K. H. Chen

Selective-area growth of indium nitride nanowires on gold-patterned Si(100) substrates

This letter reports the synthesis of indium nitride (InN) nanowires on gold-patterned silicon substrates in a controlled manner using a method involving thermal evaporation of pure indium. The locations of these InN nanowires were controlled by depositing gold in desired areas on the substrates. Scanning electron microscopy and transmission electron microscopy investigations showed that the InN nanowires are single crystals with diameters ranging from 40 to 80 nm, and lengths up to 5 μm. Energy dispersive x-ray spectrometry showed that the ends of the nanowires are composed primarily of Au, and the rest of the nanowires were InN with no detectable Au incorporations. The Raman spectra showed peaks at 445, 489, and 579 cm−1, which are attributed to the A1(transverse optical), E2, and A1(longitudinal optical) phonon modes of the wurtzite InN structure, respectively. Photoluminescence spectra of the InN nanowires showed a strong broad emission peak at 1.85 eV.

SiC-capped nanotip arrays for field emission with ultralow turn-on field

Silicon nanotips with tip diameter and height measuring 1 nm and 1 μm, respectively, and density in the range of 109–3×1011 cm−2, were fabricated monolithically from silicon wafers by electron cyclotron resonance plasma etching technique at a temperature of 200 °C. Field emission current densities of 3.0 mA/cm2 at an applied field of ∼1.0 V/μm was obtained from these silicon nanotips. High-resolution transmission electron microscope and Auger electron spectroscopy analyses concluded that the nanotips are composed of monolithic silicon and nanometer-size SiC cap at the top. A 0.35 V/μm turn-on field to draw a 10 μA/cm2 current density was demonstrated, which is much lower than other reported materials. The excellent field emission property demonstrated by these nanotips, which were fabricated by a process integrable to the existing silicon device technology at low temperatures, is a step forward in achieving low-power field emission displays and vacuum electronic devices.

Field emission from quasi-aligned SiCN nanorods

We report on the preparation and field emission properties of quasi-aligned silicon carbon nitride (SiCN) nanorods. The SiCN nanorods are formed by using a two-stage growth method wherein the first stage involves formation of a buffer layer containing high density of nanocrystals by electron cyclotron resonance plasma enhanced chemical vapor deposition and the second stage involves using microwave plasma enhanced chemical vapor deposition for high growth rate along a preferred orientation. It should be noted that growth of the SiCN nanorods is self-mediated without the addition of any metal catalyst. Scanning electron microscopy shows that the SiCN nanorods are six-side-rod-shaped single crystals of about 1–1.5 μm in length and about 20–50 nm in diameter. Energy dispersive x-ray spectrometry shows that the nanorod contains about 26 at. % of Si, 50 at. % of C, and 24 at. % of N. Characteristic current–voltage measurements indicate a low turn-on field of 10 V/μm. Field emission current density in excess of ...

Mechanism of luminescence in InGaN/GaN multiple quantum wells

We report a firm evidence of luminescence from InN clusters in InGaN/GaN multiple quantum wells. Photoluminescence, photoluminescence excitation, and Raman scattering measurements have been employed to study the optical properties of InGaN/GaN multiple quantum wells. A careful examination of the low energy shoulders of the main peak luminescence reveals the fact that their separation is in good agreement with the longitudinal optical phonon energy of pure InN film measured by Raman scattering. A large Stokes-like shift between the emission peak energy and the absorption edge is found; it increases with increasing indium content. All these observations can be explained in a consistent way by the effect of localization due to self-organized InN clusters within InGaN layers. Our results thus strongly suggest that the emission mechanism of InGaN/GaN quantum wells originates from radiation recombination within the localized states of self-organized InN clusters.

Enhanced dynamic annealing in Ga ¿ ion-implanted GaN nanowires

Ga+ ion implantation of chemical-vapor-deposited GaN nanowires (NWs) is studied using a 50-keV Ga+ focused ion beam. The role of dynamic annealing (defect-annihilation) is discussed with an emphasis on the fluence-dependent defect structure. Unlike heavy-ion-irradiated epitaxial GaN film, large-scale amorphization is suppressed until a very high fluence of 2×1016 ions cm−2. In contrast to extended-defects as reported for heavy-ion-irradiated epitaxial GaN film, point-defect clusters are identified as major component in irradiated NWs. Enhanced dynamic annealing induced by high diffusivity of mobile point-defects in the confined geometry of NWs is identified as the probable reason for observed differences.

