Lateral Composition Modulation in (InAs)n/(AlAs)m Short-Period Superlattices Investigated by High-Resolution X-Ray Scattering

The lateral composition modulation in (InAs)_n/(AlAs)_m short-period vertical superlattices was investigated by means of synchrotron grazing-incidence small-angle x-ray scattering (GISAXS) and grazing-incidence (GID) and coplanar x-ray diffraction. Our GISAXS measurements determined experimentally the interface profile of a vertical superlattice, which then served as the basis for our diffraction analysis of the lateral composition profile. We show that the interfaces in the vertical superlattice have an asymmetric profile with an average undulation wavelength of about 280 Å and an average amplitude of about 1 Å. For the analysis of the lateral composition profile, a structural model based on the measured interface profile was proposed. The model also assumes that the composition is uniform in the growth direction but modulated laterally, because we consider only the zero-order vertical x-ray superlattice peak. This model, combined with strain analysis, was employed to extract the composition information from the x-ray GID and coplanar diffraction data. For the particular sample studied, both the GID and the coplanar diffraction measurements yielded a lateral compositional wavelength of about 280 Å, which is the same as the morphological undulation wavelength of the interfaces, and a composition amplitude of about 15%-16%. From the experimentally determined interface profile, we determined that the upper limit of the amplitude of composition modulation is about 18% for this particular sample. Our results indicate that the composition modulation is predominantly caused by morphological undulation driven by the misfit strain.