Modelling second harmonic generation at mid-infrared frequencies in waveguide integrated Ge/SiGe quantum wells.

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.

Terahertz absorption-saturation and emission from electron-doped germanium quantum wells.

We study radiative relaxation at terahertz frequencies in n-type Ge/SiGe quantum wells, optically pumped with a terahertz free electron laser. Two wells coupled through a tunneling barrier are designed to operate as a three-level laser system with non-equilibrium population generated by optical pumping around the 1→3 intersubband transition at 10 THz. The non-equilibrium subband population dynamics are studied by absorption-saturation measurements and compared to a numerical model. In the emission spectroscopy experiment, we observed a photoluminescence peak at 4 THz, which can be attributed to the 3→2 intersubband transition with possible contribution from the 2→1 intersubband transition. These results represent a step towards silicon-based integrated terahertz emitters.

Modeling of second harmonic generation in hole-doped silicon-germanium quantum wells for mid-infrared sensing.

The development of Ge and SiGe chemical vapor deposition techniques on silicon wafers has enabled the integration of multi-quantum well structures in silicon photonics chips for nonlinear optics with potential applications to integrated nonlinear optics, however research has focused up to now on undoped quantum wells and interband optical excitations. In this work, we present model calculations for the giant nonlinear coefficients provided by intersubband transitions in hole-doped Ge/SiGe and Si/SiGe multi-quantum wells. We employ a valence band-structure model for Si1-xGex to calculate the confined hole states of asymmetric-coupled quantum wells for second-harmonic generation in the mid-infrared. We calculate the nonlinear emission spectra from the second-order susceptibility tensor, including the particular vertical emission spectra of valence-band quantum wells. Two possible nonlinear mid-infrared sensor architectures, one based on waveguides and another based on metasurfaces, are described as perspective application.

Photoluminescence from GeSn nano-heterostructures.

We investigate the distribution of Sn in GeSn nano-heteroepitaxial clusters deposited at temperatures well exceeding the eutectic temperature of the GeSn system. The 600 °C molecular beam epitaxy on Si-patterned substrates results in the selective growth of GeSn nano-clusters having a 1.4 ± 0.5 at% Sn content. These nano-clusters feature Sn droplets on their faceted surfaces. The subsequent deposition of a thin Ge cap layer induced the incorporation of the Sn atoms segregated on the surface in a thin layer wetting the nano-dots surface with 8 ± 0.5 at% Sn. The presence of this wetting layer is associated with a relatively strong photoluminescence emission that we attribute to the direct recombination occurring in the GeSn nano-dots outer region.

CMOS-compatible optical switching concept based on strain-induced refractive-index tuning.

In this paper we present a planar lightwave switching mechanism based on large refractive index variations induced by electrically-driven strain control in a CMOS-compatible photonic platform. Feasibility of the proposed concept, having general validity, is numerically analyzed in a specific case-study given by a Mach-Zehnder Interferometer with Ge waveguides topped by a piezoelectric stressor. The stressor can be operated in order to dynamically tune the strain into the two interferometric arms. The strain modifies the Ge band structure and can induce refractive index variations up to 0.05. We demonstrate that this approach can enable ultra-compact devices featuring low loss propagation for light wavelengths below the waveguide band gap energy, high extinction ratios (>30 dB) and low intrinsic insertion losses (2 dB). The operation wavelength can be extended in the whole FIR spectrum by using SiGe(Sn) alloy waveguides.

Three-Dimensional Reconstruction of Interface Roughness and Alloy Disorder in Ge/GeSi Asymmetric Coupled Quantum Wells Using Electron Tomography.

Interfaces play an essential role in the performance of ever-shrinking semiconductor devices, making comprehensive determination of their three-dimensional (3D) structural properties increasingly important. This becomes even more relevant in compositional interfaces, as is the case for Ge/GeSi heterostructures, where chemical intermixing is pronounced in addition to their morphology. We use the electron tomography method to reconstruct buried interfaces and layers of asymmetric coupled Ge/Ge0.8Si0.2 multiquantum wells, which are considered a potential building block in THz quantum cascade lasers. The three-dimensional reconstruction is based on a series of high-angle annular dark-field scanning transmission electron microscopy images. It allows chemically sensitive investigation of a relatively large interfacial area of about (80 × 80) nm2 with subnanometer resolution as well as the analysis of several interfaces within the multiquantum well stack. Representing the interfaces as iso-concentration surfaces in the tomogram and converting them to topographic height maps allows the determination of their morphological roughness as well as layer thicknesses, reflecting low variations in either case. Simulation of the reconstructed tomogram intensities using a sigmoidal function provides in-plane-resolved maps of the chemical interface widths showing a relatively large spatial variation. The more detailed analysis of the intermixed region using thin slices from the reconstruction and additional iso-concentration surfaces provides an accurate picture of the chemical disorder of the alloy at the interface. Our comprehensive three-dimensional image of Ge/Ge0.8Si0.2 interfaces reveals that in the case of morphologically very smooth interfaces─depending on the scale considered─the interface alloy disorder itself determines the overall characteristics, a result that is fundamental for highly miscible material systems.

Room Temperature Lattice Thermal Conductivity of GeSn Alloys.

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 Ge1-xSnx 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 Ge0.88Sn0.12 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.

