Detection of spin polarized carrier in silicon nanowire with single crystal MnSi as magnetic contacts

Uphill Diffusion Induced Point Contact Reaction in Si Nanowires

The events of repeating nucleation in point contact reactions between nanowires of Si and Ni or Co have been revisited here due to uphill diffusion as well as an extremely high supersaturation, over a factor of 1000, needed for the nucleation. Also what is the diameter of the point contact needs to be defined. The stepwise growth of nanoscale epitaxial silicide can occur because the repeating nucleation events are restricted in nanoscale wires.

Silicon Nanowires for Gas Sensing: A Review

The unique electronic properties of semiconductor nanowires, in particular silicon nanowires (SiNWs), are attractive for the label-free, real-time, and sensitive detection of various gases. Therefore, over the past two decades, extensive efforts have been made to study the gas sensing function of NWs. This review article presents the recent developments related to the applications of SiNWs for gas sensing. The content begins with the two basic synthesis approaches (top-down and bottom-up) whereby the advantages and disadvantages of each approach have been discussed. Afterwards, the basic sensing mechanism of SiNWs for both resistor and field effect transistor designs have been briefly described whereby the sensitivity and selectivity to gases after different functionalization methods have been further presented. In the final words, the challenges and future opportunities of SiNWs for gas sensing have been discussed.

A Triode Device with a Gate Controllable Schottky Barrier: Germanium Nanowire Transistors and Their Applications

Electrical contacts often dominate charge transport properties at the nanoscale because of considerable differences in nanoelectronic device interfaces arising from unique geometric and electrostatic features. Transistors with a tunable Schottky barrier between the metal and semiconductor interface might simplify circuit design. Here, germanium nanowire (Ge NW) transistors with Cu₃ Ge as source/drain contacts formed by both buffered oxide etching treatments and rapid thermal annealing are reported. The transistors based on this Cu₃ Ge/Ge/Cu₃ Ge heterostructure show ambipolar transistor behavior with a large on/off current ratio of more than 10⁵ and 10³ for the hole and electron regimes at room temperature, respectively. Investigations of temperature-dependent transport properties and low-frequency current fluctuations reveal that the tunable effective Schottky barriers of the Ge NW transistors accounted for the ambipolar behaviors. It is further shown that this ambipolarity can be used to realize binary-signal and data-storage functions, which greatly simplify circuit design compared with conventional technologies.

Monolithic Axial and Radial Metal-Semiconductor Nanowire Heterostructures

The electrical and optical properties of low-dimensional nanostructures depend critically on size and geometry and may differ distinctly from those of their bulk counterparts. In particular, ultrathin semiconducting layers as well as nanowires have already proven the feasibility to realize and study quantum size effects enabling novel ultrascaled devices. Further, plasmonic metal nanostructures attracted recently a lot of attention because of appealing near-field-mediated enhancement effects. Thus, combining metal and semiconducting constituents in quasi one-dimensional heterostructures will pave the way for ultrascaled systems and high-performance devices with exceptional electrical, optical, and plasmonic functionality. This Letter reports on the sophisticated fabrication and structural properties of axial and radial Al-Ge and Al-Si nanowire heterostructures, synthesized by a thermally induced exchange reaction of single-crystalline Ge-Si core-shell nanowires and Al pads. This enables a self-aligned metallic contact formation to Ge segments beyond lithographic limitations as well as ultrathin semiconducting layers wrapped around monocrystalline Al core nanowires. High-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and μ-Raman measurements proved the composition and perfect crystallinity of these metal-semiconductor nanowire heterostructures. This exemplary selective replacement of Ge by Al represents a general approach for the elaboration of radial and axial metal-semiconductor heterostructures in various Ge-semiconductor heterostructures.

Linear heterostructured Ni2Si/Si nanowires with abrupt interfaces synthesised in solution

Herein, we report a novel approach to form axial heterostructure nanowires composed of linearly distinct Ni silicide (Ni2Si) and Si segments via a one-pot solution synthesis method. Initially, Si nanowires are grown using Au seeds deposited on a Ni substrate with the Si delivery in the solution phase using a liquid phenylsilane precursor. Ni silicide then forms axially along the wires through progressive Ni diffusion from the growth substrate, with a distinct transition between the silicide and pure Si segments. The interfacial abruptness and chemical composition of the heterostructure nanowires was analysed through transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy, aberration corrected scanning transmission electron microscopy and atomically resolved electron energy loss spectroscopy. The method represents a versatile approach for the formation of complex axial NW heterostructures and could be extended to other metal silicide or analogous metal germanide systems.

