Terahertz response of SrTiO3 based heterostructures: Influence of strain temperature and electric field

A systematic study of strained SrTiO₃/DyScO₃ heterostructures was performed using time-domain terahertz spectroscopy in the temperature range from 20 to 294 K. Interdigitated electrodes deposited on top of the structures allowed probing of the terahertz response upon an applied electric field. We explored a possibility to increase the total thickness of SrTiO₃ layers while maintaining the epitaxial strain which induces the ferroelectric phase transition. All studied structures underwent the phase transition close to room temperature. Evolution of the complex dielectric spectra with temperature and electric bias was described by a general model involving a harmonic oscillator (soft mode) coupled to a Debye relaxation (central mode). At high temperatures, the soft mode is responsible for nearly all the changes observed in the spectra with temperature and applied electric field. At low temperatures, deep in the ferroelectric phase, the soft mode significantly hardens and loses its importance for the terahertz dynamics; the central mode becomes stronger and it almost completely determines the shape of the measured spectra. The observed variation of the phase transition temperature and of the terahertz response among investigated structures was ascribed to a partial strain relaxation.