Silicon Nano Research Articles

Modelling on c-Si/a-Si:H wire solar cells: some key parameters to optimize the photovoltaic performance

Solar cells based on silicon nano- or micro-wires have attracted much attention as a promising path for low cost photovoltaic technology. The key point of this structure is the decoupling of the light absorption from the carriers collection. In order to predict and optimize the performance potential of p - (or n -) doped c-Si/ n -(or p -) doped a-Si:H nanowire-based solar cells, we have used the Silvaco-Atlas software to model a single-wire device. In particular, we have noticed a drastic decrease of the open-circuit voltage ( V oc ) when increasing the doping density of the silicon core beyond an optimum value. We present here a detailed study of the parameters that can alter the V oc of c-Si( p )/a-Si:H ( n ) wires according to the doping density in c-Si. A comparison with simulation results obtained on planar c-Si/a-Si:H heterojunctions shows that the drop in V oc , linked to an increase of the dark current in both structures, is more pronounced for radial junctions due to geometric criteria. These numerical modelling results have lead to a better understanding of transport phenomena within the wire.

Prevention of Solid Electrolyte Interphase Damaging on Silicon by Using Polymer Derived SiCN Ceramics

A new composite anode material for lithium ion batteries, based on nano-silicon particles dispersed in a polymer-derived ceramic (PDC) matrix, was produced, characterized and electrochemically analyzed via cyclic voltammetry (CV). For this purpose a commercial preceramic polymer , namely the polysilazane HTT 1800, was mixed with silicon nano powder and pyrolyzed at 900, 1100, 1300 and 1500 °C. It was found that in comparison with pure silicon, the composite presented an enhanced electrochemical stability during lithium insertion/extraction. Moreover, dispersing the silicon into the ceramic matrix avoids a continuous energy loss related to the formation of a solid electrolyte interphase (SEI). By means of cyclic voltammetry measurements we found that for the composites synthesized at temperatures exceeding 1000 °C, no more losses were observable during subsequent insertion/extraction once the stable SEI was formed.

Efficient excitation of surface plasmons in metal nanorods using large longitudinal component of high index nano fibers

We report theoretical calculations of the mode fields of high index lead silicate and silicon nano fibers, and show that their strong longitudinal component enables efficient excitation of surface plasmons within a silver nanorod placed at the fiber tip. An excitation efficiency 1600 times higher than that of the standard single mode fibers has been achieved using a 350nm diameter silicon fiber at 1.1μm wavelength, while a factor of 640 times higher efficiency is achieved for a 400nm diameter lead silicate F2 glass fiber. The strong localized field emerging from the end of the rod serves as a nano-scale source with adjustable beam width, and such sources offer a new approach to high-resolution microscopy, particle manipulation and sensing.

Piezoresistive Sensitivity and Al Ohmic Contact of Highly Doped Polycrystalline Silicon Nano Thin Films

The piezoresistive and ohmic contact properties of polycrystalline silicon nano thin films were investigated in this paper. The polycrystalline silicon films with different thicknesses and doping concentrations were deposited by LPCVD and doped with boron highly, and then the cantilever beam samples were fabricated by photolithography and wet etching. By measuring the gauge factor and specific contact resistivity, the specific contact resistivity of Al contacts can reach 2.4×10-3Ω·cm2 after the alloying at 450 °C for 20 min; the enhanced piezoresistive effect of highly doped polycrystalline silicon nano thin films was discovered. The conclusions indicated that the enhanced piezoresistive sensitivity of PNTFs is due to the modification of depletion region barrier by ultra high doping and film thickness thinning and the enhancement of tunneling piezoresistive effect. The distinct piezoresistive phenomenon of PNTFs could be utilized for the development and fabrication of miniature piezoresistive sensors.

Formation of Ordered Silicon Nano-Textures Suitable for Field Emission Applications

We report on the use of silicon nano-grass and nano-textured structures for the field emission applications. A triode device, which can be used as a field emission transistor with gate, emitter (cathode) and a collector (anode), has been realized. Anode in this structure is a silicon substrate placed opposite to the silicon grass tips acting as the emitter. The silicon nano grass formation is based on the hydrogen assisted reactive ion etching process where a mixture of O2/H2 and SF6 gases are used in a sequential manner. The results of electrical measurements for this investigation show a sharp rise in the emission current.