Controlled Formation of Radial Core-Shell Si/Metal Silicide Crystalline Heterostructures

The highly controlled formation of "radial" silicon/NiSi  core-shell nanowire heterostructures has been demonstrated for the first time. Here, we investigated the "radial" diffusion of nickel atoms into crystalline nanoscale silicon pillar 11 cores, followed by nickel silicide phase formation and the creation of a well-defined shell structure. The described approach is based on a two-step thermal process, which involves metal diffusion at low temperatures in the range of 200-400 °C, followed by a thermal curing step at a higher temperature of 400 °C. In-depth crystallographic analysis was performed by nanosectioning the resulting silicide-shelled silicon nanopillar heterostructures, giving us the ability to study in detail the newly formed silicide shells. Remarkably, it was observed that the resulting silicide shell thickness has a self-limiting behavior, and can be tightly controlled by the modulation of the initial diffusion-step temperature. In addition, electrical measurements of the core-shell structures revealed that the resulting shells can serve as an embedded conductive layer in future optoelectronic applications. This research provides a broad insight into the Ni silicide "radial" diffusion process at the nanoscale regime, and offers a simple approach to form thickness-controlled metal silicide shells in the range of 5-100 nm around semiconductor nanowire core structures, regardless the diameter of the nanowire cores. These high quality Si/NiSi core-shell nanowire structures will be applied in the near future as building blocks for the creation of utrathin highly conductive optically transparent top electrodes, over vertical nanopillars-based solar cell devices, which may subsequently lead to significant performance improvements of these devices in terms of charge collection and reduced recombination.

Nanoscale Conductive Channels in Silicon Whiskers with Nickel Impurity

The magnetization and magnetoresistance of Si whiskers doped with <Ni, B> to boron concentrations corresponding to the metal-insulator transition (2 × 10¹⁸ cm^-3 ÷ 5 × 10¹⁸ cm^-3) were measured at high magnetic fields up to 14 T in a wide temperature range 4.2-300 K. Hysteresis of the magnetic moment was observed for Si p-type whiskers with nickel impurity in a wide temperature range 4.2-300 K indicating a strong interaction between the Ni impurities and the possibility of a magnetic cluster creation. The introduction of Ni impurity in Si whiskers leads to appearance and increase of the magnitude of negative magnetoresistance up to 10% as well as to the decrease of the whisker resistivity in the range of hopping conductance at low temperatures. The abovementioned effects were explained in the framework of appearance of magnetic polarons leading to modification of the conductive channels in the subsurface layers of the whiskers.

Europium Silicide - a Prospective Material for Contacts with Silicon

Metal-silicon junctions are crucial to the operation of semiconductor devices: aggressive scaling demands low-resistive metallic terminals to replace high-doped silicon in transistors. It suggests an efficient charge injection through a low Schottky barrier between a metal and Si. Tremendous efforts invested into engineering metal-silicon junctions reveal the major role of chemical bonding at the interface: premier contacts entail epitaxial integration of metal silicides with Si. Here we present epitaxially grown EuSi₂/Si junction characterized by RHEED, XRD, transmission electron microscopy, magnetization and transport measurements. Structural perfection leads to superb conductivity and a record-low Schottky barrier with n-Si while an antiferromagnetic phase invites spin-related applications. This development opens brand-new opportunities in electronics.

Synthesis and Three-Dimensional Magnetic Field Mapping of Co2FeGa Heusler Nanowires at 5 nm Resolution

We present the synthesis of Co2FeGa Heusler nanowires and the results of our investigations on their three-dimensional (3D) electric and magnetic internal and external fields mapped by electron holographic tomography (EHT). These fields will be of great importance in next-generation nanomagnets integrated in spintronics and memory devices. The Co2FeGa nanowires with a L21 ordered structure are prepared by a SBA-15 silica-assisted method. The magnetic dipole-like stray fields of several Co2FeGa nanowires are revealed by holographically reconstructed phase images. Based on the measured magnetic phase shifts of an individual nanowire and its 3D reconstruction using EHT, we obtain an internal magnetic induction with a magnitude of 1.15 T and a nonmagnetic surface layer of 10 nm thickness. Furthermore, we also reconstruct the 3D distribution of the electrostatic potential of the same nanowire.

