The Influence Of Misfit Strain On The Leakage Current Characteristics Of Ferroelectric Thin Films

Ferroelectric thin films have received much attention because of its promise forapplications in low-power, low-voltage, high-density nonvolatile memories. Butthere still exists many disadvantages blocking the development of ferroelectricmemories. Failure is an important obstacle to realize the application of ferroelectricmemories. It consists of fatigue，imprint and retention. Among the elements, leakagecurrent is a key factor that has relation with these three failure models. So how toregulate leakage current plays a key role in preventing ferroelectric memories failure.So we study the leakage current of ferroelectric films, and find the influencingfactors of leakage current. Finally we obtain the methods to reduce leakage currenttheoretically, and offer guidance for reducing leakage current in experiments. Themain contents are shown as follows:（1） Based on the space-charge-limited-current model, we get the relationbetween leakage current and dielectric constant from the Poisson equation. Thedielectric constant can also be obtained through P/E. Then the relationshipbetween polarization and leakage currents can be developed. We use the polarizationequation in the Hang-Ting Lue model. In the polarization equation, the coerciveelectric field is influenced by strain, temperature, stress and frequency. So thepolarization can be surely affected by these factors, and we have developed therelationship between the polarization and leakage currents, finally these factors caninfluence the leakage currents. So we obtain the relationship between the leakagecurrents and the factors. Strain and temperature have deeper influence on leakagecurrents than stress and frequency.（2） Among the influencing factors of leakage currents, strain and temperatureinfluence leakage currents more deeply. We research the leakage currents bycombining strain and temperature in terms of a single-crystal single-domainferroelectric thin film epitaxially grown in the （001）-oriented cubic phase on adissimilar cubic oriented substrate/electrode. OnlyP₃exists in the kind of film.Based on the relationship between leakage currents and polarization, dielectricconstant, we get the leakage currents diagrams. Polarization values under positiveand negative electric field are corresponding to the two working states offerroelectric memories. We study the leakage currents changing with temperatureand strain under different electric field. When positive electric field is applied, leakage currents decrease with the increase of the compressive strain. When negativeelectric field is applied, leakage currents increase with the compressive strain.（3）P₁、P₂andP₃can be different values in different phases, so we need tostudy the leakage diagrams changing with strains and temperature under differentphases. We consider the leakage currents diagrams under depolarization electric fieldand positive/negative electric field. We also obtain the polarizations and dielectricconstants diagrams changing with strain and temperature, and obtain the polarizationof different phases. Based on the diagrams, we can find the methods to regulate theleakage currents. The methods can provide guidance for the following experiments.