  The thermodynamics of the Stranski-Krastanov mode of epitaxial growth and the
effect of the sign of the lattice misfit are discussed. The Stranski-Krastanov
mode of growth represents a sequence of layer-by-layer or Frank-van der Merwe
growth followed by the formation of three-dimensional (3D) islands or
Volmer-Weber growth. The occurrence of both growth modes mentioned above is in
compliance with the wettability criterion of Bauer. The positive wetting
function required for the occurrence of the Volmer-Weber growth is originated
by the vertical displacements of the atoms close to the edges of the
two-dimensional (2D) islands as a result of the relaxation of the lattice
misfit. The monolayer high islands become unstable against bilayer islands,
bilayer islands in turn become unstable against trilayer islands, etc. beyond
some critical islands sizes. Monolayer islands appear as necessary precursors
of three-dimensional (3D) islands. The critical island size for mono-bilayer
transformation increases steeply with decreasing lattice misfit and diverges at
a critical value of the misfit. This value divides the regions of Frank-van der
Merwe and Stranski-Krastanov modes in a phase diagram of coordinates
wetting-misfit. The transformation of monolayer to multilayer islands takes
place either by consecutive nucleation and growth of 2D islands (layer-by-layer
transformation), or by nucleation and lateral (2D) growth of multilayer islands
(multilayer 2D transformation). The former occurs in the case of "stiff"
overlayer materials and mostly in compressed overlayers. The latter takes place
in the case of "soft" materials like Pb and In, mostly in tensile overlayers.
Tensile films show non-nucleation transformation compared with the
nucleation-like behavior of compressed films.