Giant Piezoresistance Effects in Silicon Nanowires and Microwires

The giant piezoresistance (PZR) previously reported in silicon nanowires is experimentally investigated in a large number of depleted silicon nano- and microstructures. The resistance is shown to vary strongly with time due to electron and hole trapping at the sample surfaces independent of the applied stress. Importantly, this time-varying resistance manifests itself as an apparent giant PZR identical to that reported elsewhere. By modulating the applied stress in time, the true PZR of the structures is found to be comparable with that of bulk silicon.

Encapsulation of methane in nanotube bundles

In this study, the authors investigate methane storage, and in particular the binding and suction energies of methane in (8, 8) carbon and (10, 10) silicon nanotube bundles. Employing the Lennard-Jones potential together with the continuous approximation, they determine the interaction energy, molecular force and suction energy for a methane molecule in nanotube bundles. The authors approximate the methane molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the centre of the sphere. This is an important assumption in terms of simplifying the calculation, since with this model the particular orientations of the hydrogen atoms do not enter the calculation. Their results indicate that silicon nano structures may be superior candidates for methane storage at high temperatures, particularly in the range 200-400 K.

The role of nano particles (Si) in gate dielectric

The progressively decreasing feature size of the circuit components, particles and electronic devices has tremendously increased the need for studying and investigating the nano structural properties of materials. We have grown titanium oxide on the Si (100) substrate at 500°C and one atmosphere pressure. Some researchers have believed that an intermediate silicon oxide film can be made between titanium oxide film and substrate but which turned to be of less use. We added silicon nano particles (100 nm) into titanium oxide film that could modify the titanium oxide morphology as revealed in x-ray diffraction patterns and scanning electron microscopy images. The obtained results show that the existence of silicon nano particles in titanium oxide film resulted in better stability. The improved film can thus be used as a good gate dielectric film for the future CMOS (Complementary metal oxide semiconductor) devices.

Dual-Gate Single-Electron Transistor with Silicon Nano Wire Channel and Surrounding Side Gates

In this paper, we propose a novel self-aligned dual-gate single-electron transistor having nano wire channel and surrounding side gates. By restricting the control gate effect to the top surface of the channel and intensifying the depletion effect through the surrounding side gates, the parasitic metal–oxide–silicon field-effect transistor (MOSFET) current and the quantum dot size are expected to be decreased. These advantages of the proposed structure are investigated through three-dimensional (3D) device simulation. In addition, the devices are fabricated by utilizing the silicon process and their electrical characteristics are analyzed at both room temperature and low temperature. Also, diverse device parameters are extracted from the measurement results, and they are systematically compared.

Current-induced recrystallization of polycrystalline silicon nano thin films deposited at different temperatures and its influences on piezoresistive sensitivity and temperature coefficients

To improve the performance and yield of piezoresistive sensors based on polycrystalline silicon nano thin films (PNTFs), the 80 nm-thick PNTFs were deposited on silicon (1 1 1) substrates by LPCVD at different temperatures and fabricated into cantilever beams. The electrical trimming characteristics and the dependences of gauge factor, temperature coefficient of resistance (TCR) and temperature coefficient of gauge factor (TCGF) on resistor trim with different deposition temperatures were measured. Based on the interstitial-vacancy model, the electrical trimming is explained as the mobility increase caused by current-induced recrystallization of grain boundaries. The conclusions indicate that the changes in gauge factor, TCR and TCGF with resistor trim are due to the grain boundary state variation (including scattering center, grain boundary width, tunneling current, localized and extended state conductions). And the electrical trimming method is testified to be applicable for the resistor trimming of PNTF-based sensors after packaging.

Silicon Eye

Silicon eye can be otherwise called as a complete eye implant. The project overview is given in brief by the description of main components. Here in the silicon eye, a biconcave gel lens is used in connection with a micro controller. A porous silicon nano photodiode is placed before the gel lens. This specially designed transparent diode will help in identifying the intensity of light receiving in the beginning of the processing chain. An effective drainage system (with the help of two valves) will control the working of gel lens. All these components together form the primary circuit to enable the process of auto focusing. The micro controller is connected with all components of the system. The primary circuit is connected to a secondary circuit which consists of an artificial silicon retina and a chemical synapse. By the combined and co-ordinate working of both these will enable vision. The power supply, which all the electrical components here need is given by a series of nano paper batteries placed beneath the retinal layer (carbon nano tubes can also be implemented instead or along with it). The converted electrical impulses (from intensity of light received) will be carried to the visual area of brain through optic nerve by the effective interaction with a number of artificial chemical synapses. Finally this silicon eye will be an effective implant for the damaged eye. And we are sure that this will be a great breakthrough in the modern medicine.

