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Thermal Conductivity Reduction in a Silicon Thin Film with Nanocones
Abstract
Modern thermoelectric devices incline toward inexpensive, environmentally friendly, and CMOS-compatible materials, such as silicon. To improve the thermoelectric performance of silicon, researchers try to decrease its thermal conductivity using various nanostructuring methods. However, most of these methods have limited efficiency because they are costly and damaging for the internal structure of silicon. Here, we propose a cost-effective, large-area, and maskless nanofabrication method that creates external nanocones on the silicon surface while preserving its interior. Our experiments show that these nanocones reduce the thermal conductivity of thin silicon membranes by more than 40%. Using a modified Callaway-Holland model, we study how the thermal conductivity is affected by various phonon scattering processes in the 4-295 K temperature range. We conclude that the nanocones generate additional surface scattering, which causes the thermal conductivity reduction. The proposed nanocones and their simple fabrication method are promising for the planar thermoelectric devices based on silicon.
... Unlike the suspension process of samples fabricated on a semiconductor-on-insulator wafer-where vapor-phase hydrofluoric (VHF) acid enters through slits opened on the semiconductor device layer, and removes specific parts of SiO 2 underneath, to suspend the structure (as demonstrated in previous works [37,38])-the suspension of graphite ribbon structures exfoliated and transferred onto SiO 2 substrate is more challenging and complex. Metal VHF stoppers are required, to protect the graphite ribbons from falling down to the substrate or peeling off during the SiO 2 removal, because the etching rate of SiO 2 under the metal is lower than that under the graphite. ...
... The in-plane thermal conductivity values of the measured graphite ribbons were thus extracted, by interpolating a linear function between the simulated thermal conductivity and the measured decay time. A detailed method for extracting the thermal conductivity can be found in our previous works [37,38]. ...
... However, the thermal conductivity enhancement was less pronounced in the 300-nm-wide and 600-nm-wide cases, indicating that diffuse boundary scattering is comparatively strong in such narrow structures, even at high temperatures. A similar invariance of the thermal conductivity with decreasing temperature has also been observed in Si thin film with nanocone structures, due to the sufficient boundary scattering of phonons [37]. At temperatures below 150 K, the thermal conductivities of the three samples generally followed the same trend, with temperature further decreasing; however, a significant difference in the thermal conductivity of the three samples was shown throughout the temperature range. ...
The super-ballistic temperature dependence of thermal conductivity, facilitated by collective phonons, has been widely studied. It has been claimed to be unambiguous evidence for hydrodynamic phonon transport in solids. Alternatively, hydrodynamic thermal conduction is predicted to be as strongly dependent on the width of the structure as is fluid flow, while its direct demonstration remains an unexplored challenge. In this work, we experimentally measured thermal conductivity in several graphite ribbon structures with different widths, from 300 nm to 1.2 µm, and studied its width dependence in a wide temperature range of 10-300 K. We observed enhanced width dependence of the thermal conductivity in the hydrodynamic window of 75 K compared to that in the ballistic limit, which provides indispensable evidence for phonon hydrodynamic transport from the perspective of peculiar width dependence. This will help to find the missing piece to complete the puzzle of phonon hydrodynamics, and guide future attempts at efficient heat dissipation in advanced electronic devices.
... Researchers lately proposed various surface nanostructures that can reduce membrane thermal conductivity. For example, nanocones, 16 amorphous surface layers, 17 aluminum droplets, 18 surface roughness, 19,20 and other surface structures were demonstrated to reduce the material thermal conductivity via incoherent phonon scattering. ...
... A detailed explanation of this approach can be found in Methods and in our previous work. 16 ...
... 34,41 Below 40 K, the thermal decay in our samples is no longer impacted by the temperature where phonon-boundary scattering is dominant, as illustrated by a temperature error bar at 4 K (Supporting Note 4). This trend is consistent with that obtained in previous theoretical 20 and experimental 16 works. ...
Nanostructuring is the dominant approach for effective thermal conduction control in nanomaterials. In the past decade, researchers have been interested in thermal conduction control by the coherent effects in phononic crystal (PnC) systems. Recent theoretical works predicted that nanopillars on the surface of silicon membranes could cause a dramatic thermal conductivity reduction due to the phonon local resonances. However, this remarkable prediction has not been experimentally verified yet with the deep-nanoscale pillar-based PnCs. Here, we fabricate nanopillars on suspended silicon membranes using damageless neutral-beam etching and investigate the impact of nanopillars on the thermal conductivity of the membranes in the 4−300 K range. We found that thermal conductivity reduction caused by the nanopillars does not exceed 16%, which is much weaker than that predicted by the theoretical works. Moreover, this reduction remains temperature independent. These facts make the coherence an unlikely reason for the observed reduction. Indeed, our Monte Carlo simulations can reproduce the experimental results under purely incoherent approximation. Our study shows that the coherent control of heat conduction by PnC nanostructures is more challenging to observe experimentally in reality than predicted in the near-ideal modeling.
... On the other hand, thermal measurements could not conclusively link localization in pillars with the reduction in thermal conductivity. Some experiments detected the reduced thermal conductivity but attributed it to diffuse surface scattering of phonons induced by the pillars 27,28 . Recent experiments with more promising nanopillars could not detect any reduction in the thermal conductivity at all 29 . ...
... Overall, the values are consistent with the theoretical predictions by Malhotra and Maldovan 40 over the 50 -300 K range. Likewise, the values are consistent with our previous measurements on the 50-nm-thick membranes 27,29 . ...
The performance of silicon-based thermoelectric energy generators is limited by the high thermal conductivity of silicon. Theoretical works have long proposed reducing the thermal conductivity by resonant phonon modes in nanopillars placed on the surface of silicon films. However, these predictions have never been confirmed due to the difficulty in the nanofabrication and measurements of such nanoscale systems. In this work, we report on the fabrication and measurements of silicon films with nanopillars as small as 12 nm in diameter. Our Brillouin light scattering spectroscopy experiments revealed that nanopillars indeed host resonant phonon modes. Yet, our thermal measurements using the micro time-domain thermoreflectance technique showed only a statistically insignificant difference between the thermal properties of silicon membranes with and without nanopillars. Results of this work contrast with the predictions of a substantial reduction in the thermal conductivity due to nanopillars and suggest refining the simulations to account for realistic experimental conditions.
... Battaglia et al. 22 formulated one-dimensional and three-dimensional models for in-plane and cross-plane diffusivities in membranes, whereas Wang et al. 23 illustrated that the size of the pump beam influences measurement sensitivity. Huang et al., 24 demonstrate a scalable, maskless nanofabrication strategy to engineer surface nanocones on silicon, enhancing thermoelectric efficiency while preserving bulk crystallinity. Experimental measurements reveal a 40% reduction in thermal conductivity at room temperature, attributed to phonon surface scattering via an adjusted Callaway Holland framework across 4-295 K. Walwil et al. 25 present a transfer-based TDTR technique to measure thermal conductivity of porous thin films by depositing flat metal transducers on holey SiO₂, bypassing challenges of direct deposition. ...
