Lattice Dynamics In Ba_xSr_(1_x)TiO_3 Thin Films Studied By Raman Spectroscopy

摘要

Thin films of Ba_xSr_(1-x)TiO_3 with Ba contents x = 0.05,0.1, 0.2 and 0.5, grown by pulsed laser deposition on SrTiO_3 and LaAlO_3 substrates with SrRuO_3 buffer layers, were studied by Raman spectroscopy. Soft phonon modes were observed, and the temperature behavior of their frequencies and Raman intensities in the range from 5 to 300 K was investigated. The temperature dependence of the soft phonon frequencies and the splitting of the triply degenerated soft mode of the cubic phase into two components of A and E symmetries indicate the ferroelectric phase transition. The E soft mode line is clearly seen in Raman spectra at temperatures away from the ferroelectric phase transition, but it becomes overdamped over a broad range of temperatures around the phase transition. The relative Raman intensity of hard phonon modes as well as the A soft mode decreases gradually with increasing the temperature over abroad range of temperatures. This indicates that the ferroelectric phase transition in the thin films is thermally broadened. Comparison of Raman data for films grown on SrTiO_3 and LaAlO_3 substrates shows the influence of strain on the temperature of the ferroelectric phase transition.