Suspended Silicon Nanowires Research Articles

TEM-compatible microdevice for the complete thermoelectric characterization of epitaxially integrated Si-based nanowires.

Nanostructured materials present improved thermoelectric properties due to non-trivial effects at the nanoscale. However, the characterization of individual nanostructures, especially from the thermal point of view, is still an unsolved topic. This work presents the complete structural, morphological, and thermoelectrical evaluation of the selfsame individual bottom-up integrated nanowire employing an innovative micro-machined device compatible with transmission electron microscopy whose fabrication is also discussed. Thanks to a design that arranges the nanostructured samples completely suspended, detailed structural analysis using transmission electron microscopy is enabled. In the same device architecture, electrical collectors and isolated heaters are available at both ends of the trenches for thermoelectrical measurements of the nanowire i.e. thermal and electrical properties simultaneously. This allows the direct measurement of the nanowire power factor. Furthermore, micro-Raman thermometry measurements were performed to evaluate the thermal conductivity of the same suspended silicon nanowire. A thermal profile of the self-heating nanowire could be spatially resolved and used to compute the thermal conductivity. In this work, heavily-doped silicon nanowires were grown on this microdevices yielding a thermal conductivity of 30.8 ± 1.7 W Km-1 and a power factor of 2.8 mW mK-2 at an average nanowire temperature of 400 K. Notably, no thermal contact resistance was observed between the nanowire and the bulk, confirming the epitaxial attachment. The device presented here shows remarkable utility in the challenging thermoelectrical characterization of integrated nanostructures and in the development of multiple devices such as thermoelectric generators.

Local Heat Dissipation and Elasticity of Suspended Silicon Nanowires Revealed by Dual Scanning Electron and Thermal Microscopies.

A novel combined setup, with a scanning thermal microscope (SThM) embedded in a scanning electron microscope (SEM), is used to characterize a suspended silicon rough nanowire (NW), which is epitaxially clamped at both sides and therefore monolithically integrated in a microfabricated device. The rough nature of the NW surface, which prohibits vacuum-SThM due to loose contact for heat dissipation, is circumvented by decorating the NW with periodic platinum dots. Reproducible approaches over these dots, enabled by the live feedback image provided by the SEM, yield a strong improvement in thermal contact resistance and a higher accuracy in its estimation. The results-thermal resistance at the tip-sample contact of 188±3.7K µW-1 and thermal conductivity of the NW of 13.7±1.6W m-1 K-1-are obtained by performing a series of approach curves on the dots. Noteworthy, the technique allows measuring elastic properties at the same time-the moment of inertia of the NW is found to be (6.1±1.0) × 10-30m4-which permits to correlate the respective effects of the rough shell on heat dissipation and on the NW stiffness. The work highlights the capabilities of the dual SThM/SEM instrument, in particular the interest of systematic approach curves with well-positioned and monitored tip motion.

All-Optical Nanosensor for Displacement Detection in Mechanical Applications.

In this paper, we propose the design of an optical system based on two parallel suspended silicon nanowires that support a range of optical resonances that efficiently confine and scatter light in the infrared range as the base of an all-optical displacement sensor. The effects of the variation of the distance between the nanowires are analyzed. The simulation models are designed by COMSOL Multiphysics software, which is based on the finite element method. The diameter of the nanocylinders (d = 140 nm) was previously optimized to achieve resonances at the operating wavelengths (λ = 1064 nm and 1310 nm). The results pointed out that a detectable change in their resonant behavior and optical interaction was achieved. The proposed design aims to use a simple light source using a commercial diode laser and simplify the readout systems with a high sensitivity of 1.1 × 106 V/m2 and 1.14 × 106 V/m2 at 1064 nm and 1310 nm, respectively. The results may provide an opportunity to investigate alternative designs of displacement sensors from an all-optical approach and explore their potential use.

Plasmonic characteristics of suspended graphene-coated wedge porous silicon nanowires with Ag partition* *Project supported by the National Natural Science Foundation of China (Grant No. 61627818), the Key Project of Henan Provincial Education Department, China (Grant No. 19A510002), the Natural Science Project of the Cultivation Foundation of Henan Provincial Normal University, China (Grant No. 2017PL04), and the Ph. D. Program of

We investigate a graphene-coated nanowire waveguide (GCNW) composed of two suspended wedge porous silicon nanowires and a thin Ag partition. The plasmonic characteristics of the proposed structure in terahertz (THz) frequency band are simulated by the finite element method (FEM). The parameters including the gap between the nanowires and Ag partition, the height of the nanowire, the thickness of the Ag partition, and the Fermi level of graphene, are optimized. The simulation results show that a normalized mode field area of ∼ 10−4 and a figure of merit of ∼ 100 can be achieved. Compared with the cylindrical GCNW and isolated GCNW, the proposed wedge GCNW has good electric field enhancement. A waveguide sensitivity of 32.28 is obtained, which indicates the prospects of application in refractive index (RI) sensing in THz frequency band. Due to the adjustable plasmonic characteristics by changing the Fermi level (E F), the proposed structure has promising applications in the electro-optic modulations, optical interconnects, and optical switches.

