Individual Si Nanowires Research Articles

Low threshold lasing from silicon Mie resonators

The unique electromagnetic response of high-index dielectric resonators with Mie resonances facilitates the selective enhancement and confinement of light, holding great promise for nanoscale lasing. However, up to now, nanolasing exploiting Mie resonances of individual Si nanowires has been limited to theoretical proposals. Here, we demonstrate a strategy to realize nanolasing with Si nanowires in the SOI platform. It is found that, by carefully tailoring the sizes of the Si nanowires, the Mie resonances of Si nanowires enhance the pumping/emission fields, leading to continuous-wave lasing at room temperature with low threshold in the visible range. The lasing of a dye at the wavelength of 523 nm assisted by the Mie resonances of Si nanowire is demonstrated experimentally. It is shown that the lasing wavelength changes with the width of nanowire due to the different scattering peak from Mie theory. In addition, the quality factor Q of lasing modes can be affected by the thickness of silica shell which determines the interaction volume of the pumping light and dyes. The proposed strategy paves the way towards practical integrated nanolasers for single-photon sources and micro-display, which have potential applications of on-chip optoelectrical devices.

Photoelectrical Properties Investigated on Individual Si Nanowires and Their Size Dependence

Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated with controllable diameters and lengths. Their photoconductive properties are investigated by photoconductive atomic force microscopy (PCAFM) on individual nanowires. The results show that the photocurrent of Si NWs increases significantly with the laser intensity, indicating that Si NWs have good photoconductance and photoresponse capability. This photoenhanced conductance can be attributed to the photoinduced Schottky barrier change, confirmed by I–V curve analyses. On the other hand, electrostatic force microscopy (EFM) results indicate that a large number of photogenerated charges are trapped in Si NWs under laser irradiation, leading to the lowering of barrier height. Moreover, the size dependence of photoconductive properties is studied on Si NWs with different diameters and lengths. It is found that the increasing magnitude of photocurrent with laser intensity is greatly relevant to the nanowires’ diameter and length. Si NWs with smaller diameters and shorter lengths display better photoconductive properties, which agrees well with the size-dependent barrier height variation induced by photogenerated charges. With optimized diameter and length, great photoelectrical properties are achieved on Si NWs. Overall, in this study the photoelectrical properties of individual Si NWs are systematically investigated by PCAFM and EFM, providing important information for the optimization of nanostructures for practical applications.

Thermal conductivity of individual Si and SiGe epitaxially integrated NWs by scanning thermal microscopy.

Semiconductor nanowires have demonstrated fascinating properties with application in a wide range of fields including energy and information technologies. In particular, increasing attention has been focused on Si and SiGe nanowires for application in thermoelectric generation after recent successful implementation in miniaturized devices. Despite this interest, an appropriate evaluation of thermal conductivity in such nanostructures still poses a great challenge, especially if the characterization of the device-integrated nanowire is desired. In this work, a spatially resolved technique based on scanning thermal microscopy has been demonstrated for the assessment of the thermal conductivity of Si and SiGe nanowires integrated in thermoelectrical microgenerators. Thermal conductivity values of 15.8 ± 0.8 W m-1 K-1 and 4.2 ± 0.3 W m-1 K-1 were measured for Si and SiGe nanowires, respectively, epitaxially grown on silicon microstructures. Moreover, the range of applicability according to the sample thermal conductance and associated errors are discussed to establish the potential of the novel technique. The results presented here show the remarkable utility of scanning thermal microscopy for the challenging thermal characterization of integrated nanostructures and the development of multiple devices such as thermoelectric generators or photovoltaic cells.

