AVS2001 Session OF+TF+EL-WeA: Growth of Organic Thin Films

Wednesday, October 31, 2001 2:20 PM in Room 131

Wednesday Afternoon

Start	Invited?	Item
2:20 PM		OF+TF+EL-WeA-2 Resonant Mid-Infrared Pulsed Laser Deposition of Polymer Films R.F. Haglund, M.R. Papantonakis (Vanderbilt University); D.M. Bubb, J.S. Horwitz, J.S. Callahan, R.A. McGill, E.J. Houser, D.B. Chrisey (Naval Research Laboratory); M. Galicia, A. Vertes (George Washington University); B. Toftmann (Risoe National Laboratory, Denmark) Resonant, picosecond-pulse, mid-infrared laser irradiation has been shown to ablate glassy and crystalline solids with high efficiency and low collateral damage.¹ We have extended this concept to show that resonant infrared (IR) pulsed-laser deposition (PLD) is an effective method for depositing polymer films with physical and chemical structure as well as optical properties virtually identical to those of the bulk starting material. This contrasts sharply with PLD at ultraviolet (UV) wavelengths, where deposited polymer material is sometimes significantly degraded. In our experiments, the organic starting material was ablated by a pulsed infrared, free electron laser tuned into resonance with various vibrational modes; the vapor was collected on a room-temperature substrate. For polyethylene glycol (PEG, MW 1450) the laser was tuned to either C-H or O-H stretching modes at 2.9 and 3.4 µm, respectively. The properties of the deposited film were determined using infrared absorption spectroscopy and mass spectrometry. When the infrared laser was detuned from resonance, the structure and optical properties of the deposited PEG film were significantly altered, showing that the off-resonance ablation process thermally damages the polymer. The potential for generalizing this technique can be seen in the successful deposition of poly(lactide co-glocolide) (PLCA, MW 67,000). PLCA films were deposited using 5.7 µm excitation (1754 cm^-1), where the laser excitation is localized by the C=O stretch. The mechanism of ablation appears to be explosive vaporization; in contrast to the photochemical mechanism typical of UV ablation, the ablated material seems to remain in the electronic ground state. The technique appears promising for biomedical and electronic applications of polymeric and organic thin films. ¹ D. R. Ermer, M. R. Papantonakis, M. Baltz-Knorr, D. Nakazawa and R. F. Haglund, Jr., Appl. Phys. A 70, 633-635 (2000).
2:40 PM		OF+TF+EL-WeA-3 Synthesizing Thin and Ultrathin Polymer Films by a Two-step Deposition/Polymerization Process J. Bai, C.M. Snively, W.N. Delgass, J. Lauterbach (Purdue University) The goal of this work is to understand and further develop an in-situ preparation method for producing high quality polymer thin films. In this two-step vacuum process, a monomer film is deposited onto a cooled substrate, and then the polymerization reaction is initiated; thus the polymerization is confined to the substrate. Compared to other preparation techniques, this process has the major advantages of allowing the fabrication of films from insoluble polymers and providing better control of film quality. This investigation focused on the molecular orientation and packing of the monomer, as well as polymerization kinetics and film quality characterization. In-situ adsorption and polymerization studies were done using Reflection-Absorption Infrared Spectroscopy (RAIRS) and Temperature-Programmed Desorption (TPD). Time resolved FTIR spectra were taken to study the factors controlling the rate of polymerization. GPC, ellipsometry and AFM provided information about molecular weight, film thickness, and morphology characterization of the films. The model systems investigated were styrene and methyl methacrylate (MMA) on platinum. For the styrene/Pt model system, kinetic studies reveal that the polymerization reaction is preferred in amorphous monomer layers. Ellipsometry measurements show that, in the range of 25-200nm, the polystyrene film thickness is a linear function of the monomer dosage. AFM data suggest that the resulting film morphology is influenced by the platinum substrate. Polarized RAIRS spectra of MMA show that, up to 0.2 micron, only p-polarized component of the beam contributes to the spectrum, indicating that RAIRS can be used to determine the molecular orientation of films with sub-micron thickness.
