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This paper introduces a photo-assisted atomic force microscope (AFM) local oxidation technique which is capable of producing highly smooth oxide patterns with heights reaching several tens of nanometres on both n- and p-types of GaN (and in principle on most semiconductors) without the use of chemicals. The novel methodology relies on UV illumination of the surface of the substrate during conventional AFM local oxidation. A low 1.2 V threshold voltage for n-type GaN was obtained, which can be explained by UV photo-generation of excess electron–hole pairs in the substrate near the junction, thereby reducing the electric field required to drive carrier flow through the tip–sample Schottky barrier. It was demonstrated that the presence or absence of light alone was sufficient to switch the growth of the oxide on or off. The photo-assisted AFM oxidation technique is of immediate interest to the semiconductor industry for the fabrication of GaN-based complementary metal–oxide–semiconductor devices and nanodevices, improves chances for AFM-type data storage, and presents new degrees of freedom for process control technique. more...

We report diameter-selective growth of GaN nanowires (NWs) by using mono-dispersed Au nanoparticles (NPs) on a ligand-modified Si substrate. The thiol-terminal silane was found to be effective in producing well-dispersed Au NPs in low density on Si substrates so that the agglomeration of Au NPs during growth could be avoided. The resultant GaN NWs exhibited a narrow diameter distribution and their mean diameter was always larger than, while keeping a deterministic relation with, the size of the Au NPs from which they were grown. A self-regulating steady growth model is proposed to account for the size-control process. more...

Large-area and high-density arrays of AlN nanorods were synthesized at low temperature via a template-free and catalyst-free chemical vapour deposition. The quasi-aligned AlN nanorods were identified to grow along the c-axis and preferentially orient with their growth direction perpendicular to the substrate. Further studies showed that the AlN nanorods were grown on a buffer layer formed at the beginning of the reaction. By changing the flow rate of the carrier gas at the beginning of the reaction, we successfully obtained nanorods with different orientations on the substrate. The Raman spectrum and cathodoluminescence spectrum of the AlN nanorods at room temperature reveal the existence of oxygen-related defects in the nanorods. more...

Top emission polymer light-emitting devices (PLEDs) with different heat treatments are investigated. Due to the high work function of an indium–tin-oxide (ITO) cathode as a top electrode, LiF is employed as an interlayer to increase electron injection. The PLEDs fabricated at room temperature show better electrical and optical characteristics such as current density–voltage, luminance, and current efficiency, etc., than that of PLED devices with heat treatment. To investigate the Li diffusion in the polyfluorene layer, X-ray photoelectron spectroscopy measurement before and after heat treatment is performed. The poor performance of PLEDs with heat treatments is observed due to the high trap density generated in the film because of the Li diffusion in the polyfluorene layer. more...

InGaN/GaN multiple quantum well structures have been grown on sapphire substrates by metalorganic chemical vapor deposition, for wide range visible light emitting diode application. The compositions and sizes within quantum wells were designed according to the requirements on the LED performance. Samples were investigated by a variety of characterization techniques. Optimization of the growth parameters and process was realized and evidenced by high resolution X-ray diffraction measurements. Optical spectroscopic properties were further studied and quantum confined stokes shift was observed from room temperature photoluminescence and photoluminescence excitation measurements. more...

Al and N K-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy (SPEM) and x-ray emission measurements were performed on AlN nanotips grown on p- and n-type Si substrates (p-AlN and n-AlN). Features and intensities in the Al and N K-edge XANES spectra of these AlN nanotips overall are similar. In contrast, the intensities of the valence-band SPEM spectra of p-AlN are apparently larger than those of n-AlN, which indicates that the valence-band density of states of p-AlN exceeds that of n-AlN. This result may be related to the observed enhancement of field-emission intensity of AlN nanotips grown on the p-type Si substrate. more...

