In-plane anisotropy in the microwave dielectric properties of $SrTiO3$ films

Microwave tunable dielectric properties of strained (001) $SrTiO3$ thin films epitaxially deposited on (110) $DyScO3$ substrates were studied for in-plane film orientations ([100], [010], [110], and $[−110]$⁠). A significant in-plane anisotropy in dielectric constant and tuning was observed in these $SrTiO3$ films. The highest dielectric constant and tuning at room temperature are observed along the [010] direction of the $SrTiO3$ film (1000 Å thick) (3500 and 70% at $1V∕μm$⁠, respectively), the lowest ones are observed along the [100] direction (i.e., 2000 and 50% at $1V∕μm$⁠, respectively). The dielectric constant and tuning along $[−110]$ and [110] are about 2500 and 30% at $1V∕μm$⁠, respectively, which are intermediary to those along the [010] and [100] directions. The dielectric $Q(=1∕tanδ)$ does not show any large difference for the four directions (i.e., $Q∼10−20$⁠). Also, the phase-transition peak for the $[−110]$ and [110] directions of the $SrTiO3$ film (300 Å thick) is observed at 275 K, which is lower than that for the [010] and [100] directions. The observed in-plane anisotropic dielectric properties have been interpreted based on the phenomenological thermodynamics of film strain.