3:00 PM		OF+TF+EL-WeA-4 Surface Growth Study of Pentacene, Perylene and CuPc S. Zorba, N.J. Watkins, L. Yan, Y. Gao (University of Rochester) Pentacene, Perylene and CuPc are widely used organic semiconductors in organic light emitting diode (OLED) and organic thin film transistor (OTFT) applications. It has been well established that the transport properties of these materials are strongly dependent upon the way they are grown as organic thin films. One of the reasons why these materials are so popular is the fact that their electronic properties, such as their mobilities and barrier to charge injection ratios, can be altered and improved by changing their morphologies. In this talk we will present our recent results on the different growth modes of these substances on different substrates and try to relate their observed electronic and transport properties to their morphologies and account for their favorable features.
3:20 PM		OF+TF+EL-WeA-5 Substrate Controlled Crystallisation of Ultra-thin Films of Perylene Q. Chen, N.V. Richardson (University of St Andrews, UK); P.J. Unwin, T.S. Jones (Imperial College of Science, Technology and Medicine, UK); T. Rada, A.J. McDowall (University of St Andrews, UK) There has been recent interest in the electronic and optoelectronic properties of polyaromatic hydrocarbons, such as tetracene, pentacene and perylene, because of their potential use as organic lasers, OLEDs and OFETs. Device performance is likely to be s trongly influenced by the structural quality of crystals or thin films. We have carried out a detailed investigation of the deposition and growth of ultra-thin films (0-30 monolayers) of perylene on a variety of substrates including Cu{110}, H/Si(111), InAs(111) and InSb(111), using STM, TPD, LEED and vibrational spectroscopies. The substrate has a profound influence on growth and subsequent crystallinity of the film. For example, films grown on a hydrogen terminated Si(111) surface show no evidence o f layer-by-layer growth and, from the earliest stages, small clusters of randomly oriented molecules are present which eventually cover the surface with a rough polycrystalline film. In contrast, vacuum deposition on a Cu{110} surface leads to large crysta l line domains upto one monolayer coverage. Electron energy loss spectroscopy confirms that the molecules are flat-lying and π-bonded to the substrate. Multilayer growth takes place epitaxially layer-by-layer on this first monolayer into a structure whic h, although commensurate with the underlying copper surface along the <110> direction is incommensurate along <100> and is unrelated to the bulk crystal structure of perylene. The 2D unit cell at all layer thicknesses (to 30 monolayers) is rectangula r with dimensions 2nm x 1.9nm containing two, flat-lying molecules per layer at 90° to each other around an axis normal to the substrate surface. The molecular rows along <110> are out-of-phase with each other in the <100> direction from layer to layer, in an ABA packing sequence of rows along the growth direction. The implications for the modification of film properties implied by this control over film structure and crystallinity will be discussed.
3:40 PM		OF+TF+EL-WeA-6 Probing the Reactivity of Multifunctional Compounds on Semiconductor Surfaces: Pyrrole and its Derivatives on Si and Ge(100)-2x1 G.T. Wang, C. Mui, C.B. Musgrave, S.F. Bent (Stanford University) While the chemistry of amines has been extensively studied on the Si(100)-2x1 surface, relatively little is known about their reactivity with the Ge(100)-2x1 surface. In this study, the reactions of pyrrole and multiple pyrrole derivatives, including methylpyrrole, pyrrolidine, and methylpyrrolidine, with the Ge(100)-2x1 and Si(100)-2x1 surface were investigated via multiple internal reflection infrared spectroscopy and ab initio quantum chemistry calculations. This series of compounds comprises a model system for examining the competition of multiple functionalities on Ge(100) and Si(100) and the role kinetics and thermodynamics play on selectively controlling growth of organics on semiconductor surfaces. Although these compounds are structurally similar, they each follow different reaction mechanisms on the Si surface, including dative bonding, N-H dissociation, cycloaddition, and electrophilic aromatic substitution. We have also found that the adsorption of pyrrole and its derivatives on the Ge surface is surprisingly different from the Si surface. While the reaction of organics on Si is typically under kinetic control, the experimental and theoretical results show that thermodynamical considerations are also necessary to explain the reaction of organics on Ge. Additionally, aromaticity and charge transfer to the surface are found to have significant effects on the adsorption behavior of these compounds.
