Uniform and position-controlled InAs nanowires on 2" Si substrates for transistor applications

This study presents a novel approach for indirect integration of InAs nanowires on 2' Si substrates. We have investigated and developed epitaxial growth of InAs nanowires on 2' Si substrates via the introduction of a thin yet high-quality InAs epitaxial layer grown by metalorganic vapor phase epitaxy. We demonstrate well-aligned nanowire growth including precise position and diameter control across the full wafer using very thin epitaxial layers (<300 nm). Statistical analysis results performed on the grown nanowires across the 2' wafer size verifies our full control on the grown nanowire with 100% growth yield. From the crystallographic viewpoint, these InAs nanowires are predominantly of wurtzite structure. Furthermore, we show one possible device application of the aforementioned structure in vertical wrap-gated field-effect transistor geometry. The vertically aligned InAs nanowires are utilized as transistor channels and the InAs epitaxial layer is employed as the source contact. A high uniformity of the device characteristics for numerous transistors is further presented and RF characterization of these devices demonstrates an f(t) of 9.8 GHz.     

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.     

Ledge-flow-controlled catalyst interface dynamics during Si nanowire growth

Self-assembled nanowires offer the prospect of accurate and scalable device engineering at an atomistic scale for applications in electronics, photonics and biology. However, deterministic nanowire growth and the control of dopant profiles and heterostructures are limited by an incomplete understanding of the role of commonly used catalysts and specifically of their interface dynamics. Although catalytic chemical vapour deposition of nanowires below the eutectic temperature has been demonstrated in many semiconductor-catalyst systems, growth from solid catalysts is still disputed and the overall mechanism is largely unresolved. Here, we present a video-rate environmental transmission electron microscopy study of Si nanowire formation from Pd silicide crystals under disilane exposure. A Si crystal nucleus forms by phase separation, as observed for the liquid Au-Si system, which we use as a comparative benchmark. The dominant coherent Pd silicide/Si growth interface subsequently advances by lateral propagation of ledges, driven by catalytic dissociation of disilane and coupled Pd and Si diffusion. Our results establish an atomistic framework for nanowire assembly from solid catalysts, relevant also to their contact formation.     

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.     

Some aspects of substrate pretreatment for epitaxial Si nanowire growth

We report on the influence of the surface pretreatment for vapor-liquid-solid growth of epitaxial silicon nanowires with gold catalyst and silane precursor on Si(111) substrates. In this paper we make it obvious that a thin native oxide layer on the Si substrate-as is present under most technological conditions-or a thin layer of oxide formed on top of the catalytic gold particle restrain nucleation and nanowire growth. High resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to demonstrate Si diffusion from the substrate through the catalytic Au layer and further the formation of a thin oxide layer atop. Based on this observation we present a sample pretreatment practice, making the catalyst insensitive for further oxide formation, thereby preserving epitaxy for nanowire synthesis.     

Repetitive growth stages in the solid phase epitaxy of silicon

We have studied the solid phase epitaxial growth of Si in the thin film system with a layer structure of Si(〈100〉 substrate)/Pd silicide/Si(amorphous). At approximately 500°C the 〈100〉Si single-crystal substrate grows with a corresponding consumption of the Si(amorphous) layer. Growth starts with nucleation of islands on the substrate. These islands then grow laterally to form a uniform layer. A second stage of island growth then develops and the process goes on repetitively. However, differences between the first and subsequent stages are observed.     

In situ observations of endotaxial growth of CoSi2 nanowires on Si(110) using ultrahigh vacuum transmission electron microscopy

We report in situ observations of the growth of endotaxial CoSi(2) nanowires on Si(110) using an ultrahigh vacuum transmission electron microscope with a miniature electron-beam deposition system located above the pole-piece of the objective lens. Metal deposition at 750-850?°C results in formation of coherently strained silicide nanowires with a fixed length/width (L/W) aspect ratio that depends strongly on temperature. Both dimensions evolve with time as L, W ～ t(1/3). To explain this behavior, we propose a fixed-shape growth mode based on thermally activated facet-dependent reactions. A second growth mode is also observed at 850?°C, with dimensions that evolve as L ～ t and W ～ constant. This mode is accompanied by formation of an array of dislocations. We expect that other endotaxial nanowire systems will follow coherently strained growth modes with similar geometrical constraints, as well as dislocated growth modes with different growth kinetics.     

