Control and Elimination of Biaxial Strain in Laser-Ablated Epitaxial BaxSr1-xTiO3 Films

We report the in-plane (a) and out-of-plane (c) lattice parameters of epitaxial laser-ablated Ba_0.4Sr_0.6TiO₃ films on MgO for a range of O₂ deposition pressures (40-250 mTorr) near the observed transition from a<c to a>c. From these lattice parameters, we calculate the residual strain and stress in terms of hydrostatic and biaxial components. Both components increase sharply with O₂ pressure between 85 and 100 mTorr, consistent with ion peening effects. Postdeposition annealing decreases the hydrostatic strain, but increases the biaxial tension. For both as-deposited and annealed films, we obtain samples with no biaxial strain (i.e., a=c), within experimental uncertainty. Overall, the strain is a combination of hydrostatic and biaxial components, both of which affect the dielectric response. Therefore, consideration and control of both types of strain is important for the optimum performance of devices such as tunable microwave devices and high-density memories.