Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO₃

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO₃ 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 CaSnO₃ using hybrid molecular beam epitaxy, which provides an adsorption-controlled growth for the phase-pure, epitaxial, and stoichiometric CaSnO₃ films. By introducing lanthanum (La) as an n-type dopant, we demonstrate the robust and predictable doping of CaSnO₃ with free electron concentrations, n_3D, from 3.3 × 10¹⁹ cm^-3 to 1.6 × 10²⁰ cm^-3. The films exhibit a maximum room-temperature mobility of 42 cm² V^-1 s^-1 at n_3D = 3.3 × 10¹⁹ 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 CaSnO₃-based field-effect transistors. The transistors show promise for CaSnO₃'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 10⁶. This work serves as a starting point for future studies on the semiconducting properties of CaSnO₃ and many devices that could benefit from CaSnO₃'s exceptionally wide band gap.