Phonon coherence in isotopic silicon superlattices

In this paper, we investigate the effect of isotopic modulation on the thermal conductivity of semiconductor nanostructures. The isotope doping is of particular interest for the application of semiconductors as thermoelectric materials as it leaves the electronic properties practically unaffected while the phononic transport is retarded. This approach could increase the figure of merit of thermoelectric generators by decreasing the thermal conductivity of semiconductors. We use non‐equilibrium molecular dynamics simulations to examine thermal transport in isotopically engineered semiconductors. The temperature profiles along the sample region deduced from the simulations allow the extraction of thermal conductivities. The reliability of the MD‐predicted thermal conductivities is studied by analyzing the influence of the input parameters on the results. The first set of samples are isotopically modified silicon samples. The influence of temperature, isotopic composition, and ordering of isotopic defects on the thermal conductivity of silicon is studied. The second material system under investigation is silicon germanium alloys. The influence of isotopic modulation on the thermal conductivity of Si–Ge alloys is examined for varying chemical composition. The thermal conductivities predicted by MD are compared to results derived from the solution of the Boltzmann transport equation in the relaxation time approach.

“…Previous results on thermal transport in

^{28}

Si/

^{30}

Si superlattices show that a minimum in thermal conductivity exists for an alternating layer sequence with a period of around 3 nm (). On the other hand, a random alloy of

^{28}

_{0.5}

–

^{30}

_{0.5}

reduces the thermal conductivity more efficiently than

^{28}

Si/

^{30}

Si superlattices.…”

Section: Resultsmentioning

confidence: 96%

Section: Silicon Samplesmentioning

confidence: 92%

Section: Thermal Conductivity Calculationsmentioning

confidence: 99%

See 1 more Smart Citation

Molecular dynamics simulations of thermal transport in isotopically modulated semiconductor nanostructures

Frieling

Eon

Wolf

et al. 2016

“…First of all, with less‐than‐perfect interfaces point scattering of phonons can occur, which reduces phonon lifetime and thus conductivity. It has been shown () that intermixed interfaces lead to a further depression of conductivity. For stronger intermixing the limit can be the conductivity of a random alloy of isotopes.…”

Section: Resultsmentioning

confidence: 99%

Ultrafast study of phonon transport in isotopically controlled semiconductor nanostructures

Issenmann

Eon²,

Bracht³

et al. 2015

Isotopically modulated silicon and germanium multilayers are analyzed by means of femtosecond spectroscopy and pulsed X‐ray scattering for determining thermal conductivity and phonon modes. Isotopic modulation decreases thermal conductivity stronger than expected from a band bending model in the coherent phonon transport regime, in particular for silicon. Femtosecond spectroscopy and X‐ray scattering resolve zone‐folded vibration modes, which are located at the edge of the new, smaller Brillouin zone due to the multilayer periodicity. These modes can contribute to the reduction of thermal conductivity by Umklapp processes within the zone‐folded mini‐bands. Color‐coded increase in ultrafast X‐ray scattering in vicinity to the mini‐zone boundary of a germanium multilayer.

“…The impact of isotopic modulation on the thermal conductivity of Si was studied by means of molecular dynamics simulations based on the Stillinger–Weber potential by Frieling et al . A direct comparison to theoretical predictions is, however, difficult since the length of the simulation cell is limited due to computational restrictions.…”

Section: Discussionmentioning

confidence: 99%

Measurement and analysis of thermal conductivity of isotopically controlled silicon layers by time‐resolved X‐ray scattering

Eon

Frieling

Plech³

et al. 2016

Self Cite

Nanostructuring is considered to be an efficient way to tailor phonon scattering and to reduce the thermal conductivity while keeping good electronic properties. This can be ideally realized by mass modulation of chemical identical elements. In this work, we report measurements of the crossplane thermal conductivity of isotopically modulated 28 Si/ 30 Si multilayer structures and of isotopically pure 28 Si layers by means of time-resolved X-ray scattering. Compared to earlier investigations, an improved measurement technique has been applied to determine the cooling behavior of a top gold metal layer after laser excitation with picosecond time resolution until thermal equilibration is established. Detailed analysis of the cooling behavior not only confirms a reduced thermal conductivity of 28 Si/ 30 Si multilayer structures compared to natural and isotopically enriched 28 Si layers but also provides evidence of direct laser heating of the Si layer. This and extrinsic effects affecting the cooling behavior of the gold layer are taken into account to determine the thermal conductivity by means of the pump-and-probe measurement technique.