Papers

AVS1997 Session SS-TuA: Surface Photochemistry and Dynamics

Tuesday, October 21, 1997 2:00 PM in Room A3/4

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS-TuA-1 Photodesorption and Photodissociation of O₂ on Stepped Pt Surfaces C.E. Tripa, J.T. Yates, Jr. (University of Pittsburgh) Photodesorption and photodissociation efficiencies of O₂(a) on stepped Pt(335) and Pt(779) surfaces have been measured for uv light in the range 3.87-4.77 eV, using photon-induced desorption (PID) and temperature programmed desorption (TPD) mass spectrometric methods. The O₂ photodepletion at the (100) step sites is faster than at the (111) terrace sites. Compared to the flat Pt(111) surface, stepped Pt surfaces strongly inhibit the O₂ photodesorption and seem to monotonically increase the photodissociation efficiency with increasing density of the step defect sites. The global cross sections over all the adsorption sites are: (a) on Pt(335): Q_diss=(8.0±1.3)x10^-21 cm² and Q_des=(0.48±0.04)x10^-21 cm²; (b) on Pt(779): Q_diss=(4.5±1.3)x10^-21 cm² and Q_des=(0.51±0.10)x10^-21 cm².
2:20 PM		SS-TuA-2 Non-Thermal Photodesorption Induced by Infrared Radiation R.M. Rao, L. Fleck, R.J. Beuhler, M. White (Brookhaven National Laboratory) The photochemistry of molecules on metal surfaces is in many cases a result of substrate photoexcitation and subsequent energy and/or charge transfer to the adsorbate. In general, substrate excitation involves the production of energetic electron-hole pairs which can interact directly with the adsorbate through scattering processes or indirectly through lattice excitations produced by the thermalization of the nascent electron-hole pair distribution. The latter is thought to be the dominant mechanism for low energy photoexcitation in the infrared and visible and is responsible for laser-induced thermal desorption (LITD) at high laser fluences. We have recently investigated laser-induced desorption of CO molecules from a Ag(111) surface by infrared (1064 nm) radiation and observed large desorption yields at laser fluences far below that required for thermal desorption. In addition, state-resolved laser techniques using coherent VUV radiation showed that the rotational and translational energy distributions of the desorbing CO molecules were power independent. More recently, we have extended these studies to both physisorbed N₂ and Kr atoms which exhibit very similar behavior, including power-independent energy distributions. These results are contrary to the predictions of the heat diffusion model governing laser-induced surface heating and suggest that weakly bound (physisorbed) adsorbates can couple directly to the photoexcited substrate without heating of the surface. Such non-adiabatic couplings between the center-of-mass motions of the adsorbate and substrate (e.g., CO--Ag stretch) have been implicated in spectrally- and temporally-resolved infrared measurements of high-frequency modes (e.g., C--O stretch) but have never been observed directly.
2:40 PM		SS-TuA-3 Nonlinear Spectroscopy and Chemical Dynamics of Surfaces. P. Guyot-Sionnest (University of Chicago) Short laser pulse excitation of surfaces and sum-frequency or second-harmonic are used to monitor the vibrational and chemical dynamics of adsorbates. Results of laser heating will be presented for CO/W(110) in UHV where the fast heating rate induces preferentially desorption over dissociation. Virational excitation and relaxation studies have been performed for CO/Pt(111) in electrochemistry and the hydrogenated silicon surfaces in UHV. Preliminary results for the effect of laser excitation of CH4/Pt(111) in UHV will be presented.
3:20 PM		SS-TuA-5 Thermal Desorption Kinetics of Methyl Radicals K.A. Briggman, P.C. Stair, E. Weitz (Northwestern University) Methyl radicals have been proposed to be important reaction intermediates in hydrocarbon surface reactions in connection with heterogeneous catalysis and thin film growth. The first measurements of the thermal desoption kinetics of methyl radicals will be presented. Neutral methyl radicals are photochemically produced in a thin film of methyl iodide on a MgO(100) surface and allowed to interact with the film. A substantial fraction of the methyl radicals become trapped and thermally accomodate to the surface (film) temperature prior to desorption. An analysis of the desorption kinetics provides a determination of the mean residence time of radicals on the surface. For a first order desorption process, the mean residence time is simply the inverse of the desorption rate. Experimentally, the residence time, t_res, is determined by comparing neutral and ion flight times in a time-of-flight mass spectrometer (TOF-MS). From measurements made at several surface temperatures, an Arrhenius analysis of the desorption rates provides a desorption barrier of Ã2.5 kcal/mol and a pre-exponential factor of Ã10¹⁰/s. For comparison to similar hydrocarbon systems, the desorption energy of methane from Pt(111) is 3.6kcal/mol with a pre-exponential of 10¹¹/s [1], and second layer ethane from ethane covered iridium is 3.8 kcal/mol [2]. [1] Surf. Sci. 297 (1993) 27-39. [2] J. Chem. Phys. 92(2) (1990) 1397.
