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We present a method of epitaxially growing thermodynamically stable gallium nitride (GaN) nanorods via metal-organic chemical vapor deposition (MOCVD) by invoking a two-step self-limited growth (TSSLG) mechanism. This allows for growth of nanorods with excellent geometrical uniformity with no visible extended defects over a 100 mm sapphire (Al2O3) wafer. An ex-situ study of the growth morphology as a function of growth time for the two self-limiting steps elucidate the growth dynamics, which show that formation of an Ehrlich-Schwoebel barrier and preferential growth in the c-plane direction governs the growth process. This process allows monolithic formation of dimensionally uniform nanowires on templates with varying filling matrix patterns for a variety of novel electronic and optoelectronic applications. A color tunable phosphor-free white light LED with a coaxial architecture is fabricated as a demonstration of the applicability of these nanorods grown by TSSLG.

We report on structural chemical state of doped Mn atoms in single crylstalline Mn doped GaN nanowires by X-ray absorption spectroscopy. Anomalous X-ray scattering and K-edge X-ray absorption fine structure measurement make it clear that Mn atoms substitute the Ga sites and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Mn L(2,3)-edges show that doped Mn has local magnetic moment and the electronic configuration of the doped Mn is mainly 3d(5) component. The structural and chemical states of the doped Mn atoms imply that they ascribe to the observed ferromagnetism in these diluted magnetic semiconductor nanowires.

We report on synthesis of Mn-doped Ge nanowires and their magnetic and electrical properties. The nanowires were grown on the silicon substrate by vapor−liquid−solid mechanism using Au as catalyst and GeCl4 and MnCl2 as precursor. Anomalous X-ray scattering measurement makes it clear that Mn atoms are substitutionally incorporated with the diamond network of host Ge sites. Superconducting quantum interference device characterization indicated that the nanowires possess ferromagnetism up to room temperature. X-ray magnetic circular dichroism spectra at Mn L2,3-edges showed that doped Mn has local spin moment with the 3d5 electronic configuration above room temperature, meaning that the ferromagnetism originates from doped Mn2+ ions. Electrical characterization of a nanowire field effect transistor revealed the improved p-type behavior with hole mobility of 4.95 cm2/V·s.

This study reports the magnetic properties of vanadium (V) doped single crystalline silicon carbide nanowires. The first principle calculation indicated that the V-doped cubic SiC phase can exhibit half-metallic ferromagnetic properties that are essential for the realization of spintronic devices. Based on this calculation, V-doped SiC nanowires were fabricated in a chemical vapor deposition process. The single crystalline β-SiC nanowires, which are doped with ca. 4 at.% of V, had diameters of < 100 nm and a length of several µm. High-resolution transmission electron microscopy observations revealed vanadium carbide (VC) phases in the nanowires, even at this low concentration of dopants. Magnetic characterization implies that the nanowires are a mixture of the diamagnetic phase of VC and ferro- or paramagnetic phases of V-doped SiC. These results suggest that the doping of transition metal having high solubility to the SiC phase can lead to the realization of dilute magnetic semiconductor behavior at very low temperature.

We demonstrated that manganese (Mn)-doped GaN nanowires (NWs) exhibit p-type characteristics using current–voltage (I–V) characteristics in both heterojunction p–n structures (GaN:Mn NWs/n-Si substrate) and p–p structures (GaN:Mn NWs/p-Si). The heterojunction p–n diodes were formed by the coupling of the Mn-doped GaN NWs with an n-Si substrate by means of an alternating current (AC) dielectrophoresis-assisted assembly deposition technique. The GaN:Mn NWs/n-Si diode showed a clear current-rectifying behavior with a forward voltage drop of 2.4 V to 2.8 V, an ideality factor of 30 to 37, and a parasitic resistance in the range of 93 kΩ to 130 kΩ. On the other hand, we observed that other heterojunction structures (GaN:Mn NWs/p-Si) showed no rectifying behaviors as seen in p–p junction structures.

The growth kinetics of Si nanowires produced by a vapour–liquid–solid (VLS) mechanism in conjunction with Pt and Au catalysts, respectively, was investigated and compared. Pt was employed as a VLS catalyst for single-crystal Si nanowires in a SiCl 4 -based chemical vapour deposition process. The growth rates were higher with Pt than with Au under all processing conditions. The activation energy was measured as 80 and 130 kJ/mol with the Pt and Au catalysts, respectively. The present results suggest that the rate-determining step is the incorporation of Si atoms in the lattice at the liquid/solid interfaces and, furthermore, the metal catalysts affect this step, resulting in different activation energy.

