An ordered Si nanowire with NiSi2 tip arrays as excellent field emitters

A method was developed to grow ordered silicon nanowire with NiSi(2) tip arrays by reacting nickel thin films on silica-coated ordered Si nanowire (NW) arrays. The coating of thin silica shell on Si NW arrays has the effect of limiting the diffusion of nickel during the silicidation process to achieve the single crystalline NiSi(2) NWs. In the meantime, it relieves the distortion of the NWs caused by the strain associated with formation of NiSi(2) to maintain the straightness of the nanowire and the ordering of the arrays. Other nickel silicide phases such as Ni(2)Si and NiSi were obtained if the silicidation processes were conducted on the ordered Si NWs without a thin silica shell. Excellent field emission properties were found for NiSi(2)/Si NW arrays with a turn on field of 0.82 V μm(-1) and a threshold field of 1.39 V μm(-1). The field enhancement factor was calculated to be about 2440. The stability test showed a fluctuation of about 7% with an applied field of 2.6 V μm(-1) for a period of 24 h. The excellent field emission characteristics are attributed to the well-aligned and highly ordered arrangement of the single crystalline NiSi(2)/Si heterostructure field emitters. In contrast to other growth methods, the present growth of ordered nickel silicide/Si NWs on silicon is compatible with silicon nanoelectronics device processes, and also provides a facile route to grow other well-aligned metal silicide NW arrays. The advantages will facilitate its applications as field emission devices.     

In situ control of atomic-scale Si layer with huge strain in the nanoheterostructure NiSi/Si/NiSi through point contact reaction

Nanoheterostructures of NiSi/Si/NiSi in which the length of the Si region can be controlled down to 2 nm have been produced using in situ point contact reaction between Si and Ni nanowires in an ultrahigh vacuum transmission electron microscope. The Si region was found to be highly strained (more than 12%). The strain increases with the decreasing Si layer thickness and can be controlled by varying the heating temperature. It was observed that the Si nanowire is transformed into a bamboo-type grain of single-crystal NiSi from both ends following the path with low-activation energy. We propose the reaction is assisted by interstitial diffusion of Ni atoms within the Si nanowire and is limited by the rate of dissolution of Ni into Si at the point contact interface. The rate of incorporation of Ni atoms to support the growth of NiSi has been measured to be 7 x 10(-4) s per Ni atom. The nanoscale epitaxial growth rate of single-crystal NiSi has been measured using high-resolution lattice-imaging videos. On the basis of the rate, we can control the consumption of Si and, in turn, the dimensions of the nanoheterostructure down to less than 2 nm, thereby far exceeding the limit of conventional patterning process. The controlled huge strain in the controlled atomic scale Si region, potential gate of Si nanowire-based transistors, is expected to significantly impact the performance of electronic devices.     

Au/Ni bilayers on n-type silicon: Metallurgical and electrical behaviour

Diffusion effects during the formation of silicides in the Ni-Au-Si system were investigated by means of ⁴He⁺ MeV Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar⁺ ion sputtering and X-ray diffraction as a function of the heat treatment temperature (280–350°C) and time (10–1000 min). Schottky barrier heights were used to identify the type of metal present at the silicon surface. Au/Ni/Si and Ni/Au/Si structures were prepared by electron gun deposition of thin gold and nickel films onto n-type Si〈111〉 single crystals. After thermal treatment only Ni₂Si and NiSi compounds were observed and their formation follows the phase order confirmed by previous investigations on the Ni/Si system, with a growth controlled by a lattice diffusion process. In the Ni/Au/Si〈111〉 structure the diffusion of the silicon through the gold film was detected during the formation of nickel silicide and the kinetics of growth of Ni₂Si and NiSi were similar to those studied in the Ni/Si〈100〉 system. A diffusion of gold towards the Si-NiSi interface was observed during the growth of NiSi in the Au/Ni/Si〈111〉 structure. The Schottky barrier height measurements confirm these findings.     

