AVS2001 Session SC+SS+EL-ThM: Interaction of Hydrogen and Organics with Silicon

Thursday, November 1, 2001 8:20 AM in Room 111

Thursday Morning

Start	Invited?	Item
8:20 AM		SC+SS+EL-ThM-1 Prepairing-dependent Desorption Kinetics of Hydrogen from Si(100)-2x1 H. Nakazawa, M. Suemitsu (Tohoku University, Japan) The reason why H₂ desorption from Si(100) surface shows a first-order desorption kinetics has been a controversial issue for more than a decade, and various desorption models have been proposed accordingly. Although most of the models assume prepairing of surface hydrogen atoms at a dimer as a precursor state, few attempts have ever been made to confirm its role in the desorption kinetics. We here show that the reaction order for the H₂ desorption from Si(100) can be varied by changing the hydrogenating gas and the thermal condition of the hydrogenation and that its behavior is systematically interpreted as a change of fractional coverage of paired hydrogen atoms. Three hydrogenating gases (atomic hydrogen (H), silane, and disilane) and three thermal conditions (room-temperature adsorption (RT), high-temperature adsorption (HT), and post-annealing (PA)) were tested. The desorption kinetics was analyzed by the peak position, the spectral shape, and their coverage dependence of the temperature-programmed desorption (TPD) spectra. As a result, the desorption kinetic order increased as H < disilane < silane and RT < HT < PA. To investigate the microscopic detail, we developed a set of rate equations for desorption with a fractional coverage of unpaired hydrogen atoms being chosen as a key parameter, which described the whole variation of the TPD spectra quantitatively. Using the obtained parameter, we argue the dependence on the hydrogenating gas in terms of different arrangements of surface H atoms. The dependence on the thermal condition is explained by a selective desorption from paired hydrogen atoms as well as the dissociation of paired hydrogen atoms during thermal treatments.
8:40 AM		SC+SS+EL-ThM-2 In situ Measurements of the Stability of H terminated Si Surfaces and Kinetics of Oxide Regrowth in Ambient V. Fomenko, D. Bodlaki, E. Borguet (University of Pittsburgh) The passivation of semiconductor surfaces is key to the operation of semiconductor devices. HF treatment removes native and deposited oxides from silicon. The resulting H-terminated surface is technologically and fundamentally important, and has been subject of a number of experimental and theoretical studies. H termination on Si surfaces has been considered to a stable in air at least for semiconductor wafer processing time scales. However, there is some disagreement as to the timescale of stability. In part this depends on the experimental probes. Using SHG, ellipsometry,contact angle, STM and AFM we have investigated the stability of the surface in ambient and under laser irradiation. in-situ second harmonic generation (SHG) experiments probe the oxide regrowth on hydrogen terminated Si(111) surfaces via SHG rotational anisotropy that is sensitive to hydrogen termination via changes the nonlinear optical response of the interface, both in the magnitude and shape of the SHG rotational anisotropy patterns. In addition, laser induced oxidation of H passivated Si(111) surfaces can be induced with intense ultrashort near IR laser pulses.
9:00 AM	Invited	SC+SS+EL-ThM-3 Hydrogen and Si(001): Adsorption/Desorption Pathways and the "Barrier Puzzle" F.M. Zimmermann (Rutgers University) Although dissociative adsorption of molecular hydrogen on Si(100) is thermodynamically favored by an adsorption energy of almost 2 eV, the sticking probability is immeasurably small (less than 10^-11) at room temperature, indicating the presence of a large energy barrier to adsorption. An adsorption barrier is expected to manifest itself in desorption as well by imparting hyperthermal amounts of kinetic energy to the desorbing molecules. Surprisingly, however, H₂ molecules associatively desorbing from Si(001) show no signs in their translational or rotational kinetic energy distributions of having traversed such a barrier, in apparent contradiction with microscopic reversibility. We have obtained experimental and theoretical results resolving this long-standing puzzle. Using surface second harmonic generation as a sensitive coverage probe, we observed that the dissociative sticking probability increases markedly with hydrogen coverage, and decreases with exposure pressure. Both dependencies are very unusual and impose severe constraints on the adsorption mechanism. By combining detailed measurements of the adsorption and desorption kinetics with statistical mechanical modeling and ab initio calculations, we arrived at a quantitative, mechanistic description of adsorption/desorption consistent with all observations and providing a natural explanation of the barrier puzzle. The model involves two distinct reaction pathways. At intermediate to high hydrogen coverages, thermal adsorption and desorption are dominated by an adsorption-barrier free, autocatalytic pathway, while a non-autocatalytic, bare-dimer pathway with a ~0.7 eV adsorption barrier dominates at very low coverages. Fitted model parameters are in quantitative agreement with density functional theory calculations.
