Molecular Quantum-dot Cellular Automata (QCA): Characterization of the bis-ferrocene molecule as a QCA device

Quantum-dot cellular automata is an emerging technology for digital computation that follows the More than Moore trends and aims to the simultaneous reduction of both device size and power consumption. In particular, the basic QCA device is a cell made of dots and in which a bunch of free charges are allowed to move without leaving the cell itself. Depending on which dots the free charges occupy inside the cell (called also charge localization inside the cell) the binary information could be encoded and the interaction between nearby cells is performed by the electrostatic interaction. This means that no current flows between QCA devices, thus strongly reducing the power dissipation. Regarding the physical implementation of the QCA technology, different solutions were proposed in literature (semiconductor, metallic, magnetic and molecular) and in some cases (metallic and magnetic) a prototype or more advanced circuits were developed. Among all the implementations proposed, molecular QCA is the most promising, since high operating frequencies (THz) and non cryogenic work temperature (room temperature) could be achieved due to the nanometer size of a molecular system. However, a molecular prototype still does not exist and in literature only preliminary attempts to demonstrate the molecular QCA feasibility were carried out. The main difficulties to achieve a molecular prototype arise from the lack of control in the fabrication processes at the molecular scale and the current resolution of the electronic instruments to read the state of a single molecular QCA cell. The work of this thesis focused on the characterization from an electronic point of view of a molecule synthesized ad hoc for QCA computing and called bis-ferrocene. The molecule was synthesized by a group of the chemical department of the University of Bologna, in collaboration with the ST Microelectronics company. This work aimed to evaluate the bis-ferrocene properties as QCA device both at the equilibrium and in presence of a bias system. In addition, the interaction between nearby molecules was evaluated and the simulation of the simplest QCA circuit, a molecular wire, was performed. The methodology adopted to carry on this analysis come from the needs to model the bis-ferrocene molecule by means of some figures of merit that could be measured by electronic instrumentations. This is because in literature all the candidate molecules proposed for QCA were characterized using chemical quantities derived from mathematical approximations (energy levels and molecular orbitals). Moreover, all the steps of this work were performed with the aim to set-up an experimental demonstration of the QCA functionalities focusing on a bis-ferrocene wire. For this reasons, the choice of the bias system, the QCA circuit and the definition of a new methodology come from the experimental scheme studied in this work. In particular, the scheme proposed here focused on the experimental evaluation of the three main mechanism involved during QCA computation: how to force the two logic states at the input (write-in system), the interaction between molecules (information propagation) and, finally, the study of system able to recognize the charge localization inside the cell (read-out stage). In addition, given the results obtained during parallel experimental activities, a fault tolerance evaluation of the bis-ferrocene wire in presence of real fabrication defects was performed.