydrothermal treatment at 110°C. Amorphous hydrous ZrO2, CaO freshly prepared by thermal decomposition of CaCO3 and MOH (M = Na, Li) mineralizer solutions were employed as precursors for the hydrothermal synthesis. Calcia-stabilized zirconia (CSZ) with different CaO content and characterized by different crystal sizes have been produced by changing the hydrothermal treatment temperature, the reaction time and the mixture composition of precursors. The combined effect of both MOH and CaO on the crystallization-stabilization of zirconia at low temperature of hydrothermal treatment is discussed.

Formation and crystallization of gels in the Na2O-SiO2-TiO2-ZrO2 system

5Na2O·95(SiO2 + TiO2 + ZrO2) gels together with Na2O-MO2 gels (MO2 = SiO2, TiO2, ZrO2) were synthesized and effect of Na2O addition to the gel formation and crystallization behavior were studied. From Si(OC2H5)4, Ti(iso-OC3H7)4, Zr(n-OC3H7)4 and NaOCH3 solutions in EtOH without H2O, transparent, opaque and heterogeneous gels were obtained. By SEM observation, it was found that the opague bulk gels, rich in TiO2 or ZrO2 component, contain agglomerated spherical particles of less than 10 μm diameter. Crystallization temperature (Tc) measured by DTA decreased sharply from 1258°C (SiO2) to 613°C (30Na2O·70SiO2) due to the increasing non-bridging oxygens in the Na2O-SiO2 system. But in the Na2O-TiO2 and Na2O-ZrO2 systems, Tc increased as Na2O content was increased up to 10 mol%. The thermal stability, defined by Tc/TL ratio, decreased from Tc/TL = 0.77 (SiO2) to 0.57 (5Na2O·95SiO2) or increased from Tc/TL = 0.30 (TiO2) to 0.67 (30Na2O·70TiO2) with Na2O addition. In the 5Na2O·95(SiO2 + TiO2 + ZrO2) system maximum Tc was 891°C at 5Na2O·76SiO2·19ZrO2 composition and decreased as TiO2 or ZrO2 contents increased. Compared with the SiO2-TiO2-ZrO2 gels, Tc of the 5 mol%Na2O containing gels (Na2O-SiO2-TiO2-ZrO2) was lower in SiO2 rich composition and higher in TiO2 or ZrO2 rich compositions. It is assumed that addition of Na2O changed the structure of amorphous TiO2 and ZrO2 someway to increase their thermal stability or amorphous forming ability. After DTA run up to 1000°C, cystalline phases precipitated in the 5Na2O·95(SiO2 + TiO2 + ZrO2) gels were determined by XRD analysis.

Changes in the Zr environment in zirconia–silica xerogels with composition and heat treatment as revealed by Zr K-edge XANES and EXAFS

X-ray absorption spectroscopy at the Zr K-edge is an important technique for probing the environment of Zr. Here it is applied to zirconia–silica xerogels with composition 0.07⩽x⩽0.40, where x is the molar ratio Zr:(Zr+Si). Reference samples of crystalline ZrO2, ZrSiO4, BaZrO3 and liquid Zr n-propoxide were also examined. New XANES (X-ray adsorption near edge structure) results are presented for zirconia–silica xerogels, and compared with previous EXAFS (extended X-ray absorption fine structure) results. For high Zr contents (x=0.4) there is a separate, amorphous ZrO2 phase, which before heat treatment is similar to Zr hydroxide, and after heat treatment at 750°C is similar to an amorphous precursor of tetragonal ZrO2. For low Zr contents (x=0.1) there is atomic mixing of Zr in the SiO2 network, and the environment of Zr is more similar to that in Zr n-propoxide compared to other reference samples. New in situ XANES and EXAFS results are presented for x=0.1 xerogels heated at 250°C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.

Microstructural characterization of plasma-sprayed hydroxyapatite-10 wt% ZrO2 composite coating on titanium.

Previous research showed that the concept of adding ZrO2 as second phase to hydroxyapatite (HA) significantly increased the bonding strength of plasma-sprayed composite material. The present work aimed to investigate the microstructural characteristics of plasma-sprayed hydroxyapatite-10 wt% ZrO2 composite coating on titanium using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). In TEM, phases such as HA, amorphous calcium phosphate, alpha-TCP, ZrO2 and minor transformed CaZrO3 are identified. The cubic phase of ZrO2 in HA-10 wt% ZrO2 powders before coating maintains during plasma spraying, and zirconia particle apparently bonds well to the calcium phosphate matrix with local crystallographic relationship. The transformed CaZrO3 does not exist as interface interphase between calcium phosphate matrix and zirconia particle. Instead, reaction of calcium phosphate and zirconia occurs rapidly to transform ZrO2 into CaZrO3. The toughening mechanism of the material studied and its biological implication of the system are discussed.