The formation of voids and bubbles in the energetic ion implantation process is an important issue in material science research, involving swelling induced embrittlement of materials in nuclear reactors, catalytic activities in the nanopores of the bubble, etc. We report here the formation and in situ dynamics of metallic nanoblisters in GaN nanowires under self-ion implantation using a Ga + focused ion beam. High-resolution transmission electron microscopes equipped with electron energy loss spectroscopy and energy filtering are used to identify the constituents of the blister. In situ monitoring, with focused ion beam imaging, revealed the translation and rotation dynamics of the blisters. more...

The density of carbon nanotubes (CNTs) deposited by microwave plasma chemical vapor deposition (MPCVD) is extremely high, which results in screening effect in the electric field. To achieve excellent field emission characteristics, an excimer laser treatment (ELT) was introduced to reduce the density of CNTs. Scanning electron microscopy (SEM) micrographs showed reduced densities of the CNTs, and the measurement of electrical characteristics results revealed the improved field emission properties under suitable ELT conditions. The turn-on field decreased from 3 V/µm to 2.1 V/µm, and the emission current density increased from 5.73 mA/cm2 to 87.13 mA/cm2 at the applied field of 5 V/µm. more...

Thermal diffusivity (α) has been correlated with the electrical and structural properties of hydrogenated microcrystalline silicon (µc-Si:H) films. In the heterogeneous microcrystalline network, α and electrical conductivity (σD) maintain a one-to-one correspondence, and both these parameters are directly related to the crystalline volume fraction (Fc) of the network. For the amorphous silicon network, α is ~ 0.2 cm2s1 and has a very low σD of ~ 10-8 Scm-1. When the crystalline volume fraction (Fc) exceeds a certain percolation threshold, both α and σD increase abruptly. Undoped µc-Si:H films having Fc~94% exhibit a high magnitude of α~0.80 cm2s-1 and a very high σD of ~ 10-3 Scm-1. more...

Nanophase compacts synthesised by different methods are comparatively studied by means of x-ray diffractometry, transmission electron microscopy and fractography. Experimental results reveal that the synthesis method has a strong influence on the microstructure of the resulting material. Although the same master alloy is used for the inert gas condensation (IGC) and severe plastic deformation (SPD), the microstructure of the pellet produced by SPD different from that of the IGC produced compact. The IGC+SPD compact, however, has a similar microstructure as that of the SPD compact. The local melting of the compacts at lower temperatures has been found, and is independent of the method used to synthesise the pellets. more...

Enhanced photoelectrochemical (PEC) performances of Ga(2)O(3) and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga(2)O(3) and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga(2)O(3) NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga(2)O(3). These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga(2)O(3) NWs, or by incorporation of indium to form InGaN NWs. more...

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications. more...

We apply several optical time-resolved techniques to investigate the dynamics of excess carriers in AlxGa1−xN nanocolumns grown on (111) silicon substrates and in thick AlxGa1−xN epitaxial layers deposited on sapphire. We demonstrate that carrier lifetime drops in nanocolumns by several times if compared to epilayers with similar Al content; in addition, recombination rate displays a strong nonlinearity on excitation. On the other hand, carrier localization effects observed in the epilayers completely disappear in nanocolumns. more...

We report the properties of low-frequency contact noise of multielectrode GaN nanowire (NW) devices. A two-port cross-spectrum technique is used to discriminate the noise of the ohmic contact from that of the NW section. The diameter of the GaN NW is around 100 nm. The Ti/Al electrodes of the NWs are defined by e-beam lithography. The typical resistance of a NW section with a length of 800 nm is about 5.5 kΩ and the two-wire resistance is below 100 kΩ. The results show that the low-frequency excess noise of the GaN NW is much smaller than that of the current-flowing contact, indicating that the contact noise dominates the noise behavior in our GaN NW devices. A careful study of the noise amplitude (A) of the 1/f noise of different types of NW and carbon nanotube devices, both in our work and in the literature, yields an empirical formula for estimating A from the two-wire resistance of the device. more...

