• nanowire
• dots
• quantum
• optoeletronics

Optoelectronic devices, such as LEDs, Lasers, Photovoltaic cells etc., are
present in everyday life. The fundamental physics of these devices lies in the
quantized energy levels and their interaction with light. Light-matter interaction
has been optimized in efficiency in continuous steps in the past years, especially
thanks to the progress at the nm scale manipulation of semiconductors. One
of the most successful ideas in this sense are Quantum Well Heterostructures,
which basically are quasi-2D materials that have enhanced transition strength
because of thee reduced dimensionality. The concept can be further exploited
even in lower dimensionality structures such as quasi 1D Nanowires and 0D
Quantum Dots.
Regarding photovoltaic cells, one of the actual challenges of research in this
scientific field, indeed is to achieve more efficient devices, in order to convert
more and more light into electrical current. Interaction with phonons, spon-
taneous emission, and other dissipation mechanisms place an upper thermo-
dynamic bound to the reachable efficiency. Among all, spontaneous emission
can be influenced by quantum interference effects. In order to overcome ther-
modynamic limitations, Scully et. al. proposed a two-coupled Quantum Dots
system, conceived with levels resonant within the continuum. By exploiting
Fano quantum interference, they demonstrated that it is possible to quench the
Spontaneous Emission probability amplitude, thus yielding a light-to-current
conversion enhancement.
Our idea was to implement the system proposed by Scully in two coupled
Quantum Dots in a Nanowire (NW). The choice of such a structure is driven
by the good NW reproducibility and the high control on design and geometry.
Indeed, QDs can be controllably grown in NWs, allowing more efficient mu-
tual coupling with respect to commonly used self-assembled QDs, for which
it is very difficult to choose the placement a priori because of the fabrication
method.
Despite these many advantages, only few photodection experiments on NW
QD devices are found in literature so far, and in a different spectral range.
In this work, we show the design and preliminary measurements of a specific
nanowire geometry to overcome the thermodynamic limitations of optoelectronic
devices by means of quantum interference. We optimized the band structure de-
sign of the NW by the use of a software for solving coupled Shroedinger-Poisson equations to obtain the desired structure. We therefore grew such studied NW
in a CBE facility. By means of several imaging techniques, such as SEM and
TEM, we verified that the NWs structural features were realized as designed,
thus confirming the achieved control on the growth. By low-temperature pho-
tocurrent measurements with a lock-in technique, we demonstrated single Dot
photodetection at 2 μm in several NWs. Once we observed the photocurrent,
we then studied the system as a function of temperature, gate potential, and po-
larization, thus confirming our theoretical predictions. The findings of our work
are a first fundamental step towards a photodetection on double dot devices
with no thermodynamic limitation.