Studies on structural and electrical properties of barium strontium titanate thin films developed by metallo-organic decomposition

Abstract

Thin films of barium strontium titanate (BST) including BaTiO₃ and SrTiO₃ end members were deposited using the metallo-organic decomposition (MOD) technique. Processing parameters such as nonstoichiometry, annealing temperature and time, film thickness and doping concentration were correlated with the structural and electrical properties of the films. A random polycrystalline structure was observed for all MOD films under the processing conditions in this study. The microstructures of the films showed multi-grains structure through the film thickness. A dielectric constant of 563 was observed for (Ba_0.7Sr_0.3)TiO₃ films rapid thermal annealed at 750° C for 60 s. The dielectric constant increased with annealing temperature and film thickness, while the dielectric constant could reach the bulk values for thicknesses as thin as ~0.3 µ m. Nonstoichiometry and doping in the films resulted in a lowering of the dielectric constant. For near-stoichiometric films, a small dielectric dispersion obeying the Curie-von Schweidler type dielectric response was observed. This behavior may be attributed to the presence of the high density of disordered grain boundaries. All MOD processed films showed trap-distributed space-charge limited conduction (SCLC) behavior with slope of ~7.5-10 regardless of the chemistry and processing parameter due to the presence of grain boundaries through the film thickness. The grain boundaries masked the effect of donor-doping, so that all films showed distributed-trap SCLC behavior without discrete-traps. Donor-doping could significantly improve the time-dependent dielectric breakdown behavior of BST thin films, mostly likely due to the lower oxygen vacancy concentration resulted from donor-doping. From the results of charge storage density, leakage current and time-dependent dielectric breakdown behavior, BST thin films are found to be promising candidates for 64 and 256 Mb ULSI DRAM applications.