Synthesis and characterization of barium silicide (BaSi2) nanowire arrays for potential solar applications

In order to utilize nanostructured materials for potential solar and other energy-harvesting applications, scalable synthetic techniques for these materials must be developed. Herein we use a vapor phase conversion approach to synthesize nanowire (NW) arrays of semiconducting barium silicide (BaSi2) in high yield for the first time for potential solar applications. Dense arrays of silicon NWs obtained by metal-assisted chemical etching were converted to single-crystalline BaSi2 NW arrays by reacting with Ba vapor at about 930 °C. Structural characterization by X-ray diffraction and high-resolution transmission electron microscopy confirm that the converted NWs are single-crystalline BaSi2. The optimal conversion reaction conditions allow the phase-pure synthesis of BaSi2 NWs that maintain the original NW morphology, and tuning the reaction parameters led to a controllable synthesis of BaSi2 films on silicon substrates. The optical bandgap and electrochemical measurements of these BaSi2 NWs reveal a bandgap and carrier concentrations comparable to previously reported values for BaSi2 thin films.

Electrical spin injection and transport in semiconductor nanowires: challenges, progress and perspectives

Spintronic devices are of fundamental interest for their nonvolatility and great potential for low-power electronics applications. The implementation of those devices usually favors materials with long spin lifetime and spin diffusion length. Recent spin transport studies on semiconductor nanowires have shown much longer spin lifetimes and spin diffusion lengths than those reported in bulk/thin films. In this paper, we have reviewed recent progress in the electrical spin injection and transport in semiconductor nanowires and drawn a comparison with that in bulk/thin films. In particular, the challenges and methods of making high-quality ferromagnetic tunneling and Schottky contacts on semiconductor nanowires as well as thin films are discussed. Besides, commonly used methods for characterizing spin transport have been introduced, and their applicability in nanowire devices are discussed. Moreover, the effect of spin-orbit interaction strength and dimensionality on the spin relaxation and hence the spin lifetime are investigated. Finally, for further device applications, we have examined several proposals of spinFETs and provided a perspective of future studies on semiconductor spintronics.

Dynamic observation on the growth behaviors in manganese silicide/silicon nanowire heterostructures

Metal silicide nanowires (NWs) are very interesting materials with diverse physical properties. Among the silicides, manganese silicide nanostructures have attracted wide attention due to their several potential applications, including in microelectronics, optoelectronics, spintronics and thermoelectric devices. In this work, we exhibited the formation of pure manganese silicide and manganese silicide/silicon nanowire heterostructures through solid state reaction with line contacts between manganese pads and silicon NWs. Dynamical process and phase characterization were investigated by in situ transmission electron microscopy (in situ TEM) and spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM), respectively. The growth dynamics of the manganese silicide phase under thermal effects were systematically studied. Additionally, Al2O3, serving as the surface oxide, altered the growth behavior of the MnSi nanowire, enhancing the silicide/Si epitaxial growth and effecting the diffusion process in the silicon nanowire as well. In addition to fundamental science, this significant study has great potential in advancing future processing techniques in nanotechnology and related applications.

