Strain Phase Diagram and Domain Orientation in ${\mathrm{SrTiO}}_3$ Thin Films

${\mathrm{SrTiO}}_3$ thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained ${\mathrm{SrTiO}}_3$ films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory; however, the increase in the phase transition temperature is much larger than predicted. Because of the epitaxial strain and substrate clamping, the ${\mathrm{SrTiO}}_3$ lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic $Cmcm$ space group but a tetragonal lattice, an interesting consequence of epitaxial constraint.

Figure 1

Temperature dependence of STO superlattice peak intensities, showing $T_s$ for different samples. For LAO (KTO) substrate, the strain in STO is compressive (tensile).

Figure 2

The presence of ($1/2$ $3/2$ $5/2$) peak but not ($1/2$ $1/2$ $7/2$) peak at 20 K in 90 nm STO/LAO sample proves that the unique axis $c$ of STO film is out of plane.

Figure 3

Structural phase diagram of strained STO. The theoretical prediction from Ref. 8 is shown as dashed lines for comparison. Solid lines are from a fitting to experimental data [16].