Marco Faverzani

This work presents an investigation of the complex dielectric function of oxygen-deficient WO3-x thin films prepared by non-reactive radio-frequency sputtering and its evolution upon post-deposition annealing in air, using Variable-Angle Spectroscopic Ellipsometry. A depth-dependent graded optical model was developed to resolve spatial variations in the dielectric response arising from oxidation-induced compositional gradients. This approach provides a more accurate physical representation than conventional models, which assume a uniform dielectric function throughout the film. The as-deposited films exhibit strong sub-bandgap absorption, likely attributable to defect states associated with oxygen vacancies. Annealing at 400–500 °C facilitates oxygen incorporation that progressively suppresses the defect-related optical loss near the film surface. Transparency in the visible and near-infrared ranges is achieved within a depth of ∼70 nm, resulting in a homogeneous optical response. In thicker films, deeper regions remain oxygen-deficient due to the limited oxygen diffusion length, resulting in a continuous depth-dependent gradient of the dielectric function that is captured by the fitting model presently developed. The graded model can be used as a quality control tool for the fine-tuning of post-deposition annealing processes, and it provides a robust and physically grounded framework for describing the optical response of oxide thin films with spatially varying defect distributions.

The design, fabrication, and comprehensive characterization of hole‐doped Ge‐rich SiGe parabolic quantum wells engineered to exhibit intersubband transitions in the mid‐infrared spectral range around 120 meV are reported. The heterostructures are grown on Si substrates by low‐energy plasma‐enhanced chemical vapor deposition, enabling finely controlled compositional profiles and high crystalline quality. Thorough structural analysis confirms the formation of parabolic potential wells despite the presence of entropic interdiffusion. Photoreflectance spectroscopy is employed to investigate interband optical transitions in these heterostructures, whereas intersubband transitions are studied by Fourier‐transform infrared spectroscopy that revealed characteristic constant‐energy TM‐polarized absorption features up to room temperature. At higher doping levels, a more structured spectral response is observed due to valence‐band non‐parabolicity. Tight‐binding band structure simulations, incorporating many‐body effects, accurately reproduce the observed spectral features. These results highlight the potential of SiGe parabolic quantum wells as a versatile and scalable platform for the development of Si‐compatible mid‐infrared optoelectronic devices based on intersubband transitions.

We present a detailed study of hole-doped Ge/SiGe multiple quantum wells as a promising material platform for mid-infrared photonics. The heterostructures were grown on SiGe virtual substrates using low-energy plasma-enhanced chemical vapor deposition. Structural and compositional analyses via atom probe tomography and x-ray diffraction were employed to assess the crystal quality and retrieve the compositional profiles. The optical response of the samples, in the 4–12-µm wavelength range, was investigated as a function of the temperature and complementary Hall-effect measurements were performed providing insights into the temperature evolution of the electrical transport properties. In addition, a well-established tight-binding model was used to reproduce and interpret the experimental results starting from the calculated valence band structure.

The thermal stability of hyper-doped Ge-on-Si and SiGe-on-Si films featuring carrier concentrations exceeding 5×10¹⁹ cm^–3 obtained by in situ doping and pulsed laser melting has been studied. The deactivation kinetics was systematically analysed through rapid thermal annealing, reflection spectroscopy and electrical characterization. The results demonstrate that while hyper-doped Ge films exhibit rapid deactivation at temperatures above 300 °C, SiGe offers enhanced thermal stability. Surface morphology analysis confirms the preservation of flatness after pulsed laser melting and thermal treatments, suggesting possible exploitation of these materials as substrates for further growths. These findings provide insights into optimizing hyper-doped material platforms for mid-infrared photonic devices and advanced semiconductor applications, emphasizing the trade-offs between the initial carrier concentration and the thermal resilience.

In this work, we assessed by means of numerical simulations the observability of mid-infrared intersubband polaritons in hole-doped Ge/SiGe multiple quantum wells embedded into stripe microcavities consisting of an upper gold grating and a bottom highly n-doped SiGe mirror.

The structural and optical properties of p-doped Ge quantum wells separated by SiGe barriers are presented. The composition profile was determined by atom probe tomography and X-ray diffraction measurements. The energy and broadening of the fundamental intersubband transition were studied by Fourier transform infrared spectroscopy which revealed a strong absorption peak around 8.5 μm making this or similar heterostructures suitable for the realization of optoelectronic devices working in the fingerprint region.

In this work we theoretically investigate the possibility of observing strong coupling at mid‐infrared frequencies within the group‐IV semiconductor material platform. Our results show that the strong coupling condition is attainable in Ge/SiGe quantum wells integrated in hybrid metal‐semiconductor microcavities, featuring a highly n‐doped SiGe layer as one of the mirrors.