Thermal Conductivity Of Silicon Research Articles

Thermal Simulation for Two-Phase Liquid Cooling 3D-ICs

This work presents an algorithm for simulating more accurate temperature distribution in two-phase liquid cooling for three-dimensional integrated circuits than the state of-the-art methods by utilizing local multi-linear interpolation techniques on heat transfer coefficients between the microchannel and silicon substrate, and considering the interdependence between the thermal conductivity of silicon and temperature values. The experimental results show that the maximum and average errors are only 9.7% and 6.7% compared with the measurements, respectively.

Microscale temperature sensing using novel reliable silicon vertical microprobe array: Computation and experiment

Microscale temperature sensing using a fabricated novel vertical silicon microprobe array was investigated for the first time using a combined simulation and experimental technique. In this context, silicon microprobes were designed with 3μm in diameter and 30μm in height. These high-aspect-ratio probes are found to be effective in microscale multisite temperature sensing. The designed microprobes (p-type) were fabricated in <111> out-of-plane orientation using vapor–liquid-solid (VLS) technique on n-type silicon substrate. The temperature dependent shift in the rectifying current–voltage (I–V) curves of the embedded p–n diode was experimentally determined and the temperature sensitivity of diode was found to be −2.3mV/K at 0.1μA. A complete 3-D model of the microprobe was created and finite element (FE) method was applied to compute the sensing capability by capturing temperature distribution taking anisotropic and phonon scattering effects on thermal conductivity of silicon into account. The obtained computational result on microscale temperature sensitivity has shown a similar trend of experimental findings. FE simulation thus can serve as a tool to a-priori predict temperature sensing capabilities of microprobes used in many applications such as artificial electronic fingertips of robotic hand/prosthetics, biological soft samples.

The effect of phonon anisotropic scattering on the thermal conductivity of silicon thin films at 300K and 400K

Previous studies have shown that anisotropy in phonon transport exist because of the difference in phonon dispersion relation due to different lattice directions, as observed by a difference in in-plane and cross-plane thermal conductivities. Our current work intends to study the effect of anisotropy scattering on silicon thermal conductivity at 300K and 400K. We adopt the Henyey and Greenstein probability density function in our phonon Monte Carlo simulation to investigate the effect of highly forward and backward scattering events. The impact of applying the anisotropy scattering using this approach is discussed in detail. While the forward and backward scattering will increase and decrease thermal conductivity respectively, the extent of the effect is non-linear such that forward scattering has a more obvious effect on thermal conductivity than backward scattering.

Thermal Diffusivity of SPS Pressed Silicon Powders and the Potential for Using Bottom–Up Silicon Quantum Dots as a Starting Material

The production of nanostructured bulk materials from silicon powders has been well documented as being one way of bringing down the thermal conductivity of silicon while still maintaining its high power factor. This reduction of thermal conductivity is predicted to lead to significant increases in its figure-of-merit, ZT. The size of the starting particles has a major effect on the nanostructuring and grain size of the final silicon-based materials. Using particles of differing size and distribution, pellets were produced using spark plasma sintering. The results show a significant lowering in the thermal diffusivity as the particle size in the powders is decreased. As the starting particle size deceases from 1 μm to 60 nm, we see a tenfold decrease in the thermal diffusivity at 300 K, from 20 mm2 S−1 to 2 mm2 S−1. Both these show a significant decrease from the thermal diffusivity of 88 mm2 S−1 observed from bulk silicon. A further decrease to 1 mm2 S−1 is observed when the particle size of the starting material is decreased from 60 nm to sub-10 nm. The results also highlight the potential of using particles from solution approaches as a potential starting point for the prediction of nanostructured bulk materials.

Origins of thermal conductivity changes in strained crystals

The strain-dependent phonon properties and thermal conductivities of a soft system [Lennard-Jones (LJ) argon] and a stiff system (silicon modeled using first-principles calculations) are predicted using lattice dynamics calculations and the Boltzmann transport equation. As is commonly assumed for materials under isotropic strain, the thermal conductivity of LJ argon decreases monotonically as the system moves from compression into tension. The reduction in thermal conductivity is attributed to decreases in both the phonon lifetimes and group velocities. The thermal conductivity of silicon, however, is constant in compression and only begins to decrease once the system is put in tension. The silicon lifetimes show an anomalous behavior, whereby they increase as the system moves from compression into tension, which is explained by examining the potential energy surface felt by an atom. The results emphasize the need to separately consider the harmonic and anharmonic effects of strain on material stiffness, phonon properties, and thermal conductivity.

