Challenges in modulation doping of n-type Ge/SiGe heterostructures. The role of epitaxial and thermal strain

N-type Ge-rich Ge/SiGe multi-quantum-wells on Si(001) substrates are a novel material system with great potential due to their compatibility with the CMOS standard. This class of strained quantum heterostructures exhibits different band edge minima in a narrow energy range. Thus, lattice deformation, quantum confinement, and band bending effects can affect the energy ordering of the associated subband states, resulting in a radical variation of their optical and transport properties. This study investigates how these factors influence the 2D subband carrier density in modulation-doped heterostructures. To this end, we focus on the recently demonstrated parabolic quantum well system because of its technological relevance. Using Fourier transform infrared and high-resolution x-ray diffraction spectroscopy in combination with numerical modeling, our results highlight the importance of the temperature dependence of the strain field in determining the efficiency of the charge transfer process. Furthermore, optimal design parameters are identified to achieve either the highest transfer efficiency or the absolute value of the carrier density as a function of the intersubband transition energy in the 3-8 THz range.