Electrical spin injection and detection in Mn5Ge3/Ge/Mn5Ge3 nanowire transistors

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

Kinetic manipulation of silicide phase formation in Si nanowire templates

The phase formation sequence of silicides in two-dimensional (2-D) structures has been well-investigated due to their significance in microelectronics. Applying high-quality silicides as contacts in nanoscale silicon (Si) devices has caught considerable attention recently for their potential in improving and introducing new functions in nanodevices. However, nucleation and diffusion mechanisms are found to be very different in one-dimensional (1-D) nanostructures, and thus the phase manipulation of silicides is yet to be achieved there. In this work, we report kinetic phase modulations to selectively enhance or hinder the growth rates of targeted nickel (Ni) silicides in a Si nanowire (NW) and demonstrate that Ni31Si12, δ-Ni2Si, θ-Ni2Si, NiSi, and NiSi2 can emerge as the first contacting phase at the silicide/Si interface through these modulations. First, the growth rates of silicides are selectively tuned through template structure modifications. It is demonstrated that the growth rate of diffusion limited phases can be enhanced in a porous Si NW due to a short diffusion path, which suppresses the formation of interface limited NiSi2. In addition, we show that a confining thick shell can be applied around the Si NW to hinder the growth of the silicides with large volume expansion during silicidation, including Ni31Si12, δ-Ni2Si, and θ-Ni2Si. Second, a platinum (Pt) interlayer between the Ni source and the Si NW is shown to effectively suppress the formation of the phases with low Pt solubility, including the dominating NiSi2. Lastly, we show that with the combined applications of the above-mentioned approaches, the lowest resistive NiSi phase can form as the first phase in a solid NW with a Pt interlayer to suppress NiSi2 and a thick shell to hinder Ni31Si12, δ-Ni2Si, and θ-Ni2Si simultaneously. The resistivity and maximum current density of NiSi agree reasonably to reported values.

A general method to measure the Hall effect in nanowires: examples of FeS2 and MnSi

We present a general methodology for measuring the Hall effect on nanostructures with one-dimensional (1D) nanowire morphology. Relying only on typical e-beam lithography, the methodology developed herein utilizes an angled electrode evaporation technique so that the nanowire itself is a shadow mask and an intimate sidewall contact can be formed for the Hall electrodes. A six-contact electrode scheme with offset transverse contacts is utilized that allows monitoring of both the longitudinal resistivity and the Hall resistivity which is extracted from the raw voltage from the transverse electrodes using an antisymmetrization procedure. Our method does not require the use of a highly engineered lithographic process to produce directly opposing Hall electrodes with a very small gap. Hall effect measurements on semiconducting iron pyrite (FeS2) nanowire devices are validated by comparing to Hall effect measurements in the conventional Hall geometry using FeS2 plate devices. This Hall effect measurement is further extended to MnSi nanowires, and the distinct anomalous Hall effect signature is identified for the first time in chiral magnetic MnSi nanowires, a significant step toward identifying the topological Hall effect due to skyrmions in chiral magnetic nanowires.

Fabrication of Ni-silicide/Si heterostructured nanowire arrays by glancing angle deposition and solid state reaction

This work develops a method for growing Ni-silicide/Si heterostructured nanowire arrays by glancing angle Ni deposition and solid state reaction on ordered Si nanowire arrays. Samples of ordered Si nanowire arrays were fabricated by nanosphere lithography and metal-induced catalytic etching. Glancing angle Ni deposition deposited Ni only on the top of Si nanowires. When the annealing temperature was 500°C, a Ni3Si2 phase was formed at the apex of the nanowires. The phase of silicide at the Ni-silicide/Si interface depended on the diameter of the Si nanowires, such that epitaxial NiSi2 with a {111} facet was formed at the Ni-silicide/Si interface in Si nanowires with large diameter, and NiSi was formed in Si nanowires with small diameter. A mechanism that is based on flux divergence and a nucleation-limited reaction is proposed to explain this phenomenon of size-dependent phase formation.

Electrical spin injection and detection in silicon nanowires through oxide tunnel barriers

We demonstrate all-electrical spin injection, transport, and detection in heavily n-type-doped Si nanowires using ferromagnetic Co/Al(2)O(3) tunnel barrier contacts. Analysis of both local and nonlocal spin valve signals at 4 K on the same nanowire device using a standard spin-transport model suggests that high spin injection efficiency (up to ~30%) and long spin diffusion lengths (up to ~6 μm) are achieved. These values exceed those reported for spin transport devices based on comparably doped bulk Si. The spin valve signals are found to be strongly bias and temperature dependent and can invert sign with changes in the dc bias current. The influence of the nanowire morphology on field-dependent switching of the contacts is also discussed. Owing to their nanoscale geometry, ~5 orders of magnitude less current is required to achieve nonlocal spin valve voltages comparable to those attained in planar microscale spin transport devices, suggesting lower power consumption and the potential for applications of Si nanowires in nanospintronics.

