We investigated the dielectric functions $\stackrel{̑}{\epsilon }\left(\omega \right)$ of Ir, Ru, Pt, and ${\mathrm{IrO}}_2$, which are commonly used as electrodes in ferroelectric thin-film applications. In particular, we investigated the contributions from bound charges ${\stackrel{̑}{\epsilon }}^b\left(\omega \right)$, since these are important scientifically as well as technologically: the ${\epsilon }_1^b\left(0\right)$ of a metal electrode is one of the major factors determining the depolarization field inside a ferroelectric capacitor. To obtain ${\epsilon }_1^b\left(0\right)$, we measured reflectivity spectra of sputtered Pt, Ir, Ru, and ${\mathrm{IrO}}_2$ films in a wide photon energy range between $3.7\mathrm{meV}$ and $20\mathrm{eV}$. We used a Kramers-Kronig transformation to obtain real and imaginary dielectric functions, and then used Drude-Lorentz oscillator fittings to extract ${\epsilon }_1^b\left(0\right)$ values. Ir, Ru, Pt, and ${\mathrm{IrO}}_2$ produced experimental ${\epsilon }_1^b\left(0\right)$ values of $48\pm 10$, $82\pm 10$, $58\pm 10$, and $29\pm 5$, respectively, which are in good agreement with values obtained using first-principles calculations. These values are much higher than those for noble metals such as Cu, Ag, and Au because transition metals and ${\mathrm{IrO}}_2$ have such strong $d\text{−}d$ transitions below $2.0\mathrm{eV}$. High ${\epsilon }_1^b\left(0\right)$ values will reduce the depolarization field in ferroelectric capacitors, making these materials good candidates for use as electrodes in ferroelectric applications.

• Received 2 July 2006

DOI:https://doi.org/10.1103/PhysRevB.74.205117