Maxime Verdier

Crystalline-amorphous silicon nano-composites: Nano-pores and nano-inclusions impact on the thermal conductivity

The thermal conductivities of nanoporous and nanocompositesilicon with incorporated amorphous phases have been computed by molecular dynamics simulations. A systematic investigation of the porosity and the width of the amorphous shell contouring a spherical pore has been made. The impact of amorphous phase nanoinclusions in a crystalline matrix has also been studied with the same amorphous fraction as the porosity of nanoporoussilicon to achieve comparison. The key parameter for all configurations with or without the amorphous phase is proved to be the interface (between the crystalline and amorphous phases or crystalline and void) to volume ratio. We obtain the sub-amorphous thermal conductivity for several configurations by combining pores, amorphous shell, and crystalline phase. These configurations are promising candidates for low cost and not toxic thermoelectric devices based on abundant semiconductors.

Effect of Amorphisation on the Thermal Properties of Nanostructured Membranes

Abstract The majority of the silicon devices contain amorphous phase and amorphous/crystalline interfaces which both considerably affect the transport of energy carriers as phonons and electrons. In this article, we investigate the impact of amorphous phases (both amorphous silicon and amorphous SiO2) of silicon nanoporous membranes on their thermal properties via molecular dynamics simulations. We show that a small fraction of amorphous phase reduces dramatically the thermal transport. One can even create nanostructured materials with subamorphous thermal conductivity, while keeping an important crystalline fraction. In general, the a-SiO2 shell around the pores reduces the thermal conductivity by a factor of five to ten compared to a-Si shell. The phonon density of states for several systems is also given to give the impact of the amorphisation on the phonon modes.

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.

Effect of the amorphization around spherical nano-pores on the thermal conductivity of nano-porous Silicon

The thermal conductivity of nano-porous Silicon with amorphous shells around the pores is computed by Molecular Dynamics simulations. For the latter property, a systematic investigation of the porosity and the thickness of the amorphous shells has been performed. Sub-amorphous thermal conductivity is reached for systems with large porosity and amorphous shell, while a non-negligible fraction of crystalline Silicon phase is still present. The thermal conductivity of all studied systems can be controlled by a key parameter which is the ratio of crystalline/amorphous or crystalline/void interface to the volume of material.

Enhanced thermal conductivity in percolating nanocomposites: a molecular dynamics investigation

In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case.