This study elucidates the intriguing phenomenon of inverse thermal anisotropy in cadmium magnesium oxide (CdMgO) thin films, characterized by cross-plane thermal conductivity being greater than in-plane thermal conductivity, essential for optimizing thermal management in next-generation optoelectronic devices. Herein, we utilized Photothermal Radiometry and Frequency Domain Thermoreflectance to precisely determine the thermal conductivity and diffusivity across various concentrations of magnesium in CdMgO alloys, thereby providing essential insights into thermophysical behavior. Atomic force microscopy and X-ray diffraction revealed a direct correlation between increasing magnesium content and progressive structural evolution within plasma-assisted molecular beam epitaxy-derived CdMgO alloys. Furthermore, heat transport mechanism, analyzed using Callaway and Abeles models, indicated key phonon interactions. This comprehensive investigation provides a framework for the precise control of CdMgO thin film thermal properties, paving the way for scalable fabrication strategies to optimize performance in high-power thermal management applications.
... As an illustrative example, the fabrication of thin-films with highly rough surfaces might result in phonon suppression beyond the Casimir limit, as a high aspect ratio of each individual protrusion effectively acts as phonon traps, suppressing them with multiple surface interactions. In the works of Pennelli et al. 54 and Huang et al., 55 highly rough thin films were employed. A roughness rms value of 4.2 and 20 nm in films of 120 and 50 nm thick enabled a thermal conductivity reduction of crystalline silicon down to 17 and 27 W/m K, respectively. ...
In this work, implementations of silicon-based thermoelectric nanomaterials are reviewed. Approaches ranging from nanostructured bulk-i.e., macroscopic materials presenting nanoscale features-to more complex low-dimensional materials are covered. These implementations take advantage of different phonon scattering mechanisms and eventual modifications of the electronic band-structure for the enhancement of the thermoelectric figure of merit. This work is focused on the recent advances in silicon and silicon-based thermoelectric nanomaterials of the last decade-at both the theoretical and experimental level-with the spotlight on the most recent works. Different nanostructures and their fabrication methods are detailed, while the thermoelectric performances and the feasibility of their integration into functional micro-harvester generators are compared and discussed. This Research Update first covers the advances in nanostructured bulk, such as nanometric-sized polycrystals or defect-induced materials. Subsequently, it reviews low-dimensional materials, namely, thin films and nanowires. Later, other complex structures based on nanoporosity, superlattices, or core-shell schemes are detailed. Finally, it is devoted to present examples of the successful implementation of nanostructured silicon into functional thermoelectric devices.
... [3][4][5] It is also used in plasma structuring. [6][7][8][9][10][11] This layer is also well-known as the passivation layer used to perform anisotropic deep etch of Si in cryogenic standard and STiGer processes. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] As far as we know, the behavior of the SiO x F y layer at cryogenic temperatures has only been investigated for passivation purpose in etching processes, so the knowledge on this layer was based on these studies. ...
A silicon oxyfluoride layer was deposited on a-Si samples using SiF4 / O2 plasma at different temperatures between -100 and -40°C. In situ X-ray photoelectron spectroscopy measurements were then performed to characterize the deposited layer. The sample was then brought back to room temperature and analyzed again. It has been shown that a temperature below 65°C is needed to significantly enhance the physisorption of SiFx species. Hence, in this condition, a F-rich oxyfluoride layer, stable at low temperature only, is physisorbed. Above this threshold temperature, the native silicon oxide layer is fluorinated and the proportion of O in the deposited layer is higher and remains stable even when the sample is brought back to room temperature.
... Nanostructuring or introducing nano-features like nano-inclusions, nanocones, nanowires, nano-fibres, and nano-voids is an effective strategy to enhance zT by reducing thermal conductivity through interfacial boundary-based phonon scattering in the materials [19][20][21][22][23]. Nano-inclusion-associated interfaces strongly scatter the phonons than electrons or holes due to their relatively high mean free path associated with thermal carriers [24,25]. ...
... Nanostructuring or introducing nano-features like nano-inclusions, nanocones, nanowires, nano-fibres, and nano-voids is an effective strategy to enhance zT by reducing thermal conductivity through interfacial boundary-based phonon scattering in the materials [19][20][21][22][23]. Nano-inclusion-associated interfaces strongly scatter the phonons than electrons or holes due to their relatively high mean free path associated with thermal carriers [24,25]. ...
Cu3SbS4 is an effective, low cost and non-toxic thermoelectric compound for intermediate temperature applications. However, its tetragonal structure needs to be tuned for efficient phonon scattering to reduce thermal conductivity and enhance zT. In this present article, the semiconductive carbon black nano-inclusions effect on Cu3SbS4 thermoelectric performance is studied. The thermoelectric properties of the fabricated samples are investigated in the temperature range of 300 - 623 K. Addition of amorphous carbon nano-inclusions in Cu3SbS4 causes a reduction in the thermal conductivity by phonon scattering and improvement in the Seebeck coefficient by carrier energy filtering mechanisms. The maximum figure of merit of 0.51 is obtained for 3 mol.% carbon nano-inclusion sample at 623 K. Additionally, enhancement of thermal stability and mechanical stability (hardness) with increased carbon nano-inclusion concentration is observed. It is found that grain boundary hardening and dispersion strengthening are the reasons for the enhancement. Moreover, our detailed studies demonstrate that the addition of carbon nano-inclusions in Cu3SbS4 can produce efficient, non-toxic, and inexpensive state-of-the-art thermoelectric devices.
... 235 However, the importance of coherent effects on the thermal conductivity has been recently questioned based on the experimental results obtained in the 4-300 K range. 225,236 Journal of Applied Physics Recently, Anufriev and Nomura have proposed a concept of ray phononics utilizing ballistic heat transport in porous membranes. 224 Such materials are envisioned for ray-like heat flow management in nanostructures regardless of their surface imperfections and foremost possible at room temperature. ...
Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals
or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible
sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal
processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and
optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs
made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks.
The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In
the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be
transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.
... To obtain a sufficient DT, there were a lot of pronounced reports about the j reduction of TE films by nanostructuring with almost no degradation of high S 2 r, for example, through the fabrication of holes or nanodots in the films using the Si process technique. [19][20][21] Now, thin films with high S 2 r are gaining importance when they are compatible with the Si process technique to use the nanostructuring technique for j reduction. ...