Novel method for convenient Seebeck coefficient measurements on individual Si nanowires

A novel characterization method is introduced to measure Seebeck coefficients on individual Si nanowires (Si NWs). By using conductive atomic force microscopy with slight modifications, without any need of microfabricated devices, the Seebeck coefficients can be conveniently measured on individual Si NWs inside a large-sized vertical array in addition to the measurements on bulk substrates. The results show that the Seebeck coefficients of Si NWs are much larger than their bulk counterparts over a wide temperature range. The temperature dependence of the Seebeck coefficients of Si NWs is found to be entirely different from that of bulk Si, yet both of them agree well with the results obtained by a macroscopic method. Particularly, with this method, the Seebeck coefficients can be easily characterized on Si NWs with varied morphology, size, or doping. As an example, the Seebeck coefficients are measured on Si NWs with different lengths, and it is found that the Seebeck coefficients decrease obviously as the nanowire length increases for all measurement temperatures but the difference becomes smaller at higher temperatures. Overall, this study provides a simple but effective novel method to measure the Seebeck coefficients on individual Si NWs, which has unique advantages on exploring the size or other parameter dependence or revealing the dominant factors in thermoelectric property studies on single nanostructures.

Si Nanowire With Integrated Space-Charge- Limited Conducted Schottky Junction for Enhancing Field Electron Emission and Its Gated Devices

Integrated field emission electron sources with low driving voltage are desired for micro/nano vacuum electronics. Herein, we demonstrate the enhanced and stable electron emission from crystalline gold-silver (Au-Ag) alloy nanoparticle capped Si nanowires (SiNWs) and the achieved gated electron sources with low driving voltage (<; 50 V). Thermally induced self-assembly hemispherical (50-80 nm) crystalline Au-Ag nanoparticles were prepared and used as masks in plasma etching to form vertically aligned high aspect ratio (~14) SiNWs. The nanoparticles were preserved after the etching, forming in situ integrated Au-Ag/Si junctions with each individual SiNWs. The SiNWs with capped nanoparticles possess superior field emission current density (45.80 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and current stability (fluctuation ~6%). A self-aligned process was developed to fabricate gated field emission devices, using the Au-Ag nanoparticle capped SiNWs as cathode. The gated devices showed field emission at low gate voltage (69.52-μA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 42.1 V). Experimental and numerical simulation studies demonstrated the Au-Ag/Si contact behaved as space-charge-limited (SCL) Schottky conduction. The idea behind the enhancing and stable emission is that the SCL-Schottky junction undergoes a process of negative feedback and thus suppressed the current and shot-noise from the outstanding SiNWs. Accordingly, more SiNWs were activated and contributed stable and higher field emission current.

Enhanced thermoelectric figure of merit of individual Si nanowires with ultralow contact resistances

Low-dimensional silicon-based materials have shown a great potential for thermoelectric applications due to their enhanced figure of merit ZT and high technology compatibility. However, their implementation in real devices remains highly challenging due to the associated large contact resistances (thermal and electrical). Herein we demonstrate ultralow contact resistance silicon nanowires epitaxially grown on scalable devices with enhanced ZT. Temperature dependent figure of merit was fully determined for monolithically integrated individual silicon nanowires achieving a maximum value of ZT = 0.2 at 620 K. Sidewise, this work accounts for the first time nearly zero thermal and electrical contact resistances in monolithically integrated bottom-up nanowires.

Thermal and kinetic analyses of silicide formation at nanostructured Si/Ni interface

The problem of the manifestation of nanostructures properties that are not typical for similar macrosystems is still relevant. First of all, this concerns the stability of the thermodynamic properties of nanomaterials, which determines the operational reliability of devices based on them. In this paper, the influence of the nanostructured Si/Ni interface on the mechanism of silicide formation is studied. For this purpose, nickel/porous silicon nanocomposites have been electrochemically synthesized and analyzed using the scanning electron microscopy and the phase X-ray diffraction analysis. The solid-state interaction during heating the nanocomposites has been investigated using differential scanning calorimetry. The evolution of phase composition in the temperature range 25–500 °C has been studied. The temperature characteristics of silicide formation processes have been estimated. Kinetic analysis of the solid-state interaction has been performed using the isoconversional Ozawa–Flynn–Wall method. The dependence of effective activation energy on conversion has been calculated from the calorimetric data. The mechanism of silicide formation in the nickel/porous silicon nanocomposite is shown to be significantly different from the bulk Ni–Si diffusion couple. At the same time, it has similarities with, but also some differences from, the silicide formation in thin-film Ni–Si systems and individual Si nanowires. Methodical aspects of the characterization of the thermal behavior of nanocomposites taking into account the nanorelief of the Si–Ni interphase boundary are discussed.

