CHARACTERIZATION OF LOCALIZED ELECTRONIC STRUCTURES ENABLING COLOSSAL PERMITTIVITY CERAMIC CAPACITORS

Open Access
Author:
Tsuji, Kosuke
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
January 31, 2017
Committee Members:
  • Clive A Randall, Thesis Advisor
Keywords:
  • Ceramic capacitors
  • Grain boundaries
  • Schottky barriers
  • Tunneling
Abstract:
In this thesis, a variety of characterization techniques were performed to investigate a localized electronic structure for controlling a colossal effective permittivity in materials. First, the development of potential barrier at the grain boundaries during oxidative annealing was investigated in (Mn, Nb)-doped SrTiO3 (STO) internal barrier layer capacitors (IBLCs). The methodology used for the (Mn, Nb)-doped STO IBLC was then applied to CaCu3Ti4O12 (CCTO), which is known as a different type of IBLCs. The motivation for these two materials was to find a correlation between the interfacial electronic states at the grain boundary and macroscopic varistor-capacitor properties. In addition, a relatively new colossal permittivity material, namely, Nb and In co-doped rutile TiO2 (NITO) was also investigated. In contrast to (Mn, Nb)-doped STO and CCTO, the research was focused on the origin of the colossal permittivity in NITO. A detailed dielectric relaxation analysis of the permittivity response of NITO at low temperatures was performed and discussed. (Mn, Nb)-doped STO IBLC (Mn, Nb)-doped STO barrier layer capacitors with a colossal permittivity (ε’~ 50,000) were synthesized with different oxidative annealing times. First, combination of microstructural analysis was performed: including scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). The elemental mapping confirmed that the IBLC was associated with internal grain boundaries instead of segregated secondary layers. A minor Mn-rich segregation layer was found at the electrode/ceramic interface. The EELS results reveal the valence change of manganese changed from a mixed Mn2+/Mn3+ to a mixed Mn3+/Mn4+ during the annealing as the oxygen was incorporated into the grains. An impedance study showed that an IBL was designed by an oxidative annealing process, leading to improved dielectric loss and breakdown voltages. The systematic change with the annealing time was then explained in terms of the Double-Schottky-barrier height and depletion layer width across the grain boundaries, which was quantified by a Capacitance-Voltage (C-V) analysis. Moreover, a charge Deep Level Transition Spectroscopy (Q-DLTS) approach was also applied to show the three electronic traps existing at the IBL. The nature of these trap states is discussed in detail in Chapter 3. The concept of a “local electric field at a grain boundary, EGB” was introduced to better account for the d.c. conduction analysis in (Mn,Nb)-doped STO. The local field was estimated from a microstructural analysis made by a scanning electron microscopy (SEM) and an electric (C-V) analysis. Then, the d.c. conduction mechanism was discussed based on the temperature dependence of J (Current density)-EGB characteristics. Three different conduction mechanisms were successively identified and transitioned with the increase of EGB. In the low EGB regime, J-EGB characteristics showed linear Ohmic behavior. In the intermediate EGB, it showed a non-linear characteristic with a thermally activated process. In the higher EGB, it became relatively insensitive to the temperature. The J- EGB in each regime is explained by Schottky Emission (SE) followed by Fowler-Nordheim (F-N) tunneling. Based on the F-N tunneling, a breakdown voltage was then scaled to the product of the depletion layer thickness and the Schottky barrier height at a grain boundary. These parameters control the breakdown strength of each GB in this system. CCTO IBLC The combination of SEM and C-V analysis was also performed to estimate EGB in CCTO IBLC. While the macroscopic varistor-capacitor properties were similar to the (Mn,Nb)-doped STO, the d.c. conduction mechanism was found to be quite different in CCTO. Ohmic, Poole-Frenkel (P-F) and SE were successively observed in CCTO in different field ranges. The transition point from P-F and SE depends on the EGB and temperature. A Q-DLTS study revealed that there are three types of trap states existing in CCTO. The first trap with E_t~0.65 eV below the conduction band showed excellent agreement with the trap level from P-F analysis. The electronic structure of the potential barrier in the CCTO was then proposed. NITO Dielectric spectroscopy was performed on NITO ceramics synthesized by a low-temperature spark plasma sintering (SPS) technique. The annealing temperature after the SPS was critical for the development of an IBL in NITO. The dielectric properties of the NITO were not found to be largely influenced by the blocking metal electrode contacts. NITO ceramics with lower heat treatment, which did not show IBLC effects, were then investigated. A large dielectric relaxation was observed at very low temperatures below 35 K. Both the activation energy and relaxation time suggested that a polaronic polarization was the underlying mechanism responsible for the colossal dielectric permittivity (CP) and its relaxation, instead of the internal barrier layer effect or a dipolar relaxation. Havriliak-Negami (H-N) fitting revealed a relaxation time with a large distribution of dielectric relaxations. The broad distributed relaxation phenomena indicated that the Nb and In ions were involved, controlling the dielectric relaxation by modifying the polaron dipole and localized states. The associated distribution function is calculated and presented. The frequency-dependent a.c. conductance was successfully explained by a hopping conduction model of the localized electrons with the distribution function. It was demonstrated that the dielectric relaxation is strongly correlated with the hopping electrons in the localized states. The CP in SPS NITO was then ascribed to a hopping polaron polarization.