This study presents the material design of Si1−xGex epitaxial films/Si for thin film thermoelectric generators (TFTEGs) by investigating their thermoelectric properties. The thermoelectric films composed of group-IV elements are advantageous due to their compatibility with the Si process. We fabricated Si1−xGex epitaxial films with various controlled x values and strains using various growth methods. Ge epitaxial films without strains exhibited the highest thermoelectric power factor (∼47 μW cm⁻¹ K⁻²) among various strain-controlled Si1−xGex (x ≠ 1) epitaxial films, which is higher at room temperature than SiGe alloy-based bulks ever reported. On the other hand, strained Si1−xGex epitaxial films showed an ultralow thermal conductivity of ∼2 W m⁻¹ K⁻¹, which is close to the value for amorphous Si. In addition to strained SiGe films with the ultralow thermal conductivity, unstrained Ge films with a high thermoelectric power factor can also be used for future TFTEGs by applying a nanostructuring technique. A preliminary TFTEG of Ge epitaxial films was realized, which generated a maximum power of ∼0.10 μW cm⁻² under a temperature difference of 20 K. This demonstrates that epitaxial films composed of group-IV semiconductors are promising materials for TFTEG applications.
... Assuming that L * c = A × L c [21], we can extract the geometrical factor A by fitting the experimentally measured thermal conductivity to the Callaway-Holland model, described in our previous work [35]. Here, we used the relaxation time representing the surface scattering (τ s ) as a fitting parameter. ...
Knowledge of the phonon mean free path (MFP) holds the key to understanding the thermal properties of materials and nanostructures. Although several experiments measured the phonon MFP in bulk silicon, MFP spectra in thin membranes have not been directly measured experimentally yet. In this work, we experimentally probe the phonon MFP spectra in suspended silicon membranes. First, we measure the thermal conductivity of membranes with arrays of slits at different temperatures. Next, we develop a fully analytical procedure to extract the accumulated thermal conductivity as a function of the MFP. The measured phonon MFP in 145-nm-thick membranes with the surface roughness of 0.2 nm is shorter than that in bulk due to the scattering at the membrane boundaries. At room temperature, the phonon MFP does not exceed 400 nm. However, at 4 K, the MFP becomes longer, and some phonons can travel ballistically for up to one micrometer. These results thus shed light on the long-lasting question of the range of ballistic phonon transport at different temperatures in nanostructures based on silicon membranes.
Research towards efficient and environmentally friendly thermoelectrics proposes silicon nanostructures as possible candidates through reduction of the phononic thermal conductivity. However, there is scarce literature about experimental measurements of the thermoelectric figure-of-merit zT on actual crystalline silicon devices. This article reports on the fabrication and full thermoelectric characterization of crystalline 60 nm thick membranes. To that end, an experiment with four types of built-in devices was designed using a silicon-on-insulator substrate to extract the Seebeck coefficient, electrical conductivity and thermal conductivity. The results show indeed a reduced thermal conductivity of 31 W m⁻¹ K⁻¹ for a 60 nm thick Si membrane and κ = 18 W m⁻¹ K⁻¹ for a porous Si membrane. This reflects an 88% reduction in thermal conductivity compared to the bulk Si material and a 42% reduction compared to plain Si membranes. In terms of power generation, the power factor of the fabricated devices surpasses that of state-of-the-art silicon thin films at room temperature. Notably, a zT figure of merit of 0.04 is reported for a 60 nm thick phonon-engineered Si membrane, which is considerably higher than that of bulk Si(0.001) but lower than previously reported results on other types of nano-objects.
Nanostructured semiconductors promise functional thermal management for microelectronics and thermoelectrics through a rich design capability. However, experimental studies on anisotropic in-plane thermal conduction remain limited, despite the demand for directional heat dissipation. Here, inspired by an oriental wave pattern, a periodic network of bent wires, we investigate anisotropic in-plane thermal conduction in nanoscale silicon phononic crystals with the thermally dead volume. We observed the anisotropy reversal of the material thermal conductivity from 1.2 at 300 K to 0.8 at 4 K, with the reversal temperature of 80 K mediated by the transition from a diffusive to a quasi-ballistic regime. Our Monte Carlo simulations revealed that the backflow of the directional phonons induces the anisotropy reversal, showing that the quasi-ballistic phonon transport introduces preferential thermal conduction channels with anomalous temperature dependence. Accordingly, the anisotropy of the effective thermal conductivity varied from 2.7 to 5.0 in the range of 4–300 K, indicating an anisotropic heat manipulation capability. Our findings demonstrate that the design of nanowire networks enables the directional thermal management of electronic devices.
Polycrystalline thin films are widely used for devices and energy-related applications, such as power electronics, solar cells, and thermal management of devices. In many cases, large-scale crystallization during thin-film growth is challenging, so columnar grains are often found in metal and semiconductor thin films. These rough columnar grain boundaries may also have different phonon specularities from that for typically smoother top/bottom film surfaces. A simple analytical model to separately treat these boundaries and interfaces for phonon scattering is currently unavailable, although the in-plane thermal transport is critical to heat spreading within thin-film devices. In this paper, we extend the effective medium formulation from three-dimensional polycrystalline bulk materials to columnar-grained thin films. The model predictions agree well with those given by frequency-dependent phonon Monte Carlo simulations, considering varied phonon specularity at top/bottom film surfaces and grain-boundary phonon transmissivity. The analytical model is further used to analyze the existing data on polycrystalline ZnO thin films with columnar grains.
Rolled‐up nanostructures provide new opportunities to control and modify thermal properties. The topology and geometry of nanoscale architectures have a strong influence on phonon spectrum and thermal conductivity. In the theoretical description, rolled‐up nanotubes are often approximated with nanotubes constructed from concentric multiwalled nanotubes. In this paper, the phonon spectrum is shown not to differ for these two nanotubes if the radius of the nanotube is much larger than the lattice constant. If the contact between layers of the rolled‐up or concentric nanotubes is weak, the thermal conductivity is not influenced by the contact, and it can be approximated by the conductivity of the one‐layered flat membrane. In the limit of weak contact, the diffusive heat transfer is shown to demonstrate an appreciable difference between rolled‐up and concentric nanotubes with large radii. The experimental measurements can be significantly distorted by the effects of the weak interlayer contact. This article is protected by copyright. All rights reserved.
Phononic metamaterials based on the idea of phonon coherent resonance attract increasing research attention because of their unique properties. In this paper, we have studied the heat transport in graphene phononic metamaterials (GPMs, pillared graphene nanoribbons) using molecular dynamics simulations. The results show that, GPMs have lower thermal conductivities in the different in-plane directions and stronger anisotropy in thermal conductance, compared with graphene nanoribbons (GNRs). As the temperature decreases or the height and width of pillar increases, the GPM thermal conductivity in both directions decreases and the anisotropic ratio increases. The pillar-driven suppression of thermal conductivity along the periodic direction is attributed to the phonon localization and the local resonance leading to the flat bands and band gaps, while the reduction in thermal conductivity along the aperiodic direction is due to the phonon boundary scattering. The thermal anisotropy results from the anisotropic phonon relaxation times and the anisotropic contributions of low-frequency-phonons. Our findings reveal the phonon transport mechanisms of GPMs in different directions, and shed some light on the GPM potential for thermoelectric and directional heat management applications.
