Abstract
Titanium dioxide (TiO2) is a high-performance material for emerging device applications, such as in resistive switching memories, in high-k capacitors, or, due to its flexoelectricity, in micro/nano-electro-mechanical systems. Enhanced electrical properties of TiO2 are ensured, especially by a careful selection of the bottom electrode material. Iridium dioxide (IrO2) is an excellent choice, as it favors the high-k rutile phase growth of TiO2. In this study, we introduce the fabrication of IrO2/TiO2/IrO2 capacitors and thoroughly characterize their electrical behavior. These capacitors show a dielectric constant for low temperature sputtered TiO2 of ∼70. From leakage current measurements, a coupled capacitive–memristive behavior is determined, which is assumed due to the presence of a reduced TiO2−x layer at the IrO2/TiO2 interface observed from transmission electron microscopy analyses. The memristive effect most probably originates from trapping and detrapping of electric charges in oxygen vacancy defects, which themselves can be generated and annihilated through an applied electric field, subsequently changing the resistance of the capacitor. The electric degradation type is identified as a filament-forming mechanism. Additionally, the temperature dependence of the leakage current is measured, demonstrating that the temperature behavior is strongly influenced by the ambient atmosphere. The latter dependency leads to the hypothesis that the oxygen evolution reaction of water incorporated in the IrO2/TiO2 interface passivates vacancies, thus significantly impacting the vacancy density in TiO2 and, as a further consequence, the electrical performance.
Highlights
Specific metal oxides offer outstanding properties for capacitors that make them unique for future electronic applications
We introduce the fabrication of IrO2/TiO2/IrO2 capacitors and thoroughly characterize their electrical behavior
A coupled capacitive–memristive behavior is determined, which is assumed due to the presence of a reduced TiO2−x layer at the IrO2/TiO2 interface observed from transmission electron microscopy analyses
Summary
Specific metal oxides offer outstanding properties for capacitors that make them unique for future electronic applications. To characterize the physical mechanisms that dominate the electrical behavior of a metal–insulator–metal (MIM) capacitor, it is common to perform current–voltage (I–V) measurements at different temperatures, which are evaluated with standard models, e.g., thermionic emission (Poole–Frenkel and Schottky) or tunneling (direct tunneling and trap assisted tunneling) These models work well for most capacitor materials, applying these models to oxygen vacancy rich materials like high permittivity oxides, such as TiO2, falls short. Through careful evaluation of leakage current measurements at different temperatures, by variation of the dielectric thickness values and by the exposure to ambient atmosphere in between deposition steps, an understanding of the underlying physical mechanisms was gained and is presented in this work This knowledge, in turn, can be used for future investigations, such as a temperaturedependent evaluation of flexoelectrically excited MEMS cantilevers
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