3:40 PM		SS-TuA-6 Fragmentation, Desorption and Surface Reactions during Photolysis of Disilane Adsorbed on a Model Silicon Surface S. Wright, E. Hasselbrink (Odense Universitet, Denmark) The spatial resolution of current semiconductor fabrication techniques is limited in part by thermal degradation of sub-micron structures during processing. Photofragmentation of disilane has emerged as a possible route to thin film growth at low substrate temperatures. Using time-of-flight (TOF) measurements and electron energy loss spectroscopy (EELS), we have investigated the photofragmentation and photodesorption of disilane physisorbed on a deuterium terminated Si(100) surface. The TOF technique provides information on the primary fragmentation pathways as well as the bimolecular reactions of the fragments. In addition to the fragmentation channel there is also photodesorption of intact disilane, which we show is initiated by hot electrons near the bottom of the conduction band. Complementary data is obtained by EELS, which we use to identify reactions of the fragments with the surface and to extract a total reaction cross-section. EELS also allows us to probe the electronic excitations of adsorbed disilane that lead to fragmentation. Specifically, we find evidence for a negative ion resonance and show that the electronic absorption spectrum is red-shifted compared with gaseous disilane.
4:00 PM		SS-TuA-7 Surface Morpholoies of Br-etched Ge/Si(001) S.J. Chey, Y. Gong, J.H. Weaver (University of Minnesota) The surface morphology of 2.0 ML Ge/Si(001) was studied using scanning tunneling microscopy before and after it was etched with Br at 650 K-900 K. Terrace etch pits produced at 650 K develop from vacancy line defects introduced by Ge deposition on Si(001). Pit growth from these vacancy lines occurs in units of two dimers(four atoms), as in step etching of Si(001) but in contrast to single-dimer growth for pits on Si(001) terraces. Growth along the dimer row direction introduces kinks. Analysis of the growth patterns makes it possible to determine the energy difference for etching by kink creation or kink propagation. Etching at 750 K produces greater roughening of S_A steps than S_B steps, again in contrast to results for Si(001). The difference is attributed to vacancy line defects that are elongated perpendicular to the S_A steps. Finally, etching at 900 K results in the growth of large terrace pits because of facile pitting and the mobile character of the surface vacancies.
4:20 PM		SS-TuA-8 Chemical Selectivity in the Abstractive Chemisorption of IBr/Si(111)-7x7 Y. Liu, A.J. Komrowski, A.C. Kummel (University of California, San Diego) The chemisorption of IBr onto Si(111)-7x7 has been studied using monoenergetic molecular beams, scanning tunneling microscopy (STM) as well as Auger and flash desorption spectroscopies. When IBr dissociatively chemisorbs, two nearest neighbor sites(SiI and SiBr) are formed which can be observed in STM. In IBr abstractive chemisorption, one of the halogen atoms chemisorbs onto the surface while the other is ejected back into the gas phase. This creates isolated reacted sites which can also be observed in STM. By comparing the number of single, double, and larger sites, it has been determined that the abstraction probability is >50% for 0.82 eV IBr. By varying the STM bias voltage, the SiBr sites can be distinguished from the SiI sites. Using this technique, we have determined that the ratio of SiBr to SiI is 1:1 +/-10%. This was confirmed with Auger and flash desorption spectroscopies. The lack of chemical selectivity in the abstractive chemisorption of IBr stands in contrast to the large selectivity for formation of SiI over SiCl in the abstractive chemisorption of both ICl and I₂Cl₆ on Si(111)-7x7. The lack of chemical selectivity cannot be ascribed to the dipole moment of IBr (0.8 D) since it is intermediate between that of ICl (1.2 D) and I₂Cl₆ (0 D). However, molecular orbital calculations show that the highest occupied molecular orbital (H.O.M.O.) on IBr lacks the large asymmetry in the H.O.M.O.s on both ICl and I₂Cl₆. In these two molecules, the H.O.M.O. lies primary on the iodine atoms since the orbital is antibonding. When a halogen molecule reacts with a Si adatom, a bond initially forms between the Si dangling bond and the halogen H.O.M.O. We proposed that the chemical selectivity results from the halogens molecules being steered into a favorable configuration prior to chemisorption. The favorable configuration is an iodine-silicon bonding geometry for ICl and I₂Cl₆, but this is absent for IBr because of its more symmetric H.O.M.O.
