Anomalous Elastic Properties of Si/Ge Superlattices: The Role of Interfaces

The elastic constant tensors of Sim/Gen superlattices with different modulation wavelengths have been determined by Brillouin light scattering spectroscopy, and their Young's moduli measured by the nanoindentation technique. Two sets of samples have been studied: m = n and m = 4n. The C33 elastic constant obtained for Sin/Gen superlattices (where bilayers are built-up by an equal number of Si and Ge monolayers) are some 13% above the predictions of the continuum elasticity theory, using the Cij values of bulk Si and Ge. On the other hand, the elastic constants tensor of Si4n/Gen superlattices (where bilayers are built-up by four times more Si monolayers than Ge ones) matches perfectly the predictions. A thorough structural analysis by low-angle X-ray reflectivity, high-angle diffraction and X-ray absorption spectroscopy has been carried out and a deep characterization of the microstructure is presented here. The unexpected value of C11 in Sin/Gen superlattices can be attributed to the compressed Si–Ge alloy found at the wide interfaces present in that set of superlattices.