Voltage offsets in (Pb,La)(Zr,Ti)O3 thin films

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...

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

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

Wang

Pang

et al. 2004

“…[7][8][9] Moreover, some recent studies have revealed that electron trapping in thin films induces a voltage offset in the polarizationvoltage characteristics. 10 This letter investigates the influence of postannealing treatments on the dielectric properties of BST thin films, after top electrode deposition, in oxygen and nitrogen atmospheres. ͑Ba 0.6 Sr 0.4 ͒TiO 3 films were prepared by the polymeric precursor method [11][12][13][14] and deposited by the spin-coating technique on a Pt/Ti/SiO 2 /Si substrate.…”

mentioning

confidence: 99%

Effects of the postannealing atmosphere on the dielectric properties of (Ba, Sr)TiO3 capacitors: Evidence of an interfacial space charge layer

Pontes

Leite

Longo

et al. 2000

145

The dielectric properties of (Ba, Sr)TiO3 films were found to be remarkably sensitive to the postannealing treatment atmosphere. This study demonstrates that postannealing in an oxygen atmosphere increases the dielectric relaxation phenomenon and that postannealing in a nitrogen atmosphere produces a slight dielectric relaxation. Such dependence of the dielectric relaxation was related both to oxygen vacancies and to the presence of negatively charged oxygen, trapped at the grain boundary and/or at the electrode/dielectric film interface.

“…Evidence in the literature suggests that epitaxial films of perovskite ferroelectrics can have unipolar character ͑strongly preferred polarization direction͒ in their as-grown state, and that the direction of the unipolarity correlates with the sense of the imprint. 4,11,12 Using AFM piezoresponse imaging, we have found that our films in their as-grown state were not composed of an equal distribution of parallel and antiparallel domains; but were unipolar. 13 The polarity of many of our films have been measured to be antiparallel to the surface normal, but films with parallel polarity have also been observed.…”

mentioning

confidence: 99%

X-ray scattering evidence for the structural nature of fatigue in epitaxial Pb(Zr, Ti)O3 films

Thompson

Munkholm

Streiffer

et al. 2001

We have probed the microscopic distribution of 180°domains as a function of switching history in 40 nm epitaxial films of Pb͑Zr 0.30 Ti 0.70 ͒O 3 by analyzing interference effects in the x-ray scattering profiles. These as-grown films exhibit voltage offsets ͑imprint͒ in the polarization hysteresis loops, coupled with a strongly preferred polarization direction in the virgin state. Our x-ray results are consistent with models attributing the loss of switchable polarization to the inhibition of the formation of oppositely polarized domains in a unipolar matrix. One of the limitations of utilizing ferroelectric films for applications such as nonvolatile memories, is the tendency of some materials, e.g., Pb͑ZrTi͒O 3 , to exhibit a loss of switchable polarization with increasing switching cycles, a phenomenon known as fatigue. Fatigue may arise from several different underlying physical mechanisms. Two mechanisms are most frequently discussed. 1 In the first, fatigue occurs by pinning of ferroelectric domain walls within the bulk of a film containing a mixture of oppositely polarized domains. In the second, fatigue is the result of inhibition of the growth of oppositely polarized domains due to nucleus suppression at the film-electrode interfaces of a uniformly polarized matrix. By analysis of the electric field dependence of permittivity as a function of degree of fatigue in polycrystalline film, Colla et al. 1 have concluded that the primary fatigue mechanism under normal cycling conditions is nucleus suppression. Piezoresponse atomic force microscopy ͑AFM͒ studies 2 have found that the fatigued state can have a preferential polarization, and it has been found that fatigue can be reduced by asymmetric switching voltages. 3 These results also support a nucleus suppression model.In this letter, we report x-ray interference characterizations of the domain configurations after successive polarization reversals in ferroelectric heterostructures incorporating top and bottom electrodes. Interference effects occurring in x-ray scattering profiles of high-quality epitaxial ferroelectric films are extremely sensitive to the distribution of oppositely polarized ͑180°͒ domains. This method has been used previously to determine the unipolar character of the native 180°d omain distribution for an epixatial PbTiO 3 film on SrTiO 3 . 4 Here, we use this method to study the structural nature of fatigue in ferroelectric films. This x-ray technique provides direct characterization of the relative population of the two domains averaged over a macroscopic region, complementary to both electrical and AFM characterizations.The interference effects upon which this scattering technique relies require very high structural coherency between the film and substrate, which serves as a phase reference. Therefore, we have investigated a model epitaxial heterostructure consisted of a 40 nm ͑001͒ Pb͑Zr 0.3 Ti 0.7 ͒O 3 ͑PZT͒ film grown by metalorganic chemical vapor deposition, 5 on a 136 nm ͑001͒ SrRuO 3 ͑SRO͒ film sputter deposited 6 on a (001) ...