4:40 PM		SS-TuA-9 Low Temperature Photochemical Attachment of Perfluoroalkyl Groups to C(110): A Multiple Internal Reflection Infrared Spectroscopic (MIRIRS) Investigation. C.S. Kim, J.N. Russell (Naval Research Laboratory) The advent of chemical vapor deposition methods for growth of large-area free-standing diamond films, coupled with the high thermal and low electrical conductivity of diamond, make it an attractive material for electronics packaging applications. Since diamond is a robust, chemically inert material at low temperatures, the development of low temperature methods for selective chemical modification of the surface would be invaluable. In this work, we investigate the attachment of perfluoroalkyl groups via the photochemical dissociation¹ of perfluoralkyl iodides (C_xF_2x+1I; x = 1-4) on a diamond (110) surface using s and p polarized MIRIRS. The surface species were identified, and the molecular orientation and thermal stability were monitored from 90 to 700 K. The perfluoroalkyl iodides were exposed to the H-free and H-terminated C(110) surfaces at 90 K. On both surfaces, the perfluoroalkyl iodides adsorbed in multilayers and desorbed from the surface above 200 K. When the C₄F₉I multilayer was adsorbed on the bare surface at 90 K and subsequently was exposed to UV radiation for 1 hr., vibrational modes associated with symmetric and asymmetric stretches for CF₃ and CF₂ (1138 - 1355 cm^-1) were observed that persisted up to 600 K and completely disappeared by 700 K. This provided direct spectroscopic evidence for the chemical attachment of the perfluorobutyl group to the surface. In contrast, no C-F_x modes were observed after exposure of the CF₃I multilayer to UV radiation. Surprisingly, C₄F₉I on the H-terminated surface yielded identical results to the bare surface, indicating the surface hydrogen does not inhibit the photochemical attachment of the perfluorobutyl group to the diamond surface. The surface chemistry of perfluoroethyl iodide and perfluoropropyl iodide are also examined. We discuss the role of chain length, molecular orientation and surface structure. ¹V. S. Smentkowski and J. T. Yates, Jr., Science 271 (1996) 193.
5:00 PM		SS-TuA-10 New Apparatus with Double Supersonic Molecular-Beams for Crystal Growth M. Ohashi, M. Ozeki, J. Cui (Joint Research Center for Atom Technology (JRCAT)-Angstrom Technology Partnership (ATP), Japan) In order to understand the dynamics of surface reactions in the growth of III-V compounds, we developed a new apparatus with several novel features. The apparatus is basically a supersonic molecular beam system for the surface analysis, but it enables us to grow high purity III-V epitaxial layers such as GaAs and InAs. This unique feature bridges the gap of between the crystal growth and the surface chemical reactions studies. Double supersonic molecular beam cells can inject group-III and V source molecules onto the substrate surface. The semiconductor-grade gas system and the shorter beam flight length provide high-purity and high-density molecular beam. The well-defined surface is prepared by molecular beam epitaxy (MBE) system connected with this apparatus. The apparatus has an analytical chamber with a rotatable differentially-pumped and liquid-nitrogen cooled quadrupole mass spectrometer (QMS) for angular distribution and time-of-flight measurements of reflected beam and with a reflection high energy electron diffraction (RHEED) for the surface structure analysis. The GaAs layer was successfully grown by alternative supplies of triethylgallium (Ga source ) and tertiarybutylarsine (As source ) molecules. The photoluminescence spectra confirmed the high density group III and V molecular beams from source cells. The dynamical measurements on the growing surface by this apparatus revealed interesting reactions which could not be observed in well-defined surface. This work is supported by New Energy and Industrial Technology Development Organization (NEDO).