Quantum confinement, carrier dynamics and interfacial processes in nanostructured direct/indirect-gap semiconductor-glass composites Page: 2 of 7
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ABSTRACT
The behavior of semiconductor clusters precipitated in an insulated matrix was investigated.
Semiconductor compositions of CdTe, Si and Ge were studies and the insulating matrix was
amorphous Si02. As a function of size, quantum confinement effects were observed in all three
composite systems. However significant differences were observed between the direct-gap
column 2-6 semiconductors and the indirect-gap column 4 semiconductors.
As observed by others, the direct-gap 2-6 semiconductors showed a distinct saturation in the
energy-gap blue shift with decreasing size. Theoretical studies using a 20-band k dot p
calculation of the electronic and valence bands for a 3-dimensionally confined CdTe
semiconductor showed that mixing of the conduction band states leads to a flattening of the
central valley. This increases the electron mass drastically and saturates the size dependent
blue shift in the bandgap.
In contrast, the blue shift in the Si and Ge nanocrystals showed no sign of saturation and
increased drastically with decreasing size. In fact, Si and Ge crystals were formed with blue shift
values that moved the bandgap to the near UV region. We examined the absorption curves to
determine whether the bandgap was direct or indirect in the quantum dots. The results are that
the absorption shows an indirect gap for all but the smallest Si crystals and an indirect gap for
all Ge crystals.
Raman studies showed negligible size dependence due to a lack of phonon confinement in the
matrix embedded clusters. Exciton saturation and recovery times were found to be very short (of
the order of 400fs) and are the fastest reported for any quantum dot system.
Work to examine the type of confinement obtained in a matrix that consists of a transparent
conductor is under way.
Studies of the photoinduced absorption change in GeSe glasses showed a significant effect of
photodarkening, regardless of composition. The photodarkening effect appears to be composed
of permanent and transient effects, presumed to be associated with photo-induced structural
changes in the glass. The transient effects appear to have recovery times in at least two
different time scales - one in minutes and one in less than a microsecond. Time-resolved
studies are under way to determine the structural origin of each photodarkening effect.1
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Simmons, Joseph H. Quantum confinement, carrier dynamics and interfacial processes in nanostructured direct/indirect-gap semiconductor-glass composites, report, August 13, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc742165/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.