Contactless Electrical and Structural Characterization of Semiconductor Nanowires with Axially Modulated Doping Profiles.

Efficient characterization of semiconductor nanowires having complex dopant profiles or heterostructures is critical to fully understand these materials and the devices built from them. Existing electrical characterization techniques are slow and laborious, particularly for multisegment nanowires, and impede the statistical understanding of highly variable samples. Here, it is shown that electro-orientation spectroscopy (EOS)-a high-throughput, noncontact method for statistically characterizing the electrical properties of entire nanowire ensembles-can determine the conductivity and dimensions of two distinct segments in individual Si nanowires with axially encoded dopant profiles. This analysis combines experimental measurements and computational simulations to determine the electrical conductivity of the nominally undoped segment of two-segment Si nanowires, as well as the ratio of the segment lengths. The efficacy of this approach is demonstrated by comparing results generated by EOS with conventional four-point-probe measurements. This work provides new insights into the control and variability of semiconductor nanowires for electronic applications and is a critical first step toward the high-throughput interrogation of complete nanowire-based devices.

The high pressure phase transformation behavior of silicon nanowires

Si nanowires of 80–150 nm and 200–250 nm diameter are pressurized up to 22 GPa using a diamond anvil cell. Raman and x-ray diffraction data were collected during both compression and decompression. Electron microscopy images reveal that the nanowires retain a nanowire-like morphology (after high pressure treatment). On compression, dc-Si was observed to persist at pressures up to 19 GPa compared to ∼11 GPa for bulk-Si. On decompression, the metallic β-Sn phase was found to be more stable for Si nanowires compared with bulk-Si when lowering the pressure and was observed as low as 6 GPa. For the smallest nanowires studied (80–150 nm), predominately a-Si was obtained on decompression, whereas for larger nanowires (200–250 nm), clear evidence for the r8/bc8-Si phase was obtained. We suggest that the small volume of the individual Si nanowires compared with bulk-Si inhibits the nucleation of the r8-Si phase on decompression. This study shows that there is a size dependence in the high pressure behavior of Si nanowires during both compression and decompression.

Impact of pore anisotropy on the thermal conductivity of porous Si nanowires

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m−1·K−1 for 90 nm diameter wires with 35–40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium-Storage Properties.

Crystalline silicon(Si)/germanium(Ge) alloy nanotubes and hollow particles are synthesized for the first time through a one-pot electrolytic process. The morphology of these alloy structures can be easily tailored from nanotubes to hollow particles by varying the overpotential during the electro-reduction reaction. The continuous solid diffusion governed by the nanoscale Kirkendall effect results in the formation of inner void in the alloy particles. Benefitting from the compositional and structural advantages, these SiGe alloy nanotubes exhibit much enhanced lithium-storage performance compared with the individual solid Si and Ge nanowires as the anode material for lithium-ion batteries.

Electrolytic Formation of Crystalline Silicon/Germanium Alloy Nanotubes and Hollow Particles with Enhanced Lithium‐Storage Properties

AbstractCrystalline silicon(Si)/germanium(Ge) alloy nanotubes and hollow particles are synthesized for the first time through a one‐pot electrolytic process. The morphology of these alloy structures can be easily tailored from nanotubes to hollow particles by varying the overpotential during the electro‐reduction reaction. The continuous solid diffusion governed by the nanoscale Kirkendall effect results in the formation of inner void in the alloy particles. Benefitting from the compositional and structural advantages, these SiGe alloy nanotubes exhibit much enhanced lithium‐storage performance compared with the individual solid Si and Ge nanowires as the anode material for lithium‐ion batteries.

Imaging Spatial Variations in the Dissipation and Transport of Thermal Energy within Individual Silicon Nanowires Using Ultrafast Microscopy.

