Strain-induced non-linear optical characteristics of pyroelectric PbVO<sub>3</sub> epitaxial thin films

Seol Hee Oh; Hae-Young Shin; Seokhyun Yoon; Jai Seok Ahn; Janghwan Cha; Suklyun Hong; Sung Jin Kang; Miyoung Kim; Sukgeun Choi; Changjae Roh; Jongseok Lee; William Jo

doi:10.1364/OME.7.000062

1. Introduction

Octahedral rotations influence the B-O bond length and B-O-B angle in ABO₃ perovskites, which play a major role in physical properties such as ferroelectricity [1], magnetic interactions, resistive behavior, and bandwidth. There have been some remarkable introductions of octahedral rotation for vanadium-based perovskite oxides, RVO₃ (R = Ca, Sr, Y, La, Sm, Eu, Lu) [2,3]. Here, PbVO₃ (PVO) is a very interesting perovskite-like pyroelectric because it has a special crystallographic structure that exhibits low-dimensional magnetic properties and giant pyroelectric polarization [4–7].

PVO has a strongly distorted tetragonal structure (c/a = 1.229), which consists of a VO₅ square-pyramidal structure with one shortened vanadyl bond of vanadium and apical oxygen [8]. Its unique feature allows two-dimensional antiferromagnetic ordering and strong pyroelectric polarization [4–6]. In this pyramidal configuration of the PVO structure, the 3d electrons of vanadium ions are localized at the d_xy orbital, leading to low-dimensional magnetic ordering [5]. Indeed, experimental results showing magnetic susceptibility at a broad maximum at 180 K support this low-dimensional antiferromagnetic ordering [4,5]. However, the orientation of magnetic moment of PVO is rather controversial because the ground energies of C- and G-type ordering for the PVO are similar [6]. In addition, PVO can be considered a potential ferroelectric driven by lone pairs of displaced A-cations, as with Bi³⁺ in BiFeO₃ [6,7]. Highly distorted tetragonal BiFeO₃ thin films with a giant axial ratio c/a = 1.23 show clear ferroelectric behavior and coexisting antiferromagnetic order [9,10] The ferroelectric polarization of PVO is enhanced by large tetragonal distortion, which is compatible with well-known ferroelectric, PbTiO₃ (c/a = 1.06). Although the estimated value for spontaneous polarization of PVO is as large as 152 μC/cm² [6], it has not been experimentally demonstrated for bulk or thin-film forms. Some reports indirectly support the possibility of ferroelectric polarization of the PVO. The pressure-induced reversible tetragonal to cubic transition of PVO supports an implicit ferroelectric property [11,12]. The reversible tetragonal to cubic phase transition in PVO is induced at approximately 2 GPa, and is associated with a significant reduction in 4 or 5 orders of magnitude [13]. Linear and nonlinear optical properties of PbVO₃ have been theoretically predicted [14,15]. The refraction (n) and extinction (k) coefficients of PVO thin films on (La_0.18Sr_0.72)(Al_0.59Ta_0.11)O₃ substrate were experimentally obtained [16]. Recently, stable PVO thin films were deposited from lead pyrovanadate, Pb₂V₂O₇, using an off-stoichiometric process under either low oxygen pressure or high argon pressure, unlike other oxide materials with equilibrium targets [17]. These thin films were epitaxially grown along the c-axis and its lattice constant was elongated along the growth direction [18,19].

In the present work, non-destructive optical measurements were performed to determine the electronic structure and related physical properties of PVO. Using Raman scattering spectroscopy, the sub-structural distortion of the PVO thin films was studied. This is critically related to the strain and formation of the interface structure, which depends on the lattice mismatch. Linear and nonlinear optical properties for the PVO thin films were characterized through spectroscopic ellipsometry (SE) and second harmonic generation (SHG), respectively. Symmetry breaking along the c-axis in PVO thin films was demonstrated using an SHG signal with nonlinear susceptibility and the Fresnel equations.

