Some aspects of the X-ray structural characterization of (Ga_1-xAl_xAs)n1(GaAs)n2/GaAs(001) superlattices

The study of the physical properties of the superlattices (Ga _{1- x}Al_xAs)n₁(GaAs)n₂/GaAs(001) requires the knowledge of their structural parameters n₁, n₂ and x and of their crystalline state. X-ray diffraction enables the existence of such artificial superstructures, obtained by molecular-beam epitaxy, to be revealed and their characteristic parameters to be determined. Two experimental techniques are used: a standard powder goniometer and a double-crystal diffractometer. The diffraction diagrams directly yield the superperiod and the average Al concentration in the superlattice. The value of x is determined by refinement between observed and calculated structure factors. The results are even more accurate when the number of observed satellite peaks for a given periodicity is greater; this number depends at the same time on the Al composition x, the n₁/n₂ ratio, the periodicity and its dispersion and the shape of the interfaces. This method is illustrated by a few examples with superlattice periods varying from 51 to 397 Å. The consequence on the X-ray diagrams of various defects (dispersion in n₁ and n₂, superperiod gradient, Al diffusion) is discussed.