Infrared Spectroscopy of Covalently Bonded Species on Silicon Surfaces: Deuterium, Chlorine, and Cobalt Tetracarbonyl
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415 0 Mat. Res. Soc. Symp. Proc. Vol. 477 1997 Materials Research Society
Organometallic and metal carbonyl reagents are known to be useful in metal deposition and silicide formation. We report here a new reaction of the H-Si(l 11) surface with Co 2(CO)8, and we show that a structurally well defined (CO) 4Co-Si species is formed on the surface. EXPERIMENTAL Preparation of Hydrogen-Terminated Si(111) and Si(100) surfaces. Double-side polished silicon pieces (5cm x 2cm, n-type, 1-10 flecm) and silicon attenuated total reflection (ATR) infrared elements (5cm x 2cm x lmm, 450 bevels, Harrick) were cleaned in piranha solution (4:1 conc. H2SO 4 : 30% H2 0 2, by volume) for 10 minutes at 100°C, followed by an SC2 clean (1:1:4 30% H 20 2 : conc. HCl:H 20, by volume) for 10 minutes at 75°C, then another 10 minutes in piranha solution again. After rinsing with ultra-pure water (Millipore filtration system, 18 MC29cm resistivity) the silicon pieces were etched for 15 minutes in 40% aqueous NH 4F in Teflon vials and then simply removed from the vials without a final water rinse [7]. For Si(100), a 5 min immersion in 50:1 HF was used followed by a short rinse in ultra-pure water. Preparation of Deuterated Si(11l) and Si(100) Surfaces. The silicon surfaces were cleaned first as for hydrogen-termination. For deuterium- terminated Si( 111) surface, we used 40% KF (99.9%, J. T. Baker) in D 20 solution and adjusted the pH to about 7.8 by addition of 37% DCI in D2 0 solution (Aldrich). For deuterium-terminated Si(100), we used 5% KF in D 20 solution and adjusted the pH to about 2.0 by addition of 37% DCI in D 20 solution. The pH was measured by a glass electrode. D 20 (99.9% Aldrich) was purified by passing through the vacuum-dehydrated adsorbent from an OrganexTM cartridge (Millipore). As a control experiment, the hydrogenterminated Si(l 11) surface was made from 40% KF in H 20 with the pH adjusted to about 7.8 by addition of conc. HC1. The hydrogen-terminated Si (100) surface was made from 5% KF in H20 with the pH adjusted to about 2.0 by addition of conc. aqueous HCl. Preparation of Chlorine-Terminated Si(11l) and Si(100) Surfaces. Freshly-prepared hydrogen-terminated Si(l II) and Si(100) surfaces were placed in a fused silica cuvette and evacuated to < 3 x 10-5 Torr on a glass vacuum line with a liquid N 2-trapped diffusion pump. Chlorine gas (Matheson, 99.999%) was then admitted to the silicon sample to a pressure of 0.2 to 2.0 torr. A broadband 350 nm lamp (Spectronics, MB-100, fluorescent flood lamp) with an average intensity of 6 mW/cm2 at a distance of 6" over a 7" diameter was used for the photoexposures [7]. For transmission FTIR, both sides of the sample were illuminated for 5 minutes at room temperature. Preparation of (CO)4 Co-Si(111) Surface. Freshly-prepared H-Si(l 11) was placed in a glass cuvette and evacuated to < 3x 10-5 Torr on a glass vacuum line with a liquid N 2-trapped diffusion pump. The vacuum was then sealed off, and the surface was exposed to the vapor of solid Co 2(CO)s (Strem Chemicals) which had been
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