Near- and far-infrared absorption and electronic structure of Ge-SiGe multiple quantum wells

We report an extensive study of strained Ge/ Si_0.2 Ge _0.8 multiquantum wells grown by ultrahigh-vacuum chemical-vapor deposition. The microstructural properties of the samples were characterized by transmission electron microscopy and Raman spectroscopy. Their electronic properties have been investigated by means of infrared absorption measurements. Both interband and intersubband transitions were analyzed. Intersubband absorption energies were found in the 20-50 meV range, depending on the quantum well width. Interband and intersubband transition energies have been successfully described by means of both a k·p approach and a tight-binding model. In particular, we found a conduction-band offset between the L edges of 124 meV, well suited for the development of optoelectronic devices operating in the terahertz range. We also found that the energy difference between the Δ₂ minima in the barrier and the L minima in the well is only ∼40 meV. This explains the observed ineffectiveness of the transfer doping in the strained heterostructures considered.