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

We have studied the optical properties (complex dielectric function) of bul k 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 i nfrared (FTIR) ellipsometry] to 8.7 eV. Ln the bulk crystals, we analyze th e interband transitions in the spectra to determine the critical-point para meters. To interpret these transitions, we performed band structure calcula tions based on ab initio pseudopotentials within the local-density approxim ation. The dielectric function was also calculated within this framework an d compared with our ellipsometry data. In the FTIR ellipsometry data, we no tice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic (LO) phonons were also identified. In SrTiO3 films on S i, the refractive index below the hand gap decreases with decreasing thickn ess because of the increasing influence of the amorphous interfacial layer between the SrTiO3 film and the Si substrate. There is also a decrease in a mplitude and an increase in broadening of the critical points with decreasi ng thickness. In SrTiO3 films on Pt, there is a strong correlation between the crystallinity and texture of the films (mostly aligned with the Pt pseu dosubstrate) and the magnitude of the refractive index, the Urbach tail bel ow the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO3 on Pt (reflection-absorption spectroscopy) show abso rption peaks at the LO phonon energies, a typical manifestation of the Berr eman effect for thin insulating films on a metal. The Urbach tail in our el lipsomety data and the broadening of the optical phonons in SrTiO3 on Pt ar e most likely caused by oxygen vacancy clusters. (C) 2000 American Vacuum S ociety. [S0734-211X(00)00904-5].