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Valence band structure engineering of thin SiGe/Si quantum wells for piezoresistive applications



In this work, we have theoretically investigated the influence of quantum confinement, biaxial stress, and high hydrostatic pressure on the valence band structure of Si1−xGex/Si quantum wells (QW) with x ranging from 0.1 to 0.75, QW thicknesses up to 50 nm, and hydrostatic pressures up to 8 GPa. Using the Nextnano simulator, we have solved the 6 × 6 k·p Hamiltonian obtaining the valence band eigenvalues (for light- and heavy-hole states) as well as their dispersion close to k = 0. We have found that for specific combinations of x, QW thickness, and hydrostatic pressure, it is possible to tailor the energy of the light- and heavy-hole states in such a way that they become almost degenerate for k = 0.

This results in a larger interaction between these sub-bands leading to an electron-like dispersion of certain hole sub-bands. We present representative examples showing that as pressure increases from 0 to 4 GPa the dispersion type of the hole states progressively evolves from electron-like to almost hole-like, which naturally produces sharp peaks in their corresponding density of states at k ≠ 0. The opposite transition from hole-like to electron-like band dispersion is also briefly discussed. This peculiar behavior of the dispersion type of the valence sub-bands induced by hydrostatic pressure results particularly interesting for piezoresistive applications, since large changes in the electrical conductivity are expected as a function of external stress.