Konstantinos Termentzidis

Molecular dynamics simulations for the prediction of thermal conductivity of bulk silicon and silicon nanowires: Influence of interatomic potentials and boundary conditions

The reliability of molecular dynamics (MD) results depends strongly on the choice of interatomic potentials and simulation conditions. Five interatomic potentials have been evaluated for heat transfer MD simulations of silicon, based on the description of the harmonic (dispersion curves) and anharmonic (linear thermal expansion) properties. The best interatomic potential is the second nearest-neighbor modified embedded atom method potential followed by the Stillinger-Weber, and then the Tersoff III. However, the prediction of the bulk silicon thermal conductivity leads to the conclusion that the Tersoff III potential gives the best results for isotopically pure silicon at high temperatures. The thermal conductivity of silicon nanowires as a function of cross-section and length is calculated, and the influence of the boundary conditions is studied for those five potentials.

Thermal conductivity of GaAs/AlAs superlattices and the puzzle of interfaces

We present a molecular dynamics investigation of the cross-plane thermal conductivity of superlattices using the non-equilibrium molecular dynamics method. The purpose is to investigate the influence of the interfaces, which is expected to be important in those nanostructures where the superlattice period is smaller than the phonon mean free path. In contrast to previous studies, more realistic interfaces are considered: interfacial roughness is modeled using atomic rectangular islands and interdiffusion is taken into account. It is shown that thermal conductivity is very sensitive to the detailed interfacial shape and to the presence of interdiffusion. This may be relevant to recent experiments.

Amorphization and reduction of thermal conductivity in porous silicon by irradiation with swift heavy ions

In this article, we demonstrate that the thermal conductivity of nanostructured porous silicon is reduced by amorphization and also that this amorphous phase in porous silicon can be created by swift (high-energy) heavy ion irradiation. Porous silicon samples with 41%-75% porosity are irradiated with 110u2009MeV uranium ions at six different fluences. Structural characterisation by micro-Raman spectroscopy and SEM imaging show that swift heavy ion irradiation causes the creation of an amorphous phase in porous Si but without suppressing its porous structure. We demonstrate that the amorphization of porous silicon is caused by electronic-regime interactions, which is the first time such an effect is obtained in crystalline silicon with single-ion species. Furthermore, the impact on the thermal conductivity of porous silicon is studied by micro-Raman spectroscopy and scanning thermal microscopy. The creation of an amorphous phase in porous silicon leads to a reduction of its thermal conductivity, up to a factor of 3 compared to the non-irradiated sample. Therefore, this technique could be used to enhance the thermal insulation properties of porous Si. Finally, we show that this treatment can be combined with pre-oxidation at 300u2009°C, which is known to lower the thermal conductivity of porous Si, in order to obtain an even greater reduction.

Thermal conductivity and thermal boundary resistance of nanostructures

AbstractWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattices interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results.PACS68.65.Cd, 66.70.Df, 81.16.-c, 65.80.-g, 31.12.xv

Characterization of the thermal conductivity of insulating thin films by scanning thermal microscopy

This paper reports on the abilities of a Scanning Thermal Microscopy (SThM) method to characterize the thermal conductivity of insulating materials and thin films used in microelectronics and microsystems. It gives a review of the previous works on the subject and gives new results allowing showing the performance of a new method proposed for reducing the thermal conductivity of meso-porous silicon by swift heavy ion irradiation. Meso-porous silicon samples were prepared by anodisation of silicon wafers and underwent irradiation by 845MeV ^2^0^8Pb ions, with fluences of 4x10^1^1 and 7x10^1^1cm^-^2. Thermal measurements show that irradiation reduced thermal conductivity by a factor of up to 2.

Thermal conductivity of meso-porous germanium

Thermal conductivity value of sponge-like meso-porous germanium (meso-PGe) layers measured by means of photoacoustic technique is reported. The room temperature thermal conductivity value is found to be equal to 0.6 W/(m K). The experimental results are in excellent agreement with molecular dynamic and Monte Carlo simulations. Both experiments and simulations show an important thermal conductivity reduction of the meso-PGe layers compared to the bulk Ge. The obtained results reveal meso-PGe as an interesting candidate for both thermoelectric and photovoltaic applications in which thermal transport is a really crucial issue.

Modeling Thermal Transport in Nano-Porous Semiconductors

Thermal transport in nano-Porous material has drawn the attention of several research groups during the last decade due to the ability of such structures to tailor efficiently the thermal properties of materials and more specifically to lower drastically the thermal conductivity of semiconductors. The present chapter recalls the basics of thermal transport in porous media from different standpoints. After a short introduction and review of the literature, analytic models that characterize heat propagation in porous media are given. Their limitations, especially in what concerns heat carriers scattering with pores when characteristic sizes become very small is pointed out and alternatives are suggested. In a second time, Monte Carlo modeling techniques, which are well designed for mesoscopic length-scales, are introduced and their use for thermal conductivity appraisal of nano-porous media is discussed. Improvement of such technique to reduce computation time and to model thin films with high porosity is then exposed with Effective Monte Carlo model. Simulation results for silicon and germanium support this part. The last section of the chapter is devoted to Molecular Dynamic (MD) modeling of nano-porous structures. Again, practical details on Equilibrium MD are proposed with a specific attention paid to crystalline and amorphous phases modeling. Then simulation results for various kinds of a-Si and c-Si nano-porous structures are discussed before concluding on all these methods and models.

Impact of screw and edge dislocations on the thermal conductivity of individual nanowires and bulk GaN: a molecular dynamics study

We report on thermal transport properties of wurtzite GaN in the presence of dislocations, by using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration were analyzed and found to reduce considerably the thermal conductivity while impacting its temperature dependence in a different manner. We demonstrate that isolated screw dislocations reduce the thermal conductivity by a factor of two, while the influence of edge dislocations is less pronounced. The relative reduction of thermal conductivity is correlated with the strain energy of each of the five studied types of dislocations and the nature of the bonds around the dislocation core. The temperature dependence of the thermal conductivity follows a physical law described by a T

Size dependence of the surface tension of a free surface of an isotropic fluid

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Nanoscale and Microscale Heat Transfer V (NMHT-V) EUROTHERM seminar No 108

variation in combination with an exponent factor which depends on the materials nature, the type and the structural characteristics of the dislocations core. Furthermore, the impact of the dislocations density on the thermal conductivity of bulk GaN is examined. The variation and even the absolute values of the total thermal conductivity as a function of the dislocation density is similar for both types of dislocations. The thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are analyzed. The discrepancy of the anisotropy of the thermal conductivity grows in increasing the density of dislocations and it is more pronounced for the systems with edge dislocations.