SiNW Arrays Research Articles

Effect of porous layer engineered with acid vapor etching on optical properties of solid silicon nanowire arrays

In this paper, we report, for the first time, an investigative study involving the engineering of lightly doped porous silicon nanowire arrays (pSiNWs) by exposing solid silicon nanowire arrays (SiNWs) to an acid vapor emanating from HF/HNO3 hot solution. SEM and TEM images exhibit vertically distributed SiNW arrays on the whole silicon (Si) surface with relatively smooth surface sidewalls. By submitting the SiNW arrays to Acid Vapor Etching (AVE), they become porous with a substantial decrease in their densities and lengths. Increasing etching duration leads to a higher porosity without affecting the wire diameter which remains almost constant nearly 100nm. Exceeding a critical etching duration, a porous structure is observed superseding the SiNW structure. The morphological characterizations have been correlated to the optical properties. We note a blue shift of the strong visible photoluminescence (PL) bands after AVE treatment due to the decrease of the silicon quantum dots diameter (Si-QDs). UV–Visible measurement shows a decrease of the total reflectivity by 5% after AVE treatment.

Decoration of silicon nanowires with silver nanoparticles for ultrasensitive surface enhanced Raman scattering

Silicon nanowires (Si NWs), produced by the chemical etching technique, were decorated with silver nanoparticles (NPs) produced at room temperature by the pulsed laser deposition (PLD) technique. Silver NPs were obtained by means of nanosecond pulsed laser ablation of a target in the presence of a controlled Ar atmosphere. Two different laser pulse numbers and Si NWs having different lengths were used to change the NP number density on the Si NW surface. The resulting Ag NP morphologies were studied by scanning electron microscopy imaging. The results show that this industrially compatible technological approach allows the coverage of the Si NW walls with Ag NPs with a strong control of the NP size distribution and spatial arrangement. The obtained Ag NP decorated Si NWs are free from chemicals contamination and there is no need of post deposition high temperature processes. The optical properties of Si NW arrays were investigated by reflectance spectroscopy that showed the presence of a plasmon related absorption peak, whose position and width is dependent on the Ag NP surface morphology. Coupling the huge surface-to-volume ratio of Si NW arrays with the plasmonic properties of silver nanoparticles resulted in a 3D structure suitable for very sensitive surface enhanced Raman scattering (SERS) applications, as demonstrated by the detection of Rhodamine 6G in aqueous solution at a concentration level of 10−8 M.

Precise and selective sensing of DNA-DNA hybridization by graphene/Si-nanowires diode-type biosensors.

Single-Si-nanowire (NW)-based DNA sensors have been recently developed, but their sensitivity is very limited because of high noise signals, originating from small source-drain current of the single Si NW. Here, we demonstrate that chemical-vapor-deposition-grown large-scale graphene/surface-modified vertical-Si-NW-arrays junctions can be utilized as diode-type biosensors for highly-sensitive and -selective detection of specific oligonucleotides. For this, a twenty-seven-base-long synthetic oligonucleotide, which is a fragment of human DENND2D promoter sequence, is first decorated as a probe on the surface of vertical Si-NW arrays, and then the complementary oligonucleotide is hybridized to the probe. This hybridization gives rise to a doping effect on the surface of Si NWs, resulting in the increase of the current in the biosensor. The current of the biosensor increases from 19 to 120% as the concentration of the target DNA varies from 0.1 to 500 nM. In contrast, such biosensing does not come into play by the use of the oligonucleotide with incompatible or mismatched sequences. Similar results are observed from photoluminescence microscopic images and spectra. The biosensors show very-uniform current changes with standard deviations ranging ~1 to ~10% by ten-times endurance tests. These results are very promising for their applications in accurate, selective, and stable biosensing.

Location-Controlled Growth of Vertically Aligned Si Nanowires using Au Nanodisks Patterned by KrF Stepper Lithography.

The location-controlled epitaxial growth of vertically aligned Si nanowire (v-SiNW) arrays over large surface area was investigated with Au nanodisks (AuNDs) patterned by KrF stepper lithography. There are two steps for synthesizing v-SiNWs from an AuND pattern: annealing and growth. The annealing process induces the formation of a single Au nanoparticle (AuNP) from an AuND pattern, which consists of several cracked AuNPs. Here, the oxide layer between the AuNDs and Si substrate is necessary for impeding the diffusion of Si atoms into the AuNPs. However, the oxide layer must be removed for properly aligned epitaxial SiNW growth. These SiNW arrays in large area can contribute highly to improve a nanowire-based engineering by controlling the location of SiNWs with consistent pitch.