Defect engineering can effectively modulate the band structure of a thermoelectric (TE) material, thereby enhancing its power factor S²σ. Furthermore, residual stress engineering influences the film performance, especially in the planar technologies. For the TE Mg-doped CuCrO2-based materials, the limitations in achieving an outstanding figure of merit, ZT, arise from their characteristically low charge carrier mobility and high thermal conductivity. Herein, we propose a combination of defect engineering and stress engineering via heavy doping CuCr1-xMgxO2 with x = 0.15 at different deposition temperatures to overcome the aforementioned limitations. Combining the compressive residual stress with multiscale defects (point defects, grain boundaries, and nano-inclusions) significantly reduces the thermal conductivity (κ) to 0.44 W/mK. The σ of the films shows a remarkable enhancement because of point defects introduced via heavy doping. Notably, the compressive-stressed films exhibit higher ZT values, compared to the tensile-stressed films. As a result, an outstanding approximated ZT of 0.66 is observed in the compressive-stressed CuCr0.85Mg0.15O2 films, overcoming the limitations of its ZT value observed for the past two decades.
Thermal transport at the nanoscale level is attracting attention not only because of its physically interesting features such as the peculiar behavior of phonons due to their pronounced ballistic and wave-like properties but also because of its potential applications in alleviating heat dissipation problems in electronic and optical devices and thermoelectric energy harvesting. In the last quarter-century, researchers have elucidated the thermal transport properties of various nanostructured materials, including phononic crystals (PnCs): artificial periodic structures for phonons. PnCs are excellent platforms for investigating thermal transport owing to their well-defined structural parameters. In addition, it is interesting to control thermal transport by interference, as demonstrated in the low-frequency regime with elastic waves and sounds. In this article, we focus on high-frequency phonons and review the thermal transport in semiconductor PnCs. This comprehensive review provides an understanding of recent studies and trends, organized as theoretical and experimental, in terms of the quasiparticle and wave aspects.
Thermoelectricity has been proven as a potential technology for the conversion of waste heat into usable electricity. It involves primarily three parameters, namely the Seebeck coefficient, electrical conductivity, and thermal conductivity. However, there are many other interrelated parameters, such as carrier concentration, mobility, effective mass, multi-valley bands, relaxation time, reduced Fermi energy, phonon modes, scattering parameters, and the number of neighbouring atoms in a given structure. The understanding of these parameters is equally important in order to optimize a high figure of merit (ZT). This article addresses the basics of electronic and thermal transport with the help of Boltzmann transport equation, fundamental concepts for the design of thermoelectric (TE) materials, and implementation of several strategies such as alloying, the phonon-glass electron-crystal (PGEC) approach, band engineering and nanostructuring to optimize the ZT of materials, and finally ends with a discussion of the future prospects of heat extraction through different heat sources.
Based on the semi-continuum model, the effect of temperature on Young’s modulus in the presence of oxide layer in silicon nano-films was studied theoretically by using the anharmonic Keating deformation potential, and the effect of oxide layer on Young’s modulus was also studied. The results show that Young’s modulus of the nano-film is inversely proportional to its temperature, which decreases with the increase of temperature. And with the number of oxide layer increasing, Young’s modulus of silicon nano-film increases. At the same thickness and layer numbers, Young’s modulus of the films with oxide layer is larger than that of pure silicon nano-films. The existence of oxide layer leads to the increase of Young’s modulus of the silicon nano-film.
A Si-based superlattice is one of the promising thermoelectric films for realizing a stand-alone single-chip power supply. Unlike a p-type superlattice (SL) achieving a higher power factor due to strain-induced high hole mobility, in the n-type SL, the strain can degrade the power factor due to lifting conduction band degeneracy. Here, we propose epitaxial Si-rich SiGe/Si SLs with ultrathin Ge segregation interface layers. The ultrathin interface layers are designed to be sufficiently strained, not to give strain to the above Si layers. Therein, a drastic thermal conductivity reduction occurs by larger phonon scattering at the interfaces with the large atomic size difference between Si layers and Ge segregation layers, while unstrained Si layers preserve a high conduction band degeneracy leading to a high Seebeck coefficient. As a result, the n-type Si0.7Ge0.3/Si SL with controlled interfaces achieves a higher power factor of ∼25 μW cm-1 K-2 in the in-plane direction at room temperature, which is superior to ever reported SiGe-based films: SiGe-based SLs and SiGe films. The Si0.7Ge0.3/Si SL with controlled interfaces also exhibits a low thermal conductivity of ∼2.5 W m-1 K-1 in the cross-plane direction, which is ∼5 times lower than the reported value in a conventional Si0.7Ge0.3/Si SL. These results demonstrate that strain and atomic differences controlled by ultrathin layers can bring a breakthrough for realizing high-performance light-element-based thermoelectric films.
We measure the thermal conductivity of silicon phononic crystals with asymmetric holes at room and liquid helium temperatures and study the effect of thermal rectification, phonon boundary scattering, neck transmission, and hole positioning. Also, we compare the influence of asymmetric holes on thermal conductivity reduction with the one of conventional circular holes. This reduction is almost 40% larger in the case of pacman shaped holes as compared with circular ones for the same parameters of phononic crystals. Our experimental results can be used to significantly improve the efficiency of thermoelectric devices by using pacman-shaped holes in phononic crystals.
Phononic crystals have been studied for the past decades as a tool to control the propagation of acoustic and mechanical waves. Recently, researchers proposed that nanosized phononic crystals can also control heat conduction and improve the thermoelectric efficiency of silicon by phonon dispersion engineering. In this review, we focus on recent theoretical and experimental advances in phonon and thermal transport engineering using pillar-based phononic crystals. First, we explain the principles of the phonon dispersion engineering and summarize early proof-of-concept experiments. Next, we review recent simulations of thermal transport in pillar-based phononic crystals and seek to uncover the origin of the observed reduction in the thermal conductivity. Finally, we discuss first experimental attempts to observe the predicted thermal conductivity reduction and suggest the directions for future research.
The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high electrical conductivity. We have designed two different nanostructures for this purpose. One structure is connected Si nanodots (NDs) with the same crystal orientation. The phonons scattering at the interfaces of these NDs occurred and it depended on the ND size. As a result of phonon scattering, the thermal conductivity of this nanostructured material was below/close to the amorphous limit. The other structure is Si films containing epitaxially grown Ge NDs. The Si layer imparted high electrical conductivity, while the Ge NDs served as phonon scattering bodies reducing thermal conductivity drastically. This work gives a methodology for the independent control of electron and phonon transport using nanostructured materials. This can bring the realization of thermoelectric Si-based materials that are compatible with large scale integrated circuit processing technologies.
