Influence of Water Vapor on the Response of Mg-Doped SrTio3 Ceramic Oxygen Sensors

Mg-doped SrTio₃ thick film sensors fabricated by screen-printing proved to be very promising for the use as oxygen sensors. A study of the influence of water on the response of these sensors gives an important basis for understanding their behavior in practical applications. The influence of water on the sensor response was measured in the oxygen partial pressure region from air (0.21 bar) to pure N₂ (2.5 × 10^–5)and the temperature range from 600 to 800°C. The relative humidity was varied from 1 to 95% RH. The resistance variation as a function of temperature and the activation energy were evaluated under different dry and wet conditions. The results obtained show that the resistance of these sensors generally decreases with increasing water content in the carrier gas and that the effect of water was strongest at lower temperatures as well as at lower oxygen pressures. To explain this behavior, it is proposed that a partial proton conduction is introduced in the water-containing atmospheres and that this contributes to the total conductivity leading to a reduction of the total resistance. Finally, the measurements also show that the response of these sensors still depends on the oxygen partial pressure according to the standard expression even in the presence of water vapor. Therefore, these sensors can still be used as oxygen sensors in humid atmospheres.     

Effects of water on the properties of Fe3O4@C superparamagnetic nanospheres

ABSTRACT

Fe₃O₄@C nanospheres have potential applications in the field of magnetic separation, magnetic hyperthermia, magnetic targeting and magnetic resonance imaging (MRI). The carbon layers formed around Fe₃O₄ nanopaticles determined the adsorption properties of this material. In this study, Fe₃O₄@C was prepared by a simple one-pot solvothermal method, in which different amount of water was added to investigate the influences of water on the properties of Fe₃O₄@C nanospheres. The resulted samples were characterized by techniques of TEM, FT-IR, UV–Vis and BET, and found that the thickness of carbon layer, the intensity of surface carboxyl group and the adsorption characters were significantly changed by introducing additional water in autoclave.     

Characterization of the CeO2 thin films for insulation layer and Pt/SrBi2Ta2O9/CeO2/Si MFISFET structure

Abstract

CeO₂ and SrBi₂Ta₂O₉ (SBT) thin films for MFISFET (metal-fcrroelectrics-insulator-semiconductor field effect transistor) were deposited by rf sputtering and pulsed laser deposition method, respectively. The effects of oxygen partial pressure during deposition for CeO₂ films were investigated. The oxygen partial pressure significantly affected the preferred orientation, grain size and electrical properties of CeO₂ films. The CeO₂ thin films with a (200) preferred orientation were deposited on Si(100) substrates at 600°C. The films deposited under the oxygen partial pressure of 50 % showed the best C-V characteristics among those under various conditions. The leakage current density of films showed order of the 10^?7～10^?8 A/cm² at 100 kV/cm. The SBT thin films on CeO₂/Si substrate showed dense microstructure of polycrystalline phase. From the C-V characteristics of MFIS structure composed of the SBT film annealed at 800°C, the memory window width was 0.9 V at ±5 V. The leakage current density of Pt/SBT/CeO₂/Si structure annealed at 800°C was 4×10^?7 A/cm² at 5 V.     

The origin and elimination of random single-bit failure in high density 1t1c fram

Abstract

The COB (capacitor-over-bit line) structure is essential for integrating high density FRAM (ferroelectric random access memory) device. It is very difficult in the COB cell structure to maintain low contact resistance between plug material and bottom electrode during the integration processes. We have used CoSi_x/poly-Si and Pt/IrO₂/Ir as plug materials and bottom electrode stack, respectively. However, random single-bit failure was observed due to the high contact resistance, which was caused by the formation of amorphous oxide layer, especially, during high temperature anneal for PZT (Pb[Ti_x,Zr_1-x]O₃) crystallization. It was observed that silicon atoms diffuse even into Ir oxygen barrier, thus leading to the oxide formation between the plug and Ir oxygen barrier. The approach we have taken to prevent contact resistance failure is to optimize the plug material that has high endurance against oxidation reaction. In this study, we developed tungsten plug technology to eliminate the random single-bit failure. It is expected that this tungsten plug technology can provide highly reliable COB structure for 32Mb FRAM device and beyond.     

