ISSN 10637834, Physics of the Solid State, 2011, Vol. 53, No. 5, pp. 1062–1066. © Pleiades Publishing, Ltd., 2011. Original Russian Text © O.A. Maslova, Yu.I. Yuzyuk, N. Ortega, A. Kumar, R.S. Katiyar, 2011, published in Fizika Tverdogo Tela, 2011, Vol. 53, No. 5, pp. 999–1003.

LATTICE DYNAMICS

Raman Spectroscopy of BaTiO3/(Ba,Sr)TiO3 Superlattices O. A. Maslovaa, *, Yu. I. Yuzyuka, N. Ortegab, A. Kumarb, and R. S. Katiyarb a

Southern Federal University, ul. Bolshaya Sadovaya 105/42, RostovonDon, 344006 Russia * email: [email protected] b Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, PO Box 23334, San Juan, Puerto Rico, 009313334 United States Received September 14, 2010

Abstract—A series of BaTiO3/Ba1 – xSrxTiO3 (BT/BST) superlattices were prepared by pulsed laser deposi tion on MgO substrates with a constant period of 80 Å (40 Å BT and 40 Å BST) and varying compositions of the BST layer so that the Ba/Sr concentration ratios were 0/100, 30/70, 40/60, 50/50, 60/40, 70/30, 100/0. The soft mode E(1TO) of the polarized Raman spectra transformed depending on the Ba/Sr ratio in the BST layer. As the Sr concentration in the BST layers increased from 0 to 100%, the E(1TO) soft mode halfwidth varied from 171 to 103 cm–1 and its frequency increased from 31 to 109 cm–1 due to the interaction between the epitaxial layers forming the superlattices. DOI: 10.1134/S1063783411050192

1. INTRODUCTION In recent years, artificial superlattices consisting of alternating layers of various polar and nonpolar per ovskites such as BaTiO3 (BT), PbTiO3 (PT), SrTiO3 (ST), and BaZrO3 (BZ) are very popular subjects of studies [1–10]. The increased interest in these struc tures is due to their quite unique properties such as low losses, high dielectric permittivities, significant polar ization, and high Curie temperatures. Such properties permit the application of the superlattices for produc ing functional elements of the dynamic random access memory optoelectronics, and tuned microwave devices. The physical properties of perovskite layerbased superlattices can be artificially controlled by, e.g., varying the film deposition conditions, substrate type, layer thickness, and lattice parameters by doping the layers. The use of the layers of various compositions allows the controlling the layer deformation and thus artificially varying the ferroelectric properties of the structures. Since the stresses induced by the lattice misfits in adjacent layers change the ion positions, some lattice vibrations, in particular, the ferroelectric soft mode, are usually very sensitive to the existence of a strain in the thin films [11]. Therefore, information on the behavior of the soft mode in ferroelectric super lattices is very important from the standpoint of the fundamental physics of these structures and their practical application. In this work, we study the polarized Raman spectra of the BaTiO3/(Ba,Sr)TiO3 superlattices grown on MgO substrates with the constant periodicity of the BT/BST

layers and variable Ba/Sr ratio in the BST layers. The soft mode frequency is found to be changed with the Ba/Sr ratio, which is due to the stresses induced between the epitaxial layers of the superlattices. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Thin films of the BaTiO3/Ba1 – xSrxTiO3 (BT/BST) superlattices were grown on the (001) MgO substrates by a pulsed laser deposition when the laser beam was alternatively focused on the BT and BST stoichiomet ric targets. The layers was deposited with a constant periodicity of 80 Å (40 Å BT and 40 Å BST) with a par tial substitution of Sr for Ba in the BST layer in such a manner that the Ba/Sr concentration ratios were 0/100, 30/70, 40/60, 50/50, 60/40, 70/30, 100/0, where 0/100 corresponds to the superlattice with the BT/ST composition, and 100/0 to the pure BT film. The total thickness of the BT/BST films was 600 nm for each of the samples under study. The superlattice were deposited using an excimer laser (KrF, 248 nm) with the laser fluence of 1.5 J/cm2 and the pulserepetition rate of 10 Hz. During the deposition, the substrate temperature was 830°C and the oxygen pressure was 20 mTorr. Whether the films deposited are singlephase, it was examined by Xray diffraction on a Siemens D500 diffractometer with Cu Kα radiation. The Raman spectra were excited by a polarized radiation of an argon laser (λ = 514.5 nm) and were measured by a Renishaw spectrometer equipped with a CCD detector. The exciting radiation was focused on

