Effect of Misfit Strain Relaxation on Nonlinear Dielectric Properties of Epitaxial (Ba0.60 Sr0.40) Thin Films on NdGaO3

Strain relaxation in (Ba_0.60 Sr_0.40)TiO₃ epitaxial films on <110>-oriented NdGaO₃ substrates is investigated in the thickness range, h= 25-1200 nm. The BST films prepared by PLD show that residual strains mostly relax by h~200 nm, and for h>600 nm films are essentially strain free. Two independent dislocation mechanisms operate to relax anisotropic strains along the principal directions. The critical thickness for misfit dislocation formation along [001] and [010] were found to be 11 and 15 nm, respectively. Deviation from linear elasticity for h<200 was observed, and increased as thickness decreased. Films with h<25 nm are monoclinic due to a finite principal shear stress along [110] of the BST cell. The effects of misfit strain relaxation on the nonlinear dielectric response and tunability will be discussed as well. The in plane dielectric response demonstrates a directional dependence that increases with the magnitude of the residual strain.