Tuning Optical Properties of Ge Nanocrystals by Si Shell

We
present a theoretical study of Ge-core/Si-shell nanocrystals
in a wide bandgap matrix and compare the results with experimental
data obtained from the samples prepared by co-sputtering. The empirical
tight-binding technique allows us to account for the electronic structure
under strain on the atomistic level. We find that a Si shell as thick
as one monolayer is enough to reduce the radiative recombination rate
as a result of valley L–X crossover. Thin Si shells lead to a dramatic reduction of the optical
bandgap from the visible to the near-infrared range, which is promising
for photovoltaics and photodetector applications. Our detailed analysis
of the structure of the confined electron and hole states in real
and reciprocal spaces indicates that the type-II heterostructure is
not yet achieved for Si shells with thicknesses below 0.8 nm, despite
some earlier theoretical predictions. The energy levels of holes are
affected by the Si shell more than the electron states, even though
holes are completely confined to the Ge core. This occurs due to a
strong influence of strain on the band offsets.