Si Nanowire Arrays Research Articles

Strained silicon based complementary tunnel-FETs: Steep slope switches for energy efficient electronics

Electrical characteristics of silicon nanowire tunnel field effect transistors (TFETs) are presented and benchmarked versus other concepts. Particular emphasis is placed on the band to band tunneling (BTBT) junctions, the functional core of the device. Dopant segregation from ion implanted ultrathin silicide contacts is proved as a viable method to achieve steep tunneling junctions. This reduces defect generation by direct implantation into the junction and thus minimizes the risk of trap assisted tunneling. The method is applied to strained silicon, specifically to nanowire array transistors, enabling the realization of n-type and p-type TFETs with fairly high currents and complementary TFET inverters with sharp transitions and good static gain, even at very low drain voltages of VDD=0.2V. These achievements suggest a considerable potential of TFETs for ultralow power applications. Gate-all-around Si nanowire array p-type TFETs have been fabricated to demonstrate the impact of electrostatic control on the device performance. A high on-current of 78μA/μm at VD=VG=1.1V is obtained.

Optical simulations of P3HT/Si nanowire array hybrid solar cells.

An optical simulation of poly(3-hexylthiophene) (P3HT)/Si nanowire array (NWA) hybrid solar cells was investigated to evaluate the optical design requirements of the system by using finite-difference time-domain (FDTD) method. Steady improvement of light absorption was obtained with increased P3HT coating shell thickness from 0 to 80 nm on Si NWA. Further increasing the thickness caused dramatic decrease of the light absorption. Combined with the analysis of ultimate photocurrents, an optimum geometric structure with a coating P3HT thickness of 80 nm was proposed. At this structure, the hybrid solar cells show the most efficient light absorption. The optimization of the geometric structure and further understanding of the optical characteristics may contribute to the development for the practical experiment of the promising hybrid solar cells.

Two-Sided Silicon Nanowire Array/Bulk Thermoelectric Power Generator

To improve the output power of n-and p-type Si nanowire array (NWA)/bulk thermoelectric power generators (TEGs) fabricated using metal assisted chemical etching, post-nanowire-etch spin-on-doping (SOD), and arrays on both sides of the bulk are used. Post-process SOD increases the power factor and removes surface depletion/inversion effects while maintaining crystalline NWs. A maximum output power of 3.5 μW for a p-NWA/bulk/NWA TEG with NWA length of 23 and 24 μm is obtained for an external temperature difference of 37 °C and a TEG area of 25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Laterally assembled nanowires for ultrathin broadband solar absorbers

We studied optical resonances in laterally oriented Si nanowire arrays by conducting finite-difference time-domain simulations. Localized Fabry-Perot and whispering-gallery modes are supported within the cross section of each nanowire in the array and result in broadband light absorption. Comparison of a nanowire array with a single nanowire shows that the current density (J(SC)) is preserved for a range of nanowire morphologies. The J(SC) of a nanowire array depends on the spacing of its constituent nanowires, which indicates that both diffraction and optical antenna effects contribute to light absorption. Furthermore, a vertically stacked nanowire array exhibits significantly enhanced light absorption because of the emergence of coupled cavity-waveguide modes and the mitigation of a screening effect. With the assumption of unity internal quantum efficiency, the J(SC) of an 800-nm-thick cross-stacked nanowire array is 14.0 mA/cm², which yields a ~60% enhancement compared with an equivalent bulk film absorber. These numerical results underpin a rational design strategy for ultrathin solar absorbers based on assembled nanowire cavities.

Morphology control of ordered Si nanowire arrays by nanosphere lithography and metal-assisted chemical etching

Metal-assisted chemical etching (MACE) using a nanosphere lithography (NSL) technique is regarded as a general fabrication method for silicon nanowire (SINW) arrays. However, morphology control of SiNWs using this method has not been reported. In this study, silicon nanowire (SINW) and silicon nanocone (SINC) arrays were fabricated by MACE using a NSL. Depending on the concentration of etchants in the etching solution, the morphology of the wires and etching rate were systemically changed. At high concentrations, the wires were etched cylindrically and at low concentrations, tapered (cone-like) wires were obtained. To quantify the degree of tapering, the volume ratio of the etched part was calculated from their morphology. The degree of tapering increased as the concentrations of etchants decreased. We suggest that the mechanism of the formation of nanocone is related to the degree of hole diffusion under the metal layer, which was supported by field emission scanning electron microscope (FE-SEM) images.

