Effect of lattice-misfit strain on the process-induced imprint behavior in epitaxial Pb(Zr0.52Ti0.48)O3 thin films

Abstract

The effect of lattice-misfit strain on the process-induced imprint behavior in Pb͑Zr 0.52 Ti 0.48 ͒O 3 (PZT) capacitors with Pt (top), and SrRuO 3 , La 0.7 Sr 0.3 MnO 3 or LaNiO 3 (bottom) electrodes has been studied. With the different oxide electrodes and by changing the deposition oxygen pressure, various lattice-misfit strains in the epitaxial PZT films have been produced. It was found that after in situ annealing at reduced oxygen pressures, the capacitors showed an increased voltage offset in the polarization-electric field hysteresis loops with increasing the misfit strain, irrelevant to the oxide electrodes employed, while lattice disorder at the bottom interface can effectively eliminate the voltage shift. Our results suggest that the imprint behavior is caused by oxygen loss via dislocations generated by the misfit strain relaxation at the growth temperature. Ferroelectric thin films possess a unique set of physical properties and have great potential for use in various electronic devices. 1 For successful implementation of ferroelectric devices based on polarization reversal, symmetric switching between two opposite polarization states must be ensured. However, it is often observed that ferroelectric films exhibit significant imprint, or asymmetry of switching parameters, such as the coercive field ͑E c ͒ and the remnant polarization ͑P r ͒. 2-10 The cause of the imprint is generally attributed to the presence of an internal electric field in the capacitor which supports the given polarization state while opposing the antiparallel one. However, there is a lack of understanding on the formation of the internal field. Different models, such as the aligned dipolar defect complexes and asymmetric distribution of charged defects through a bulk thin film, 2-5 and a built-in electric field or a nonswitching layer at the ferroelectric-electrode interface, 6-9 have been proposed to explain the asymmetric behavior of ferroelectric capacitors.Pb͑Zr, Ti͒O 3 (PZT) and their derivatives are the most extensively studied ferroelectric materials due to their large P r , low E c and processing temperature. Since the PZT capacitors with noble metal electrodes like Pt exhibit a significant polarization loss when subject to bipolar switching pulses, in the past decade, for the growth of fatigue-free PZT capacitors and especially in the epitaxial form, various oxide electrodes such as La 0.5 Sr 0.5 CoO 3 (LSCO), SrRuO 3 (SRO), La 0.7 Sr 0.3 MnO 3 (LSMO), and LaNiO 3 (LNO) have been employed. [3][4][5][6][7][8]11,12 For epitaxial LSCO/PZT/LSCO capacitors, however, when exposed to reducing atmosphere for device fabrication, a large voltage offset was observed. [3][4][5] This process-induced imprint behavior was greatly complicated by the instability of the LSCO layer at reduced oxygen pressures, 5,13 and rarely examined for the other oxide electrodes. Comparatively, SRO, LSMO, and LNO are very stable and have different lattice constants. 14,15 Abe et al. demonstrated that the lattice-misfit strain is important for the voltag...

Effect of electrode configurations on the process-induced imprint behavior of epitaxial Pb(Zr0.52Ti0.48)O3 capacitors

“…13 Although the origin of such a layer is still under debate, Abe et al 14 have attributed it to an asymmetrical relaxation of the epitaxial lattice-misfit strain between BaTiO 3 and the SrRuO 3 bottom electrode, and we also showed that an enlarged misfit can increase the voltage shift in epitaxial Pb͑Zr 0.52 Ti 0.48 ͒O 3 ͑PZT͒ capacitors. 15 This strain-related scenario seems consistent with the imprint induced by mechanical stress, where a simultaneous backswitching of ferroelectric domains, as revealed by the local piezoresponse method, was observed during the polarization reversal. 16 Recently, a theoretical model based on two interface layers of unequal conductivity was proposed, which results in both positive and negative voltage offsets.…”

Effect of substrate temperature on the microstructure and transport properties of highly (100)-oriented LaNiO3−δ films by pure argon sputtering

“…Due to the different lattice parameters and thermal expansion coefficients (between Si and LNO), internal defect induced strains and non-good crystallization, LNO film deposited at 500 1C showed an enlarged lattice constant of a ¼ 3.93 Å . Increase of deposition temperature will involve higher energy and mobility for adatoms, resulting the release of defect induced strains and will facilitate the formation of dislocations at the film/substrate interface [12], which will further release the mismatch induced strains, and finally make the lattice constant gradually decrease and approach to the bulk value of a ¼ 3.84 Å .…”

Ferroelectric vs. structural properties of large-distance sputtered epitaxial LSMO/PZT heterostructures

“…The observed imprint is well known in literature, especially in cases where the TE and BE materials differ, 34 with interpretations and explanations including misfit strain effects and/or oxygen loss at the LSMO/PZT interface, 10,35 electrons and/or Pb vacancies being trapped at the top interface, 36,37 charge compensation for oxygen vacancies generated at the TE interface 37,38 and defect-dipole complexes due to oxygen vacancies. 37,39,40 A fundamental feature of the ferroelectric is the genuine remanent polarization P r which can be extracted from the FE hysteresis loops by subtracting parasitic contributions, such as linear capacitances and leakage currents (Fig.…”

“…Along with conductive and superconductive oxides like IrO 2 , RuO 2 , ZnO and YBa 2 Cu 3 O 7−x 1-4 as electrode materials, conductive perovskites like LaNiO 3 , SrRuO 3 , (La,Sr)CoO 3 , Sr(Ti,Sb)O 3 (SSTO), (La,Sr)SnO 3 (LSSO) and (La,Sr)MnO 3 (LSMO) have been in the focus of research, since they allow for heteroepitaxial growth with the PZT. [5][6][7][8][9] Yet in the course of time it became apparent that the good match of the lattice constants between the electrode and PZT is not the only decisive characteristic determining the usefulness of a particular conducting oxide as an electrode; 10 what also matters is the type of the majority charge carriers in the electrode. Chen et al 8 found that n-type perovskites (LSSO, SSTO) cause the ferroelectric hysteresis loops to broaden and the fatigue lifetime to be reduced dramatically.…”