Photovoltaic measurements in single-nanowire silicon solar cells

Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.     

Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions

Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths.     

High-resolution detection of Au catalyst atoms in Si nanowires

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.     

Single-crystal CdSe nanowires prepared via vapor-phase growth assisted with silicon

Hexagonal cadmium selenide (CdSe) nanowires, with diameter around 20 nm, were synthesized using a simple vapor-phase growth. Silicon (Si) powder acts as a source material assisting the synthesis, which is very important to the formation of the CdSe nanowires. We also suggest that self-catalysis at the Cd-terminated (0001) surface, together with the assistance action of Si, leads to the formation of wire-like structures to be formed. Meanwhile, the assistance of Si is responsible for the fineness and uniformity of the CdSe nanowires. The possible growth mechanism of the CdSe nanowires is proposed, and the optical property of the as-grown CdSe nanowires is characterized.     

化学气相沉积法制备GaN纳米线和纳米棒

采用浸渍法在未抛光的硅衬底上涂抹一层NiCl2薄膜,通过化学气相沉积法(CVD)制备出高质量的GaN纳米线和纳米棒.X射线衍射(XRD)、傅立叶红外吸收光谱(FTIR)、选区电子衍射(SAED)和高分辨透射电子显微镜(HRTEM)的分析结果表明,采用此方法得到了六方纤锌矿结构的GaN单晶纳米线.通过扫描电镜(SEM)观察发现纳米线的形貌,纳米线的直径在50～200nm之间,纳米棒的直径在200～800nm之间.     

Ni-silicide growth kinetics in Si and Si/SiO2 core/shell nanowires

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from ～ 10 to 100 nm is presented. For temperatures between 300 and 440?°C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Ni flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for (111) orientated SiNWs. In situ TEM silicidation experiments show that NiSi(2) is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.     

Dramatic reduction of surface recombination by in situ surface passivation of silicon nanowires

Nanowires have unique optical properties and are considered as important building blocks for energy harvesting applications such as solar cells. However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude, often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. Reducing the recombination by surface passivation is crucial for the realization of high-performance nanosized optoelectronic devices but remains largely unexplored. Here we show that a thin layer of amorphous silicon (a-Si) coated on a single-crystalline silicon nanowire, forming a core-shell structure in situ in the vapor-liquid-solid process, reduces the surface recombination nearly 2 orders of magnitude. Under illumination of modulated light, we measure a greater than 90-fold improvement in the photosensitivity of individual core-shell nanowires, compared to regular nanowires without shell. Simulations of the optical absorption of the nanowires indicate that the strong absorption of the a-Si shell contributes to this effect, but we conclude that the effect is mainly due to the enhanced carrier lifetime by surface passivation.     

Broadband photodetector of high quality Sb2S3 nanowire grown by chemical vapor deposition

Low dimensional semiconductors can be used for various electronic and optoelectronic devices because of their unique structure and property.In this work,one-dimensional Sb2S3 nanowires(NWs)with high crystallinity were grown via chemical vapor deposition(CVD)technique on SiO2/Si substrates.The Sb2S 3 NWs exhibited needle-like structures with inclined cross-sections.The lengths of Sb2S3 nanowires changed from 7 to 13 pm.The photodetection properties of Sb2S3 nanowires were comprehensively and systematically characterized.The Sb2S3 photodetectors show a broadband photoresponse ranging from ultraviolet(360 nm)to near-infrared(785 nm).An excellent specific detectivity of 2.1×1014 Jones,high external quantum efficiency of 1.5×104％,sensitivity of 2.2×104 cm2W-1 and short response time of less than 100 ms was achieved for the Sb2S3 NW photodetectors.Moreover,the Sb2S3 NWs showed out-standing switch cycling stability that was beneficial to the practical applications.The high-quality Sb2S3 nanowires fabricated by CVD have great application potential in semiconductor and optoelectronic fields.     

Doping concentration of GaN nanowires determined by opto-electrical measurements

The influence of n-doping on the electrical transport properties of GaN nanowires is investigated by photoconductivity measurements on wires with different diameters. The electrical transport in nanowires is extremely sensitive to the wire diameter because of the size dependent barrier for surface recombination. This effect is used to determine the doping level of the nanowires and to complete and consolidate our previously developed surface recombination model for GaN nanowires.     

