Ordered structures and metastable alloys grown by OMVPE

The 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 (ge)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 was indeed observed in a number of systems. Nevertheless, such alloys can be grown by OMVPE, including the highly metastable alloys GaPSb and InPSb. Ordered structures are now observed in several III/V alloy systems including the ternary systems GaAsSb, GaInP, AlInAs, AlGaAs, and GaInAs, and the quaternaries GaInAsP, GaInAsSb, and AlGaInP. 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, InAsSb, and GaAsP involves ordering along the (111) directions, forming the CuPt (L1 sub 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 the other ordered structures.