2. Experimental methods

2.1 Growth of epitaxial PbVO₃ thin films

Epitaxial PVO thin films were deposited on LAO (001) and STO (001) substrates using pulsed laser deposition [19]. This study focused on the tensile train because the c-lattice constant of PVO thin films on LAO substrate is elongated even for tiny tensile strains, with lattice mismatch of 0.26% [19]. The extreme structural distortion in the PVO lattice leads to VO₅ square pyramidal structures rather than VO₆ octahedral structure with the shortened vanadyl bond V-O_a, which might limit the switchable ferroelectric polarization of PVO. The epitaxial growth of the PVO thin films along the c-axis was determined by X-ray diffraction. Deposition was performed at a constant 570°C, Ar pressure of 200 mTorr, and laser fluence of 1.5 J/cm² for 15 minutes, with a frequency of 5 Hz and using a KrF excimer laser with a wavelength of 248 nm and a pulse duration of 25 ns. The thickness of the deposited PVO thin films was measured using a surface profilometer (Dektak, Bruker) and was found to be approximately 200 nm. An off-stoichiometric Pb₂V₂O₇ target was ablated to deposit the PVO thin films, owing to the chemical stability of PVO bulk. Before the ablation, all substrates were cleaned with acetone, isopropyl alcohol, ethyl alcohol, and distilled water to prevent any artificial contributions.

2.2 Raman scattering spectroscopy

The micro-Raman spectra of the samples were obtained with a McPherson 207 spectrometer equipped with a nitrogen-cooled charge-coupled device array detector and a 488-nm diode laser at room temperature. The power of the laser illuminating the sample was set to 1 mW and was focused onto a ~1-μm-diameter spot using a microscope objective lens ( × 100).

2.3 Spectroscopic ellipsometry

SE has been applied to determine the complex dielectric function spectra (ε = ε₁ + iε₂), in the photon energy range of 0.75 to 6.45 eV at room temperature using a spectroscopic rotating compensator-type ellipsometer (M2000-DI model, J. A. Woollam Inc.) [20]. Data were recorded after averaging 1000 cycles of the compensator (1000 revolutions per measurement) to increase the signal-to-noise ratio.

2.4 Second harmonic generation

An optical second harmonic signal is generated when a light source with frequency ω creates a nonlinear polarization by going through a nonlinear medium. The SHG experiments were performed using a Ti-sapphire laser with a wavelength of 800 nm. A typical single pulse has a width of 100 fs, with a repetition rate of 80 MHz. The average power of each fundamental light pulse is about 1.4 mW and is focused on the sample surface with a spot diameter of ~100 μm.

3. Results and discussion

3.1 Crystallinity of epitaxial PVO thin films

Figure 1(a) shows the typical XRD patterns of PVO thin films deposited on two different substrates in the θ-2θ scan, exhibiting only the (00l) reflection of PVO, which indicates highly c-axis-oriented growth of the PVO thin films. Figure 1(b) shows φ scans of the PVO/LAO and PVO/STO thin films for the {110} planes of PVO and LAO. A 4-fold in-plane symmetric structure was demonstrated by the φ scans, presenting a periodicity of 90°. Therefore, the epitaxial growth along the c-axis of the tetragonal PVO thin films was confirmed by the θ-2θ scans and 4-fold symmetric φ scan results. Figure 1(c) and 1(d) show topographic images with root mean square roughness 1.42 nm for PVO/LAO and 8.84 nm for PVO/STO thin films, obtained using atomic force microscopy. Figure 1(e) and 1(f) show HAADF-STEM images of PVO thin films on LAO (001) and STO (001) substrates, respectively. The well-defined crystal structure of the PVO thin films on LAO and STO substrates is obtained at the atomic scale by a fast Fourier transform, which indicates a tetragonal structure as shown inset in Fig. 1(e) and 1(f).

Fig. 1 The XRD patterns of the PVO films deposited on LAO and STO substrates. (a) θ-2θ scans and (b) φ scans. The φ scan are for the {110} planes of PVO and LAO {110}. Topographic images of (c) PVO/LAO and (d) PVO/STO. HAADF-STEM images of PVO thin films on (e) LAO and (f) STO with the fast Fourier transform patterns.

