Optical properties of bulk and thin-film SrTiO3 on Si and Pt

Abstract

We have studied the optical properties (complex dielectric function) of bulk SrTiO3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV [using Fourier transform infrared (FTIR) ellipsometry] to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic (LO) phonons were also identified. In SrTiO3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO3 film and the Si substrate. There is also a decrease in amplitude and an increase in broadening of the critical points with decreasing thickness. In SrTiO3 films on Pt, there is a strong correlation between the crystallinity and texture of the films (mostly aligned with the Pt pseudosubstrate) and the magnitude of the refractive index, the Urbach tail below the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO3 on Pt (reflection–absorption spectroscopy) show absorption peaks at the LO phonon energies, a typical manifestation of the Berreman effect for thin insulating films on a metal. The Urbach tail in our ellipsomety data and the broadening of the optical phonons in SrTiO3 on Pt are most likely caused by oxygen vacancy clusters.

Metal-induced charge transfer, structural distortion, and orbital order in SrTiO3thin films

“…Using the experimentally measured value of 2 . 5 = ∞ ε for STO [43], we obtain S = 0.36 in good agreement with our result. The values of SB and S are the same for both interface structures (hollow and oxygen site interfaces), suggesting that the relaxation has a similar effect on the electronic structure of STO for both geometries.…”

Interplay Between Thin Film Ferroelectric Composition, Microstructure and Microwave Phase Shifter Performance

“…For films with Ba≤0.70, the refractive indices at 632.8 nm varied from 2.36 to 2.40 (see figure 2). These values are close to that of bulk, single-crystal SrTiO 3 (2.39) [10,11], but relatively very high for thin films. The refractive indexes of BSTO films in the literature were found to vary considerably with the film growth method [10,12].…”

“…These values are close to that of bulk, single-crystal SrTiO 3 (2.39) [10,11], but relatively very high for thin films. The refractive indexes of BSTO films in the literature were found to vary considerably with the film growth method [10,12]. In general, we expect that a lower refractive index will indicate a lower film quality, as the change of the refractive index versus the Ba:Sr ratio is much less than the change in the refractive index versus film quality.…”

Final-state effect on x-ray photoelectron spectrum of nominallyd1andn-dopedd0transition-metal oxides

“…n core = 0), all TM d levels are shifted down by U dc to screen the core hole. By fitting to published experimental data from XPS of SrTiO 3 , ellipsometry, and angle-resolved photoemission spectroscopy (ARPES) [15,32,[41][42][43], we take ǫ d (e g ) = 2.0 eV, ǫ d (t 2g ) = 0 eV, ǫ p (Γ) = −3.0 eV, V (e g ) = 2.5eV, V (t 2g ) = −1.3 eV, U = 6.0 eV, and U dc = 8.0 eV [44]. This problem can be solved exactly by the technique introduced by Gunnarsson and Schönhammer [12,45], and the details are provided in Appendix B.…”