  We show that mismatch of the piezoelectric parameters between layers of
multiple-quantum well structures leads to modification of the electron-phonon
interaction. In particular, short-wavelength phonons propagating perpendicular
to the layers with wavevector close to $2\pi n/d$, where $d$ is the period of
the structure, induce a strong smoothly-varying component of the
piezo-potential. As a result, they interact efficiently with 2D electrons. It
is shown, that this property leads to emission of collimated
quasi-monochromatic beams of high-frequency acoustic phonons from hot electrons
in multiple-quantum well structures. We argue that this effect is responsible
for the recently reported monochromatic transverse phonon emission from
optically excited GaAs/AlAs superlattices, and provide additional experimental
evidences of this.
