Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures Page: 36
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required to produce a spectrally resolvable change in the bandgap can be small, and
hence effective change in the lattice constant of the quantum structure compared to the
surrounding material may be small. In these cases, techniques such as HR-XRD and
Raman will measure both the strain of the embedding material (bulk) and the active
region simultaneously. As the volume of the bulk tends to be significantly larger than the
active region itself, the strain of the quantum structure can become lost in the signal of
the bulk. In many cases, however, the quantum structure may be independently
spectrally resolved from the bulk emission. This provides an opportunity for measuring
the strain from spectral information retrieved directly from the active area.
One example of strain information in spectral data is the dependence of the
energy of emission of excitons on strain in the sample. This energy shift may be
measured and related directly the strain of the region of emission . Generally,
however, excitons can only be observed at low temperatures, increasing the complexity
of such experiments for functional devices. Furthermore such measurements must take
into account the change in the strain in the system at low temperatures .
4.2 The Huang-Rhys Parameter (S)
In contrast to the exciton energy dependence, electron-phonon interactions
become stronger at higher temperatures as the population of phonons increases . In
binary semiconductors with optical phonon modes, interaction between longitudinal
optical (LO) phonons and electron-hole pairs can play a significant role in
recombination. This results in "phonon-replicas" of the main emission, i.e. secondary
peaks lowered in energy by an amount AE = n x hwLO, an integral multiple of the LO
phonon energy. The intensities of subsequent replicas can be related directly to the
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Llopis, Antonio. Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures, dissertation, May 2012; Denton, Texas. (digital.library.unt.edu/ark:/67531/metadc115113/m1/46/: accessed April 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .