  Transparent electronic materials are increasingly in demand for a variety of
optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band
gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits
unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and
superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys.
Express. 5, 061102 (2012)]. Following that work, we report various physical
properties of (Ba,La)SnO3 single crystals and films including
temperature-dependent transport and phonon properties, optical properties and
first-principles calculations. We find that almost doping-independent mobility
of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping
range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4
ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest
value. We attribute the high mobility to several physical properties of
(Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding,
small disorder effects due to the doping away from the SnO2 conduction channel,
and reduced carrier scattering due to the high dielectric constant. The
observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly
attributed to additional carrier-scatterings which are presumably created by
the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main
optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the
in-gap states only slightly increased, thus maintaining optical transparency in
the visible region. Based on these, we suggest that the doped BaSnO3 system
holds great potential for realizing all perovskite-based, transparent
high-frequency high-power functional devices as well as highly mobile
two-dimensional electron gas via interface control of heterostructured films.