Contactless measurement of surface dominated recombination in gold- and aluminum-catalyzed silicon vapor-liquid-solid wires

Carrier lifetimes of Si micro/nanowires grown by the vapor-liquid-solid method are measured using an extension of the classic contactless photoconductivity decay method. The samples measured consist of a thin aggregated film of oxide passivated wires on a fused silica carrier. Au catalyzed wires in the 392-730 nm diameter range are studied. Recombination in these wires is controlled by the surface or near surface effects, not bulk Au impurities. The lifetimes of Au- and Al-catalyzed wires of comparable diameter are measured. The Al wires are found to have slightly longer lifetimes than those grown with Au at a comparable diameter. Across all samples, the lifetimes measured range was from 0.2 to 1.0 ns. The surface controlled nature of the recombination measured implies larger diameter wires will offer better performance in devices that rely on minority carrier transport.     

Au／SiO2复合纳米颗粒膜及SiO2纳米线的制备

室温下用磁控溅射法在Si（111）衬底上生成Au／SiO2复合纳米颗粒膜,并分不同温度进行退火处理。1000℃退火时自组装生成空间分布均匀（直径约为70nm）的Au纳米点,从而用自组装生长方法制备了生长一维纳米材料的模板,然后,将Au催化剂模板在1100℃下退火处理,生成纳米线,SEM和TEM测试,制备的SiO2纳米线直径约为100nm,长度约为4μm,表面光滑,直且粗细均匀。     

Thickness optimized nanocrystalline ZnO-coated silicon nanowires for cold cathode application

Silicon nanowire is an important material for the potential use as a cold cathode, but there are some bottlenecks like oxidation of the surface during field emission thereby degradation of its performance. To compete with carbon based nanostructures in this field the performance of Si nanowires as field emitter should be improved. Here, we report a simple technique for the significant improvement of field emission properties of Si nanowires by ZnO nanoparticle coating. Boron-doped p-type Si wafers were chemically etched to synthesize vertically aligned silicon nanowires and they were coated with different thicknesses of ZnO layer by radio frequency magnetron sputtering technique. The nanostructured thin films were studied by X-ray photoelectron spectroscopy for compositional and valence states information while their morphological information was obtained by a field emission scanning electron microscope and a high resolution transmission electron microscope. The field assisted electron emission performance of Si nanowires significantly improved for the thickness optimized ZnO coating. The photoluminescence spectra showed a peak at ~558 nm assigned to surface defect states of ZnO and the field emission from Si nanowires coated with ZnO for different times were correlated with the surface defect structures. The mechanism of such improvement is also discussed.     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Diameter-dependent or independent: toward a mechanistic understanding of the vapor-liquid-solid si nanowire growth rate

Si nanowires have received continued increased attention because they keep the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most Si nanowires are grown by vapor-liquid-solid mechanism, and despite many years of study, this growth mechanism remains under lively debate. For instance, contradictory results have been reported on the effect of diameter size on nanowire growth rate. Here, we developed a universal kinetic model of Si nanowire growth based on surface diffusion which takes into account adatom diffusion from the sidewall and substrate surface into the liquid droplet as well as the Gibbs-Thomson effect. Our analysis shows that the diameter independence for Si nanowires is affected by the interplay between the Gibbs-Thomson effect and the surface diffusion, whereas the diameter dependence is mainly influenced by the Gibbs-Thomson effect. The results based on the proposed model are in good agreement with experimental data.     

Catalyst-free synthesis of ZnO nanowire arrays on zinc substrate by low temperature thermal oxidation

High density ZnO nanowire arrays were successfully synthesized without catalyst by direct oxidation of zinc substrate in air below 400 °C, lower than the melting point of zinc metal. The as-grown ZnO nanowires are single crystalline with a Wurzite structure extending in <110> direction. The diameters of the ZnO nanowires range from 20 to 150 nm and the lengths from several to over ten micrometers. Room temperature photoluminescence measurements reveal an intense ultraviolet emission at 397 nm. The present work highlights the promise of the low temperature, direct oxidation process in the high-yield synthesis of high quality semiconductor nanowire arrays for nano-devices.     