Electrical probing of magnetic phase transition and domain wall motion in single-crystalline Mn₅Ge₃ nanowire

In this Letter, the magnetic phase transition and domain wall motion in a single-crystalline Mn(5)Ge(3) nanowire were investigated by temperature-dependent magneto-transport measurements. The ferromagnetic Mn(5)Ge(3) nanowire was fabricated by fully germaniding a single-crystalline Ge nanowire through the solid-state reaction with Mn contacts upon thermal annealing at 450 °C. Temperature-dependent four-probe resistance measurements on the Mn(5)Ge(3) nanowire showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferromagnetism to paramagnetism. The transition temperature was able to be controlled by both axial and radial magnetic fields as the external magnetic field helped maintain the magnetization aligned in the Mn(5)Ge(3) nanowire. Near the magnetic phase transition, the critical behavior in the 1D system was characterized by a power-law relation with a critical exponent of α = 0.07 ± 0.01. Besides, another interesting feature was revealed as a cusp at about 67 K in the first-order derivative of the nanowire resistance, which was attributed to a possible magnetic transition between two noncollinear and collinear ferromagnetic states in the Mn(5)Ge(3) lattice. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric, and stepwise axial magnetoresistance of the Mn(5)Ge(3) nanowire. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn(5)Ge(3) nanowire and the annihilation of domain walls driven by the magnetic field. The Kurkijärvi model was used to describe the domain wall depinning as thermally assisted escape from a single energy barrier, and the fitting on the temperature-dependent depinning magnetic fields yielded an energy barrier of 0.166 eV.

Electrical spin injection into InN semiconductor nanowires

We report on the conditions necessary for the electrical injection of spin-polarized electrons into indium nitride nanowires synthesized from the bottom up by molecular beam epitaxy. The presented results mark the first unequivocal evidence of spin injection into III-V semiconductor nanowires. Utilizing a newly developed preparation scheme, we are able to surmount shadowing effects during the metal deposition. Thus, we avoid strong local anisotropies that arise if the ferromagnetic leads are wrapping around the nanowire. Using a combination of various complementary techniques, inter alia the local Hall effect, we carried out a comprehensive investigation of the coercive fields and switching behaviors of the cobalt micromagnetic spin probes. This enables the identification of a range of aspect ratios in which the mechanism of magnetization reversal is single domain switching. Lateral nanowire spin valves were prepared. The spin relaxation length is demonstrated to be about 200 nm, which provides an incentive to pursue the route toward nanowire spin logic devices.

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.

Ferromagnetic germanide in Ge nanowire transistors for spintronics application

To explore spintronics applications for Ge nanowire heterostructures formed by thermal annealing, it is critical to develop a ferromagnetic germanide with high Curie temperature and take advantage of the high-quality interface between Ge and the formed ferromagnetic germanide. In this work, we report, for the first time, the formation and characterization of Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire transistors, in which the room-temperature ferromagnetic germanide was found through the solid-state reaction between a single-crystalline Ge nanowire and Mn contact pads upon thermal annealing. The atomically clean interface between Mn(5)Ge(3) and Ge with a relatively small lattice mismatch of 10.6% indicates that Mn(5)Ge(3) is a high-quality ferromagnetic contact to Ge. Temperature-dependent I-V measurements on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) nanowire heterostructure reveal a Schottky barrier height of 0.25 eV for the Mn(5)Ge(3) contact to p-type Ge. The Ge nanowire field-effect transistors built on the Mn(5)Ge(3)/Ge/Mn(5)Ge(3) heterostructure exhibit a high-performance p-type behavior with a current on/off ratio close to 10(5), and a hole mobility of 150-200 cm(2)/(V s). Temperature-dependent resistance of a fully germanided Mn(5)Ge(3) nanowire shows a clear transition behavior near the Curie temperature of Mn(5)Ge(3) at about 300 K. Our findings of the high-quality room-temperature ferromagnetic Mn(5)Ge(3) contact represent a promising step toward electrical spin injection into Ge nanowires and thus the realization of high-efficiency spintronic devices for room-temperature applications.

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.

Coaxial metal-silicide Ni2Si/C54-TiSi2 nanowires

One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core-shell Ni(2)Si/C54-TiSi(2) nanowires, 2 μm in length, were grown controllably via a solid-liquid-solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core-shell Ni(2)Si/C54-TiSi(2) nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 μΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core-shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits.

