INTERFACIAL EFFECTS IN OXIDE-POLYMER LAMINAR COMPOSITE THIN FILM DIELECTRICS FOR CAPACITOR APPLICATIONS
Open Access
- Author:
- Tewari, Pratyush
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 12, 2009
- Committee Members:
- Michael T Lanagan, Dissertation Advisor/Co-Advisor
Michael T Lanagan, Committee Chair/Co-Chair
Osama O Awadelkarim, Committee Member
Mark William Horn, Committee Member
Qing Wang, Committee Member
Eugene Furman, Committee Member
Ramakrishnan Rajagopalan, Committee Member - Keywords:
- Composite dielectrics
Interfacial effects
Energy density
Polymer - Abstract:
- Continuous increase in the density of active components on microelectronic chip/circuit board requires development of new capacitors with smaller size, weight and cost. Miniaturization in the size of capacitors demands development of high energy density dielectric materials, which are the core of parallel plate capacitors. Nano composite dielectrics comprising high polarizibility oxide fillers randomly dispersed in high breakdown strength polymer matrix are considered as a potential high energy density materials for capacitor applications. Large interfacial volume, generated due to introduction of nano fillers in polymer matrix, might have significant positive contribution towards energy storage in nano composites. However, percolation issues associated with nano fillers and generation of large interfacial volume in nano composites, where complex electric field distribution overlaps with interfacialy modified polymer lead to unclear understanding of polymer-filler interfacial interactions in nano composites. Hence, in the current work laminar composite double layered dielectric structures, which provide relatively simple local field distribution at the interface and ideal series connectivity between oxide and polymer, are used as a model system to understand polymer-oxide interfacial interactions. Interfacial effects are reported for both low permittivity (SiO2-Parylene C) and medium permittivity (ZrO2–P(VDF-TrFE)) laminar composite dielectrics. Pyrolytic vapor decomposition polymerization process was used to grow Parylene C thin films on gold and thermally grown SiO2 surfaces. Enhancement in crystallite dimension with post deposition annealing treatments of Parylene C thin films was found to reduce dielectric loss tangent and hence enhance its dielectric properties. Electric field and temperature dependant leakage current analysis suggested hopping as dominant conduction mechanism in Parylene C thin films. Parylene C thin films in laminar composites showed enhanced segmental chain mobility or reduced degree of co-operative chain motion in comparison to Parylene C itself. These effects were found to be more pronounced for Parylene C in the vicinity of SiO2. AC electric field dependant impedance spectroscopy analysis of SiO2-Parylene C laminar composites showed gradual reduction in segmental chain motion relaxation rate in Parylene C with an increase in AC oscillation amplitude. Reduced segmental chain motion relaxation rate at a fixed temperature, suggesting higher stability of glassy phase at high AC oscillation amplitude, was explained based on its similar dependence on external pressure, previously shown for polyisoprene and PMMA. Lesser effect of AC oscillation amplitude on interfacial polymer segmental chain motion relaxation rate in comparison to bulk polymer was found to be consistent with reported higher activation energy for segmental chain motion for thicker film in comparison to thinner. With an increase in AC oscillation amplitude a new sub-ohmic AC conduction mechanism associated with minority carrier injection was found to be dominant at lower frequency. The relaxation associated minority carrier injection was found to be strongly coupled with AC electric field amplitude. Thermally stimulated depolarization current measurements (TSDC) of SiO2-Parylene C laminar composites revealed the presence of interfacial dipolar energy state with activation energy in the range of 1.05 to 1.30 eV. Surface functionalization of thermally grown SiO2 with various organosilane was carried out to control the polarity of SiO2 surface and modify interfacial characteristics of laminar composites. Shift in TSDC interfacial peak maxima, signifying presence of deeper interfacial state, with surface functionalization was found to be directly related to surface polarity of functionalized SiO2. Overall increase in polarizibility of SiO2-Parylene C laminar composite dielectric with surface functionalization was found to be controlled by surface polarity of functionalized SiO2 surface and interfacial surface tension between functionalized SiO2 and Parylene C. Impedance spectroscopy measurements of laminar composites with functionalized SiO2 surface suggested a coupling between interfacial dipole and interfacial charge transportation mechanism. Electrical properties of medium permittivity composite comprising, solution cast P(VDF-TrFE) film on sputtered ZrO2 thin films, were found to be controlled by P(VDF-TrFE) in high frequency region and by ZrO2 thin film in low frequency region. Impedance spectroscopy and equivalent circuit modeling showed that additional polarization in ZrO2-P(VDF-TrFE) composites, shown as a difference in experimentally measured and theoretically calculated real part of permittivity using series mixing rule, was primarily due to large scale structural modification of interfacial P(VDF-TrFE) grown on ZrO2 thin films. Presence of additional polarization mechanism in ZrO2-P(VDF-TrFE) laminar composite was found to be consistent with enhanced polarization observed in ZrO2-fuloropolymer particulate composites.