Gai-Ying Yang

Oxygen nonstoichiometry and dielectric evolution of BaTiO3. Part II—insulation resistance degradation under applied dc bias

The microchemical and microstructural origins of insulation-resistance degradation in BaTiO3-based capacitors are studied by complementary impedance spectroscopy and analytical transmission electron microscopy. The degradation under dc-field bias involves electromigration and accumulation of oxygen vacancies at interfaces. The nonstoichiometric BaTiO3−δ becomes locally more conducting through increased oxygen vacancy concentration and Ti ion reduction. The symmetry across the dielectric layer and locally across each grain is broken during the degradation process. Locally, the nonstoichiometry becomes so severe that metastable lattice structures are formed. The degradation in insulation resistance at the grain boundaries and electrode interfaces is associated with the double Schottky-barrier potential lowering and narrowing. This may correlate with an effective decrease in net acceptor charge density at the grain boundaries.

Oxygen nonstoichiometry and dielectric evolution of BaTiO3. Part I-improvement of insulation resistance with reoxidation

Impedance spectroscopy, transmission electron microscopy, and electron energy-loss spectroscopy are used to correlate local electrical properties with the microstructure and microchemistry of BaTiO3 in Ni-electrode multilayer ceramic capacitors. High densities of linear defects and some grains with structural modulations are observed in BaTiO3 grains in the as-cofired capacitors. The modulated structure is formed on {111} planes of the BaTiO3. Both types of structural defects are associated with high concentrations of oxygen vacancies. In particular, the oxygen content in the BaTiO3 grains that are in direct contact with the internal Ni electrodes is less uniform with a systematic decrease in oxygen content towards the electrode. In the capacitors that are reoxidized in a higher oxygen partial pressure at lower temperature, the BaTiO3 grains are almost free of linear defects and structural modulations and the oxygen content is homogeneous throughout the BaTiO3 active layers. A concomitant improvement in t...

Modulated and ordered defect structures in electrically degraded Ni–BaTiO3 multilayer ceramic capacitors

Structural defects formed on {111} planes of BaTiO3 during the degradation of high performance multilayer Ni–BaTiO3 X7R ceramic capacitors are studied using transmission electron microscopy and electron energy loss spectroscopy (EELS). Regular pseudocubic barium titanate grains are present in as-produced (virginal) base-metal electrode capacitors. However, there is a coexistence of regular, modulated, and long-range ordered structures in intentionally electrically degraded devices. The EELS analysis demonstrates that the concentration of oxygen vacancies in barium titanate with modulated or ordered structures is higher than that in the regular perovskite grains. The clustering or accumulation of oxygen vacancies in the structural framework of BaTiO3 gives rise to the formation of new metastable structures. These observations are consistent with earlier models for degradation, but demonstrate that the details of the process may be more complex than originally assumed. Here we introduce new details on the n...

Evidence for Increased Polaron Conduction Near the Cathodic Interface in the Final Stages of Electrical Degradation in SrTiO3 Crystals

The nature of charge carriers in degraded SrTiO3 is studied by using impedance spectroscopy (IS) and electron energy loss spectroscopy (EELS). Impedance response from the degraded crystal is fitted with a modified three-element lumped circuit, and these three elements represent a leaky anode capacitance (p-type conducting), an intermediate region, and a leaky cathodic capacitance (n-type conducting) with additional elements (R–C) placed in the cathode region. The temperature dependence of an additional resistive element in the cathode region follows the relation of σ=(A/T1.5)exp (-Ea/KBT) with the activation energy of 0.096 eV, indicating the existence of a parallel conduction path via a non-adiabatic small polaron hopping mechanism in the degraded cathode region. The thickness of the n-type conducting region calculated from impedance data is in good agreement with oxygen vacancy profile directly measured by EELS. All the above data shows excellent self consistency and give insight to the important role of the polaron conductivity controlling the degradation behavior.

Electric Conduction of Thin-Layer Ni-Multilayer Ceramic Capacitors with Core–Shell Structure BaTiO3

The electric conduction mechanism for multilayer ceramic capacitors with Ni internal electrodes (Ni-MLCCs) was investigated, utilizing impedance spectroscopy (IS) and thermally stimulated current (TSC) measurement techniques. A modified 4RC equivalent circuit model was proposed to analyze the IS data for the Ni-MLCCs. This model revealed that electrode/ceramics interfaces (E/C-I) and grain boundaries (GBs) have a Schottky type conduction mechanism controlling the leakage behavior at low electric field. The Schottky barrier height at E/C-I and surface level height at GB were calculated being 1.43 and 1.06 eV, respectively. The Ni-MLCCs showed a tunneling conduction occurs with high dc electric fields of more than 10 V/µm. The onset electric field for the tunneling conduction shifted toward high electric fields as the Mn content of the capacitors increased. TSC measurements revealed that a low Mn content resulted in high mobile oxygen vacancies concentration in the Ni-MLCCs. Mn also played a role in preventing oxygen vacancies from migrating to cathode electrodes, which resulted in a long lifetime for the Ni-MLCCs.

