Single Si Nanowires Research Articles

Nanograin network memory with reconfigurable percolation paths for synaptic interactions

The development of memory devices with functions that simultaneously process and store data is required for efficient computation. To achieve this, artificial synaptic devices have been proposed because they can construct hybrid networks with biological neurons and perform neuromorphic computation. However, irreversible aging of these electrical devices causes unavoidable performance degradation. Although several photonic approaches to controlling currents have been suggested, suppression of current levels and switching of analog conductance in a simple photonic manner remain challenging. Here, we demonstrated a nanograin network memory using reconfigurable percolation paths in a single Si nanowire with solid core/porous shell and pure solid core segments. The electrical and photonic control of current percolation paths enabled the analog and reversible adjustment of the persistent current level, exhibiting memory behavior and current suppression in this single nanowire device. In addition, the synaptic behaviors of memory and erasure were demonstrated through potentiation and habituation processes. Photonic habituation was achieved using laser illumination on the porous nanowire shell, with a linear decrease in the postsynaptic current. Furthermore, synaptic elimination was emulated using two adjacent devices interconnected on a single nanowire. Therefore, electrical and photonic reconfiguration of the conductive paths in Si nanograin networks will pave the way for next-generation nanodevice technologies.

Water splitting with silicon p-i-n superlattices suspended in solution.

Photoelectrochemical (PEC) water splitting to produce hydrogen fuel was first reported 50 years ago1, yet artificial photosynthesis has not become a widespread technology. Although planar Si solar cells have become a ubiquitous electrical energy source economically competitive with fossil fuels, analogous PEC devices have not been realized, and standard Si p-type/n-type (p-n) junctions cannot be used for water splitting because the bandgap precludes the generation of the needed photovoltage. An alternative paradigm, the particle suspension reactor (PSR), forgoes the rigid design in favour of individual PEC particles suspended in solution, a potentially low-cost option compared with planar systems2,3. Here we report Si-based PSRs by synthesizing high-photovoltage multijunction Si nanowires (SiNWs) that are co-functionalized to catalytically split water. By encoding a p-type-intrinsic-n-type (p-i-n) superlattice within single SiNWs, tunable photovoltages exceeding 10 V were observed under 1 sun illumination. Spatioselective photoelectrodeposition of oxygen and hydrogen evolution co-catalysts enabled water splitting at infrared wavelengths up to approximately 1,050 nm, with the efficiency and spectral dependence of hydrogen generation dictated by thephotonic characteristics of the sub-wavelength-diameter SiNWs. Although initial energy conversion efficiencies are low, multijunction SiNWs bring the photonic advantages of a tunable, mesoscale geometry and the material advantages of Si-including the small bandgap and economies of scale-to the PSR design, providing a new approach for water-splitting reactors.

Low-noise, high-detectivity, polarization-sensitive, room-temperature infrared photodetectors based on Ge quantum dot-decorated Si-on-insulator nanowire field-effect transistors

A CMOS-compatible infrared (IR; 1200–1700 nm) detector based on Ge quantum dots (QDs) decorated on a single Si-nanowire channel on a silicon-on-insulator (SOI) platform with a superior detectivity at room temperature is presented. The spectral response of a single nanowire device measured in a back-gated field-effect transistor geometry displays a very high value of peak detectivity ∼9.33 × 1011 Jones at ∼1500 nm with a relatively low dark current (∼20 pA), which is attributed to the fully depleted Si nanowire channel on SOI substrates. The noise power spectrum of the devices exhibits a with the exponent, γ showing two different values of 0.9 and 1.8 owing to mobility fluctuations and generation-recombination of carriers, respectively. Ge QD-decorated nanowire devices exhibit a novel polarization anisotropy with a remarkably high photoconductive gain of ∼104. The superior performance of a Ge QDs/Si nanowire phototransistor in IR wavelengths is potentially attractive to integrate electro-optical devices into Si for on-chip optical communications.