EMRS Spring Meeting 2014 Symposium D: Phonons and fluctuations in low dimensional structures

The E-MRS 2014 Spring meeting, held from 26-30th May 2014 in Lille included the Symposium D entitled ‘‘Phonons and Fluctuations in Low Dimensional Structures’’, the first edition of its kind. The symposium was organised in response to the increasing interest in the study of phonons in the context of advances in condensed matter physics, electronics, experimental methods and theory and, in particular, the transfer of energy across atomic interfaces and the propagation of energy in the nm-scale. Steering heat by light or vice versa and examining nano-scale energy conversion (as in thermoelectricity and harvesting e.g. in biological systems) are two aspects that share the underlying science of energy processes across atomic interfaces and energy propagation in the nanoscale and or in confined systems. The nanometer scale defies several of the bulk relationships as confinement of electrons and phonons, locality and non-equilibrium become increasingly important. The propagation of phonons as energy carriers impacts not only heat transfer, but also the very concept and handling of temperature in non-equilibrium and highly localised conditions. Much of the needed progress depends on the materials studied and this symposium targeted the interface material aspects as well as the emerging concepts to advance in this field.The symposium had its origins in a series of meetings and seminars including: (1) the first Phonon Engineering Workshop, funded by Catalan Institute for Research and Advanced Studies (ICREA), the then MICINN, the CNRS, VTT, and several EU projects, held in Saint Feliu de Guixols (Girona, Spain) from 24th to 27th of May 2010 with 65 participants from Europe, the USA and Japan; (2) the first Phonons and Fluctuations workshop, held in Paris on 8th and 9th November 2010, supported by French, Spanish and Finnish national projects and EU projects, attended by about 50 researchers; (3) the second Phonon and Fluctuations workshop, held in Paris on 8th and 9th September 2011, attended by 55 researchers and (4) the 3rd Workshop on Phonons & Fluctuations, held in Saint Feliux de Guixols (Girona, Spain) during 21 to 24th May 2012 attended by 65 participants from Europe and the USA.These papers in this proceeding are examples of the work presented at the symposium. They represent the tip of the iceberg, as the symposium attracted over 100 abstracts. The meeting room was usually full with an audience varying between 40 and 100 participants. The plenary presentation was given by Prof. Gang Chen (MIT) on ‘‘Ballistic and Coherent Phonon Heat Conduction in Bulk Materials and Nanostructures’’, which was warmly welcome by an eager and highly motivated audience. The invited speakers were: Prof. Thomas Dehoux (U. Bordeaux), Dr S. Chung (U. New South Wales, Australia), Prof. A. Goni (CSIC-ICMAB), Prof. Giuliano Benenti (U. Insubria), Dr. Davide Donadio (Max Planck Institute for Polymer Research, Mainz), Prof. George Fytas (University of Crete), Prof. Dr. Tobias Kippenberg (EPFL, Switzerland), Prof. Bernard Perrin (INSP, Paris), Prof. Gyaneshwar P. Srivastava, U. Exeter) and Prof. Dr. Achim Kittel (U. Oldenburg). The organisers are very grateful to them for supporting the symposium and sharing their latest research results with the symposium participants.The symposium organisers recognised the participation of students and awarded prizes to the two Best Student Presentations, which went to Valeria Lacatena (IEMN, Lille) with an invited presentation entitled ‘‘Efficient reduction of thermal conductivity in silicon using phononic-engineered membranes’’ and to Yan Qing Liu (Institute Neel, Grenoble) who presented the talk entitled ‘‘Sensitive 3-omega measurements of epitaxial thermoelectric thin films’’. The poster session had about fifty posters and the four best poster prizes went to: Konstanze Hahn et al. (U. Cagliari) poster title ‘‘Determination of Thermal conductivity in (nanostructured) SiGe materials’’, Florian Doehring et al. (U. Goettingen) poster title ‘‘Phonon blocking in Multilayers produced by Pulsed Laser Deposition’’, Jordi Gomis-Bresco et al. (ICN2) poster ‘‘ A 1D PhoXonic Crystal’’, Barcelona and Benjamin J Robinson et al. (U. Lancaster) poster ‘‘Scanning Thermal microscopy studies of 2D materials’’.The symposium organisers are grateful to the Scientific Committee members, Prof. Bahram Djafari-Rouhani (France), Prof. Dr. Thomas Dekorsy (Germany), Prof. Anthony Kent (UK), Prof. Fabio Marchesoni (Italy), Dr. Natalio Mingo (France), Prof. Pascal Ruello, (France) and Prof. Javier Viejo-Rodriguez (Spain), for their help with all aspects of evaluation of the scientific level of the presentations in the symposium.The symposium was sponsored by the FP7 ICT FET Open Coordination Action EUPHONON (GA. 612086) and by the CNRS GDR ‘‘Thermal Nanosciences and NanoEngineering’’. The symposium organisers express their sincere thanks to the staff of the E-MRS for continuous support and timely advice in all organisational aspects. We are indebted to Dr. Erwan Guillotel (ICN2) for his assistance with the organisation of the symposium.

