Silicon-based semiconductors are attractive for thermoelectric devices, but their phonon-dominated thermal conductivity is preventing their use. Here, Alaie et al. observe coherent phonon boundary scattering in phononic crystals at room temperature by introducing large (=100 nm) air holes in the Si matrix.

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Thermal transport in phononic crystals and the observation of coherent phonon scattering at room temperature

We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.

Profiling the Thermoelectric Power of Semiconductor Junctions with Nanometer Resolution

This paper will show that, while ZT is an appropriate performance index when optimizing the heat conversion rate, it may mislead research in view of applications aiming at large electric power production. This is of special relevance when related to the surge of research in the area of low-dimensionality semiconductors where ZT is increased by lowering the thermal conductivity κ. It will be shown that, when operating between sources at fixed temperature, the highest power output can be obtained by increasing κ, not decreasing it, the larger electric power output economically enabling thermoelectric generators for massive electric power production.

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Do we really need high thermoelectric figures of merit? A critical appraisal to the power conversion efficiency of thermoelectric materials

A large thermoelectric power factor in heavily boron-doped p-type nanograined Si with grain sizes ~30 nm and grain boundary regions of ~2 nm is reported. The reported power factor is ~5 times higher than in bulk Si. It originates from the surprising observation that for a specific range of carrier concentrations, the electrical conductivity and Seebeck coefficient increase simultaneously. The two essential ingredients for this observation are nanocrystallinity and extremely high boron doping levels. This experimental finding is interpreted within a theoretical model that considers both electron and phonon transport within the semiclassical Boltzmann approach. It is shown that transport takes place through two phases so that high conductivity is achieved in the grains, and high Seebeck coefficient by the grain boundaries. This together with the drastic reduction in the thermal conductivity due to boundary scattering could lead to a significant increase of the figure of merit ZT. This is one of the rare observations of a simultaneous increase in the electrical conductivity and Seebeck coefficient, resulting in enhanced thermoelectric power factor.

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Simultaneous increase in electrical conductivity and Seebeck coefficient in highly boron-doped nanocrystalline Si

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

https://ddd.uab.cat/pub/artpub/2015/210987/tunthetra_a2015v9n4p3820.pdf

Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering

We measured the Seebeck coefficient of P-doped ultrathin silicon-on-insulator (SOI) layers with thicknesses of 2–100 nm. The dependence of the coefficient on the impurity concentration was investigated, and was shown to be in good agreement with that of bulk Si for SOI thicknesses above 6 nm. In addition, it was found to decrease with increasing impurity concentration, which is usually observed in semiconductor materials. However, for doping levels above 3.5×1019 cm-3, the Seebeck coefficient was observed to increase. This is likely to be due to the influence of an impurity band.

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Seebeck Coefficient of Ultrathin Silicon-on-Insulator Layers

We measured the Seebeck coefficient of heavily P-doped silicon-on-insulator layers with P concentrations above 1×1019 cm−3. The coefficient decreased with increasing P concentration, and with a peak of the Seebeck coefficient around 5×1019 cm−3. We calculated the density-of-states (DOS) of bulk Si based on theoretical models of impurity-band formation, ionization-energy shift, and conduction-band tailing. The calculated impurity-concentration dependence of the energy derivative of the DOS at the Fermi energy also showed a peak. Consequently, the Seebeck coefficient of the heavily doped Si is ruled by the DOS distribution, similar to metallic materials.

Influence of heavy doping on Seebeck coefficient in silicon-on-insulator

The contribution of the phonon drag effect to the Seebeck coefficient of P-doped ultrathin Si-on-insulator (SOI) layers with a thickness of 9–100 nm is investigated for near-room-temperature applications. The contribution is found to be significant in the lightly doped region and to depend on the carrier concentration with increasing carrier concentration above ∼5 × 1018 cm−3. Moreover, the contribution is not influenced by SOI thickness above 9 nm. On the basis of phonon mean-free-path calculations considering phonon scattering processes, the phonon drag part of the SOI Seebeck coefficient in the lightly doped region is mainly governed by phonon-phonon scattering. Furthermore, in higher concentration regions, the dependence of phonon drag can be qualitatively explained by the interaction between phonons and doped impurities.

Phonon drag effect on Seebeck coefficient of ultrathin P-doped Si-on-insulator layers

Influence of TiO2 layer's nanostructure on its thermoelectric power factor

We have performed a theoretical calculation of the Seebeck coefficient in heavily P-doped Si, taking into account the influence of impurity-band formation, ionization-energy shift, and conduction-band tailing. It was found that the calculated Seebeck coefficient decreased with increasing impurity concentration, with a peak around 1 × 1019 cm−3. This result is qualitatively in good agreement with experimental results, indicating that the Seebeck coefficient of the heavily doped Si is strongly influenced by the electronic density of states in the impurity band. Therefore, to enhance the Seebeck coefficient by introducing nanostructures, the Fermi energy has to be controlled without the influence of the impurity band.

Faiz Salleh

Papers

Seebeck Coefficient of Ultrathin Silicon-on-Insulator Layers

Influence of heavy doping on Seebeck coefficient in silicon-on-insulator

Phonon drag effect on Seebeck coefficient of ultrathin P-doped Si-on-insulator layers

Influence of TiO2 layer's nanostructure on its thermoelectric power factor

Seebeck Coefficient of Heavily P-Doped Si Calculated from an Alteration in Electronic Density of States