Engineering self-assembled SiGe islands for robust electron confinement in Si

The confinement potential and the energy of localized electron states in the Si matrix surrounding self-assembled SiGe/Si(001) islands are evaluated with realistic structural parameters. For homogeneously alloyed islands overgrown with Si at low substrate temperatures, a nonmonotonic dependence of the energy levels on size and composition is obtained and conditions to achieve the deepest confinement potential are derived within the available parameters. The influence of the experimentally reported composition distributions on the electron confinement is considered and confined states are found to lie as deep as 120 meV below the Si  Δ conduction-band edge. Finally, shape changes occurring during Si capping at high substrate temperatures are shown to lead to a substantial reduction in the confinement potential. This work guides the design of structures able to provide robust single-electron confinement in Si.