Morphological and electrical properties of SrTiO₃/TiO₂/SrTiO₃ sandwich structures prepared by plasma sputtering

SrTiO₃ (STO) and TiO₂ are insulating materials with large dielectric constants and opposite signs of the quadratic coefficient of voltage (α). Insertion of a TiO₂ thin film between STO layers increases the linearity of the capacitance in response to an applied voltage, to meet the increasing demand of large-capacitance-density dynamic random access memory capacitors. Both STO and TiO₂ suffer from the problem of high leakage current owing to their almost equivalent and low bandgap energies. To overcome this, the thickness of the thin TiO₂ film sandwiched between the STO films was varied. A magnetron sputtering system equipped with radio frequency and direct current power supply was employed for depositing the thin films. TiN was deposited as the top and bottom metal electrodes to form a metal-insulator metal (MIM) structure, which exhibited a very large linear capacitance density of 21 fF/um² that decreased by increasing the thickness of the TiO₂ film. The leakage current decreased with an increase in the thickness of TiO₂, and for a 27-nm-thick film, the measured leakage current was 2.0×10^-10 A. X-ray diffraction and Raman spectroscopy revealed that TiN, STO, and TiO₂ films are crystalline and TiO₂ has a dominant anatese phase structure.