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In the molecular beam epitaxy of oxide films, the cation (Sn, Ga) or dopant(Sn) incorporation does not follow the vapor pressure of the elemental metalsources, but is enhanced by several orders of magnitude for low sourcetemperatures. Using line-of-sight quadrupole mass spectrometry, we identify thedominant contribution to the total flux emanating from Sn and Ga sources atthese temperatures to be due to the unintentional formation and evaporation ofthe respective suboxides SnO and Ga$_{2}$O. We quantitatively describe thisphenomenon by a rate-equation model that takes into account the O backgroundpressure, the resulting formation of the suboxides via oxidation of the metalsource, and their subsequent thermally activated evaporation. As a result, thetotal flux composed of the metal and the suboxide fluxes exhibit an\textsf{S}-shape temperature dependence instead of the expected linear one inan Arrhenius plot, in excellent agreement with the available experimental data.Our model reveals that the thermally activated regimes at low and hightemperatures are almost exclusively due to suboxide and metal evaporation,respectively, joined by an intermediate plateau-like regime in which the fluxis limited by the available amount of O. An important suboxide contribution isexpected for all elemental sources whose suboxide exhibits a higher vaporpressure than the element, such as B, Ga, In, La, Si, Ge, Sn, Sb, Mo, Nb, Ru,Ta, V, and W. This contribution can play a decisive role in the molecular beamepitaxy of oxides, including multicomponent or complex oxides, from elementalsources. Finally, our model predicts suboxide-dominated growth in low-pressurechemical vapor deposition of Ga$_{2}$O$_{3}$ and In$_{2}$O$_{3}$.

Drastically enhanced cation incorporation in the epitaxy of oxides due to formation and evaporation of suboxides from elemental sources