Surface Instability in High Surface Area Complex Oxides: BaTiO3 Study

High quality BaTiO3 powders that were synthesized with hydrothermal and solid state methods are characterized with respect to surface chemistry. Different characterization techniques are used to obtain a physical picture for the BaTiO3 powders with respect to different synthesis routes and thermal histories. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) are used to understand the BaTiO3 powders after heat and humidity treatments. Formation of amorphous and crystalline BaCO3 in local heterogeneous regions depletes the surrounding surface of Ba ions to create larger areas that are Ti-rich. BaCO3 formation and adventitious carbon adsorption occur extremely quickly on the surface of crushed single-crystal BaTiO3 powder, but for manufactured powders, the degree of carbonate formation and the impact on surface stoichiometry varies with thermal history.

Stability of Ni electrodes and Ni-BaTiO3 interface evolution in ultrathin BME MLCCs

Microstructural control in thin-layer multilayer ceramic capacitors (MLCC) is one of the present day challenges for maintaining an increase in capacitive volumetric efficiency. It is observed that the continuity of the Ni electrodes increases with increasing heating rate but behaves non-linearly on sintering temperature. It is proposed that an interfacial liquid alloy layer initiates when the Ni electrodes are under tension. This accelerates a stress-induced diffusion which is the key cause of the severe electrode discontinuities during heating. Kinetic and thermodynamic approaches based on the control of sintering profiles or the control of Ni-BaTiO3 interface chemistry are proposed to prevent the Ni electrode discontinuity.

Thin film capacitors fabricated by chemical solution deposition

It was found that the dielectric properties of BaTiO3 films on Ni foil substrates varied with oxygen partial pressure and composition. 200 nm thick Mn doped BaTiO3 films annealed at 1000°C in 10¿12 atm pO2 showed a dielectric constant of 950 and low loss up to 250kV/cm bias electric field. Mn doping may trap electrons introduced by oxygen vacancies formed due to low oxygen partial pressures during heat-treatment. High dielectric constant values were achieved for 200nm thick films, so scaling of the dielectric to around 100nm should be possible. As the oxygen partial pressure during firing drops, the dielectric loss of Mn doped BaTiO3 film suddenly increased at low electric field. Carbon residue as well as oxygen partial pressure and composition can affect the dielectric properties. Removal of carbon residue was retarded from 750°C in air to 1000°C in nitrogen ambients. Also, residual carbon was found after 1000°C annealing in reducing ambient by electron energy loss spectroscopy (TEM-EELS) analysis. This residual carbon led to local reductions in the oxygen partial pressure during firing, reduced titanium ions, and to the presence of a Ni-Ba interfacial alloy layer on the surface of the Ni foils.

Microstructure and Interfaces of Thin Film Capacitors on Base Metal Foils

The microstructure and interface quality of chemical solution deposited barium titanate thin films on Ni foil were studied. Cross-sectional transmission electron microscopy shows that a ~200 nm thick barium titanate film annealed in a controlled oxygen partial pressure consists of equiaxed grains with grain size range of 24-75 nm (~ 42 nm average). NiO was detected after re-oxidation by X-ray diffraction, but not by transmission electron microscopy, suggesting that the oxide is not a continuous barrier layer, but is spatially distributed in the films. Oxygen non-stoichiometry and the existence of C in barium titanate films were observed by electron energy loss spectrometry. In addition, it was found that there is a 5-8 nm thick Ni-Ba alloy developed at the interface between the barium titanate film and Ni foil.

Interfacial Reactions Between Ni and BaTiO3: An Analytical Transmission Electron Microscopy Study

The structure and chemistry of metal-ceramic interfaces are of immense technological importance in multilayer ceramic capacitors (MLCCs), as they can profoundly affect physical and electrical properties of the devices. Only recently it is becoming fully appreciated that interface structure can be a strong function of local oxygen activity [1]. Since it is important to have a high interfacial Shottkey barrier to prevent electron injection into the dielectric layer, the chemistry of the electrode interface is of critical importance to the electrical behaviors of the capacitor. In this work we study the chemical stability of Ni-BaTiO3 interfaces processed in heavily reducing atmospheres, which are typical of commercial processes.