Surface Science Letters最新文献

The adsorption of Cs on Al(111) has been studied by electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). Two series of Cs overlayers with various coverages were prepared by two methods; as-deposition at 85 K (AD series) and thermally desorbing excess Cs (TD series). We find an abrupt change of EEL spectra for the TD series at a coverage of approximately 0.5; the nature of a Cs-induced loss peak changes from a surface plasmon to a Cs 6s → 6p like excitation, which is indicative of a metallic-to-ionic change. This transition is associated with a simultaneous structural transformation. Both EELS and LEED for the TD series are different from those for the AD series at the coverages between ca. 0.5 and 0.2. Irreversible structural and electronic transformations from the AD to the TD series also occur at these coverages by heating above 200 K without desorbing Cs. This indicates that the transformation is induced by a thermally activated process.

The desorption and dissociation of ammonia adsorbed on (GaAs(100) occurs readily upon activation with 50 eV electrons. Complementary results from temperature programmed desorption and X-ray photoelectron spectroscopy indicate that the cross-sections for parent desorption and for dissociation to NH_x (= 1, 2) are similar (10⁻¹⁶−10⁻¹⁷ cm²), whereas that for nitride production is two orders of magnitude smaller. All these are more than an order of magnitude higher than observed in analogous photon-driven reactions.

Disilane (Si₂H₆) is used for silicon chemical vapor deposition (CVD) and is a potential precursor for atomic layer epitaxy (ALE) on silicon surfaces. In this study, Fourier transform infrared (FTIR) transmission spectroscopy was employed to examine the adsorption and decomposition of disilane on high surface area porous silicon surfaces. The FTIR spectra revealed that disilane dissociatively adsorbs on porous silicon surfaces at 200 K to form a large fraction of SiH₃ surface species and some SiH₂ and SiH surface species. The SiH_x stretching modes between 2125–2156 cm⁻¹, the SiH₃ degenerate deformation mode at 862 cm⁻¹ and the SiH₂ scissor mode at 907 cm⁻¹ were then employed to monitor the decomposition of the surface hybride species during thermal annealing. Between 200–600 K, the SiH₃ species decreased and were depleted from the silicon surface. Concurrently, the SiH₂ surface species were observed to increase between 200–440 K and subsequently to decrease between 440–620 K. Only monohydride species remained on the porous silicon surface at 620 K. Above 640 K, the SiH surface species decreased concurrently with H₂ desorption. Adsorption studies were also conducted at various isothermal temperatures. These disilane adsorption and decomposition experiments provide important insights to the surface chemistry during silicon CVD and ALE processing.

The adsorption of CO on a Pt(111) surface held at 20 K under ultrahigh vacuum was investigated using vibrational and thermal desorption spectroscopies. In contrast to adsorption at higher temperatures, surface diffusion of chemisorbed CO across the surface was suppressed and vibrational spectroscopy showed non-equilibrium occupation of top, bridge and three-fold hollow sites. The saturation coverage of chemisorbed CO at 20 K was 0.5 ML. The infrared absorption band attributed to three-fold hollow site CO was centered at 1736 cm⁻¹. The integrated absorbance of the 1736 cm⁻¹ band grew in and saturated as the exposure was increased from 0.35 to 1 ML.