Silicon Nanowire on Mirror Nanoantennas: Engineering Hybrid Gap Mode for Light Sources and Sensing Platforms

It has been demonstrated recently that a nanowire (NW) of high refractive index dielectric materials works as a nanoantenna because of the excitation of the Mie-type resonances. Here, we explore the capability of a silicon (Si) NW as an optical nanoantenna by combining it with an external metallic component. We investigate the light scattering property of a single Si NW placed on a gold (Au) mirror via a very thin dielectric spacer and demonstrate strong confinement of electromagnetic fields in the gap because of the coupling of the resonance modes with a Au mirror. We demonstrate that the resonance wavelength of the hybrid mode can be tuned by the gap length, and the hybrid gap mode strongly modifies the spectral shape of the emission from a quantum dot (QD) monolayer incorporated in the gap. Quantitative analyses of the data in combination with numerical simulations reveal that the enhancement and the polarization control of the QD emission are achieved by the coupling.

Diameter-dependent photoresponse with high internal gain in a back gated single Si nanowire phototransistor

Size-dependent photodetection properties of back-gated single Si nanowire (NW) phototransistor with different nanowire diameters have been investigated. The nanowire transistor has been fabricated on silicon-on-insulator wafers with a buried oxide with a thickness of 300 nm and a top silicon layer with a thickness of 50 nm. The fabricated devices show 103 times change in photocurrent under the exposure of small optical power ~1 nW, which shows the high sensitivity of the devices. A high internal gain of the order of 103–104 has been observed from these devices. Diameter-dependent photoresponse was also observed from the devices, as the smaller NWs have shown a higher gain compared to larger NWs. NWs with a 50 nm diameter has shown maximum gain of 1.2 × 104 with a rise time of 120 µs. Finite-difference time-domain simulation shows that the confinement of incident optical energy is much more effective in the smaller NWs due to wave guiding effect in the NWs, which leads to confinement of photon energy and associated electric field.

Unique Scattering Properties of Silicon Nanowires Embedded with Porous Segments.

The development of advanced imaging tools is important for the investigation of the fundamental properties of nanostructures composed of single or multiple nanomaterials. However, complicated preparation processes and irreversible alterations of the samples to be examined are inevitable in most current imaging techniques. In this work, we developed a simple method based on polarization-resolved light scattering measurements to characterize the structural and optical properties of complex nanomaterials. In particular, we examined a single Si nanowire embedded with porous Si segments, in which the porous Si could not be easily distinguished from solid Si by scanning electron microscopy. The dark-field optical images and polarization-resolved scattering spectra showed unique optical features of porous and solid Si. In particular, the porosity, diameter, and number of porous Si segments in the single Si nanowire were identified from the scattering measurements. In addition, we performed systematic optical simulations based on the effective medium model in individual porous and solid Si nanowires. A good agreement between the simulation and measurement results enabled the estimation of the structural parameters of the nanowires, such as diameter and porosity. We believe that our method will be useful for analyzing the structural and optical properties of nanomaterials prior to using complicated and uneconomical imaging tools.

Interfacial shear stress transfer at nanowire-polymer interfaces with van der Waals interactions and chemical bonding

Interfacial mechanics between nanostructures and matrix is of critical importance for a range of device applications of the nanostructures. However, it is challenging to characterize interfacial shear stress transfer at such nanoscale interfaces. In this work, we report a new method to study the interfacial shear stress transfer of single Si nanowires on top of a polymer substrate that is subjected to uniaxial tensile loading. In-situ atomic force microscopy (AFM) testing is used to measure the nanowire deformation, more specifically, average axial strain, as a function of the applied strain to the substrate. Two types of substrates, as-prepared and chemically treated, are selected to examine the effect of van der Waals interactions and chemical bonding. It is found that nonlinear and bilinear cohesive shear-lag models can well capture the interfacial shear stress transfer characteristics associated with the two types of interactions, respectively. For each type, the interface parameters such as interfacial stiffness, shear strength, and/or fracture toughness are identified by fitting the experimental results. This work provides valuable insights into fundamental mechanisms underlying the interfacial shear-lag models. In addition, a parametric study with different nanowire dimensions is carried out, which can provide a guide to experimental design of elastic strain engineering and fracture of Si nanowires.