The oxidation behaviors of Si1-xGex nanowires were investigated using TEM analysis. Thicker nanowires had higher oxide growth rates. Si1-xGex and silicon nanowires exhibited a saturation tendency as the oxidation time increased. The oxide growth rate of Si1-xGex nanowires was slightly faster than that of silicon nanowires. Differences in germanium content also affect the oxidation behavior.

Oxidation characteristics of Si 0.85 Ge 0.15 nanowires were investigated using transmission electron microscopy (TEM) analyses. Si 0.85 Ge 0.15 nanowires were grown in a tube furnace by vapor–liquid–solid (VLS) method and thermally oxidized at 925 °C for 1–8 h. After oxidation, oxide thicknesses were measured using TEM images. Si 0.85 Ge 0.15 nanowires showed a thicker oxide than Si nanowires, for the whole range of oxidation time. The oxidation rate of Si 0.85 Ge 0.15 nanowires significantly decreased in nanowires with diameters less than 150 nm. Long-term oxidation in Si 0.85 Ge 0.15 nanowire resulted in the oxidation of germanium atoms.

We have investigated the size-effect of the nano-Schottky diodes on the electrical transport properties and the temperature-dependent current transport mechanism in a metal-semiconductor nanowire junction (a Ti/GaN nano-Schottky diode) using current-voltage characterization in the range of 300-423 K. We found that the modified mean Schottky barrier height (SBH) was approximately 0.7 eV with a standard deviation of approximately 0.14 V using a Gaussian distribution model of the barrier heights. The slightly high value of the modified mean SBH (approximately 0.11 eV) compared to the results from the thin-film based Ti/GaN Schottky diodes could be due to an additional oxide layer at the interface between the Ti and GaN nanowires. Moreover, we found that the abnormal behavior of the barrier heights and the ideality factors in a Ti/GaN nano-Schottky diode at a temperature below 423 K could be explained by a combination of the enhancement of the tunneling current and a model with a Gaussian distribution of the barrier heights.

The electrochemical characteristics of single crystalline SnO2, ZnO and Si nanowires and their driven mechanism are reported as nanostructural anode materials. As intercalation and deintercalation of Li, Si nanowires are converted to amorphous phases of shorter wire shapes caused by the lattice expansion of the single crystalline Si, resulting in the fading of discharge capacity, although the reversible capacity (2500 mAh/g) in the first cycle is very high. However, oxide nanowries (SnO2 and ZnO) are transformed from a single crystalline structure into a polycrystalline form consisting of nano-sized metallic particles and Li2O crystals within the wires, which maintain their discharge capacity. The results of this study imply that the large surface area and high electrochemical activity of nanowires and nano-sized polycrystalline particles can provide a method to develop a new class of one-dimensional anode nanostructures in lithium-ion rechargeable batteries.

Several vapor phase processes for the preparation of GaN nanowires, such as chemical vapor deposition (CVD), direct reaction (DR), and hydride vapor phase epitaxial growth (HVPE), have been previously reported. To determine the most appropriate route for fabrication and engineering of GaN nanowires, we prepared nanowires via the three aforementioned routes and characterized their microstructures and photoluminescence (PL) properties. All prepared nanowires were single-crystalline, whowing well-defined crystal structure in X-ray diffraction and transmission electron microscopic analyses. However, high-quality nanowires could most readily be obtained by DR. Large-scale and selective area growth of nanowires could most readily be achieved by CVD and HVPE. PL spectra for the nanowires prepared by HVPE showed a red-shifted center wavelength and wider full width-half maximum (FWHM) value as compared to those prepared by DR or CVD. This indicates the presence of unknown impurities and/or defects in the nanowires prepared by HVPE. Our results indicate that high-quality nanowires can be prepared by DR and CVD, while large-scale selective growth can be achieved by CVD and HVPE.

The growth of amorphous silica nanowires by on-site feeding of silicon and oxygen is reported. The nanowires were grown on a nickel-coated oxidized silicon substrate without external silicon or oxygen sources. Transmission electron microscopy observation revealed that the nanowires, which have diameters of less than 50 ㎚ and a length of several micrometers, were grown using a traditional vapor-liquid-solid mechanism. Blue photoluminescence was observed from these nanowires at room temperature. An approach to grow nanowires without external precursors may be useful when integrating nanowires into devices structures. This can benefit the fabrication of nanowire-based nanodevices.