InP HBT Thermal Management by Transferring to High Thermal Conductivity Silicon Substrate

We report about the self-heating management of an InP double heterojunction bipolar transistor (DHBT) by the way of the thermal resistance. In order to reduce this latter, an AlInP/GaAsSb DHBT has been transferred on a silicon substrate offering a high thermal conductivity. According to our thermal resistance measurements on a 0.8 × 6 μm 2 DHBT, a low thermal resistance of 1625 K/W is obtained, 65 % lower than for the same device fabricated on its own substrate of InP and which exhibited a value of 4452 K/W.

Phonon coherence in isotopic silicon superlattices

Recent experimental and theoretical investigations have confirmed that a reduction in thermal conductivity of silicon is achieved by isotopic silicon superlattices. In the present study, non-equilibrium molecular dynamics simulations are performed to identify the isotope doping and isotope layer ordering with minimum thermal conductivity. Furthermore, the impact of isotopic intermixing at the superlattice interfaces on phonon transport is investigated. Our results reveal that the coherence of phonons in isotopic Si superlattices is prevented if interfacial mixing of isotopes is considered.

Efficiency and Droop Improvement in GaN-Based High-Voltage Flip Chip LEDs

The GaN-based high-voltage flip chip light-emitting diode (HVFC-LED) is designed and developed for the purpose of efficiency enhancement. In our design, the distributed Bragg reflector (DBR) is deposited at the bonded substrate to increase the light extraction. After the flip chip process, the general current-voltage characteristics between the flip chip sample and the traditional sample are essentially the same. With the help of great thermal conductive silicon substrate and the bottom DBR, the HVFC-LED is able to enhance the power by 37.1% when compared to the traditional high-voltage LEDs. The wall-plug efficiencies of the HVFC-LED also show good droop reduction as high as 9.9% compared to the traditional devices.

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO2/Si3N4/SiO2 thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N2) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm2 and of 1.1 μW/cm2, respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N2 flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10−3 μW/cm2 K2.

