Anthony J Muscat
GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH3)(3)) and titanium tetrachloride (TiCl4) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 +/- 1.6 angstrom-thick mixed oxide consisting primarily of As2O3 with a small Ga2O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place. TMA deposited an Al2O3 layer, but TiCl4 exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO2 coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 degrees C to 135 degrees C, but no TiO2 was present from 170 degrees C to 230 degrees C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO2 deposition even after 50 full ALD cycles of TiCl4 and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237-297 degrees C. This allowed TiO2 deposition at 300 degrees C in single TiCl4 pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl4 exposure deposited TiO2. (C) 2011 Elsevier B.V. All rights reserved.
The dissociation of NH3 on a Si(l 00) surface activated with Cl atoms was investigated using X-ray photoelectron spectroscopy. Gas phase UV-Cl-2 (0.1-10 Torr Cl-2 for 10-600 s under 1000 W Xe lamp illumination) completely replaced the H-termination on aqueous-cleaned Si(100) with 0.82 +/- 0.06 ML of Cl at 298 K. A single spin-orbit split Cl 2p doublet indicated that the Cl atoms were bound to Si dimer atoms, forming silicon monochloride (Cl-Si-Si-Q. Exposing the Cl-terminated surface at 348 K to NH3 (1-1000 Torr for 5-60 min) replaced one Cl atom with one N atom up to a coverage of 0.33 +/- 0.02 ML. Cl atoms lowered the activation energy barrier for reaction to form a primary amine (Si-NH2). Oxygen was coadsorbed due to competition by H2O contamination. The presence of Cl on the surface even after high NH3 exposures is attributed to site blocking and electrostatic interactions among neighboring Cl-Si-Si-NH2 moieties. The results demonstrate a low temperature reaction pathway for depositing N-bearing molecules on Si surfaces. (c) 2006 Elsevier B.V. All rights reserved.
The impact of annealing on the kinetic parameters for CO desorption from Ni(100) was investigated using isotopic mixing, temperature-programmed desorption (TPD), and work function Δφ measurements. Isotopic labeling TPD demonstrated that all of the CO molecules in a layer are kinetically equivalent above 200 K regardless of the order in which they were adsorbed; the CO molecules that initially desorb from the surface are consequently representative of all of the CO molecules in the layer at that coverage. Using the threshold TPD technique to sample the initial CO distribution, values of the activation energy Ed and preexponential factor Vd derived for the layers prepared with or without annealing converged to 135 ±5 kJ/mol and 1015±1 s-1, respectively, in the limit of zero coverage. Increasing the CO coverage in a layer adsorbed at 110 K without annealing prior to TPD produced a sharp decrease in Ed and vd from the zero coverage limits. At θ0 = 0.37 monolayer, values of 26 ± 5 kJ/mol and 103.4±1 s-1 were obtained. These values are not representative of the true binding parameters for CO on Ni(100) because the Polanyi-Wigner equation was applied to a reaction in which two steps, desorption and site conversion, occur simultaneously and with comparable rates. This analysis yielded kinetic parameters for the nonequilibrated adlayers that compensated strongly. The desorption rate parameters derived from CO layers adsorbed at 296 K without annealing prior to TPD show a much less pronounced change with coverage. At θ0 = 0.39 monolayer, values of 76 ± 6 kJ/mol and 109.3±1 s-1 were obtained. Annealing CO layers adsorbed at 110 K prior to TPD produced more normal kinetic behavior. At θ0 = 0.38 monolayer, an Ed of 121 ± 4 kJ/mol and a vd of 1015.3±1.5 s-1 were calculated. Layers saturated with CO at 110 K and annealed above approximately 300 K were quasi-equilibrated as shown by the close agreement between the Ed results and the steady-state measurements of the isosteric heat qst made using the Kelvin technique. The qst values fell gradually from 139 ± 7 kJ/mol near 0 eV to 106 ± 13 kJ/mol at 1.1 eV (0.5 monolayer). The kinetic parameters calculated from the TPD spectra of the annealed layers consequently reflect the true binding properties for CO on the Ni(100) surface. © 1996 American Chemical Society.