An idea for simultaneously manipulating spin and charge in a single semiconductor medium has resulted in the development of diluted magnetic semiconductors (DMSs), which exhibits surprisingly room temperature ferromagnetic signatures despite having controversial ferromagnetic origin. However, achievement of truly room temperature ferromagnetism by carrier mediation is still the subject of intense research to develop the practical spin-based devices. Nanowires with one-dimensional nanostructure, which offers thermodynamically stable features and typically single crystalline and defect free, have a number of advantages over thin films with respect to studying ferromagnetism in DMSs. This review focuses primarily on our works on GaN -based DMS nanowires, i.e., Mn -doped GaN , Mn -doped AlGaN and Cu -doped GaN nanowires. These DMS nanowires have room temperature ferromagnetism by the local magnetic moment of doping elements that are in a divalent state and in tetrahedral coordination, thus substituting Ga in the wurtzite-type network structure of host materials. Importantly, our evidences indicate that the magnetism is originated from the ferromagnetic interaction driven by the carrier. These outcomes suggest that nanowires are ideal building blocks to address the magnetism in DMS due to their thermodynamic stability, single crystallinity, free of defects and free standing nature from substrate. Nanowires themselves are ideal building blocks for nanodevices and, thus, it would also be helpful in developing DMS-based spin devices.

We demonstrate the fabrication and the electrical transport properties of single crystalline 3C silicon carbide nanowires (SiC NWs). The growth of SiC NWs was carried out in a chemical vapor deposition (CVD) furnace. Methyltrichlorosilane (MTS, CH3SiCl3) was chosen as a source precursor. SiC NWs had diameters of less than 100 nm and lengths of several μm. For electrical transport measurements, as-gown SiC NWs were prepared on a highly doped silicon wafer, pre-patterned by a photo-lithography process, with a 400 nm thick SiO2 layer. Source and drain electrodes were defined by e-beam lithography (EBL). Prior to the metal deposition (Ti/Au : 40 nm/70 nm) by thermal evaporation, the native oxide on SiC NWs was removed by buffered HF. The estimated mobility of carriers is 15 cm2/(Vs) for a source-drain voltage (VSD) of 0.02 V. It is very low compared to that expected in bulk and/or thin film 3C-SiC. The electrical measurements from nanowire-based field effect transistor (FET) structures illustrate that SiC NWs are weak n-type semiconductor. We have also demonstrated a powerful technique, a standard UV photo-lithography process, for fabrication of SiC nanowires instead of using EBL process.

We report on simple techniques for extracting the electrical properties of 1-dimensional semiconductor nanowires using standard ultraviolet (UV) photo-lithography instead of e-beam lithography (EBL), which is a commonly used technique for the fabrication of nanoscale electrical devices. For electrical transport measurement the gallium nitride nanowires (GaN NWs) were prepared by a horizontal hot-wall chemical vapor deposition (CVD) with metallic Ga and NH3 gas for Ga and N sources, and GaN nanowire field effect transistor (FET) structures on a 8×8 mm2 silicon wafer were fabricated by ordinary 2-mask photo-lithography processes. The estimated carrier mobility from the gate-modulation characteristics is on the order of 60 ∼ 70 cm2/V⋅s. We found that our approach is a powerful and simple technique to extract the electrical properties of semiconductor nanowires. The material characteristics of GaN nanowires are also discussed.

We report on a simple and effective ac and dc dielectrophoresis (DEP) method that can be used to align and manipulate semiconductor gallium nitride (GaN) nanowires (NWs) with variations in the type of electrical fields as well as variations of frequency. We observed that the ability of the alignment and the formation of the assembling nanowires (single or a bundle configuration) strongly depend on the magnitude of both the ac and dc electric fields. The yield results indicate that the GaN NWs, using ac DEP, are better aligned with a higher yield rate of approximately 80% over the entire array in the chip than by using dc DEP. In addition, we first demonstrated the simple hybrid p-n junction structures assembled by n-type GaN nanowires together with a p-type silicon substrate (n-GaN NW/p-Si substrate) using dielectrophoresis. From the transport measurements, the p-n junction structures show well-defined current rectifying behaviour with a low reverse leakage current of approximately 3 x 10(-4) A at -25 V. We believe that our unique p-n junction structures can be useful for electronic and optoelectronic nanodevices such as rectifiers and UV nano-LEDs.

