Temperature dependence of the elastic and inelastic moduli of YBa2Cu3O7−δ ceramics

Elastic and inelastic moduli of the superconducting yttrium-barium oxide ceramics YBa₂Cu₃O_7−δ (YBCO) were studied using the ultrasonic resonance technique. In the range from room temperature up to the temperature of the ortho-tetra phase transition, the elastic and inelastic moduli of YBCO samples exhibit a number of anomalies. It is shown that these features are related to the behavior of active oxygen in the ceramics, the phase inhomogeneity of the material, and the phase transitions.     

Ultrasonic study and physical properties of some borate glasses

The structure of the glass system (75−x)B₂O₃–xBi₂O₃–25Li₂O, where x=5, 10, 15, and 20 mol% was investigated by using pulse-echo technique. Elastic properties of the glass system have been calculated together with Poisson’s ratio and Debye temperature from the measured densities as well as longitudinal and shear ultrasonic velocities at room temperature. Ultrasonic velocity measurements were taken at 4 MHz. Estimated parameters based on Makishima–Mackenzie theory and bond compression model were calculated in order to analyse the experimental elastic moduli. Addition effect of Bi₂O₃ on the elastic moduli was investigated in terms of the number of network bonds and the mean cross-link density of the glass systems. The average atomic ring size of the network was also calculated and it was found that it depends on the concentration of the modifiers. Composition dependence of the elastic moduli and Poisson’s ratio from one hand and the trend of both the ring diameter and the packing density from the other hand showed structural change at x=15 mol%.     

Effect of grain size on elastic modulus and hardness of nanocrystalline ZrO2-3 wt% Y2O3 ceramic

Dense nanocrystalline ZrO₂-3 wt% Y₂O₃ ceramics with grain sizes ranging between 23 to 130 nm were tested by ultrasonic pulse echo and Vickers hardness. The elastic modulus and hardness results were corrected for the residual porosity and the phase content. The corrected elastic moduli exhibited continuous decrease with decrease in the grain size. In contrast, no correlation was found between the corrected hardness and grain size. The percolative composite model was used to describe the changes in the elastic moduli in terms of percolation of the elastic wave through the intercrystalline phase at the percolation threshold. The absence of correlation with the hardness results was explained due to the other energy absorbing mechanisms such as microcracking beneath the indenter.     

Enhanced leakage current properties of HfO2/GaN gate dielectric stack by introducing an ultrathin buffer layer

The band alignments of HfO₂/GaN, HfO₂/SiO₂/GaN and HfO₂/Al₂O₃/GaN gate dielectric stacks were comparatively investigated by using X-ray photoelectron spectroscopy. It was observed that the introduction of an ultrathin buffer layer film (SiO₂ or Al₂O₃) in HfO₂/GaN stack can make the band alignments more symmetrical with larger barrier height as identified by the valence band offsets and electron energy loss spectrum measurements. At room temperature, the leakage current density as function of temperature is 4.1 × 10^?6, 3 × 10^?7 and 9.8 × 10^?8 A cm^?2 at the bias of 1 V for the HfO₂/GaN, HfO₂/Al₂O₃/GaN and HfO₂/SiO₂/GaN gate dielectric stacks, correspondingly. With temperature increase from room temperature to 300 °C, the HfO₂/SiO₂/GaN gate dielectric stack exhibits lowest lower leakage current density than that of others. The HfO₂/GaN high-k gate dielectric stack with an ultrathin SiO₂ buffer layer appears to be a promising candidate for future GaN based high temperature metal-oxide-semiconductor (MOS) devices applications.     

Study of electrical and micro-structural properties of high-κ gate dielectric stacks deposited using pulse laser deposition for MOS capacitor applications

The electrical properties of hafnium oxide (HfO₂) gate dielectric as a metal–oxide–semiconductor (MOS) capacitor structure deposited using pulse laser deposition (PLD) technique at optimum substrate temperatures in an oxygen ambient gas are investigated. The film thickness and microstructure are examined using ellipsometer and atomic force microscope (AFM), respectively to see the effect of substrate temperatures on the device properties. The electrical J–V, C–V characteristics of the dielectric films are investigated employing Al–HfO₂–Si MOS capacitor structure. The important parameters like leakage current density, flat-band voltage (V_fb) and oxide-charge density (Q_ox) for MOS capacitors are extracted and investigated for optimum substrate temperature. Further, electrical studies of these MOS capacitors have been carried out by incorporating La₂O₃ into HfO₂ to fabricate HfO₂/La₂O₃ dielectric stacks at an optimized substrate temperature of 800 °C using a PLD deposition technique under oxygen ambient. These Al–HfO₂–La₂O₃–Si dielectric stacks MOS capacitor structure are found to possess better electrical properties than that of HfO₂ based MOS capacitors using the PLD deposition technique.     