Nanoscale Mapping of the 3D Strain Tensor in a Germanium Quantum Well Hosting a Functional Spin Qubit Device.

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at <100 nm and >1 µm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 µeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology.

Tight-binding calculation of optical gain in tensile strained [001]-Ge/SiGe quantum wells.

It is known that under a tensile strain of about 2% of the lattice constant, the energy of the bottom conduction state of bulk Ge at the Gamma point falls below the minimum at the L point, leading to a direct gap material. In this paper we investigate how the same condition is realized in tensile strained Ge quantum wells. By means of a tight-binding sp(3)d(5)s(*) model, we study tensile strained Ge/Si(0.2)Ge(0.8) multiple quantum well (MQW) heterostructures grown on a relaxed SiGeSn alloy buffer along the [001] direction. We focus on values of the strain fields at the crossover between the indirect and direct gap regime of the MQWs, and calculate band edge alignments, electronic band structures, and density of states. We also provide a numerical evaluation of the MQW material gain spectra for TE and TM polarization under realistic carrier injection levels, taking into account the leakages related to the occupation of the electronic states at the L point. The analysis of the different orbital contributions to the near-gap states of the complete structure allows us to give a clear interpretation of the numerical results for the strain-dependent TM/TE gain ratio. Our calculations demonstrate the effectiveness of the structures under consideration for light amplification.

Clinical governance for elderly patients with renal insufficiency. Community care programs.

From a clinical governance perspective, process management is essential because it allows attention to be focused on the health problems of the people affected by illness, creating care programs that arise out of a holistic vision. This is all the more true when the people involved have specific care needs, like the elderly and patients with chronic illnesses whose primary place of care is outside the hospital and who, in any case, require continuity and coordination of care. This group certainly includes elderly patients with chronic kidney disease, the management of which has significant effects on health care settings. The national and regional dialysis and transplant registers currently provide partial data on this phenomenon, but our information is incomplete. What we lack is an unambiguous, uniform care program which addresses itself to community care for the elderly with chronic kidney disease and which, above all, places the nephrologist in a leading role. The issue is to provide a suitable solution for this anomaly, so that by putting aside an anachronistic hospital-centered vision, the nephrologist can move out into the community and come into contact with the sorts of cases which currently remain outside his or her field of vision. It is to be hoped that the Italian Society of Nephrology will spearhead this initiative by becoming more aware of the structural and organizational changes that the Italian health system is currently undergoing.

Chronic kidney disease certification process manual by the Italian Society of Nephrology (SIN): Part I: clinical care delivery and performance measurements and improvement.

Chronic kidney diseases (CKD) has now emerged as a public health priority, and there is an increasing demand by patients and health care organisations that the quality of care delivered by renal units to CKD patients be systematically monitored and evaluated. The Italian Society of Nephrology (SIN) has started an initiative aimed at promoting a quality certification process specifically focused on CKD. To this end, SIN started a collaboration with an independent Italian company which is a partner of Joint Commission International (JCI), a nonprofit international organisation dedicated to the promotion of quality improvement and safety of health services. As a result of this collaboration, a document describing a voluntary certification process developed based on JCI criteria was produced by SIN. This document comprises 2 parts. Herein (Part I) we deal with standards for clinical care delivery and performance measurements related to CKD care. Programme management and clinical information management will be presented in a separate manuscript (Part II).

Chronic kidney disease certification process manual by the Italian Society of Nephrology (SIN): part II: programme management and clinical information management.

This is the second part of a document describing a voluntary certification process based on Joint Commission International (JCI) criteria developed by the Italian Society of Nephrology (SIN) and JCI representatives. In the first part we discussed standards for clinical care delivery and performance measurements related to chronic kidney disease care. Herein (Part II), we complete the description of Performace measurements and CKD care by describing issues related the management and clinical information management.

Strain-modulated Ge superlattices.

We present a numerical study of the electronic and optical properties of a model single-element superlattice made of a periodic sequence of relaxed and strained regions of a germanium crystal, realized by means of an externally applied strain. We adopt the tight-binding model to evaluate the strain-driven modifications of the band structure and the optical properties. Superlattice band gaps, spatial confinement of near-gap valence and conduction states, and analysis of their symmetry character, have been obtained for different superlattice periodicities and strain intensities. Our results indicate that, for suitable choices of spatial periodicity and strain values, type-I and direct-gap superlattices, with strong dipole matrix elements, can be realized. Conceptually, we demonstrate that Ge single-element strained superlattices could be active materials for novel Si-compatible optical devices.

Conduction intersubband transitions at normal incidence in Si(1-x)Ge(x) quantum well devices.

We show theoretically that it is possible to design SiGe-based quantum well structures in which conduction intersubband transitions are induced by normal incidence infrared radiation. A sp(3)d(5)s(*) tight binding model has been adopted to evaluate the electronic states and optical transitions between lowest conduction confined states of a superlattice composed of one pure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We find that significant optical coupling between confined states in the Ge wells is achieved at normal incidence radiation by the off-diagonal elements of the mass tensor. The minimum energy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. For comparison we show that no such coupling can be realized for the conduction states confined in a similar structure composed by Si quantum wells because the ellipsoids of the lowest conduction valleys are oriented along the growth direction.