We report on the growth and modulation of silicon carbide nanowires (SiC NWs). The NWs were fabricated by chemical vapor deposition (CVD) process, and had diameters of

We report on the fabrication of GaN/GaN:Mn core/shell nanowires (NWs) using a two-step metalorganic chemcial vapor deposition (MOCVD) and chloride-based chemical vapor transport (CVT) process. Structural analyses indicated that the heterostructure NWs were single crystalline and exhibited a core/shell and lozenge structure. The photoluminescence (PL) of the core/shell NWs showed a peak at a center wavelength of 454 nm, which was red-shifted compared to those of GaN and GaN:Mn NWs. This outcome indicates the accumulation of excited carriers at the interfaces that would be helpful in developing novel magnetism in diluted magnetic GaN:Mn semiconductors.

Single-crystalline mullite (3Al2O3·2SiO2) nanowires have been synthesized on silicon substrates by forcing aluminum and chromium chloride powders to react under an ammonia gas flow. The diameter and length of the nanowires have uniform diameters of

We report the conversion of one dimensional (1D) single crystalline SnO2 nanobelts into 1D polycrystalline nanobelts by intercalation of Li. Much improved electrochemical behavior of these 1D superstructures was observed due to “synergy” effect of high surface area and high electrochemical activity of 1D and zero dimensional (0D) structures, respectively.

Vertically aligned InGaN/GaN nanorod (NR)-based phosphor-free light emitting diodes (LEDs) using SiO2 nanohole patterns are demonstrated. The highly ordered SiO2 nanoholes were realized on a 2 μm-thick n+GaN template by a two-step dry etching process. The use of C4F8/O2/Ar plasma chemistries under the low pressure is found to greatly enlarge the bottom diameter of each hole, exhibiting high aspect ratio (AR ∼ 9) and vertical etch profile (∼89°). SAG technique was used to define the height of the GaN NRs while the width is determined by the trimethylgallium flow rate and growth temperature. An LED structure consisted of three-pairs of InGaN/GaN quantum well and AlGaN electron blocking layer on the sidewall of the nanorod in a core-shell structure. The wavelengths were successfully tuned by controlling pitches of the rods, which was caused by the different growth rate and indium incorporation of conformally overgrown InGaN multiquantum wells. At the operating current density of 1.5 A/cm2 (65 mA), NR-based s...

We report on synthesis of Mn-doped Ge nanowires and their magnetic and electrical properties. The vertically aligned Mn:Ge nanowires wer grown on the germanium substrate by vapor-liquid-solid mechanism using Au as catalyst and GeCl 4 and MnCl 2 as precursor. Anomalous X-ray scattering measurement makes it clear that Mn atoms are substitutionally incorporated with the diamond network of host Ge sites. Also X-ray magnetic circular dichroism spectra at Mn L 2,3 -edges showed that doped Mn has local spin moment with the 3d5 electronic configuration above room temperature, meaning that the ferromagnetism originates from doped Mn2+ ions.

An idea for simultaneously manipulating spin and charge in a single semiconductor medium has resulted in the development of diluted magnetic semiconductors (DMSs), which exhibits surprisingly room temperature ferromagnetic signatures despite having controversial ferromagnetic origin. However, achievement of truly room temperature ferromagnetism by carrier mediation is still the subject of intense research to develop the practical spin-based devices. Nanowires with one-dimensional nanostructure, which offers thermodynamically stable features and typically single crystalline and defect free, have a number of advantages over thin films with respect to studying ferromagnetism in DMSs. This review focuses primarily on our works on GaN-based DMS nanowires, i.e., Mn-doped GaN, Mn-doped AlGaN and Cu-doped GaN nanowires. These DMS nanowires have room temperature ferromagnetism by the local magnetic moment of doping elements that are in a divalent state and in tetrahedral coordination, thus substituting Ga in the wurtzite-type network structure of host materials. Importantly, our evidences indicate that the magnetism is originated from the ferromagnetic interaction driven by the carrier. These outcomes suggest that nanowires are ideal building blocks to address the magnetism in DMS due to their thermodynamic stability, single crystallinity, free of defects and free standing nature from substrate. Nanowires themselves are ideal building blocks for nanodevices and, thus, it would also be helpful in developing DMS-based spin devices.