We present experimental and theoretical investigations on the roles of the limiting dimensions, such as the smallest dimension, surface roughness, and density of holes in the reduction of thermal conductivity of one-dimensional phononic nanostructures at temperatures of 4 and 295 K. We dis- cover that the thermal conductivity does not strongly depend on the period of the phononic crystal nanostructures whereas the surface roughness and the smallest dimension of the structure—the neck—play the most important roles in thermal conductivity reduction. Surface roughness is a very important structural parameter in nanostructures with a characteristic length less than 100nm in silicon. The importance of the roughness increases as the neck size decreases, and the thermal conductivity of the structure can differ by a factor of four, reaching the thermal conductivity of a small nanowire. The experimental data are analyzed using the Callaway–Holland model of Boltzmann equation and Monte Carlo simulation providing deeper insight into the thermal phonon transport in phononic nanostructures.
Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.
Wave effects of phonons can give rise to controllability of heat conduction in nanostructures beyond that by particle scattering at surfaces and interfaces. In this paper, we propose a new class of three-dimensional nanostructures: a silicon-nanowire-cage (SiNWC) structure consisting of silicon nanowires (SiNWs) connected by nano-cross-junctions. We perform equilibrium molecular dynamics simulations and find an ultralow value of thermal conductivity of SiNWC, 0.173Wm−1K−1, which is one order lower than that of SiNWs. By further modal analysis and atomistic Green's function calculations, we identify that the large reduction is due to significant phonon localization induced by the phonon local resonance and hybridization at the junction part in a wide range of phonon modes. This localization effect does not require the cage to be periodic, unlike the phononic crystals, and can be realized in structures that are easier to synthesize, for instance in a form of randomly oriented SiNW network.
It has been proposed for a long time now that the reduction of the thermal conductivity by reducing the phonon mean free path is one of the best way to improve the current performance of thermoelectrics. By measuring the thermal conductance and thermal conductivity of nanowires and thin films, we show different ways of increasing the phonon scattering from low-temperature up to room-temperature experiments. It is shown that playing with the geometry (constriction, periodic structures, nano-inclusions), from the ballistic to the diffusive limit, the phonon thermal transport can be severely altered in single crystalline semiconducting structures; the phonon mean free path is in consequence reduced. The diverse implications on thermoelectric properties will be eventually discussed.
Electrical and thermal properties of polycrystalline Si thin films with two-dimensional phononic patterning were investigated at room temperature. Electrical and thermal conductivities for the phononic crystal nanostructures with a variety of radii of the circular holes were measured to systematically investigate the impact of the nanopatterning. The concept of phonon-glass and electron-crystal is valid in the investigated electron and phonon transport systems with the neck size of 80 nm. The thermal conductivity is more sensitive than the electrical conductivity to the nanopatterning due to the longer mean free path of the thermal phonons than that of the charge carriers. The values of the figure of merit ZT were 0.065 and 0.035, and the enhancement factors were 2 and 4 for the p-doped and n-doped phononic crystals compared to the unpatterned thin films, respectively, when the characteristic size of the phononic crystal nanostructure is below 100 nm. The greater enhancement factor of ZT for the n-doped sample seems to result from the strong phonon scattering by heavy phosphorus atoms at the grain boundaries.
The ultralow thermal conductivity, κ, observed experimentally in intentionally roughened silicon nanowires (SiNWs) is reproduced in phonon Monte Carlo simulations with exponentially correlated real-space rough surfaces similar to measurement [J. Lim et al., Nano Lett. 12, 2475 (2012)]. Universal features of thermal transport are revealed by presenting κ as a function of the normalized geometric mean free path λ ¯ ( 0 < λ ¯ < 1 ) ; the diffusive (Casimir) limit corresponds to λ ¯ = 1 / 2 . κ vs λ ¯ is exponential at low-to-moderate roughness (high λ ¯ ), where internal scattering randomly interrupts phonon bouncing across the SiNW, and linear at high roughness (low λ ¯ ), where multiple scattering events at the same surface results in ultralow, amorphous-limit thermal conductivity.
Black Silicon nanostructures are fabricated by Inductively Coupled Plasma Reactive Ion Etching
(ICP-RIE) in a gas mixture of SF6and O2at non-cryogenic temperatures. The structure evolution
and the dependency of final structure geometry on the main processing parameters gas composition
and working pressure are investigated and explained comprehensively. The optical properties of
the produced Black Silicon structures, a distinct antireflection and light trapping effect, are resolved
by optical spectroscopy and conclusively illustrated by optical simulations of accurate models of
the real nanostructures. By that the structure sidewall roughness is found to be critical for an
elevated reflectance of Black Silicon resulting from non-optimized etching processes. By analysis
of a multitude of structures fabricated under different conditions, approximate limits for the range
of feasible nanostructure geometries are derived. Finally, the technological applicability of Black
Silicon fabrication by ICP-RIE is discussed.
The mechanism of the reduced thermal conductivity of fishbone-type phononic crystal (PnC) nanostructures, in which ballistic phonon transport is dominant, was investigated with consideration of both the wave and particle nature of phonons. Phononic band diagrams were calculated for an Si nanowire and a fishbone-type PnC structure with a period of 100 nm, and a clear reduction of the group velocity of phonons, because of a zone-folding effect, was shown. Air-suspended Si nanowires and fishbone-type PnC structures were fabricated by electron beam (EB) lithography, and their thermal conductivities were mea-sured by use of the originally developed micro time-domain thermoreflectance method. The PnC structure had a much lower thermal conductivity. We measured the thermal conductivity of a variety of PnC structures with dif-ferent fin widths to investigate the mechanism of the reduced thermal con-ductivity observed. The result indicates that the increase of the phonon traveling distance. as a result of the fins, also results in reduced thermal conductivity.
Knowledge of the mean free path distribution of heat-carrying phonons is key
to understanding phonon-mediated thermal transport. We demonstrate that thermal
conductivity measurements of thin membranes spanning a wide thickness range can
be used to characterize how bulk thermal conductivity is distributed over
phonon mean free paths. A non-contact transient thermal grating technique was
used to measure the thermal conductivity of suspended Si membranes ranging from
15 to 1500 nm in thickness. A decrease in the thermal conductivity from 74% to
13% of the bulk value is observed over this thickness range, which is
attributed to diffuse phonon boundary scattering. Due to the well-defined
relation between the membrane thickness and phonon mean free path suppression,
combined with the range and accuracy of the measurements, we can reconstruct
the bulk thermal conductivity accumulation vs. phonon mean free path, and
compare with theoretical models.
