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Phononic and Photonic Semiconductor Nanostructures - Transport, Coherence, Confinement, and Dynamics of Phonons and Excitons in Single and Periodic Nanostructures
Abstract
Semiconductor nanostructures form the basic building blocks for the majority of modern electronic and photonic components. The continuous reduction of their dimensions in the past decades has opened possibilities to manipulate phononic and photonic properties at the micro- and nanoscale. With decreasing structure sizes, heat in the form of atomic vibrations can drastically modify material properties and limit the performance of devices. This fact poses a major challenge for the design of nanoscale devices due to the complexity of controlling the heat flow at the nanoscale.
The present work provides an overview of the experimental methods and physical processes that are relevant for thermal investigations of semiconductor nanostructures. The experimental methods dis-cussed in this work include optical techniques such as micro-Raman thermometry in its one- and two- laser version, frequency and time-domain thermoreflectance, and femtosecond pump-probe spec-troscopy based on asynchronous optical sampling, as well as contact-based techniques such as the 3-omega method, scanning thermal microscopy, and the microchip suspended platform. A comparative overview of the state-of-the-art of these techniques highlights the most suitable experimental ap-proach for materials with different structures and dimensions and identifies their main advantages and disadvantages with regard to spatial resolution and temperature sensitivity.
Following the discussion of the experimental techniques, the effects of geometry and artificial perio-dicity on the thermal properties of materials with reduced dimensionality are examined. The text pro-vides an overview of phonon scattering mechanisms, phonon lifetimes, and the effects of phonon confinement on the modification of thermal properties in nanostructures. In addition, recent advances in the study of coherent and non-coherent phonon heat conduction in materials with reduced dimen-sionality are explained. These include thin films and quasi-2D membranes, nanostructures with sec-ond-order periodicity such as two-dimensional phononic crystals and superlattices, nanowires, and quantum dots. The work highlights own publications that are part of this thesis in conjunction with recent advances in the understanding and control of phonon mediated heat propagation.
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The notion of a locally resonant metamaterial—widely applied to light and sound—has recently been introduced to heat, whereby the thermal conductivity is reduced primarily by intrinsic localized atomic vibrations rather than scattering mechanisms. This article reviews and analyzes this new emerging concept, termed nanophononic metamaterial (NPM), and contrasts it with the competing concept of a nanophononic crystal (NPC) in which thermal conductivity reduction is realized primarily via nanoscale Bragg scattering. Both the NPM and NPC core mechanisms require the presence of a sufficient level of wave behavior, which is possible when there is a relatively wide distribution of the phonon mean free path (MFP). Silicon serves as a perfect material to form NPMs and NPCs given its relatively large average phonon MFP. This offers a unique opportunity considering silicon's abundance and mature fabrication technology. It is shown in this comparative study that while both the NPM and NPC nanosystems may be rendered to serve as extreme insulators of heat, an NPM may do so without excessive reduction in the minimum feature size–which is key to keeping the electrical properties intact. This trait makes a silicon‐based NPM poised to serve as a low‐cost thermoelectric material with exceptional performance. Nanophononic metamaterials and nanophononic crystals are nanostructured materials with unique features suitable for the reduction of the thermal conductivity and the increase of the thermoelectric energy conversion figure‐of‐merit. An in‐depth literature and technical review on these materials is presented with a focus on membrane‐based configurations. Attention is given to the underlying physics and performance, in contrast to a corresponding standard uniform membrane.
We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.
Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.
A cool way to use isotopes
Thermal management of electronics requires materials that can efficiently remove heat. Several promising materials have been found recently, but diamond remains the bulk material with the highest thermal conductivity. Chen et al. found that isotopically pure cubic boron nitride has an ultrahigh thermal conductivity, 75% that of diamond. Using only boron-11 or boron-10 allows the crystal vibrations that carry heat to move more efficiently through the material. This property could be exploited for better regulating the temperature of high-power devices.
