  We calculate the spectrum of elementary excitations in a two-dimensional
exciton superfluid in the vicinity of a two-dimensional electron gas. We show
that attraction of excitons due to their scattering with free electrons may
lead to formation of a roton minimum. The energy of this minimum may go below
the ground state energy which manifests breaking of the superfluidity. The
Berezinsky-Kosterlitz-Thouless phase transition temperature decreases due to
the exciton-exciton attraction mediated by electrons.