Electronic structure of the carbon nanotube tips studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

Angle-dependent x-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been performed to differentiate local electronic structures of the tips and sidewalls of highly aligned carbon nanotubes. The intensities of both π*- and σ*-band C K-edge XANES features are found to be significantly enhanced at the tip. SPEM results also show that the tips have a larger density of states and a higher C 1s binding energy than those of sidewalls. The increase of the tip XANES and SPEM intensities are quite uniform over an energy range wider than 10 eV in contrast to earlier finding that the enhancement is only near the Fermi level.

Spectroscopic studies of nitrogenated amorphous carbon films prepared by ion beam sputtering

Spectroscopic analysis of the unhydrogenated amorphous carbon nitride (a-CNx) films, prepared by ion beam sputtering, was done by ellipsometry, Raman scattering and x-ray photoelectron spectroscopic (XPS) studies. The optical gap of the films was estimated from the Tauc’s analysis of the (n,k) data obtained from spectroscopic ellipsometry. In addition to the commonly observed D and G bands at ∼1350 cm−1 and 1550 cm−1, respectively, we detected a separate band at ∼1450 cm−1 in the Raman spectrum of a-CNx films. This intermediate peak was unambiguously identified as the N band arising out of the nitrogen–nitrogen bonding, due to the incorporation of nitrogen in the a-C network. High resolution XPS C 1s and N 1s peaks were used to estimate the nitrogen content in the films and various bonding configurations were identified from their Gaussian deconvolution. An increase in the intensity ratio of CN and CC components, I(CN)/I(CC), in the C 1s spectra signified increasing nitrogenation of the carbon network. De...

Preparation and characterization of carbon nanotubes encapsulated GaN nanowires

A novel two-step catalytic reaction is developed to synthesize gallium nitride nanowires encapsulated inside carbon nanotubes (GaN@CNT). The nanowires are prepared from the reaction of gallium metal and ammonium using metals or metal alloys as a catalyst. After the formation of the nanowires, carbon nanotubes are subsequently grown along the nanowires by chemical vapor deposition of methane. The structural and optical properties of pure GaN nanowires and GaN@CNT are characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy. The results show that GaN nanowires are indeed encapsulated inside carbon nanotubes. The field emission studies show that the turn-on field of GaN@CNT is higher than that of carbon nanotubes, but substantially lower than that of pure GaN nanowires. This work provides a wide route toward the preparation and applications of new one-dimensional semiconductor nanostructures.

Electronic structure of GaN nanowire studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

X-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been employed to obtain information on the electronic structures of the GaN nanowires and thin film. The comparison of the XANES spectra revealed that the nanowires have a smaller (larger) N (Ga) K edge XANES intensity than that of the thin film, which suggests an increase (decrease) of the occupation of N 2p (Ga 4p) orbitals and an increase of the N (Ga) negative (positive) effective charge in the nanowires. The SPEM spectra showed that the Ga 3d band for the nanowires lies about 20.8 eV below the Fermi level and has a chemical shift of about −0.9 eV relative to that of the thin film.

Field emission properties of two-layer structured SiCN films

Abstract The electron emission characteristics of two-layer structured silicon carbon nitride (SiCN) films, which were composed of amorphous and nanocrystalline phases, were studied. Rutherford backscattering spectroscopy (RBS) was used to determine the composition of the SiCN film. The ratio (Si;C)/N of the SiCN film was kept at approximately 0.75, which is identical to that of Si 3 N 4 film. High resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to investigate the bonding structures of the SiCN films. In comparison with silicon nitride films, the turn-on voltage (for an emission current of 0.01 mA/cm 2 ) of the SiCN films was lower and the emission current densities of the SiCN significantly enhanced. The promising emission properties of the SiCN film could be due to the unique two-layer structure wherein nanocrystalline SiCN was grown on top of the amorphous interlayer with sp 2 CN bond in the SiCN film.