High temperature conductivity behavior of doped SrTiO3 thin films

Abstract

In order to investigate the dominant charge transport mechanisms of doped SrTiO₃ thin films, high temperature measurements were performed under varying oxygen partial pressures. To meet specific demands of SrTiO₃ thin films, a common 4-point measuring setup was improved profoundly by full triaxial shielding and the use of a solid state oxygen pump (made of YSZ). This allowed a precise analysis in the temperature range from 700°C to 1000°C and at oxygen partial pressures (pO₂) between 10^?20 bar and 1 bar. The conduction behavior of (doped) SrTiO₃ thin films, as a function of pO₂, revealed characteristics that substantially differ from those of bulk ceramics and cannot be explained by point defect chemistry. Additionally, segregation effects have been observed which lead to a restructuring of the film's morphology to a significant extent.     

BIPOLAR RESISTIVE SWITCHING IN Au/TiO2/Pt THIN FILM STRUCTURES

ABSTRACT

The conditions and physical mechanisms of electroforming and subsequent resistive switching in Au/TiO₂/Pt thin film structures were investigated. It was concluded that the electroforming, being a current-limited electric breakdown of the TiO₂ films, resulted in a considerable increase of the oxygen vacancy concentration in the bulk and on the surface of the films. The resistive switching implemented by short voltage pulses of different polarities is proposed to be due to the change of the Schottky barrier height at the Pt/TiO₂ interface as a result of the current-induced variation of the occupancy by electrons of the surface states in the band gap of TiO₂.     

Integration of Novel Capacitor Structure for High Density FeRAM With Barrier Metal TiAlN and (Bi,La)4Ti3O12

16M 1T1C ferroelectric random access memory was successfully integrated by newly proposed scheme named Recessed Open Barrier (ROB) structure with planar Pt/BLT/Pt/IrOx/Ir stacked capacitor. In conventional barrier metal scheme which bottom electrode was stacked, oxygen diffused through the bottom electrode interface formed during MTP structure integration, and oxidized the edge of barrier metal resulting compressive stress. The failure was accelerated by free oxygen diffusion as the stress built up to make the bottom electrode pop-up. In this paper, the barrier metal TiAlN was recessed and refilled by ALD Al₂O₃ after bottom electrode patterning. By this novel structure, oxidation resistance was greatly improved, because oxygen diffusion path to BM was much longer than conventional scheme. The contact resistance of storage node was below 10 kΩ/plug after all the thermal budget relevant to BLT capacitor.     

Growth and characterization of epitaxial SrBi2Ta2O9 films on (110) SrTiO3 substrates

Abstract

SrBi₂Ta₂O₉ (SBT) is an attractive material for nonvolatile ferroelectric memory applications. In this paper we report on the deposition of highly epitaxial and smooth SrBi₂Ta₂O₉ films on (110) SrTiO₃substrates. The films were grown by pulsed laser deposition at temperatures ranging from 600 to 800°C and at various laser fluences from a Bi-excess SBT target. The background oxygen pressure was maintained at 28 Pa during the film deposition. Structural characterization of the films was performed by x-ray diffraction. Atomic force microscopy was used to investigate morphology and growth of the films. The films grew with preferred (115) or (116) orientation. The roughness was of the order of unit cell height. The films display a growth pattern resulting in corrugated film morphology.     

Praseodymium-cerium oxide thin film cathodes: Study of oxygen reduction reaction kinetics

Praseodymium-Cerium Oxide (Pr_xCe_1-xO_2−δ; PCO), a potential three way catalyst oxygen storage material and solid oxide fuel cell (SOFC) cathode, exhibits surprisingly high levels of oxygen nonstoichiometry, even under oxidizing (e.g. air) conditions, resulting in mixed ionic electronic conductivity (MIEC). In this study we examine the redox kinetics of dense PCO thin films using impedance spectroscopy, for x = 0.01, 0.10 and 0.20, over the temperature range of 550 to 670°C, and the oxygen partial pressure range of 10⁻⁴ to 1 atm O₂. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to electrode area, pointing to surface exchange rather than bulk diffusion limited kinetics. The large electrode capacitance (10⁻²F) was found to be consistent with an expected large electrochemically induced change in stoichiometry for x = 0.1 and x = 0.2 PCO. The PCO films showed surprisingly rapid oxygen exchange kinetics, comparable to other high performance SOFC cathode materials, from which values for the surface exchange coefficient, k ^q , were calculated. This study confirms the suitability of PCO as a model MIEC cathode material compatible with both zirconia and ceria based solid oxide electrolytes.     

Water molecule adsorption properties on surfaces of MVO4 (M = In, Y, Bi) photo-catalysts

Water molecule adsorption properties at the surface of InVO₄, YVO₄, and BiVO₄, which have indicated promising photocatalytic properties, have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V (3c-V) sites on the (001) surface and 4c-In sites on the (100) surface of InVO₄, however, in a non-dissociated molecular form on the 5c-Bi site on the (100) surface of BiVO₄ and the 7c-Y site on the (010) surface of YVO₄. The adsorption energies of water molecule and geometries of the systems with adsorbed water molecules are reported in detail.     