Growth of silicon nanowires of controlled diameters by electrodeposition in ionic liquid at room temperature

Silicon nanowires were fabricated for the first time by electrochemical template synthesis at room temperature. This innovative, cheap, and simple process consists of electroreduction of Si ions using a nonaqueous solvent and insulating nanoporous membranes with average pore diameters from 400 to 15 nm which fix the nanowires diameters. Characterization techniques such as scanning and transmission electron microscopies, infrared absorption measurements, X-ray diffraction experiments, energy dispersive X-ray, and Raman spectrometries show that the as-deposited silicon nanowires are amorphous, composed of pure Si and homogeneous in sizes with average diameters and lengths well matching with the nanopores' diameters and the thicknesses of the membranes. Thanks to annealing treatments, it is possible to crystallize the Si nanowires, demonstrating the potentiality for this innovative electrochemical process to obtain a wide range of Si nanowires with well controlled diameters and lengths.     

A generic approach for vertical integration of nanowires

We report on the collective integration technology of vertically aligned nanowires (NWs). Si?and ZnO NWs have been used in order to develop a generic technological process. Both mineral and organic planarizations of the as-grown nanowires have been achieved. Chemical vapour deposition (CVD) oxides, spin on glass (SOG), and polymer have been investigated as filling materials. Polishing and/or etching of the composite structures have been set up so as to obtain a suitable morphology for the top and bottom electrical contacts. Electrical and optical characterizations of the integrated NWs have been performed. Contacts ohmicity has been demonstrated and specific contact resistances have been reported. The photoconducting properties of polymer-integrated ZnO NWs have also been investigated in the UV-visible range through collective electrical contacts. A small increase of the resistivity in the ZnO NWs under sub-bandgap illumination has been observed and discussed. A comparison of the photoluminescence (PL) spectra at 300?K of the as-grown and SOG-integrated ZnO nanowires has shown no significant impact of the integration process on the crystal quality of the NWs.     

Synthesis and characterization of ZnO nanowires for nanosensor applications

In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy (RS) have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H₂ of a gas nanosensor based on an individual ZnO nanowire is also reported.     

Synthesis and characterization of amorphous silicon oxide nanowires embedded with Ni nanoparticles

The growth of silicon oxide nanowires (SiOxNWs) was obtained by thermal process of nickel (Ni) nanoparticles (NPs) deposited on silicon (Si) wafer in mixed gases of nitrogen (N₂) and hydrogen (H₂). TEM analysis showed that SiOxNWs had diameters ranging from 100 to 200 nm with lengths extending up to a few μm and their structure was amorphous. SiOxNWs were grown by the reaction between Ni NPs and Si wafer and Ni NPs acted as catalysts. Ni silicides (NixSi) were also formed inside the wafer by Ni diffusion into Si wafer.     

Method for electrical characterization of nanowires

The ability to control conductivity in semiconductor nanostructures is often challenged by surface states trapping the majority of the charge carriers. Addressing this challenge requires a reliable method for assessing electrical properties such as carrier concentration and mobility. Unfortunately, here we are facing another challenge, as the Hall effect is geometrically inapplicable to nanowires while the field effect model is also challenged by the geometry of the common nanowire field effect transistor, and can only yield channel mobility which is very different from Hall mobility. In this paper, we propose a method that combines resistivity and photovoltage measurements with a chemical perturbation to the surface to measure carrier concentration and mobility, as a function of wire diameter, and also to measure the surface state density and the surface band bending before and after the chemical treatment. We apply this method to CVD grown GaN nanowires, before and after a mild HCl etch. Using transmission electron microscope and x-ray photoelectron spectrometry we find that HCl removes the native gallium oxide. The etch is found to reduce the surface state density from 1 × 10(12) to 5.3 × 10(11) cm(2), which is calculated from a reduction of the critical radius for full depletion from 7.6 to 4 nm, and a reduction of the surface band bending from 0.53 to 0.29 eV, observed using surface photovoltage. On the average, the values of carrier concentration we obtain are about ten times smaller, and the mobility about ten times greater, than values obtained using field effect transistors. Interestingly, the weak size dependence of the mobility disappears after etching, suggesting a causal linkage between the as-grown size dependence of the mobility and the density of surface states. The proposed method provides an experimental handle to the study of surface states and their effects on the electrical properties of nanowires.