Alloyed Materials for PHOtonics: exploring dielectric REsponse in group IV nanowires – AMPHORE

Achieving light emission from group IV elements has been representing a pot of gold in modern Si technology for decades, involving generations of scientists trying to make Si and SiGe alloys optically active. Over the last few years, the emergence of data-intensive technological areas has made the need of implementing telecommunication functionalities (1.3-1.8 µm range) in group IV materials even more urgent, encouraging researchers to develop novel direct bandgap systems — where recombination of electrons and holes occurs with the same momentum — while keeping their compatibility with Si CMOS electronics. Among the variety of materials investigated, hexagonal-diamond Si1-xGex alloyed nanowires (2H-Si1-xGex NWs) are extremely attractive due to their easiness of fabrication and peculiar optical features. Indeed, although 2H-Si bulk is stable only at pressures larger than 12 GPa, recent studies demonstrated that, at the nanoscale, 2H-SiGe crystals can exist as pure wires. These nanostructures offer the unique possibility to combine an indirect semiconductor (2H-Si) with a direct one (2H-Ge) to obtain a light-emitting direct bandgap material between 1.3-1.8 µm. In fact, photoluminescence proved that increasing the Ge fraction (xGe) in 2H-Si lowers the minimum of the conduction band at G until the bandgap becomes direct — with an allowed optical transition — for xGe larger than 0.65.

Despite these significant advances, the potentialities of 2H-Si1-xGex NWs remain widely unexplored and a unified description of their optical response is fundamentally lacking. On the one hand, current modelling techniques used, though accurate, cannot take into account the influence of intrinsic (size and composition) and extrinsic parameters (strain, substrates and dopants) on the NW dielectric response because of the high computational demand and convergence problems. On the other hand, most of the optical experimental characterization of such NWs is affected by an intrinsic significant difficulty in separating the role of different physical environmental variables. In this regard, since ultra-high-resolution STEM-EELS allows for the investigation of individual nano-objects it can hence provide unique information that to a considerable degree is obscured in many optical spectroscopic methods.

The main breakthrough of the AMPHORE project is to investigate, through the effectual combination of ab initio approaches, semi-empirical methods, and computer simulations, the great potential as light emitters of 2H-Si1-xGex NWs in close integration with targeted advanced nanometer-scale optical measurements. The proposal presents two key challenging objectives: (i) the deep theoretical understanding, via precise quantum-mechanical modelling beyond the state-of-the-art, of the dielectric response of 2H-Si1-xGex NWs in a realistic environment, including morphology, substrates, and dopants and (ii) the design and accurate interpretation of targeted experiments using advanced nanometer-scale optical measurements with STEM-EELS. This funding will place the principal investigator in a unique scientific context to create an independent research line in the field of multiscale modelling of nanostructures in close connection with advanced experiments.