The electronic structure of Ni doped rutile TiO2

The electronic structure of 6.25% Ni doped rutile titanium dioxide (Ni/TiO₂) was investigated by ab initio ultrasoft pseudopotential plane wave method based on density functional theory. When a Ni atom was substituted for a Ti site of TiO₂ lattice, two additional unoccupied 3d states appeared as defect states in band gap, and occupied 3d states were hybridized with oxygen 2p states. From the calculation of band decomposed charge density and density of states, it was found that the lower defect state (D₁), a Ni 3d-orbital, was hybridized with unoccupied oxygen 2p states and the higher one (D₂), localized 3d-orbital of Ni, was located at the middle of band gap.     

Co-Doping Effect of Mn and Y on Charge and Mass Transport Properties of BaTiO3

BaTiO₃-based multilayer-ceramic capacitors (MLCC) using base metal (Ni) electrodes normally contains Mn and Y each approximately on the order of 0.5 mol%. It is only empirically known that the co-doping of Y and Mn facilitates sintering with the base-metal electrodes as well as improves the device performance and life time. In order to understand the effect of the co-doping, we have measured the electrical conductivity and chemical diffusivity on polycrystalline BaTiO₃ that is co-doped with Y and Mn each by 0.5 mol% against oxygen partial pressure at elevated temperatures. It is found that while the n-type conductivity in reducing atmospheres (e.g., Po₂ < 10^{– 6} atm at 1000C) remains similar to that of undoped or acceptor-doped BaTiO₃, its p-type conductivity in oxidizing atmospheres (e.g., Po₂ > 10^{– 6}at 1000C) is remarkably suppressed compared to the latter. The chemical diffusivity is also similar to that of the latter in magnitude (e.g., 10^{– 2}–10^{– 5} cm²/s at 1000C), but its trend of variation with oxygen partial pressure is rather opposite. These variations of the conductivity and chemical diffusivity are mainly attributed to Mn ions changing their valence from +2 to +3 to +4 with increasing oxygen partial pressure. It is explained from a defect-chemical view why the codoping of fixed-valent donor (Y) and variable-valent acceptor (Mn) has been practiced in MLCC processing.     

Protonic Conduction in SrY0.5Ta0.5O3-Based Ceramics at Elevated Temperatures

The perovskite-type-oxide solid solution SrY_0.5+x Ta_0.5+x O_3– was synthesized and its properties were investigated. The single phase character of the samples was confirmed by X-ray diffraction when 0 x 0.02, while lines from the impurity phase SrY₂O₄ appeared in patterns of x 0.03. The conductivities of SrY_0.52Ta_0.48O_3– were about an order of magnitude higher than those of SrY_0.50Ta_0.50O₃. The results of electrochemical measurements such as emf measurements of gas concentration cells, isotope effect in conductivity, and oxygen partial pressure dependence of conductivity showed that SrY_0.52Ta_0.48O_3– exhibited pure protonic conduction in reducing atmospheres and p-type electronic conduction under high oxygen partial pressure conditions.     

Poisoning of Temperature Independent Resistive Oxygen Sensors by Sulfur Dioxide

Sulfur dioxide strongly affects temperature independent resistive oxygen sensors of SrTi_1-xFe_xO_3-. Time dependent sensor deterioration was investigated for lanthanum doped SrTi_0.65Fe_0.35O_3- (STF35). Parameters were sulfur dioxide concentration, oxygen partial pressure, temperature, and sensor morphology. The sensor poisoning consists of two steps. At lower temperatures sulfur dioxide adsorption and sulfate ion formation at the grain surface is suggested. At higher temperatures the material decomposes into SrSO₄, iron depleted STF35, and Fe₂TiO₅.     

Improvement in D-E Hysteresis Curve Squareness of PZT Thin Films—Effect of Oxygen Partial Pressure in High-Pressure Post-Annealing

A high-pressure annealing was applied to a post-annealing process for sol-gel derived PZT thin films. The squareness of D-E hysteresis curves changes depending on both total pressure and oxygen concentration. Moreover, the change follows the product of the total pressure and the oxygen concentration, which correspond to oxygen partial pressure P_O2. Where the P_O2 is higher than 0.03 MPa, few of the squareness of the hysteresis curve are excellent. The squareness of the hysteresis curves dramatically improve as the P_O2 decreases. Where the P_O2 is lower than 0.01 MPa, the squareness deteriorates slightly. These changes in the D-E hysteresis curves are thought to be explained by the generation of lead and oxygen vacancies as a function of the P_O2.     

