  In this Letter, we demonstrate that it is possible to form a two-dimensional
(2D) silicene-like Si$_5$Ge compound by replacing the Si atoms occupying on-top
sites in the planar-like structure of epitaxial silicene on ZrB$_2$(0001) by
deposited Ge atoms. For coverages below 1/6 ML, the Ge deposition gives rise to
a Si$_{6-x}$Ge$_{x}$ alloy (with $x$ between 0 and 1) in which the on-top sites
are randomly occupied by Si or Ge atoms. The progressive increase of the
valence band maximum with $x$ observed experimentally originates from a
selective charge transfer from Ge atoms to Si atoms. These achievements provide
evidence for the possibility of engineering the bandgap in 2D SiGe alloys in a
way that is similar for their bulk counterpart.
