Direct measurements of surface scattering in Si nanosheets using a microscale phonon spectrometer: implications for Casimir-limit predicted by Ziman theory

Self-Calibrating Superconducting Pair-Breaking Detector

We propose and experimentally demonstrate a self-calibrating detector of Cooper pair depairing in a superconductor based on a mesoscopic superconducting island coupled to normal metal leads. On average, exactly one electron passes through the device per broken Cooper pair, independent of the absorber volume, device, or material parameters. The device operation is explained by a simple analytical model and verified with numerical simulations in quantitative agreement with experiment. In a proof-of-concept experiment, we use such a detector to measure the high-frequency phonons generated by another, electrically decoupled superconducting island, with a measurable signal resulting from less than 10 fW of dissipated power.

Probing ballistic thermal conduction in segmented silicon nanowires

Ballistic heat conduction in semiconductors is a remarkable but controversial nanoscale phenomenon, which implies that nanostructures can conduct thermal energy without dissipation. Here, we experimentally probed ballistic thermal transport at distances of 400-800 nm and temperatures of 4-250 K. Measuring thermal properties of straight and serpentine silicon nanowires, we found that at 4 K heat conduction is quasi-ballistic with stronger ballisticity at shorter length scales. As we increased the temperature, quasi-ballistic heat conduction weakened and gradually turned into diffusive regime at temperatures above 150 K. Our Monte Carlo simulations illustrate how this transition is driven by different scattering processes and linked to the surface roughness and the temperature. These results demonstrate the length and temperature limits of quasi-ballistic heat conduction in silicon nanostructures, knowledge of which is essential for thermal management in microelectronics.

Quasi-Ballistic Heat Conduction due to Lévy Phonon Flights in Silicon Nanowires

Future of silicon-based microelectronics depends on solving the heat dissipation problem. A solution may lie in a nanoscale phenomenon known as ballistic heat conduction, which implies conduction of heat without heating the conductor. However, attempts to demonstrate this phenomenon experimentally are controversial and scarce, whereas its mechanism in confined nanostructures is yet to be fully understood. Here, we experimentally demonstrate quasi-ballistic heat conduction in silicon nanowires (NWs). We show that the ballisticity is the strongest in short NWs at low temperatures but weakens as the NW length or temperature is increased. Yet, even at room temperature, quasi-ballistic heat conduction remains visible in short NWs. To better understand this phenomenon, we probe directions and lengths of phonon flights. Our experiments and simulations show that the quasi-ballistic phonon transport in NWs is essentially the Lévy walk with short flights between the NW boundaries and long ballistic leaps along the NW. Thus, we conclude that ballistic heat conduction is present in silicon even at room temperature in sufficiently small nanostructures and may yet improve thermal management in silicon-based microelectronics.

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m⁻¹·K⁻¹ for 90 nm diameter wires with 35–40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Thermal transport in Si and Ge nanostructures in the 'confinement' regime

Reducing semiconductor materials to sizes comparable to the characteristic lengths of phonons, such as the mean-free-path (MFP) and wavelength, has unveiled new physical phenomena and engineering capabilities for thermal energy management and conversion systems. These developments have been enabled by the increasing sophistication of chemical synthesis, microfabrication, and atomistic simulation techniques to understand the underlying mechanisms of phonon transport. Modifying thermal properties by scaling physical size is particularly effective for materials which have large phonon MFPs, such as crystalline Si and Ge. Through nanostructuring, materials that are traditionally good thermal conductors can become good candidates for applications requiring thermal insulation such as thermoelectrics. Precise understanding of nanoscale thermal transport in Si and Ge, the leading materials of the modern semiconductor industry, is increasingly important due to more stringent thermal conditions imposed by ever-increasing complexity and miniaturization of devices. Therefore this Minireview focuses on the recent theoretical and experimental developments related to reduced length effects on thermal transport of Si and Ge with varying size from hundreds to sub-10 nm ranges. Three thermal transport regimes - bulk-like, Casimir, and confinement - are emphasized to describe different governing mechanisms at corresponding length scales.

Lifetime of high-order thickness resonances of thin silicon membranes

Femtosecond laser pulses are used to excite and probe high-order longitudinal thickness resonances at a frequency of ∼270 GHz in suspended Si membranes with thickness ranging from 0.4 to 15 μm. The measured acoustic lifetime scales linearly with the membrane thickness and is shown to be controlled by the surface specularity which correlates with roughness characterized by atomic force microscopy. Observed Q-factor values up to 2400 at room temperature result from the existence of a local maximum of the material Q in the sub-THz range. However, surface specularity would need to be improved over measured values of ∼0.5 in order to achieve high Q values in nanoscale devices. The results support the validity of the diffuse boundary scattering model in analyzing thermal transport in thin Si membranes.

Computational study of in-plane phonon transport in Si thin films

We have systematically investigated the in-plane thermal transport in Si thin films using an approach based on the first-principles calculations and lattice dynamics. The effects of phonon mode depletion induced by the phonon confinement and the corresponding variation in interphonon scattering, which may be important for the thermal conductivities of ultra-thin films but are often neglected in precedent studies, are considered in this study. The in-plane thermal conductivities of Si thin films with different thicknesses have been predicted over a temperature range from 80 K to 800 K and excellent agreements with experimental results are found. The validities of adopting the bulk phonon properties and gray approximation of surface specularity in thin film studies have been clarified. It is found that in ultra-thin films, while the phonon depletion will reduce the thermal conductivity of Si thin films, its effect is largely offset by the reduction in the interphonon scattering rate. The contributions of different phonon modes to the thermal transport and isotope effects in Si films with different thicknesses under various temperatures are also analyzed.

Cubic lattice nanosheets: thickness-driven light emission

Silicon has a diamond-like cubic crystal lattice for which two-dimensional (2D) nanometer thickness nanosheet crystallization appears not to be trivial. However, in this issue of ACS Nano, the group led by Heon-Jin Choi demonstrates the gas-phase dendritic growth of Si nanosheets, only 1 to 13 nm thick. Moreover, such nanosheets display strong thickness-dependent photoluminescence in a visible range with red, green, and blue emission each documented.