FABRICATION AND CHARACTHERIZATION OF NANOSTRUCTURED INTEGRABLE SILICON DEVICES

This thesis work investigates the design, fabrication, and characterization of nanostructured silicon devices that can be integrated on chip, with the aim of understanding and controlling the mechanisms of thermal and electronic transport in semiconductor materials at the nanoscale. The research originates from the growing interest in technologies capable of improving the energy efficiency of electronic devices and of exploiting or managing heat more effectively, since in modern electronic systems heat represents both an energy loss and a limitation to performance. In this context, silicon is a particularly important material because it forms the basis of the microelectronics industry and is compatible with standard CMOS fabrication processes; however, its thermal properties are not always ideal for applications such as thermoelectric energy conversion or advanced thermal management. Nanostructuring the material, however, makes it possible to significantly modify phonon transport, that is, the quanta of vibration of the crystal lattice responsible for heat conduction in semiconductors, reducing thermal conductivity without drastically compromising the electronic properties. The first part of the thesis is therefore devoted to the study of the physical principles of thermal transport in silicon and the role of phonons, analyzing how scattering phenomena with defects, impurities, surfaces, and interfaces can limit the propagation of lattice vibrations. When the dimensions of the structures become comparable to the phonon mean free path, surfaces and boundaries become dominant in determining the thermal behavior of the system. For this reason, the realization of controlled nanometric patterns represents an effective tool for engineering heat conduction. In this work, several nanostructured silicon configurations are designed and fabricated using micro- and nanofabrication techniques, including electron-beam lithography and controlled etching processes, with the aim of creating lattices and surface patterns capable of modifying the phonon pathways. Particular attention is devoted to so-called shallow-etched structures, that is, nanostructures etched only partially into the material, which allow the reduction of thermal conductivity while maintaining good mechanical integrity and compatibility with chip integration processes. Through experimental measurements and comparative analyses, it is shown how parameters such as the distance between nanostructures, their size, and their geometry significantly influence heat transport. The results indicate that the presence of periodic patterns can increase phonon scattering and therefore reduce the effective thermal conductivity of the material compared to bulk silicon. This type of phonon transport engineering is particularly relevant for the development of thermoelectric devices, which directly convert a temperature difference into electrical energy, since low thermal conductivity is one of the key conditions for improving the efficiency of such devices. Another important result of the thesis is the realization and study of a silicon-integrated thermal diode. A thermal diode is a device capable of allowing heat to flow preferentially in one direction rather than the other, similarly to how an electrical diode allows current to pass mainly in a single direction. This behavior, called thermal rectification, can be obtained by introducing a geometric or structural asymmetry in the system. In this work, asymmetric nanostructures are designed to produce different degrees of phonon scattering depending on the direction of the heat flow. Experimental measurements demonstrate that rectification strongly depends on the spacing between the nanostructures and on the geometric configuration of the device, and that it is possible to obtain directional thermal transport even using a relatively simple material such as silicon. This result is significant because it demonstrates the possibility of realizing phononic components that can be integrated into conventional electronic circuits, opening the way to the development of heat-control systems at the micro- and nanoscale. The second part of the thesis broadens the scope of the study by including other aspects related to advanced materials and devices. In particular, the role of doping in silicon is analyzed, namely the controlled introduction of impurities that modify the concentration of charge carriers. Doping is fundamental for the operation of electronic devices, but it also affects thermal transport because impurities can interact with phonons and increase scattering processes. The study shows how the choice of the doping level can be used to simultaneously optimize electronic and thermal properties, a crucial aspect in the design of multifunctional devices. Another line of research concerns the realization of conductive structures through three-dimensional printing techniques. These structures are developed as prototypes for sensing devices and electronic components that can be fabricated through rapid and low-cost manufacturing methods. Electrical characterization measurements show that these systems can exhibit interesting conductive properties and may be useful for applications in sensors or flexible electronic devices. Finally, the thesis includes a study on photoconduction in two-dimensional hybrid perovskites, emerging materials that have been widely investigated in recent years for optoelectronic applications such as solar cells, photodetectors, and photonic devices. Perovskites exhibit particularly promising optical and electronic properties, including strong light absorption and good charge carrier mobility. Through photoconduction experiments, the behavior of these materials under illumination is analyzed, highlighting their ability to generate and transport electrical charges in response to light. The results confirm the potential of two-dimensional perovskites for the development of advanced optoelectronic devices and demonstrate how the integration of new materials with micro- and nanostructured fabrication techniques can open innovative perspectives in the design of functional devices. Overall, the thesis provides a significant contribution to the understanding and control of thermal transport in nanostructured semiconductors and demonstrates the possibility of using silicon, a material already widely employed in the electronics industry, to realize phononic devices and heat-management systems that can be integrated on chip. The results obtained show that nanostructuring represents a powerful tool for modifying the physical properties of materials and that phonon transport engineering can have important applications in fields such as energy conversion, heat dissipation in integrated circuits, and the development of new thermal and optoelectronic devices. This research therefore contributes to the advancement of energy and microelectronic technologies by proposing solutions that combine industrial compatibility, innovation in materials, and control of physical properties at the nanoscale.