Thermal conductivity of nanocrystalline SiGe alloys using molecular dynamics simulations

Cruz, Carolina Abs da; Katcho, Nebil A.; Mingo, Natalio; Veiga, Rga

doi:10.1063/1.4826526

Cited by 11 publications

(15 citation statements)

References 26 publications

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“…Ideal Reyleigh scattering (n = 4) corresponds to α = 7/4 and produces the κ ∼ L 1/4 trend previously reported in molecular dynamics simulations [24]. It also offers a good approximation for the AlGaN results presented here (α 1.72).…”

supporting

confidence: 54%

Cross-plane heat conduction in thin films with ab-initio phonon dispersions and scattering rates

Vermeersch

Carrete

Mingo

2016

Applied Physics Letters

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We present a first-principles study of the cross-plane thermal conductivity κ ⊥ in a wide variety of semiconductor thin films. We introduce a simple suppression model that matches variance-reduced Monte Carlo simulations with ab-initio phonon dispersions and scattering rates within ≤ 5% even for anisotropic compounds. This, in turn, enables accurate κ ⊥ reconstruction from tabulated cumulative conductivity curves κΣ(Λ ⊥ ). We furthermore reveal, and explain, a distinct quasiballistic regime characterised by a fractional thickness dependence κ ⊥ ∼ L 2−α in alloys (where α is the Lévy exponent) and logarithmic dependence κ ⊥ ∼ ln(L) in single crystals. These observations culminate in the formulation of two compact parametric forms for κ ⊥ (L) that can fit the first-principles curves across the entire ballistic-diffusive range within a few percent for all investigated compounds.Phonon-mediated heat conduction in thin films plays an important role in nanoscale devices [1, 2] and received increasing theoretical attention [3][4][5][6][7][8][9][10]. Interestingly, cross-plane thermal transport has posed a considerably harder challenge than its in-plane counterpart. An exact solution of the Boltzmann transport equation (BTE) in cross-plane geometries has been obtained only very recently [7] and provides the film conductivity at thickness L as an integral over phonon frequencyHere S is a 'suppression function' that contains the thin film physics and κ(ω) denotes the bulk spectral conductivity. Evaluation of the latter often relies on analytical models with isotropic dispersions [8-10] for mathematical convenience. More realistic phonon spectra can also be utilised through frequency binning, but this procedure becomes problematic in anisotropic compounds. In this work, we adopt a first-principles framework [11,12] that goes well beyond the predictive power of spectral formulations. We subsequently show that the resulting cross-plane thin film conductivity curves κ(L) can be captured by compact models that effortlessly maintain their accuracy in anisotropic compounds. In the process, we also reveal quasiballistic regimes in both alloys and single crystals with distinct film thickness dependences.We performed our ab-initio study for a variety of semiconductors selected for their technological relevance. Thin Si films play a key role in silicon-on-insulator de- We calculated ab-initio heat capacities C( k), group velocities v( k) and scattering rates τ −1 ( k) using the procedure described in Ref. 17. The adopted method has been tested on a variety of bulk compounds and provides results in good agreement with experiments [11]. Phonon properties are resolved over a uniform wavevector grid with N 3 k points, where we have set N k to 24 for diamond/zincblende crystals, 16 for wurtzites and 12 for Pnma SnSe. Scattering rates τ −1 = τ −1 anh + τ −1 har and mean free paths (MFPs) Λ = v τ account for anharmonic (three-phonon) and harmonic (isotope/alloy) scattering mechanisms. Alloys were treated under the virtual crystal approxim...

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supporting

confidence: 54%

Cross-plane heat conduction in thin films with ab-initio phonon dispersions and scattering rates

Vermeersch

Carrete

Mingo

2016

Applied Physics Letters

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“…Previous studies have shown that decreasing the grain size can effectively reduce the thermal conductivity. 15,16 The decreasing trend of thermal conductivity as grain size decreases in Fig. 4(c) confirm this observation.…”

Section: Resultssupporting

confidence: 72%

Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon–germanium thermoelectrics

Hua¹,

Minnich²

2014

Semicond. Sci. Technol.

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Nanocrystalline silicon and silicon-germanium alloys are promising thermoelectric materials that have achieved substantially improved figure of merits compared to their bulk counterparts. This enhancement is typically attributed to a reduction in lattice thermal conductivity by phonon scattering at grain boundaries. However, further improvements are difficult to achieve because grain boundary scattering is poorly understood, with recent experimental observations suggesting that the phonon transmissivity may depend on phonon frequency rather than being constant as in the commonly used gray model. Here, we examine the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys in a realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo simulations. We find that the grain boundary may not be as effective as predicted by the gray model in scattering certain phonons, with a substantial amount of heat being carried by low frequency phonons with mean free paths longer than the grain size. Our result will help guide the design of more efficient thermoelectrics.

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“…The MFP can be also calculated from l ¼ v g s, which must result in same value. Therefore, the relaxation time was estimated from the latter relation, which agrees with the decay of the autocorrelation of the average heat flux versus time for each material system, 34,35 such as the plot shown in Figure 4. According to kinetic relaxation time approximation, the thermal conductivity depends on the lattice specific heat per unit volume, average phonon group velocity, and the phonon mean free path.…”

Section: Discussion and Resultsmentioning

confidence: 99%

Thermal conductivity of nanostructured SixGe1−x in amorphous limit by molecular dynamics simulation

Norouzzadeh

Nozariasbmarz

Krasiński

et al. 2015

Journal of Applied Physics

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We report the thermal conductivity of amorphous SixGe1−x compound calculated versus composition and temperature. The result sets the minimum value of thermal conductivity which is achievable by nanostructuring. We employed molecular dynamics with Tersoff's potential for the calculations. It was found that, contrary to the crystalline SixGe1−x, the thermal conductivity of amorphous phase is a weak function of the material composition. For the most popular composition Si0.8Ge0.2, the thermal conductivity of the amorphous phase is less than 1 W m−1 K−1 with small reduction as the temperature increases from 300 K to 1400 K. The thermal conductivity of amorphous SixGe1−x for any value of x is approximately an order of magnitude smaller than the minimum thermal conductivity of crystalline SixGe1−x alloy, which occurs near x = 0.5. It is known that alloying with germanium is more efficient than nanostructuring to reduce the thermal conductivity of silicon; however, it was found that the amorphization process is even more effective than alloying for that purpose. It was also shown that the reduction of the thermal conductivity of silicon due to alloying with germanium is more efficient in crystalline phase than in amorphous phase.

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Thermal conductivity of nanocrystalline SiGe alloys using molecular dynamics simulations

Cited by 11 publications

References 26 publications

Cross-plane heat conduction in thin films with ab-initio phonon dispersions and scattering rates

Cross-plane heat conduction in thin films with ab-initio phonon dispersions and scattering rates

Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon–germanium thermoelectrics

Thermal conductivity of nanostructured SixGe1−x in amorphous limit by molecular dynamics simulation

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