Papers

ICMCTF2003 Session H6: Novel Materials and Deposition Strategies

Wednesday, April 30, 2003 1:30 PM in Room San Diego

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		H6-1 Carbon Nanotubes: Complex Structures by Simple Techniques P. Ajayan (Rensselaer Polytechnic Institute) The talk will review the various forms of carbon nanotube structures that have been grown in our laboratory using simple thermal CVD processes and subsequent surface modification treatments. Aligned nanotube arrays (both singlewalled and multiwalled), doped nanotubes, nanotube based meso-structures (such as long strands), surface modified nanotubes and nanotube junctions can be fabricated by combinations of simple techniques. The rationale behind growing such structures, possible applications and the challenges that still face this field in terms of growth and assembly, will be discussed.
2:10 PM		H6-3 Synthesis and Properties of Doped Nanotubes and Nonowires A.M. Rao (Clemson University) One-dimensional materials have attracted great scientific interest because of their unique fundamental properties and potential technological applications. Here we describe synthesis techniques for the synthesis of modified carbon nanotubes using thermal CVD and pulsed laser vaporization methods. Boron doped nanotubes (p-type) were prepared by pulsed laser vaporization of carbon targets containing boron with concentrations ranging between 0.5 - 10 at %. Above a threshold boron concentration of 3 at%, the growth of nanotube bundles ceases due to the low solubility of boron in carbon at ~1200 °C. The nitrogen (n-type) doped tubes were prepared by introducing melamine vapors during the growth of multiwalled carbon nanotubes from the decomposition of the xylene-ferrocene mixtures. The changes in the morphology of the nanotube products as monitored using Raman spectroscopy, SEM, high resolution TEM, and EELS measurements will be presented. In addition, gallium oxide nanostructures, such as nanobelts and nanowires using the thermal or microwave plasma CVD reactor were also prepared. High-resolution TEM, electron diffraction and Raman analysis showed the structure of the nanowires to be that of the monoclinic β-Ga₂O₃3 phase. The nanowire diameters ranged from 10-100 nm in diameter with lengths up to several microns. Preliminary micro-Raman analysis of the nanowires (produced by the plasma CVD) using laser excitation wavelengths of 442nm, 514.5nm and 785nm showed an upshift of ~10-15 cm-1 in the frequency of the dominant ~200 cm-1 (A_g) peak. Room temperature photoluminescence spectra of these nanowires suspended in iso-propyl alcohol showed a dominant emission band ~325 nm.
2:50 PM		H6-5 Selective Metallization of Modified Silica Surfaces by Copper CVD P. Doppelt (CNRS, France) Recent work [1] has shown that SiO₂ and SiLK adhesive < 2-nm-thick films of self-assembled molecular layers (SAMs) hold promise for serving as diffusion barriers to inhibit Cu diffusion in dielectric layers for sub-65-nm device structures. However, these monolayers must be compatible with advanced copper metallization processes such as Chemical Vapor Deposition (CVD) which exhibits conformal coverage in non planar geometries. Generally, copper CVD where volatile -diketonate Cu(I) precursors are employed [2], proceeds as uniform sticky thin films on conductive materials such as gold, tantalum or barriers such as TiN or TaN via a thermally induced disproportionation reaction. On non-conductive materials, the copper films are obtained at higher temperature and are not adhesive. We achieved sticky continuous copper CVD on derivatized silica surfaces with the 3-mercaptopropyltrimethoxysilane. The effect of the deposition route (gas-phase vs. liquid phase SAM formation) on copper adhesion has been studied and the best results were obtained when the gas-phase SAMs deposition was employed. Moreover, UV-exposure of the sulfur terminated molecules employing a mask resulted in a controlled pattern of the surface affinity towards the copper complex and hence selective Cu CVD was achieved. The nature of the interaction between the chemically functionnalized, patterned surface and the copper precursor at the initial step of the film growth is discussed. [1]A. Krishnamoorthy, K. Chanda, S. P. Murarka, G. Ramanath, J. G. Ryan, Appl. Phys. Let., 78, 2467 (2001). [2] P. Doppelt, T. H. Baum, MRS Bulletin, XIX(8), 41 (1994).
3:50 PM		H6-8 Spontaneous Generation of Charged Cu Clusters by Thermal Evaporation S. Maikap, Nong-Moon Hwang, J.-H. Lee (Seoul National University, South Korea); J.-H. Song, W.-K. Choi (Korea Institute of Science and Technology, South Korea); D.