Pyroelectric properties of barium strontium titanate films: Effect of thermal stresses

A nonlinear thermodynamic model is used to investigate the influence of thermal stresses on the room temperature (RT) pyroelectric properties of (001)-textured polycrystalline barium strontium titanate (BaxSr1−xTiO3, BST) films with four different compositions on three integrated circuit-friendly substrates: Si, (0001) and (1¯1¯20) sapphire. It is found that the in-plane tensile thermal strain in BST increases with increasing deposition/processing temperature TG due to the difference of thermal expansion coefficients between the BST film and the selected substrate materials. Considering the variations in the ferroelectric phase transformation due to thermal strains, our findings show that relatively lower processing temperatures would result in better pyroelectric response. For BST 60/40 and 70/30 on all the substrates, the film is in the paraelectric state for TG=25–800 °C and the pyroelectric response is due to the interplay of the electric field and thermal strain dependence of the dielectric constant and the TG-dependence of the thermal strains. While the RT pyroelectric response of these films is expected to be relatively low (0.02–0.05 μC/cm2 K at ∼200 kV/cm), at high applied fields the pyroelectric properties do not display a significant dependence on the temperature at which the material was treated. On the other hand, BST 80/20 and BST 90/10 films may be in the ferroelectric state at RT depending on TG. If TG can be adjusted such that the ferroelectric phase transformation is near RT, a relatively high bulklike pyroelectric response (>0.5–1.0 μC/cm2 K) can be realized. However, the pyroelectric coefficient decreases sharply above and below these “critical” TG, and thus a precise temperature control during processing would be required.