Stabilizing the Ferroelectric Phase of KNO3 Thin Films Using Substrate Electrodes

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This research investigated the possibility of stabilizing the ferroelectric phase of KNO3 thin films using substrate electrodes. The substrate electrodes used for this investigation were nickel (Ni), stainless steel (SS) and tantalum (Ta). The dip coating technique was used to deposit the films from the molten form of KNO3. The ferroelectric properties of the films were characterized by means of polarization-voltage measurements, current density-voltage measurements, and dielectric constant temperature dependence measurements. From the results obtained, the e_ect of the di_erent substrate electrodes on the KNO3 thin film ferroelectric phase stability were determined. UV-Visible absorption spectroscopy was used to analyse the optical bandgap of KNO3. An optical bandgap of 3.79 eV was obtained for the film deposited at 480.0 oC. The P-V hysteresis loops exhibited by the KNO3 films deposited on the substrate electrodes and the temperature range over which the hysteresis loops extended revealed the possibility of stabilizing and extending the ferroelectric phase of KNO3 thin films by means of an appropriate choice of substrate electrode. Well-defined hysteresis loops characterized the ferroelectric phase of the films deposited on SS and Ni but the films deposited on Ta exhibited unsaturated hysteresis loops. The Ni and SS substrate electrodes stabilized and extended the ferroelectric phase to 40.0 oC and beyond room temperature respectively. Ta extended the ferroelectric phase to about 50.0 oC. However, it did not stabilize the ferroelectric phase since it severely degraded polarization in the film a condition which led to poorly define hysteresis loops. The characteristic hysteresis loops exhibited by the di_erent samples was attributed to the nature of the current (either displacive or leakage) present in the sample. Displacive current is responsible for hysteresis loop formation while leakage current results in elliptical loop formation. The level of oxygen vacancies at the SS/KNO3 and Ni/KNO3 interface coupled with the electronegativity and d-shell occupancy of SS and Ni caused displacive current to be dominant in the films deposited on them. On the other hand, the films deposited on Ta su_ered large leakage current e_ect due to the electronegativity and d-shell occupancy of Ta as well as the level of oxygen vacancies present at the Ta/KNO3 interface. The dielectric constant behaviour of KNO3 as temperature cooled through the paraelectric-ferroelectric phase revealed anomalies in the vicinity of the Curie point Tc. These dielectric constant temperature dependence curves obeyed the Curie-Weiss law just above Tc in the paraelectric region. The Curie point Tc and Curie temperature To were the same for all samples irrespective of the substrate electrode and this was evidence that the samples underwent a second-order transition. The films deposited on SS exhibited a shift in their inherent Tc from the reported range of between 120.0 oC and 124.0 oC to temperatures above 140.0 oC. The films deposited on Ni and Ta however exhibited no shift. The observed shift in Tc was attributed to strong polarization-strain coupling. The strain is however not due to misfit as a result of thermal stress and lattice misfit but rather due to lattice defects caused by the di_usion of Fe3+ ions into the deposited film layer during the deposition at elevated temperatures. These Fe3+ ions occupied a relatively smaller interstitial sites of KNO3. This condition deformed the lattice of the films prepared on SS causing the development of strain in the film layer. This strain is believed to be the strain that coupled with polarization and caused the shift in Tc. The results obtained form this research have clearly demonstrated and proven the viability of stabilizing the ferroelectric phase of KNO3 thin films by means of an appropriate choice of substrate electrode. With respect to this research, austenitic stainless steel substrate electrode has stabilized and extended the ferroelectric phase of KNO3 thin films beyond room temperature.
A Thesis Submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology in Partial Fulfilment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY (Solid State Physics).