Black silicon (BSi) represents a very active research area in renewable energy materials. The rise of BSi as a focus of study for its fundamental properties and potentially lucrative practical applications is shown by several recent results ranging from solar cells and light-emitting devices to antibacterial coatings and gas-sensors. In this paper, the common BSi fabrication techniques are first reviewed, including electrochemical HF etching, stain etching, metal-assisted chemical etching, reactive ion etching, laser irradiation and the molten salt Fray-Farthing-Chen-Cambridge (FFC-Cambridge) process. The utilization of BSi as an anti-reflection coating in solar cells is then critically examined and appraised, based upon strategies towards higher efficiency renewable solar energy modules. Methods of incorporating BSi in advanced solar cell architectures and the production of ultra-thin and flexible BSi wafers are also surveyed. Particular attention is given to routes leading to passivated BSi surfaces, which are essential for improving the electrical properties of any devices incorporating BSi, with a special focus on atomic layer deposition of Al 2 O 3 . Finally, three potential research directions worth exploring for practical solar cell applications are highlighted, namely, encapsulation effects, the development of micro-nano dual-scale BSi, and the incorporation of BSi into thin solar cells. It is intended that this paper will serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in research currently being undertaken for renewable energy applications.
Thermal conduction in periodically porous nanostructures is strongly influenced by phonon boundary scattering, although the precise magnitude of this effect remains open to investigation. This work attempts to clarify the impact of phonon-boundary scattering at room temperature using electrothermal measurements and modeling. Silicon nanobeams, prepared using electron beam lithography, were coated with a thin palladium overlayer, which serves as both a heater and thermometer for the measurement. The thermal conductivity along the length of the silicon nanobeams was measured using a steady-state Joule heating technique. The thermal conductivities of the porous nanobeams were reduced to as low as 3% of the value for bulk silicon. A Callaway-Holland model for the thermal conductivity was adapted to investigate the relative impact of boundary scattering, pore scattering, and phonon bandgap effects. Both the experimental data and the modeling showed a reduction in thermal conductivity with increasing pore diameter, although the experimentally measured value was up to an order of magnitude lower than that predicted by the model.
We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.
We investigate thermal transport in Si/Ge and Si 1−x Ge x /Si 1−y Ge y alloy superlattices based on solving the single-mode phonon Boltzmann transport equation in the relaxation-time approximation and with full phonon dispersions. We derive an effective interface scattering rate that depends both on the interface roughness (captured by a wave-vector-dependent specularity parameter) and on the efficiency of internal scattering mechanisms (mass-difference and phonon-phonon scattering). We provide compact expressions for the calculations of in-plane and cross-plane thermal conductivities in superlattices. Our numerical results accurately capture both the observed increase in thermal conductivity as the superlattice period increases and the in-plane vs cross-plane anisotropy of thermal conductivity. Owing to the combined effect of interface and internal scattering, an alloy/alloy superlattice has a lower thermal conductivity than bulk SiGe with the same alloy composition. Thermal conductivity can be minimized by growing short-period alloy/alloy superlattices or Si/Si 1−x Ge x superlattices with the SiGe layer thicker than the Si one.
Self heating diminishes the reliability of silicon-on-insulator (SOI) transistors, particularly those that must withstand electrostatic discharge (ESD) pulses. This problem is alleviated by lateral thermal conduction in the silicon device layer, whose thermal conductivity is not known. The present work develops a technique for measuring this property and provides data for layers in wafers fabricated using bond-and-etch-back (BESOI) technology. The room-temperature thermal conductivity data decrease with decreasing layer thickness, ds, to a value nearly 40 percent less than that of bulk silicon for ds = 0.42 μm. The agreement of the data with the predictions of phonon transport analysis between 20 and 300 K strongly indicates that phonon scattering on layer boundaries is responsible for a large part of the reduction. The reduction is also due in part to concentrations of imperfections larger than those in bulk samples. The data show that the buried oxide in BESOI wafers has a thermal conductivity that is nearly equal to that of bulk fused quartz. The present work will lead to more accurate thermal simulations of SOI transistors and cantilever MEMS structures.
Non-metallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events-their mean free paths. Due to the breadth of the phonon mean free path spectrum, nanostructuring materials can reduce thermal conductivity from bulk by scattering long mean free path phonons, whereas short mean free path phonons are unaffected. Here we use a breakdown in diffusive phonon transport generated by high-frequency surface temperature modulation to identify the mean free path-dependent contributions of phonons to thermal conductivity in crystalline and amorphous silicon. Our measurements probe a broad range of mean free paths in crystalline silicon spanning 0.3-8.0 μm at a temperature of 311 K and show that 40±5% of its thermal conductivity comes from phonons with mean free path >1 μm. In a 500 nm thick amorphous silicon film, despite atomic disorder, we identify propagating phonon-like modes that contribute >35±7% to thermal conductivity at a temperature of 306 K.
The thermal conductance of straight and corrugated monocrystalline silicon
nanowires has been measured between 0.3 K and 5 K. The difference in the
thermal transport between corrugated nanowires and straight ones demonstrates a
strong reduction in the mean free path of the phonons. This averaged mean free
path is remarkably smaller than the smaller diameter of the nanowire,
evidencing a phonon thermal transport reduced below the Casimir limit. Monte
Carlo simulations highlight that this effect can be attributed to significant
multiple scattering of ballistic phonons occuring on the corrugated surfaces.
This result suggests an original approach to transforming a monocrystalline
material into a phonon glass.
The effect of surface roughness on phonon transport in a nanowire has often been described by treating the surface as flat with a specularity parameter (p) in the range between 0 and 1. A lower thermal conductivity limit is approached at p=0 for diffuse surface. It is demonstrated here by Monte Carlo simulation that sawtooth roughness on a nanowire can cause phonon backscattering and suppress the thermal conductivity below the diffuse surface limit. The backscattering effect can be accounted for only by a negative p if the detail of the surface roughness is ignored.
The mechanism for phonon scattering by nanostructures and by point defects in nanostructured silicon (Si) and the silicon germanium (Ge) alloy and their thermoelectric properties are investigated. We found that the thermal conductivity is reduced by a factor of 10 in nanostructured Si in comparison with bulk crystalline Si. However, nanosize interfaces are not as effective as point defects in scattering phonons with wavelengths shorter than 1 nm. We further found that a 5 at. % Ge replacing Si is very efficient in scattering phonons shorter than 1 nm, resulting in a further thermal conductivity reduction by a factor of 2, thereby leading to a thermoelectric figure of merit 0.95 for Si95Ge5, similar to that of large grained Si80Ge20 alloys. National Science Foundation Department of Energy
Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
We propose a simple, low-cost, and large-area method to increase the thermoelectric figure of merit (ZT) in silicon membranes by the deposition of an ultrathin aluminum layer. Transmission electron microscopy showed that short deposition of aluminum on a silicon substrate covers the surface with an ultrathin amorphous film, which, according to recent theoretical works, efficiently destroys phonon wave packets. As a result, we measured 30-40% lower thermal conductivity in silicon membranes covered with aluminum films while the electrical conductivity was not affected. Thus, we have achieved 40-45% higher ZT values in membranes covered with aluminum films. To demonstrate a practical application, we applied this method to enhance the performance of a silicon membrane-based thermoelectric device and measured 42% higher power generation.