Czochralski-Grown Silicon Crystals for Microelectronics

In the era of semiconductor electronics, monocrystalline silicon remains a basic and widely used material [1 4]. It will probably continue to dominate in the world of electronics for many years to come. The rst material in history, though, on the basis of which the invention of a transistor was made (1948) was germanium (Ge). In order to produce a germanium single crystal, the molten germanium was held in a graphite crucible, and the crystal grew from a seed initially immersed in the liquid and later slowly pulled upwards. This method of crystallization was described in 1916 by Jan Czochralski, a scientist and inventor (for his life and scienti c achievements, see [5]). During the rst years after invention, Czochralski focused his studies on determination of the rate of metal crystallization. In later investigations, he proved that materials produced using this method are single crystals. They were not single crystals in today understanding, though, they were threads of such metals as zinc, tin and lead. At the time there was no need whatsoever for monocrystaline materials, as all materials industrially exploited at that time were polycrystalline. Properties were studied such as textures, martensitic structures; recrystallization, mostly in order to improve the mechanical properties. Returning to the subject of this article, it must be pointed out that despite technological progress and development in the examination of semiconductor devices, the Czochralski crystallization method applied to semiconductor materials, in particular to silicon, remained essentially the same since the 1950s. It is commonly called the Czochralski method (for a detailed description of the method see, e.g. [6 8]). On the following pages, the characteristics of the method, contributing to its success are discussed, taking into consideration the mechanisms of crystallization and technological progress observed in the last decades. The discussion is focused on crystallization of silicon, a material which became irreplaceable in electronics and microelectronics. At the present time, silicon crystals are produced in tens of thousands of tons, annually. The broad use of silicon is due to both, its physical and technological characteristics as well as availability of resources (the primary resource of silicon is SiO2). Silicon has a unique characteristics of forming, on its surface, an ultrathin passivating oxide lm. Thermal conductivity of silicon (140 163 W/(mK)) is higher in comparison to other semiconductors (GaAs 46 55 W/(mK), SiC 16 55 W/(mK)). This property is of particular signi cance as it leads to (advantageous) heat dissipation at semiconductor junctions. Silicon became a leading material in electronics due to its energy gap (1.1 eV at 300 K) which guarantees the work of semiconductor devices at high temperatures.

Surface chemistry mediates thermal transport in three-dimensional nanocrystal arrays

Arrays of ligand-stabilized colloidal nanocrystals with size-tunable electronic structure are promising alternatives to single-crystal semiconductors in electronic, optoelectronic and energy-related applications. Hard/soft interfaces in these nanocrystal arrays (NCAs) create a complex and uncharted vibrational landscape for thermal energy transport that will influence their technological feasibility. Here, we present thermal conductivity measurements of NCAs (CdSe, PbS, PbSe, PbTe, Fe3O4 and Au) and reveal that energy transport is mediated by the density and chemistry of the organic/inorganic interfaces, and the volume fractions of nanocrystal cores and surface ligands. NCA thermal conductivities are controllable within the range 0.1-0.3 W m(-1) K(-1), and only weakly depend on the thermal conductivity of the inorganic core material. This range is 1,000 times lower than the thermal conductivity of silicon, presenting challenges for heat dissipation in NCA-based electronics and photonics. It is, however, 10 times smaller than that of Bi2Te3, which is advantageous for NCA-based thermoelectric materials.

Effects of quantum statistics of phonons on the thermal conductivity of silicon and germanium nanoribbons

We present molecular dynamics simulation of phonon thermal conductivity of semiconductor nanoribbons with an account for phonon quantum statistics. In our semiquantum molecular dynamics simulation, dynamics of the system is described with the use of classical Newtonian equations of motion where the effect of phonon quantum statistics is introduced through random Langevin-like forces with a specific power spectral density (color noise). The color noise describes interaction of the molecular system with the thermostat. The thermal transport of silicon and germanium nanoribbons with atomically smooth (perfect) and rough (porous) edges are studied. We show that the existence of rough (porous) edges and the quantum statistics of phonon change drastically the low-temperature thermal conductivity of the nanoribbon in comparison with that of the perfect nanoribbon with atomically smooth edges and classical phonon dynamics and statistics. The rough-edge phonon scattering and weak anharmonicity of the considered lattice produce a weakly pronounced maximum of thermal conductivity of the nanoribbon at low temperature.

Changes of Thermal Conductivity , Optical Conductivity and Electric Conductivity of Porous Silicon with Porosity

Changes of Thermal Conductivity , Optical Conductivity and Electric Conductivity of Porous Silicon with Porosity

Predictive calculation of the lattice thermal conductivity with temperature-dependent vibrational parameters

This paper presents a predictive model for the lattice thermal conductivity. The model is based on Callaway's solution to the Boltzmann equation for phonons which discriminates between the natures of the resistive and nonresistive phonon processes. However, the present model uses temperature-dependent lattice vibrational parameters and sound group velocities calculated on the basis of a dynamical matrix. No adjustment to thermal conductivity measurements is required. The model requires only the material mechanical properties as inputs to yield the material thermal conductivity as a function of temperature. A precise transmission probability function is introduced in the model in order to widen its application for the cases where interfaces are present. The importance of all the features of the developed model is demonstrated clearly with reference to reported data regarding the effects of surface orientation and isotope composition in single crystals, the effect of alloy composition in alloys, and the effect of grains boundaries in polycrystals. Namely, the developed model accounts for (i) the effects of surface orientation and isotope composition on the thermal conductivity of silicon and germanium single crystals, (ii) the effect of alloy composition on the thermal conductivity of silicon-germanium alloys, and (iii) the effect of phonon scattering at grains boundaries on the thermal conductivity of polycrystalline silicon.

