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

Abstract

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 (

Magnetodielectric effect and phonon properties of compressively strained EuTiO3thin films deposited on (001)(LaAlO3)0.29Kamba1, Goian2, Орлита3 et al. 2012Phys. Rev. BSelf Cite220181View full textAdd to dashboardCite

“…Moreover, the possible anisotropic strain from the non-cubic LSAT substrate should as well split the phonons in SrTiO 3 /LSAT (SrTiO 3 and EuTiO 3 have the same lattice constant and therefore the strain in both films is the same), but this was not detected. 33 On the other hand, we have revealed the same new phonon activated in IR spectra near 150 K in EuTiO 3 films deposited on NdGaO 3 and LaAlO 3 . Compressive strains in both films are slightly relaxed and have the value of about 0.9%, 10 , i.e.…”

Far infrared and terahertz spectroscopy of ferroelectric soft modes in thin films: A review

“…Since both the FTIR and THz techniques probe the in-plane (non-FE) response, for such films presumably only SM stiffening can be expected and the static dielectric response is expected to be smaller without pronounced maxima at T C . This was actually observed in films on both the substrates [37,45,46,47], without any anomaly in the SM behavior (see Fig. 2).…”

Ultrafast X-ray diffraction probe of terahertz field-driven soft mode dynamics in SrTiO3

“…Overlaid with our results (solid markers) are values from IR measurements taken on 107 nm STO films on LSAT from Ref. 19 (hollow markers). We find that the low frequency peak goes from 3.3 THz at 105 K to 3.80 THz at 270 K, in good agreement with the values reported from IR reflectivity 19 and ellipsometry 30 .…”

“…2(c). We observe two sharp peaks at frequencies consistent with known IR-active phonons at zone center 19 and label them TO 1 and TO 2 . These peaks also explain the persistent oscillations in the X-ray scattering signal after the THz pulse has propagated out of the film.…”