Understanding phonon transport mechanisms in nanostructures is of great importance for delicately tailoring thermal properties. Combining phonon particle and wave effects through different strategies, previous studies have obtained ultra-low or super-high thermal conductivity in different nanostructures. However, phonon particle and wave effects are coupled together, and thus, their individual contributions to phonon transport have not been figured out. In this article, we present how to quantify the particle and wave effects on phonon transport by combining Monte Carlo and atomic Green's function methods. We apply it to one-dimensional silicon nanophononic metamaterial with cross junctions, where it has been thought that the wave effect was the main modulator to block phonon transport and the particle effect was negligibly weak. Surprisingly, we find that the particle effect is quite significant as well and can contribute as much as 39% to the total thermal conductivity reduction. Further phonon transmission analysis by reducing the junction leg length also qualitatively demonstrates the strong particle effect. The results highlight the importance of mutually controlling particle and wave characteristics.
Thermoelectric modules based on silicon nanowires (Si-NWs) have recently attracted significant attention as they show improved thermoelectric efficiency due to a decrease in thermal conductivity. Here, we adopt a top-down fabrication method to dramatically reduce the thermal conductivity of vertical Si-NWs. The thermal conductivity of vertical Si-NW is significantly suppressed with increasing surface roughness, decreasing diameter, and increasing doping concentration. This large suppression is caused by enhanced phonon scattering, which depends on phonon wavelength. The boron and phosphorus doped rough Si-NWs with diameter of 200 nm and surface roughness of 6.88 nm show the lowest thermal conductivity of 10.1 and 14.8 W∙m-1∙K-1, respectively, which are 3.4 and 2.4 fold lower than that of smooth intrinsic nanowire and 14.8 and 10.1 fold lower than that of bulk silicon. A thermoelectric module was fabricated using these doped rough Si-NW array and its thermoelectric performance is compared with previously reported Si-NW modules. The fabricated module exhibits excellent performance with an open circuit voltage of 216.8 mV·cm-2 and a maximum power of 3.74 uW·cm-2 under a temperature difference of 180 K, the highest reported for Si-NW thermoelectric modules.
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.
Phonon engineering is expected to contribute to further development of various fields and technologies such as electronics, photonics, thermal engineering, and materials science. Although phonons inherently exist in condensed matter, their behavior strongly depends on the scale of the system and the materials, and they play a major role in electrical, optical, thermal, and mechanical properties. Therefore, researchers have been attempting to find effective ways to control phonons to modify the material properties — in this regard, nanostructuring was found to be highly effective. Here, we review recent advances in the simulations and experiments on phonon transport and thermal conduction control in nanostructures. We mainly focus on tailored nanostructures, especially phononic crystals of which the design is based on the nanoscale phonon transport property.
In search of efficient thermoelectric nanostructures, many theoretical works predicted that nanopillars, placed on the surface of silicon membranes, nanobeams or nanowires, can reduce the thermal conductivity of these nanostructures. To verify these predictions, we experimentally investigate heat conduction in suspended silicon nanobeams with arrays of aluminium nanopillars. Our room temperature time-domain thermoreflectance experiments show that the nanobeams with nanopillars have 20% lower thermal conductivity as compared to pristine nanobeams. We discuss possible explanations of these data, including coherent effects, and conclude that the thermal conductivity is reduced mainly by phonon scattering at the pillar/beam interface due to the intermixing of aluminium and silicon atoms, as supported by the transmission electron microscopy. As this intermixing does not only reduce thermal conductivity but may also increase electrical conductivity, these nanostructures are exceptionally promising for thermoelectric applications.
The in-plane thermal conductivity of silicon phononic membranes is investigated by micro time domain thermoreflectance and Monte Carlo simulations. Strong reduction of thermal conductivity is observed mainly due to phonon boundary scattering for both aligned and staggered lattices of holes. The measured and calculated thermal conductivities of the porous membranes with cylindrical holes are found to be in good quantitative agreement (at 4 K and 300 K). A significant difference between thermal conductivities of aligned and staggered lattice of identical porosities is observed. This difference is shown to arise from ballistic phonons that acquired directionality by propagating between the holes. The directionality effect strengthens when the temperature is decreased or when the diameter of the holes becomes close to the period. Finally, we propose a model, which quantifies and explains the difference between thermal conductivities of aligned and staggered lattices based on geometric considerations.
Phonon-surface scattering is the fundamental mechanism behind thermal transport phenomena at the nanoscale. Despite its significance, typical approaches to describe the interaction of phonons with surfaces do not consider all relevant physical quantities involved in the phonon-surface interaction, namely, phonon momentum, incident angle, surface roughness, and correlation length. Here, we predict thermal conduction properties of thin films by considering an accurate description of phonon-surface scattering effects based on the rigorous Beckmann-Kirchhoff scattering theory extended with surface shadowing. We utilize a Boltzmann transport based reduced mean-free-path model for phonon transport in thin-films to predict the wavelength and mean-free-path heat spectra in Si and SiGe films for different surface conditions and show how the thermal energy distribution can be tailored by the surface properties. Using the predicted wavelength spectra, we also introduce a measure to quantify phonon-confinement effects and show an enhanced confinement in Ge alloyed Si thin films. The impact of surface roughness and correlation lengths on thermal conductivities is also studied, and our numerical predictions show excellent agreement with experimental measurements. The results allow to elucidate and quantitatively predict the amount of thermal energy carried by different phonons at the nanoscale, which can be used to design improved optoelectronic and thermoelectric devices.
Block copolymer patterned holey silicon (HS) was successfully integrated into a microdevice for simultaneous measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity of the same HS microribbon. These fully integrated HS microdevices provided excellent platforms for the systematic investigation of thermoelectric transport properties tailored by the dimensions of the periodic hole array, that is, neck and pitch size, and the doping concentrations. Specifically, thermoelectric transport properties of HS with a neck size in the range of 16-34 nm and a fixed pitch size of 60 nm were characterized, and a clear neck size dependency was shown in the doping range of 3.1 × 10(18) to 6.5 × 10(19) cm(-3). At 300 K, thermal conductivity as low as 1.8 ± 0.2 W/mK was found in HS with a neck size of 16 nm, while optimized zT values were shown in HS with a neck size of 24 nm. The controllable effects of holey array dimensions and doping concentrations on HS thermoelectric performance could aid in improving the understanding of the phonon scattering process in a holey structure and also in facilitating the development of silicon-based thermoelectric devices.