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Applying θ-2θ XRD peaks to Bragg’s law, the c-lattice constants of PVO/LAO and PVO/STO were calculated to be 4.989 and 5.024 Å. The c-lattice parameter of the PVO/STO thin films was contrary to our expectation that it should be reduced owing to the tensile strain of the STO substrate. Therefore, the in-plane strain did not show conventional changes, despite the possibility that the lattice mismatch causes structural transformation. A vanadium-based oxide thin film, LaVO₃, showed similar behavior; that is, the expansion of volume rather than the compression of the out-of-plane lattice parameter [21]. The PVO thin films were vertically elongated as much as 6.83% (PVO/LAO) and 7.58% (PVO/STO), compared with the c-lattice constant of bulk PVO, even when the PVO/LAO thin films were deposited on only 0.26% mismatched LAO substrates. The excessive c-elongation of the PVO thin films may have been influenced by the relatively weak interaction between neighboring layers in the PVO structure, compared with the interlayer interaction. Another possibility is that the electron density in the PVO lattice overlaps between Pb and O atoms, unlike other vanadium-based perovskite compounds such as SrVO₃ and BaVO₃, which do not have covalent bonds between Sr or Ba sites and O atoms [8,22]. In a bulk study containing constant in-plane lattice parameters and carefully controlled out-of-plane lattice parameters under artificial conditions [12], a- and b-lattice parameters showed only slight changes and the c-lattice parameter changed significantly.

3.2 Dynamic phonon structure

The structural distortion in the PVO samples was investigated by characterizing the local structure of the PVO through Raman scattering spectroscopy. Figure 2 illustrates the Raman spectra of the PVO thin films on LAO and STO substrates, with a non-centrosymmetric structure (space group P4mm). In Fig. 2(a), the peaks of the Raman spectra for the PVO/LAO film were observed at 175 cm⁻¹ (Pb-O vibration modes), 220 and 320 cm⁻¹ (V-O-V bending mode), 668 cm⁻¹, and near 950 cm⁻¹, while the Raman signal for PVO/STO film can be observed only at 950 cm⁻¹, because the Raman signal of the STO substrate is large enough to mask the signal of the PVO thin film, as shown in Fig. 2(b). The peak identifications are in agreement with those presented in [18] and [19]. In particular, the peaks at 950 cm⁻¹ split into two peaks, as shown in Fig. 2(c). The peaks were qualitatively identified as V-O_a (apical oxygen) stretching with A_g symmetric vibration mode [6,20], and the dominant intensities can be related to c-axis-oriented PVO films, which differ from bulk PVO [4]. These two peaks are also observed in the cubic KCrFe₃, in which there is a melting of the orbital ordering [23].

Fig. 2 Raman spectra of the PVO thin films on (a) LAO and (b) STO from 100 to 1,100 cm⁻¹. (c) Raman spectra in specific region for V-O apical vibration signal near 950 cm⁻¹ of PVO/LAO and PVO/STO. (d) Raman shift positions and FWHM depending on the substrate (i.e. lattice mismatch between PVO and the substrates). P and W represent position and FWHM, respectively.

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To obtain a better understanding of the high-frequency modes, we calculated the theoretical phonon modes of PVO with the first principles method using the CRYSTAL09 code. A C-type antiferromagnetic cell was assumed and a hybrid functional B1WC was chosen to reproduce the tetragonal structure and volume [24]. The calculated modes are nearly consistent with the experimental results, within 7.5%. The atom-decomposed eigenmodes are presented in Fig. 3. In particular, doublets at 950 cm⁻¹ and a single line at 760 cm⁻¹ are remarkable. The doublet peaks near 950 cm⁻¹ can be assigned to V-O_a stretching modes, of which the lower- and higher-frequency peaks are anti-symmetric (p1 in Fig. 2(c)) and symmetric (p2 in Fig. 2(c)) motions of two V-sites in the C-AFM cell, respectively. Additionally, two V-O_p (planar oxygen) modes are assigned near 760 cm⁻¹ and 550 cm⁻¹. A singlet near 650 cm⁻¹ is assigned as a LO mode corresponding to a V-O_p TO mode at near 550 cm⁻¹ by considering some vanadium oxide compounds such as Ca_0.33V₂O₅ [25]. For further analysis, the FWHM of the p1 and p2 peaks was considered, as shown in Fig. 2(d). The two distinguished peaks become closer to each other and overlap as the lattice mismatch increases, indicating that atomic displacement becomes larger. The broadened peaks represent the deterioration of the crystallinity, which is consistent with the mosaic spreads in rocking curves.

Fig. 3 Atom-decomposed phonon eigenmodes of PVO. LO modes (in red-lines) were obtained through the non-analytic correction using the calculated anisotropic dielectric constants: ε_a = 5.5024 and ε_c = 5.6384.

