  Chiral magnetic Mn$_x$Fe$_{1-x}$Ge compounds have an antisymmetric exchange
interaction that is tunable with the manganese stoichiometric fraction, $x$.
Although millimeter-scale, polycrystalline bulk samples of this family of
compounds have been produced, thin-film versions of these materials will be
necessary for devices. In this study, we demonstrate the growth of epitaxial
Mn$_x$Fe$_{1-x}$Ge thin films on Si (111) substrates with a pure B20 crystal
structure in the stoichiometric fraction range x from 0 to 0.81. Following
systematic physical and magnetic characterization including microwave
absorption spectroscopy, we quantify the antisymmetric exchange interaction and
helical period as a function of $x$, which ranges from 200 nm to 8 nm. Our
results demonstrate an approach to engineering the size of magnetic skyrmions
in epitaxial films that are grown using scalable techniques.
