Wetting droplet instability and quantum ring formation.

InAs islands on GaAs substrates undergo a morphological change into ring-shaped configurations upon deposition of a GaAs layer after island growth. By invoking an analogy of the InAs islands to wetting droplets on solid substrates, we suggest that this transition might be brought about by a change of the surface free-energy balance at the three-phase contact-line between GaAs, InAs, and vacuum (or As atmosphere). Our scenario can also be tested in conventional liquid systems (e.g., polymers).