Photon-Triggered Current Generation in Chemically-Synthesized Silicon Nanowires.

A porous Si segment in a Si nanowire (NW), when exposed to light, generates a current with a high on/off ratio. This unique feature has been recently used to demonstrate photon-triggered NW devices including transistors, logic gates, and photodetection systems. Here, we develop a reliable and simple procedure to fabricate porous Si segments in chemically synthesized Si NWs for photon-triggered current generation. To achieve this, we employ 100 nm-diameter chemical-vapor-deposition grown Si NWs that possess an n-type high doping level and extremely smooth surface. The NW regions uncovered by electron-beam resist become selectively porous through metal-assisted chemical etching, using Ag nanoparticles as a catalyst. The contact electrodes are then fabricated on both ends of such NWs, and the generated current is measured when the laser is focused on the porous Si segment. The current level is changed by controlling the power of the incident laser and bias voltage. The on/off ratio is measured up to 1.5 × 104 at a forward bias of 5 V. In addition, we investigate the porous-length-dependent responsivity of the NW device with the porous Si segment. The responsivity is observed to decrease for porous segment lengths beyond 360 nm. Furthermore, we fabricate nine porous Si segments in a single Si NW and measure the identical photon-triggered current from each porous segment; this single NW device can function as a high-resolution photodetection system. Therefore, our fabrication method to precisely control the position and length of the porous Si segments opens up new possibilities for the practical implementation of programmable logic gates and ultrasensitive photodetectors.

Size-Dependent Localized Phonon Population in Semiconducting Si Nanowires.

The optical phonons in semiconductor nanostructures play an indispensable role in fundamental phenomenon and device applications based on these nanostructures. We study the Raman spectroscopy of optical phonons in Si nanowires (NWs) whose sizes are beyond the phonon confinement regime. The peak shift and unusual asymmetric broadening by one-phonon mode in Si NWs is observed during far-field Raman studies. Using an appropriate thermal anchoring and localized Raman measurements on single NWs by near-field tip-enhanced Raman spectroscopy (TERS), we demonstrate the decoupling of multiple origins responsible for the peak shift accompanied by asymmetric broadening of the one-phonon mode and the appearance of multiple phonon peaks from a single measurement area. A model based on the localized phonon population induced by NW size-dependent charge depletion is proposed to explain the observed dependence of phonon characteristics on NW size. The scanning Kelvin probe force microscopy measurements confirm the size-dependent intrinsic semiconductor surface and interface states-induced charge depletions in single Si NWs. The study clearly suggests the size-dependent phonon characteristics of Si NWs which are crucial for several NW-based photovoltaic and thermoelectric devices.

In situ TEM observation of preferential amorphization in single crystal Si nanowire

The nanoinstability of a single crystal Si nanowire under electron beam irradiation was in situ investigated at room temperature by the transmission electron microscopy technique. It was observed that the Si nanowire amorphized preferentially from the surface towards the center, with the increasing of the electron dose. In contrast, in the center of the Si nanowire the amorphization seemed much more difficult, being accompanied by the rotation of crystal grains and the compression of d-spacing. Such a preferential amorphization, which is athermally induced by the electron beam irradiation, can be well accounted for by our proposed concepts of the nanocurvature effect and the energetic beam-induced athermal activation effect, while the classical knock-on mechanism and the electron beam heating effect seem inadequate to explain these processes. Furthermore, the findings revealed the difference of amorphization between a Si nanowire and a Si film under electron beam irradiation. Also, the findings have important implications for the nanoinstability and nanoprocessing of future Si nanowire-based devices.