Formation of epitaxial NiSi2 nanowires on Si(100) surface by atomic force microscope nanolithography

Ni nanowries were fabricated by atomic force microscope nanolithography, evaporation, lift-off and annealing processes. Epitaxial NiSi2 nanowires on a Si(100) surface along Si(110) and (100) directions were formed by the rapid thermal annealing treatment of the Ni nanowires at 400 degrees C. The silicide nanowires along the Si(110) direction had coherent type-A Si(111) and Si(100) interfaces, while those along the Si(100) direction had a type-A Si(110) interface. Silicide nanowires were agglomerated when the Ni nanowires were annealed at high temperature (> or = 500 degrees C). The mechanism of formation of a faceted nanowire was discussed based on the minimization of the total surface energy.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Ultralong single-crystal metallic Ni2Si nanowires with low resistivity

Ultralong, single-crystal Ni2Si nanowires sheathed with amorphous silicon oxide were synthesized on a large scale by a chemical vapor transport (CVT) method, using iodine as the transport reagent and Ni2Si powder as the source material. Structural characterization using powder X-ray diffraction, electron microscopy, and energy-dispersive spectroscopy shows that the nanowires have Ni2Si-SiOx core-shell structure with single-crystal Ni2Si core and amorphous silicon oxide shell. The oxide shell is electrically insulating and can be removed by HF etching. Four-terminal electrical measurements show that the single-crystal nanowire has extremely low resistivity of 21 muOmega.cm and is capable of supporting remarkably high failure current density >108 A/cm2. These unique Ni2Si nanowires are very attractive nanoscale building blocks for interconnects and fully silicided (FUSI) gate applications in nanoelectronics.     

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.     

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.     

The influence of surface oxide on the growth of metal/semiconductor nanowires

We report the critical effects of oxide on the growth of nanostructures through silicide formation. Under an in situ ultrahigh vacuum transmission electron microscope, it is observed from the conversion of Si nanowires into the metallic PtSi grains epitaxially through controlled reactions between lithographically defined Pt pads and Si nanowires. With oxide, instead of contact area, single crystal PtSi grains start forming either near the center between two adjacent pads or from the ends of Si nanowires, resulting in the heterostructure formation of Si/PtSi/Si. Without oxide, transformation from Si into PtSi begins at the contact area between them, resulting in the heterostructure formation of PtSi/Si/PtSi. The nanowire heterostructures have an atomically sharp interface with epitaxial relationships of Si(20-2)//PtSi(10-1) and Si[111]//PtSi[111]. Additionally, it has been observed that the existence of oxide significantly affects not only the growth position but also the growth behavior and the growth rate by two orders of magnitude. Molecular dynamics simulations have been performed to support our experimental results and the proposed growth mechanisms. In addition to fundamental science, the significance of the study matters for future processing techniques in nanotechnology and related applications as well.     

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.     

Kinetic competition model and size-dependent phase selection in 1-D nanostructures

The first phase selection and the phase formation sequence between metal and silicon (Si) couples are indispensably significant to microelectronics. With increasing scaling of device dimension to nano regime, established thermodynamic and kinetic models in bulk and thin film fail to apply in 1-D nanostructures. Herein, we present an unique size-dependent first phase formation sequence in 1-D nanostructures, with Ni-Si as the model system. Interfacial-limited phase which forms the last in thin film, NiSi(2), appears as the dominant first phase at 300-800 °C due to the elimination of continuous grain boundaries in 1-D silicides. On the other hand, θ-Ni(2)Si, the most competitive diffusion-limited phase takes over NiSi(2) and wins out as the first phase in small diameter nanowires at 800 °C. Kinetic parameters extracted from in situ transmission electron microscope studies and a modified kinetic growth competition model quantitatively explain this observation. An estimated critical diameter from the model agrees reasonably well with observations.     

