Physical properties of transparent perovskite oxides (Ba,La)SnO3with high electrical mobility at room temperature

Abstract

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications, ranging from passive transparent conductive windows to active thin-film transistors. BaSnO${}_{3}$ is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that BaSnO${}_{3}$ doped with a few percent of La exhibits an unusually high electrical mobility of $320\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at room temperature and superior thermal stability at high temperatures [H. J. Kim et al., Appl. Phys. Express 5, 061102 (2012)]. Following that paper, here, we report various physical properties of (Ba,La)SnO${}_{3}$ single crystals and epitaxial films including temperature-dependent transport and phonon properties, optical properties, and first-principles calculations. We find that almost doping-independent mobility of $200\ensuremath{-}300\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ is realized in the single crystals in a broad doping range from $1.0\ifmmode\times\else\texttimes\fi{}{10}^{19}$ to $4.0\ifmmode\times\else\texttimes\fi{}{10}^{20}$ cm${}^{\ensuremath{-}3}$. Moreover, the conductivity of $\ensuremath{\sim}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ reached at the latter carrier density is comparable to the highest value previously reported in the transparent conducting oxides. We attribute the high mobility to several physical properties of (Ba,La)SnO${}_{3}$: a small effective mass coming from the ideal Sn-O-Sn bonding in a cubic perovskite network, small disorder effects due to the doping away from the SnO${}_{6}$ octahedra, and reduced carrier scattering due to the high dielectric constant. The observation of the reduced mobility of $\ensuremath{\sim}70\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ in the epitaxial films is mainly attributed to additional carrier scattering due to dislocations and grain boundaries, which are presumably created by the lattice mismatch between the substrate SrTiO${}_{3}$ and (Ba,La)SnO${}_{3}$. The main optical gap coming from the charge transfer from O 2$p$ to Sn 5$s$ bands in (Ba,La)SnO${}_{3}$ single crystals remained at about 3.33 eV, and the in-gap states only slightly increased, thus, maintaining optical transparency in the visible spectral region. Based on all these results, we suggest that the doped BaSnO${}_{3}$ system holds great potential for realizing all perovskite-based transparent high-temperature high-power functional devices as well as highly mobile two-dimensional electron gas via an interface control of heterostructured films.