An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demonstrated. The high-density array of Si nanotips (SiNTs) is employed as a template for depositing La(0.7)Sr(0.3)MnO(3) (LSMO) rods with a pulsed-laser deposition method. Compared with the planar LSMO/Si thin film, the heterojunction shows a large enhancement of room temperature magnetoresistance (MR) ratio up to 20% under 0.5 T and a bias current of 20 µA. The MR ratio is found to be tunable, which increases with increasing external bias and the aspect ratios of the nanotips. Electric-field-induced metallization, in conjunction with nanotip geometry, is proposed to be the origin for the giant MR ratio. more...

We report measurements of magnetoresistance (MR) fluctuations in a weak disorder indium nitride nanowire. The MR fluctuations are reproducible, aperiodic and symmetric in magnetic field but are asymmetric upon reversal of bias direction. The fluctuations are analysed for both perpendicular and parallel external magnetic field configurations in the light of tunnel magnetoresistance at low field and impurity scattering at higher field. The asymmetry in bias reversal is caused by breakdown of time reversal symmetry. more...

The enhancement of light extraction from Si(0.5)Ge(0.5)/Si multiple quantum wells (MQWs) with nanowall structures fabricated by electron cyclotron resonance (ECR) plasma etching is presented. It is shown that the ECR plasma treatment does not damage the crystalline quality. At a driving current of 5.5 × 10(6) A m(-2), the light output intensity of the MQWs with nanowall structures shows an enhancement of about 50% compared with that of the original MQWs. In addition to the enhanced light extraction, the improved optoelectronic properties are also attributed to the strain relaxation in nanowall structures. more...

Fast charging–discharging performances of well-dispersed RuO2 nanoparticles (NPs) on N-doped carbon nanotubes (CNTs) have been demonstrated. As regards the structural properties, the N 1s x-ray photoelectron spectroscopy spectrum shows two characteristic bonding peaks of N within CNTs. Cross-sectional scanning electron microscope images show that N-doped CNTs provide good support for uniform distribution of RuO2 NPs. As regards the capacitive behavior, N-doped CNTs covered with well-dispersed RuO2 NPs lead to a high charging–discharging property performed at the scan rate of 2000 mV s−1. The role of N doping is to create preferential sites of CNTs for capturing RuO2 NPs. Such a fast sweep capability established by the N-incorporation method is very useful in supercapacitor applications. more...

The authors demonstrate the coalescence overgrowth of GaN nano-columns on a (111) Si substrate with metalorganic chemical vapor deposition to show high-quality optical properties in the overgrown film. Plan-view scanning electron microscopy (SEM) shows coalesced surface morphology, although hexagonal structures are still visible in the images. The cross-section cathodoluminescence (CL) image shows more efficient emission in the overgrowth layer than from the nano-column layer. The plan-view CL image demonstrates that the emitted light is mainly from the hexagonal structures. The photoluminescence measurement result indicates that the emission efficiency of the overgrown layer is even higher than that of an undoped GaN thin film of high quality. The presence of hexagonal structures correlates to surface roughness values in the range of several nanometres. more...

Long-range ferromagnetic ordering at room temperature is reported for post-annealed TiO2 nanorods doped with Co2+ by an ion implantation technique. The catalyst free anatase TiO2 nanorods are grown normal to the substrate plane. A Curie temperature of ~320?K was recorded for a ~9?at.% Co doped sample. Electronic structural studies confirm the substitutional Co in the Ti lattice site. Microstructural studies show the presence of extended defects, where a minor nonuniformity in doping concentration helps to sustain the intrinsic ferromagnetic ordering in Co doped TiO2 at room temperature. The projected nanorod tip size is found to be smaller than a single magnetic domain, which may lead to an increased signal-to-noise ratio for magnetic recording. more...

