Substrate-driven ordering microstructure in Ga_xIn_1-xP alloys

The effect of substrate misorientation on {111} ordering in Ga _xIn_1-xP epilayers grown on (001) GaAs substrates by metalorganic vapor-phase epitaxy has been studied by transmission electron microscopy. Among the four possible variants, only the { 1/2 1/2 1/2 } _B are observed, and the substrate misorientation toward (11̄0) can disymmetrize the two B variants to a point such that only one remains. We show that the diffuse intensity observed along (001) originates from a modulation of the degree of order. When the domain size is comparable to the characteristic length of the modulation, domain boundaries and antiphase boundaries contribute to the diffuse scattering, giving rise to the reported wavy diffuse intensity. We propose a few simple rules controlling the growth process at the atomistic level, from which the absence of the { 1/2 1/2 1/2 }_A variants, as well as the selection effect of the [11̄0] steps on the two remaining variants may be explained. From existing literature, the validity of these rules seems to be rather general. Finally, considering reports that the ordered structure observed here is not at equilibrium, we suggest that the latter is growth induced. The necessary conditions for such an ordering mechanism to operate are identified.