Michele Virgilio

The optical generation of an out-of-equilibrium spin population is a keystone process for quantum technologies and spintronics alike. Although this is an established technique for studying direct band-gap semiconductors, it has been proven limited in materials that possess weak oscillator strengths for the optical transitions. We address the problem by presenting an all-optical analog of the spin-pumping method. By applying this concept to a Ge-on-Si heterostructure, we observe luminescence from Si with a polarization degree as high as 9%. The progressive etching of the absorbing layer, assisted by magneto-optic experiments, allows us to ascertain that the polarized emission is determined by effective spin injection aided by the carrier lifetime shortening due to extended defects. These findings can facilitate the use of highly promising spin-dependent phenomena of Si, whose optical exploitation has been hampered by fundamental limitations due to its peculiar electronic structure.

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.

N-type Ge-rich Ge/SiGe multi-quantum-wells on Si(001) substrates are a novel material system with great potential due to their compatibility with the CMOS standard. This class of strained quantum heterostructures exhibits different band edge minima in a narrow energy range. Thus, lattice deformation, quantum confinement, and band bending effects can affect the energy ordering of the associated subband states, resulting in a radical variation of their optical and transport properties. This study investigates how these factors influence the 2D subband carrier density in modulation-doped heterostructures. To this end, we focus on the recently demonstrated parabolic quantum well system because of its technological relevance. Using Fourier transform infrared and high-resolution x-ray diffraction spectroscopy in combination with numerical modeling, our results highlight the importance of the temperature dependence of the strain field in determining the efficiency of the charge transfer process. Furthermore, optimal design parameters are identified to achieve either the highest transfer efficiency or the absolute value of the carrier density as a function of the intersubband transition energy in the 3–8 THz range.

We investigate the nanoheteroepitaxy (NHE) of SiGe and Ge quantum dots (QDs) grown on nanotips (NTs) substrates realized in Si(001) wafers. Due to the lattice strain compliance, enabled by the nanometric size of the tip and the limited dot/substrate interface area, which helps to reduce dot/substrate interdiffusion, the strain and SiGe composition in the QDs could be decoupled. This demonstrates a key advantage of the NHE over the Stranski–Krastanow growth mechanism. Nearly semi-spherical, defect-free, ∼100 nm wide SiGe QDs with different Ge contents were successfully grown on the NTs with high selectivity and size uniformity. On the dots, thin dielectric capping layers were deposited, improving the optical properties by the passivation of surface states. Intense photoluminescence was measured from all samples investigated with emission energy, intensity, and spectral linewidth dependent on the SiGe composition of the QDs and the different capping layers. Radiative recombination occurs in the QDs, and its energy matches the results of band-structure calculations that consider strain compliance between the QD and the tip. The NTs arrangement and the selective growth of QDs allow to studying the PL emission from only 3–4 QDs, demonstrating a bright emission and the possibility of selective addressing. These findings will support the design of optoelectronic devices based on CMOS-compatible emitters.

CMOS-compatible materials for efficient energy harvesters at temperatures characteristic for on-chip operation and body temperature are the key ingredients for sustainable green computing and ultralow power Internet of Things applications. In this context, the lattice thermal conductivity (κ) of new group IV semiconductors, namely Ge_1-xSn_x alloys, are investigated. Layers featuring Sn contents up to 14 at.% are epitaxially grown by state-of-the-art chemical-vapor deposition on Ge buffered Si wafers. An abrupt decrease of the lattice thermal conductivity (κ) from 55 W/(m·K) for Ge to 4 W/(m·K) for Ge_0.88Sn_0.12 alloys is measured electrically by the differential 3ω-method. The thermal conductivity was verified to be independent of the layer thickness for strained relaxed alloys and confirms the Sn dependence observed by optical methods previously. The experimental κ values in conjunction with numerical estimations of the charge transport properties, able to capture the complex physics of this quasi-direct bandgap material system, are used to evaluate the thermoelectric figure of merit ZT for n- and p-type GeSn epitaxial layers. The results highlight the high potential of single-crystal GeSn alloys to achieve similar energy harvest capability as already present in SiGe alloys but in the 20 °C-100 °C temperature range where Si-compatible semiconductors are not available. This opens the possibility of monolithically integrated thermoelectric on the CMOS platform.

A parabolic potential that confines charge carriers along the growth direction of quantum wells semiconductor systems is characterized by a single resonance frequency, associated to intersubband transitions. Motivated by fascinating quantum optics applications leveraging on this property, we use the technologically relevant SiGe material system to design, grow, and characterize n‐type doped parabolic quantum wells realized by continuously grading Ge‐rich Si _{1− x} Ge _x alloys, deposited on silicon wafers. An extensive structural analysis highlights the capability of the ultra‐high‐vacuum chemical vapor deposition technique here used to precisely control the quadratic confining potential and the target doping profile. The absorption spectrum, measured by means of Fourier transform infrared spectroscopy, revealed a single peak with a full width at half maximum at low and room temperature of about 2 and 5 meV, respectively, associated to degenerate intersubband transitions. The energy of the absorption resonance scales with the inverse of the well width, covering the 2.5–5 THz spectral range, and is almost independent of temperature and doping, as predicted for a parabolic confining potential. On the basis of these results, we discuss the perspective observation of THz strong light–matter coupling in this silicon compatible material system, leveraging on intersubband transitions embedded in all‐semiconductor microcavities.

A promising alternative to bulk materials for the nonlinear coupling of optical fields is provided by photonic integrated circuits based on heterostructures made of asymmetric-coupled quantum wells. These devices achieve a huge nonlinear susceptivity but are affected by strong absorption. Here, driven by the technological relevance of the SiGe material system, we focus on Second-Harmonic Generation in the mid-infrared spectral region, realized by means of Ge-rich waveguides hosting p-type Ge/SiGe asymmetric coupled quantum wells. We present a theoretical investigation of the generation efficiency in terms of phase mismatch effects and trade-off between nonlinear coupling and absorption. To maximize the SHG efficiency at feasible propagation distances, we also individuate the optimal density of quantum wells. Our results indicate that conversion efficiencies of ≈ 0.6%/W can be achieved in WGs featuring lengths of few hundreds µm only.

A promising alternative to bulk materials for the nonlinear coupling of optical fields is provided by photonic integrated circuits based on heterostructures made of asymmetric-coupled quantum wells. These devices achieve a huge nonlinear susceptivity but are affected by strong absorption. Here, driven by the technological relevance of the SiGe material system, we focus on Second-Harmonic Generation in the mid-infrared spectral region, realized by means of Ge-rich waveguides hosting p-type Ge/SiGe asymmetric coupled quantum wells. We present a theoretical investigation of the generation efficiency in terms of phase mismatch effects and trade-off between nonlinear coupling and absorption. To maximize the SHG efficiency at feasible propagation distances, we also individuate the optimal density of quantum wells. Our results indicate that conversion efficiencies of ≈ 0.6%/W can be achieved in WGs featuring lengths of few hundreds µm only.