Thermal management is an important consideration for most nanoelectronic devices, and an understanding of the thermal conductivity of individual device components is critical for the design of thermally efficient systems. However, it can be difficult to directly probe local changes in thermal conductivity within a nanoscale system. Here, we utilize the time-resolved and diffraction-limited imaging capabilities of ultrafast pump-probe microscopy to determine, in a contact-free configuration, the local thermal conductivity in individual Si nanowires (NWs). By suspending single NWs across microfabricated trenches in a quartz substrate, the properties of the same NW both on and off the substrate are directly compared. We find the substrate has no effect on the recombination lifetime or diffusion length of photogenerated charge carriers; however, it significantly impacts the thermal relaxation properties of the NW. In substrate-supported regions, thermal energy deposited into the lattice by the ultrafast laser pulse dissipates within ∼10 ns through thermal diffusion and coupling to the substrate. In suspended regions, the thermal energy persists for over 100 ns, and we directly image the time-resolved spatial motion of the thermal signal. Quantitative analysis of the transient images permits direct determination of the NW's local thermal conductivity, which we find to be a factor of ∼4 smaller than in bulk Si. Our results point to the strong potential of pump-probe microscopy to be used as an all-optical method to quantify the effects of localized environment and morphology on the thermal transport characteristics of individual nanostructured components.

Percolating silicon nanowire networks with highly reproducible electrical properties

Here, we report the morphological and electrical properties of self-assembled silicon nanowires networks, also called Si nanonets. At the macroscopic scale, the nanonets involve several millions of nanowires. So, the observed properties should result from large scale statistical averaging, minimizing thus the discrepancies that occur from one nanowire to another. Using a standard filtration procedure, the so-obtained Si nanonets are highly reproducible in terms of their morphology, with a Si nanowire density precisely controlled during the nanonet elaboration. In contrast to individual Si nanowires, the electrical properties of Si nanonets are highly consistent, as demonstrated here by the similar electrical properties obtained in hundreds of Si nanonet-based devices. The evolution of the Si nanonet conductance with Si nanowire density demonstrates that Si nanonets behave like standard percolating media despite the presence of numerous nanowire-nanowire intersecting junctions into the nanonets and the native oxide shell surrounding the Si nanowires. Moreover, when silicon oxidation is prevented or controlled, the electrical properties of Si nanonets are stable over many months. As a consequence, Si nanowire-based nanonets constitute a promising flexible material with stable and reproducible electrical properties at the macroscopic scale while being composed of nanoscale components, which confirms the Si nanonet potential for a wide range of applications including flexible electronic, sensing and photovoltaic applications.

The Role of Elasto-Plasticity on Phase Kinetics in Lithiated Silicon Nanowires: Phase Field Simulation

Silicon (Si) is a promising anode material for Li-ion batteries, since it absorbs large amounts of Li. In addition to volumetric expansion, huge stresses, and fracture, recent experiments reveal that the kinetics of Li insertion and extraction, which critically control the rates of charging and discharging, are of great importance for the high performance of LIBs. Here, we first propose a phase field model coupled to elasto-plasticity to study the lithiation kinetics in individual Si nanowires. In contrast to other models, the proposed phase field model is thermodynamically consistent, and the corresponding thermodynamic and kinetics parameters can be obtained from the experiments or Density function theory (DFT) calculations. The developed model describes well the experimental known self-limiting lithiation phenomenon, which is attributed to the retardation effect of the lithiation-induced stress. Further, the proposed model has general character and is applicable to any electrode systems.

Colossal injection of catalyst atoms into silicon nanowires

The incorporation of impurities during the growth of nanowires from the vapour phase alters their basic properties substantially, and this process is critical in an extended range of emerging nanometre-scale technologies. In particular, achieving precise control of the behaviour of group III and group V dopants has been a crucial step in the development of silicon (Si) nanowire-based devices. Recently it has been demonstrated that the use of aluminium (Al) as a growth catalyst, instead of the usual gold, also yields an effective p-type doping, thereby enabling a novel and efficient route to functionalizing Si nanowires. Besides the technological implications, this self-doping implies the detachment of Al from the catalyst and its injection into the growing nanowire, involving atomic-scale processes that are crucial for the fundamental understanding of the catalytic assembly of nanowires. Here we present an atomic-level, quantitative study of this phenomenon of catalyst dissolution by three-dimensional atom-by-atom mapping of individual Al-catalysed Si nanowires using highly focused ultraviolet-laser-assisted atom-probe tomography. Although the observed incorporation of the catalyst atoms into nanowires exceeds by orders of magnitude the equilibrium solid solubility and solid-solution concentrations in known non-equilibrium processes, the Al impurities are found to be homogeneously distributed in the nanowire and do not form precipitates or clusters. As well as the anticipated effect on the electrical properties, this kinetics-driven colossal injection also has direct implications for nanowire morphology. We discuss the observed strong deviation from equilibrium using a model of solute trapping at step edges, and identify the key growth parameters behind this phenomenon on the basis of a kinetic model of step-flow growth of nanowires. The control of this phenomenon provides opportunities to create a new class of nanoscale devices by precisely tailoring the shape and composition of metal-catalysed nanowires.

