The Role of Defect Chemistry in DC Resistance Degradation of Lead Zirconate Titanate Thin Films

Open Access
- Author:
- Akkopru Akgun, Betul
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 09, 2019
- Committee Members:
- Susan E Trolier-Mckinstry, Dissertation Advisor/Co-Advisor
Susan E Trolier-Mckinstry, Committee Chair/Co-Chair
Suzanne E Mohney, Committee Member
Clive A Randall, Committee Member
Zoubeida Ounaies, Outside Member
Michael T Lanagan, Dissertation Advisor/Co-Advisor
John C Mauro, Program Head/Chair
Michael T Lanagan, Committee Chair/Co-Chair - Keywords:
- PZT
thin film
defect chemistry
electrical degradation - Abstract:
- This thesis describes the roles of defect chemistry and internal electric fields on the long-term stability of the properties of piezoelectric films. The correlation between defect chemistry, aging, leakage currents, and time dependent dielectric breakdown was studied for Nb and Mn doped PbZr0.52Ti0.48O3 (PZT) thin films. It was demonstrated that the magnitude of the internal field is much higher in Mn doped PZT (PMZT) films compared to Nb doped PZT (PNZT) films after poling in the temperature range of 25-200°C under an electric field of -240 kV/cm. The development of the internal field is thermally activated, with activation energies from 0.5±0.06 to 0.8±0.1 eV in Mn doped films and from 0.8±0.1 to 1.2±0.2 eV in Nb doped films. The different activation energies for imprint suggests that the physical mechanism underlying the evolution of the internal field in PMZT and PNZT films differs; the enhanced internal field upon poling is attributed to (1) alignment of oxygen vacancy – acceptor ion defect dipoles (〖〖(Mn〗_Ti^''-V_O^())〗^x, 〖〖(Mn〗_Ti^'-V_O^())〗^')) in PMZT films, and (2) thermionic injection of electron charges and charge trapping in PNZT films. In either case, the internal field reduces back switching, enhances the remanent piezoelectric properties, and dramatically improves the aging behavior. PMZT films exhibited the greatest enhancement, with reduced high temperature (180°C) aging rates of 2-3%/decade due to improved stability of the poled state. In contrast, PNZT films showed significantly larger high temperature aging rates (15.5%/decade) in the piezoelectric coefficient, demonstrating that the fully poled state was not retained with time. The correlation between defect chemistry, leakage currents, and time dependent dielectric breakdown was also studied for PbZr0.52Ti0.48O3 (PZT) films doped with 0.5, 1, 2, or 4% Nb. As the samples are nearly intrinsic, signatures for both hole hopping between Pb2+ and Pb3+ and electron trapping by Ti4+ were observed. For all doping levels, the dominant conduction mechanism transitioned from Poole-Frenkel emission at lower electric fields to Schottky emission at higher electric fields. The electric field for this transition diminishes from 172 to 82 kV/cm with decreasing Nb concentration. The concomitant modification of Schottky barrier height from 1.24 to 0.95 eV with decreasing Nb concentration is attributed to Fermi level pinning via oxygen vacancies. The DC resistance degradation was controlled by Schottky emission from 250-400 kV/cm; the lifetime of the films increases with increasing Nb level. The effective Schottky barrier height for 2% Nb doped PZT films decreased from 1.12 to 0.85 eV during degradation. This is related to the migration of oxygen vacancies towards the cathode, accompanied by the observation of Ti3+ near the cathode. Furthermore, the electric field range over which Schottky conduction increased with degradation. The mechanisms for time-dependent dielectric breakdown in PZT films will thus be a strong function of the initial film defect chemistry. The electrical reliability of lead zirconate titanate (PZT) films was improved by incorporating Mn; the time dependent dielectric breakdown lifetimes and the associated activation energy both remarkably increased with Mn concentration. The correlation between the defect chemistry and the resistance degradation was studied to understand the physical mechanism(s) responsible for enhanced electrical reliability. At lower electric fields, Poole-Frenkel emission was responsible for the leakage current. Beyond a threshold electric field, Schottky emission controlled the leakage. After degrading 2 mol% Mn doped PZT film under an electric field of 350 kV/cm at 180°C for 12 h, no significant change in potential barrier height for injecting electrons from the cathode into the anode was observed. This suggests that the degradation is mostly controlled by Poole-Frenkel conduction via some combination of hole migration between lead vacancies, small polaron hopping between Mn sites and hole hopping between Pb2+ and Pb3+. The oxygen vacancy concentration after degrading PZT films under an electric field of 350 kV/cm at 180°C for 12 h was found to be 1x1020 cm-3 for 2 mol% Mn doped PZT films. No variation in the valence state of Ti near the cathode was observed in degraded Mn doped PZT films. Additionally, thermally stimulated depolarization current results did not show any depolarization peak arising from relaxation of trapped electrons on Ti4+. This suggests that multivalent Mn provides trap sites for electrons and holes; free electron generation due to compensation of oxygen vacancies at the cathode and free hole formation at the anode region might be suppressed by the valence changes from Mn3+ to Mn2+ and Mn2+ to Mn3+ respectively. Finally, the role of interfacial defect chemistry in time dependent breakdown and associated charge transport mechanisms was investigated for Pb0.99(Zr0.52Ti0.48)0.98Nb0.02O3 (PNZT) films. Electrical degradation was strongly dependent on the sign of the electric field; a significant increase in the median time to failure from 4.8±0.7 to 7.6±0.4 hours was observed when the top electrode was biased negatively compared to the bottom electrode. The improvement in electrical reliability of Pt/PNZT/Pt films is attributed to (1) a V_O^(••) distribution across the film due to PbO non-stoichiometry, and (2) Ti/Zr segregation in PNZT films. Compositional mapping indicates that PbO loss is more severe near the bottom electrode, leading to a V_O^(••) gradient across the film thickness. Upon degradation, V_O^(••) migration towards the bottom Pt electrode is enhanced. The concentration of V_O^(••) accumulated near the bottom Pt interface (6.2×1018 /cm3) after degradation under an electric field of 350 kV/cm for 12 h was two times higher than that near the top Pt/PNZT interface (3.8×1018 /cm3). The V_O^(••) accumulation near the bottom Pt/PNZT interface causes severe band bending and a decrease in potential barrier height, which in turn accelerates the electron injection, followed by electron trapping by Ti4+. This causes a dramatic increase in the leakage current upon degradation. In contrast to the bottom Pt/PNZT interface, only a small decrease in potential barrier height for electron injection was observed at the top Pt/PNZT interface following degradation. It is also possible that a Zr-rich layer near the top interface reduces electron trapping by Ti4+.