First-order Raman scattering in a free-standing GaN nanowire with ring geometry

We have investigated one phonon resonant Raman scattering in GaN nanowires (NWs) with ring geometry. We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum approach. The selection rules are studied. For the GaN NWs with small radius, results reveal that the main contribution to the differential cross-section (DCS) stems from the surface optical (SO) phonons especially from the high-frequency of SO phonons, with a minor contribution from the longitudinal optical (LO) phonons. Meanwhile, dispersions of the two branches of the SO phonon modes are obvious when the wire is thin. Moreover, compared to GaAs NWs, the GaN NWs make more contribution to the DCS in the small quantum size.     

Simplified fabrication and electrical analysis of crystalline silver nanowire

Silver nanowires have been fabricated using electrochemical deposition within the confined nanochannels of anodic alumina membrane on aluminum substrate. The characterization of structure, morphology, and composition indicates that the silver nanowires are uniform with about 50?nm diameter corresponding to the pore diameter of the anodic alumina membrane and almost single crystal with preferred growth along the [111] direction. Electrical measurements at room temperature showed that these silver nanowires were high conductive with a conductivity of approximately 0.13×10⁵?S/cm. Also, individual silver nanowires were investigated by measuring current as a function of bias voltage until the point of failure. This study will be very useful for the development of future nanoelectronic devices and circuits that employ Ag nanowires.     

The physics and fabrication of ultra-thin free-standing wires

Free-standing fine wires of metals, insulators and semiconductors are expected to show both quantum-size phenomena and one-dimensional behaviour in their electrical and thermal properties at low temperatures. We summarise some of these effects and describe the methods for fabricating test structures.     

Quasi-horizontal GaN nanowire array network grown by sublimation sandwich technique

Quasi-horizontal GaN nanowire array network has been grown on Au-film-coated MgO substrates via a sublimation sandwich technique. These GaN nanowire arrays principally grew along two directions which were perpendicular to each other and nearly parallel to the substrate, forming a regular network. The formation of the nanowire network was a hetero-epitaxial vapor-liquid-solid (VLS) process assisted by Au catalysts and was dependent on the substrates. Transmission electron microscopy revealed that the nanowires were single-crystalline wurtzite GaN. Raman scattering spectrum of the nanowire network presented some new features.     

The synthesis of highly oriented GaN nanowire arrays

Highly oriented GaN nanowire arrays have been achieved by the catalytic reaction of gallium with ammonium. The resulting materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). SEM images show that the resulting materials are nanowire arrays with a uniform length of about 10 μm. XRD, EDS, TEM and SAED indicate that the nanowire arrays are single-crystal hexagonal GaN with a wurtzite structure. They have diameters of 10 to 20 nm. Received: 2 October 2002 / Accepted: 7 October 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. E-mail: wwwangjc@sina.com     

大规模制备Ni80Fe20纳米线阵列及其磁学特性研究

利用电化学沉积方法在高度有序纳米孔氧化铝模板中大规模制备了Ni80 Fe20纳米线阵列.该方法得到的Ni80Fe20纳米线产率高(约1012-1013/cm2),而且这些纳米线阵列具有(111)择优生长取向和很高的纵横比.与体材料相比,这些Ni80Fe20纳米线阵列具有更高的矫顽力和较大的剩磁比等性能,在微型磁性元件领域将具有广泛应用前景.     

First-principles study on structural and electronic properties of copper nanowire encapsulated into GaN nanotube

We present a systemic study of the structural and electronic properties of Cu_n nanowires (n=5, 9 and 13) encapsulated in armchair (8,8) gallium nitride nanotubes (GaNNTs) using the first-principles calculations. We find that the formation processes of these systems are all exothermic. The initial shapes are preserved without any visible changes for the Cu₅@(8,8) and Cu₉@(8,8) combined systems, but a quadratic-like cross-section shape is formed for the outer nanotube of the Cu₁₃@(8,8) combined system due to the stronger attraction between nanowire and nanotube. The electrons of Ga and N atoms in outer GaN sheath affect the electron conductance of the encapsulated metallic nanowire in the Cu₁₃@(8,8) combined system. But in the Cu₅@(8,8) and Cu₉@(8,8) combined systems, the conduction electrons are distributed only on the copper atoms, so charge transport will occur only in the inner copper nanowire, which is effectively insulated by the outer GaN nanotube. Considering the maximal metal filling ratio in nanotube, we know that the Cu₉@(8,8) combined system is top-priority in the ultra-large-scale integration (ULSI) circuits and micro-electromechanical systems (MEMS) devices that demand steady transport of electrons.     

Cathodoluminescent and electrical properties of an individual ZnO nanowire with oxygen vacancies

A single ZnO nanowire with intrinsic oxygen vacancies is utilized to fabricate four-contact device with focus ion beam lithography technique. Cathodoluminescent spectra indicate strong near-UV and green emission at both room temperature and low temperatures. Experimental measurement shows the temperature-dependent conductivity of the ZnO nanowire at low temperatures （below 100 K）. The further theoretical analysis confirms that weak localization plays an important role in the electrical transport, which is attributed to the surface states induced by plenty of oxygen vacancies in ZnO nanowire.     