An effective surface-enhanced Raman scattering template based on gold nanoparticle/silicon nanowire arrays

A large-scale Si nanowire array (SiNWA) is fabricated with gold (Au) nanoparticles by simple metal-assisted chemical etching and metal reduction processes. The three-dimensional nanostructured Au/SiNWA is evaluated as an active substrate for surface-enhanced Raman scattering (SERS). The results show that the detection limit for rhodamine 6G is as low as 10−7 M, and the Raman enhancement factor is as large as 105 with a relative standard deviation of less than 25%. After the calibration of the Raman peak intensities of rhodamine 6G and thiram, organic molecules could be quantitatively detected. These results indicate that Au/SiNWA is a promising SERS-active substrate for the detection of biomolecules present in low concentrations. Our findings are an important advance in SERS substrates to allow fast and quantitative detection of trace organic contaminants.

Ultra‐Broadband Photodetector for the Visible to Terahertz Range by Self‐Assembling Reduced Graphene Oxide‐Silicon Nanowire Array Heterojunctions

Advantages of reduced graphene oxide (RGO) and Si nanowire (SiNW) array are integrated into a single photodetector by simple drop-casting of the suspension of graphene oxide (GO) nanosheets on top of the SiNW array followed by heat-treatment. The RGO-SiNW array heterojunction photodetector is photoresponsive to the visible, near-infrared, mid-infrared, and terahertz wave irradiations at room temperature.

Self-assembled nanowire array capacitors: capacitance and interface state profile

Direct characterization of the capacitance and interface states is very important for understanding the electronic properties of a nanowire transistor. However, the capacitance of a single nanowire is too small to precisely measure. In this work we have fabricated metal–oxide–semiconductor capacitors based on a large array of self-assembled Si nanowires. The capacitance and conductance of the nanowire array capacitors are directly measured and the interface state profile is determined by using the conductance method. We demonstrate that the nanowire array capacitor is an effective platform for studying the electronic properties of nanoscale interfaces. This approach provides a useful and efficient metrology for the study of the physics and device properties of nanoscale metal–oxide–semiconductor structures.

A chemical route to fabricate Lu2Si2O7:Ce3+ micro-column structured films

Lu2Si2O7 (LPS):Ce3+ micro-column structured films have been prepared on silicon substrate by a chemical method which combines Si nanowire array wafers with dip-coating method. In the processing, the Si nanowire array wafers are obtained via a metal-assisted chemical etching route on single-crystal silicon (100) substrate at first step. The produced Si nanowire array wafers not only serve as guide–templates for the fabrication of LPS:Ce3+ films in a columnar fashion, but also provide Si sources for further reacting with lutetium sources at high temperatures to form LPS:Ce3+ micro-column structured films. XRD analysis indicates that as-prepared films are crystallized into a monoclinic Lu2Si2O7 phase along with an amorphous silicon oxide phase, and the latter phase ascribes to the oxidation of Si nanowire arrays. The as-prepared LPS:Ce3+ films exhibit a columnar-array structure and a uniform morphology according to SEM and TEM observations. The UV photoluminescence spectra illustrate the characteristic excitation and emission spectrum of Ce3+ ions in the host of LPS. X-ray induced emission spectrum reveals a similar shape with UV excited emission spectrum peaking at 378nm. Furthermore, the LPS:Ce3+ micro-column structured films present a fast decay time of about 3425ns. These experiments have demonstrated that the film technique thus developed is possible to fabricate column-like luminescent films of large scale with low cost.

High-Density Silicon Nanowires Prepared via a Two-Step Template Method

High density ordered Si nanowire arrays can be fabricated from a Fe2O3 template annealed from polystyrene (PS) microsphere layers via a metal-assisted chemical etching method. The metal mesh films, containing position- and density-defined pores that determine the position and density of the remaining structures after etching, are extremely important for achieving high quality Si nanowires. By adding a structural inversion process, a Au metal mesh with arrays of high density nanopores is devised as a catalyst for metal-assisted chemical etching of silicon. The density of Si nanowires can be increased to two times that of the single-layer PS microspheres and further to three times when a double layer of PS microspheres is introduced. The two-step template method for the preparation of high-density Si nanowires shows great potential in the fields of nanofabrication and nanoelectronics.

