Voltage Shifts and Defect-Dipoles in Ferroelectric Capacitors

Abstract

We review the processes and mechanisms by which voltage offsets occur in the hysteresis loop of ferroelectric materials. Simply stated, voltage shifts arise from nearinterfacial charge trapping in the ferroelectric. We show that the impetus behind voltage shifts in ferroelectric capacitors is the net polarization, with the net polarization being determined by the perovskite and the aligned defect-dipole components. Some common defect-dipoles in the PZT system are lead vacancy-oxygen vacancy complexes. One way to change the net polarization in the ferroelectric is to subject the PZT capacitor to a dc bias at elevated temperature; this process is spectroscopically shown to align defect-dipoles along the direction of the applied electric field. The alignment of defect-dipoles can strongly impact several material properties. One such impact is that it can lead to enhanced voltage shifts (imprint). It is proposed that the net polarization determines the spatial location of the asymmetrically trapped charge that are the cause for the voltage shifts. An'enhanced polarization at one electrode interface can lead to larger voltage shifts since it lowers the electrostatic potential well for electron trapping, i.e., more electron trapping can occur. Defect-dipole alignment is also shown to increase the UV sensitivity of the ferroelectric.

Ferroelectricity in AlScN: Switching, Imprint and sub-150 nm Films

“…While imprint is found to become reduced under repeated cycling of the material, temperature and the majority polarization of the film exert a profound effect on its sign and magnitude. The measurement summarized in Figure 6 suggests that imprint appears due to alignment of charged defects with the majority polarization -similar to what is attributed to oxygen vacancies in PZT [6]. Therefore, changing the majority polarization from N-polar to metal polar tends to slowly realign these defects -which can be greatly sped up by providing thermal energy.…”

The effect of imprint on remanent piezoelectric properties and ferroelectric aging of PbZr0.52Ti0.48O3 thin films

“…Additionally, evaporation of PbO could, in principle, lead to formation of defect associates. Oxygen vacancies are the most mobile ions in PZT and short‐range migration of oxygen vacancies around the acceptor can generate an internal field in PZT films at poling temperatures and fields . Presumably, the concentration of defect associates increases with Mn concentration in the PZT, enhancing the propensity of defect‐dipole alignment upon thermal poling of PZT films.…”

“…Oxygen vacancies are the most mobile ions in PZT and short-range migration of oxygen vacancies around the acceptor can generate an internal field in PZT films at poling temperatures and fields. 41,42 Presumably, the concentration of defect associates increases with Mn concentration in the PZT, enhancing the propensity of defect-dipole alignment upon thermal poling of PZT films. Indeed, the E a for imprint in Mn doped PZT films varies between 0.5 and 0.8 eV, which is close to the activation energy required for migration of oxygen vacancies (Table 1).…”

Stability of 71° stripe domains in epitaxial BiFeO3 films upon repeated electrical switching

“…For example, it can lead to enhanced voltage shifts, that is, imprint. It was proposed that the net polarization determines the spatial location of the asymmetrically trapped charges that are the cause for the voltage shifts 28.…”

“…Lee et al 13 showed that the oxygen vacancies move and redistribute during in‐plane switching of epitaxial BFO. The effects of the oxygen vacancy motion and the formation of defect‐dipoles in ferroelectric perovskites were extensively studied for other ferroelectrics, such as BaTiO 3 and PZT 28, 30. Oxygen vacancies and the defect‐dipoles they may form were correlated with the degradation of the electrical resistance and also, indirectly, with the imprint of the ferroelectric capacitors, by affecting the potential wells for trapped electrons.…”