4:00 PM		OF+TF+EL-WeA-7 Chemisorption of Vinylacetic Acid on Si(001) and Its Subsequent Reaction with Iron Pentacarbonyl K. An, S.S. Lee, Y. Kim (Korea Research Institute of Chemical Technology, South Korea) Chemisorption of vinylacetic acid (VAA) on clean Si(001) substrate at room temperature and the change of its adsorption with temperature have been investigated by x-ray and ultraviolet photoelectron spectroscopy. It was hoped that the C=C double bond of the chemisorbed VAA may be utilized in the subsequent reaction with the second adsorbate, iron pentacarbonyl, Fe(CO)₅, in this study. VAA first adsorbs on Si(001) molecularly, and as the substrate temperature is increased to 350°C, loses the acidic hydrogen resulting in the configuration that has two equivalent oxygen atoms according to the change in the O 1s XPS peak. The curve-fitted Si 2p peak indicates formation of the Si²⁺ state on the surface. UPS also showed that the photoemission peak due to the OH group of VAA loses its intensity significantly. This surface species is stable up to about 350°C. As the temperature is raised even more to ~450°C, the adspecies are decomposed leaving a mixture of oxidized silicon and silicon carbide. At about 700°C, only tenacious oxygens remain on the carbided substrate surface. Iron pentacarbonyl was introduced into the XPS analysis chamber to 35000 L with the monolayer of the anionic species of VAA at 350°C. The surface, after the introduction of Fe(CO)₅, was found to consist of the acid anion and an iron species that shows metallic behavior according to XPS and UPS data. The Fe(CO)₅ molecules must have been completely dissociated since no accumulation of C or O adspecies has been detected. Also the surface iron atoms do not seem to have been oxidized judging from the shape of the Fe 2p level and the appearance of the Fermi level crossing. However, it is not yet clear whether the iron species have formed microscopic structures or not, while the change in the C 1s peak suggests that they may each have a chemical bond with the surface C=C double bond. At the moment a scanning tunneling microscopy experiment is in preparation to examine the surface structure of the iron species.
4:20 PM		OF+TF+EL-WeA-8 Possibilities of Electron Beam Nanometer-scale Fabrication of Si(111) Using Alkyl Monolayers T. Yamada, N. Takano, K. Yamada, S. Yoshitomi, T. Inoue, T. Osaka (Waseda University, Japan) Utilization of monolayer materials covering Si wafer surfaces is prospective in mass-production of nanometer-scale patterns generated by electron beam drawing, maintaining the spatial resolution. This paper presents application or monolayers of organic moieties bonded on Si(111) for electron-beam patterning and successive chemical metal deposition processes over the patterns. For the purpose of passivation and chemical alteration of Si(111) surface, alkyl groups were deposited by contacting H:Si(111)(1x1) with Grignard reagents (C_nH_2n+1MgX, X=Cl, Br, I, as tetrahydrofuran solutions).¹ Vibrational spectroscopy by FT-IR and HREELS indicated that the bonds in alkyl groups delivered as the Grignard reagent remained unbroken, and that most of the terminating H atoms were replaced by the alkyl groups. STM revealed the original step/terrace structure of Si(111) uniformly covered with a monolayer of organics. The alkyl groups were arranged in a rather disordered manner within the monolayer. Auger electron spectroscopy resulted that the number of alkyl groups per a unit area is constant with respect to the chain length n in the range of 1 ~ 18. All these facts indicate that the alkyl groups are covalently bonded to Si(111). Electron bombardment in vacuum did not introduce useful alteration of the adlayer, and patterning by electron beam (incident energy <3 kV) with ambient O₂ atmosphere (<10^-6 Torr) deposited SiOx on the irradiated portions. Metals such as Ni and Cu were deposited only over the patterns by immersion into aqueous solutions including metal ions. The electron beam patterns were even visualized as metal-deposited area limited by the passivation effect of alkyl monolayer. Application of this entire process to the patterns of <100 nm is now attempted to explore the spatial resolution limit on the nanometer scale. ¹R. Boukherroub, S. Morin, F. Bensebaa and D. D. M. Wayner: Langmuir 15 (1999) 3831.