Transformation of Na2O-HfO2-B2O3 glass into a material having interconnected pores

Substitution of SiO₂ in the ternary sodium borosilicate system with HfO₂ was found to produce glasses, which after heat treatment decomposed into immiscible microphases, one of which was water soluble. The structure of the leached material after heat treatment was either glassy (mainly in the presence of Al₂O₃) or crystalline. Crystalline forms found during X-ray diffraction analysis of heat treated and leached material (melted in Pt/Rh crucibles) were monoclinic HfO₂. Monoclinic HfO₂ was also found in heat treated, leached and then fired materials melted in Pt/Rh or Al₂O₃ crucibles, in the latter an additional 9Al₂O₃ · 2B₂O₃ phase was detected. The higher solubility of HfO₂ in a Na₂O-B₂O₃ matrix than that of ZrO₂ (30 wt% against 15 wt%) resulting in clear glasses is of interest. The specific surface areas of the leached materials ranged between 41.3 and 290 m²g^–1, while the mean radii of interconnected pores were calculated to be 1.2 and 15.2nm. A firing temperature between 1450 and 1500° C is estimated from void volume and bulk density measurements.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Combined spectroscopic ellipsometry and attenuated total reflection analyses of Al2O3/HfO2 nanolaminates

The microstructure of thin HfO₂-Al₂O₃ nanolaminate high κ dielectric stacks grown by atomic vapor deposition has been studied by attenuated total reflection spectroscopy (ATR) and 8 eV spectroscopic ellipsometry (SE). The presence of Al₂O₃ below HfO₂ prevents the crystallisation of HfO₂ if an appropriate thickness is used, which depends on the HfO₂ thickness. A thicker Al₂O₃ is required for thicker HfO₂ layers. If crystallisation does occur, we show that the HfO₂ signature in both ATR and 8 eV SE spectra allows the detection of monoclinic crystallites embedded in an amorphous phase.     

Elastic properties of phosphotungstate glasses

Elastic properties of potassium and lead phosphotungstate glasses have been investigated using ultrasonic velocity measurements. The composition dependence of elastic moduli in WO₃-K₂O-P₂O₅ glasses suggests that at low alkali oxide concentrations the atomic ring size increases by network modification, which results in the decrease of elastic moduli. In the highly modified regime, due to the presence of coulombic interaction, the rate of decrease of elastic moduli is reduced. In the WO₃-PbO-P₂O₅ glasses the behaviour of elastic moduli suggests that PbO behaves both as a network former and network modifier. The incorporation of PbO into the network is quantitatively determined by the concentration of P₂O₅ in the system. The results are consistent with the structural model proposed earlier, based on characterization studies.     

Simultaneous determination of the thermal expansion coefficient and the elastic modulus of Ta2O5 thin film using phase shifting interferometry

Abstract

This paper reports on the application of a phase shifting interferometry technique for the concurrent measurement of the thermal expansion coefficient α_f and the elastic modulus E_f /1 - V_f of Ta₂O₅ thin film. The Ta₂O₅ films were prepared by ion beam sputter deposition. The stresses in the thin films were measured with the phase shifting interferometry technique using two types of circular discs with known thermal expansion coefficients, Young's moduli and Poisson's ratios. The temperature-dependent stress behaviour of Ta₂O₅ films was obtained by heating samples in the range from room temperature to 70°C. The internal stresses of Ta₂O₅ thin films deposited on the BK-7 and Pyrex glass substrates were plotted against the stress measurement temperature, showing a linear dependence. From the slopes of the two lines in the stress versus temperature plot, the thermal expansion coefficient and the elastic modulus of Ta₂O₅ thin film are then calculated.     

