AVS2004 Session SS2-WeA: Surface Collision Dynamics

Wednesday, November 17, 2004 2:00 PM in Room 210C

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS2-WeA-1 Atomic-Scale Analysis of the SiH₃ Surface Reactivity During Plasma Deposition of Amorphous Silicon Thin Films M.S. Valipa, E.S. Aydil (University of California, Santa Barbara); D. Maroudas (University of Massachusetts, Amherst) Hydrogenated amorphous silicon (a-Si:H) thin films grown by plasma-assisted deposition are used widely in the fabrication of solar cells and flat panel displays. The dominant precursor for deposition of device-quality a-Si:H films is the SiH₃ radical. Development of systematic strategies for depositing a-Si:H films with desirable properties requires a fundamental understanding of surface reactions of the SiH₃ radical leading to a-Si:H growth and H incorporation. This presentation focuses on detailed atomic-scale analysis of the surface reactions of SiH₃ using molecular-dynamics simulations of repeated impingement of SiH₃ radicals on growth surfaces of smooth a-Si:H films. The corresponding surface reaction probability, β, is determined over the temperature (T) range 475-800 K. SiH₃ can either incorporate into the film by adsorbing onto a dangling bond or inserting into Si-Si bonds (sticking), or abstract surface H through Eley-Rideal (ER) or Langmuir-Hinshelwood (LH) pathways to produce SiH₄ gas, or react with another surface SiH₃ to desorb as Si₂H₆ (recombination), or leave the film by reflection or desorption. The overall β (sticking + recombination) is almost constant over the T range studied, as are the probabilities for sticking and recombination, s and γ, respectively. Energetic analysis shows that SiH₃ adsorption and insertion and ER abstraction are barrierless processes, which explains the measured T independence of β. LH abstraction is activated at high T, but competes with Si₂H₆ formation, yielding a T-independent γ. Also, LH abstraction leads to H elimination from a-Si:H during growth and explains the T dependence of H content in the a-Si:H film.
2:20 PM		SS2-WeA-2 Adsorption Induced Phase Separation followed in Real Time by High-pressure Scanning Tunneling Microscopy J. Knudsen, R.T. Vang, E.K. Vestergaard, F. Besenbacher (University of Aarhus, Denmark) The validity of surface science experiments as an efficient tool in catalysis research is often questioned due to the enormous difference in pressure conditions. For single metal systems the issue is most often concerned with the equivalence between adsorption phases obtained under low pressure and low temperature versus high pressure and high temperature phases. But for the case of more complex model systems such as bimetallic alloys the stability of the model catalyst becomes an additional important factor. We have developed a high-pressure fast-scanning STM which serves as an ideal tool for studies of high pressure induced morphological and structural changes in real time by the acquisition of STM movies. In this way we have been able to describe in great detail a CO induced phase separation of a Au/Ni(111) surface alloy. The STM movies reveal how a removal of nickel atoms from the topmost layer of the alloy surface is nucleated at the step edges of the surface. The gold atoms are left behind on the surface, and small gold clusters are formed in the wake of the moving step edge. Based on these experimental findings we propose a model, in which nickel atoms are removed by the formations of nickel carbonyls; a reaction that is well known from studies of clean nickel surfaces. Finally, we present recent studies of the stability of a Cu/Pt(111) surface alloy under high CO pressures. Exposure of the Cu/Pt(111) surface alloy to CO leads to the formation of clusters on the surface. The atomic details of this adsorbate-induced structural change of the Cu/Pt(111) alloy are currently being investigated.
2:40 PM	Invited	SS2-WeA-3 Quantum State Resolved Studies of Gas/Surface Reaction Dynamics R.D. Beck (Ecole Polytechnique Federale de Lausanne (EPF), Switzerland) The dissociation of methane on a nickel catalyst is a key step in steam reforming of natural gas for hydrogen production. Despite substantial effort in both experiment and theory, there is still no atomic scale description of this important gas-surface reaction. To elucidate its dynamics, we have performed quantum state resolved studies of vibrationally excited methane reacting on the Ni(100) surface using pulsed laser and molecular beam techniques. We observed up to a factor of 5 greater reaction probability for methane-d2 with two quanta of excitation in one C-H bond versus a nearly isoenergetic state with one quanta in each of two C-H bonds. For CH4, stimulated Raman pumping is used to probe the reactivity of the totally symmetric C-H stretch vibration for comparison with reactivity of CH4 excited to the infrared active antisymmetric C-H stretch vibration. The observed reactivities point to a transition state structure which has one of the C-H bonds significantly elongated. Our results also clearly exclude the possibility of statistical models correctly describing the mechanism of this process and emphasize the importance of full-dimensional calculations of the reaction dynamics.