Three-dimensional vertical Si nanowire MOS capacitor model structure for the study of electrical versus geometrical Si nanowire characteristics

Three-dimensional (3D) Si surface nanostructuring is interesting towards increasing the capacitance density of a metal-oxidesemiconductor (MOS) capacitor, while keeping reduced footprint for miniaturization. Si nanowires (SiNWs) can be used in this respect. With the aim of understanding the electrical versus geometrical characteristics of such capacitors, we fabricated and studied a MOS capacitor with highly ordered arrays of vertical Si nanowires of different lengths and thermal silicon oxide dielectric, in comparison to similar flat MOS capacitors. The high homogeneity and ordering of the SiNWs allowed the determination of the single SiNW capacitance and intrinsic series resistance, as well as other electrical characteristics (density of interface states, flat-band voltage and leakage current) in relation to the geometrical characteristics of the SiNWs. The SiNW capacitors demonstrated increased capacitance density compared to the flat case, while maintaining a cutoff frequency above 1 MHz, much higher than in other reports in the literature. Finally, our model system has been shown to constitute an excellent platform for the study of SiNW capacitors with either grown or deposited dielectrics, as for example high-k dielectrics for further increasing the capacitance density. This will be the subject of future work.

Engineering the thermal conductivity along an individual silicon nanowire by selective helium ion irradiation

The ability to engineer the thermal conductivity of materials allows us to control the flow of heat and derive novel functionalities such as thermal rectification, thermal switching and thermal cloaking. While this could be achieved by making use of composites and metamaterials at bulk length-scales, engineering the thermal conductivity at micro- and nano-scale dimensions is considerably more challenging. In this work, we show that the local thermal conductivity along a single Si nanowire can be tuned to a desired value (between crystalline and amorphous limits) with high spatial resolution through selective helium ion irradiation with a well-controlled dose. The underlying mechanism is understood through molecular dynamics simulations and quantitative phonon-defect scattering rate analysis, where the behaviour of thermal conductivity with dose is attributed to the accumulation and agglomeration of scattering centres at lower doses. Beyond a threshold dose, a crystalline-amorphous transition was observed.

Multiple Schottky Barrier-Limited Field-Effect Transistors on a Single Silicon Nanowire with an Intrinsic Doping Gradient.

In comparison to conventional (channel-limited) field-effect transistors (FETs), Schottky barrier-limited FETs possess some unique characteristics which make them attractive candidates for some electronic and sensing applications. Consequently, modulation of the nano Schottky barrier at a metal-semiconductor interface promises higher performance for chemical and biomolecular sensor applications when compared to conventional FETs with ohmic contacts. However, the fabrication and optimization of devices with a combination of ideal ohmic and Schottky contacts as the source and drain, respectively, present many challenges. We address this issue by utilizing Si nanowires (NWs) synthesized by a chemical vapor deposition process which yields a pronounced doping gradient along the length of the NWs. Devices with a series of metal contacts on a single Si NW are fabricated in a single lithography and metallization process. The graded doping profile of the NW is manifested in monotonic increases in the channel and junction resistances and variation of the nature of the contacts from ohmic to Schottky of increasing effective barrier height along the NW. Hence multiple single Schottky junction-limited FETs with extreme asymmetry and high reproducibility are obtained on an individual NW. A definitive correlation between increasing Schottky barrier height and enhanced gate modulation is revealed. Having access to systematically varying Schottky barrier contacts on the same NW device provides an ideal platform for identifying optimal device characteristics for sensing and electronic applications.

Effect of Ag/Au bilayer assisted etching on the strongly enhanced photoluminescence and visible light photocatalysis by Si nanowire arrays.

