Phonon-phonon interactions: First principles theory

2018

The interactions between heat flow and an oxide layer in Si are studied within two temperature windows using non‐equilibrium ab initio molecular‐dynamics (MD). The model system is a H‐saturated Si nanowire containing an amorphous SiOx layer. The nanowire is in a large 1‐D periodic box which prevents thermal contamination between image nanowires. The results show that the oxide acts as barrier to heat flow and substantially increases the time required for the system to reach thermal equilibrium. This effect is caused by the higher‐frequency vibrational modes in the oxide relative to Si, and is unrelated to the low thermal conductivity of SiOx. A new first‐principles method to calculate the Kapitza resistance of the interface directly from the MD data is proposed.

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

“…A comparison of the fluctuations in conventional and supercell‐preparation MD runs is also in ref. . In the present calculations, we average over n = 30 runs.…”

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Phonon Dynamics at an Oxide Layer in Silicon: Heat Flow and Kapitza Resistance

2018

Heat flow across an oxide layer in Si

2017

Oxide layers are ubiquitous in Si technology including nanostructures. How such layers interact with heat flow is not well understood. In this contribution, we present the preliminary results of ab initio molecular-dynamic simulations of such interactions. We show that oxide layers reflect (part of) the incoming heat, which results in the accumulation of energy on the warmer side of the layer for longer times than without the presence of the oxide. The results are consistent with earlier predictions that phonon-defect interactions are determined by the vibrational properties of the defect (here, the oxide layer).

Removing Heat from Si with a “Thermal Circuit”: An Ab‐Initio Study

Vincent

2018

Electronic devices generate unwanted heat. The removal of this heat often involves a layer of a high‐thermal‐conductivity material X deposited at a strategic location on the Si chip. The layer is then cooled using a mechanical fan or more sophisticated means. The authors present here the results of ab‐initio molecular‐dynamics simulations which examine the thermal interactions between heat generated in Si and three materials X: Ge, C, and Si itself. The geometry is a very thin wire of atoms X located within a Si slab. Heat is periodically removed from the wire as if it were connected to a heat sink. The wire then absorbs heat from the warmer Si slab, thus cooling it. The rate at which the Si temperature drops depends on how efficiently heat crosses the Si|X boundary and then is absorbed by the wire. The interactions involve the coupling of Si‐Si vibrational modes with Si‐X (interface) and then X‐X (layer) modes. The worst performance occurs for XC as lower‐frequency Si‐Si modes must decay into higher‐frequency Si‐C then CC modes. This involves slow two‐ (or more) phonon processes. The fastest cooling rate occurs when the wire is made of Si itself: there is no Si|Si interface and the vibrational interactions involve only Si bulk modes which couple to each other resonantly. The Ge wire performs quite well, as the Si‐Si modes easily decay into lower‐frequency Si‐Ge and Ge‐Ge modes.