Growing Oxide Nanowires and Nanowire Networks by Solid State Contact Diffusion into Solution‐Processed Thin Films

New techniques to directly grow metal oxide nanowire networks without the need for initial nanoparticle seed deposition or postsynthesis nanowire casting will bridge the gap between bottom‐up formation and top‐down processing for many electronic, photonic, energy storage, and conversion technologies. Whether etched top‐down, or grown from catalyst nanoparticles bottom‐up, nanowire growth relies on heterogeneous material seeds. Converting surface oxide films, ubiquitous in the microelectronics industry, to nanowires and nanowire networks by the incorporation of extra species through interdiffusion can provide an alternative deposition method. It is shown that solution‐processed thin films of oxides can be converted and recrystallized into nanowires and networks of nanowires by solid‐state interdiffusion of ionic species from a mechanically contacted donor substrate. NaVO₃ nanowire networks on smooth Si/SiO₂ and granular fluorine‐doped tin oxide surfaces can be formed by low‐temperature annealing of a Na diffusion species‐containing donor glass to a solution‐processed V₂O₅ thin film, where recrystallization drives nanowire growth according to the crystal habit of the new oxide phase. This technique illustrates a new method for the direct formation of complex metal oxide nanowires on technologically relevant substrates, from smooth semiconductors, to transparent conducting materials and interdigitated device structures.     

Controlled growth of Si nanowire arrays for device integration

Silicon nanowires were synthesized, in a controlled manner, for their practical integration into devices. Gold colloids were used for nanowire synthesis by the vapor-liquid-solid growth mechanism. Using SiCl4 as the precursor gas in a chemical vapor deposition system, nanowire arrays were grown vertically aligned with respect to the substrate. By manipulating the colloid deposition on the substrate, highly controlled growth of aligned silicon nanowires was achieved. Nanowire arrays were synthesized with narrow size distributions dictated by the seeding colloids and with average diameters down to 39 nm. The density of wire growth was successfully varied from approximately 0.1-1.8 wires/microm2. Patterned deposition of the colloids led to confinement of the vertical nanowire growth to selected regions. In addition, Si nanowires were grown directly into microchannels to demonstrate the flexibility of the deposition technique. By controlling various aspects of nanowire growth, these methods will enable their efficient and economical incorporation into devices.     

Effects of stress on the interfacial reactions of metal thin films on (0 0 1)Si

The influences of stress on the interfacial reactions of Ti and Ni metal thin films on (0 0 1)Si have been investigated. Compressive stress present in the silicon substrate was found to retard significantly the growth of Ti and Ni silicide thin films. On the other hand, the tensile stress present in the silicon substrate was found to enhance the formation of Ti and Ni silicides. For Ti and Ni on stressed (0 0 1)Si substrates after rapid thermal annealing, the thicknesses of TiSi₂ and NiSi films were found to decrease and increase with the compressive and tensile stress level, respectively. The results clearly indicated that the compressive stress hinders the interdiffusion of atoms through the metal/Si interface, so that the formation of metal silicide films was retarded. In contrast, tensile stress facilitates the interdiffusion of atoms. As a result, the growth of Ti and Ni silicide is promoted.     

Structure and oxidation resistance of plasma sprayed Ni–Si coatings on carbon steel

Ni–Si coatings consisting of mainly NiSi₂ and NiSi were deposited on a carbon steel by air plasma spraying. Isothermal oxidation tests of the carbon steel substrates with the Ni–Si coatings at 500–800 °C have been carried out. The result indicated that a protective SiO₂-based oxide scale was formed on the surface of the coatings after oxidation. On the other hand, during oxidation, phase transformation occurred among the NiSi₂, NiSi and Ni₂Si phases constructing the Ni–Si coatings. This was caused by the extraction of silicon from the silicides and the reformation of silicides at the silcide/Si-blocks interface. Above 700 °C, the outward diffusion of iron and carbon became very fast and consequently decarburization happened at the coating/substrates interface, which induced the formation of pores in the substrates near the interface. In addition, grain boundary oxidation of Cr in the steel substrate was observed above 700 °C.     