We report on the strongly enhanced photoluminescence (PL) and visible light photocatalysis by arrays of vertically aligned single crystalline Si nanowires (NWs) grown by Ag/Au bilayer assisted etching. High resolution FESEM and TEM imaging reveals that the Si NWs are decorated with ultra-small size arbitrary shaped Si nanocrystals (NCs) due to the lateral etching of the NWs. A strong broad band and tunable visible to near-infrared (NIR) photoluminescence (PL) in the range 1.3-2.4 eV are observed for these Si NWs/NCs at room temperature, depending on the etching conditions. Our studies reveal that the visible-NIR PL intensity is about two orders of magnitude higher and it exhibits faster decay dynamics in the bilayer assisted etching case as compared to the Ag or Au single layer etching case. The enhanced PL in the bimetal case is attributed to the longer length and higher density of the Si NWs/NCs, surface plasmon resonance enhanced absorption by residual bimetal NPs and the enhanced radiative recombination rate. Studies on the time evolution of PL spectral features with laser exposure under ambient conditions and laser power dependence reveal that both the quantum confinement of carriers in Si NCs and the nonbridging oxygen hole defects in the SiOx layer contribute to the tunable PL. Interestingly, Si NWs grown by Ag/Au bilayer assisted etching exhibit enhanced photocatalytic degradation of methylene blue in comparison to Si NWs grown by single layer Ag or Au assisted etching. The Schottky barrier present between bimetallic NPs and nanoporous Si NWs with Si-H bonds facilitates the photocatalytic activity by efficient separation of photogenerated e-h pairs. Our results demonstrate the superiority of the Si NW array grown by bilayer assisted etching for their cutting edge applications in optoelectronics and environmental cleaning.

Direct Electrodeposition of Crystalline Si Nanowires at Low Temperatures

Crystalline Si nanowires (SiNWs) were synthesized through an electrochemical liquid-liquid-solid (ec-LLS) deposition process from dissolved SiCl4 in propylene carbonate at 90 °C using Ga nanoelectrodes. Scanning electron micrographs showed controlled growth of nanowire structured deposits from an array of Ga nanoelectrodes after the ec-LLS process. Energy dispersive X-ray spectroscopy indicated that the chemical composition of as deposit nanowires was Si with low Ga incorporation. X-ray diffraction and Raman spectroscopy separately indicated that the film of as-deposited SiNWs were crystalline. Electron beam diffraction of a single SiNW was taken within a transmission electron microscope and patterns for a diamond cubic crystal structure was obtained. Scanning electron micrographs of the orientation of nanowires on Si(111) and Si(100) substrates with different crystalline orientations suggested that epitaxial nanowire growth was achieved under these conditions. Cyclic voltammogram of SiCl4 electrolyte on Ga nanoelectrodes and potential response of the chronopotentiometry experiment that produced SiNWs indicated that the SiNW deposition is energetically favorable to other parallel processes in the deposition electrolyte. Temperature dependence study showed that SiNWs could be produced using ec-LLS at as low as 60 °C, at which temperature Si solubility in Ga and Si diffusivity in Ga is sufficient for heterogeneous nucleation to occur at Ga/substrate interface.

Spatially-resolved and polarized Raman scattering from a single Si nanowire

We report spatially-resolved and polarized Raman scattering results from a single Si nanowire (NW). Transmission electron microscope images show that the surface morphology of the Si NW varies from smooth to rough along the long axis. As the NW grows, the smooth surface becomes rough because of Au diffusion to the surface, resulting in the formation of facets and stacking faults. Spatially-resolved Raman spectra along the NW long axis reveal variations in tensile strain related to the morphological changes in NW surface. The tensile strain in the top segment of the NW with a smooth surface is greater than that in the bottom segment with a rough surface. Despite the formation of facets and stacking faults, polarized Raman scattering results both from the top and bottom segments of the NW are consistent with the Raman polarization selection rules expected for a cubic crystal. Copyright © 2015 John Wiley & Sons, Ltd.

A four-probe thermal transport measurement method for nanostructures.

