Showing papers by "Shuit-Tong Lee published in 2000"

Synthesis of Large Areas of Highly Oriented, Very Long Silicon Nanowires

Abstract: Silicon is one of the most important electronic materials. Its nanoscale forms, such as nanocrystals, porous silicon, quantum wells, and nanowires, have stimulated great interest among scientists because of their peculiar physical properties, such as light emission, field emission, and quantum confinement effects. The progress made in the synthesis of silicon nanostructures and nanowires in recent years has attracted considerable attention. Today, large quantities of silicon nanowires can be produced by the laser ablation of metalor SiO2-containing silicon targets, [8] and a few properties, such as electric and thermal conductivity and optical properties, have also been studied. However, the experimental characterization and application of silicon nanowires, for example, the measurement of the elastic properties, the realization of efficient field emission of nanoscale silicon, and the fabrication of nanometer field effect transistors and planar displays, have been hampered so far because of the difficulty in growing oriented silicon nanowires. As a result, the production of highly oriented and very long silicon nanowires is a very important and challenging issue. In this communication, we report the successful synthesis of highly oriented, largescale, and very long silicon nanowires on flat silicon substrates by thermal evaporation of silicon monoxide (SiO). The growth mechanism and optical properties of the oriented silicon nanowires are also discussed. To the best of our knowledge, the synthesis of oriented silicon nanowires has not yet been reported. The equipment used for the present work is similar to that described previously. An alumina tube was mounted inside a tube furnace. The SiO powders (Gooodfellow, 99.95 %) were placed near the middle of the high-temperature zone of the furnace. The polished silicon (100) substrates about 5 mm in width and 50 mm in length were ultrasonically cleaned in acetone, ethanol, and deionized water for 20 min each, dipped in 20 % HF for 20 min, and finally rinsed in deionized water for 20 min before they were placed abreast at one end of the alumina tube. The tube had previously been evacuated to a base pressure of 10 torr by a mechanical pump before the starting materials were heated. The carrier gas of argon mixed with 5 % H2 admitted at the other end of the alumina tube flowed at 50 sccm (standard cubic centimeters per minute) at 400 torr. The temperature of the furnace was increased to 1300 C at 6 C/min and kept at this temperature for 7 h. The temperature of the silicon substrate surface where the oriented silicon nanowires grew was found to be approximately 930 C, which differed from that at the center due to the temperature gradient within the tube. The product was first directly examined by scanning electron microscopy (SEM, Philips XL 30 FEG). Microstructural characterization was carried out in a conventional Philips CM 20 transmission electron microscope (TEM) at 200 kV. The high-resolution transmission electron microscopy (HRTEM) study was performed in a Philips CM200 FEG transmission electron microscope, operated at 200 kV accelerating voltage at room temperature. The chemical compositions of the samples were determined by an energy dispersive X-ray (EDX) spectrometer attached to both the SEM and HRTEM instruments. Raman scattering spectra were measured with a Renishaw micro-Raman spectrometer at room temperature. Excitation was by means of the 514 nm line of an Ar laser, and the Raman signals were measured in a backscattering geometry with a spectral resolution of 1.0 cm. The deposited silicon nanowire product is light yellow in color. SEM images at different magnifications of a typical sample in Figures 1a, 1b, 1c, and 1d clearly show the large area of highly oriented nanowires on the surface of the silicon substrate. The low magnification SEM image (Fig. 1a) shows that the area of highly oriented silicon nanowires is about 2 mm ́ 3 mm and the lengths of individual nanowires are up to 1.5±2 mm. The thickness of the oriented nanowire product was about 10 lm, as estimated from the cross-sectional image (Fig. 1d) of the sample prepared by focused ion beam cutting. The highly oriented array of Si nanowires can also be observed from the cross-sectional image. The EDX results show that the nanowires are composed of silicon and oxygen. No metal was found in the sample. Such results are consistent with our previous theory that silicon nanowire growth is enhanced by silicon oxide instead of a metal particle catalyst. Because of local charging effects, the diameters observed from SEM images appear larger than the actual wire diameters. More information about the morphology of silicon nanowires is given by the following TEM characterization Small pieces of oriented silicon nanowire samples were peeled off from a silicon substrate and mounted on a folding grid for TEM and HRTEM observations. Figure 2 shows the typical morphology of silicon nanowires. As reported previously, these nanowires show a better orientation than those synthesized by laser ablation. Silicon nanowires as observed by TEM are quite clean, with very few particles attached to their surfaces, and are relatively homogeneous. Analysis of a number of nanowires shows that the diameters of these silicon nanowires vary from 18 to 46 nm, and the mean value is about 30 nm. The selected-area electron dif-

