When a strained film is grown on a vicinal substrate, the steps advance like a train when the deposited atoms have sufficient mobility to reach the step edges. However, as the steps advance, the strain-induced force monopoles associated with the steps cause the steps to attract to each other (J. Tersoff, PRL 74, 4962, (1995)), resulting in a thermodynamic instability of the steps in the form of step bunching (J. Tersoff, et al., PRL 75, 2730 (1995)).

In growth of essentially every compound material such as GaN, one element always diffuses faster than the other(s) at the growth front. To grow good-quality materials, even the most sluggish element has to be sufficiently mobile, forcing materials growers to go to higher growth temperatures. Higher growth temperatures, in turn, are acompanied with undesirable drawbacks: the element(s) with intrinsically higher mobilities would desorb from the surface (higher mobility means weaker binding to the surface and easier desorption), the interfaces would be rougher because of more intermixing, etc.

When a metal is grown onto a substrate of itself (homoepitaxy), the growth front is typically smooth, or at most is roughened by the formation of shallow hills (called surface mounds). The underlying reason for the roughening has been recognized to be of kinetic nature: Atoms landed on an upper terrace do not have enough time to overcome the "road blocks" provided by the steps and fill all the valleys (known as the Villian instability).

When a metal system shrinks its dimension(s), the conduction electrons inside the metal feel the squeezing, and are forced into (discrete) quantum states. Such confined motion of the conduction electrons may influence the global or local stability of the low dimensional systems, and in the case of a thin film on a foreign substrate this "quantum energy" of electronic origin can easily overwhelm the strain effects in definging the film stability, thereby severely influencing the preferred growth mode (see, e.g., Suo and Zhang, Phys. Rev. B 58, 5116 (1998)).