Understanding nanoscale electron and phonon transport is critical for the development of next- generation semiconductor technologies, where deviations from macroscopic behaviors can either limit or enhance device performance. While transient gratings generated by the interference of visible lasers can directly excite microscopic, nonequilibrium charge and heat distributions in metals and traditional semiconductors, extending this noncontact approach to ultrawide-bandgap materials involves added complexities. To address these challenges, here we introduce a tabletop deep-ultraviolet (DUV; 6.3 eV) transient grating setup, and show that it supports sub-300 nm spatial and subpicosecond temporal resolution. As an initial demonstration, we excite and probe gigahertz surface acoustic waves in thin gold films. We then perform DUV transient grating measurements of nanoscale carrier transport in diamond and discuss the carrier concentration-dependent diffusion coefficient. This DUV transient grating capability provides a versatile, noncontact tool for investigating transport at length scales below the visible diffraction limit and in wide-bandgap materials, and bridges the gap between visible and facility-scale extreme-ultraviolet transient grating capabilities.

Next-generation nanoelectronic, energy, and quantum technologies require increasingly stringent thermal, optical, mechanical, and electrical properties of component materials, often surpassing the limits of widely used materials such as silicon. Diamond, an ultrawide bandgap semiconductor, is a promising material for these applications because of its very high stiffness, thermal conductivity, and electron mobility. However, incorporating diamond into devices that require high-quality metal-diamond interfaces is challenging. In this work, we use a suite of electron microscopy measurements to reveal an ultrathin amorphous carbon layer that emerges at metal-diamond interfaces after electron beam lithography. Using extreme ultraviolet scatterometry, we nondestructively determine lower bounds on the layer's Young's modulus and thermal conductivity, which at >230 GPa and >1.1 W/(m K) are indicative of a diamondlike form of amorphous carbon with high sp3 bonding. However, extreme ultraviolet coherent diffractive imaging reflectometry and energy-dispersive x-ray spectroscopy measurements indicate a low and likely inhomogeneous density in the range of 1-2 g/cm(3). The low density of such a stiff and conductive layer could indicate that it contains nanometer-scale voids or atomic-scale vacancies. The appearance of this unusual layer illustrates the nanofabrication challenges for diamond and highlights the need for better techniques to characterize surfaces and interfaces in nanoscale devices.

We use ultrafast, extreme ultraviolet high harmonic beams to fully characterize a silicon metalattice, a novel nanostructured semiconductor material. We then develop a general, predictive model for the highly-confined heat flow behavior in nanostructured materials.

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible to engineer their thermal properties. However, the influence of boundaries limits the validity of bulk models, while first principles calculations are too computationally expensive to model real devices. Here we use extreme ultraviolet beams to study phonon transport dynamics in a 3D nanostructured silicon metalattice with deep nanoscale feature size, and observe dramatically reduced thermal conductivity relative to bulk. To explain this behavior, we develop a predictive theory wherein thermal conduction separates into a geometric permeability component and an intrinsic viscous contribution, arising from a new and universal effect of nanoscale confinement on phonon flow. Using experiments and atomistic simulations, we show that our theory applies to a general set of highly-confined silicon nanosystems, from metalattices, nanomeshes, porous nanowires to nanowire networks, of great interest for next-generation energy-efficient devices.

Nanostructured semiconductors can exhibit thermal properties unachievable in bulk systems and will play a crucial role in next-generation nanoelectronics and energy efficient devices, where heat evacuation poses a critical limitation. However, first principles models are too computationally challenging for 3D nanostructured geometries, while ballistic phonon descriptions make overly-simplistic assumptions about the nature of phonon-boundary interactions. Here, we study heat flow in a 3D nanostructured silicon metalattice of <<100nm feature size, using an infrared pump laser to excite the sample and an extreme ultraviolet probe to monitor its relaxation. Analyzing surface acoustic waves launched by metallic gratings fabricated on the sample surface, we nondestructively extract the metalattice’s elastic properties and porosity, validated by electron tomography, which are required parameters to model the heat flow. With the same measurement, we observe the heat flow dynamics to exhibit a Fourier-like behavior with an ultra-low apparent thermal conductivity. Through an analogy to rarefied gas flow in porous media, we explain this and similar measurements of phonon transport in silicon nanomeshes, porous nanowires and nanowire networks. We separate the geometry-dependent permeability contribution to thermal conduction from an intrinsic viscous component, which scales universally with porosity across all systems where feature sizes are much smaller than the dominant phonon mean free paths. This leads to an analytic description of thermal conduction in highly-confined silicon nanosystems, enabling their representation as effective media in engineering applications.

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counterintuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfvén waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, α = 2 as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed >600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: preflare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that α = 1.63 ± 0.03. This is below the critical threshold, suggesting that Alfvén waves are an important driver of coronal heating.

Semiconductor metalattices consisting of a linked network of three-dimensional nanostructures with periodicities on a length scale <100 nm can enable tailored functional properties due to their complex nanostructuring. For example, by controlling both the porosity and pore size, thermal transport in these phononic metalattices can be tuned, making them promising candidates for efficient thermoelectrics or thermal rectifiers. Thus, the ability to characterize the porosity, and other physical properties, of metalattices is critical but challenging, due to their nanoscale structure and thickness. To date, only metalattices with high porosities, close to the close-packing fraction of hard spheres, have been studied experimentally. Here, we characterize the porosity, thickness, and elastic properties of a low-porosity, empty-pore silicon metalattice film (∼500 nm thickness) with periodic spherical pores (∼tens of nanometers), for the first time. We use laser-driven nanoscale surface acoustic waves probed by extreme ultraviolet scatterometry to nondestructively measure the acoustic dispersion in these thin silicon metalattice layers. By comparing the data to finite element models of the metalattice sample, we can extract Young's modulus and porosity. Moreover, by controlling the acoustic wave penetration depth, we can also determine the metalattice layer thickness and verify the substrate properties. Additionally, we utilize electron tomography images of the metalattice to verify the geometry and validate the porosity extracted from scatterometry. These advanced characterization techniques are critical for informed and iterative fabrication of energy-efficient devices based on nanostructured metamaterials.