Silicon Nanomeshes Research Articles

Spectral analysis of coherent and incoherent phonon transport in silicon nanomeshes

Spectral analysis of coherent and incoherent phonon transport in silicon nanomeshes

Elucidating optimal nanohole structures for suppressing phonon transport in nanomeshes

Nanomeshes, often referred to as phononic crystals, have been extensively explored for their unique properties, including phonon coherence and ultralow thermal conductivity (κ). However, experimental demonstrations of phonon coherence are rare and indirect, often relying on comparison with numerical modeling. Notably, a significant aspect of phonon coherence, namely the disorder-induced reduction in κ observed in superlattices, has yet to be experimentally demonstrated. In this study, through atomistic modeling and spectral analysis, we systematically investigate and compare phonon transport behaviors in graphene nanomeshes, characterized by 1D line-like hole boundaries, and silicon nanomeshes, featuring 2D surface-like hole boundaries, while considering various forms of hole boundary roughness. Our findings highlight that to demonstrate a disorder-induced reduction in κ of nanomeshes, optimal conditions include low temperature, smooth and planar hole boundaries, and the utilization of thick films composed of 3D materials.

Nanomeshed Si nanomembranes

One of the main challenges in stretchable electronics is to achieve high-performance stretchable semiconductors. Here, we introduce an innovative concept of nanomeshed semiconductor nanomembrane which can be regarded almost as intrinsically stretchable to conventional microelectronic layouts. By making a silicon film into homogeneous nanomeshes with spring-like nano traces, we demonstrated a high electron mobility of 50 cm2/V·s, and moderate stretchability with a one-time strain of 25% and cyclic strain of 14% after stretching for 1000 cycles, further improvable with optimized nanomesh designs. A simple analytic model covering both fractional material and trace sidewall surfaces well predicted the transport properties of the normally on silicon nanomesh transistors, enabling future design and optimizations. Besides potential applications in stretchable electronics, this semiconductor nanomesh concept provides a new platform for materials engineering and is expected to yield a new family of stretchable inorganic materials having tunable electronic and optoelectronic properties with customized nanostructures.

Hydrogen Sensors: Palladium‐Decorated Silicon Nanomesh Fabricated by Nanosphere Lithography for High Performance, Room Temperature Hydrogen Sensing (Small 10/2018)

Hydrogen Sensors: Palladium‐Decorated Silicon Nanomesh Fabricated by Nanosphere Lithography for High Performance, Room Temperature Hydrogen Sensing (Small 10/2018)

Palladium-Decorated Silicon Nanomesh Fabricated by Nanosphere Lithography for High Performance, Room Temperature Hydrogen Sensing.

A hydrogen (H2 ) gas sensor based on a silicon (Si) nanomesh structure decorated with palladium (Pd) nanoparticles is fabricated via polystyrene nanosphere lithography and top-down fabrication processes. The gas sensor shows dramatically improved H2 gas sensitivity compared with an Si thin film sensor without nanopatterns. Furthermore, a buffered oxide etchant treatment of the Si nanomesh structure results in an additional performance improvement. The final sensor device shows fast H2 response and high selectivity to H2 gas among other gases. The sensing performance is stable and shows repeatable responses in both dry and high humidity ambient environments. The sensor also shows high stability without noticeable performance degradation after one month. This approach allows the facile fabrication of high performance H2 sensors via a cost-effective, complementary metal-oxide-semiconductor (CMOS) compatible, and scalable nanopatterning method.

(Invited) Thermal Transport and Thermal Management By Silicon Nanostructures

Thermal transport in silicon nanostructures has captured the attention of scientists for understanding phonon transport at the nanoscale, and the competitive thermoelectric figure-of-merit has inspired engineers to develop efficient power generation and solid-state cooling devices. Firstly, our work focuses on identifying fundamental mechanisms of thermal transport in silicon nanostructures. The thermal transport interpretation remains unclear at the nanoscale and is often called into doubt due to the coupled effects of phonon coherence, boundary scattering, backscattering, and contact resistance. We isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the particle-related backscattering effect by comparing variable-pitch nanomeshes. The lithographically-patterned silicon nanomeshes provide a unique opportunity for this progress because the relevant dimensions including the artificial periodicity, pitch, and neck were independently controlled within a monolithic measurement device. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Our simulations based on a ray tracing technique support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K. In other words, the wave nature of phonons does not need to be considered to describe transport in the regime where periodicities are greater than the dominant wavelengths but smaller than inelastic mean free paths. We also show that phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction in silicon nanomeshes. This work provides valuable insights for describing phonon transport in complex nanostructured geometries and improves a fundamental understanding of nanoscale thermal transport that is critical for a broad range of semiconductor technologies. Lastly, our work provides a novel thermal management design based on holey silicon, in which nanoscopic holes are lithographically patterned and vertical trenches are formed to offer tunable transport properties. Due to the unique thermal conductivity anisotropy including the low in-plane thermal conductivity and the high cross-plane thermal conductivity preserved by long-wavelength phonons, the holey silicon structure will provide innovative thermoelectric cooling performance even at a confined lateral space, which is unachievable by conventional isotropic materials. The thermal conductivity anisotropy is attractive for lateral thermoelectric cooling designs because the low in-plane thermal conductivity sets required temperature gradients for thermoelectric Peltier effects in the lateral direction and the high cross-plane conductivity enables effective heat dissipation from the hotspot in the vertical direction. This work investigates the role of thermal conductivity anisotropy in thermoelectric cooling through theoretical, numerical, and experimental approaches. The work bridges the gap between fundamental thermal sciences and device engineering, in which the holey silicon-based thermoelectric cooling device benefits from the unique transport phenomena at the nanoscale. The outcomes of this research present the anisotropic silicon nanostructures as efficient and effective thermoelectric materials that can offer transformative solutions to thermal management of nanoscale electronics.

