The hole subband structures and effective masses of tensile strained Si/Sil-yGey quantum wells are calculated by using the 6 × 6 k·p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 （100） substrates shows a large absorption coefficient of 8400 cm^-1.
The waveguide design is one of the most important parts in a terahertz quantum cascade laser（QCL）. Si/SiGe QCL waveguides, based on the Drude model and finite-difference time-domain （FDTD） method, are designed by the traditional refractive index waveguide structure, the single-sided metal structure, the double-metal clad structure, and a novel metal/metal silicide structure. The metal/metal silicide structure, showing high modal confinement,is convenient in process engineering and is expected to be a viable waveguide solution for Si/SiGe QCLs in the THz range.
