  The distinction between absolute and convective instabilities is well known
in the context of hydrodynamics and plasma physics. In this Letter, we examine
an epitaxial crystal growth model from this point of view and show that a
strain-induced step bunching instability can be convective. Using analytic
stability theory and numerical simulations, we study the response of the
crystal surface to an inhomogeneous deposition flux that launches impulsive and
time-periodic perturbations to a uniform array of steps. The results suggest a
new approach to morphological patterning.