Simulation of GaN micro-structured neutron detectors for improving electrical properties

Nowadays,the superior detection performance of semiconductor neutron detectors is a challenging task.In this paper,we deal with a novel GaN micro-structured neutron detector(GaN-MSND) and compare three different methods such as the method of modulating the trench depth,the method of introducing dielectric layer and p-type inversion region to improve the width of depletion region(W).It is observed that the intensity of electric field can be modulated by scaling the trench depth.On the other hand,the electron blocking region is formed in the detector enveloped with a dielectric layer.Furthermore,the introducing of p-type inversion region produces new p/n junction,which not only promotes the further expansion of the depletion region but also reduces the intensity of electric field produced by main junction.It can be realized that all these methods can considerably enhance the working voltage as well as W.Of them,the improvement on W of GaN-MSND with the p-type inversion region is the most significant and the value of W could reach 12.8 μm when the carrier concentration of p-type inversion region is 10~(17) cm~(-3).Consequently,the value of W is observed to improve200% for the designed GaN-MSND as compared with that without additional design.This work ensures to the researchers and scientific community the fabrication of GaN-MSND having superior detection limit in the field of intense radiation.     

The effect of hydrostatic pressure on material parameters and electrical transport properties in bulk GaN

Experimental data for temperature dependence of electron transport properties in a bulk, low dislocation density, GaN sample at atmospheric pressure and 7.1 kbar have been presented. The data are representing a weak hydrostatic pressure dependence. Our quantitative analysis on its material parameters including: high and low dielectric constants (ε_∞,εs), longitudinal and transverse optical phonons (ωLO,ωTO), and electronic effective mass show a small fractional change of −0.12,−0.14,0.05,0.058 and 0.089 (percent/kbar), respectively. These results are confirmed by the Hall-effect data analysis on the basis of charge neutrality condition and various scattering mechanisms.     

Fabrication and electrical properties of axial and radial GaAs nanowire pn junction diode arrays

We report on the fabrications and characterizations of axial and radial Ga As nanowire pn junction diode arrays.The nanowires are grown on n-doped Ga As（111）B substrates using the Au-catalyzed vapor–liquid–solid mechanism by metal–organic chemical vapor deposition（MOCVD）. Diethyl–zinc and silane are used as p- and n-type dopant precursors,respectively. Both the axial and radial diodes exhibit diode-like J–V characteristics and have similar performances under forward bias. Under backward bias, the axial diode has a large leakage current, which is attributed to the bending of the pn junction interface induced by two doping mechanisms in Au-catalyzed nanowires. The low leakage current and high rectification ratio make the radial diode more promising in electrical and optoelectronic devices.     

Structural and local electrical properties of AlInN/AlN/GaN heterostructures

GaN layers and Al_1−xIn_xN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures.     

Optical and electrical properties of GaN: Si-based microstructures with a wide range of doping levels

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (～2.5 × 10¹⁸ cm^?3) and to the overlap of the impurity band with the conduction band (～2 × 10¹⁹ cm^?3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.     

The concept and realization of nanostructure fabrication using free-standing metallic wires with rapid thermal annealing

Free-standing metallic nanostructures are considered to be highly relevant to many branches of science and technology with applications of three dimensional metallic nanostructures ranging from optical reflectors,actuators,and antenna,to free-standing electrodes,mechanical,optical,and electrical resonators and sensors.Strain-induced out-of-plane fabrication has emerged as an effective method which uses relaxation of strain-mismatched materials.In this work,we report a study of the thermal annealing-induced shape modification of free-standing nanostructures,which was achieved by introducing compositional or microstructural nonuniformity to the nanowires.In particular gradient,segmented and striped hetero-nanowires were grown by focused-ion-beam-induced chemical vapor deposition,followed by rapid thermal annealing in a N2 atmosphere.Various free-standing nanostructures were produced as a result of the crystalline/grain growth and stress relief.     

Influence of dislocations in the GaN layer on the electrical properties of an AlGaN/GaN heterostructure

This paper reports on a comparative study of the spatial distributions of the electrical, optical, and structural properties in an AlGaN/GaN heterostructure. Edge dislocation density in the GaN template layer is shown to decrease in the regions of the wafer where the heterostructure sheet resistance increases and the GaN photoluminescence band-edge energy peak shifts to a high wavelength. This phenomenon is found to be attributed to the local compressive strain surrounding edge dislocation, which will generate a local piezoelectric polarization field in the GaN layer in the opposite direction to the piezoelectric polarization field in the AlGaN layer and thus help to increase the two-dimensional electron gas concentration.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

Characterization of free-standing GaN substrate grown through hydride vapor phase epitaxy with a TiN interlayer

Large-scale GaN free-standing substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The bottom surface N-face and top surface Ga-face showed great difference in anti-etching and optical properties. The variation of optical and structure characteristics were also microscopically identified using spatially resolved cathodoluminescence and micro-Raman spectroscopy in cross-section of the GaN substrate. Three different regions were separated according to luminescent intensity along the film growth orientation. Some tapered inversion domains with high free carrier concentration of 5 × 10¹⁹ cm⁻³ protruded up to the surface forming the hexagonal pits. The dark region of upper layer showed good crystalline quality with narrow donor bound exciton peak and low free carrier concentration. Unlike the exponential dependence of the strain distribution, the free-standing GaN substrate revealed a gradual increase of the strain mainly within the near N-polar side region with a thickness of about 50 μm, then almost kept constant to the top surface.