Davide Impelluso

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 employ several X-ray based techniques, including X-ray diffraction, X-ray absorption spectroscopy and resonant inelastic X-ray scattering, to disentangle the contributions of individual chemical species to the structural, electronic and magnetic properties of high-entropy oxides. In the benchmark compound Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O and related systems, we unambiguously resolve a sizable Jahn–Teller distortion at the Cu sites, more pronounced in the absence of Ni²⁺ and Mg²⁺, suggesting that these ions promote positional order, whereas Cu²⁺ ions act to destabilize it. Moreover, we detect magnetic excitations and estimate the strength of the interactions between pairs of different magnetic elements. Our results provide valuable insights into the role of various chemical species in shaping the physical properties of high-entropy oxides.

Mid-infrared spectroscopy is a powerful technique for identifying chemical and biological substances, increasing the demand for compact and cost-effective integrated systems. In this context, integrated electro-optic modulators are essential to implement advanced techniques such as synchronous detection or dual-comb spectroscopy. In this paper, we present a waveguide-integrated electro-optic modulator operating in a broad spectral range from 5 to 9 µm wavelength, based on a Schottky diode embedded in a graded silicon germanium waveguide. By using the free-carrier plasma absorption effect, the device achieves an extinction ratio of up to 1.9 dB at a wavelength of 9 µm. Two different methods have been used to evaluate the device speed. In both cases, high-speed operation up to 7 GHz is obtained, while the optical bandwidth is estimated to be around 3 GHz. These results represent a significant step forward in both the development of characterization techniques and in the performance of mid-IR integrated modulators, paving the way for high-performance on-chip spectroscopic systems.

We study the molecular beam epitaxy of GaSb layers on Ge-based templates grown on Si substrates by low-energy plasma-enhanced chemical vapor deposition. First, we study the growth mechanism to obtain a GaSb surface free of emerging antiphase boundaries on Ge substrates, and then we transfer this mechanism to Ge-based templates. Our investigations reveal that this mechanism is hindered by the crosshatch pattern formed during the Ge-based template growth. We demonstrate that using low-miscut (1°) Si substrates or chemical-mechanical polishing mitigates the effects of the crosshatch pattern, enabling the growth of GaSb layers free of emerging antiphase boundaries. These results represent a further step towards the monolithic integration of III-V on mid-infrared photonic platforms.

The development of on-chip spectroscopic systems has attracted a lot of attention in recent years. Such systems require broadband mid-infrared sources, resulting in the exploration of mid-infrared supercontinuum generation. This study focuses on reducing the pump peak power to obtain on-chip octave-spanning supercontinua at wavelengths from 4 to 13 μm, leveraging the high nonlinear refractive index and low propagation losses of Ge-rich graded SiGe photonic circuits. Two distinct platforms are explored, one grown in an industrial-scale epitaxy tool and the other using low-energy plasma-enhanced chemical vapor deposition. These silicon-compatible platforms are used to design nonlinear waveguides exhibiting anomalous group velocity dispersion, enabling soliton fission based octave-spanning supercontinuum generation at a record-low peak power of 311 W in this wavelength range. This work paves the way toward chip-based commercially available supercontinuum sources in the mid-infrared.

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.