Joule-Heated and Suspended Silicon Nanowire Based Sensor for Low-Power and Stable Hydrogen Detection.

We developed self-heated, suspended, and palladium-decorated silicon nanowires (Pd-SiNWs) for high-performance hydrogen (H2) gas sensing with low power consumption and high stability against diverse environmental noises. To prepare the Pd-SiNWs, SiNWs were fabricated by conventional complementary metal-oxide-semiconductor (CMOS) processes, and Pd nanoparticles were coated on the SiNWs by a physical vapor deposition method. Suspended Pd-SiNWs were simply obtained by etching buried oxide layer and Pd deposition. Joule heating of Pd-SiNW (<1 mW) enables the detection of H2 gas with a faster response and without the reduction of sensitivity unlike other Pd-based H2 gas sensors. We proposed a H2 sensing model using oxygen adsorption on the Pd nanoparticle-coated silicon oxide surface to understand the H2 response of Joule-heated Pd-SiNWs. A suspended Pd-SiNW showed a similar transient sensing response with around four times lower Joule heating power (147 μW) than the substrate-bound Pd-SiNW (613 μW). The effect of interfering gas on the Pd-SiNW was investigated, and it was found that the Joule heating of Pd-SiNW helps to maintain the H2 sensing performance in humid or carbon monoxide environments.

Direct and Fourier space travelling: multi-dimensional mapping of lattice strain and tilt of a suspended silicon nanowire in a monolithic system

Direct and Fourier space travelling: multi-dimensional mapping of lattice strain and tilt of a suspended silicon nanowire in a monolithic system

Sanitization of Data in Nanoscale Flash Memory by Thermal Erasing and Reuse of Storage

This study, a thermal method for erasing and permanently destroying data stored in flash memory fabricated on a suspended silicon nanowire is demonstrated. An intentionally applied heat treatment is used to erase the data stored in the charge trap layer of the flash memory. The data destruction is verified and analyzed at a unit cell level as well as in a commercial off‐the‐shelf chip. Characteristics of memory performance and reliability are also investigated. Then, the feasibility of the proposed method is further evaluated for next generation 3‐dimensional V‐NAND.

Arrays of suspended silicon nanowires defined by ion beam implantation: mechanical coupling and combination with CMOS technology

We present the fabrication, operation, and CMOS integration of arrays of suspended silicon nanowires (SiNWs). The functional structures are obtained by a top–down fabrication approach consisting in a resistless process based on focused ion beam irradiation, causing local gallium implantation and silicon amorphization, plus selective silicon etching by tetramethylammonium hydroxide, and a thermal annealing process in a boron rich atmosphere. The last step enables the electrical functionality of the irradiated material. Doubly clamped silicon beams are fabricated by this method. The electrical readout of their mechanical response can be addressed by a frequency down-mixing detection technique thanks to an enhanced piezoresistive transduction mechanism. Three specific aspects are discussed: (i) the engineering of mechanically coupled SiNWs, by making use of the nanometer scale overhang that it is inherently-generated with this fabrication process, (ii) the statistical distribution of patterned lateral dimensions when fabricating large arrays of identical devices, and (iii) the compatibility of the patterning methodology with CMOS circuits. Our results suggest that the application of this method to the integration of large arrays of suspended SiNWs with CMOS circuitry is interesting in view of applications such as advanced radio frequency band pass filters and ultra-high-sensitivity mass sensors.

Heavily Boron-Doped Silicon Layer for the Fabrication of Nanoscale Thermoelectric Devices.

Heavily boron-doped silicon layers and boron etch-stop techniques have been widely used in the fabrication of microelectromechanical systems (MEMS). This paper provides an introduction to the fabrication process of nanoscale silicon thermoelectric devices. Low-dimensional structures such as silicon nanowire (SiNW) have been considered as a promising alternative for thermoelectric applications in order to achieve a higher thermoelectric figure of merit (ZT) than bulk silicon. Here, heavily boron-doped silicon layers and boron etch-stop processes for the fabrication of suspended SiNWs will be discussed in detail, including boron diffusion, electron beam lithography, inductively coupled plasma (ICP) etching and tetramethylammonium hydroxide (TMAH) etch-stop processes. A 7 μm long nanowire structure with a height of 280 nm and a width of 55 nm was achieved, indicating that the proposed technique is useful for nanoscale fabrication. Furthermore, a SiNW thermoelectric device has also been demonstrated, and its performance shows an obvious reduction in thermal conductivity.

