Structural stability of bulk and epitaxial In0.5Ga0.5P-alloy-based ordered superlattices.

Abstract

The stability of bulk and epitaxial ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P alloys is examined for three ordered structures with use of self-consistent ab initio pseudopotential calculations. In both bulk and epitaxial forms, a chalcopyritelike structure is the lowest-energy state, more stable than the (001) and (111) monolayer superlattices (MLS). The [001]-oriented MLS is generally found to be more stable than the (111) MLS. Our calculations indicate that the ordered structures in bulk form are unstable against phase segregation into binary constituents at T=0. This instability mainly results from the incomplete release of elastic energy, a general feature in superlattices of lattice-mismatched constituents. For epitaxial growth, the formation enthalpy is significantly reduced. We find that the epitaxial formation enthalpy decreases as the substrate lattice constant increases. The chalcopyrite-structure phase is found to be stabilized while the (111) superlattice is most unstable when grown epitaxially. Thus, the observed [1\ifmmode\bar\else\textasciimacron\fi{}11]- or [11\ifmmode\bar\else\textasciimacron\fi{}1]-ordered ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$P should be related to chemical-bonding effects at the growing surface.