Ge quantum dots in Si: Self-assembly, stacking and level spectroscopy

Ge quantum dots in Si were fabricated by molecular beam epitaxy in the Stranski-Krastanow growth mode at low substrate temperatures and were investigated by optical and electrical spectroscopy. The dot size is about 20 nm in width and 2 nm in height for a substrate temperature of about 510°C. The effective valence band structure of such Si/Ge quantum dots is consistently analyzed by type-II interband photoluminescence transitions between electrons in the Si host and holes localized within the Ge dots, by bound-to-quasibound intra-valence band transitions of localized holes in the mid-infrared spectral range and by C-V and admittance spectroscopy. Holes localized in the dot ground states reveal an ionization energy of about 350 meV corresponding to the effective Si/Ge dot valence band offset. The energy separation of zero-dimensional ground and first excited levels due to lateral confinement is about 40 meV, and the Coulomb charging energy for the second hole within the ground state is about 15 meV. A slow thermal excitation rate of holes out of the dots at low temperature strongly affects optoelectronic properties like photocurrent response. The impact of elastic and electronic coupling of dots in self-aligned stacks on band structure is discussed.