Competing relaxation mechanisms in strained layers

We show that strained epitaxial layers can relax by two competing mechanisms. At large misfit, the surface becomes rough, allowing easy nucleation of dislocations. However, strain-induced surface roughening is thermally activated, and the energy barrier increases very rapidly with decreasing misfit ɛ. Thus below some misfit ${\mathrm{\varepsilon }}_{\mathit{c}}$, the strain relaxes by nucleation of dislocations at existing sources before the surface has time to roughen. Relaxation via surface roughening is technologically undesirable; we discuss how temperature, surfactants, or compositional grading change ${\mathrm{\varepsilon }}_{\mathit{c}}$ and so control the mode of relaxation.