Ultraviolet Raman Spectroscopy of Ferroelectric Thin Films and Superlattices

Ferroelectrics are materials possessing a spontaneous and switchable electrical polarization, which are exploited for various electronic devices, such as non-volatile memories or tunable microwave devices. In recent years science and technology of ferroelectricity has been moving toward ferroelectrics at the nanoscale. Ultraviolet Raman spectroscopy is a useful tool for the study of wide-bandgap materials such as ferroelectric oxides. We have applied this technique to study nanoscale ferroelectric materials including BaTiO3 ultrathin films and BaTiO3/SrTiO3 superlattice structures. We studied the effects of strain on the lattice vibrations and phase diagram of these materials. Using UV excitation we were able to study the effects of strain on a series of ultrathin BaTiO3 films with layer thicknesses varied from 4 to 25 unit cells. We found that the ferroelectric phase transition varied as a function of film thickness in a very broad temperature range, from ~70 to over 900K. We have also studied the strain effect on phase diagram of BaTiO3/SrTiO3 superlattices that were grown on high quality rare earth scandate (GdScO3, DyScO3, SmScO3, NdScO3) substrates. Our Raman data indicated the presence of different ferroelectric phases with out-of-plane and in-plane components of polarization in superlattices, depending on strain and temperature