-Y. Kim (Seoul National University, South Korea) It is well known that the crystals grow by atomic or the molecular unit. The atom first adsorbs on the terrace, diffuses to the ledge and finally becomes the crystal at the kink. A new technique, charged cluster model (CCM), for growing crystal has been proposed by Hwang et al. The CCM is practically the same concept as the new mechanism suggested by Glasner et al. The only difference is that the charged cluster model deals with the thin film growth. The purpose of this study is to test whether the charged Cu clusters are also generated in the gas phase under a processing condition by thermal evaporation. In order to investigate the charged Cu cluster and also its size, the time-of-flight (TOF) experiment has been performed. The TOF analyzer consisted of a pulse generator with 10 micros pulse width at every 1 ms and a multi-channel plate (MCP) for amplification of cluster ion current. The MCP was connected to a 500 MHz oscilloscope to measure the flight time of monomers and also clusters. The negative pulse voltage was applied to the mesh to cut off clusters and monomers. The length of flight tube was 1.7 m. The TOF mass spectrum of Cu cluster has been observed with the acceleration voltage of 10 keV. The cluster size is obtained from the ratio of flight time of clusters to that of monomers. The estimated mean cluster size from TOF mass spectrum is about 6 atoms per clusters at the initial evaporation. After evaporation of 5 minutes, the new larger cluster with the size of 4450 atoms per clusters has been observed. It is reported that the interaction between charged clusters of similar size tends to be repulsive while that between small and large ones can be less repulsive or even attractive. Therefore, the larger clusters can be explained by considering the Coulomb interaction between charged clusters. These clusters are attributed to the presence of charge and might be applied to the fabrication of nano-structure.
4:10 PM		H6-9 Direct Evaporative Deposition of Design Topology in Aluminum Coatings A.F. Jankowski, J.P. Hayes (Lawrence Livermore National Laboratory) The direct deposition of metals with a well-defined topology by design presents a challenge for the use of high-rate, physical vapor deposition technology. An application for this type of novel material is found in advancing the evaluation of equation-of-state predictions for material behavior under extreme loading conditions. Specimens of constant cross-section, in the form of coated flats and as free standing, can be produced through electron-beam evaporation. For example, single crystal deposits of low melting point metals as aluminum are routinely produced with a 25 to 200 micron thickness range and areas in excess of 1 sq cm. However, to deposit a design topology in cross-section without the use of post deposition processing requires some innovative use of existing evaporation methods. As a starting point, the basic conditions for growth of the evaporated deposit are found in the classic zone model. The columnar structure intrinsic to vapor deposits evolves from the sub-micrometer to millimeter scale as the substrate temperature increases to the melting point. To produce the desired thickness gradient in cross-section, the exposure of the substrate is coupled to the highly collimated evaporation source through a variable position shutter. As driven by a computer-controlled stepper motor, the shuttered exposure provides a means to reproduce the desired cross-section shape. As an example, results will be presented to for the deposition of aluminum with a single crystallographic orientation onto lithium fluoride substrates wherein the film thickness increases from 30 to 100 microns over a 0.8 mm path length.
4:30 PM		H6-10 Lithium Fluoride Thin Films Grown by Off--axis Pulsed Laser Ablation S.J. Henley, M.N.R. Ashfold (University of Bristol, United Kingdom) Thin coatings of LiF are currently of interest for significantly reducing the work function of metal electrodes, allowing for more efficient electron injection and field emission. Alkali halide coatings have been found to act as effective dipole layers to lower the surface work function and induce a negative electron affinity of diamond surfaces. Here the results of the analysis of films grown by pulsed laser ablation from a commercially available sintered disk of LiF are reported. The morphology, composition and crystallinity of films grown on silicon and quartz substrates are examined and suitable deposition parameters for optimising the growth are suggested. Hydrodynamic sputtering is shown to be the dominant mechanism of material ejection, at high laser fluences, and a substrate position off the axis of the ablation plume is suggested for improving the morphology of the films. This position is shown to grow films with a Li:F ratio identical to that of bulk LiF.
4:50 PM		H6-11 Vanadium Oxide Thin Films Produced by Hot Filament Metal Oxide Deposition M.A. Bica de Moraes (LPP-DFA-IFGW-UNICAMP, Brazil); J. Scarminio (Depto. de Fisica, CCE, UEL, Brazil); B.C. Trasferetti (UNICAMP, Brazil); F.P.M. Rouxinol (LPP, DFA, IFGW, UNICAMP, Brazil); S.F. Durrant (UNIVAP, Brazil) Vanadium oxide (V₂O₅) films find application as cathodes in re-chargable micro-batteries and in electro-chromic devices. The latter depend on the modification of the color of such films upon intercalation with H⁺ or Li⁺ ions. In this work, thin films were deposited using a recently developed variant of Hot Filament Chemical Vapor Deposition in which a metallic filament, in this case of V, is evaporated in an oxygen atmosphere. This approach has been designated Hot Filament Metal Oxide Deposition (HFMOD). Deposition rates and film structure were examined as a function of the key deposition parameters, namely the filament current and the partial pressure of oxygen. From deposition times and film thicknesses, deposition rates were determined. Film structure was probed using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The potential of the films for applications in electrochromism are assessed. The proposed procedure is a novel and relatively cheap route for the production of vanadium oxide and similar (MoO_x and WO_x) films.