Processing-Induced Electrically Active Defects in Black Silicon Nanowire Devices.

Silicon nanowires (Si NWs) are widely investigated nowadays for implementation in advanced energy conversion and storage devices, as well as many other possible applications. Black silicon (BSi)-NWs are dry etched NWs that merge the advantages related to low-dimensionality with the special industrial appeal connected to deep reactive ion etching (RIE). In fact, RIE is a well established technique in microelectronics manufacturing. However, RIE processing could affect the electrical properties of BSi-NWs by introducing deep states into their forbidden gap. This work applies deep level transient spectroscopy (DLTS) to identify electrically active deep levels and the associated defects in dry etched Si NW arrays. Besides, the successful fitting of DLTS spectra of BSi-NWs-based Schottky barrier diodes is an experimental confirmation that the same theoretical framework of dynamic electronic behavior of deep levels applies in bulk as well as in low dimensional structures like NWs, when quantum confinement conditions do not occur. This has been validated for deep levels associated with simple pointlike defects as well as for deep levels associated with defects with richer structures, whose dynamic electronic behavior implies a more complex picture.

Ultra-low reflection loss for silicon nanowire-array-textured based photovoltaic devices

A two-step, metal-assisted, electroless etching technique based on the sputtering of silver particles is described, using phosphorus silicate glass, doping, and screen printing processes to fabricate photovoltaic devices. Si-NW arrays on pyramid surfaces show a low reflectivity of ∼1.11%, while the average length of the Si-NW arrays is ∼375nm, with an aspect ratio of ∼5.4. Superior photovoltaic characteristics of the PCE (∼10.26%) with a Jsc of 28.2mA/cm2 and a Voc of 540mV are achieved. Power conversion efficiency shows a dependence on shunt resistance while the series resistance and the NWs aspect ratio are respectively below 2.41Ω and 10.

Photovoltaic performance of Gallium-doped ZnO thin film/Si nanowires heterojunction diodes

In this work, photovoltaic performance of Ga-doped ZnO thin film/Si NWs heterojunction diodes was investigated. Highly dense and vertically well-aligned Si NW arrays were successfully synthesised on a p-type (1 0 0)-oriented Si wafer through cost-effective metal-assisted chemical etching technique. Ga-doped ZnO thin films were deposited onto Si NWs via radio frequency magnetron sputtering to construct three-dimensional heterostructures. Photovoltaic characteristics of the fabricated diodes were determined with current density (J)–voltage (V) measurements under simulated solar irradiation of AM 1.5 G. The optimal open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were found to be 0.37 V, 3.30 mA cm−2, 39.00 and 0.62%, respectively. Moreover, photovoltaic diodes exhibited relatively high external quantum efficiency over the broadband wavelengths between 350 and 1100 nm interval of the spectrum. The observed photovoltaic performance in this study clearly indicates that the investigated device structure composed of Ga-doped ZnO thin film/Si NWs heterojunctions could facilitate an alternative pathway for optoelectronic applications in future, and be a promising alternative candidate for high-performance low-cost new-generation photovoltaic diodes.

Structure and antireflection properties of SiNWs arrays form mc-Si wafer through Ag-catalyzed chemical etching

A simple and low cost MACE method was demonstrated for efficiently texturing commercial mc-Si wafer at room temperature. The effects of fabrication parameters (deposition time, HF concentration, H2O2 concentration, and etching time) on the morphology structure, antireflection property of textured mc-Si were carefully studied. The large scale SiNWs arrays with different structure can be obtained under various fabrication conditions. Meanwhile, the results indicate that the fabricate parameters have important effect on the reflectance of textured mc-Si sample in the order of etching time>deposition time>H2O2 concentration>HF concentration. The comprehensive research results indicate that it is more beneficial for the nanowire arrays with tapering structure and the length of 13μm to obtain excellent antireflection property. Under these optimization conditions, the textured mc-Si shows an outstanding anti-reflectance ability of ∼5.6%, which indicates that the Ag-catalysis etched mc-Si shows a huge potential application in high-efficiency polysilicon solar cells.

