Acceptor and compensating donor doping of single crystalline SnO (001) films grown by molecular beam epitaxy and its perspectives for optoelectronics and gas-sensing

(La and Ga)-doped tin monoxide (stannous oxide, tin (II) oxide, SnO) thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from $\approx 5\times {10}^{18}$cm${}^{-3}$ to $2\times {10}^{21}$cm${}^{-3}$. In this concentration range, the incorporation of Ga into SnO was limited by the formation of secondary phases observed at $1.2\times {10}^{21}$cm${}^{-3}$ Ga, while the incorporation of La showed a lower solubility limit. Transport measurements on the doped samples reveal that Ga acts as an acceptor and La as a compensating donor. While Ga doping led to an increase of the hole concentration from $1\times {10}^{18}$cm${}^{-3}-1\times {10}^{19}$cm${}^{-3}$ for unintentionally (UID) SnO up to $5\times {10}^{19}$cm${}^{-3}$, La-concentrations well in excess of the UID acceptor concentration resulted in semi-insulating films without detectable $n$-type conductivity. Ab-initio calculations qualitatively agree with our dopant assignment of Ga and La, and further predict In${}_{\text{Sn}}$ to act as an acceptor as well as Al${}_{\text{Sn}}$ and B${}_{\text{Sn}}$ as donor. These results show the possibilities of controlling the hole concentration in $p$-type SnO, which can be useful for a range of optoelectronic and gas-sensing applications.