Structure of Ferroelectric (Ba,Sr)TiO3 Thin Films for Tunable Microwave Devices

Ferroelectric (Ba,Sr)TiO₃ films have been deposited on (001) MgO single crystals by a pulsed laser deposition with oxygen background while heating the substrates. Deposited BST films exhibit epitaxial growth along (001), which are confirmed by x-ray diffraction measurement. Structure of (Ba,Sr)TiO₃ along in-plane and surface normal direction have been investigated and found to have a tetragonal distortion depend on the deposition conditions, such as oxygen pressure. Lattice parameter decreases with increasing oxygen pressure, and tetragonallity (c/a) changes from 1.005 to 0.997 as oxygen pressure increase. Interestingly, energy gap measured by FTIR decreases with decreasing oxygen pressure until it reach a certain oxygen pressure, then increases again with increasing oxygen pressure. Furthermore, microwave properties of devices measured by a HP 8510C vector network analyzer from 0.045–20 GHz suggest that the least distorted films exhibit a larger dielectric constant changes with dc bias field.     

Comparisons of direct-current breakdown characteristics in dry air and SF6 gaps using a parallel-plane arrangement with variable height protrusion

Breakdown voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to nonuniform gap in dry air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge crossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the dc voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset. The experimental observations have been explained qualitatively on the basis of a streamer model and precise electric field calculations of gap.     

Effects of Oxide Additives Coating on Microstructure and Dielectric Properties of BaTiO3

Effects of mixing methods (mechanical mixing, chemical coating) on microstructure and dielectric properties of Ho, Mg and Mn doped BaTiO₃ have been studied. BaTiO₃ particles coated with Ho, Mn and Mg were prepared by a homogeneous precipitation method using urea, and then silica was coated by the sol-gel technique. The adsorption of additives on the BaTiO₃ surface was confirmed. Temperature characteristics of capacitance were satisfied by mechanical mixing and chemical coating techniques, both of which yield different sintering and microstructure behaviors. Pyrochlore phase (Ho₂Ti₂O₇) was observed on the mechanically mixed sample, whereas none of pyrochlore phase was observed at the coated sample after thermal etching. Those different behaviors caused by the degree of homogeneous distribution of the additives in BaTiO₃ matrix were confirmed by EDS analysis.     

Degradation mechanisms of SrBi2Ta2O9 ferroelectric thin film capacitors during forming gas annealing

Abstract

The effects of annealing in forming gas (5% hydrogen, 95% nitrogen; FGA) are studied on spin coated SrBi₂Ta₂O₉ (SBT) thin films. SBT films on platinum bottom electrode are characterized with and without platinum top electrode by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), High Temperature X-Ray Diffraction (HT-XRD) and Secondary Ion Mass Spectrometry (SIMS).

High Temperature X-Ray Diffraction (HT-XRD) of blanket Ti/Pt/SBT films in forming gas revealed that the bismuth layered perovskite structure of the SBT is stable up to approx. 500°C. SIMS analysis of Pt/SBT/Pt samples annealed in deuterated forming gas (5% D₂, 95% N₂) showed that the hydrogen accumulates in the SBT layer and at the platinum interfaces next to the SBT. After FGA of blanket SBT films, tall platinum-bismuth whiskers are seen on the SBT surface.

Performing the FGA of the whole Pt/SBT/Pt/Ti stack, two different results are found. For the samples with a high temperature annealing (HTA) step in oxygen after top electrode patterning, top electrode peeling is observed after FGA. For the samples without a HTA step after top electrode patterning, no peeling is observed after FGA.     

NO2 adsorption behaviour on LaFeO3 electrodes of YSZ-based non-nernstian electrochemical sensors

X-ray photoelectron spectroscopy (XPS) was used to examine the NO₂ adsorption behaviour on the LaFeO₃ and Pt electrodes of planar yttria stabilized zirconia non-Nernstian gas sensors. The electrochemical sensors were exposed to the same gas atmosphere containing 1000 ppm NO₂ at 650°C. XPS of the as-prepared sensors and sensors after exposure to NO₂ revealed bonded nitrogen peaks on the surface of the semiconducting oxide but no nitrogen peaks on the Pt electrode. Therefore, NO₂ adsorption on a LaFeO₃ electrode plays an important role in the NO₂ detection mechanism.