Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO 3 .

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO ₃ has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable". Here we redeem CaSnO ₃ using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO ₃ films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO ₃ with free electron concentrations, n _3D , from 3.3 × 10 ¹⁹ cm ^-3 to 1.6 × 10 ²⁰ cm ^-3 . The films exhibit a maximum room-temperature mobility of 42 cm ² V ^-1 s ^-1 at n _3D = 3.3 × 10 ¹⁹ cm ^-3 . Despite having a comparable radius as the host ion, La expands the lattice parameter. Using density functional calculations, this effect is attributed to the energy gain by lowering the conduction band upon volume expansion. Finally, we exploit robust doping by fabricating CaSnO ₃ -based field-effect transistors. The transistors show promise for CaSnO ₃ 's high-voltage capabilities by exhibiting low off-state leakage below 2 × 10 ^-5 mA/mm at a drain-source voltage of 100 V and on-off ratios exceeding 10 ⁶ . This work serves as a starting point for future studies on the semiconducting properties of CaSnO ₃ and many devices that could benefit from CaSnO ₃ 's exceptionally wide band gap.