Thermoelectric Enhancement of Silicon Membranes by Ultrathin Amorphous Films

Defect scattering can lead to enhanced phonon transport at nanoscale

Defect scattering is well known to suppress thermal transport. In this study, however, we perform both molecular dynamics and Boltzmann transport equation calculations, to demonstrate that introducing defect scattering in nanoscale heating zone could surprisingly enhance thermal conductance of the system by up to 75%. We further reveal that the heating zone without defects yields directional nonequilibrium with overpopulated oblique-propagating phonons which suppress thermal transport, while introducing defects redirect phonons randomly to restore directional equilibrium, thereby enhancing thermal conductance. We demonstrate that defect scattering can enable such thermal transport enhancement in a wide range of temperatures, materials, and sizes, and offer an unconventional strategy for enhancing thermal transport via the manipulation of phonon directional nonequilibrium.

Boosting Thermoelectric Performance in Epitaxial GeTe Film/Si by Domain Engineering and Point Defect Control

This study demonstrates a simultaneous realization of ultralow thermal conductivity and high thermoelectric power factor in epitaxial GeTe thin films/Si substrates by a combination of the interface introduction by domain engineering and the suppression of Ge vacancy generation by point defect control. We formed epitaxial Te-poor GeTe thin films having low-angle grain boundaries with a misorientation angle close to 0° or twin interfaces with a misorientation angle close to 180°. The control of interfaces and point defects gave rise to ultralow lattice thermal conductivity of ∼0.7 ± 0.2 W m^-1 K^-1. This value was the same in the order of magnitude as the theoretical minimum lattice thermal conductivity of ∼0.5 W m^-1 K^-1 calculated by the Cahill-Pohl model. At the same time, the GeTe thin films exhibited a high thermoelectric power factor because of the suppression of Ge vacancy generation and a small contribution of grain boundary carrier scattering. The outstanding combined technique of domain engineering and point defect control can be a great approach for developing high-performance thermoelectric films.

Impact of nanopillars on phonon dispersion and thermal conductivity of silicon membranes

The performance of silicon-based thermoelectric energy generators is limited by the high thermal conductivity of silicon. Theoretical works have long proposed reducing the thermal conductivity by resonant phonon modes in nanopillars placed on the surface of silicon films. However, these predictions have never been confirmed due to the difficulty in the nanofabrication and measurements of such nanoscale systems. In this work, we report on the fabrication and measurements of silicon films with nanopillars as small as 12 nm in diameter. Our Brillouin light scattering spectroscopy experiments revealed that nanopillars indeed host resonant phonon modes. Yet, our thermal measurements using the micro time-domain thermoreflectance technique showed only a statistically insignificant difference between the thermal properties of silicon membranes with and without nanopillars. Results of this work contrast with the predictions of a substantial reduction in the thermal conductivity due to nanopillars and suggest refining the simulations to account for realistic experimental conditions.

Thermoelectric Energy Harvesters: A Review of Recent Developments in Materials and Devices for Different Potential Applications

The thermoelectric effect encompasses three different effects, i.e. Seebeck effect, Peltier effect, and Thomson effect, which are considered as thermally activated materials that alter directions in smart materials. It is currently considered one of the most challenging green energy harvesting mechanisms among researchers. The ability to utilize waste thermal energy that is generated by different applications promotes the use of thermoelectric harvesters across a wide range of applications. This review illustrates the different attempts to fabricate efficient, robust and sustainable thermoelectric harvesters, considering the material selection, characterization, device fabrication and potential applications. Thermoelectric harvesters with a wide range of output power generated reaching the milliwatt range have been considered in this work, with a special focus on the main advantages and disadvantages in these devices. Additionally, this review presents various studies reported in the literature on the design and fabrication of thermoelectric harvesters and highlights their potential applications. In order to increase the efficiency of equipment and processes, the generation of thermoelectricity via thermoelectric materials is achieved through the harvesting of residual energy. The review discusses the main challenges in the fabrication process associated with thermoelectric harvester implementation, as well as the considerable advantages of the proposed devices. The use of thermoelectric harvesters in a wide range of applications where waste thermal energy is used and the impact of the thermoelectric harvesters is also highlighted in this review.

Coherent and Incoherent Impacts of Nanopillars on the Thermal Conductivity in Silicon Nanomembranes

Nanostructuring is the dominant approach for effective thermal conduction control in nanomaterials. In the past decade, researchers have been interested in thermal conduction control by the coherent effects in phononic crystal (PnC) systems. Recent theoretical works predicted that nanopillars on the surface of silicon membranes could cause a dramatic thermal conductivity reduction due to the phonon local resonances. However, this remarkable prediction has not been experimentally verified yet with the deep-nanoscale pillar-based PnCs. Here, we fabricate nanopillars on suspended silicon membranes using damageless neutral-beam etching and investigate the impact of nanopillars on the thermal conductivity of the membranes in the 4-300 K range. We found that thermal conductivity reduction caused by the nanopillars does not exceed 16%, which is much weaker than that predicted by the theoretical works. Moreover, this reduction remains temperature independent. These facts make the coherence an unlikely reason for the observed reduction. Indeed, our Monte Carlo simulations can reproduce the experimental results under a purely incoherent approximation. Our study shows that the coherent control of heat conduction by PnC nanostructures is more challenging to observe experimentally in reality than predicted in near-ideal modeling.

Thermal Conductivity Reduction in a Silicon Thin Film with Nanocones

Modern thermoelectric devices incline toward inexpensive, environmentally friendly, and CMOS-compatible materials, such as silicon. To improve the thermoelectric performance of silicon, researchers try to decrease its thermal conductivity using various nanostructuring methods. However, most of these methods have limited efficiency because they are costly and damaging for the internal structure of silicon. Here, we propose a cost-effective, large-area, and maskless nanofabrication method that creates external nanocones on the silicon surface while preserving its interior. Our experiments show that these nanocones reduce the thermal conductivity of thin silicon membranes by more than 40%. Using a modified Callaway-Holland model, we study how the thermal conductivity is affected by various phonon scattering processes in the 4-295 K temperature range. We conclude that the nanocones generate additional surface scattering, which causes the thermal conductivity reduction. The proposed nanocones and their simple fabrication method are promising for the planar thermoelectric devices based on silicon.