1D X-ray speckle patterns: A novel probe of interfacial disorder in semiconductor superlattices

The high brilliance of third generation undulator sources offers an unprecedented opportunity to perform coherent X-ray diffraction studies of the structure and dynamics of materials on length scales down to interatomic spacings. We have measured the coherent diffraction or ''speckle'' pattern from a GaAs/AlAs superlattice. The speckle diffraction is consistent with height fluctuations of the superlattice of similar to 10 Angstrom over lengths of order 60 mu m. Unlike other methods of characterizing the roughness of semiconductor material, such as scanning probe microscopies (STM, AFM) or scanning electron microscopy (SEM), this novel coherent diffraction method is sensitive to lateral variations of the interface height buried within the small illuminated volume of the material, and thus can offer information unavailable from other non-destructive techniques.