The optical properties of several commonly used single-crystal oxide substrates were explored by spectroscopic ellipsometry over a wide spectral range from 0.74 eV to 8.8 eV. The crystals examined are (100) SrTiO3, 0.7 % wt Nb-doped (100) SrTiO3,(100) (LaAlO3)0.29(SrAl0.5Ta0.5O3)0.7, (011) DyScO3, (100) MgAl2O4, (100) MgO, and (100) LaAlO3, all of which enable epitaxial growth of numerous perovskite-type and other optical thin films. An analytic form for the complex dielectric function was derived from ellipsometric data through a physically consistent modeling process. The obtained dielectric spectra were further utilized to calculate the complex index of refraction and absorption coefficient for each substrate material. The absorption spectra and optical band gap were analyzed using Tauc plots. The parameters for reconstructing the dielectric functions are given in detail, allowing for extensive applications of the results of this work.

In this work, we determine the dielectric functions of multi-target, reactively sputtered Ba(Zr0·2Ti0.8)O3 by spectroscopic ellipsometry in the photon energy range from 0.8 to 8.7eV. Based on this data, we discuss the interband transition energies and the spectral dependencies of the refractive index and the absorption coefficient.

Relaxor ferroelectric Ba(Zr,Ti)O 3 thin films are a promising active material for lead-free electrocaloric refrigerators. In this work, in-situ crystallized Ba(Zr,Ti)O 3 thin films with fractions of 12, 20 and 35% BaZrO 3 and a thickness of 200 to 300 nm were fabricated on 6-inch Pt(111)-coated Si wafers by multi-target reactive sputtering (MTRS) using three metallic targets (Ba, Zr, and Ti) for the first time. The advantage of MTRS is the possibility to control the BaZrO 3 fraction of deposited thin films by process parameters like power and temperature. The base of the Ba target was directly connected with the RF (13.56 MHz) generator. A unit for pulsed DC (100 kHz, duty cycle ~ 50 %) and arc suppression was connected between power supply and Ti target. High-power-pulse sputtering (50 kHz, duty cycle 50 %) was performed by connecting a switching unit between the DC power supply and the Zr-target. Composition analysis was performed by means of XPS and EDX. SEM images show a dense textured film microstructure. XRD analysis evidenced a single cubic perovskite phase with a dominant (111)/[001] texture. Higher order (00l) peaks indicate a weak peak splitting. The optical properties of the film were investigated by VUV variable-angle spectroscopic ellipsometry. The optical band gap of the films amounted to 3.4 eV. Absorption maxima at about 5.2 and 7.6 eV were attributed to the O 2p valence band to Ti 3d conducting band transition and O 2p to Zr 4d transitions, respectively. Due to the short-range disorder, a pronounced Urbach-tail occurred with a tail width of about 0.3 eV. Films with higher BaZrO 3 fractions exhibit the characteristic for relaxor ferroelectrics: a large and frequency-dependent peak in the temperature dependence of dielectric permittivity. Thus, they provide the large and reversible polarization change required for a large electrocaloric response.

The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K0.5Na0.5NbO3 films. The 20-nm-thick cube-on-cube-type epitaxial films are grown on SrTiO3(001) and DyScO3(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15-4.30 eV and differ by 0.2-0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films.

Cube-on-cube epitaxy of perovskite sub-cell of Pr-doped and undoped NaNbO3 is obtained in 130-nm-thick films on top of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrates. Experimental studies show that the edge of optical absorption red-shifts and some interband transitions change in the films compared to crystals. Bright red luminescence is achieved at room-temperature under ultraviolet excitation in the Pr-doped film. An interband mechanism of luminescence excitation is detected in the film, which is in contrast to the intervalence charge transfer mechanism in the crystal. The results are discussed in terms of epitaxially induced changes of crystal symmetry and ferroelectric polarization in the films. It is suggested that the band structure and interband transitions in NaNbO3 and the transition probabilities in the Pr ions can be significantly modified by these changes.

A lattice strain of 0.3%–1.3% is achieved in epitaxial tetragonal BaTiO3 films grown on (001)-oriented SrTiO3 single-crystal substrates. Our experimental studies of absorption spectra in the range of 0.74–9.0 eV demonstrate that epitaxy produces significant changes in the optical properties of the films compared with those of a reference polydomain BaTiO3 crystal: the absorption edge and the peak at 5 eV strongly blue-shift by 0.2–0.4 eV, the magnitude of the peak at 5 eV drops, and certain spectral features disappear, whereas the absorption peak at 8.5 eV remains unchanged. The observed behavior is attributed to ferroelectric polarization, which is enhanced by epitaxial strain in the films. Our results indicate that epitaxially induced variations of ferroelectric polarization may be used to tailor the optical properties of thin films for photonic and optoelectronic applications.