Optical and electrical properties of lanthanum-modified lead titanate thin films with various lanthanum concentrations

We have studied the effects of La concentration on the optical, dielectric and ferroelectric properties of lead lanthanum titanate (PLT) thin films using the sol-gel method. Most of the properties are greatly affected by the La concentration. The refractive index at λ=632.8 nm increases from 2.39 to 2.44 as the La concentration increases from 15 to 33 mol%, whereas the extinction coefficient is independent of the La concentration. The dielectric constants of the films varied from 340 to 870 with varying La concentration in the range from 15 to 33 mol%. The hysteresis loop becomes narrower with the increase of La concentration from 15 to 28 mol% and slightly wider again with the increase of La concentration from 28 to 33 mol%. Among the films investigated in this research, PLT(28) thin film shows the best dielectric properties for application to the dielectrics of ULSI dynamic random access memories (DRAMs). At the frequency of 100 Hz, the dielectric constant and the loss tangent of PLT(28) thin film are 940 and 0.08, respectively. The leakage current density at 1.5×10 5 V/cm is 1×10 -6 A/cm 2 . The comparison between the simulated and the experimental curves for the switching transient characteristics shows that PLT(28) thin film behaves like a normal dielectric.