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3.3 Linear optical characterization

SE data were analyzed using a multilayer model consisting of ambient pressure, a surface roughness layer, the PVO film, and the substrate. The optical properties of the surface roughness layer are represented by the Bruggeman effective medium approximation (BEMA) [26] using a 50-50 mix of the underlying material and void. The best results with minimum discrepancy between the experimental data and the model were obtained with BEMA layers with thicknesses of 2.6 nm for PVO/LAO and 11.7 nm for PVO/STO systems.

The complex dielectric function ε spectra for PVO layers were constructed using basis (B)-spline formulation [27]. A spline function is a series of polynomial segments constructed in a manner to maintain continuity up to a certain degree of differentiation. This set of polynomial functions can describe the optical structures in the ε₂ spectrum, and the corresponding ε₁ spectrum is calculated at the same time through the Kramers–Kronig transform. Experimental data (Ψ and Δ) and their best-fit curves, compared in Fig. 4(a) and 4(b), show excellent agreement. The modeled ε₁ and ε₂ are presented as red and black solid lines, respectively, in Fig. 4(c) and 4(d).

Fig. 4 Complex dielectric function spectra (ε = ε₁ + iε₂) corresponding to epitaxial PVO films prepared on different substrates.

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The ε₂ spectrum for PVO/LAO exhibits three broad spectral features near 2.5, 4.5, and 6.0 eV, which is overall in good agreement with the calculated data [14,15]. In their density-functional theory calculations, Milosevic et al. attributed the first structure at ~2.5 eV to the transition from the top of the valence band consisting mainly of a single V (3d) state to the bottom of the conduction band consisting of the rest of the V (3d) states [14]. The major broad structure spanning from 3.5 eV to 6 eV has been identified as the transition from the O (2p⁶) states in the valence band to the bottom of the conduction band.

There are two noticeable differences in the ε spectra between PVO/LAO and PVO/STO samples. First, the optical structures in ε₂ spectrum for PVO/STO shift to lower energy with respect to the PVO/LAO data. Second, the structure at ~6 eV appears much weaker for PVO/STO. Ju and Cai calculated the optical responses of PVO as a function of the applied strain and found that the peaks in ε exhibit a systematic redshift when the system is under more tensile strain [15]. As PVO/STO experiences larger tensile strain than PVO/LAO, our observation is consistent with the theoretical predictions. Reduction of the optical structure at high energies may be a result of the inferior crystalline quality caused by a large misfit strain.

3.4 Nonlinear optical characterization

PVO was expected to have large ferroelectric polarization induced by an exotic tetragonal distortion. This polarization can be inferred by checking inversion symmetry breaking. Here, the SHG experiment is a suitable tool because the second harmonic signals can be generated only for non-centrosymmetric samples [28]. The nonlinear optical characteristics of the epitaxial PVO films were examined via SHG with an incident light source of wavelength 800 nm, injected into the sample at incident angle φ = 45° from the surface normal. Incident light is linearly polarized, and its polarization axis is continuously changed by rotating a half-wave plate by angle θ. The SHG experiment is shown in Fig. 5(c). The intensity of the s and p-polarized output SHG signal, I_s^out and I_p^out, at wavelength 400 nm was detected as a function of polarization angle θ of incident light at room temperature. The results show that the PVO/STO thin films have a larger SHG intensity than do the PVO/LAO thin films (Fig. 5(a) and 5(b)). Thus, the PVO/STO thin films undergo harder inversion symmetry breaking than do PVO/LAO thin films. The SHG results are analyzed based on the symmetry analyses considering the 4mm point group where four independent nonzero second-order susceptibility tensor components are given as χ_xzx = χ_yzy, χ_xxz = χ_yyz, χ_zxx = χ_zyy, χ_zzz [11,16,29] Measured quantities of the SHG intensity I_s^out and I_p^out are expressed as

I_{s}^{o u t} = {| {\tilde{f}}_{y}^{R} P_{y}^{N L} |}^{2}

I_{p}^{o u t} = {| {\tilde{f}}_{x}^{R} P_{x}^{N L} + {\tilde{f}}_{z}^{R} P_{z}^{N L} |}^{2}

where

Fig. 5 (a) and (b) The s- and p-polarized output SHG signals of the PVO/LAO and PVO/STO thin films, respectively.