Large-area ordered P-type Si nanohole arrays as photocathode for highly efficient hydrogen production by photoelectrochemical water splitting

Large-area highly ordered P-type silicon nanohole (SiNH) arrays have been fabricated by the combination of metal assisted electroless etching and nanospheres lithography technique, and systematic investigations of photoelectrochemical hydrogen evolution behavior of SiNH arrays were also reported in this study. The SiNH arrays exhibited a maximum limiting photocurrent density of 28 mA cm−2, higher than that of SiNW arrays (24 mA cm−2) and the planar Si (21.5 mA cm−2). The improved performance was attributed to SiNH arrays providing a patterned and ordered nanoholes structure as a result of enhancement of the light harvesting as well as charge transportation and collection efficiency. Furthermore, we have demonstrated that the photoelectrode consisting of longer SiNH yielded a higher limiting current. However, when the length of SiNH was increased further, the limiting current dramatically reduced, which is due to an increased interface recombination and scattering resulting from the increased surface area of SiNH begin to play a dominant role. The relationship between the diameter of nanoholes and the photoconversion efficiency of SiNH arrays was also discussed. By implementing this ordered SiNH arrays into electrode design, one may be able to advantageously impact PEC and photovoltaic device performance.

Effect of acid vapor etching on morphological and opto-electric properties of flat silicon and silicon nanowire arrays: A comparative study

In this paper, we report a comparative study between porous silicon (pSi) and porous silicon nanowires (pSiNWs). Acid Vapor Etching (AVE) treatment has been used to perform porous structure on flat Si and SiNWs array substrates respectively. SiNW structure is prepared by the widely used Silver catalyzed etching method. SEM and TEM images show that AVE treatment induces porous structure in the whole Si wafer and the SiNW sidewall. Comparatively to pSi, pSiNWs exhibit a low reflectivity in the whole spectral range which decreases with etching duration. However, the reflectivity of pSi changes with porous layer thickness. Both pSi and pSiNWs exhibit a significant PL peak situated at 2 eV. PL peaks are attributed to the quantum confinement effect in the silicon nanocrystallites (SiNCs). We discussed the significant enhancement in the peak intensities and a shift toward lower energy displayed in Raman spectra for both pSi and pSiNWs. We reported a correlative study of the AVE treatment effect on the minority carrier life time of flat silicon and SiNW arrays with the passivation effect of chemical induced silicon oxides highlighted by FTIR spectra.

A vertical tip-tip contact silicon nanowire array for gas sensing.

Novel chemiresistive gas sensors based on a vertical tip-tip contact silicon nanowire (TTC-SiNW) array were constructed. The welding of TTC-SiNWs after joule heating treatment was confirmed by a current-voltage curve (I-V curve). The TTC-SiNW structure not only resolved the problem of electrode contact encountered in conventional nanowire sensors, but also elongated the nanowire length to increase the void space for fast gas diffusion. The TTC-SiNW sensor comprising the same two types of SiNW arrays responded to NO2 very sensitively. The LOD for the p-p and n-n contact SiNW arrays is around 150 ppb and 3 ppb (S/N = 3), respectively. Furthermore, the highly oriented nano-junction formed on the TTC structure provided solid evidence to clarify the contribution of the nanojunction to gas sensing behavior. The TTC-SiNW sensor with a p-n junction displays a significant rectification effect. The sensitive response towards NO2 (LOD is about 18 ppb) was observed at a reverse bias voltage, whereas the response at a forward bias voltage was insignificant. Finally, the mechanism of gas sensing behavior on different types of TTC structures was proposed.

