Mechanism of high temperature hydrogen reduced polarization degradation in Pb(Zr_1−xTi_x)O₃ ferroelectric thin film capacitors

Abstract

Annealing Pb(Zr 1Àx Ti x )O 3 (PZT) thin films in the hydrogen ambient can result in the unacceptable degradation of ferroelectric properties due to interaction of H 2 forming gas with the ferroelectric materials. Since the understanding of this degradation mechanism is limited, this issue arouses a big problem for the application of PZT in high-density non-volatile ferroelectric random access memories (NVFRAM). In order to resolve this problem, we investigate Pt/PZT/Pt capacitors before and after H 2 annealing at various temperatures. The capacitors lost their ferroelectric properties during hydrogen annealing. The measurements of polarization hysteresis loops and leakage currents suggest that there are different degradation mechanisms for the reduced PZT films. Although hydrogen induced degradation can be partially recovered by postannealing of the thin films in the O 2 ambient, all the remanent polarizations after recovery are much smaller than their virgin values, which indicates the irreversible damage of the ferroelectricity during the forming gas annealing (FGA) process. In order to investigate the degradation mechanism, X-ray diffraction was performed before and after FGA, and atomic force microscopy images were taken to observe the topograghy and microstructure changes. A conductive Pt-coated piezoelectric force microscope probe was used as a mobile top electrode to check ferroelectric and piezoelectric properties within microregions of PZT films before and after FGA.

Effect of Al-doped ZnO or Sn-doped In2O3electrode on ferroelectric properties of (Pb,La)(Zr,Ti)O3capacitors

“…H 2 degradation results from the reductive chemical reaction of ferroelectric material caused by the catalytic effect of these materials. 12,[16][17][18][19] The primary mechanism for the ferroelectric property loss is the formation of OH − bonds between the ionized hydrogen and oxygen ions along the polarization axis in the octahedral structure. [18][19][20] The incorporation of H containing species into the PZT lattice is one of the mechanisms for H 2 degradation.…”

“…12,[16][17][18][19] The primary mechanism for the ferroelectric property loss is the formation of OH − bonds between the ionized hydrogen and oxygen ions along the polarization axis in the octahedral structure. [18][19][20] The incorporation of H containing species into the PZT lattice is one of the mechanisms for H 2 degradation. 19) Aggarwal et al observed the characteristic peak of polar hydroxyl OH − bond stretching at ∼3700 cm −1 in Raman spectra, which is absent from PZT before forming gas annealing (FGA) with H 2containing gas.…”

“…[18][19][20] The incorporation of H containing species into the PZT lattice is one of the mechanisms for H 2 degradation. 19) Aggarwal et al observed the characteristic peak of polar hydroxyl OH − bond stretching at ∼3700 cm −1 in Raman spectra, which is absent from PZT before forming gas annealing (FGA) with H 2containing gas. 18) cm −3 , respectively.…”

“…The oxygen loss produces vacancies and electrons, thereby increasing the film conductivity. 19,22) Third, other defect charges created in the FGA process can also pin PZT domains. Hui et al reported that the oxygen loss is a good explanation for the changes induced by FGA.…”

“…Hui et al reported that the oxygen loss is a good explanation for the changes induced by FGA. 19) The pining has effects similar to the OH − bonds, which serve as defect dipoles. 19,23) FGA causes lower P r on polarization vs voltage (P-V) hysteresis loops, especially when capacitors have a Pt top electrode.…”