• SETs
• Single-electron tunnel coupling frequency
• Tunnel coupling

This thesis focused on InAs/InP heterostructured semiconductor nanowires (NWs), with the aim to explore the electrostatic control of the electron transport in strongly-confined quantum dots (QDs). The recent developments in semiconductor NW growth have opened the way to new fundamental and applied research on single-electron devices obtained using heterostructured NWs. In particular, InAs/InP QDs are considered as very promising nanostructures for the implementation of strongly confined single-electron transistors. These systems are obtained by defining a few-nanometer long conductive InAs region between two InP tunnel barriers which, having a higher band gap separate it from the rest of the NWs. In these devices it is possible to reach a very good control of the electron transport, down to the individual electron, and both charging and confinement energies are relatively large (of the order of 10 meV). Unfortunately, they are also very difficult to tune since the energy spectrum is mostly determined by the hard-wall confinement of the InAs and the barrier transparency is fixed by the InP thickness. Recent studies (see Stefano Roddaro, Andrea Pescaglini,Daniele Ercolani, Lucia Sorba, and Fabio Beltram. Manipulation of Electron Orbitals in Hard-Wall InAs/InPNanowire QDs. Nano Lett. 2011, 11, 1695–16994 (2011)) have demonstrated that the electronic spectrum of an InAs/InP QD can be modified by further confining the electrons in the InAs island by means of a transverse electric field. The lack of control of the tunnel barrier transparency instead remained so far an open issue and a major limit of single-electron systems based on the InAs/InP technology.

The thesis contains an experimental study, supported also by modeling and simulations, aimed at demonstrating the control of the tunnel coupling in InAs/InP single-electron systems, by populating QD orbitals with a different quantum confinement in the direction perpendicular to the the barrier and, consequently, a different barrier penetration. Through the application of a transverse electric field, a continuous tuning of the energy levels can be obtained thanks to the mixing of orbitals with different tunneling rates. The investigation of tunnel coupling is relevant for the implementation of various quantum devices based on the InAs/InP QDs technology, and in particular for the development of novel photonic sensor based on double quantum dot system, as illustrated in the thesis. A set of devices with a specific architecture was designed and realized, integrating all the necessary circuital elements to control the energy spectrum of the QD and electrical conduction through the device.

Differently from what reported so far in the literature, the work was focused on a "high-filling'' and electronic transport has been carefully mapped from the limit of an empty QD up to a top filling of $36$ electrons. By exploiting the manipulation of the orbital energy and by measuring the finite-bias Coulomb blockade characteristics, the most important parameters of each QD orbital are extracted. In particular single-electron tunneling rates are determined and a clear modulation is reported as a function of the electron filling, with a sharp increase occurring at the filling of axially-excited orbitals having a larger kinetic energy in the direction perpendicular to the barrier. Using lateral gates, the tunneling rates can be further modulated by inducing a hybridization between the different orbitals. In particular I obtained a tunneling rates from 0.85 +- 0.02 GHz for weakly-coupled orbitals up to 28.8 +- 0.3 GHz for the strongly-coupled ones. Perspectives of this research include: (i) a further investigation of the modulation mechanism of the single-electron tunneling rate and (ii) to the realization of novel photonic sensors based on double quantum dot system, where one QD acts as a ``detector'' changing its charge state upon absorption of a single photon and the other QD acts as a ``charge detector'' that translates the charge signal into a measurable current.