Complete Report - University of New South Wales

ias to about 2.2V (Figure 4.5.23c). This is further tentative evidence for the collection of
hot carriers, as the energies appropriate for collection by the contact are generated at lower
bias voltages with optical assistance. However it should be pointed out that measurements
are made at present without the presence of additional fi lters to control the spectrum of
light illuminating the sample. In subsequent measurements, such fi lters - either providing a
narrow range of energies or a variable sharp long wavelength pass cut-off – will allow more
distinct quantitative data to be collected.
Figure 4.5.23: a) Dark and illuminated, or optically assisted, I-V measurements.
Detail of b) dark and c) illuminated I-V traces.
4.5.6.2 Slowed carrier cooling
Researchers:
Gavin Conibeer, Jean-Francois Guillemoles, Martin Green
Slowing of carrier cooling has been observed in bulk semiconductors by several authors, but
only at very high illumination intensities – typically equivalent to 250 to 2500 suns [4.5.17].
However, the reduced dimensionality in nanostructures offers a promising route for such a
role as this restricts the number of acoustic phonon modes available. because of the Brillouin
zone folding that occurs as a result of the periodicity of the superlattice. Slowed carrier
cooling in QW superlattices has been demonstrated experimentally [4.5.17, 4.5.18] and is
described as an enhancement of the phonon bottleneck effect. Some of the current authors
describe the likelihood of an increased effect in QD superlattices elsewhere [4.5.19]. This
work is principally theoretical at present.
4.5.6.2.1 Modelling of phononic band structures
Investigation in this area is currently focussed on obstructing the phonon decay mechanisms
for carrier cooling. This potentially has relevance to any device which relies on reducing
thermal transport, including thermoelectrics.
The current work [4.5.19] stems from the realisation that hot carriers lose their energy by
scattering with optical phonons which in turn build up a hot non-equilibrium population. Thus,
the critical feature in the overall carrier cooling is the rate of decay of the hot optical phonons.
It has been identifi ed by other workers that the principal and perhaps only mechanism for this
is the decay of an optical phonon into two longitudinal acoustic (LA) phonons of energy half
that of the optical phonon and of equal and opposite momenta.
In some bulk semiconductors, with a large difference in their anion and cation masses, there
can be a large gap between the highest acoustic phonon energy and the lowest optical phonon
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