9:40 AM	Invited	SC+SS+EL-ThM-5 Noncontact AFM Study for Hydrogen Termination on Silicon Surfaces Y. Sugawara, S. Morita (Osaka University, Japan) In order to most effectively apply the noncontact atomic force microscopy (AFM) using frequency modulation (FM) technique as a science tool in a variety of fields such as surface science, it is very important to understand fully the imaging mechanisms of the noncontact AFM on various samples. The imaging mechanism has been investigated on a chemically reactive surface such as semiconductor surface and an insensitive surface such as pure metal surface and layered material surface. However, there is no report of a comparative study between a reactive surface and an insensitive surface using same tip. For instance, Si(100)2x1:H monohydride surface is that a Si(100)2x1 reconstructed surface is terminated by a hydrogen atom, and do not newly reconstruct as metal deposited semiconductor surface, and the surface structure hardly change. Thus, Si(100)2x1:H monohydride surface is one of most useful surface for a model system to investigate the imaging mechanism, experimentally and theoretically. However, there is no report for noncontact AFM imaging on Si(100)2x1:H monohydride surface, and whether the interaction between a very small atom as hydrogen and a tip apex is observable with noncontact AFM do not have been clarified. In the present experiments, we compared the noncontact AFM images obtained for the Si(100)2x1 reconstructed surface with that for Si(100)2x1:H monohydride surface to investigate the role of chemical reactivity on the surface. It is found that the distance between bright spots is increased by the hydrogen termination. On Si(100)2x1 reconstructed surface, the noncontact AFM atomically resolved the dangling bonds localized outside the silicon dimer with a clear contrast. On the other hand, on Si(100)2x1:H monohydride surface, the noncontact AFM atomically resolved the individual hydrogen atoms on top most layer.
10:20 AM		SC+SS+EL-ThM-7 Making Organic Molecules on Cu(100) and GaAs(100) N.K. Singh, N. Paris, P. Gatland (The University of New South Wales, Australia) Alkyl coupling reactions, to form longer chain hydrocarbons, form the basis of many catalysed industrial processes. Surface studies carried out to date to understand the mechanisms by which carbon-carbon bonds form during the coupling process have been restricted to reactions of alkyl halides on coinage metal surfaces. Our recent investigations have shown that GaAs(100), a compound semiconductor, is also capable of catalysing alkyl coupling reactions, which had not been realised previously. Coupling products form irrespective of whether the alkyl groups are derived from alkanethiols or alkyl halides. However, on GaAs(100) the respective higher alkenes form, whereas it is known higher alkanes form on coinage metals. In this paper the surface reactions of a select group of alkanethiols (propanethiol, 1,1,1-trifluoroethanethiol) and alkyl halides (iodoethane, 2-iodo-1,1,1-trifluoroethane) on GaAs(100) and Cu(100), studied by thermal desorption and X-ray photoelectron spectroscopies, will be presented in order to establish the trend in the product mixtures on the two surfaces. We will show that both surfaces exhibit disproportionation and coupling reactions. Disproportionation reactions of the adsorbed alkyl fragments form the corresponding gaseous alkene, alkane and hydrogen. The coupling reactions however on the two surfaces differ. So for example, on Cu(100) CF₃CH₂I forms CF₃CH₂CH₂CF₃ as the coupling product while on GaAs(100) it forms CF₂=CHCH₂CF₃, the corresponding alkene. In the case of coupling reactions of CF₃CH₂SH, on Cu(100) CF₃CH₂CH₂CH₃ is formed while on GaAs(100) , CF₂=CHCH₂CH₃ is formed. These products are inconsistent with the products formed by CF₃CH₂I reactions. We will discuss the mechanisms by which these coupling reactions occur, and postulate reasons for the differences in the observed product mixtures on GaAs(100) and Cu(100).
