Don Greg Walker

Context: In classical molecular dynamics, thermal transport via electrons is typically non-existent. Therefore, thermal property determination in metals or material systems that include metals is inaccessible. We have developed a two-temperature model for use with non-equilibrium molecular dynamics to predict thermal interface resistance across metal–metal and metal–insulator interfaces. Using LAMMPS and a modified module for the diffusion of thermal energy via electrons, we systematically examine the effects of including a second transport pathway through material systems. We found that inclusion of an electronic transport pathway reduces the phonon-only thermal conductivity because of electron–phonon scattering. Moreover, the presence of electrons eliminates temperature jumps at the boundary but still admits interface resistance, which is reduced in some cases by an order of magnitude. Method: We developed a module for LAMMPS that estimates thermal transport via the diffusion equation with a specified electron thermal conductivity. The electronic energy is transferred to/from the atomic system using velocity rescaling with appropriate momentum perturbation. The atomistic motion is governed by the NiU3-EAM potential.

Energetic carriers in semiconductors thermalize by impact-ionization, which generates electron–hole pairs (EHPs), and by energy losses to phonons. The average EHP creation energy is typically about three times the energy gap. In 1960, Shockley derived a simple equation for the average EHP creation energy with a single free parameter that fits experimental values for a wide range of materials, but the underlying assumptions, as stated, have been widely criticized as lacking justification. Modified expressions derived by improved approximations have been proposed but do not fare better. Here, we revisit the foundations of Shockley's equation and provide a robust justification for the kinetic-energy component as a model averaging procedure and then apply a similar procedure to the phonon component of the equation. The phonon result retains Shockley's form, but the interpretation and justification are now on par with those of the kinetic-energy term. The single-parameter fit to the data remains unchanged, i.e., the present analysis accounts for the exceptional applicability of Shockley's equation.

There are a broad range of applications for narrowband long-wave infrared (LWIR) sources, especially within the 8–12 μm atmospheric window. These include infrared beacons, free-space communications, spectroscopy, and potentially on-chip photonics. Unfortunately, commercial light-emitting diode (LED) sources are not available within the LWIR, leaving only gas-phase and quantum cascade lasers, which exhibit low wall-plug efficiencies and in many cases require large footprints, precluding their use for many applications. Recent advances in nanophotonics have demonstrated the potential for tailoring thermal emission into an LED-like response, featuring narrowband, polarized thermal emitters. In this work, we demonstrate that such nanophotonic IR emitting metamaterials (NIREMs), featuring near-unity absorption, can serve as LWIR sources with effectively no net power consumption, enabling their operation entirely by waste heat from conventional electronics. Using experimental emissivity spectra from a SiC NIREM device in concert with a thermodynamic compact model, we verify this feasibility for two test cases: a NIREM device driven by waste heat from a CPU heat sink and one operating using a low-power resistive heater for elevated temperature operation. To validate these calculations, we experimentally determine the temperature-dependent NIREM irradiance and the angular radiation pattern. We purport that these results provide a first proof-of-concept for waste heat-driven thermal emitters potentially employable in a variety of infrared application spaces.

Thermal transport in amorphous silicon dioxide (a-SiO₂) is traditionally treated as random walks of vibrations owing to its greatly disordered structure, which results in a mean free path (MFP) approximately the same as the interatomic distance. However, this picture has been debated constantly and in view of the ubiquitous existence of thin a-SiO₂ layers in nanoelectronic devices, it is imperative to better understand this issue for precise thermal management of electronic devices. Different from the commonly used cross-plane measurement approaches, here we report on a study that explores the in-plane thermal conductivity of double silicon nanoribbons with a layer of a-SiO₂ sandwiched in-between. Through comparing the thermal conductivity of the double ribbon samples with that of corresponding single ribbons, we show that thermal phonons can ballistically penetrate through a-SiO₂ of up to 5 nm thick even at room temperature. Comprehensive examination of double ribbon samples with various oxide layer thicknesses and van der Waals bonding strengths allows for extraction of the average ballistic phonon penetration depth in a-SiO₂. With solid experimental data demonstrating ballistic phonon transport through a-SiO₂, this work should provide important insight into thermal management of electronic devices.