Laser Ablation Synthesis and Optical Characterization of Silicon Carbide Nanowires

Abstract: Silicon carbide (SiC) nanowires were synthesized at 900°C by the laser ablation technique. The growth morphology, microstructure, and defects in SiC nanowires were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The Raman scattering study indicated that the Raman peaks corresponding to the TO and LO phonon modes of the SiC nanowires had larger red shifts compared to those of bulk SiC material. The red shift, broadening peak, and the asymmetry of the Raman peak could be explained by the size confinement effect in the radial and growth directions. The growth mechanism of SiC nano-wires was discussed based on the vapor–liquid–solid reaction.

Germanium nanowires sheathed with an oxide layer

Abstract: We report how the structural arrangement of germanium nanowires optimizes surface stability and enables the formation of very thin free-standing crystalline wires. Ge nanowires consisting of a crystalline Ge core and an amorphous ${\mathrm{GeO}}_{2}$ sheath have been formed by laser ablation of a mixture of Ge and ${\mathrm{GeO}}_{2}.$ The crystalline Ge core lies in the axial [211] direction and is terminated by the {111} facets on the surface. The ${\mathrm{GeO}}_{2}$ sheath saturates the surface bonds of the core, adapts to the core surface roughness, and prohibits the growth of the nanowire in the lateral direction. With such a core and sheath, the surface energy of the nanowire is reduced and the formation of very thin nanowires is thereby permitted. Phonon confinement in the Ge nanowires has been observed by Raman scattering.

Semiconductor nanowires: synthesis, structure and properties

Abstract: Highly pure, ultra long and uniform-sized semiconductor nanowires in bulk-quantity have been synthesized by novel methods of laser ablation and thermal evaporation of semiconductor powders mixed with metal or oxide catalysts. Transmission electron microscopic study shows that decomposition of semiconductor sub-oxides and the defect structure play an important role in enhancing the formation and growth of high-quality semiconductor nanowires. The morphology, microstructure, optical and electrical properties of the nanowires have been characterized systematically by Raman scattering, photoluminescence and field emission. A new growth mechanism, namely oxide-assisted growth, is proposed based on the microstructure and different morphologies of the nanowires observed.

Improved performance of electroluminescent devices based on an europium complex

Abstract: Electroluminescent (EL) devices using an europium complex Eu(DBM)3 bath as the electron-transport emitting layer were fabricated. The quenching effect of the metal cathode and the unstable nature of the Eu complex under EL operation markedly influence the EL efficiency. By keeping the emitting area far from the metal cathode and partly doping the Eu(DBM)3 bath layer with a hole-transport material, the EL performance was significantly improved. Sharp-band red emissions with turn-on voltage of 3 V, brightness of 820 cd/m2, and external quantum efficiency of 1% were achieved.

A Nucleation Site and Mechanism Leading to Epitaxial Growth of Diamond Films

Abstract: A diamond nucleation site responsible for epitaxial growth of diamond on silicon by chemical vapor deposition (CVD) is identified in high-resolution transmission electron microscopic images. Other sites in the same sample leading to polycrystalline growth, but deleterious to epitaxial CVD growth, are also described. A mechanism for the heteroepitaxial growth of diamond is suggested, in which etching of the nondiamond carbon binder exposes and removes nonadherent nanodiamond nuclei, leaving intact only those directly nucleated on the silicon substrate. This work enhances our understanding of diamond nucleation and heteroepitaxial growth and its potential applications.

Modification of the hole injection barrier in organic light-emitting devices studied by ultraviolet photoelectron spectroscopy

Abstract: Ultraviolet photoelectron spectroscopy has been applied to the investigation of modified hole injection barriers in organic light-emitting devices (OLEDs). Different from those reported previously, the indium tin oxide (ITO) surface treated in situ by oxygen plasma possesses a work function of 5.2 eV, and the organic ITO interface thereafter formed shows a 0.5 eV smaller hole injection barrier compared to that on untreated ITO. Insertion of an ultrathin SiO2 layer between the organic and ITO results in a similar reduction of the barrier. This indicates that improved hole injection favors efficient operation of OLEDs, as manifested by enhanced efficiency by the SiO2 insertion.

