Progetti di ricerca

The HD-PIT project aims to understand and engineer the processes inducing a phase transition from the diamond cubic (dc) into the hexagonal diamond (hd) phase by nanoindentation of SiGe layers epitaxially grown on Si wafers. The final goal is to realize… Leggi tutto hd SiGe pits in the films, which could be subsequently used as a template for the realization of hd SiGe epitaxial dots. Silicon technology drives phenomenal advances in nano-electronics, but the indirect nature of Si bandgap hinders exploitation for photonic integrated circuits. Interestingly, theoretical studies have shown that the hd phase of SiGe, with a relatively high Ge content, turns the alloy into a direct bandgap material. Recently, SiGe with the hd structure has been obtained by epitaxial growth on III-V nanowires. Confirming the predictions, a direct bandgap has been demonstrated. Despite the high potential of these polytypes for photonics, the present synthesis approaches are not fully compatible with Si technology. Another approach based on pressure-induced phase transition has been exploited, obtaining occasionally the Si and Ge hd phase by nanoindentation. However, a clear control of the hd phase appearance in this context is still lacking neither it is verified for SiGe alloys. The project will explore the possibility to realize a controlled and extended transition to the hd phase by nanoindentation of Ge-rich SiGe films, being an optimal solution to obtain desired electronic properties in a system fully integrated into the Si platform. A synergic approach leveraging multiscale atomistic simulations, micro-Raman and electron microscopy experiments, will be used for a rational design of the nanoindentation experiments. In fact, it will allow for an efficient search through the large parameter space that includes a wide range of nanoindentation conditions and many different characteristics of the SiGe films. Because of the high Ge content of the SiGe films, they will be grown under strong out-of-equilibrium conditions on Si substrate by low-energy plasma-enhanced chemical vapour deposition (LEPECVD). While Raman micro-spectroscopy and transmission electron microscopy will be used to assess the formation of the hd phase, the multiscale atomistic simulations will guide the experiments. They will involve first-principle calculations of the electronic, structural, and thermodynamic properties of the hd compounds at the atomic scale, but also larger-scale classical molecular dynamics simulations of the whole indentation process, exploiting state-of-the-art interatomic potential based on machine learning approaches. The project will provide fundamental knowledge to boost the monolith integration of hd group IV compounds for on-chip optical components, such as nano-laser and optical amplifiers, thus enabling optical network-on-chip and being breakthroughs for quantum computing, high performance and green ICT.