Strain and thermal conductivity in ultrathin suspended silicon nanowires

We report on the uniaxial strain and thermal conductivity of well-ordered, suspended silicon nanowire arrays between 10 to 20 nm width and 22 nm half-pitch, fabricated by extreme-ultraviolet (UV) interference lithography. Laser-power-dependent Raman spectroscopy showed that nanowires connected monolithically to the bulk had a consistent strain of $\ensuremath{\sim}0.1%$, whereas nanowires clamped by metal exhibited variability and high strain of up to 2.3%, having implications in strain engineering of nanowires. The thermal conductivity at room temperature was measured to be $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{m}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for smooth nanowires and $\ensuremath{\sim}0.1\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{m}\phantom{\rule{4pt}{0ex}}\mathrm{K}$ for rougher ones, similar to results by other investigators. We found no modification of the bulk properties in terms of intrinsic scattering, and therefore, the decrease in thermal conductivity is mainly due to boundary scattering. Different types of surface roughness, such as constrictions and line-edge roughness, may play roles in the scattering of phonons of different wavelengths. Such low thermal conductivities would allow for very efficient thermal energy harvesting, approaching and passing values achieved by state-of-the-art thermoelectric materials.

Demonstration of highly efficient forward stimulated Brillouin scattering in partly suspended silicon nanowire racetrack resonators

We demonstrate the forward stimulated Brillouin scattering (FSBS) in a partly suspended silicon nanowire racetrack resonator. To realize the tight confinement of the transverse acoustic modes in the nanoscale silicon core, the racetrack resonator is supported by the tiny pillar. The Brillouin amplification of 2.25 dB is achieved with the resonator radius of 100 μm under a low-power pump laser of 8 mW. The influences of the waveguide width and the top width of the tiny pillar on the Brillouin frequency shift and Brillouin gain are presented and analyzed. The Brillouin frequency shift is conveniently manipulated by the changes in waveguide widths. Our proposed approach furnishes an alternative towards harnessing FSBS in integrated photonic circuits.

Stimulated Brillouin Scattering in Photonic Integrated Circuits: Novel Applications and Devices

The last few years have seen major progress in harnessing on-chip photon-phonon interactions, leading to a wide range of demonstrations of new functionalities. Utilizing not only the optical response of a nonlinear waveguide-but also acoustic resonances-enables the realization of microwave devices with unprecedented performance, otherwise hard to achieve in all-optical processing schemes or electronically. Here, we overview on-chip stimulated Brillouin scattering (SBS) with special emphasis on microwave sources and microwave signal processing schemes. We review the different material platforms and structures for on-chip SBS, ranging from chalcogenide rib waveguides to hybrid silicon/silicon-nitride structures, high-Q photonic-phononic silica microresonators, and suspended silicon nanowires. We show that the paradigm shift in SBS research-from long length of fibers to chip-scale devices-is now moving toward fully integrated photonic-phononic CMOS chips.

Stimulated Brillouin scattering in silicon/chalcogenide slot waveguides.

We study slot waveguide geometries, comprising a combination of soft glasses and high-index guiding structures, for enhancing stimulated Brillouin scattering (SBS). We show that strong optical and acoustic mode confinement in these waveguides can lead to a substantial increase in SBS gain, comparable to or greater than recently proposed suspended silicon nanowire structures. We compute the optimal parameters of the structure and examine the physics of optical and acoustic confinement within slot waveguides. Finally, we compute the effects of linear and nonlinear loss mechanisms on optimum pump/Stokes powers and waveguide lengths.

Resonant tunnelling features in a suspended silicon nanowire single-hole transistor

Suspended silicon nanowires have significant potential for a broad spectrum of device applications. A suspended p-type Si nanowire incorporating Si nanocrystal quantum dots has been used to form a single-hole transistor. Transistor fabrication uses a novel and rapid process, based on focused gallium ion beam exposure and anisotropic wet etching, generating &amp;lt;10 nm nanocrystals inside suspended Si nanowires. Electrical characteristics at 10 K show Coulomb diamonds with charging energy ∼27 meV, associated with a single dominant nanocrystal. Resonant tunnelling features with energy spacing ∼10 meV are observed, parallel to both diamond edges. These may be associated either with excited states or hole–acoustic phonon interactions, in the nanocrystal. In the latter case, the energy spacing corresponds well with reported Raman spectroscopy results and phonon spectra calculations.

Net on-chip Brillouin gain based on suspended silicon nanowires

The century-old study of photon–phonon coupling has seen a remarkable revival in the past decade. Driven by early observations of dynamical back-action, the field progressed to ground-state cooling and the counting of individual phonons. A recent branch investigates the potential of traveling-wave, optically broadband photon–phonon interaction in silicon circuits. Here, we report continuous-wave Brillouin gain exceeding the optical losses in a series of suspended silicon beams, a step towards selective on-chip amplifiers. We obtain efficiencies up to the highest to date in the phononic gigahertz range. We also find indications that geometric disorder poses a significant challenge towards nanoscale phonon-based technologies.

