Ordered structures and metastable alloys grown by OMVPE

Abstract

Our understanding of the thermodynamics of mixing in semiconductor solids has evolved from the purely empirical regular solution model to models based on the electron energy states in the solid, including the delta-lattice parameter (DLP) model and, more recently, first principles calculations. These models are in agreement that the enthalpy of mixing is invariably ⩾ 0 for III/V and II/VI alloys, and increases with increasing difference in lattice constant for the constituent binary compounds. In terms of the simple thermochemical mixing models, this suggests the occurrence of miscibility gaps. Solid phase immiscibility has indeed been observed in a number of systems. Nevertheless, such alloys can be grown by OMVPE, including the highly metastable alloys GaPSb and InPSb. Initially surprising was the occurrence of ordered structures in these same alloys. The regular solution model apparently specifically excludes immiscibility and ordering in the same system. However, when the positive enthalpy of mixing is due to strain energy effects, as in III/V and II/VI alloys, Hume-Rothery recognized very early that such phenomena should be anticipated. This was later confirmed by the detailed first principles calculations. In fact, the tendency for ordering is anticipated to increase as the difference in tetrahedral radii of the elements sharing a common sublattice increases. Ordered structures have now been observed in several III/V alloy systems including the ternary systems GaAsSb, GaInP, AllnAs, AlGaAs, and GaInAs, and the quarternaries GaInAsP, GaInAsSb, and AlGaInP. In this paper, ordering in other alloy systems such as GaPSb, InPSb, InAsSb, and GaAsP will be described. An unexpected observation is that the preferred ordered structure for the ternaries GaInP, GaPSb, InPSb, InAsSb, and GaAsP involves ordering along the <111 directions, forming the Cu-Pt (L1 1) structure. This is also true of GaAsSb grown by MBE. Both the first principles calculations and simple strain energy calculations indicate that such ordered structures are more stable than the disordered solid solution, but less stable than other ordered structures. The chalcopyrite structure (E1 0), with ordering along <210 directions, and the Cu-Au structure (L1 0), where ordering occurs along the <100 directions, are considerably more stable. The occurrence of these structures must be explained in terms of the relative strain energies of the various ordered structures at the surface and/or kinetic processes occurring during epitaxial growth. The importance of kinetic effects is indicated by the importance of growth parameters such as growth rate, temperature, and substrate orientation in determining both the degree of ordering and the specific ordered structures observed.