Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures Page: 19
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Figure 3.1 Excitation in a direct (a), and indirect (b) bandgap
semiconductor. The upper parabola represents the conduction band and
the lower parabola represents the valence band. The final result of
excitation in both cases is an electron (blue circle) in the conduction band
and a hole (red circle) in the valence band. In the indirect case, emission or
absorption of a phonon with wavevector Ak is required to complete the
the bandgap. This energy is the minimum energy required for a carrier to be excited
from the valance to the conduction band. If we were to provide an electron in the
valence band with this amount of energy, we are left with the excited electron in the
conduction band, and an absence of an electron, known as a hole, in the valence band.
Eventually, in the simplest case, the electron will recombine with the hole freeing up an
amount of energy equal to the bandgap energy. If the semiconductor is a "direct gap"
semiconductor, then the momentum of the excited electron and hole match up, and
recombination can occur via the production of a photon. In the case of an "indirect gap"
semiconductor, momentum conservation cannot be fulfilled solely by emission of a
photon, and so the difference in momentum must be compensated for by emission of
one or more phonons (quanta of vibrations in the lattice).
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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/m1/29/: accessed April 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .