N-type SiGe heterostructures for THz intersubband transitions

Research on band-gap engineering of Silicon-Germanium heterostructures for the realization of Quantum Cascade (QC) structures emitting in the Terahertz (THz) spectral region has recently attracted a vast interest. While several successful attempts have been reported using hole-based (p-type) intersubband transitions, very few results have been published on systems exploiting electrons (n-type). In this work we present the optical and structural characterization of n-type heterostructures made either of tensely-strained Si (sSi) quantum well (QW) confined between low Ge content Si_1-xGe_x barriers [0.2<x<0.5] or of compressively-strained Ge (sGe) QW confined between high Ge content Si_1-xGe_x barriers [0.8<x<0.9]. The structural and morphological characterizations of the samples have been made by atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), transmission electron microscopy (TEM), and Raman spectroscopy. Intersubband transitions have been experimentally investigated by absorption spectroscopy and compared with the theoretical results of a tight-binding model, which provides the electronic band structure of the complete multi quantum well system throughout the whole Brillouin zone.