Metal-induced crystallization of amorphous Si thin films assisted by atomic layer deposition of nickel oxide layers

Atomic layer deposition (ALD) of nickel oxide was applied to the nickel-induced crystallization of amorphous Si thin films. The nickel-induced crystallization was monitored as a function of annealing temperature and time using Raman spectroscopy. Since Raman spectroscopy allows for the numerical quantification of structural components, the incubation time and the crystallization rates were estimated as functions of the annealing temperature. The spatial locations of a nickel-based species, probably NiSi2, were investigated using X-ray photoelectron spectrometry. The formed NiSi2 seeds appeared to accelerate the crystallization kinetics in amorphous Si thin films deposited onto glass substrates. The ramifications of the atomic layer deposition are discussed with regard to large-panel displays, with special emphasis on the sophisticated control of the catalytic elements, especially nickel.     

The growth mechanism for silicon oxide nanowires synthesized from an Au nanoparticle/polyimide/Si thin film stack

During pyrolysis of polyimide (PI) thin film, amorphous silicon oxide nanowires (SiO(x)NWs) were produced on a large scale through heat treatment of an Au nanoparticle/PI/Si thin film stack at 1000?°C. It was shown that carbonization of the PI film preceded the nucleation of the SiO(x)NWs. The formation of the SiO(x)NWs was sustained by the oxygen derived from carbonization of the polyimide thin film while Si was provided from the substrate. Au nanoparticles promoted the SiO(x)NW growth by inducing localized melting of the Si substrate and by catalyzing the nanowire growth.     

不同射频溅射功率下直接制备Ca_2Si薄膜(英文)

磁控溅射系统在恒定Ar气压和Ar气流流量下,使用不同射频溅射功率在Si(100)衬底上分别沉积Ca薄膜;随后,800℃真空退火1 h.立方相的Ca2Si薄膜首次、单独、直接生长在Si(100)衬底上.实验结果指出,在多相共生的Ca-Si化合物中,沉积Ca薄膜时的射频溅射功率影响了立方相Ca2Si薄膜的质量;最优化的溅射功率是85 W.另外,退火温度为800℃时,有利于单一相Ca2Si的独立生长.并且,退火时间也是关键因素.     

Silicidation of silicon nanowires by platinum

The solid-state reaction between platinum and silicon nanowires grown by the vapor-liquid-solid technique was studied. The reaction product PtSi is an attractive candidate for contacts to p-type silicon nanowires due to the low barrier height of PtSi contacts to p-type Si in the planar geometry, and the formation of PtSi was the motivation for our study. Silicidation was carried out by annealing Pt on Si nanowires from 250 to 700 degrees C, and the reaction products were characterized by transmission electron microscopy. Strikingly different morphologies of the reacted nanowires were observed depending on the annealing temperature, platinum film thickness, silicon nanowire diameter, and level of unintentional oxygen contamination in the annealing furnace. Conversion to PtSi was successfully realized by annealing above 400 degrees C in purified N2 gas. A uniform morphology was achieved for nanowires with an appropriate combination of Si nanowire diameter and Pt film thickness to form PtSi without excess Pt or Si. Similar to the planar silicidation process, oxygen affects the nanowire silicidation process greatly.     

Dynamics of dysprosium silicide nanostructures on Si(001) and (111) surfaces

The growth and coarsening dynamics of dysprosium silicide nanostructures are observed in real-time using photoelectron emission microscopy. The annealing of a thin Dy film to temperatures in the range of 700–1050 °C results in the formation of epitaxial rectangular silicide islands and nanowires on Si(001) and triangular and hexagonal silicide islands on Si(111). During continuous annealing, individual islands are observed to coarsen via Ostwald ripening at different rates as a consequence of local variations in the size and relative location of the surrounding islands on the surface. A subsequent deposition of Dy onto the Si(001) surface at 1050 °C leads to the growth of the preexisting islands and to the formation of silicide nanowires at temperatures above where nanowire growth typically occurs. Immediately after the deposition is terminated, the nanowires begin to decay from the ends, apparently transferring atoms to the more stable rectangular islands. On Si(111), a low continuous flux of Dy at 1050 °C leads to the growth of kinked and jagged island structures, which ultimately form into nearly equilateral triangular shapes.