Temperature-dependent structural characterization of sol-gel deposited strontium titanate (SrTiO3) thin films using Raman spectroscopy

The dynamics of the lattice optical phonons in strontium titanate (SrTiO3) thin films deposited on M-plane sapphire [Al2O3(10(1) over bar 0)] substrate using the sol-gel route were investigated. A detailed study of the temperature dependence of first-order zone-center optical phonons in the 70-300 K temperature range was carried out by means of Raman spectroscopy. The transverse optical phonons (denoted by TO1,2,3,4 in order of increasing wavenumber), which are symmetry forbidden in single crystals at room temperature, become Raman active in thin films. The lowering of symmetry is attributed either to the micro-stress/strain or the presence of defects (such as oxygen vacancies V-0), intrinsic defects in oxidic perovskites. The temperature dependence of the lowest wavenumber mode (TO1) is discussed and its Raman cross-section was utilized to demonstrate the first-order nature of Raman spectra. The Fano line profile and the associated asymmetric factor denoted by q of the TO2 phonon is interpreted as evidence of micro- or nanopolar regions (also known as ferroelectric microregions) in thin films, possibly arising due to highly polarized oxygen ions, likely to exist because of incomplete oxygenation during the film deposition. These optical phonon lines and the asymmetry of TO2 persist up to room temperature, another implication of the violation of the k conservation rule by breaking either the inversion or/and translation symmetry. The significant utility of micro-Raman scattering in contrast to x-ray diffraction as an important structural probe is also demonstrated. Copyright (C) 2001 John Wiley & Sons, Ltd.