Improved Time-of-Flight Technique for Measuring Carrier Mobility in Thin Films of Organic Electroluminescent Materials

Abstract: Using an improved time-of-flight (TOF) technique, the drift mobilities of electrons and holes in organic films prepared on silicon or indium-tin-oxide (ITO)-coated glass substrates have been determined. For the samples on silicon, the silicon was also used as a carrier-generating layer. This substantially increased the number of charge carriers generated and thus resulted in a higher intensity electrical signal. Consequently, the thickness of the organic layers can be reduced to less than 1/10 of the typical values (several microns) required in the conventional TOF measurement. The typical thickness of the organic layer in the present work is 400 nm. For organic materials with a high optical absorption coefficient, samples for the TOF measurement can be prepared by directly depositing these materials onto ITO glass substrates with a thickness of about 1000 nm. For both types of substrate, the thickness of the organic layer is much closer to the typical value used in organic electroluminescent devices. The signal, and thus the accuracy, in the present measurement were much improved over those of the conventional TOF measurement. The logarithm of the drift mobility changed linearly with the square root of the applied electric field.

Electronic structure of silicon nanowires: A photoemission and x-ray absorption study

Abstract: Photoemission and x-ray absorption spectroscopy have been used to study silicon nanowires prepared by a laser ablation technique together with Si(100) and porous silicon. Si $2p$ and valence-band spectra show that the Si nanowires are essentially crystalline Si encapsulated by silicon oxide. HF etching removes the surface oxide but leaves the morphology intact. Si K-edge x-ray absorption near-edge structures show that the characteristic Si K-edge whiteline doublet in Si(100) smears out in the nanowires and blurs entirely in porous silicon and that the whiteline exhibits a small blueshift. This observation indicate a progressive degradation in long-range order going from bulk Si to nanowires to porous Si and a wider band gap for a fraction of the nanowires. The extended x-ray absorption fine structures show that despite an increased disorder relative to bulk Si, Si nanowire remains essentially crystalline, in good accord with recent transmission electron microscopy and x-ray powder diffraction studies.

Straight β-SiC nanorods synthesized by using C–Si–SiO2

Abstract: Straight beta-silicon carbide nanorods have been grown on silicon wafers using hot filament chemical vapor deposition with iron particles as catalyst. A plate made of a C–Si–SiO2 powder mixture was used as carbon and silicon sources. Hydrogen, which was the only gas fed into the deposition system, acts both as a reactant and as a mass transporting medium. The diameter of the β-SiC nanorod ranged from 20 to 70 nm, while its length was approximately 1 μm. A growth mechanism of beta-silicon carbide nanorods was proposed. The field emission properties of the beta-silicon carbide nanorods grown on the silicon substrate are also reported.

Smallest diameter carbon nanotubes

Abstract: Mass-selected carbon ion beam deposition (MSIBD) was used to demonstrate that the diameter of a carbon nanotube could be as small as 0.4 nm, the theoretical limit predicted but never experimentally reached so far. The deposition was performed at an elevated temperature much lower than the high temperatures (800–1000 °C) needed for deposition of carbon nanotubes by conventional methods. High-resolution transmission electron microscopy showed that the combination of the stress induced by the ion impact and the C migration at the temperature applied formed graphitic sheets with their normal (c axis) parallel to the surface of the silicon substrate. Some sheets closed to form multiwall nanotubes. The smallest diameter of the innermost tube was found to be 0.4 nm. The novel use of MSIBD (a pure method, catalyst free, low deposition temperature, easily applied to large surfaces without surface pretreatment capable of pattern-writing) may significantly advance the carbon nanostructure technology.

Growth and emission properties of β-SiC nanorods

Abstract: A one step procedure has been developed to grow beta-silicon carbide (β-SiC) nanorods from a solid carbon and silicon source on silicon substrates using a hot filament chemical vapor deposition. The growth process was catalyzed by the impurities of metallic particles confined in the solid source plate made of a mixture of graphite and silicon powders pressed at 150°C. Hydrogen was introduced into a reaction chamber to react with the solid source. The resulting process produced, hydrocarbon and hydrosilicon radicals, which subsequently reacted on the Si substrate surface and presumably formed SiC nanorods. The nanorods consisted of a crystalline β-SiC core with an amorphous silicon oxide shell layer. The nanorods were 10–30 nm in diameter and less than 1 μm in length. Field emission characteristics of the β-SiC nanorods were investigated using current-voltage measurements and the Fowler–Nordheim equation. The silicon carbide nanorods exhibited high electron field emission with high stability. Along with the ease of preparation, these silicon carbide nanorods are believed to have potential application in electron field emitting devices.

Bubble formation in organic light-emitting diodes

Abstract: Bubbles in organic light-emitting diodes can be formed from gas release due to Joule heating effect at localized electrical shorts during operation, which could be simulated by a rapid thermal annealing. The gases in the bubbles consist of not only adsorbed moistures but also the decomposed organic species, which are detected in situ in an ultrahigh vacuum chamber. In the device of Al/tris-(8-hydroxyquinoline) aluminum (Alq/N,N′-diphenyl-N.N′-bis-{3-methylphenyl}{1,1′biphenyl}-4,4′-diamine/indium tin oxide (ITO), the gases released from ITO surface were mainly of adsorbed moistures, while those released from the organic layers were of both the decomposed products from Alq and the trapped moistures. The decomposition of Alq could not be easily avoided if there were severe localized electrical shorts in the devices.

Investigation of Si-doped diamond-like carbon films synthesized by plasma immersion ion processing

Abstract: Silicon (Si)-doped diamond-like carbon (DLC) was prepared on Si(100) and polymethyl metha_crylate (PMMA) substrates using a C2H2–SiH4–Ar plasma immersion ion processing (PIIP) method. The chemical composition of the films was varied by adjusting the reactive gas-flow ratio of SiH4 to C2H2 during PIIP depositions. The influence of the Si dopant on the bonding structure, stress, and properties of the DLC films was investigated by using ion beam analysis techniques, Raman shift, ultraviolet/visible spectroscopy, and by analyzing the measured properties. The incorporation of Si up to 17.3 at. % produced a reduction in film stress and increased the density and optical band gap. The Si-doped DLC films also exhibited increased sp3 bonding and higher hardness (25–28 GPa). Further increase in Si dopant, to above 22 at. %, caused a transformation from DLC to amorphous silicon carbide (a-SiC) that showed high hydrogen capacity, low hardness, and low stress. Pin-on-disk tribological tests of Si-doped DLC on PMMA show...

The electronic structures and properties of Alq3 and NPB molecules in organic light emitting devices: Decompositions of density of states

Abstract: A decomposition treatment of density of states in combination with PM3 molecular orbital calculations was used to reveal the fingerprints of electronic structures of two prototypical electroluminescent molecules, tris(8-hydroxy-quinoline)aluminum (Alq3) and N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). High convenience and accuracy of such a treatment were found for these large organic molecules in the determinations of (1) the distribution of important molecular orbitals such as the highest occupied orbital and the lowest unoccupied orbital; (2) the contribution to valence and conduction bands as well as gap states from constituent atoms, and thus the attribution of ultraviolet photoemission spectrum; (3) the sites and properties of reaction and excitation of a molecule; and (4) the localization property of electronic states. In particular, this study indicates that Alq3 is most possibly attacked by other atoms at the oxygen atoms while the reaction site for NPB is at the nitrogen ...

Direct evidence for interaction of magnesium with tris(8-hydroxy-quinoline) aluminum

Abstract: The interaction between magnesium (Mg) and tris(8-hydroxy-quinoline) aluminum (Alq3) has been studied using high-resolution electron energy-loss spectroscopy (HREELS). It was found that deposition of magnesium on the Alq3 film gave rise to clear changes in the HREELS spectra. The changes are attributed to the weakly bounded Mg atoms on the Alq3 layer. Interestingly, for a given amount of magnesium (Mg to Al atoms ratio=3) on Alq3 film, remarkable changes were observed in the HREELS spectra when the sample was heated. A loss peak at 81 meV, which was assigned to Mg–O stretch mode, appeared upon annealing and increased in intensity as the annealing temperature increased up to about 360 K. This suggested that the diffusion of Mg atoms into the Alq3 layer and the reaction between Mg and Alq3 molecule occurred at the temperature range investigated. The present work has provided direct evidence for the strong interaction between magnesium and Alq3.