Germanium Quantum Wells in the Conduction Band for Terahertz Emission on Silicon Substrates

Terahertz (THz) Quantum Cascade Lasers (QCLs) are presently made from III-V semiconductor heterostructures. The use of Ge/SiGe heterostructures in the conduction band, instead, could lead to room temperature operation of such devices [1]. Also, the non-polarity of Ge and Si lattices may help filling the emission wavelength gap between THz and mid-IR QCLs, due to the existence of polar optical phonons in III-V materials. Here we study the THz absorption spectrum of n-type strained Ge/SiGe multi-quantum wells (MQWs) grown on Si (001) substrates by means of ultra-high vacuum chemical vapour deposition. We measure the intersubband transition lines by FTIR spectroscopy of MQWs, symmetric- and asymmetric-coupled MQWs at 70° single-metal waveguide geometry [2]. Supported by theoretical modeling, we discuss the role on the line broadening of electron scattering due to interface roughness, ionized donors, phonons, and extended defects. Experiments with THz pump on three-level Ge/SiGe asymmetric-coupled MQWs are also planned, using a tunable free electron laser as optical pump