Wide bandgap oxides such as tin-doped indium oxide (ITO), zinc oxide (ZnO), and tin oxide (SnO₂) are currently used in a variety of technologically important applications, including gas sensors and transparent conducting films for devices such as flat panel displays and photovoltaics. Due to the focus on industrial applications, prior research did not investigate the basic material properties of SnO₂ films due to unoptimized growth methods such as RF sputtering and pulsed laser deposition which produced low resistance, polycrystalline films. Beyond these applications, few attempts to enhance and control the fundamental SnO₂ properties for semiconducting applications have been reported.

This work develops the heteroepitaxy of SnO₂ thin films on r-plane Al₂O₃ by plasma-assisted molecular beam epitaxy (PA-MBE) and demonstrates control of the electrical transport of those films. Phase-pure, epitaxial single crystalline films were controllably and reproducibly grown. X-ray diffraction measurements indicated that these films exhibited the highest structural quality reported. Depending on the epitaxial conditions, tin- and oxygen-rich growth regimes were observed. An unexpected growth rate decrease in the tin-rich regime was determined to be caused by volatile suboxide formation. Excellent transport properties for naturally n-type SnO₂ were achieved: the electron mobility, μ, was 103 cm²/V s at a concentration, n, of 2.7 × 10¹⁷ cm^-3. To control the bulk electron density, antimony was used as an intentional n-type dopant. Antimony-doped film properties showed the highest reported mobilities for doped films (μ = 36 cm²/V s for n = 2.8 × 10 ²⁰ cm^-3). Films doped with indium had resistivities over five orders-of-magnitude greater than undoped films. These highly resistive films provided a method to control the electrical transport properties. Further research will facilitate detailed studies of the fundamental properties of SnO₂ and its development as an oxide with full semiconducting properties.