3:20 PM		SS2-WeA-5 Gas-Surface Reaction Dynamics at the Inorganic-Organic Interface A. Dube, P.F. Ma, A.S. Killampalli, M. Sharma, J.R. Engstrom (Cornell University) Inorganic-organic interfaces play an important role in a number of technologies. Much of the work to date in the area of gas-surface reaction dynamics has involved study of the reaction of small organic molecules with transition metal surfaces. Here, we have chosen to examine the inverse problem: the reaction of coordination compounds of transition metals with model organic surfaces using supersonic molecular beam techniques. Such reactions are the first key step to barrier formation on organic surfaces, and they may result in a superior method for the formation of contacts to molecular electronics. In the work we report here we examine explicitly the reaction of Ti- and Ta- containing coordination compounds with a variety of self-assembled monolayers (SAMs) possessing different terminal endgroups (e.g.,-CH₃,-OH, -NH₂,-COOH), using both HX-R-SiCl₃/SiO₂ and HX-R-SH/Au based SAM chemistries. For example, for the reaction of Ti[N(CH₃)₂]₄ and for molecular kinetic energies E_i = 0.5-2.0 eV, we find that the reaction probability on -OH and -NH₂ terminated R-SiCl₃/SiO₂ type SAMs passes through a minimum, near E_i ~ 1 eV. Since we have shown in other work that penetration of the SAM by the coordination compound is possible, these results suggest that penetration is enhanced at sufficiently high E_i. On the other hand, variation of both the substrate temperature and the angle of incidence indicates that reaction with these two terminal endgroups follows a trapping-mediated chemisorption channel. In selected cases we also make comparison to results from ab initio quantum chemistry calculations of the potential energy surface. For example, these calculations are consistent with a barrier near the vacuum level for the reaction on -OH terminated SAMs, which we observe experimentally, yet they suggest that reaction with an isolated -NH₂ may be activated by as much as 15 kcal-mol^-1.
3:40 PM		SS2-WeA-6 Site-Selective Abstraction in the Reaction of 5- to 20-eV O⁺ with a Self-Assembled Monolayer X. Qin, T.D. Tzvetkov, D.C. Jacobs (University of Notre Dame); D. Lee, L. Yu (University of Chicago) The reaction of hyperthermal (5-20 eV) O⁺ with alkanethiolate self-assembled monlayers (SAM) is studied under UHV conditions. To learn about the site-specificity to hydrogen abstraction in this system, we deposit SAM layers for which the hydrogen atoms located on the C-12, C-11, or C-10 positions of 1-dodcanethiol are substituted with deuterium atoms. By comparing the yields of OH to OD emerging from these three isotopomers, we find that hyperthermal O⁺ initially abstracts only H(D)-atoms bound to the top two carbon atoms within the SAM layer. Continued bombardment with O⁺ ions significantly disorders the structure of the SAM.
4:00 PM	Invited	SS2-WeA-7 Current Research and Development Topics on Gas Cluster Ion Beam Processes I. Yamada (University of Hyogo, Japan) It has been 15 years now since the idea of the gas cluster ion beam (GCIB) process first came up, but the interest in GCIB process has increased only recently, driven by the nano-technology program, especially in Japan. This ion beam process uses a beam of ions consisting of clusters of a few hundreds to thousands of atoms generated from gaseous materials. The impact of these accelerated cluster ions with the surface produces a low energy bombardment at very high density. The cluster-surface collisions were found non-linear effects and to have unique characteristics which were found to be useful for applications in novel surface processing. These characteristics include surface effects such as shallow implantation, lateral sputtering, cleaning and smoothing, as well as low temperature thin film formation. This paper reviews the current fundamental research related to the GCIB-surface interactions as well as their applications in modern magnetic, optical and semiconductor device fabrications. These presently include: (i). IC Back End of the Line (BEOL) materials processing, (ii). surface smoothing of metals, dielectrics, superconductors, and diamond films for optical and magnetic devices, (iii). selective smoothing of SOI, SiC and compound semiconductor films and (iv). high quality thin multi-layer film deposition for reliable and durable optical filters.