Article

Refractive index and interband transitions in strain modified NaNbO3 thin films grown by MOCVD

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The influence of lattice strain on the refractive index and optical band gap of NaNbO3 thin films, deposited by the liquid-delivery spin metalorganic chemical vapor deposition method, was investigated by spectroscopic ellipsometry. Epitaxial growth of coherently strained NaNbO3 films was confirmed by high-resolution x-ray diffraction and transmission electron microscopy. Incorporated lattice strain in the films was varied by the use of the oxide substrates NdGaO3, SrTiO3 and DyScO3, which exhibit lattice mismatches to NaNbO3 with different sign, magnitude and anisotropy. The Sellmeier dispersion was employed to analyze the ellipsometry data in energy region of 1.49–2.75 eV. The refractive index at 632.8 nm of the pseudomorphically grown NaNbO3 films critically depends on the incorporated elastic lattice strain and results in a continuous decrease from 2.46 to 2.18 by varying the in-plane strain from compressive to tensile. Band gap energies for films grown under compressive and tensile lattice strain were determined by collecting spectroscopic ellipsometry data in a larger energy range between 0.73–6.48 eV and evaluating them by the Tauc-Lorentz dispersion. We observed that for tensily strained NaNbO3 films deposited on DyScO3 and SrTiO3, the band gap energies increased to 3.60-/+0.01 and 3.64-/+0.02 eV, respectively. For the compressively strained NaNbO3 film deposited on NdGaO3 the band gap is shifted to still higher energies (3.80-/+ 0.01 eV).
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... Recently, we have reported epitaxial growth of high quality NaNbO 3 thin films by MOVPE 14,[17][18][19][20] and found a sequence of phase transitions when the films are grown on various substrates with varying lattice mismatch. 17,21 In particular, fully compressively strained NaNbO 3 thin films grown on the NdGaO 3 substrate exhibit an orthorhombic c phase with exclusive vertical electrical polarization, which transforms into the inclined monoclinic M A phase when the epitaxial strain is partially relaxed. ...
... The experimental spectra agree with those typically observed in the transparency range of perovskite oxide ferroelectrics and comparable to that of the NaNbO 3 film deposited on DyScO 3 . 19,29 In the nearinfrared range, the real part slightly increases with temperature, while at the UV-visible region, the imaginary part shows a redshift tendency. This behavior is attributed to electron-phonon interaction that is enhanced with the increasing temperature, as observed in other ferroelectric materials. ...
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We have investigated high temperature phase transitions in NaNbO 3 thin films epitaxially grown under tensile lattice strain on (110) DyScO 3 substrates using metal-organic vapor phase epitaxy. At room temperature, a very regular stripe domain pattern consisting of the monoclinic a 1 a 2 ferroelectric phase was observed. Temperature-dependent studies of the refractive index and the optical bandgap as well as in situ high-resolution x-ray diffraction measurements prove a ferroelectric–ferroelectric phase transition in the range between 250 and 300 °C. The experimental results strongly suggest that the high-temperature phase exhibits a distorted orthorhombic a 1 /a 2 crystal symmetry, with the electric polarization vector lying exclusively in the plane. A second phase transition was observed at about 500 °C, which presumably signifies the transition to the paraelectric phase. Both phase transitions show a pronounced temperature-dependent hysteresis, indicating first-order phase transitions.
... Recently, we have reported epitaxial growth of high quality NaNbO 3 thin films by MOVPE 14,[17][18][19][20] and found a sequence of phase transitions when the films are grown on various substrates with varying lattice mismatch. 17,21 In particular, fully compressively strained NaNbO 3 thin films grown on the NdGaO 3 substrate exhibit an orthorhombic c phase with exclusive vertical electrical polarization, which transforms into the inclined monoclinic M A phase when the epitaxial strain is partially relaxed. ...
... The experimental spectra agree with those typically observed in the transparency range of perovskite oxide ferroelectrics and comparable to that of the NaNbO 3 film deposited on DyScO 3 . 19,29 In the nearinfrared range, the real part slightly increases with temperature, while at the UV-visible region, the imaginary part shows a redshift tendency. This behavior is attributed to electron-phonon interaction that is enhanced with the increasing temperature, as observed in other ferroelectric materials. ...
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We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite LaInO3, employing density functional theory and many-body perturbation theory. We find the lowest-energy excitation at 0.2 eV below the fundamental gap (5 eV), reflecting a sizable electron-hole attraction. Since the transition from the valence band maximum (Γ point) is, however, dipole forbidden, the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not well enough resolved by the ellipsometry measurement. Through temperature-dependent measurements of the optical gap, we assess renormalization effects by electron-phonon coupling, enhancing the quantitative comparison between theoretical and experimental results.
... NaNbO 3 has an orthorhombic symmetry at room temperature with lattice constants a o = 5.569 Å, b o = 5.505 Å, c o = 15.523 Å, and we also can calculate the pseudocubic lattice parameters of NaNbO 3 to a c = 3.881 Å, b c = 3.915 Å, c c = 3.915, and ␣ c = 90.67 • [7,8]. As a photocatalyst, it has the extensive potential applications in solar fuel production and organic pollutant removal [9][10][11][12][13][14][15][16]. ...
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