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.     

Fabrication of vertical ZnO nanowires on silicon (100) with epitaxial ZnO buffer layer

Vertical ZnO nanowires were successfully grown on epitaxial ZnO (002) buffer layer/Si (100) substrate. The nanowire growth process was controlled by surface morphology and orientation of the epitaxial ZnO buffer layer, which was deposited by radio-frequency (rf) sputtering. The copper catalyzed the vapor-liquid-solid growth of ZnO nanowires with diameter of approximately 30 nm and length of approximately 5.0 microm. The perfect wurtzite epitaxial structure (HCP structure) of the ZnO (0002) nanowires synthesized on ZnO (002) buffer layer/Si (100) substrate results in excellent optical characteristics such as strong UV emission at 380 nm with potential use in nano-optical and nano-electronic devices.     

Growth of nickel silicides in Si and Si/SiOx core/shell nanowires

We exploited the oxide shell structure to explore the structure confinement effect on the nickel silicide growth in one-dimensional nanowire template. The oxide confinement structure is similar to the contact structure (via hole) in the thin film system or nanodevices passivated by oxide or nitride film. Silicon nanowires in direct contact with nickel pads transform into two phases of nickel silicides, Ni31Si12 and NiSi2, after one-step annealing at 550 °C. In a bare Si nanowire during the annealing process, NiSi2 grows initially through the nanowire, followed by the transformation of NiSi2 into the nickel-rich phase, Ni31Si12 starting from near the nickel pad. Ni31Si12 is also observed under the nickel pads. Although the same phase transformations of Si to nickel silicides are observed in nanowires with oxide confinement structure, the growth rate of nickel silicides, Ni31Si12 and NiSi2, is retarded dramatically. With increasing oxide thickness from 5 to 50 nm, the retarding effect of the Ni31Si12 growth and the annihilation of Ni2Si into the oxide confined-Si is clearly observed. Ni31Si12 and Ni2Si phases are limited to grow into the Si/SiOx core-shell nanowire as the shell thickness reaches 50 nm. It is experimental evidence that phase transformation is influenced by the stressed structure at nanoscale.     

Magnetic properties of single-crystalline CoSi nanowires

We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.     

Cobalt silicide nanocables grown on Co films: synthesis and physical properties

Single-crystalline cobalt silicide/SiO(x) nanocables have been grown on Co thin films on an SiO(2) layer by a self-catalysis process via vapor-liquid-solid mechanism. The nanocables consist of a core of CoSi nanowires and a silicon oxide shell with a length of several tens of micrometers. In the confined space in the oxide shell, the CoSi phase is stable and free from agglomeration in samples annealed in air ambient at 900?°C for 1?h. The nanocable structure came to a clear conclusion that the thermal stability of the silicide nanowires can be resolved by the shell encapsulation. Cobalt silicide nanowires were obtained from the nanocable structure. The electrical properties of the CoSi nanowires have been found to be compatible with their thin film counterpart and a high maximum current density of the nanowires has been measured. One way to obtain silicate nanowires has been demonstrated. The silicate compound, which is composed of cobalt, silicon and oxygen, was achieved. The Co silicide/oxide nanocables are potentially useful as a key component of silicate nanowires, interconnects and magnetic units in nanoelectronics.     