Preparation and Thermal Conductivity Behavior of Elastomeric Silicon Rubber-Aluminum Nitride Composite

The thermal conductive silicon rubber have been prepared by using aluminum nitride as filler. The morphology and dispersion propriety of the filler were investigated by Scanning electron microscopy. The thermal conductivity of the composites was analysized by thermal measurement system. The results show that The larger particles give rise to a higher thermal conductivity of composites, at the same time the thermal conductivity of the composites increase with increase of aluminum nitride filler fraction. When the particle size is 60 nm, the thermal conductivity of composites at 40% volume fraction is about 3.5times that of pure silicon rubber matrix.

Thermal conductivity of nanocrystalline silicon by direct molecular dynamics simulation

The thermal conductivity simulation of nanocrystalline silicon is conducted on a three-dimensional configuration of nanocrystalline silicon with random grain shape for molecular dynamics simulation. The configuration is formed by the Voronoi tessellation method and the thermal conductivity is calculated by the Green-Kubo method. The effects of random grain distribution, periodic boundary, and the simulation system size are examined. Their effects on the simulation results can be neglected. The conductivity at temperature range from 300 K to 1100 K is obtained. The results indicate that the nanocrystalline thermal conductivity of silicon is far below the bulk single crystal and increases quickly with increasing grain size. The average grain boundary thermal resistance varies from 1.0 × 10−9 m2 KW−1 to 1.16 × 10−9 m2 KW−1. The restrain of the phonon mean free path by the nano-grain boundary is responsible for the sharp decrease in thermal conductivity. The effective phonon mean free path plays an important role in determining the thermal conductivity of nanocrystalline materials.

Reduced thermal conductivity of isotopically modulated silicon multilayer structures

We report measurements of the thermal conductivity of isotopically modulated silicon that consists of alternating layers of highly enriched silicon-28 and silicon-29. A reduced thermal conductivity of the isotopically modulated silicon compared to natural silicon was measured by means of time-resolved x-ray scattering. Comparison of the experimental results to numerical solutions of the corresponding heat diffusion equations reveals a factor of three lower thermal conductivity of the isotope structure compared to natural Si. Our results demonstrate that the thermal conductivity of silicon can be effectively reduced with isotopically modulated structures. This offers a promising approach to optimize silicon for thermoelectric applications.

High Thermal Conductivity Silicon Nitride Ceramics

This paper deals with the recent developments of high thermal conductivity silicon nitride ceramics. First, the factors that reduce the thermal conductivity of silicon nitride are clarified and the potential approaches to realize high thermal conductivity are described. Then, the recent achievements on the silicon nitride fabricated through the reaction bonding and post sintering technique are presented. Because of a smaller amount of impurity oxygen, the obtained thermal conductivity is substantially higher, compared to that of the conventional gas-pressure sintered silicon nitride, while the microstructures and bending strengths are similar to each other between these two samples. Moreover, further improvement of the thermal conductivity is possible by increasing β/α phase ratio of the nitrided sample, resulting in a very high thermal conductivity of 177 W/(m·K) as well as a high fracture toughness of 11.2 ㎫· m 1/2 .

Simulated thermal conductivity of silicon-based random multilayer thin films

Reverse nonequilibrium molecular dynamics simulations have been used to quantify the impact of randomly placed mass-altered atomic planes, such as those produced in pseudomorphically grown heterostructures, on the thermal conductivity of silicon. The results indicate that the room temperature thermal conductivity of these silicon-based structures can be reduced to values below 0.050 W/m-K. These values are significantly less than those found in random alloy or superlattice structures containing the same percentage of mass-altered atoms and are attributed to Anderson localization of phonons. Such low lattice thermal conductivity in these silicon-based structures could dramatically improve the thermoelectric efficiency of this earth abundant material.