Variations of the leakage current density and the dielectric constant of Pt/(Ba,Sr)TiO3/Pt capacitors by annealing under a N2 atmosphere

This article reports variations of the leakage current density of Pt/40 nm-${\mathrm{(Ba,Sr)TiO}}_{\mathrm{3}}\mathrm{/Pt}$ capacitors after annealing under a ${\mathrm{N}}_2$ atmosphere containing oxygen of about 0.01 ppm. Annealing a capacitor under a ${\mathrm{N}}_2$ atmosphere at 750 °C for 30 min results in an increase of the leakage current density at 1.5 V by more than six orders of magnitude compared to that of a capacitor annealed under the optimum oxygen concentration (about 5% in ${\mathrm{O}}_2$ ) at the same temperature. The dielectric constant was decreased after annealing under the ${\mathrm{N}}_2$ atmosphere. The composition of ${\mathrm{(Ba,Sr)TiO}}_3$ (BST) films, measured by Auger electron spectroscopy, did not show any difference between the two samples. The major reason for the huge increase in the leakage current density is due to the roughening of top Pt electrodes by the annealing under ${\mathrm{N}}_2$ atmosphere. Such roughening imposes a strong stress to the BST film surface and generates surface trap states. The stress-induced trap states reduce the interfacial potential barrier height for Schottky emission and result in increased leakage current densities. Capping of the top Pt electrode with a very thin BST layer (about 10 nm) before the annealing eliminates surface roughening, preserves a low leakage current level, and increases dielectric constants.