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a sample using a Leica optical microscope, the focused beam diameter on the sample was 2 μm. The spectra were measured at room temperature in the backscat tering geometries from the film surface (Z(YX)Z ) and Z(YY)Z and in sideview backscattering geometries (scattering from a film crosssection) such as Y(ZX)Y , Y(XX)Y , and Y(ZZ)Y at which the wave vector orienta tion and the incident wave polarization corresponded to the crystallographic axes of the cubic substrate X || [100], Y || [010], and Z || [001]. The intensities of all the spec trograms presented in this work were corrected with allowance for the temperature population factor.

(a) E(1TO)

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E(3TO) E(2LO) B

E(4TO)

A1(2TO)

E(1TO) A

E(1LO) E(2TO)

3. RESULTS AND DISCUSSION Figure 1 shows the polarized Raman spectra of the BT/BST superlattices with the Ba/Sr ratio of 50/50 measured at room temperature in various scattering geometries. The Raman spectra of the tetragonal fer roelectric phase in BT are known to contain active optical phonons 3A1 + B1 + 4E. These phonons occur as the threefold degenerated three F1u and the F2u modes of the paraelectric cubic phase split into 3A1 + 3E and B1 + E modes, respectively [12]. The long range electrostatic forces split all the A1 and E modes of symmetry into transverse (TO) and longitudinal (LO) components. It follows from the selection rules for the point group C4v (the fourfold axis is directed along Z) that the A1 modes are active only for the diag onal components of the polarization tensor axx = ayy and azz; the B1 mode is active for the components axx and ayy; and the E mode are only allowed for the com ponents azx = axz and ayz = azy. In the scattering geom etry related to the component axy = ayx, the Raman spectra of the tetragonal ferroelectric phase have no modes allowed by the selection rules [12]. It is seen from Fig. 1a that the diagonal scattering geometries (Y(XX)Y ), Z(YY)Z , and Y(ZZ)Y show the transverse (183, 271, 304, and 521 cm–1) and longitudinal (185, 474, and 725 cm–1) A1 optical modes of symmetry, and also the contribution of the E(TO) (75 cm–1) modes leaked from the nondiagonal scattering geometry (Y(ZX)Y ). In the spectrum measured in the Y(ZX)Y scattering geometry (Fig. 1b), the E(TO) (74, 175, 307, and 495 cm–1) and E(LO) (175, 307, and 725 cm–1) modes are dominant, but also there are the A1(2TO) (278 cm–1) and A1(3TO) (520 cm–1) bands that leak from the fullsymmetric diagonal scattering geome tries. The two most intense fullsymmetric modes occur practically always, because of polarization dis tortions, in both the spectra of specially single domainized BT single crystals [13] and the spectra of cdomain BST films [14, 15]. However, in the super

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400 ν, cm−1

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800

Fig. 1. Polarized Raman spectra of the BT/BST superlat tice with the Ba/Sr ratio of 50/50 in the BST layers mea sured in various scattering geometries at room tempera ture. The inset shows the system of coordinates with respect to the sample. In the sideview backscattering geometry, the wavevector of incident and scattered light is parallel to axis Y, the polarization of incident/scattered light is parallel to axis X or axis Z; the film axis c (in the tet ragonal phase) is parallel to axis Z.