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P_{x}^{N L} = I_{0} \cdot \sin ϕ_{i n} \cos ϕ_{i n} 2 f_{x} f_{z} χ_{x x z} \cos 2 θ,

P_{y}^{N L} = I_{0} \cdot \sin ϕ_{i n} 2 f_{y} f_{z} χ_{x x z} \sin θ \cos θ,

P_{z}^{N L} = I_{0} \cdot [\begin{matrix} χ_{z x x} (f_{x}^{2} \cos 2 ϕ_{i n} \cos 2 θ + f_{y}^{2} \sin 2 θ) \\ + χ_{z z z} f_{z}^{2} \sin 2 ϕ_{i n} \cos 2 θ \end{matrix}] .

Here, $I_{0}$ is the intensity of the incident fundamental light. $f_{i}^{}$ and $f_{i}^{R}$ with $i = x, y, z$ are the Fresnel coefficients for fundamental and second-harmonic light, respectively, at the air–film interface. These coefficients are determined by considering the refractive index of the film and the angle of incidence $φ_{i}$ [11,16,29]. The symmetry analysis results, plotted in Fig. 5 as solid lines, reproduce the experimental results quite well. In particular, the tensor components used for the PVO/STO thin films are almost twice as large as those of the PVO/LAO thin films; (χ_xzx, χ_zxx, χ_zzz) = (2.4, 15, −125) for PVO/LAO and (χ_xzx, χ_zxx, χ_zzz) = (4.4, 30, −300) for PVO/STO. Here, all the values are given in arbitrary units. This confirms that the symmetry breaking along the c-axis occurs more strongly for PVO/STO than for PVO/LAO, which could be attributed to the elongation of the c-axis lattice constant for the films grown on the STO substrate.

4. Conclusion

We present the effects of strain on the structural properties of PVO thin films by investigating the dynamic phonon structure, and linear and nonlinear optical properties. The doublet Raman spectra, which indicate V-O_a stretching vibration mode, are caused by octahedral rotation of VO₆. An abnormal interfacial layer is induced by the lattice mismatch with the STO (001) substrate. Spectroscopic ellipsometry was used to determine parameters describing the dielectric constant and absorption coefficient of the PVO thin films on LAO and STO substrates. The ellipsometric spectra of the PVO thin films exhibit a redshift tendency with increasing misfit strain. As a result of c-lattice elongation, PVO thin films have stronger inversion symmetry breaking along c-axis, demonstrated by fitting SHG responses with nonlinear susceptibility and the Fresnel equations. This study can be a guideline for further investigation of the magnetic and ferroelectric properties in this material. Highly strained PVO films are potentially applicable to pyroelectric sensors and optoelectronic devices.

Funding

National Research Foundation (NRF) of Korea (NRF-2013H1A8A1004287, 2014R1A2A2A01004070, 2015001948, 2013R1A1A2007951; NRF-2013-Fostering Core Leaders of the Future Basic Science Program; NRF-2010-00453; 2012M3A7B4049888; 2010-0020207; 2015R1A5A1009962, 2015R1A1A1A05001560); Korea Institute of Science and Technology Information Supercomputing Center (KSC-2014-C2-032).

Acknowledgments

This research was supported by a grant from the National Research Foundation (NRF) of Korea, funded by the South Korean government (MSIP) (NRF-2013H1A8A1004287, 2014R1A2A2A01004070, 2015001948, 2013R1A1A2007951). One of the authors (SHO) was supported by an NRF grant funded by the South Korean government (NRF-2013-Fostering Core Leaders of the Future Basic Science Program). This research was also supported by Leading Foreign Research Institute Recruitment Program through an NRF grant funded by the Ministry of Science, ICT and Future Planning of Korea (NRF-2010-00453). The authors SH and JC were supported by NRF grants (Nano Material Technology Development Program 2012M3A7B4049888 and Priority Research Center Program 2010-0020207). The computation (JSA) is supported by the Korea Institute of Science and Technology Information Supercomputing Center through Contract No. KSC-2014-C2-032. This work was supported in part by the Science Research Center and the Basic Science Research Program through the NRF and funded by the Ministry of Science, ICT and Future Planning (Nos. 2015R1A5A1009962 and 2015R1A1A1A05001560).

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Strain-induced non-linear optical characteristics of pyroelectric PbVO₃ epitaxial thin films

Abstract

1. Introduction