(Invited) Phonon Coherence and Backscattering in Thermal Transport of Silicon Nanostructures

Understanding thermal transport at the nanoscale is critical for a broad range of semiconductor technologies involving nanoelectronics, phase change data storage, heat-assisted magnetic recording, solid-state lighting, and thermoelectric energy conversion. Past studies on the thermal conductivity of silicon materials have substantially contributed to establishing microscopic understanding of thermal transport. Classical theories based on the Boltzmann Transport Equation that treat phonons as incoherent particles have accurately described the size-dependent thermal conductivity in silicon thin films and high purity nanowires; these thermal conductivity reductions are due to mean free path suppression from boundary scattering. However, recent thermal conductivity measurements of silicon nanomeshes, which are thin membranes with a fabricated periodic mesh of nanoscopic holes, have called into question the validity of this particle-based Boltzmann Transport Equation approach when applied to periodic nanostructures. Phonons can display both wave-like and particle-like behavior during thermal transport. While the prospect of controlling phonon waves is attractive for potential phononic systems, experimental reports remain inconclusive on the relative importance of wave effects versus particle-based boundary scattering effects in the nanomesh. Although there are several mechanisms proposed to explain the thermal conductivity reduction in silicon nanostructures, possible phonon coherence (wave) and backscattering (particle) effects are often coupled, and previous experimental studies were unable to isolate the dominant mechanism. Here, we measure thermal conductivity of silicon nanomesh structures defined by electron-beam lithography to isolate these competing mechanisms of thermal conductivity reduction. By comparing nanomeshes with periodic and aperiodic holes of the same average dimension, our experimental design decouples the phonon coherence effect from boundary scattering effects. Similarly, by controlling the hole-to-hole pitch from 1 µm to 100 nm, our experiments evaluate the phonon backscattering effect. The thermal conductivity measurements are performed on monolithic silicon devices using an established technique that was first developed for nanowires. The measurement temperature is controlled from 325 K to 14 K; these low temperature measurements are crucial because the important phonon wavelength and the phonon mean free path are larger at low temperature, which facilitates observation of potential coherence and backscattering effects. We perform ray tracing simulations to rigorously capture the boundary scattering required for particle model thermal conductivity predictions. By comparing experimental results between samples and against particle model predictions, we show that phonon coherence effects are not necessary to describe thermal transport in the regime where the nanomesh pitch is greater than the phonon wavelength but smaller than the phonon mean free, and that the backscattering effect leads to the thermal conductivity reductions. The insights obtained from this work will be valuable in understanding phonon transport in complicated nanostructured geometries and advancing breakthroughs in silicon nanostructure-based thermoelectric devices

Investigation of phonon coherence and backscattering using silicon nanomeshes

Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.

Tunable thermoelectric transport in nanomeshes via elastic strain engineering

Recent experimental explorations of silicon nanomeshes have shown that the unique metastructures exhibit reduced thermal conductivity while preserving bulk electrical conductivity via feature sizes between relevant phonon and electron mean free paths, aiding in the continued promise that nanometer-scale engineering may further enhance thermoelectric behavior. Here, we introduce a strategy for tuning thermoelectric transport phenomena in semiconductor nanomeshes via heterogeneous elastic strain engineering, using silicon as a model material for demonstration of the concept. By combining analytical models for electron mobility in uniformly stressed silicon with finite element analysis of strained silicon nanomeshes in a lumped physical model, we show that the nonuniform and multiaxial strain fields defined by the nanomesh geometry give rise to spatially varying band shifts and warping, which in aggregate accelerate electron transport along directions of applied stress. This allows for global electrical conductivity and Seebeck enhancements beyond those of homogenous samples under equivalent far-field stresses, ultimately increasing thermoelectric power factor nearly 50% over unstrained samples. The proposed concept and structures—generic to a wide class of materials with large dynamic ranges of elastic strain in nanoscale volumes—may enable a new pathway for active and tunable control of transport properties relevant to waste heat scavenging and thermal management.

An Efficient Photoelectrochemical Hydrogen Evolution System using Silicon Nanomaterials with Ultra‐High Aspect Ratios

AbstractWe fabricated ultra‐high aspect ratio silicon nanomaterials, including a silicon nanomesh and silicon nanowire array, on a wafer scale for efficient photoelectrochemical hydrogen production. These silicon nanomaterials (feature size≈20 nm) possess a high aspect ratio to increase the optical absorptivity of the cells to approximately 95 % over a broad range of wavelengths. The silicon nanomesh and Si nanowire cells achieved high photocurrent values of 13 and 28 mA cm−2, respectively, which are increased by 200 % and 570 % in comparison to their bulk counterparts. In addition, these scalable Si nanomaterials remained stable for up to 100 min of hydrogen evolution. Detailed studies on the doping and geometrical structures of the resulting hydrogen evolution cells suggest that both the n+ pp+ doping and thickness of nanostructures are keys to the enhancement of the hydrogen evolution efficiency. The results obtained in this work show that these silicon nanomaterials can be used for high‐performance water‐splitting system applications.

Thermal conductivity of silicon nanomeshes: Effects of porosity and roughness

We theoretically investigate thermal conductivity in silicon nanomeshes using Monte Carlo simulations of phonon transport. Silicon membranes of 100 nm thickness with randomly located pores of 50 nm diameter are considered. The effects of material porosity and pore surface roughness are examined. Nanomesh porosity is found to have a strong detrimental effect on thermal conductivity. At room temperature, a porosity of 50% results in ∼80% reduction in thermal conductivity. Boundary roughness scattering further degrades thermal conductivity, but its effect is weaker. Thermal transport can additionally be affected by the specific arrangement of the pores along the transport direction.