Abstract
We look at the relationship between the preparation method of Si and Ge
nanostructures (NSs) and the structural, electronic, and optical
properties in terms of quantum confinement (QC). QC in NSs causes a blue
shift of the gap energy with decreasing NS dimension. Directly measuring
the effect of QC is complicated by additional parameters, such as
stress, interface and defect states. In addition, differences in NS
preparation lead to differences in the relevant parameter set. A
relatively simple model of QC, using a `particle-in-a-box'-type
perturbation to the effective mass theory, was applied to Si and Ge
quantum wells, wires and dots across a variety of preparation methods.
The choice of the model was made in order to distinguish contributions
that are solely due to the effects of QC, where the only varied
experimental parameter was the crystallinity. It was found that the hole
becomes de-localized in the case of amorphous materials, which leads to
stronger confinement effects. The origin of this result was partly
attributed to differences in the effective mass between the amorphous
and crystalline NS as well as between the electron and hole. Corrections
to our QC model take into account a position dependent effective mass.
This term includes an inverse length scale dependent on the displacement
from the origin. Thus, when the deBroglie wavelength or the Bohr radius
of the carriers is on the order of the dimension of the NS the carriers
`feel' the confinement potential altering their effective mass.
Furthermore, it was found that certain interface states (Si-O-Si) act to
pin the hole state, thus reducing the oscillator strength.
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