The effects of oxygen plasma posttreatment (PPT) on the morphology and field emission properties of carbon nanotube (CNT) arrays grown on silicon substrates are proposed and experimental results are reported. Oxygen PPT led to an enhancement in the emission properties of CNTs, which showed an increase in total emission current density and a decrease in turn-on field after plasma treatment. Scanning electron microscopy (SEM) images showed reduced densities of the CNTs, which resulted in a decrease of the screening effect in the electric field. Raman spectra showed an increase in the number of defects which served as field-emission sites when the plasma power or treatment time with the plasma increased. Transmission electron microscopy (TEM) images were used to identify the quality of the nanotubes, so that we could clearly find evidences of improvement in the field emission properties after plasma treatment. The measurement of electrical characteristics revealed improved field emission properties under proper plasma conditions. The turn-on field decreased from 4.8 to 2.5 V/µm, and the emission current density increased from 78.7 µA/cm2 to 18 mA/cm2 at an applied field of 5.5 V/µm. more...

We have proposed and fabricated a vertical lateral field emission device (LFED) based on carbon nanotubes (CNTs). It combines high-performance nanomaterials with mature solid-state fabrication technology to produce miniaturized vacuum devices with superior field emission characteristics. The techniques employed are very simple and allow for good reproducibility in controlling the short distance from the polysilicon anode to the CNTs cathode inter-electrode distance. The inter-electrode gap can be easily fabricated to be less than 1 µm by a wet etching process without using fine lithography. The CNTs were selectively grown using a microwave-plasma enhanced chemical vapor deposition system (MPCVD). The anode-to-emitter gap distance and the length of carbon nanotubes are well controlled to enable investigation of their effect on the field emission properties. The turn-on voltage of the fabricated device with an inter-electrode gap of 0.53 µm is as low as 0.2 V, and the emission current is as high as 9.72 mA at 10 V. The emission current fluctuation is approximately ±3.5% for 1500 s. more...

The density distribution of CNTs is one of the crucial parameters determing the field-emission property of CNTs. To effectively control the density of CNTs, an inactive thin-film layer was deposited on a catalyst. The results showed that improved field emission property could be obtained with a thin SiO layer on the catalyst layer as the precursor. For 3.5 nm Fe and 3.5 nm SiO on 3.5 nm Fe as a catalyst, the turn-on field could be decreased from 3.7 V/µm. to 2.2 V/µm and the field-emission current density increased from 2.6×10-8A/cm2to 2.4×10-4A/cm2when the applied field was 4 V/µm more...

To achieve high emission current density, carbon nanotube triodes have been proposed using recessed oxide as the insulator between the gate and the carbon nanotubes to replace the conventional spacer. An anode current of 10 µA was achieved with a gate voltage of 98 V for a nanotube sample prepared with a growth time of 12 min. Carbon nanotubes synthesized with various growth times were analyzed using a scanning electron microscope and Raman spectroscopy. By increasing the growth time, longer carbon nanotubes were obtained. Effects of the length of the carbon nanotubes on the field emission of the triodes are discussed. Enhanced luminance was obtained as the anode voltage increased from 600 V to 1000 V. Such carbon nanotube triodes are promising for utilization in future field-emission displays. more...

We present a simple, but highly sensitive, method for obtaining both the sign and magnitude of the nonlinear refractive index and the nonlinear absorption coefficient as a function of wavelength. It is shown theoretically that these parameters can be easily obtained from the linear and nonlinear transmission spectra of the medium as well as the laser intensity distribution on the frequency. This technique is demonstrated for ZnSe-ZnS multiple quantum wells on transparent CaF2 substrate at 77 K. more...

The excitonic optical nonlinearities and optical bistability are observed in ZnSe-ZnS superlattices optical waveguide at room temperature for the first time. The excitonic absorption band is broadened and tends to saturating absorption, and has a blue shift towards short wavelength side with increasing incident intensities. At higher incident intensity, the switching response time of the optical bistability is 500 ps. The results suggest that the optical nonlinearities are due to the phase space filling of excitonic states, excitonic saturating absorption and excitonic band broadening, while the major contribution to the optical bistability is due to increasing absorption effect caused by phase space filling of excitonic states and excitonic band broadening. more...