Several experimental techniques reported in recent years have enabled the measurement of thermal transport properties of nanostructures. However, eliminating the contact thermal resistance error from the measurement results has remained a critical challenge. Here, we report a different four-probe measurement method that can separately obtain both the intrinsic thermal conductance and the contact thermal resistance of individual nanostructures. The measurement device consists of four microfabricated, suspended metal lines that act as resistive heaters and thermometers, across which the nanostructure sample is assembled. The method takes advantage of the variation in the heat flow along the suspended nanostructure and across its contacts to the four suspended heater and thermometer lines, and uses sixteen sets of temperature and heat flow measurements to obtain nine of the thermal resistances in the measurement device and the nanostructure sample, including the intrinsic thermal resistance and the two contact thermal resistances to the middle suspended segment of the nanostructure. Two single crystalline Si nanowires with different cross sections are measured in this work to demonstrate the effectiveness of the method. This four-probe thermal transport measurement method can lead to future discoveries of unique size-dependent thermal transport phenomena in nanostructures and low-dimensional materials, in addition to providing reliable experimental data for calibrating theoretical models.

Photoluminescence signature of resonant energy transfer in ZnO coated Si nanocrystals decorated on vertical Si nanowires array

We investigate the mechanism of red shift in photoluminescence (PL) and reduction in the PL lifetime from Si nanocrystals (NCs) decorated on vertical Si nanowires (NWs) array due to ZnO over layer coating. Arrays of vertically aligned single crystalline Si NWs decorated with arbitrary shaped Si NCs have been fabricated by a silver assisted wet chemical etching method. A strong broad band and tunable visible to near-infrared PL is observed from these Si NWs at room temperature and the Si NCs on the surface of the Si NWs are primarily responsible for the PL emission. Higher band gap ZnO film is sputter deposited on the Si NCs decorated Si NWs to form heterostructure. Bare Si NW/NCs and Si NCs/ZnO heterostructures show extremely high broad band absorption in the entire visible region. PL studies on the Si NCs/ZnO heterostructures reveal significant red shift and in some cases reduced intensity of the PL band due to the ZnO layer in close proximity of the Si NCs. This is accompanied by a reduction in the PL lifetime of the Si NCs after ZnO coating. Interestingly, no measurable red shift in PL is observed in absence of the resonance in the visible PL emission energy of ZnO and that of Si NCs. The modified PL from the heterostructures is explained through an energy band diagram on the basis of resonant energy transfer from the defect assisted recombination of the carries in the ZnO overlayer that excites the Si NCs in the close proximity and subsequent de-excitation process via radiative recombination. These findings have important bearing on the development of cost effective Si-based hybrid optoelectronic devices using wide band gap heterostructured oxide semiconductors.

Radial photovoltaic junction with single Si nanowire core–shell structure

A single silicon nanowire core–shell structure has been elaborated. Technological stages of the process are presented. The device results in a P-i-N radial junction: the core is a P-type silicon nanowire encapsulated in an intrinsic thin silicon layer and an N-type doped silicon layer. Scanning electron microscopy observations, as well as the electrical I(V) characterisation on single nanowires, are presented.

Formation of Single Crystal Si-Nanowire by Electric Field Self-Redistribution Effect in Anodic Oxidation for Multilayer Array Application

Single crystal silicon nanowire was formed successfully by electric field self-redistribution effect in anodic oxidation (ANO) with room temperature process. By using the ANO process with large electric field and long oxidation time, nanowires with a diameter of 9 nm can be achieved in this study. The silicon nanowire with tunable diameter in double layer array was automatically formed due to the self-ending oxidation blocking caused by the self-redistribution nature of electric field. The E-beam lithography with different photoresist widths from 60 to 90 nm was designed for observing the formation of nanowire. It can be observed that the convex profile of silicon shows larger electric field than concave profile in the ANO process and therefore induces thicker oxide. The single crystal Si-nanowire array by the ANO process is potential for the complementary metal–oxide–semiconductor devices application in the next generation.