Doping Effect on Ferroelectric Soft Optic Mode of SrTiO3 Crystals with Cubic Perovskite Structure

In order to clarify the correlation between the polar instability and carrier electrons in provskite oxides, the terahertz dielectric response was studied in A- and B-site replaced strontium titanate SrTiO3 (STO) crystals. Ferroelectric soft optic modes with T1u symmetry and carrier electrons in the Pm-3m phase were investigated by the measurement of the complex dielectric constants using the terahertz time-domain spectroscopic ellipsometry (THz-TDSE). The observed real and imaginary parts of a complex dielectric constant were analyzed by the damped harmonic oscillator model for the ferroelectric soft optic mode and the Drude-Smith model for carrier electrons introduced by heterovalent doping. For the A-site replaced La doped STO and B-site replaced Nb doped STO crystals, the soft mode frequency increases as the La and Nb content increases. The good correlation is found between the ferroelectric soft mode frequency and the carrier electron concentration. It is concluded from this fact that the increase of the carrier electron concentration induced by donor doping essentially suppresses the ferroelectric instability and the hardening of a ferroelectric soft mode frequency occurs.