On elastic relaxation and long wavelength microstructures in spinodally decomposed In_xGa_1−x.As_yP_1−y epitaxial layers

The origin of the long-wavelength (100–300 nm) quasi-periodic microstructure observed in transmission electron microscopy studies of spinodally decomposed In Ga_1−xAs_yP_1−y alloys has been investigated. The contrast is well explained by diffraction effects arising from lattice plane bending near the surfaces of the thinned specimens, such as would be induced by elastic relaxation of shear stresses accompanying a quasi-periodic lattice modulation. Excellent qualitative agreement between calculated and experimental contrasts lends weight to the claim that these contrasts, observed over part of the composition range of liquid phase epitaxy grown In_xGa_1−xAs_yP_1−y alloys, are associated with spinodal decomposition. The accompanying strain modulation amplitude is of the order of 10⁻³. It is suggested that speckle contrast on a smaller scale (5–15 nm) in this material may also be related to composition variations. More generally, the nature of near-surface elastic relaxation in compositionally modulated thin foils is discussed. It is emphasized that resultant distortions must be allowed for when structure-property relationships of modulated structures are to be determined from thinned samples by techniques such as transmission electron microscopy.