Thickness modulation of InGaAs/GaAs superlattices studied by large angle x-ray scattering

Superlattices of 100-period In_xGa_1-xAs (15 Å)/GaAs(100 Å) grown on GaAs (100) substrates by molecular beam epitaxy were studied by using large angle x-ray scattering techniques. In contrast to the usual superlattice satellite peaks corresponding to structural periodicity along the growth direction, unusual satellite peaks in the lateral direction parallel to the sample surface were observed in a sample with x=0.535 grown at 480°C, indicating an in-plane ordering. This result is confirmed by high resolution transmission electron microscopy observations that thickness modulation in the In_xGa_1-xAs layers gives rise to long-range lateral periodic arrays of clusterlike microstructures with spacing on the order of a few hundred angstroms. This thickness modulation is found to occur only in the [110] direction, thus the material can be viewed as a somewhat disordered array of grown-in parallel quantum wires.