Intact sublimation of silicon nanocrystals evidenced via HREM imaging and EELS in a dedicated STEM

Silicon nano crystals (NCs) have attracted considerable interest for possible uses in optoelectronics1 As the particle size decreases the properties of NCs become increasingly sensitive to the surface termination.2, 3 Monolayer chemistries4-10 have been exploited to control the physicochemical properties. NCs are often prepared by vapour-phase deposition techniques; using these they can be conveniently analysed via gas phase analysis techniques, such as mass spectrometry. This cannot be employed, however, if NCs are not synthesized in the gas phase. Here we present a STEM study of undecyl-capped SiNCs, evaporated intact upon heating in ultrahigh vacuum at 200 °C and collected on a variety of solid substrates, including carbon-coated TEM grids. The BF- and HAADF lattice images confirm that the particles have a crystalline core with Si-lattice spacings. The presence of Si in the core is also confirmed by Si-L edge EELS, which reveals furthermore the presence of a surface oxide.

Silicon nanodot arrays patterned using diblock copolymer templates

Dense and periodic arrays of holes and Si nano dots were fabricated on silicon substrate. The holes were approximately 20–40 nm wide, 40 nm deep and 40–80 nm apart. To obtain nano-size patterns, self-assembling resists were used to produce layer of hexagonally ordered parallel cylinders of polymethylmethacrylate (PMMA) in polystyrene (PS) matrix. The PMMA cylinders were degraded and removed with acetic acid rinse to produce a PS. 10 nm-thick Au thin film was deposited by using electron beam evaporator. PS template was removed by lift-off process. Arrays of Au nano dot were transferred by using fluorine-based reactive ion etching. Au nano dots were removed by sulfuric acid. Si nano dots size and height were 24–70 nm and 20–30 nm respectively. Sequential oxidation-wet etching method reduced size of Si nano dots. Reduced sized silicon nano dots diameter and height were 18 nm and 12 nm, respectively. Nanopatterned holes sizes were observed by field emission scanning electron microscope (FESEM) and atomic force microscopy.

Silicon nano particles: Surface characterization, defects and electronic properties

AbstractNano scaled silicon particles (nc‐Si) having a diameter of about 3 to 9 nm were synthesized by direct reduction of SiCl4 with H2 in a LaCl3 matrix by a vapour deposition (CVD) route. The particles were characterized by diffuse reflection fourier transform spectroscopy (DR/FT‐IR), photoluminescence (PL), electron paramagnetic resonance (EPR) and microwave detected photo induced current transient spectroscopy (MD‐PICTS). EPR measurements identified dangling bond defects on the surface of as‐prepared nc‐Si particles in the LaCl3 matrix. The number of dangling bonds significantly decreases upon extraction of the nc‐Si with ethanol. As expected, the dangling bonds disappear completely after treatment with 1% HF solution. A strong correlation between presence of dangling bond defects and electronic properties was found. The temperature dependent photoconductivity in conjunction with the EPR and PL results allowed us to analyze the defect structure of the nc‐Si particles. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and characterization of NEMS

We proposed a called “nano-fabrication based on MEMS technology” approach to realize the typical nano-electromechanical structures, such as integrated nano probe, ultrathin cantilever, silicon nano wire, and doubly clamped nano beam, to demonstrate the feasibility and advantages. We also introduced the characterization of nano structures based on laser vibrometer and piezoresistive effect, the latter method was first time applied to investigate a doubly daped nanobeam with a thickness of about 200 nm.

Strain Analysis of Silicon Nanolayers by Grazing Incidence X-ray Diffraction

Grazing incident X-ray diffraction was observed from a thin silicon nano scale overlayer fabricated by oxidation and etch-back in a separation by implantation oxygen wafer at incident angles between 0.01o and 0.1o below the critical angle of total reflection (0.18o). We measured {220} reflections by probing the sample with respect to surface normal and found that the silicon nano scale overlayer has finite domains under strain close to the surface. We also found that annealing the sample up to 1000oC significantly reduced inhomogeneous strain and increased the size of the domains in the surface region of the silicon nano scale overlayer.