lattices under study, the contribution of these modes forbidden in the tetragonal phase is very significant as compared to that of allowed modes with E symmetry. Moreover, the same bands that are observed in the Y(ZX)Y scattering geometry are observed in the nondi agonal Z(YX)Z scattering geometry (Fig. 1b); although, as noted above, the polarization tensor com ponents αyx in the Raman spectrum has no modes active in the tetragonal phase. The Raman spectrum of the tetragonal BT single crystal [12] has a specific feature in the Y(ZZ)Y geom etry, namely, a clear interference dip at 178 cm–1 due to the interaction of two A1(TO) modes. The Y(XX)Y spectrum has a clear peak with a maximum at ~180 cm ⎯1 and has no dip. Similar polarization fea tures of the spectra are also observed in the cdomain

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E(1TO)

BT/BST Y(ZX)Y

Ba/Sr

Intensity, arb. units

0/100

30/70 40/60 50/50 60/40 70/30 100/0 0

200

400 ν, cm−1

600

800

Fig. 2. Raman spectra of the BT/BST superlattices mea sured in the Y(ZX)Y scattering geometry as a function of the Ba/Sr ratio.

BST films, but the interference dip is shifted to higher frequencies [15, 16], which is likely due to a change in the balance of the force constants of the interacting modes in the crystal lattice with changing the inter atomic distances as Sr is substituted for Ba. The spec tra of the superlattices with BT/BST = 50/50 (Fig. 1a) also demonstrate evidence of the mode interaction, but the interference dip at ~160 cm–1 exists in the Y(XX)Y and Z(YY)Z spectra, and it is practically absent in the Y(ZZ)Y spectrum, which does not agree with the polarization characteristics of the tetragonal BT spectra [12]. Moreover, the Raman spectra of the superlattice contain additional bands labeled as A, B, C, and D in Fig. 1; the band appearance is related to local distor tions of the crystal structure that lead to the translation symmetry violation as Sr is substituted for Ba in the BST layer. Similar lines are also observed in the BST film spectra [16, 17]. The A band (138 cm–1) is inter preted [18] as the density of phonon states of the acoustic transverse (TA) and longitudinal (LA) branches with high density near the Brillouin zone

boundary induced by an disorder; the band is observed in the BST films in all scattering geometries. In the c domain BST films, the B, C, and D bands (340, 567, and 635 cm–1, respectively) are most intense in the Y(XX)Y scattering geometry. In the superlattice spec trum shown in Fig. 1a, these bands have practically the same intensity in all the scattering geometries, includ ing nondiagonal those. Thus, we may conclude that the polarization char acteristics of the Raman spectra of the BT/BST super lattice with the Ba/Sr ratio of 50/50 do not correspond to the tetragonal symmetry with the c axis directed normally to the substrate. The existence of the A1type modes in nondiagonal scattering geometries and also the transition of modes of the Etype symmetry to the spectra measured in diagonal scattering geometries implies a reduction of symmetry from the tetragonal to orthorhombic, and even monoclinic. The most important feature of the Raman spectra of the BT/BST superlattices is a substantial transfor mation of the ferroelectric E(1TO) soft mode depend ing on the Ba/Sr ratio in the BST layer. Figure 2 depicts the polarized Raman spectra of the superlat tices measured at various Ba concentrations in the BST layer in the Y(ZX)Y scattering geometry. A detailed analysis with the spectrum expansion into component profiles shows that, as the Ba content in the BST layers forming the superlattice increases, the E(1TO) maximum frequency decreases regularly and its halfwidth increases. Figure 3 depicts the E(1TO) mode frequency and its halfwidth (FWHM) as func tions of the Ba concentrations with allowance for the absolute error. In the BT film, the E(1TO) mode is overdamped, i.e., it is a vibration with a high damping and has a sig nificant halfwidth (about 170 cm–1) at a low fre quency of ~30 cm–1. As seen from Fig. 2, the over damped E(1TO) soft mode in the BT film is trans formed to an underdamped peak (that is a vibration with a low damping) in the superlattices as the Sr con centration in the BST layers increases, and its half width is substantially changed (from 171 cm–1 in BT to 103 cm–1 in BT/ST). The significant decrease in the peak halfwidth of the mode can be explained as fol lows. The structural transformations in a BT crystal can be described in terms of socalled eightminimum model [19]. According to the model, the Ti ions ini tially displaced from the centrosymmetric positions in the oxygen octahedron centers occupy one of eight minima along the threefold axes of the cubic lattice in the paraelectric phase. In the tetragonal ferroelectric phase, the ions occupy only four from the eight posi tions; all the position are arranged in the same plane and related by the fourfold axis; as a result, the spon

