Physical properties of transparent perovskite oxides (Ba,La)SnO ₃ with high electrical mobility at room temperature

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 320cm2V ^-1s ^-1 at room temperature and superior thermal stability at high temperatures. 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 200-300cm2V ^-1s ^-1 is realized in the single crystals in a broad doping range from 1.0×1019 to 4.0×1020 cm ^-3. Moreover, the conductivity of ∼104Ω ^-1cm ^-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 ∼70cm2V ^-1s ^-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 2p to Sn 5s 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.