ON THE ELECTRON-DIFFRACTION CONTRAST OF COHERENTLY STRAINED SEMICONDUCTOR LAYERS

The electron diffraction contrast of coherently strained semiconductor layers viewed in cross-section has been investigated. A number of contrast effects have been observed which are associated with either structure factor and/or strain variations between heterostructure layers. Firstly, the relaxation of built-in elastic strains near thin foil free surfaces has been treated using two elasticity solutions which, from two-beam dynamical image simulations, are shown to accurately predict the observed diffraction contrast features in both single and multilayer structures. Secondly, it has been shown that the contrast of strained epitaxial layers is primarily due to structure factor differences which are manifest by: (i) extinction distance shifts and (ii) delta-fringe patterns when the layers are inclined relative to the electron beam. More generally, it has been pointed out that the visibility of any compositional modulation for a given reflection depends on the orientation of the interfaces with respect to the electron beam; certain tilt orientations can completely eliminate contrasts arising from compositional variations. The general features described above have been studied using Ge(x)Si1-x/Si heterostructures as a model system.