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 x 10(19) cm(-3) to 1.6 x 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 x 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 CaSnO3-based field-effect transistors. The transistors show promise for CaSnO3's high-voltage capabilities by exhibiting low off-state leakage below 2 x 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 CaSnO3 and many devices that could benefit from CaSnO3's exceptionally wide band gap.

Automated summary

The alkaline earth stannates, specifically CaSnO3, have been proven to be a promising ultrawide band gap semiconductor with high room-temperature electron mobilities. Previous theoretical studies and doping attempts had labeled CaSnO3 as an undopable material, but using hybrid molecular beam epitaxy, the researchers successfully demonstrated predictable doping of CaSnO3 using lanthanum as an n-type dopant. The resulting films exhibited high electron concentrations and impressive room-temperature mobility, leading to the fabrication of CaSnO3-based field-effect transistors with low off-state leakage and high on-off ratios. This work serves as a starting point for further exploration of CaSnO3's semiconducting properties and potential applications.