The effects of elastic relaxation on TEM studies of thinned composition-modulated materials.

Composition modulations in defect-free crystalline materials are generally accompanied by a lattice distortion where unit cell dimensions are not characteristic of the local composition. The distortion originates form the constraint that the inter- atomic spacings perpendicular to the modulation direction be commensurate from one layer to the next. Thus there is an internal stress field set up in the material which attempts to suppress unit cell expansions and contractions in these perpendicular directions In this paper we discuss how this important, and oft-neglected relaxation affects TEM bright- field and dark-field image contrasts, high resolution lattice images, and interpretation of selected area diffraction data. We also derive expressions, relevant for all sample thicknesses, describing the relaxation strain field in cubic anisotropic materials, thinned perpendicularly to the modulation direction. The results are discussed in the context of two classes of modulated materials, namely spinodally-decomposed InGaAsP alloys and GeSi/Si strained layer superlattices.