Elastic constants of the iron oxide doped yttria-stabilized zirconia

Ultrasonic compressional and shear wave velocity measurements have been made on the (ZrO₂)_1−A (Y₂O₃)_A−X (Fe₂O₃) _X system at 5 MHz. The amounts of Fe₂O₃ dopant ranging from 0 up to 10 mol% were mixed with the ceramic matrix. In this paper the ultrasonic compressional and shear wave velocity measurements made on this system at room temperature are discussed. The elastic moduli, Poisson's ratio and density are found to be sensitive to the additions of the Fe₂O₃ dopant and they show an anomalous behaviour. A qualitative explanation of the compositional dependence of the elastic moduli and Poisson's ratio is given in terms of changes in packing density, bond strength, co-ordination number and cross-link density.     

Study of rigidity of semiconducting vanadate glasses and its importance in use of coatings

The elastic moduli of some multicomponent vanadate based glasses were analysed in terms of the bond compression model by some physical parameters such as, the density, average stretching force constant and average atomic ring size. These parameters were calculated for all the glass series and for all the glass composition to estimate the rigidity of these glasses. The results showed that the average force constant and the elastic moduli of these glasses are sensitive to the decrease in PbO content. This behaviour was attributed to the increase in the molar volume and the role of different modifiers. These parameters along with the coordination number of the glasses affect the glass transition temperature. The correlation between the elastic moduli and thermal properties of these samples showed that 0·25MoO₃–0.25PbO–0·5V₂O₅ glass is the most rigid and has an applicable glass transition temperature for coating.     

Discussion of the indentation hardness of a glass-ceramic with participate microstructure

On the basis of a theory previously developed by the authors for the indentation hardness of glass matrix, particulate composites, an attempt was made to interpret published hardness data for a ZnO-Al₂O₃-SiO₂ glass-ceramic in which gahnite (ZnAl₂O₄) crystal particles are dispersed in a glass matrix as a major crystalline phase. The elastic moduli for gahnite were estimated using both the bulk modulus-molar volume relationship and the density-Poisson's ratio relationship, established for oxide crystals. After determining the variation of the matrix Young's modulus with heat-treatment, the variation of the overall hardness with volume fraction of crystal phase as well as the crystal-size effect were discussed. The hardness behaviour of the present glass-ceramic could be interpreted well in terms of the properties and amounts of the constituent phases and the microstructural effects.     

Synthesis, characterization and devitrification behaviour of an yttrium containing boroaluminate glass

The glass forming region in the B₂O₃-Al₂O₃-Y₂O₃ composition diagram has been determined by a melting and quenching procedure at temperatures up to 1800°C. Different physical characteristics (density, coefficient of thermal expansion, glass transition and crystallization peak temperatures) have been determined for a 35B₂O₃-40Al₂O₃-25Y₂O₃ glass composition (in mol.%). By using a predictive model and some NMR structural data, different elastic moduli (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) have been calculated. The devitrification behaviour has also been studied. Internal crystallization is the dominant mechanism and a new (Y, Al)BO₃ ternary phase has been characterized by X-ray powder diffraction. The temperature and time nucleation dependence have been determined from DTA experiments as well as the crystallization kinetics (i.e. the Avrami exponent and the activation energy for crystal growth).     

Effect of yttrium on the physical,elastic, and structural properties of neodymium-doped lead borotellurite glass

In this study, a series of glass systems with a composition of (49???x)H₃BO₃–35TeO₂–15PbO–1.0Nd₂O₃–xY₂O₃ (x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol%) was prepared via melt-quench technique. The effect of Y₂O₃ content on the density, molar volume, oxygen packing density, elastic, and structural properties were investigated. The density and molar volume were found to vary concerning Y₂O₃ content. Ultrasonic non-destructive testing was employed at room temperature for measuring the elastic properties of the prepared glass samples. The density data, ultrasonic velocities, and measured elastic moduli were found to be composition dependent with Y₂O₃ content. The Y₂O₃ modifier was expected to change the physical properties of neodymium-doped lead borotellurite glasses. The structure analyzed using X-ray diffraction was confirmed the amorphous nature of the prepared glasses. Fourier Transform Infrared Spectroscopy has been carried out and reveals that the inclusion of Y₂O₃ into the glass system tends to increase the number of bridging oxygen atoms. All element study of the glass sample was proved by energy dispersive X-ray analysis.