10:40 AM		SC+SS+EL-ThM-8 Adsorption and Thermal Decomposition of Iodoethane on Si(100)-2x1: Kinetically-Favored Adsorbate Ordering A.V. Teplyakov, K.M. Bulanin, A.G. Shah (University of Delaware) The adsorption and chemical transformation of iodoethane were studied on a Si(100)-2x1 surface using multiple-internal reflection Fourier-transform infrared spectroscopy (MIR-FTIR). Although ethyl groups are stable on the Si(100)-2x1 surface at room temperature, thermal annealing studies suggest the formation of ethylene, a major hydrocarbon reaction product, accompanied by the loss of hydrogen, which is left on the surface until the temperature of recombinative desorption is reached. Adsorption of iodoethane on Si(100), followed by annealing to 570 K, leaves only hydrogen and iodine on the surface. MIR-FTIR spectroscopy shows that hydrogen is bound in several different types of site at temperatures between 295 K and 570 K. Annealing to higher temperatures produces a distribution dominated by a single hydrogen configuration. First-principles theory and polarization-dependent infrared spectra are consistent with the identification of this configuration as a dimer occupied by one hydrogen and one iodine atom. Calculations show that this configuration is not thermodynamically favored relative to other possible configurations. The observed ordering is attributed to kinetics, a consequence of slow pairing of iodine atoms.
11:00 AM		SC+SS+EL-ThM-9 Are Silicon and Germanium Surfaces Chemically Similar? Reactions of Amines C. Mui, G.T. Wang, J.H. Han, C.B. Musgrave, S.F. Bent (Stanford University) The organic chemistry at silicon and germanium surfaces has been studied in the past, and the chemical similarity between the two materials is often exploited. In this study, we will present an example in which the surface chemistry of silicon and germanium are notably different. We have used surface infrared spectroscopy and temperature programmed desorption to investigate the chemistry of amines at the Si(100)-2x1 and the Ge(100)-2x1 surfaces. We find that surface reaction of methylamine and dimethylamine on the Si(100)-2x1 surface results in facile N-H dissociation, whereas molecular adsorption occurs on the Ge(100)-2x1 surface. We also show that molecular adsorption of amines on both the Si(100)-2x1 and the Ge(100)-2x1 surfaces occurs through the formation of surface dative bonds which are stable at room temperature. Quantum chemistry calculations are used to explain the observed reactivity difference between the two surfaces. We find that N-H dissociation of dimethylamine is kinetically favored compared to N-CH3 cleavage on both surfaces. However, while N-H dissociation on the Si(100)-2x1 surface is unactivated, the overall activation energy for N-H cleavage on the Ge(100)-2x1 surface is above the vacuum level, explaining the lack of reactivity on the Ge(100)-2x1 surface. We will also discuss our theoretical approach for modeling reactions at semiconductor surfaces, including the effect of surface cluster models and basis sets.
11:20 AM		SC+SS+EL-ThM-10 The Influence of Conjugation in Attachment of π-Electron Containing Organic Molecules to the Si(001) Surface: Acrylonitrile vs. Allyl Cyanide M.P. Schwartz, S.K. Coulter, R.J. Hamers (University of Wisconsin-Madison) Organic chemists have developed methods for controlling chemical reactions of complex molecules by defining a broad and detailed set of rules for reactivity. Controlling electron density within a molecule through the use of electron donating or withdrawing groups is a very important way in which to influence product distributions. While a wide variety of unsaturated organic molecules can be tethered to the Si(001) surface, little is known about the role of conjugation of π-electrons in influencing product distribution. In this study, we have investigated the attachment of acrylonitrile (CH2=CH-CN) and allyl cyanide (CH2=CH-CH2-CN) to the Si(001) surface to determine how conjugation of an electron withdrawing group to a vinyl group influences the final surface products. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) measurements show that the attachment chemistry differs significantly for these two molecules. Allyl cyanide adsorbs primarily through the vinyl group while acrylonitrile attaches predominantly via the cyano group. Acetonitrile and benzonitrile were also studied to help determine the nature of the final surface products. The role of conjugation in determining product distributions for attachment of allyl cyanide and acrylonitrile to the Si(001) surface will be discussed.
11:40 AM		SC+SS+EL-ThM-11 Scanning Tunneling Microscopy of a Conjugated {C}3-oligomer on Si(100) B. Grandidier (ISEN, France); C. Krzeminski, J.P. Nys, C. Delerue, D. Stievenard (IEMN/ISEN, France); C. Martineau, P. Blanchard, J. Roncali (IMMO, France) Scanning tunneling microscopy (STM) has been used to study the adsorption of a {C}3 p-conjugated oligomer on the Si(100) surface. The symmetry of the molecule is resolved with the STM and different conformations are observed. As the oligomer is made up of vinyl groups and aromatic constituents, which are all known to react with the Si(100) surface through cycloaddition reactions, ab initio calculations are performed to determine the occupied molecular orbitals of the free and covalently bound oligomers. The results are compared with the occupied state STM images to characterize the adsorption state of the different conformations.