Infrared phonon spectroscopy of a compressively strained (001) SrTiO3 film grown on a (110) NdGaO3 substrate

Polarized infrared reflectivity was measured between 5 and 300 K on a 17 nm thick, 1.1% compressively strained epitaxial (001) SrTiO3 film and the orthorhombic (110) NdGaO3 substrate upon which it was grown. A strong in-plane infrared anisotropy of the NdGaO3 substrate was observed and polar modes with B-1u-and a mixture of B-2u + B-3u-symmetry were seen. At low temperatures three new modes arose in the 90-130 cm(-1) range, which we assigned to 4f Nd electronic transitions. The in-plane SrTiO3 film phonons showed strong stiffening compared to the phonon frequencies of bulk unstrained SrTiO3, particularly the soft mode, and the in-plane phonon peaks were found to split. No anomalies were detected as a function of temperature in either the infrared response or lattice parameters of the compressively strained SrTiO3 film, providing an absence of evidence for the out-of-plane ferroelectric phase transition predicted by theory.