Relaxor ferroelectric-like high effective permittivity in leaky dielectrics/oxide semiconductors induced by electrode effects: A case study of CuO ceramics

摘要

The electrical behavior of copper oxide (CuO) ceramics sintered at 920 ℃ has been characterized by a combination of fixed, radio frequency (rf) capacitance measurements, and impedance spectroscopy (IS). Fixed rf capacitance measurements on ceramics with sputtered Au electrodes revealed a temperature- and frequency-dependent high effective permittivity of ～10~4 in the temperature range of 150-320 K. The response is similar to that observed for relaxor-ferroelectrics, however, the magnitude of the effect can be suppressed by thermal annealing of the ceramics with Au electrodes in air at 300 ℃ or by changing the work function of the electrode material by using In-Ga as opposed to Au. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductors with a room temperature dc resistivity <10~4 Ω cm, consisting of semiconducting grains with relative permittivity, ε_r <10 and slightly more resistive grain boundaries with "effective" permittivity, ε_(eff), of ～110. Samples with Au electrodes exhibited an additional low frequency response with ε_(ef) ～ 10~4. dc bias experiments showed the capacitance behavior of this additional response to obey the Mott-Schottky law and thus confirm it to be a non-Ohmic electrode contact. We conclude, therefore, that an electrode rather than a grain boundary effect is the primary source for the high effective permittivity in CuO ceramics, although the latter is also present and does give additional effective permittivity. This work demonstrates how an extrinsic effect associated with non-Ohmic electrode contacts can; (ⅰ) dominate the rf capacitance spectra of leaky dielectrics/oxide semiconductors over a wide temperature and frequency range; and, (ⅱ) manifest a dielectric response more typically associated with relaxor-ferroelectrics.