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taneous polarization occurs along this axis. As the symmetry is reduced further, the number of possible positions occupied with Ti ions is still halved, and there are two positions in the orthorhombic phase and one position in the rombohedral phase. At room tem perature, BT must have the tetragonal structure and BST with the BT content lower than 70% cubic struc ture; however, the layers in the superlattices are strongly distorted, and the soft mode has the shape characteristic of the lowsymmetric orthorhombic phase which is formed in bulk BT and BST materials at temperatures lower than room temperature [20]. The underdamped character of the soft mode in the superlattices is likely a result of the Ti ion ordering even at room temperature. In the eightminimum model, the Ti ions can be disordered only in not more than two positions, which is characteristic of the orthorhombic phase in which, as known, the perovs kite subcell is monoclinically distorted. Since the soft mode is very sensitive to twodimen sional stresses, the observed shift of the soft mode to higher frequencies is likely due to an internal two dimensional stress induced by the lattice parameter misfit between the BT and BST layers in the superlat tices. The lattice parameter of the BST solid solutions decreases regularly as the Sr content increases, and it almost precisely follows the Vegard linear law [20]. Varying the Sr concentration in the BST layer can be used to change the lattice parameter in the layer con jugation plane from a = 3.905 Å characteristic of ST to a = 3.990 Å realized in the tetragonal BT. Thus, there is a possibility of varying the stresses induced between the layers and, thus, modifying the superlattice prop erties by varying the concentration of certain atoms in the layers. Since the soft mode is immediately related to the magnitude of the static permittivity of ferroelec trics, its frequency in the Raman spectra of the super lattices allows one to diagnose their properties. In our case, the superlattice consists of approximately ten BT unit cells and the same amount of the BST unit cells. In the superlattices, the BST layers are stretched by adjacent BT layers which, in turn, are compressed by surrounding BST layers. The strain induced is maxi mal in the BT/ST superlattice where the layer param eter misfit is 2.2%. In the BT layers, a tetragonal dis tortion with the polarization in the direction normal to the substrate (c phase in [21]) occurs. In the BST layers stretched, an analog of the aa phase with the polariza tion in the plane occurs. The electrostatic interaction between the layers leads to the appearance of the resulting polarization in the unit cell of the BT/BST superlattice that is sloped with respect to a normal to the substrate, and the symmetry is reduced to the monoclinic that (analog of r phase in [21]). Such a symmetry reduction leads precisely to a partial depo PHYSICS OF THE SOLID STATE

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2 140

ν, cm−1

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100

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80

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Fig. 3. (1) Frequency and (2) halfwidth of the E(1TO) soft mode vs. the Ba/Sr ratio in the BST layers of the BT/BST superlattices.

larization of the Raman spectra that is manifested in activation of the lines with the A1 symmetry in nondi agonal geometries and appearance of the E modes in the diagonal spectra. 4. CONCLUSIONS We have studied the BT/BST superlattices grown on (001) MgO substrates by pulsed laser deposition with the constant layer periodicity of 80 Å. As the Sr ion concentration in the BST layers increases, the E(1TO) soft mode is found to take an underdamped character and is shifted to higher frequencies. The soft mode frequency in the BT/BST superlattices is found to be determined by the lattice misfit of the BT and BST layers, and it can be varied with a factor of more than three, which allows one to change the static per mittivity by an order of magnitude. Thus, varying the concentration of ions forming the superlattice, and, thus, varying the lattice parameters of the layers, we can artificially induce a deformation of the layer in the superlattice, which allows the control of the ferroelec tric properties of the structures within wide limits.

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