  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 metal
sources, but is enhanced by several orders of magnitude for low source
temperatures. Using line-of-sight quadrupole mass spectrometry, we identify the
dominant contribution to the total flux emanating from Sn and Ga sources at
these temperatures to be due to the unintentional formation and evaporation of
the respective suboxides SnO and Ga$_{2}$O. We quantitatively describe this
phenomenon by a rate-equation model that takes into account the O background
pressure, the resulting formation of the suboxides via oxidation of the metal
source, and their subsequent thermally activated evaporation. As a result, the
total flux composed of the metal and the suboxide fluxes exhibit an
\textsf{S}-shape temperature dependence instead of the expected linear one in
an Arrhenius plot, in excellent agreement with the available experimental data.
Our model reveals that the thermally activated regimes at low and high
temperatures are almost exclusively due to suboxide and metal evaporation,
respectively, joined by an intermediate plateau-like regime in which the flux
is limited by the available amount of O. An important suboxide contribution is
expected for all elemental sources whose suboxide exhibits a higher vapor
pressure 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 beam
epitaxy of oxides, including multicomponent or complex oxides, from elemental
sources. Finally, our model predicts suboxide-dominated growth in low-pressure
chemical vapor deposition of Ga$_{2}$O$_{3}$ and In$_{2}$O$_{3}$.
