Dielectric Response of Soft Modes in Ferroelectric Thin Films

Petzelt, J.; Kužel, Petr; Rychetský, I.; Pashkin, A.; Ostapchuk, T.

doi:10.1080/00150190390211891

Cited by 35 publications

(31 citation statements)

References 38 publications

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“…As pointed out earlier, the accuracy of the determination of the permittivity of thin films is usually rather low. 20 In addition, in order to limit the probing THz beam to the bare part of the sample, an aperture of 3 mm was used in THz transmission experiments; this may lead to diffraction effects and the lowfrequency part of the THz spectra ͑which was subsequently found to be very important for the interpretation͒ may suffer from larger systematic errors, especially if the sample is handled in the cryostat ͑problems arise with precise positioning of the sample and reference͒. For this reason, the sample 4 ϫ 50B was prepared following the same experimental protocol and the temperature-dependent zero-bias THz transmission experiments were carried out with this sample by using an aperture of 6 mm.…”

Section: Methodsmentioning

confidence: 99%

Temperature and electric field tuning of the ferroelectric soft mode in a strainedSrTiO3/DyScO3heterostructure

et al. 2009

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We studied THz dielectric spectra of SrTiO 3 / DyScO 3 epitaxial multilayers with four 50-nm-thick layers of each compound deposited on DyScO 3 substrate as a function of temperature and electric field. At room temperature, SrTiO 3 is in the paraelectric phase and, due to the in-plane tensile strain within the films, the soft-mode frequency is shifted down to 47 cm −1 . The ferroelectric phase transition occurs in the strained SrTiO 3 layers around 270 K and in the ferroelectric phase the soft-mode hardening upon temperature decrease is observed. The soft mode is linearly coupled to an excitation of relaxation type at 10 cm −1 at all temperatures. This relaxation is silent in the paraelectric phase, but it becomes coupled to the polarization below the ferroelectric transition temperature and its strength progressively increases. The recently proposed universal model of soft-mode behavior in strained SrTiO 3 films ͓C. Kadlec et al., J. Phys.: Condens. Matter 21, 115902 ͑2009͔͒ was generalized to describe the observed spectra at all temperatures and applied fields.

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Section: Methodsmentioning

confidence: 99%

Temperature and electric field tuning of the ferroelectric soft mode in a strainedSrTiO3/DyScO3heterostructure

et al. 2009

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“…The IR spectroscopy has proven to be a very powerful technique for studying the polar phonon properties, particularly soft modes, in thin ferroelectric films [5,13]. Until recently, only transmission spectroscopy in the far IR was used, which limited the technique to the use of far-IR transparent substrates, such as sapphire, MgO, highly resistive Si, SiO 2 etc.…”

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Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

et al. 2008

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Several SrTiO 3 (STO) thin films without electrodes processed by pulsed laser deposition, of thicknesses down to 40 nm, were studied using infrared transmission and reflection spectroscopy. The complex dielectric responses of polar phonon modes, particularly ferroelectric soft mode, in the films were determined quantitatively. The compressed epitaxial STO films on (100) La 0.18 Sr 0.82 Al 0.59-Ta 0.41 O 3 substrates (strain 0.9%) show strongly stiffened phonon responses, whereas the soft mode in polycrystalline film on (0001) sapphire substrate shows a strong broadening due to grain boundaries and/or other inhomogeneities and defects. The stiffened soft mode is responsible for a much lower static permittivity in the plane of the compressed film than in the bulk samples.Keywords Soft mode . Thin film . Infrared spectroscopy SrTiO 3 (STO) is a classical incipient ferroelectric with strongly increasing permittivity on cooling-up to more than 20,000 in single crystals [1]. It is well known since 1994 [1] that the permittivity in STO thin films is strongly reduced and does not increase on cooling as much as in single crystals. It is thickness dependent, but even in the limit of very thick films it does not reach the crystal values [2]. The thickness dependence was assigned to lowpermittivity (dead) interfacial layers between the film and electrodes [2]. However, grain boundaries also play an important role in bulk ceramics as well as in thin films [3][4][5][6]. Low-permittivity grain boundaries and possible nanocracks in the films and/or ceramics decrease the effective permittivity, which is accompanied by a remarkable stiffening of the effective soft mode.Strain induced by the substrate may also influence the dielectric response of epitaxial films. The phase diagram of STO under uniaxial (biaxial) pressure was theoretically calculated from the known thermodynamic parameters [7,8]; it appears to be very sensitive to both compressive and tensile stresses. The ferroelectric transition may be also induced by the choice of appropriate substrate. For instance, the tensile in-plane stress produced by a (110) DyScO 3 substrate may induce the in-plane ferroelectricity up to room temperature [9]. Quasi-epitaxial film on (0001) sapphire exhibited ferroelectric transition near 120 K as evidenced by a softer TO 1 mode above 120 K compared to the single crystals [5]. Below the structural transition occurring probably at the same temperature, the TO 1 mode couples with the structural soft mode doublet and hardens on further cooling. On the other hand, the compressive stress induced by a (110) NdGaO 3 (NGO) substrate is responsible for the out-of plane ferroelectricity near 150 K [10] whereas the in-plane mode softening with decreasing temperature is only very weak. In the latter case, the in-J Electroceram (

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“…13͒ using software for evaluation of the dielectric function of a two-slab ͑in general, multislab͒ system. 10,22 Fitting of the FTIR reflectance was performed simultaneously with the complex permittivity obtained from the terahertz data. The resulting dielectric functions below ϳ250 cm −1 of the film are shown in Fig.…”

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Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions

Nuzhnyy

Petzelt

Kamba

et al. 2009

Applied Physics Letters

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Infrared reflectance, terahertz transmittance, and microwave resonance measurements show that SrTiO3 films, strained by ∼1% in biaxial tension by growing them on (110) DyScO3 substrates, undergo a pronounced phonon softening near 270 K. This in-plane soft-mode drives the ferroelectric transition. The appearance of two new low-frequency modes and splitting of the high-frequency TO4 mode provide evidence of an antiferrodistortive phase below ∼180 K.

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Dielectric Response of Soft Modes in Ferroelectric Thin Films

Cited by 35 publications

References 38 publications

Temperature and electric field tuning of the ferroelectric soft mode in a strainedSrTiO3/DyScO3heterostructure

Temperature and electric field tuning of the ferroelectric soft mode in a strainedSrTiO3/DyScO3heterostructure

Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films

Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions

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