LAUSR

When it comes to providing the unusual combination of optical transparency, $p$-type conductivity, and relatively high mobility, ${\mathrm{Sn}}^{2+}$-based oxides are promising candidates. Epitaxial films of the simplest ${\mathrm{Sn}}^{2+}$ oxide, SnO, are grown in an adsorption-controlled regime at $380{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates by molecular-beam epitaxy, where the excess volatile ${\mathrm{SnO}}_{x}$ desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch ($\ensuremath{-}12%$). The unintentionally doped films exhibit $p$-type conductivity with carrier concentration $2.5\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and mobility $2.4\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at room temperature. Additional physical properties are measured and linked to the ${\mathrm{Sn}}^{2+}$ valence state and corresponding lone-pair charge-density distribution.