Science , this issue p. 555
The thesis tackles one of the most difficult problems of modern nanoscale science and technology - exploring what governs thermal phenomena at the nanoscale, how to measure the temperatures in devices just a few atoms across, and how to manage heat transport on these length scales. Nanoscale heat generated in microprocessor components of only a few tens of nanometres across cannot be effectively fed away, thus stalling the famous Moore's law of increasing computer speed, valid now for more than a decade. In this thesis, Jean Spièce develops a novel comprehensive experimental and analytical framework for high precision measurement of heat flows at the nanoscale using advanced scanning thermal microscopy (SThM) operating in ambient and vacuum environment, and reports the world’s first operation of cryogenic SThM. He applies the methodology described in the thesis to novel carbon-nanotube-based effective heat conductors, uncovers new phenomena of thermal transport in two- dimensional (2D) materials such as graphene and boron nitride, thereby discovering an entirely new paradigm of thermoelectric cooling and energy production using geometrical modification of 2D materials.
Thermal conduction in semiconductor nanowires is controlled by the transport of atomic vibrations also known as thermal phonons. The ability of nanowires to tailor the transport of thermal phonons stems from their precise atomic scale growth coupled with high structural surface to volume ratios. Understanding and manipulating thermal transport properties at the nanoscale is central for progress in the fields of microelectronics, optoelectronics, and thermo-electrics. Here, we review state-of-the-art advances in the understanding of nanowire thermal phonon transport and the design and fabrication of nanowires with tailored thermal conduction properties. We first introduce the basic physical mechanisms of thermal conduction at the nanoscale and details recent developments in employing nanowires as thermal materials. We discuss and provide insight on different strategies to modulate nanowire thermal properties leveraging the underlying phonon transport processes occurring in nanowires. We also highlight challenges and key areas of interest to motivate future research and create exceptional capabilities to control heat flow in nanowires.
“Second sound” has been observed in graphite
Starting from our previous work in which we obtained a system of coupled integrodifferential equations for acoustic sound waves and phonon density fluctuations in two-dimensional (2D) crystals, we derive here the corresponding hydrodynamic equations, and we study their consequences as a function of temperature and frequency. These phenomena encompass propagation and damping of acoustic sound waves, diffusive heat conduction, second sound, and Poiseuille heat flow, all of which are characterized by specific transport coefficients. We calculate these coefficients by means of correlation functions without using the concept of relaxation time. Numerical calculations are performed as well in order to show the temperature dependence of the transport coefficients and of the thermal conductivity. As a consequence of thermal tension, mechanical and thermal phenomena are coupled. We calculate the dynamic susceptibilities for displacement and temperature fluctuations and study their resonances. Due to the thermomechanical coupling, the thermal resonances such as the Landau-Placzek peak and the second-sound doublet appear in the displacement susceptibility, and conversely the acoustic sound wave doublet appears in the temperature susceptibility, Our analytical results not only apply to graphene, but they are also valid for arbitrary 2D crystals with hexagonal symmetry, such as 2D hexagonal boron nitride, 2H-transition-metal dichalcogenides, and oxides.
Graphite gets a second sound
Between the two extremes of ballistic and diffusive lattice thermal transport is the potential for an exotic wave-like state known as second sound. Huberman et al. used fast, transient thermal grating measurements to show the existence of second sound in graphite between 85 and 125 kelvin (see the Perspective by Shi). Previous observations of second sound have been rare, confined to isotopically pure materials at very low temperatures. The observation of second sound in graphite is likely due to its layered nature, suggesting that this thermal transport mode may be accessible in other two-dimensional materials.
Science , this issue p. 375 ; see also p. 332
The vibrational landscape of an ∼250 nm-radius single gold nanoparticle dropped on a silica substrate is imaged with an ultrafast pump-probe experiment in a transient reflectivity configuration. A movie of the picosecond dynamics of the nanoparticle is recorded over 20 ns. A spatially resolved analysis of the spectrum of the transient reflectivity is also achieved. In addition to the axial oscillation of the nanoparticle driven by the normal contact stiffness and to the breathing mode of the nanoparticle, ultrafast microscopy allows us to reveal higher order acoustic eigenmodes otherwise hidden by the noise in single point measurements. These results are confirmed by calculations of the acoustic eigenfrequencies of the nanoparticle. The shear component of the particle surface displacement associated with the higher order modes is of strong interest for probing the elasticity of the surrounding medium in the GHz to THz range.