  We investigate lanthanum (La) as an n-type dopant in the strain-stabilized
tetragonal phase of SrSnO3 grown on GdScO3 (110) using a radical-based hybrid
molecular beam epitaxy approach. Fully coherent, epitaxial films with
atomically smooth film surface were obtained irrespective of doping density. By
combining secondary ion mass spectroscopy and Hall measurements, we demonstrate
that each La atom contributes to one electron to the film confirming it
occupies Sr-site in SrSnO3 and that it is completely activated. Carrier density
exceeding 1 x 10^20 cm-3 was achieved in LSSO films, which is in excellent
agreement with the dopant-solubility limit predicted by the density functional
theory calculations. A record-high room-temperature mobility of 70 cm2V-1s-1 at
1 x 10^20 cm-3 was obtained in 12 nm La-doped SrSnO3 film making this the
thinnest perovskite oxide semiconductor with a reasonably high electron
mobility at room temperature. We discuss the structure-dopant-transport
property relationships providing essential knowledge for the design of
electronic devices using these materials.