Reduction of Cr(VI) to Cr(III) using silicon nanowire arrays under visible light irradiation

We report an efficient visible light-induced reduction of hexavalent chromium Cr(VI) to trivalent Cr(III) by direct illumination of an aqueous solution of potassium dichromate (K2Cr2O7) in the presence of hydrogenated silicon nanowires (H-SiNWs) or silicon nanowires decorated with copper nanoparticles (Cu NPs-SiNWs) as photocatalyst. The SiNW arrays investigated in this study were prepared by chemical etching of crystalline silicon in HF/AgNO3 aqueous solution. The Cu NPs were deposited on SiNW arrays via electroless deposition technique. Visible light irradiation of an aqueous solution of K2Cr2O7 (10−4M) in presence of H-SiNWs showed that these substrates were not efficient for Cr(VI) reduction. The reduction efficiency achieved was less than 10% after 120min irradiation at λ>420nm. Addition of organic acids such as citric or adipic acid in the solution accelerated Cr(VI) reduction in a concentration-dependent manner. Interestingly, Cu NPs-SiNWs was found to be a very efficient interface for the reduction of Cr(VI) to Cr(III) in absence of organic acids. Almost a full reduction of Cr(VI) was achieved by direct visible light irradiation for 140min using this photocatalyst.

Glucose oxidase immobilization platform based on ZnO nanowires supported by silicon nanowires for glucose biosensing

A nanostructured platform composed of ZnO nanowires supported by silicon nanowires (ZnO/SiNWs) is utilized to immobilize glucose oxidase on a glucose enzymatic biosensor. The SiNWs arrays are fabricated by a simple metal-catalyzed solution method under normal conditions and ZnO nanowires about 650nm long are produced on the wall of the silicon nanowires by aqueous thermal decomposition. The large specific surface area, abundant microspace, small channels, and high isoelectric point (IEP) fracture of ZnO enable effective fluid circulation and good biocompatibility boding well for immobilization of enzymes. The nanocomposite materials show good electrocatalytic activity exhibiting strong and sensitive response to glucose even in the presence of high concentrations of interfering species besides excellent long-term stability and reproducibility.

Maximizing Transfection Efficiency of Vertically Aligned Silicon Nanowire Arrays

Vertically aligned silicon nanowire (VA‐SiNW) arrays are emerging as a powerful new tool for gene delivery by means of mechanical transfection. In order to utilize this tool efficiently, uncertainties around the required design parameters need to be removed. Here, a combination of nanosphere lithography and templated metal‐assisted wet chemical etching is used to fabricate VA‐SiNW arrays with a range of diameters, heights, and densities. This fabrication strategy allows identification of critical parameters of surface topography and consequently the design of SiNW arrays that deliver plasmid with high transfection efficiency into a diverse range of human cells whilst maintaining high cell viability. These results illuminate the cell‐materials interactions that mediate VA‐SiNW transfection and have the potential to transform gene therapy and underpin future treatment modalities.

Catalytic activity of silicon nanowires decorated with silver and copper nanoparticles

The paper reports on the elaboration of silicon nanowires decorated with silver (SiNWs–Ag NPs) or copper (SiNWs–Cu NPs) nanoparticles and the investigation of their catalytic properties for the reduction of 4-nitrophenol to 4-aminophenol. The SiNW arrays were produced through chemical etching of crystalline silicon in HF/AgNO3 aqueous solution. The metal nanoparticles were deposited on the SiNW substrates through chemical bath immersion in a metal salt/hydrofluoric acid aqueous solution. The SiNWs decorated with Ag NPs and Cu NPs were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The catalytic activity of the SiNWs loaded with metal nanoparticles was evaluated for the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride (NaBH4). The substrates exhibited good catalytic performance toward nitrophenol with a full reduction in less than 30 s for the SiNWs–Cu NPs.

Wafer-level site-controlled growth of silicon nanowires by Cu pattern dewetting

An approach for the wafer-level synthesis of size- and site-controlled amorphous silicon nanowires (α-SiNWs) is presented in this paper. Microscale Cu pattern arrays are precisely defined on SiO2 films with the help of photolithography and wet etching. Due to dewetting, Cu atoms shrink to the center of patterns during the annealing process, and react with the SiO2 film to open a diffusion channel for Si atoms to the substrate. α-SiNWs finally grow at the center of Cu patterns, and can be tuned by varying critical factors such as Cu pattern volume, SiO2 thickness, and annealing time. This offers a simple way to synthesize and accurately position a SiNW array on a large area.

3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in Li- doped ionic liquid.