Photoluminescence and Electroluminescence Studies on Tb-Doped Silicon Rich Oxide Materials and Devices

Photoluminescence (PL) characteristics of Tb-doped silicon rich oxide (SRO) films prepared by DC-sputtering and post-annealing processes were studied. The silicon richness of the SRO film could be controlled by varying the sputtering power and oxygen concentration in the sputtering chamber. PL emission from the as-deposited sample was found to be composed of Tb 3+ intra 4f transition-related emission and the silicon nano particle-related broad bandwidth emission. Thermal annealing could significantly improve the material properties as well as the PL signals. PL properties depended strongly upon the annealing scheme and silicon richness. Annealing at high temperatures (900 ∼ 1050 °C) enhanced Tb-related emission and suppressed the silicon nano particle-related emission. For samples with different silicon richness, annealing at 950 °C was found to produce higher PL signals than at other temperatures. It was attributed more to lifetime quenching than to concentration quenching. Electroluminescent (EL) devices with a capacitor structure were fabricated, the optimized process condition for the EL device was found to be different from that of PL emission. Among the annealing schemes that were used, wet oxidation was found to improve device performance the most, whereas, dry oxidation was found to improve material property the most. Wet oxidation allowed the breakdown electrical field to increase significantly and to reach above 10 mV·cm −1. The EL spectra showed a typical Tb 3+ emission, agreeing well with the PL spectra. The I-V measurements indicated that for 100 nm thick film, the Fowler-Nordheim tunneling started at an electrical field of around 6 mV·cm −1 and the light emission became detectable at a current density of around 10 −4 A ·cm −2 and higher. Strong electroluminescence light emission was detected when the electrical field was close to 10 mV·cm −1.

Wide Bandgap Semiconductors - Nanowires of p- and n-type Silicon Carbide

ABSTRACTMonocrystalline nanowires of cubic silicon carbide were synthesized using a combined sol-gel and carbothermal reduction process in which tetraethoxysilane was used as primary sili-con and sucrose as carbon source. The diameters of the as-grown nanowires varied depending on process parameters from several tens to several hundreds nanometers, whereas the length of the wires was located in the millimetre region. By precisely controlling the atomic ratio of Si / C, silicon carbide nano wires were synthesized exclusively and pure without the presence of resid-ual carbon or unwanted silica, thus leads to semi-insulating behaviour. Supported by their consis-tence the silicon carbide micro or nano wires can be processed to textile or felt structures and are therefore usable for many applications such as for fireproof clothing, high temperature or chemi-cal filters and composite materials. Additionally during sol-gel synthesis the silicon carbide mi-cro / nano wires were easily doped to achieve p- or n-conduction, guiding to new applications in the field of wide bandgap semiconductors. The structure of 3C-SiC micro and nano wires was determined using scanning electron microscopy, X-ray diffraction, nuclear magnetic resonance and fourier transform infrared spectroscopy. The electronic properties were studied using elec-tron paramagnetic resonance spectroscopy, Hall effect and current-voltage measurements.

Capacitive-type gas sensors combining silicon semiconductor and NaNO 2-based solid electrolyte for NO 2 detection

A capacitive-type NO 2 gas sensor, developed by replacing metal in metal–insulator–semiconductor structure with solid electrolyte as sensing material, was investigated to improve the response and recovery rates and the operating temperature by modifying NaNO 2-sensing material. Binary sensing materials of NaNO 2–Ca 3(PO 4) 2 and NaNO 2–WO 3 were tested for auxiliary phase of the device. The Ca 3(PO 4) 2-containing phase showed a response closer to ideal Nernstian behavior at 140 °C as compared to WO 3-containing phase. On the other hand, the WO 3-containing phase showed faster response and recovery rates at 160 °C, while the response deviated from Nernstian correlation. In order to obtain optimum sensor response, a ternary phase of NaNO 2–Ca 3(PO 4) 2–WO 3 was used as sensing phase. The device with ternary auxiliary phase was found to show more stable and faster response and recovery rates at 130 °C, as compared to previous devices attached with binary phases, corresponding Nernstian correlation in the concentration range of 20–500 ppb NO 2. Furthermore, the effects of modification process on the auxiliary phase were investigated through SEM observation of the surface structures and measurement of ionic conductivity.

Fabrication of individual aligned carbon nanotube for scanning probe microscope

This paper demonstrates the fabrication of a micro-cantilever equipped with a high aspect ratio carbon nanotube (CNT) tip to perform surface characterization with high spatial resolution. A single vertical CNT, synthesized on a freestanding tip-lessmicro cantilever, is intended to be used for scanning probe microscopy (SPM). E-beam lithography and lift-off technique was adopted to define a nano-sized catalyst particle. An individual CNT was then synthesized by using an inductively coupled plasma chemical vapor deposition (ICP-CVD) system. The freestanding individual aligned CNT probe was then used to characterize a silicon nano pillar structure. The surface morphology was scanned in a contact mode and 0.1 nN set-point force. The ultra fine resolution can be characterized without wearing the probe and sample. The CNT probe shows a great potential as a high sensitive sensor operating in an optimally adjusted atomic force microscopy (AFM) system.