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 (a). 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/um(2) 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 x 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.