Automatic Release of Silicon Nanowire Arrays with a High Integrity for Flexible Electronic Devices

Automatic release and vertical transferring of silicon/silicon oxide nanowire arrays with a high integrity are demonstrated by an Ag-assisted ammonia etching method. By adding a water steaming step between Ag-assisted HF/H2O2 and ammonia etching to form a SiOx protective layer sheathing Si nanowires, we can tune the composition of the nanowires from SiOx (0 ≤ x ≤ 2) to Si nanowires. Ag plays a key role to the neat and uniform release of Si/SiOx nanowire arrays from Si wafer in the ammonia etching process. The vertical Si nanowire array device, with both sides having high-quality Ohmic contact, can be transferred to arbitrary substrates, especially on a flexible substrate. The method developed here offers a facile method to realize flexible Si nanowire array functional devices.

Light trapping and surface plasmon enhanced high-performance NIR photodetector

Heterojunctions near infrared (NIR) photodetectors have attracted increasing research interests for their wide-ranging applications in many areas such as military surveillance, target detection, and light vision. A high-performance NIR light photodetector was fabricated by coating the methyl-group terminated Si nanowire array with plasmonic gold nanoparticles (AuNPs) decorated graphene film. Theoretical simulation based on finite element method (FEM) reveals that the AuNPs@graphene/CH3-SiNWs array device is capable of trapping the incident NIR light into the SiNWs array through SPP excitation and coupling in the AuNPs decorated graphene layer. What is more, the coupling and trapping of freely propagating plane waves from free space into the nanostructures, and surface passivation contribute to the high on-off ratio as well.

Optical Characterization of Si Nanowires: Dependence with Substrate Orientation and Light Polarization

ABSTRACTOptical properties of Si nanowire arrays (SiNWs) prepared on p-doped Si(111) and Si(100) substrates are studied. The SiNWs were synthesized by self-assembly electroless metal deposition nanoelectrochemistry in an ionic silver HF solution through selective etching. Total reflectance (Rt) and total diffuse reflectance (Rdt) of SiNWs change drastically in comparison to polished Si. To understand these changes diffuse reflectance (Rd) with polarized incident light was studied. For samples prepared on Si(111), the wavelength integrated Rd (wIRd) shows maxima at certain angle of incidence θ and it does not depend on light polarization. Moreover, Rdt of SiNWs prepared on Si(111) can be modeled as an ensemble of diffuse reflectors. For samples prepared on Si(100) wIRd increases with θ, being greater when the light electric field is parallel to the plane of incidence. Also, Rd spectra show structures due to interference effects. For these reasons SiNWs prepared on Si(100) can be considered as a thin film whose refractive index depends on light polarization.

Regulation of the morphology and photoluminescence of silicon nanowires by light irradiation

We successfully synthesized porous Si nanowire arrays with excellent photoluminescence properties.

White light emission from heterojunction diodes based on surface-oxidized porous Si nanowire arrays and amorphous In-Ga-Zn-O capping

A novel heterojunction white light emitting diode (LED) structure based on an array of vertically aligned surface-passivated p-type porous Si nanowires (PSiNWs) with n-type amorphous In-Ga-Zn-O (a-IGZO) capping is introduced. PSiNWs were initially synthesized by electroless etching of p-type Si (100) wafers assisted by Ag nanoparticle catalysts and then surface-passivated by thermal oxidation. The nanowires synthesized by metal-assisted electroless etching were found to have longitudinally varying nanoporous morphologies due to differences in the duration of exposure to etching environment. These PSiNWs were optically active with orange red photoluminescence consisting of dark red to yellow emissions attributable to quantum confinement effects and to modified band structures. The LED structures emitted visible white light while exhibiting rectifying current-voltage characteristics. The white light emission was found to be the result of the combination of dark red to yellow emissions originating from the quantum confinement effect within the PSiNWs and green to blue emissions due to the oxygen-deficiency-related recombination centers introduced during the surface oxidation.

