Stefano Roddaro

We present the operation with moderate radiation shield in a surface laboratory of BULky and Low-threshold Kinetic Inductance Detector) (BULLKID), a cryogenic detector for searches of light dark matter or coherent elastic neutrino-nucleus scattering. The detector consists of an array of 60 cubic silicon particle absorbers of 0.34 g each, sensed by cryogenic kinetic inductance detectors. The analysis presented focuses on data from 15 elements of the array, with two central units used to evaluate the background and with their surrounding elements used as veto. The low energy spectrum resulting from an exposure of 290 hours to ambient backgrounds, acquired with the use of external and internal radiation shields, is compatible with the simulations at the level of ( 6.8 ± 0.4 stat . ± 0.1 syst . ) × 10 4 counts / keV kg days from 2 keV down to an energy of 600 eV. The region between 225 eV and 600 eV shows a rise in background in disagreement with the simulations, while not sharing some of the key traits of the low energy excess observed in other cryogenic experiments. The high energy spectrum shape is in overall agreement with the simulations and displays the typical particle-induced x-ray emission of the surrounding lead.

We present a comprehensive theoretical study of n-type Ge/SiGe resonant tunneling diodes integrated on Si substrates, aimed at developing CMOS-compatible terahertz oscillators. Leveraging non-equilibrium Green’s function simulations combined with semi-analytical modeling, we investigate the influence of band structure engineering, strain, and heterostructure composition on device performance. Optimal double-barrier quantum well profiles are identified within experimentally feasible constraints, highlighting a trade-off between the device current density and the current peak-to-valley ratio (PVR) in the negative differential resistance (NDR) region of the J–V characteristic. Optimized designs feature PVRs of up to ∼10 at low current densities or, alternatively, higher current densities of up to 100 kA/cm2 accompanied by lower PVR values. Predicted cut-off frequencies exceed 1 THz, with output powers on the order of tenths of milliwatts. Furthermore, a drift-based model highlights the importance of lateral bias uniformity in mesa devices, setting critical design limits for stable negative differential resistance behavior. These results indicate the potential of Ge/SiGe resonant tunneling diodes as a viable route toward silicon-integrated THz electronic sources, bridging the gap between III–V and group-IV semiconductor platforms.

The design of soft actuators remotely controlled by optical triggering and propelling schemes requires materials that might combine ease of device fabrication and miniaturization with robust performance of moving parts and versatile and precise response to light stimuli. Here, light-triggered photochromic-doped poly(methyl methacrylate)/reduced graphene-oxide bilayers are proposed as effective actuation architectures, showing multi-photoaddressable bending and distinctive photothermal properties. By interlayer thermomechanical contrast and intrinsic mechanical amplification of photoinduced strain mismatch, these hybrid systems show good bending and load-lifting performance, broadband photothermal triggering, as well as faster thermal and actuation response compared to pristine photochromic films. Under UV illumination, measured displacements, exerted force, and response time reach values of about 200 μm, 2 mN, and the scale of few seconds, respectively. The photothermal conversion efficiency of the system is estimated to be at least 55%. Such properties make these architectures appealing as actuation elements for various device platforms, including segments of biomimetic components, and biomedical devices with precise structure-function design of the optical control.

The integration of thermoelectric devices into mainstream microelectronic technological platforms could be a major breakthrough in various fields within the so-called Green-IT realm. In this article, the thermoelectric properties of heteroepitaxial SiGeSn alloys, an emergent CMOS-compatible material system, are evaluated to assess their possible application in thermoelectric devices. To this purpose, starting from the experimentally low lattice thermal conductivity of SiGeSn/Ge/Si layers of about ∼1 to 2 W/m·K assessed by means of 3-ω measurements, the figure of merits are calculated through the use of Boltzmann transport equation, taking into account the relevant intervalley scattering processes, peculiar of this multivalley material system. Values for the figure of merit ZT exceeding 1 have been obtained for both p- and n-type material at operating temperatures within the 300–400 K range, i.e., at typical on-chip temperatures. In this interval, the predicted power factor also features very competitive values on the order of 20 μW/cm ·K². Our finding indicates that this emergent class of Si-based materials has extremely good prospects for real-world applications and can further stimulate scientific investigation in this ambit.

In the emergent field of quantum technology, the ability to manage heat at the nanoscale and in cryogenic conditions is crucial for enhancing device performance in terms of noise, coherence, and sensitivity. Here, active cooling and refrigeration of the electron gas in graphene are demonstrated, by taking advantage of nanoscale superconducting tunnel contacts able to pump or extract heat directly from the electrons in the device. These structures achieved a top cooling of electrons in graphene of ∼15.5 mK at a bath temperature of ∼448 mK, demonstrating the viability of the proposed device architecture. These experimental findings are backed by a detailed thermal model that accurately replicated the observed behavior. Alternative cooling schemes and perspectives are discussed in light of the reported results. Finally, graphene electron cooling could find application in superconducting hybrid quantum technologies, such as radiation detectors, logic gates and qubits.

In the emergent field of quantum technology, the ability to manage heat at the nanoscale and in cryogenic conditions is crucial for enhancing device performance in terms of noise, coherence, and sensitivity. Here, we demonstrate the active cooling and refrigeration of the electron gas in graphene, by taking advantage of nanoscale superconductive tunnel contacts able to pump or extract heat directly from the electrons in the device. Our structures achieved a top cooling of electrons in graphene of ∼15mK at a bath temperature of ∼450 mK, demonstrating the viability of the proposed device architecture. Our experimental findings are backed by a detailed thermal model that accurately replicated the observed behavior. Alternative cooling schemes and perspectives are discussed in light of the reported results. Finally, graphene electron cooling could find application in superconducting hybrid quantum technologies, such as radiation detectors, logic gates and qubits.

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.