Electronic structure and impurity-limited electron mobility of silicon superlattices

With the utilization of a new complex-band-structure technique, the electronic structure of [100]-oriented model Si-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$ and metal-oxide-silicon superlattices have been obtained over a wide range of layer thickness l (11≤l≤110 Å), complementing previous results obtained for very thin layer systems (l≤11 Å). For l≥44 Å, it is found that these systems exhibit a direct fundamental band gap, produced in large part by the Brillouin-zone folding of the bulk conduction-band edges. In the same range, the transverse band-edge electron effective mass is reduced to a limiting value of 0.73 of the bulk conductivity effective mass, supporting our previous suggestion that a band-structure-driven enhancement in transverse electron mobility over bulk silicon may be possible. Simple effective-mass scaling yields an enhancement of about 1.2 in the (low-temperature) impurity-scattering limit and about 2.2 in the (high-temperature) phonon-scattering limit. Detailed consideration is made of the simpler of the two cases, impurity-scattering-limited electron mobility, with the result that enhanced mobility is indeed predicted for sufficiently high carrier concentrations.