Chemical synthesis and magnetotransport of magnetic semiconducting Fe1-xCoxSi alloy nanowires

We report single-crystal nanowires of magnetic semiconducting Fe1-xCoxSi alloys synthesized using a two-component single source precursor approach. Extending our previous syntheses of FeSi and CoSi nanowires from Fe(SiCl3)2(CO)4 and Co(SiCl3)(CO)4 precursors, we found that a homogeneous solution formed upon mixing these two precursors due to melting point suppression. This liquid constitutes the single-source precursor suitable for delivery through chemical vapor deposition, which enables the chemical synthesis of Fe1-xCoxSi alloy nanowires on silicon substrates covered with a thin (1-2 nm) SiO2 layer. Using scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy and mapping, we demonstrate two homogenously mixed alloy nanowire samples with very different Co substitution concentrations (x): 6+/-5%, the ferromagnetic semiconductor regime, and 44+/-5%, the helical magnetic regime. The magnetotransport properties of these alloy nanowires are pronouncedly different from that of the host structures FeSi and CoSi, as well as from one another, and consistent with the physical properties as expected for their corresponding compositions. These novel magnetic semiconducting silicide nanowires will be important building blocks for silicon-based spintronic nanodevices.     

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.     

Silicide formation in contacts to Si nanowires

Silicides, intermetallic compounds formed by the reaction of a metal and Si, have long been used as contacts for metal oxide semiconductor (CMOS) transistors and have more become interesting for other Si nanowire (SiNW) devices. In the following, experimental results for the Ti, V, Pt, Pd, Fe, and Ni–Si systems are reported and placed in the context of prior work on silicide formation from metal films on Si wafers. For the early transition metals Ti and V, the silicide is formed only underneath the contact pad and is Si-rich (MSi₂). For the middle transition metal Fe and late transition metals Pt and Pd, a metal-rich silicide was the first phase observed to form, but poor morphologies were common, making it a challenge to incorporate these contacts into nanowire devices. Nickel contacts were the only ones with well-behaved axial silicide growth away from the contact pad, and silicide formation was strongly dependent on the original SiNW orientation. These findings are discussed in terms of kinetic features of the metal-SiNW systems.     

Epitaxial insertion of gold silicide nanodisks during the growth of silicon nanowires

Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.     

Crystallinity control of ferromagnetic contacts in stressed nanowire templates and the magnetic domain anisotropy

We report the controlled growth of single-crystalline ferromagnetic contacts through solid state reaction at nanoscale. Single-crystal Mn(5)Si(3) and Fe(5)Ge(3) contacts were grown within stressed Si and Ge nanowire templates, where oxide-shells were used to exert compressive stress on the silicide or germanide. Compared to polycrystalline silicide and germanide structures observed within bare nanowires, the built-in high strain in the oxide-shelled nanostructures alters the nucleation behavior of the ferromagnetic materials, leading to single crystal growth in the transverse/radial direction. Interestingly, the compressive stress is also found to affect the magnetic anisotropy of the ferromagnetic contacts. In-plane and out-of-plane magnetization were observed in Fe(5)Ge(3) for different crystal orientations, showing distinctly preferred domain orientations. These interesting results display the capability to control both the crystallinity and the magnetic anisotropy of ferromagnetic contacts in engineered nanostructures.     

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.     

Selective growth of silica nanowires in silicon catalysed by Pt thin film

Selective growth of amorphous silica nanowires on a silicon wafer deposited with Pt thin film is reported. The mechanism of nanowire growth has been established to follow the vapour liquid solid (VLS) model via the PtSi phase acting as the catalyst. Nanowires grow with diameters ranging from 50 to 500?nm. These bottom-up grown nanowires exhibit photoluminescence with a stable emission of blue light at 430?nm under excitation. The effect of varying the seed layer thickness (Pt film) from 2 to 100?nm has been studied. It is observed that, above 10?nm thickness, a continuous layer of Pt(2)Si re-solidifies on the surface, inhibiting the growth of nanowires. The selectivity to the Pt thickness has been exploited to create regions of nanowires connected to conducting silicide (Pt(2)Si) simultaneously in a single furnace treatment. This novel approach has opened the gateways for realizing hybrid interconnects in silicon for various nano-optical applications such as the localization of light, low-dimensional waveguides for functional microphotonics, scanning near-field microscopy, and nanoantennae.     

The morphology of axial and branched nanowire heterostructures

We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.