Dielectric and electrical properties of sputter grown (Ba,Sr)TiO3 thin films

Abstract

The dielectric behavior containing the film thickness dependent dielectric constant, capacitance–voltage $\mathrm{(C–V)}$ variation, and the electrical conduction behavior containing the film thickness dependent current density-electric field variation of rf sputter deposited $(\mathrm{Ba},\mathrm{Sr}){\mathrm{TiO}}_3$ (BST) thin films are analyzed based on a fully depleted film and a combined Schottky-tunneling conduction model. The fact that the capacitance values in $\mathrm{C–V}$ increase with the increasing film thickness clearly indicates that the films are fully depleted. Also, the decreasing and nonvariant dielectric constants of the BST film on the Pt and ${\mathrm{IrO}}_2$ electrodes, respectively, with decreasing film thickness are attributed to the intrinsic interfacial layers at the interfaces with electrodes which have low dielectric constants and very small thicknesses. The increased leakage current density of a thicker film under a given electric field originates from the increased interfacial field strength due to the high space charge density in the sputtered film. The leakage current density under the electric field strength smaller than 120 kV/cm is controlled by the Schottky conduction mechanism (thermionic emission), and over that field strength, the tunneling related mechanism (thermionic field emission) is predominant. The calculated leakage current density on the basis of the combined Schottky-tunneling conduction model fits well to the measured current density at various temperatures and over a wide field range. For all the calculations, the electric field dependent dielectric constant is always taken into account. The electric field dependent dielectric constant and high defect or space charge density of the BST film make the conventional analysis of the dielectric and conduction behavior based on the classical metal/semiconductor contact theory invalid. Therefore, a more comprehensive formalism is developed to take into account the field dependent dielectric properties and high charge density of the sputter deposited BST films.