Resonance effects in the Raman analysis of sige nanostructures

In the last two decades, attention has raised towards SiGe nanostructures for applications in microelectronics and optoelectronics. Many efforts have been payed in order to control the growth of the nanostructures and characterize their structural properties: for this purpose, a wide series of different experimental techniques have been developed. Raman spectroscopy of SiGe is commonly used to obtain information about the average values of composition and strain inside the nanostructures. Actually, due to the lack of data about the Raman efficiency of SiGe as a function of the alloy composition and the excitation light frequency, it is not possible to extract information about the composition profiles which are known to be present inside the structures and deeply affect their optoelectronic properties. In this work, the Raman efficiency of SiGe is measured across the whole compositional range for six different excitation light energies in the UV and VIS range. Strong resonances in the Raman efficiency of SiGe are observed, related to the direct electronic transitions in the material: their origin is explained by means of the theory and confirmed by semiempirical calculations. Then, a numerical tool is presented, which is able to analyze the Raman spectrum of an inhomogeneous SiGe structure and extract information about its internal composition profile by taking into account the effects of the resonance on the SiGe Raman efficiency. The methodology is validated by means of a calibrated SiGe sample, and applied to the study of SiGe islands. The internal composition profile of the islands obtained with this methodology is in good agreement with the results of independent techniques. Finally, the powers and limitations of the methodology are investigated.