ZnTe/Zn(S,Te) superlattices: A relaxation study by x-ray diffraction and reflectometry

Abstract

Two different x-ray techniques, high resolution x-ray diffraction and reflectometry, are applied to investigate crystallinity and interface properties of ${\mathrm{ZnS}}_{0.8}{\mathrm{Te}}_{0.2}/\mathrm{Z}\mathrm{n}\mathrm{T}\mathrm{e}$ strain-balanced superlattices as a function of their period. To gain a conclusive picture, the reciprocal space is probed in different regions: Measurements around the (004) Bragg peak allow conclusions on island formation and relaxation: For a superlattice period below 2 nm the layers and interfaces are nearly perfect. At periods above 2 nm there is a spread in the average lattice constant, indicating the formation of ZnTe islands. Finally, at periods above 2.4 nm the critical thickness of the ZnTe layers is exceeded, and dislocations are observed as a spread in the lattice plane orientation. Reflectometry scans, that probe a region close to the origin of reciprocal space, give complementary information and help to quantify the interface roughness: The nearly perfect sample with the lowest period exhibits a pronounced structure of very regular terraces with bunched steps, so that the interfaces are strongly anisotropic. The roughness in the vertical direction is very low, and is inherited from layer to layer. With increasing period, one observes an increase of the vertical roughness due to island formation and a decrease of the anisotropy due to step wandering. Direct imaging methods like atomic force microscopy and transmission electron microscopy essentially confirm these results, but do not give much additional information.