This study reports the electrical transport characteristics of Si(1-x)Gex (x=0-0.3) nanowires. Nanowires with diameters of 50-100 nm were grown on Si substrates. The valence band spectra from the nanowires indicate that energy band gap modulation is readily achievable using the Ge content. The structural characterization showed that the native oxide of the Si(1-x)Gex nanowires was dominated by SiO2; however, the interfaces between the nanowire and the SiO2 layer consisted of a mixture of Si and Ge oxides. The electrical characterization of a nanowire field effect transistor showed p-type behavior in all Si(1-x)Gex compositions due to the Ge-O and Si-O-Ge bonds at the interface and, accordingly, the accumulation of holes in the level filled with electrons. The interfacial bonds also dominate the mobility and on- and off-current ratio. The large interfacial area of the nanowire, together with the trapped negative interface charge, creates an appearance of p-type characteristics in the Si(1-x)Gex alloy system. Surface or interface structural control, as well as compositional modulation, would be critical in realizing high-performance Si(1-x)Gex nanowire devices.

We report magnetism in Cu doped single-crystalline GaN nanowires. The typical diameter and the length of the Ga1-xCuxN nanowires (x = 0.01, 0.024) are 10-100 nm and tens of micrometers, respectively. The saturation magnetic moments are measured to be higher than 0.86 microB/Cu at 300 K, and the Curie temperatures are far above room temperature. Anomalous X-ray scattering and X-ray diffraction measurement make it clear that Cu atoms substitute the Ga sites, and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Cu L(2,3) edges show that doped Cu has local magnetic moment and the electronic configuration of it is mainly 3d9 but mixed with a small portion of trivalent component. It seems that the ionocovalent bonding nature of Cu 3d orbital with surrounding semiconductor medium makes Cu atom a mixed electron configuration and local magnetic moments. These outcomes suggest that the Ga1-xCuxN system is a room-temperature ferromagnetic semiconductor.

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.

Single-crystalline diluted magnetic semiconductor GaN:Mn nanowires with controlled Mn concentrations have been successfully synthesized and incorporated into devices. These nanowires exhibit Curie temperatures above room temperature, magnetoresistances near room temperature, and spin-dependent transport. The nanowires are used as building blocks for the fabrication of GaN:Mn/n-SiC based light-emitting diodes.

Semiconductors doped with magnetic ion, the so-called diluted magnetic semiconductors, are promising candidates for spintronics. Herein, we report on magnetic anisotropy in Mn:Ge diluted magnetic semiconductor nanowires. We grew single crystal Mn:Ge nanowires vertically on a Ge substrate and found the anisotropy in ratios of orbital to spin magnetic moments in the angle-dependent x-ray magnetic circular dichroism measurements. Our further characterization indicates that this anisotropy comes from the unique characteristics of nanowires, i.e., very high aspect ratio in their shape and tensile stress along the longitudinal direction, which confine the spins along the longitudinal direction and make an easy axis in that direction.

We have fabricated Si(1-x)Ge(x) alloy nanowire devices with Ni and Ni/Au electrodes. The electrical transport characteristics of the alloy nanowires depended strongly on the annealing temperature and contact metals. Ni/Au-contacted devices annealed at 400 degrees C showed p-type transistor behavior as well as a resistance switching effect, while no switching was observed from Ni-contacted alloy nanowire devices. To identify the origin of such a hysteretic resistance switching effect, we constructed nanowire devices on a 40 nm Si(3)N(4) membrane. Transmission electron microscopy analysis combined with electrical transport measurements revealed that devices contacted with Ni/Au, and thereby showing resistance switching, have Au atoms right next to the alloy nanowire.

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The authors discussed the photoluminescence (PL) spectra of GaN and GaMnN nanowires grown on sapphire substrates. Comparison of the excitonic PL peak energy with bulk GaN indicates that the strain of the nanowires is fully relaxed. For GaMnN nanowires, the redshift of the PL peak with increasing temperature was larger than that of the GaN nanowires, which was explained by redistribution of carriers into localization sites. The absence of Zeeman shift and circular polarization in GaMnN nanowires indicates that the exchange interaction between carriers and Mn2+ has to be at least one order of magnitude smaller than that in Cd0.94Mn0.06S nanowires.

We have studied the spin dependent tunneling properties of Mn-doped GaN nanowires with ferromagnetic Co contacts. The magnetoresistances were measured between two ferromagnetic Co electrodes, or Co and Ti∕Au electrodes through Mn-doped GaN nanowires. The magnetoresistances of nanowire with the Co electrode indicate hysteretic behaviors, which are commonly observed in tunnel magnetoresistance devices. The magnetoresistance ratio increases from −0.6% at 20K to −9.4% at 1.74K. It is believed that the hysteretic magnetoresistances originate from the tunnel magnetoresistance effect between the ferromagnetic phases of nanowire and Co electrode.