X-ray diffraction study of phase transformation in hydrolyzed zirconia nanoparticles

The occurrence of metastable tetragonal ( t′) phase and its transformation into monoclinic (m) in nanocrystalline ZrO 2 were systematically studied by using X-ray diffraction with respect to the grain size and processing conditions. The zirconia particles in the average size range 7–20 nm were prepared by hydrolyzis of zirconium oxy-chloride solution for different hydrolyzing periods. Drastic changes in grain size and the phase formation with the hydrolyzing time were observed. The amorphous ZrO 2 crystallizes in the t′ phase around 718 to 753 K on heating. However, if the crystallization occurs during the hydrolyzis itself, the monoclinic phase is formed even the grain size is less than 10 nm. The microstructural defects seem to dominate the grain size effect causing the reduction in crystal symmetry in the nanocrystalline zirconia obtained after 72 h of hydrolyzis. ©

Influence of oxygen flow rate on the structural and mechanical properties of reactively magnetron sputter-deposited Zr–B–O coatings

Zr–B–O coatings were deposited on both construction steel and stainless steel substrates by magnetron sputtering of ZrB 2 targets in reactive Ar–O 2 gaseous mixtures. In this paper, the deposition rate and the structural and mechanical properties of Zr–B–O films will be investigated as a function of the inlet oxygen flow rate. We will show that two domains are available owing to the sputtering phenomena. In the former, the coating enrichment is quite proportional to the oxygen flow rate. Even for a weak oxygen enrichment, amorphization of the Zr–B–O films is observed which also decreases both their intrinsic internal stress and microhardness, thus improving the physical quality of the coatings. As the oxygen flow rate increases, the amorphous coatings, initially composed of a single-phase ZrB 2− x O x solid solution, present an increasing fraction of amorphous ZrO 2, and then, of amorphous B 2O 3 simultaneously with the disappearance of the metallic solid solution. Higher oxygen flow rates allow the synthesis of optically transparent amorphous oxide mixture films, the composition and mechanical properties of which become independent of the inlet oxygen flow rate.

Photon- and electron-stimulated desorption of O + from zirconia

We present a study of the photon- and electron-simulated desorption (PSD and ESD) of cations from yttria-stabilized cubic ZrO 2(100) and undoped amorphous ZrO 2 surfaces. For both types of zirconia, O + is the primary ionic desorption product. A weak threshold for ESD of O + from yttria-stabilized cubic zirconia (YSZ) is observed at ∼26–27 eV with a rapid rise above ∼30 eV. The PSD threshold from YSZ is ∼31±1 eV. This is essentially the same as the O + ESD and PSD thresholds (∼30±1 eV) from undoped amorphous ZrO 2 surfaces. The PSD O + kinetic energy distributions extend from 0 to ∼7 eV with a peak at ∼2 eV and are similar from both surfaces. A comparison of the ion threshold data with photoelectron spectra indicates that desorption of O + is primarily initiated by excitation of the Zr(4p) core level. All of the evidence is consistent with a desorption mechanism, in which the O + ions are produced and ejected from the surface via a multi-electron Auger decay process.

Structure of (ZrO2)x(SiO2)1-x xerogels (x=0.1, 0.2, 0.3 and 0.4) from FTIR, 29Si and 17O MAS NMR and EXAFS

A combination of 29Si and 17O MAS NMR, EXAFS and FTIR spectroscopy has been used to study the atomic structure of (ZrO2)x(SiO2)1-x (x=0.1, 0.2, 0.3 and 0.4) xerogels prepared by reacting partially hydrolysed tetraethyl orthosilicate with zirconium(IV) propoxide. Results from (ZrO2)0.1(SiO2)0.9 samples reveal the oxides to be atomically mixed with no evidence of phase separation. In these samples, the nearest neighbour environment of zirconium is similar to that found in cubic zirconia. In the (ZrO2)0.4(SiO2)0.6 samples, phase separation occurs with a significant proportion of the zirconium present as amorphous ZrO2 with a local structure similar to that of monoclinic zirconia. 17O MAS NMR and EXAFS have proven valuable techniques for gauging the level of atomic mixing in these materials.

