Role of PZT/metal interface in ferroelectric capacitor

Lead zirconate titanate (PZT) films are extensively used in FRAM (ferroelectric random access memory), piezoelectric MEMS (micro-electro-mechanical systems), flexible electronics, high-energy storage, and other integrated ferroelectrics application areas. Ferroelectric/electrode interface strongly effects device performance. These properties could be to high extent influenced by defect chemistry in PZT. The migration of oxygen vacancies and the corresponding charge redistribution in PZT films is most responsible for leakage current, electrical degradation, fatigue, retention, imprint, and other electrical characteristics of PZT ceramic films. The report contains a detailed analysis of electrical properties of PZT thin film capacitors with different electrode materials, including Pt, Ir, Au, LaNiO₃ (LNO), Hg (mercury probe).

The steady-state current-voltage techniques and low-frequency relaxation techniques were used to examine PZT capacitors with different electrode materials. To determine the presence and distribution of local electric fields inside the ferroelectric film we used the electron beam-induced current (EBIC) technique. In the case of impermeable to oxygen vacancies interface (e.g. Ir/PZT or Pt/PZT), their accumulation near the interface causes electrons injection with formation of an induced p-n junction. We propose a model that adequately describes leakage current in these structures. In contrast, if the interface is transparent to oxygen vacancy (e.g. Au/PZT or PZT/LNO) the space charge region is not created at the interface. As vacancies leave the PZT volume, an injection of electrons decreases and an experimental current-voltage dependence demonstrates a nonlinear behavior with a region of negative differential conductivity at high electric fields. The capacitors with interfaces transparent to oxygen vacancies demonstrate 1.5–2 times higher relaxation charge and more than two times lower relaxation times in contrast to those ones with impermeable interfaces. We suggest that proposed model of induced p-n junction can be useful for engineering of FRAM and MEMS devices based on PZT films.