Two-dimensional phase separation in ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{As}}_{\mathit{y}}$${\mathrm{P}}_{1\mathrm{-}\mathit{y}}$ epitaxial layers

Microstructures of lattice-matched ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{As}}_{\mathit{y}}$ ${\mathrm{P}}_{1\mathrm{-}\mathit{y}}$ epitaxial layers, grown on (001), (110), (111${)}_{\mathit{A}}$, and (123) InP substrates by use of liquid-phase epitaxy, have been examined in detail by transmission electron microscopy. Results indicate that the fine-scale speckle microstructure is caused by two-dimensional phase separation occurring at the surface while the layer is growing; decomposition along the growth direction is not observed in either of the four cases examined. The decomposition is found to take place along those directions in the growth plane along which the elastic work associated with the transformation is a minimum. The temperature dependence of the wavelength of the fine-scale structure in (001) ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$As epitaxial layers grown by use of molecular-beam epitaxy indicates that the wavelength evolution is controlled by the surface diffusion of As atoms.