  At low strain, SiGe films on Si substrates undergo a continuous
nucleationless morphological evolution known as the Asaro-Tiller-Grinfeld
instability. We demonstrate experimentally that this instability develops on
Si(001) but not on Si(111) even after long annealing. Using a continuum
description of this instability, we determine the origin of this difference.
When modeling surface diffusion in presence of wetting, elasticity and surface
energy anisotropy, we find a retardation of the instability on Si(111) due to a
strong dependence of the instability onset as function of the surface
stiffness. This retardation is at the origin of the inhibition of the
instability on experimental time scales even after long annealing.
