• thz
• detector
• quantum dots
• Coulomb blockade

In the past decades, the progress in nanostructured materials has opened many new research avenues, ranging from fundamental physics and material science to novel device
concepts in nanoelectronics, optoelectronics, and computing. Among the many new concepts that have emerged, the exploitation of Coulomb blockade in so-called “artificial
atoms”, or quantum dots (QDs), pushes the control of the electronic structure, optical
transitions and charge transport characteristics to the extreme. In particular, it allows
the creation of devices, such as single-electron transistors (SETs), where the flow of electrons can be controlled down to the single carrier. In a QD, electrons are strongly confined
in a nanometric region of the nanostructure and have a zero-dimensional quantized energy
spectrum, which can be almost designed at will by controlling the strength and geometry
of the quantum confinement, for instance by tuning the material composition or also by
field effect. In very recent years, the progress of epitaxial growth of self-assembled heterostructured and polytypic semiconductor nanowires has offered a new ideal substrate to
implement these concepts in devices that – despite still operating in cryogenic conditions
– are characterized by large enough energy scales as not to require ultra-low milliKelvin
temperatures that can only be reached in a dilution refrigerator.
This thesis moves from the general idea of developing a novel class of THz radiation
detectors that exploit SETs to convert the absorption of photons into an electrical signal.
This approach offers in fact a number of advantages in terms of design and sensitivity,
which could even reach the single-photon limit. In the specific architecture we envisioned
for this work, the photon absorption occurs between the orbitals of a tightly confined
double QD (DQD) which – similarly to the case of a molecule – can support bonding (B)
and antibonding (AB) orbitals emerging from the tunnel coupling between the individual
QD. These electronic structures are particularly interesting in the context of radiation
detectors since they easily allow the design of the correct selection rules and large dipole
moments that are needed to maximize photon absorption. In the devices studied in the
thesis, transport is thus mediated by the “molecular” orbitals existing in DQDs, which are
able or not to mediate charge transport, based on the absorption of single THz photons.
In this thesis, we leveraged recent results obtained at Lund University on the study of polytypic InAs nanowire embedding 10 nm-thick parallel-coupled DQDs, which allowed
very fine control of molecular orbitals with a B-AB energy gap on the order of a few
meV, making them an interesting candidate for the detection of THz radiation. While
these studies were performed on conventional devices working at dilution temperatures
(≈ 100 mK), here we designed a new architecture including THz antennas and targeted
the demonstration of a similar effect in an OptistatDry cryostat with optical access and
a much higher nominal operational temperature of ≈ 3 K. The first aim has been the
observation of the B-AB anticrossing between orbitals localized in the multi dot structure
hosted by the nanowire, thus demonstrating the configuration in which it is possible to
operate the detector.
Initial studies focused on the design of nanowires equipped with bow-tie antennas,
capable of collecting low THz frequencies, and on the study of charge transport in dark
conditions. The discussion is based on results from two single-electron devices (out of
about ten measured in two fabrication batches) that allowed the observation of a single QD
and a DQD behavior at a nominal cryostat temperature of 2.4 K. By studying the Coulomb
diamonds, measurements allowed the extraction of the capacitive coupling parameters of
the device with the electrodes and demonstrated full control of the electronic occupancy
in the double dot architecture. In particular, by measuring the full stability diagram of
the DQD close to the complete pinch-off of the device, we achieved the limit of the first
B and AB orbitals of the “molecule”, as desired. By exploiting the possibility of using
QDs as absolute thermometers for electronic temperature, we also verified that the device
operated at an electronic temperature of about 4.2 K. The second part of the study focused
on the effect of the opening of the THz optical access, which posed a set of challenges in
terms of perturbation and heating of the device. The study allowed the optimization of the
optical filtering by introducing, beyond the obvious room-temperature optical windows, a
thin PTFE layer anchored to the intermediate stage of the cryogenic system: this solution
allowed the device temperature to be maintained at the base cryostat temperature of 2.4 K.
The sought-after B-AB anticrossing was observed in the stability diagram under these new
conditions, demonstrating the actual possibility of studying the response of the system in
the presence of THz radiation. Finally, a 500 GHz source was characterized, for use in
the induction of the intraband transition for THz detection. Unfortunately, in the final
step of measuring the device performance when irradiated by the 500 GHz could not be
performed because the available devices stopped working properly during the last cooling
cycle and no new ones could be fabricated in time.
In conclusion, the work demonstrated the feasibility of the proposed DQD detection
architecture for THz radiation, the observation and control of the desired B-AB orbital
structure in a modified device equipped with THz antennas. Immediate perspectives will include the study of Coulomb blockade in the presence of free-space excitation at 500 GHz
and the quantification of the performance of the detector. Thanks to the foundations laid
by this work, we also expect that further device architecture will be explored, for instance
including multiple QD systems that have been shown to potentially allow the detection of
single photons in the meV energy range.