Black silicon
plasma technology begins to be integrated into the process flow of silicon solar cells. However, most of the current technology is used at cryogenic or very low substrate temperatures. Here, the authors investigate the temperature-dependent
properties of black silicon prepared by two different plasma etching techniques for black silicon, a pure capacitively coupled process (CCP), and an inductively and capacitively coupled process (ICP + CCP). It turns out that the ICP + CCP process at room-temperature yields black silicon samples with 93% absorption and minority carrier lifetime above 1 ms. The authors show that these optoelectronic properties are comparable to samples obtained at low temperatures.
In this review, we discuss some of the representative strategies of phonon engineering by categorizing them into the methods affecting each component of phonon thermal conductivity, i.e., specific heat, phonon group velocity, and mean free path. In terms of specific heat, a large unit cell is beneficial in that it can minimize the fraction of thermal energy that can be transported since most of the energy is stored in the optical branches. In an artificial structure such as the superlattice, phonon bandgaps can be created through constructive interference by Bragg reflection, which reduces phonon group velocity. We further categorize the mean free path, i.e., scattering processes, into grain boundary scattering, impurity scattering, and phonon–phonon scattering. Rough-surfaced grains, nano-sized grains, and coated grains are discussed for enhancement of the grain boundary scattering. Alloy atoms, vacancies, nanoparticles, and nano-sized holes are treated as impurities, which limit the phonon mean free path. Lone pair electrons and acoustical-to-optical scattering are suggested for manipulating phonon–phonon scattering. We also briefly mention the limitation and temperature range in which the Wiedemann–Franz law is valid in order to achieve a better estimation of electronic thermal conductivity. This paper provides an organized view of phonon engineering so that this concept can be implemented synergistically with power factor enhancement approaches for design of thermoelectric materials.
Responding to the need for thermoelectric materials with high efficiency in both conversion and cost, we developed a nanostructured bulk silicon thermoelectric materials by sintering silicon crystal quantum dots of several nanometers in diameters synthesized by plasma-enhanced chemical vapor deposition (PECVD). The material consists of hybrid structures of nano-grains of crystalline silicon and amorphous silicon oxide. The percolated nanocrystalline region gives rise to high power factor with the high doping concentration realized by PECVD, and the binding amorphous region reduces thermal conductivity. Consequently, the non-dimensional figure of merit reaches 0.39 at 600°C, equivalent to the best reported value for silicon thermoelectrics. The thermal conductivity of the densely-packed material is as low as 5 Wm-1K-1 in a wide temperature range from room temperature to 1000°C, which is beneficial not only for the conversion efficiency but also for material cost by requiring less material to establish certain temperature gradient.
The thermoelectric properties of n type nanoscale three dimensional (3D) Si
phononic crystals (PnCs) with spherical pores are studied. Density functional
theory and Boltzmann transport equation under the relaxation time approximation
are applied to study the electronic transport coefficients, electrical
conductivity, Seebeck coefficient and electronic thermal conductivity. We found
that the electronic transport coefficients in 3D Si PnC at room temperature
(300 K) change very little compared with that of Si, for example, electrical
conductivity and electronic thermal conductivity is decreased by 0.26 to 0.41
and 0.39 to 0.55 depending on carrier concentration, respectively, and the
Seebeck coefficient is similar to that of bulk Si. However, the lattice thermal
conductivity of 3D Si PnCs with spherical pores is decreased by a factor of 500
calculated by molecular dynamics methods, leading to the ZT of 0.76, which is
about 30 times of that of porous Si. This work indicates that 3D Si PnC is a
promising candidate for high efficiency thermoelectric materials.
We report the development of a Si-based micro thermogenerator build from silicon-on-insulator by using standard CMOS processing. Ultrathin single-crystalline Si membranes, 100 nm in thickness, with embedded n and p-type doped regions electrically connected in series and thermally in parallel, are the active elements of the thermoelectric device that generate the thermopower under various thermal gradients. This proof-of-concept device produces an output power density of 4.5 µW/cm2, under a temperature difference of 5 K, opening the way to envisage integration as wearable thermoelectrics for body energy scavenging.
A phenomenological model is developed to facilitate calculation of lattice thermal conductivities at low temperatures. It is assumed that the phonon scattering processes can be represented by frequency-dependent relaxation times. Isotropy and absence of dispersion in the crystal vibration spectrum are assumed. No distinction is made between longitudinal and transverse phonons. The assumed scattering mechanisms are (1) point impurities (isotopes), (2) normal three-phonon processes, (3) umklapp processes, and (4) boundary scattering. A special investigation is made of the role of the normal processes which conserve the total crystal momentum and a formula is derived from the Boltzmann equation which gives their contribution to the conductivity. The relaxation time for the normal three-phonon processes is taken to be that calculated by Herring for longitudinal modes in cubic materials. The model predicts for germanium a thermal conductivity roughly proportional to T-3/2 in normal material, but proportional to T-2 in single-isotope material in the temperature range 50°-100°K. Magnitudes of the relaxation times are estimated from the experimental data. The thermal conductivity of germanium is calculated by numerical integration for the temperature range 2-100°K. The results are in reasonably good agreement with the experimental results for normal and for single-isotope material.
An analysis of thermal conductivity is presented which differs from that of Klemens and of Callaway in that it considers explicitly the conduction by both transverse and longitudinal phonons. This approach is then used to provide a very good fit to the data on silicon from 1.7 to 1300°K and on germanium from 1.7 to 1000°K, and is also used to fit the data on isotropically pure germanium. A comparison of the analysis with that due to Callaway shows that the same results are obtained in the impurity scattering and boundary scattering regions. A discussion of the approximations used in the various analyses is included. A more complete expression for the umklapp scattering relaxation time, valid for materials with a very disperse transverse acoustic phonon spectrum, is derived in an appendix. The question of the validity of the addition of inverse relaxation times and the coupling due to normal three-phonon processes is considered in another appendix.
Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si(80)Ge(20))(70)(Si(100)B(5))(30) by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si(70)Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.
Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.
This work investigated the thermoelectric properties of thin silicon membranes that have been decorated with high density of nanoscopic holes. These "holey silicon" (HS) structures were fabricated by either nanosphere or block-copolymer lithography, both of which are scalable for practical device application. By reducing the pitch of the hexagonal holey pattern down to 55 nm with 35% porosity, the thermal conductivity of HS is consistently reduced by 2 orders of magnitude and approaches the amorphous limit. With a ZT value of ∼0.4 at room temperature, the thermoelectric performance of HS is comparable with the best value recorded in silicon nanowire system.
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