Hydrogen-induced bandgap engineering: tailoring the optoelectronic properties of InN nanowires

The monolithic integration of III-nitride semiconductors on Silicon platforms is central to the development of next-generation optoelectronic devices. In this framework, semiconductor nanowires (NWs) represent a particularly powerful platform, as their geometry can accommodate large lattice-mismatch through strain relaxation at the free lateral surfaces. Among III-nitride compounds, Indium Nitride (InN) is particularly attractive owing to its narrow direct bandgap, low carrier effective mass and high electron mobility, making it a great candidate for a diverse range of applications. Realizing the full potential of InN in this context, however, requires precise control over its bandgap energy. Conventional approaches based on tuning the optical properties during growth can be limited, motivating the search for alternative post-growth engineering strategies. This thesis introduces a post-growth strategy that bypasses these limitations by exploiting low-energy hydrogen irradiation as a non-destructive tool for bandgap tuning in InN NWs grown directly on Silicon substrates. Building on prior evidence that hydrogen incorporation in bulk InN induces a genuine bandgap shift beyond simple band filling effects, this work extends the hydrogenation strategy to InN NWs, demonstrating controlled modulation of the electronic and optical properties with high precision. The experimental investigation relies on μ-photoluminescence (μ-PL) and μ-Raman spectroscopy on single NWs. We show that hydrogen incorporation induces a widening of the bandgap and a peak energy shift from ~ 0.6 eV up to ~ 1.1 eV, also covering the telecommunications wavelength window, without altering the structural integrity of the lattice. To assess the microscopic origin and spatial distribution of these effects, multi-wavelength μ-PL measurements were performed, shedding light on the diffusion dynamics of hydrogen complexes within the NWs. The thermodynamic stability and reversibility of the hydrogenation process were systematically evaluated through different annealing treatments. Thermal annealings established specific temperature thresholds required for the "on-demand" sequential recovery of the pristine material properties. Concurrently, localized laser annealing experiments performed on single isolated NWs demonstrated the feasibility of achieving site-specific bandgap modulation. Pristine and hydrogenated NWs were also investigated through electron microscopy, in order to examine the morphological effects of hydrogen on the NWs’ surface. In conclusion, this work establishes a comprehensive and non-destructive strategy for customizing the optoelectronic properties of InN NWs on demand, paving the way for novel compact and efficient technological micro-devices.