FORMATION OF STRAINED SUPERLATTICES WITH A MACROSCOPIC PERIOD VIA SPINODAL DECOMPOSITION OF III-V SEMICONDUCTOR ALLOYS

The theory of (in)stability of III-V A1-xBxC1-yDy-type quaternary alloys with respect to the coherent phase separation is developed on the basis of the regular solution approximation. It is shown that, in spite of the stabilizing factor of coherency elastic strains, the III-V quaternary alloys which have positive formation enthalpy become at some temperature unstable with respect to coherent phase separation. The equilibrium structure resulting after the phase separation is a one-dimensional modulated two-phase structure with alternating alloy composition ( a system of compositional domains, or, in the other words, a superlattice with a macroscopic periodicity). Alloy compositions of both phases are determined by the conditions of the minimal homogeneous bulk free energy; the superlattice periodicity is governed by the interplay of the interface free energy and the inhomogeneous part of the elastic free energy. The coherent phase separation diagram and the periodicity of spontaneously formed superlattices are calculated for Ga1-xInxAs1-yPy.