Structural, dielectric, and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films with conductive LaNiO3 bottom electrodes

We report structural, dielectric, and ferroelectric properties of compositionally graded $(\mathrm{Pb,La}){\mathrm{TiO}}_3$ thin films on ${\mathrm{LaNiO}}_{\mathrm{3}}{\mathrm{/SiO}}_{\mathrm{2}}\mathrm{/Si}$ substrates, where all the films were prepared by the sol-gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of film layer. Upgraded film exhibited (100) preferentially oriented growth, whereas downgraded film showed a randomly oriented growth, where the films with La content increasing or decreasing gradually along film thickness from the substrate to the top surface are called “upgraded” or “downgraded” films, respectively. The dielectric constants, for upgraded and downgraded films annealed at 650 °C for 60 min, were found to be 659 and 641, respectively. The thin films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of the offsets displayed a power law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The leakage current of the upgraded film was ${\text{3.43×10}}^{\text{−8}}\hspace{0.17em}{\mathrm{A/cm}}^2$ at the voltage of 3 V. These results showed that the compositionally graded $(\mathrm{Pb,La}){\mathrm{TiO}}_3$ thin films had excellent dielectric properties and abnormal ferroelectric properties which can be used in various microelectronic devices.