Experimental Investigations on New Characterization Method for Giant Piezoresistance Effect and Silicon Nanowire Piezoresistive Detection

This paper presents a novel and effective characterization method for giant piezoresistive properties of silicon nanowires by using the reference structures. This contrast detection approach investigates the influences of quantum size effect and surface defects effect on piezoresistive coefficients of silicon nanowires by direct comparison of the resistivity change ratio of silicon wires with nanoscale-to-microscale width under the same applied stress conditions. The characterization experiments based on four-point bending tensile test demonstrate that piezoresistive coefficient of small nanowidth silicon nanowire can be significantly increased to about five times higher levels than that of bulk silicon under the same impurity concentration, which indicates that the silicon nanowire can have giant piezoresistive effect. On the other hand, to solve the problem on nanowires pick-up, we proposed a nanowire piezoresistive detection approach, whose validity is confirmed by the dynamic LDV resonance test. Meanwhile, to investigate the influence of undercut arising from the wet chemical release process of the suspended silicon nanowire, a three-dimensional finite element simulation is also carried out for the fundamental resonant frequency using ANSYS software. The numerical and experimental results show that our piezoresistive detection is accurate and effective and the undercut should be carefully considered in the design of the high frequency resonator and mixer. The findings of this paper provide some useful references for the piezoresistive effect measurement and the piezoresistive pick-up in nanoelectromechanical system.

Fabrication and characterisation of suspended narrow silicon nanowire channels for low-power nano-electro-mechanical (NEM) switch applications

Suspended silicon nanowires with narrow (∼10nm) conduction channel are fabricated and characterised for further development of low power nano-electro-mechanical (NEM) switching devices using CMOS compatible fabrication. Double suspension fabrication process using an amorphous silicon sacrificial layer and xenon difluoride etching is employed for thermally-oxidised suspended Si nanowire channels. Device current–voltage characteristics demonstrate depletion mode operation of heavy doped nanowires with an on/off ratio of 105 and a threshold voltage of −1.8V. In plane electromechanical pull-in to side gate is demonstrated and confirmed to be consistent with finite element analysis.

Modelling of the Non-linearity and Hysteresis of Silicon Nanowire Electrical Response for Large Self-heating

In this paper, we study by numerical and graphical modeling the electro-thermal behavior of heavily p-doped (NA=5×1019cm-3) suspended silicon nanowires for large self-heating. Computed results are correlated with experimental ones for top-down fabricated nanowires. The computed temperature rise along a Si nanowire due to Joule effect exhibits a complex nonlinearity for large biasing current when the thermal dependence of silicon nanowire properties is considered. The agreement with experimental results is improved by taking into account thermal conductivity variation resulting from size effects. The proposed model also enables a graphical interpretation of the hysteretic behavior of the silicon nanowire self-heating. Complementary phenomena and possible application are eventually discussed.

Ammonia Sensors Based on Suspended Silicon Nanowires

Resistors based on parallel suspended polycrystalline silicon nanowires are fabricated using the classical top down CMOS silicon technologies. Results show potential use as sensitive units of silicon nanowires for charged chemical species (ammonia) detection. Resistors are promising as low-cost manufacturing gas sensors, for very high sensitive ammonia detection over a large concentration range.

High frequency top-down junction-less silicon nanowire resonators

We report here the first realization of top-down silicon nanowires (SiNW) transduced by both junction-less field-effect transistor (FET) and the piezoresistive (PZR) effect. The suspended SiNWs are among the smallest top-down SiNWs reported to date, featuring widths down to ∼20 nm. This has been achieved thanks to a 200 mm-wafer-scale, VLSI process fully amenable to monolithic CMOS co-integration. Thanks to the very small dimensions, the conductance of the silicon nanowire can be controlled by a nearby electrostatic gate. Both the junction-less FET and the previously demonstrated PZR transduction have been performed with the same SiNW. These self-transducing schemes have shown similar signal-to-background ratios, and the PZR transduction has exhibited a relatively higher output signal. Allan deviation (σA) of the same SiNW has been measured with both schemes, and we obtain σA ∼ 20 ppm for the FET detection and σA ∼ 3 ppm for the PZR detection at room temperature and low pressure. Orders of magnitude improvements are expected from tighter electrostatic control via changes in geometry and doping level, as well as from CMOS integration. The compact, simple topology of these elementary SiNW resonators opens up new paths towards ultra-dense arrays for gas and mass sensing, time keeping or logic switching systems on the SiNW–CMOS platform.