Ultra-high aspect ratio Si nanowires fabricated with plasma etching: plasma processing, mechanical stability analysis against adhesion and capillary forces and oleophobicity

Room-temperature deep Si etching using time-multiplexed deep reactive ion etching (DRIE) processes is investigated to fabricate ultra-high aspect ratio Si nanowires (SiNWs) perpendicular to the silicon substrate. Nanopatterning is achieved using either top-down techniques (e.g. electron beam lithography) or colloidal polystyrene (PS) sphere self-assembly. The latter is a faster and more economical method if imperfections in diameter and position can be tolerated. We demonstrate wire radii from below 100 nm to several micrometers, and aspect ratios (ARs) above 100:1 with etching rates above 1 μm min−1 using classical mass flow controllers with pulsing rise times of seconds. The mechanical stability of these nanowires is studied theoretically and experimentally against adhesion and capillary forces. It is shown that above ARs of the order of 50:1 for spacing 1 μm, SiNWs tend to bend due to adhesion forces between them. Such large adhesion forces are due to the high surface energy of silicon. Wetting the SiNWs with water and drying also gives rise to capillary forces. We find that capillary forces may be less important for SiNW collapse/bending compared to adhesion forces of dry SiNWs, contrary to what is observed for polymeric nanowires/nanopillars which have a much lower surface energy compared to silicon. Finally we show that SiNW arrays have oleophobic and superoleophobic properties, i.e. they exhibit excellent anti-wetting properties for a wide range of liquids and oils due to the re-entrant profile produced by the DRIE process and the well-designed spacing.

KOH Etching for Tuning Diameter of Si Nanowire Arrays and Their Field Emission Characteristics

Large-area, vertical, periodic silicon nanowire (SiNW) arrays on Si(100) substrates were fabricated by nanosphere lithography and metal-assisted catalytic etching. Immersion in KOH solution uniformly reduced the diameter of SiNWs, enabling the diameter to be tuned from 60 to 150 nm. The etching rates of SiNW in 20 wt% KOH solution at different temperatures were measured, and the activation energy was calculated from Arrhenius plots to be about 0.6 eV. The field emission properties were improved by reducing the diameter of the SiNWs. Excellent field emission properties of the SiNW arrays were found: they had a turn-on field of 4 V μm−1 and a field enhancement factor of 3821. The excellent field emission characteristics are attributed to the vertically well-aligned, high aspect ratio and highly ordered arrangement of the single crystalline SiNW field emitters.

Graded Silicon Nanostructure Arrays for Tailoring Antireflection Performances

Advances in nanofabrication have resulted in great potentials for improving in both device performance and the manufacturing process of various applications. One revolutionary example is silicon (Si) nanostructures, typically using Si nonopore arrays or Si nonowire arrays, to construct high efficient and low-cost solar cells. In this work, we develop the innovative combined nanostructure arrays with tailored structural profiles using inexpensive, simple and rapid etching processes, whose total reflection is suppressed to 1.6%, approximately 39% less than Si nonopore arrays, and 20% less than Si nanowire arrays. In addition, systematic investigations on wettability of textured Si surfaces reveal the inherent surface oxidation during etching process. These combined nanostructure arrays with tailored antireflection performances, along with the in-depth studies of underlying etching mechanisms, may benefit both the yield and cost efficiently in industrial standard of silicon solar cells.

Fabrication of Ordered, Large Scale, Horizontally‐Aligned Si Nanowire Arrays Based on an In Situ Hard Mask Block Copolymer Approach

A simple technique is demonstrated to fabricate horizontal, uniform, and hexagonally arranged Sinanowire arrays with controlled orientation and density at spatially well defined locations on a substrate based on an in situ hard-mask pattern-formation approach by microphase-separated block-copolymer thin films. The technique may have significant application in the manufacture of transistor circuitry.

Electrical and optoelectronic properties of graphene Schottky contact on Si-nanowire arrays with and without H2O2 treatment

Developing better contacts on Si-nanowire (SiNW) arrays is one of the main challenges for Si-based device technology. The present work reports the fabrication and detailed electrical properties of graphene/SiNWs/n-type Si Schottky diodes. The graphene/H2O2-treated SiNWs/n-type Si Schottky diode shows a better rectifying behavior in the dark than the graphene/SiNWs/n-type Si diode. Such an improvement indicates that a good passivation is formed at the interface. The graphene/H2O2-treated SiNWs/n-type Si Schottky diode also shows a higher photocurrent under illumination than the graphene/SiNWs/n-type Si diode. Results revealed that SiNW surface passivation influences the photoconductivity by reducing the number of electron traps that serve to decrease the photocurrent time constant.