     

Synthesis of ultrafine pure and yttria-stabilized hafnia by solid-state reaction at relatively low temperature

Highly reactive, ultrafine powders of pure hafnia as well as stabilized hafnia (8 mol% yttria) have been synthesized at relatively low temperature (500 °C) by solid-state reaction between standard coarse-grained powders of sodium metaphosphate (NaPO₃) and sodium hafnate (Na₂HfO₃). The reaction product was found to be a two-phase mixture of sodium orthophosphate (Na₃PO₄) and hafnia. Subsequently, the orthophosphate phase was removed by washing with dilute nitric acid. The as-prepared powders were extremely fine (particle size < 20 nm) and crystalline (cubic fluorite structure). After cold pressing and heating to 1650 °C, the materials retained a fine-grained microstructure. Stable cubic HfO₂ with Y₂O₃ addition (8 mol%) was obtained at a temperature which was not too high (1650 °C).     

Phase equilibria and ordering in the system HfO2-Yb2O3

The system HfO₂-Yb₂O₃ was investigated in the 0 to 100 mol % Yb₂O₃ range using X-ray diffraction analysis, linear thermal expansion measurements and melting point studies. At high temperatures, the system is dominated by wide regions of solid solutions based on HfO₂ and Yb₂O₃ separated by a two-phase field which appears to extend to the solidus. The extent of the cubic hafnia and ytterbia C-type solid solution fields was established using the precision lattice parameter method. At low temperature (< 1800° C) two ordered phases were found in the system, one at 40 mol % ytterbia with ideal formula Yb₄Hf₃O₁₂, and another at 70 mol % ytterbia with formula Yb₆HfO₁₁. Four eutectoid reactions and a peritectic reaction cubic ytterbia solid solution cubic hafnia solid solution + liquid at 67 mol % and 2380° C have been established in the system. By incorporating the known tetragonal-cubic hafnia and C-type-hexagonal ytterbia transition temperatures, and the melting points data in the system, a tentative phase diagram is given for the system HfO₂-Yb₂O₃.     

Ultrasonic studies of (TeO2)50–(V2O5)50−x(TiO2)x glasses

New ternary tellurite glasses in the form (TeO₂)₅₀–(V₂O₅)_50−x(TiO₂)_x have been prepared. Both longitudinal and shear ultrasonic velocities were measured in different compositions of the glass system by using the pulse-echo method at 5 MHz frequency and at room temperature. The elastic properties of ternary telluirte glasses (TeO₂)₅₀–(V₂O₅)_50−x(TiO₂)_x were measured as a function of composition. The ultrasonic velocity data, the density, the calculated elastic moduli, micro-hardness, softening temperature, and Debye temperature depend on the glass composition. By calculating the number of network bonds per unit volume, the average stretching force constant, and the average ring size, information about the structure of the glass can be deduced. Comparison between the calculated and the experimental elastic moduli and Poisson's ratio have been carried out.     

Low-temperature ozone passivation for improving the quality of sputtered HfOx thin-film

In this study, we proposed the low temperature (150 °C) oxygen (O₂) and ozone (O₃) treatments with a high pressure of 68 atm to improve the quality of sputtered hafnium-oxide (HfO_x) thin-film dielectric. XPS results indicate that the binding energy of Hf-O became stronger and traps in dielectric film could be effectively passivated after the O₂ and O₃ treatments. The accumulation capacitance for the as-deposited HfO_x sample can be improved from 261 pF to 303 pF and 366 pF for the O₂ and O₃ treated samples, respectively. The O₃ treated sample also exhibits the negligible hysteresis phenomenon and slight flatband voltage shift.     

Atomic layer deposition grown composite dielectric oxides and ZnO for transparent electronic applications

In this paper, we report on transparent transistor obtained using laminar structure of two high-k dielectric oxides (hafnium dioxide, HfO₂ and aluminum oxide, Al₂O₃) and zinc oxide (ZnO) layer grown at low temperature (60 °C-100 °C) using Atomic Layer Deposition (ALD) technology. Our research was focused on the optimization of technological parameters for composite layers Al₂O₃/HfO₂/Al₂O₃ for thin film transistor structures with ZnO as a channel and a gate layer. We elaborate on the ALD growth of these oxides, finding that the 100 nm thick layers of HfO₂ and Al₂O₃ exhibit fine surface flatness and required amorphous microstructure. Growth parameters are optimized for the monolayer growth mode and maximum smoothness required for gating.