Leakage current measurements of STO and BST thin films interpreted by the "dead" layer model

Electrical properties of Strontium-Titanate (STO) and Barium-Strontium-Titanate (BST) thin films capacitors were investigated. The STO films were fabricated by chemical solution deposition (CSD) with thickness between 50 and 150 nm, while the BST films were deposited by metal organic chemical vapor deposition (MOCVD) with thickness between 20 and 110 nm. All films were grown on platinized and oxidized silicon wafers. As top electrodes platinum (Pt) was deposited on top of the ceramic film by sputtering. The electrode size varied between 8*10(-3) to 1 mm(2) . The leakage current measurements were performed at different temperatures ranging from 15 to 200degreesC and the applied voltage varied between 0 and +/-4 V. Capacitance was measured at RT up to +/-3 V bias at 1 kHz and 50 mV oscillation voltage. The main results are: The effective barrier heights extracted from the temperature dependence of leakage current are about 1.35 eV for STO and 0.94 eV for BST for the temperature region >100degreesC. The field dependencies of the leakage current show almost perfect linear behavior in a "Schottky" plot for BST while STO reveals 2 separated Schottky regions. The permittivity extracted from the field dependence using the simple thermionic emission model with Schottky lowering results in rather improbable values of the effective Richardson constant A(*) and unphysical values of the relative optical permittivity, epsilon(r,opt) <1. The use of a modified model with low permittivity interface layers ("dead layers"), as suggested by the thickness dependence of the inverse capacitance, solved these difficulties. The parameters extracted from fits of this model are discussed.