Self-Limiting Lithiation in Silicon Nanowires

The rates of charging and discharging in lithium-ion batteries (LIBs) are critically controlled by the kinetics of Li insertion and extraction in solid-state electrodes. Silicon is being intensively studied as a high-capacity anode material for LIBs. However, the kinetics of Li reaction and diffusion in Si remain unclear. Here we report a combined experimental and theoretical study of the lithiation kinetics in individual Si nanowires. By using in situ transmission electron microscopy, we measure the rate of growth of a surface layer of amorphous Li(x)Si in crystalline Si nanowires during the first lithiation. The results show the self-limiting lithiation, which is attributed to the retardation effect of the lithiation-induced stress. Our work provides a direct measurement of the nanoscale growth kinetics in lithiated Si, and has implications on nanostructures for achieving the high capacity and high rate in the development of high performance LIBs.

Toward Local Growth of Individual Nanowires on Three-Dimensional Microstructures by Using a Minimally Invasive Catalyst Templating Method

We present a novel minimally invasive postprocessing method for catalyst templating based on focused charged particle beam structuring, which enables a localized vapor-liquid-solid (VLS) growth of individual nanowires on prefabricated three-dimensional micro- and nanostructures. Gas-assisted focused electron beam induced deposition (FEBID) was used to deposit a SiO(x) surface layer of about 10 × 10 μm(2) on top of a silicon atomic force microscopy cantilever. Gallium focused ion beam (FIB) milling was used to make a hole through the SiO(x) layer into the underlying silicon. The hole was locally filled with a gold catalyst via FEBID using either Me(2)Au(tfac) or Me(2)Au(acac) as precursor. Subsequent chemical vapor deposition (CVD)-induced VLS growth using a mixture of SiH(4) and Ar resulted in individual high quality crystalline nanowires. The process, its yield, and the resulting angular distribution/crystal orientation of the silicon nanowires are discussed. The presented combined FIB/FEBID/CVD-VLS process is currently the only proven method that enables the growth of individual monocrystalline Si nanowires on prestructured substrates and devices.

Scanning confocal Raman spectroscopy of silicon phase distribution in individual Si nanowires

AbstractSilicon nanowires were grown by gold enhanced CVD self‐assembly technique. In the Raman spectra of nanowires, along with phonon band of cubic diamond phase (Si I) at near 521 cm‐1, additional phonon bands at 516 and 496 cm‐1 related to hexagonal wurtzite phase (Si IV) were observed. Spatial distribution of intensities of these Raman bands around single nanowire reveals that both phases coexist along the wire length, even if with variable contributions. In addition, there is a strong alternating shift of Si I phonon band between the two ends of the nanowire (the one end is tipped with Au and the other end is the one that was connected to the substrate). Additionally, reversibility of Si IV phase transformation during laser heating at temperatures about 100 ºC was found. The intensity of Si IV band strongly depends on exciting power in the range of 0.05 to 1 mW/µm2. The observed phenomena are discussed in relation with peculiarities of nanowire growth. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Three‐Dimensional Carrier Profiling of Individual Si Nanowires by Scanning Spreading Resistance Microscopy

Three-dimensional profiling of nanowires is carried out by repeatedly scanning the same nanowire with controlled force of the probing tip. This method abrades material from the measured cross section during each scan, allowing three-dimensional profiling. The typical shape of an individual Si nanowire, measured by SEM, and the respective resistivity images of several cross sections obtained by scanning spreading resistance microscopy are shown.