SURFACE ENERGETICS AND ORDERING IN GAINP

Although bulk III-V semiconductor alloys generally exhibit phase separation, vapor-phase epitaxial growth of Ga0.5In0.5P on GaAs (001) shows spontaneous ordering into a [111] oriented monolayer superlattice. This ordering is believed to be induced at the surface during growth. We have performed total-energy first-principles pseudopotential calculations for the surface layers of cation- and anion-terminated (001) surfaces of Ga0.5In0.5P on GaAs substrates and valence-forcefield model calculations for deeper layers. These calculations show that the relative stability of the various Ga/In arrangements is a strong function of the layer depth below the surface. At the cation-terminated surface a Ga/In atomic arrangement corresponding to the observed bulk ordering is preferred over other arrangements by 90 meV per surface atom. This preference is intimately tied to electronically driven surface reconstructions. For subsurface Ga/In layers the energy differences are generally small (<30 meV per atom), but at the fourth subsurface layer they play a critical role in controlling the three-dimensional stacking of the ordered (001) layers. Finally, thermodynamic calculations based on a configurational Hamiltonian whose interaction energies are fit to the total energy calculations show that the observed ordering can be explained as a thermodynamically stable surface phase at growth temperatures, which, depending on the atomic mobilities, may remain as a metastable bulk phase as the growth proceeds. On the basis of these results we propose a possible mechanism for the development of the ordered phase.