Abstract
SrTiO3 (STO) thin films of different thicknesses were deposited on MgAl2O4 (MAO) substrates to investigate the in-plane strain effect on the soft-mode frequency of the STO films. X-ray reciprocal space mapping (X-RSM) results indicate that there was no relaxation of the in-plane lattice strain of the STO films on MAO. Shifts in the soft-mode frequencies with a decrease in the film thickness were observed using terahertz time-domain spectroscopy (THz-TDS). However, despite the larger lattice mismatch between STO and MAO than that between STO and DyScO3 (DSO), the shifts in the soft-mode frequencies of the STO films on MAO were smaller than those on DSO. The results indicate that the soft-mode frequencies of the STO films on MAO are affected by the c-axis (out-of-plane) lengths.
Highlights
Strontium titanate (STO) is a transition metal oxide that has a perovskite structure
That the strain in the STO thin film on MAO used in that experiment was somewhat relaxed, and sufficient strain required for the generation of ferroelectricity at room temperature was not induced in the film
The c-axis length of the STO film on LSAT is longer than that of a bulk STO single crystal owing to in-plane compressive strain, while that on DSO was compressed owing to in-plane tensile strain
Summary
Strontium titanate (STO) is a transition metal oxide that has a perovskite structure. Lattice strain in STO changes the phase transition temperature drastically and induces ferroelectricity [2], and, Haeni et al [3] have reported room-temperature ferro-electricity in a strained STO thin film deposited on a DyScO3 (DSO) substrate. There have only been a few reports concerning the ferroelectric and dielectric properties of STO thin films induced by large in-plane strain. The temperature dependence of the dielectric dispersion of a 360 nm thick STO film on MAO in the terahertz region showed signs of a ferroelectric phase transition at around 170 K, much lower than that expected from the lattice mismatch [10]. That the strain in the STO thin film on MAO used in that experiment was somewhat relaxed, and sufficient strain required for the generation of ferroelectricity at room temperature was not induced in the film. Further experimental study is necessary to reveal the strain effects in STO thin films on MAO substrates
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