Domain wall motion in synthetic Co2Si nanowires

We report the synthesis of single crystalline Co(2)Si nanowires and the electrical transport studies of single Co(2)Si nanowire devices at low temperature. The butterfly shaped magnetoresistance shows interesting ferromagnetic features, including negative magnetoresistance, hysteretic switch fields, and stepwise drops in magnetoresistance. The nonsmooth stepwise magnetoresistance response is attributed to magnetic domain wall pinning and depinning motion in the Co(2)Si nanowires probably at crystal or morphology defects. The temperature dependence of the domain wall depinning field is observed and described by a model based on thermally assisted domain wall depinning over a single energy barrier.

Degenerate electronic structure of reconstructed MnSi(1.7) nanowires on Si(001)

Higher manganese silicide nanowires have been grown on the Si(001)-2 × 1 surface by the pre-growth of Bi nanolines. Scanning tunnelling microscope (STM) observations show that the nanowire has a linear surface reconstruction with a periodicity of 0.56 nm, and we propose a reconstruction on their surface to reduce the density of dangling bonds, which forms linear structures matching the dimensions from STM. Scanning tunnelling spectroscopy (STS) data agree with previous calculation results and reveal that the nanowires are degenerate semiconductors, with potential application for spintronics.

The growth and applications of silicides for nanoscale devices

Metal silicides have been used in silicon technology as contacts to achieve high device performance and desired device functions. The growth and applications of silicide materials have recently attracted increasing interest for nanoscale device applications. Nanoscale silicide materials have been demonstrated with various synthetic approaches. Solid state reaction wherein high quality silicides form through diffusion of metal atoms into silicon nano-templates and the subsequent phase transformation caught significant attention for the fabrication of nanoscale Si devices. Very interestingly, studies on the diffusion and phase transformation processes at the nanoscale have indicated possible deviations from the bulk and the thin film system. Here we present a review of fabrication, growth kinetics, electronic properties and device applications of nanoscale silicides formed through solid state reaction.

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.

Joule-assisted silicidation for short-channel silicon nanowire devices

We report on a technique enabling electrical control of the contact silicidation process in silicon nanowire devices. Undoped silicon nanowires were contacted by pairs of nickel electrodes, and each contact was selectively silicided by means of the Joule effect. By a real-time monitoring of the nanowire electrical resistance during the contact silicidation process we were able to fabricate nickel-silicide/silicon/nickel-silicide devices with controlled silicon channel length down to 8 nm.

Oxide-confined formation of germanium nanowire heterostructures for high-performance transistors

Over the past several years, the formation of nanowire heterostructures via a solid-state reaction between a semiconductor nanowire and metal contact pads has attracted great interest. This is owing to its ready application in nanowire field-effect transistors (FETs) with a well-controlled channel length using a facile rapid thermal annealing process. We report the effect of oxide confinement on the formation of Ge nanowire heterostructures via a controlled reaction between a vapor-liquid-solid-grown, single-crystalline Ge nanowire and Ni pads. In contrast to the previous formation of Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni(2)Ge and Ge with the confinement of Al(2)O(3) during annealing. Significantly, back-gate FETs based on this Ni(2)Ge/NiGe/Ge/NiGe/Ni(2)Ge heterostructure demonstrated a high-performance p-type transistor behavior, showing a large on/off ratio of more than 10(5) and a high normalized transconductance of 2.4 μS/μm. The field-effect hole mobility was extracted to be 210 cm(2)/(V s). Temperature-dependent I-V measurements further confirmed that NiGe has an ideal ohmic contact to p-type Ge with a small Schottky barrier height of 0.11 eV. Moreover, the hysteresis during gate bias sweeping was significantly reduced after Al(2)O(3) passivation, and our Ω-gate Ge nanowire FETs using Al(2)O(3) as the top-gate dielectric showed an enhanced subthreshold swing and transconductance. Therefore, we conclude that the Al(2)O(3) layer can effectively passivate the Ge surface and also serve as a good gate dielectric in Ge top-gate FETs. Our innovative approach provides another freedom to control the growth of nanowire heterostructure and to further achieve high-performance nanowire transistors.

Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.

Diffusion-driven crystal structure transformation: synthesis of Heusler alloy Fe3Si nanowires

We report fabrication of Heusler alloy Fe(3)Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe(3)Si nanowire ensemble show high-temperature ferromagnetic properties with T(c) >> 370 K. This methodology is also successfully applied to Co(2)Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize.