Showing papers by "Shuit-Tong Lee published in 1998"

Silicon nanowires prepared by laser ablation at high temperature

Abstract: Silicon nanowires have been synthesized in high yield and high purity by using a high-temperature laser-ablation method with growth rates ranging from 10 to 80 μm/h. Transmission electron microscopic investigation shows that the nanowires are crystalline Si, and have diameters ranging from 3 to 43 nm and length up to a few hundreds microns. Twins and stacking faults have been observed in the Si core of the nanowires. The lattice structure and constant of the nanowires as determined from x-ray diffraction (XRD) are nearly identical to those of bulk Si, although the relative XRD peak intensities are different from those of randomly oriented Si crystallites. Raman scattering from the nanowires shows an asymmetric peak at the same position as that of bulk crystalline silicon.

Nucleation and growth of Si nanowires from silicon oxide

Abstract: Nucleation and growth of Si nanowires by laser ablation and thermal evaporation of Si powder sources mixed with ${\mathrm{SiO}}_{2}$ have been investigated by means of transmission electron microscopy. At the initial nucleation stage, Si oxide vapor condensed on the substrate and formed Si nanoparticles (the nuclei of nanowires). Each Si nanowire nucleus consisted of a polycrystalline Si core containing a high density of defects and a Si oxide shell. A growth mechanism was proposed based on the microstructure and different morphologies of the Si nanowires observed.

Synthesis of Boron nitride nanotubes by means of excimer laser ablation at high temperature

Abstract: Boron nitride nanotubes (BN-NTs) were synthesized by using excimer laser ablation at 1200 °C in different carrier gases. The main characteristic of the BN-NTs produced by this method is that nanotubes are of only one to three atomic layers thick, which could be attributed to the dominance of the axial growth rate over the radial growth rate. The diameter of the BN-NTs ranged from 1.5 to 8 nm. The tips of the BN-NTs are either a flat cap or of polygonal termination, in contrast to the conical ends of carbon nanotubes. The atomic ratio of boron to nitrogen as measured by means of parallel electron energy loss spectroscopy is 0.8, which is within the experimental error of the stoichiometry of hexagonal BN structure.

SiO2-enhanced synthesis of Si nanowires by laser ablation

Abstract: Si nanowires with uniform size have been synthesized by laser ablation of highly pure Si powder targets mixed with SiO2. A bulk quantity of Si nanowires was successfully obtained by mixing 30%–70% of SiO2 into the Si powder target. SiO2 played a crucial role in enhancing the formation and growth of the Si nanowires. The morphology and microstructure of the Si nanowire tips have been systematically characterized by means of high-resolution transmission electron microscopy. No evidence of metal was found at the tips. The results suggest that Si oxide is more important than metal in catalyzing the formation of Si nanowires.

Energy level alignment at Alq/metal interfaces

Abstract: The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

Synthesis, composition, surface roughness and mechanical properties of thin nitrogenated carbon films

Abstract: In this study, we explored the properties of amorphous nitrogenated carbon films grown in a standard magnetron sputter-deposition system without a magnetic trap. Silicon wafers and hard disk substrates were used. To simplify process control, we operated the graphite target at constant voltage and used 3% nitrogen in argon from a premixed feed. Under these conditions, most nitrogenated carbon films contain about 6–8 a/o of nitrogen. Films with the highest hardness values (28 GPa) and lowest root-mean-square surface roughness (0.2 nm on hard disk substrates) were obtained at low sputtering pressure (5 mTorr) and large substrate pulse bias (−300 V). These observations demonstrate the importance of ion bombardment in controlling film morphology and properties. The harder films appear to have better resistance against scratch damage, as is demonstrated by the nano-scratch tests using a two-dimensional nanoindentor.

Large scale synthesis of silicon nanowires by laser ablation

Abstract: Quasi one-dimensional materials have attracted considerable attention in recent years because of its potential to both fundamental physics and nanoelectronic applications. More recently, we have achieved large scale synthesis of silicon nanowires (SINW) at a high growth rate by laser ablation of Si target at 1200 °C. The laser source was a pulsed KrF excimer laser and the Si targets were made by pressing Si powder of 5 microns in size. 50 sccm Ar was used as a carrying gas flowing from the side near the Si target towards a water-cooled copper finger. Si nanowires have been grown with diameters ranging from 3 to 43 nm and several hundreds microns in length after 2 hours of laser ablation of Si target. The SLNWs were analyzed by XRD, Raman, EDS, TEM and HRTEM. Successful large scale synthesis of SINW by laser ablation extends the pulsed laser ablation method from depositing thin films to synthesis of nanowires.

Local strain in interface: Origin of grain tilting in diamond (001)/silicon (001) heteroepitaxy

Abstract: Diamond grain tilting, one of the central problems of diamond heteroepitaxy on silicon (001) surface, has been studied by means of atomic-force and high-resolution electron microscopic observations, and by theoretical simulation using the molecular-orbital method. It is shown that, due to interface mismatch-induced local lattice strain and three-dimensional stacking, diamond nucleation in small areas results naturally in grain tilting. For more perfect heteroepitaxy, nucleation with reduced silicon surface damage and over relatively large lateral domains is required.

Oriented diamond growth on silicon (111) using a solid carbon source

Abstract: Textured diamond films have been grown on silicon (111) substrate by using hot filament chemical vapor deposition. A graphite plate immersed in hydrogen was used as the carbon source rather than the conventional gaseous methane source. During the nucleation period, a negative bias relative to the filaments was applied to the substrate. An epitaxial β-SiC layer was deposited during the bias treatment. Textured diamond film was subsequently grown on the β-SiC layer from the mixture of hydrogen and hydrocarbon species etched from the graphite.

Epitaxial growth of β-SiC on silicon by bias-assisted hot filament chemical vapor deposition from solid graphite and silicon sources

Abstract: Epitaxial β–SiC film has been grown on a mirror-polished Si(111) substrate using bias-assisted hot filament chemical vapor deposition (BA-HFCVD) at a substrate temperature of 1000 °C. A graphite plate was used as the only carbon source, and hydrogen was the only feeding gas to the deposition system. Atomic hydrogen, produced by hot filaments, reacted with the graphite to form hydrocarbon radicals which further reacted with the silicon substrate and deposited as β–SiC. The effect of negatively biasing the substrate is the key factor for epitaxial growth. Under the same growth conditions without negative bias, polycrystalline β–SiC resulted.

Effect of negative rf bias on electrophotographic properties of hard diamond-like carbon films deposited on organic photoconductors

Abstract: Hard diamond-like carbon (DLC) films were prepared on organic photoconductor (OPC) and PMMA (polymethyl methacrylate) samples by electron cyclotron resonance (ECR) plasma deposition with low substrate bias. The films exhibited a specific electrical resistivity above and very high transmittance for the visible spectrum of 370-800 nm. The hardness of the OPC samples was increased at least fourfold after deposition of DLC films. The acceptance voltage, photodischarge rate and dark decay rate of the DLC-coated OPC samples were all improved, and reached optimal values for rf biases around . The results indicated that the deposition of hard DLC films on OPC is a very promising technique to lengthen the working lifetimes of the OPC materials.

Diamond-like carbon protective films for organic photoconductors

Abstract: The deposition of a protective film to increase the hardness of an organic photoconductor (OPC) surface is an effective method of lengthening the lifetime of the OPC. In this work, diamond-like carbon (DLC) protective films were deposited onto OPC samples by the electron cyclotron resonance (ECR)-microwave plasma chemical vapor deposition (MPCVD) method with low substrate temperature. The DLC films were deposited with optimized deposition conditions and exhibited high transmissivity and high electrical resistivity. The films caused a remarkable increase in the hardness of the OPC surface, by a factor of 2.5∼5.4. The acceptance voltage, dark decay rate, photodischarge rate, and contrast potential of the OPC protected by DLC film were improved. These results show DLC is a suitable protective film for OPC.

Silicon nanowire: A new shape of crystalline silicon

Abstract: Silicon nanowires have been synthesized by using a high-temperature laser ablation method. Transmission electron microscopic investigation shows that the nanowires are crystalline Si, and have diameters ranging from 3 to 43 nm and lengths up to a few hundreds μm. Raman scattering spectrum shows an asymmetric peak at the same position as that of bulk crystalline silicon. The x-ray diffraction revealed an important contribution from surface oxide. This is due to the high surface-to-volume ratio. Visible photoluminescence (PL) was observed in nanowires with sub-5 nm diameter.

The origin of mis-oriented diamond grains nucleated directly on (001) silicon surface

Abstract: The origin of mis-oriented diamond grains frequently observed in heteroepitaxial diamond films on (001) silicon surfaces was studied. By statistically analyzing the in-plane rotation angles of diamond grains in scanning electron microscopy observations, it was found that the distribution of the grain orientation is not random and two satellite distribution peaks at about 20° and 30° accompany the main distribution peak at zero degree referenced to the direction of substrate. The interface structure corresponding to the main distribution peak at zero degree of oriented diamond growth has been proposed in our previous studies. In this study, our molecular orbital PM3 simulation of a step-by-step diamond nucleation further reveals two other metastable diamond/silicon interfacial structures. The orientations of the corresponding diamond grains are parallel to the (001) silicon surface but with in-plane rotations of 20° and 30° respectively with respect to the direction. We relate these two mis-oriented growths to the two satellite peaks of grain orientation distribution. Based on this study, the possibility in experiment to reduce the formation of mis-oriented configurations and to obtain a perfectly oriented diamond growth is discussed.

Bonding Regeneration: The Driving Force of Hetero-Epitaxial Diamond Grain Coalescence on (001) Silicon

Abstract: The grain coalescence phenomenon in the growth of heteroepitaxial diamond film on (001) silicon substrate by microwave plasma chemical vapor deposition was examined by using high-resolution electron microscopy. It was shown that this phenomenon evidently occurs between two diamond grains with a small-angle tilt. The coalescence was completed after some more growth steps following the meeting of such two grains, indicating the difficulty for the lattice matching in grain boundary. By performing simulation of a step-by-step growth of two diamond grains on a (001) silicon substrate with molecular orbital PM3 method, it was shown that the bonding regeneration between the two grains is essential for the coalescence and the coalescence is only possible when the orientation difference between the grains is sufficiently small so as to allow efficient overlap of electron cloud in the grain boundary. This study indicates that single crystal diamond growth may be possible by the current CVD growth techniques via further reduction of the surface roughness to gain a heteroepitaxy with very small grain tilting.

LEDs on diamond

Abstract: Fabrication of various types of diamond LEDs, their characteristics and the operation mechanisms are reported. All the devices are based on the double injection electroluminescence: charge carriers are injected into intrinsic (i) diamond from adjacent semiconducting p- and n- type areas. The i-area is activated with appropriate optical centers. In the first step a survey of the appropriate optical centers and their spectra is given. The devices are fabricated in the form of either planar or vertical p-i-n or p-i-p structures. It is shown that the electrical and optical properties of the devices are controlled by the type, concentration, and spatial distribution of the deep traps in the i-region. The selection of diamond substrates suitable for the fabrication of LEDs is an essential point and it can be performed with a microwave photoconductivity technique. Several fields of possible application of diamond LEDs are shown: high temperature LEDs, optical sensors of magnetic field, color switches, color indicators of temperature, the high temperature applications being the most promising application area. Perspectives of creation of diamond based laser diodes is shortly discussed.

Excimer Laser Ablation of Silicon at High Temperature

Abstract: Pulsed laser ablation of granulated Si target was carried out at 1200 °C in an Ar atmosphere. Multishot ablated target surface forms intensity dependent features, including porous, skeleton, and columnar structures. Very long columnar structures were observed when the angle of the target surface with respect to the direction of the laser beam was small. Evidence on preferable remove of smaller particles has been observed. Formation of the columnar structures started from the biggest particles at the surface and grew deeper, straight in the laser beam direction, by consuming the removed Si species from the deep channel between columns. The Si species ablated off the granulated Si target deposited as Si nanowires or nanoparticles down stream of the Ar flow. Significant decrease in the deposition rate of Si nanostructures has been observed upon the formation of the columnar structures at the target surface.