Building of hierarchical core-shell hetero-structures is currently the subject of intensive research in the electrochemical field owing to its potential for making improved electrodes for high-performance micro-supercapacitors. Here we report a novel architecture design of hierarchical MnO2@silicon nanowires (MnO2@SiNWs) hetero-structures directly supported onto silicon wafer coupled with Li-ion doped 1-Methyl-1-propylpyrrolidinium bis(trifluromethylsulfonyl)imide (PMPyrrBTA) ionic liquids as electrolyte for micro-supercapacitors. A unique 3D mesoporous MnO2@SiNWs in Li-ion doped IL electrolyte can be cycled reversibly across a voltage of 2.2 V and exhibits a high areal capacitance of 13 mFcm−2. The high conductivity of the SiNWs arrays combined with the large surface area of ultrathin MnO2 nanoflakes are responsible for the remarkable performance of these MnO2@SiNWs hetero-structures which exhibit high energy density and excellent cycling stability. This combination of hybrid electrode and hybrid electrolyte opens up a novel avenue to design electrode materials for high-performance micro-supercapacitors.

Effects of Preparation on the Silicon Nanowires Grown By Electroless Etching

Highly oriented SiNWs arrays prepared by electroless etching method can significantly suppress light reflection across a broad spectrum [1]. The antireflective nature of Silicon nanowires (SiNW’s) revealed by their black color and dull appearances have drawn the attention of researchers [2]. Reflectivity studies on these show a significantly decrease in optical reflectivity down to 1.3% for 10 µm long SiNWs arrays [3]. Electroless etching is a top-down technique for growing SiNWs and does not need costly and expensive equipment to grow SiNWs as bottom-up methods usually do. The electroless process begins with the cleaning of silicon substrates. They were cleaned in acetone, methanol and deionized water, then blow-dried with N2. They are dipped in hydrofluoric acid (HF) 49% solution for 1 min to remove any remaining oxide. The silicon (Si) wafers are dipped for 15 minutes at room temperature in an electroless solution prepared in a petri dish containing silver nitrate (AgNO3) and hydrofluoric acid (HF) [4]. The substrates are dipped and galvanic reactions disperse silver ions on their surface, losing silicon as SiF6. As a consequence of this etching process, the removal of Si via Ag ions results in vertically aligned arrays of SiNWs covered with silver dendrites. At the end of the etching process, the samples were dipped in two baths of deionized water and placed in diluted nitric acid (HNO3) 20% to remove the Ag dendrite layer formed on top of the SiNW arrays. Reflectance on different methods to remove the residual silver particles from SiNW’s when using an unstirred, stirred and sonicated diluted nitric acid (HNO3) 20% solution are presented Fig 1. Figure 1

Fabrication and characterization of p-type SiNW/n-type ZnO heterostructure for optoelectronics application

Semiconductor hybrid structure, known as core-shell heterostructures was fabricated and optical properties were analyzed to make it applicable in future optoelectronic and photonic devices. Large-area, high density, vertically oriented silicon nanowire arrays, synthesized by means of metal-assisted chemical etching of p-type silicon (100) substrate was used as the core and zinc oxide (ZnO) layer, deposited on the SiNW arrays by atomic layer deposition (ALD) was used as shell. The XRD peaks of the heterostructure confirmed the subsequent growth of ZnO film on the template of SiNW arrays having similar crystalline quality. The photoluminescence (PL) spectra showed a very sharp peak at 378 nm, corresponding to the band gap of ZnO material and another broad emission band almost throughout the entire visible range with a peak around 550 nm. The structure also showed a very good antireflection property. The results present that the SiNW/ZnO heterostructure can have potential application in future nanoscale electronic and photonic devices.

Demonstration of higher electron mobility in Si nanowire MOSFETs by increasing the strain beyond 1.3%

In this work we demonstrate the benefit of high uniaxial tensile strain on the performances of Si nanowire (NW) MOSFETs. High uniaxial tensile strained Si NWs were realized by exploiting a “bridge technology” based on patterning of an initial biaxial tensile strained Si on insulator (sSOI) into thin NWs with large pads. Strain relaxation on the large pads amplifiers the strain in the NWs. Strained Si NW-arrays along 〈110〉/(100) direction with tensile strain values up to 2.2% were achieved. We have fabricated n-type Si NW-array MOSFETs with HfO2/TiN gate stack and NiSi2 source/drain contacts. An increase of Ion current by a factor of 1.35 was observed then the uniaxial strain was increased from 1.3% to 1.8% in NW MOSFETs. The enhanced device performance is primarily attributed to a higher electron mobility in the highly strained Si NWs.