Synthesis of ZrO2 and Y2O3‐Doped ZrO2 Thin Films Using Self‐Assembled Monolayers

Undoped or Y2O3‐doped ZrO2 thin films were deposited on self‐assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single‐crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO4)·4H4O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal‐ (t‐) ZrO2 and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t‐ZrO2. Amorphous Y2O3‐containing ZrO2 films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y2(SO4)38·H2O), and urea (NH2CONH2) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.

TEM characterization of plasma-sprayed thermal barrier coatings and ceramic–metal interfaces after hot isostatic pressing

Plasma-sprayed thermal barrier coatings (TBCs) consisting of both a Ni–5%wt. Al bond coat (Ni–Al) and a thermally insulating 8%wt. Y 2O 3 partially stabilized ZrO 2 ceramic top coat were subjected to hot isostatic pressing (HIP). Transmission electron microscopy (TEM) with qualitative microanalysis was utilized to characterize the microstructural changes occurring during HIP treatment and to clarify the nature of the ceramic–metal interface [ZrO 2–(Ni–Al)] both in the as-sprayed and hipped states. During HIP treatment, the as-sprayed ZrO 2 experienced a certain degree of hot plastic deformation and recrystallization, forming dense and crackless fine-equiaxed tetragonal grains. However, due to nonuniformities of the as-sprayed structure, the degree of recrystallization of the original columnar grains changed from location to location, resulting in a recrystallized microstructure ranging from coarse equiaxed grains 1–2 μm in size to co-existing grains of equiaxed and columnar shapes, including still intact columnar grains. Through TEM observations of a plasma-sprayed ZrO 2–(Ni–Al) interface more than 20 μm long, it was found that the interface is amorphous in nature and unevenly thick and even discontinuous in some locations. The amorphous interfacial layer is composed largely of the Ni–Al bond coat materials together with some ZrO 2 ceramic material. After HIP treatment, the amorphous ZrO 2–(Ni–Al) interfacial layer is replaced by a continuous, dense and polycrystalline Al 2O 3 layer which can act as a diffusion barrier for elements both from the ZrO 2 coating to the bond coat and to the substrate, and also in the opposite direction.

Atomic-scale structure of ZrO 2 xerogels by X-ray diffraction and reverse Monte Carlo simulations

Structure functions for two amorphous ZrO 2 xerogels have been determined by X-ray diffraction experiments. Three-dimensional structure models consistent with the experimental data have been constructed by reverse Monte Carlo simulations. Partial distribution functions, interatomic distances and coordination numbers have been extracted from the model atomic configurations. Local atomic ordering in the as dried ZrO 2 xerogel resembles crystalline monoclinic ZrO 2 and the xerogel annealed at elevated temperatures resembles that in crystalline tetragonal ZrO 2. A comparison between structure data for zirconia xerogels obtained under different preparation conditions was made and evidence was found that thermal treatment at elevated temperatures and preparation conditions affect the atomic arrangement of amorphous ZrO 2. A structural mechanism explaining why zirconia gels with different local atomic arrangements may be obtained under different preparation and thermal conditions is advanced.

Growth of oriented anodic films on single grains of Zr: structure and epitaxy from anisotropy-micro-ellipsometry

Anodic passive films formed potentiodynamically (20 mV s −1) on polycrystalline zirconium in 0.5 M H 2SO 4 have been investigated on single grains by Anisotropy-Micro-Ellipsometry (AME). Substrate grain orientations were determined from AME-curves ( Δ Ψ vs rotation angle α around the surface normal) recorded on freshly electropolished samples. For Zr ZrO 2 , an increase in amplitude along with a shift in the phase relation of the AME curves is observed with increasing anodic layer thickness on all grain orientations except (0001). These effects are caused by the formation of crystalline, optically anisotropic oxide films exhibiting grain-dependent epitaxy relations with the Zr substrate. From the resulting curves, the crystal orientation of the passive film in addition to layer thickness was determined for several substrate grains. For basal (0001)-oriented grains ( ϑ MET = 0 °), amorphous or nanocrystalline ZrO 2 films are formed as indicated by the absence of anisotropy effects. On less densely packed surfaces ( ϑ s = 10 °–60 °), ordered crystalline ZrO 2 films of the monoclinic phase are grown exhibiting orientations of 45 and 22.5 deg. to (001), respectively, in systematic dependence on the substrate grain orientation. Similar differences found in the oxide formation factors correlate with packing density and structure of the anodic oxides. Comparison with photocurrent measurements at 4.8 eV sub-bandgap photon energy yields additional differences in the electronic properties of the films. The results prove a strong texture-dependence of passive film growth on polycrystalline Zirconium.