Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films

Abstract

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol–gel-derived Sr1−1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Structure, Microstructure, and Dielectric Response of Polycrystalline Sr1-xZnxTiO3 Thin Films

“…In the case of ST thin films, these changes can be significantly enhanced, resulting e.g., in ferroelectricity induced at room temperature in epitaxial ST films by DyScO 3 substrate lattice misfit strain [9]. In addition to the strain [5,9,10], dopants can also induce a variety of phases, ranging from dipolar glass and relaxor to ferroelectric, at isovalent substitution for Sr 2+ by Ba 2+ [6,11], Pb 2+ [6], Ca 2+ [12,13], and Mn 2+ ions [8,14] as well as at heterovalent substitution for Sr 2+ by Bi 3+ [7,[15][16][17], Y 3+ [18][19][20], Dy 3+ [21] and Gd 3+ ions [22,23]. We would wish to note here, that since the perovskite structure of SrTiO 3 is tolerant to substitutional dopants, whereas interstitial doping is not likely to occur in SrTiO 3 [24], doping in this work means ionic substitution.…”