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

Abstract

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.