Fermi energy, electrical conductivity, and the energy gap of NaNbO3

Abstract

The energy of the valence band maximum of ${\mathrm{NaNbO}}_{3}$ is determined from the Schottky barrier heights at the contacts with low work function Sn-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and high work function ${\mathrm{RuO}}_{2}$ by means of x-ray photoelectron spectroscopy with in situ interface preparation. The measurements reveal a valence-band edge energy, which is comparable to that of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$. The energy gap of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$ is $3.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and comparable to the values of $3.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\mathrm{to}\phantom{\rule{0.16em}{0ex}}3.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, which are determined by means of optical and electron energy loss spectroscopy for ${\mathrm{NaNbO}}_{3}$. It is therefore expected that the conduction band minimum of ${\mathrm{NaNbO}}_{3}$ is also located at a similar energy as the conduction band minimum of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$. If this is the case, it can be expected that donor doping of ${\mathrm{NaNbO}}_{3}$ leads to an electrical conductivity, which is comparable to those of donor-doped ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$ (up to $\ensuremath{\sim}$ $1\phantom{\rule{0.16em}{0ex}}\mathrm{S}/\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$). In contrast, Sr- and Ca-doped ${\mathrm{NaNbO}}_{3}$ bulk ceramics exhibit a room temperature conductivity up to $10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{S}/\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, only slightly higher than that of ${\mathrm{NaNbO}}_{3}$. High-field conductivity measurements and impedance spectroscopy give no indication that the low conductivity is caused by insulating grain boundaries separating electrically conductive grains. It is therefore suggested that the energy gap of ${\mathrm{NaNbO}}_{3}$ is substantially higher than the gap of $3.4\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\mathrm{to}\phantom{\rule{0.16em}{0ex}}3.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ determined from optical spectroscopy reported in literature and from electron energy loss spectroscopy within this paper, as also suggested from electronic structure calculations of ${\mathrm{LiNbO}}_{3}$ [Phys. Rev. B 77, 035106 (2008)].