Hyperthermal H Atom Reactions on D/Si(100)
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ABSTRACT A photolytic source of H atoms is used to probe abstraction and adsorption reactions on the D/Si(100) monodeuteride surface. Surface H and D coverages are detennined for H atom exposures, with incident average kinetic energies of 1.0 and 2.8 eV. The D atom depletion probability per incident H atom is 0.3±0.2 for both kinetic energies, and is likely due to abstraction. These results, together with previous studies, indicate that the rates of depletion of surface D and the uptake of H are nominally independent of H atom kinetic energy over the range of about 0.3 to nearly 3 eV. INTRODUCTION Radicals play an important role in the chemistry of materials processing. Even for these highly reactive species, however, kinetic energy can serve to enhance greatly surface reaction rates, particularly for activated processes [1]. Studies with atomic hydrogen have demonstrated the potential of this species in UHV-compatible surface cleaning [2] and atomic layer epitaxy [3]. While aspects of the kinetics of hydrogen atom reactions with halide [4,5] and hydride [6] adlayers on silicon have been characterized, the effects of H
atom kinetic energy in silicon surface chemistry remain to be examined. Characterization of the incident kinetic energy and other dynamical parameters is vital to both the understanding of the mechanisms and the full exploitation of radical/surface chemistry. EXPERIMENTAL The experiments were conducted in an ultrahigh vacuum (UHV) chamber that was evacuated by a liquid nitrogen-trapped diffusion pump, and equipped with facilities for Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). A 10xl0 mm Si(100) crystal (P-doped, 0.2-0.8 ohm-cm) was cleaned initially by repeated sputter (2 keV Kr÷, 71.A, 900s)/anneal (1050K, 300s) cycles until C and 0 contaminants were below the AES detection limit. Iodine was found to be present on the sample following each HI exposure run, and one sputter/anneal cycle reduced the I concentration to a few atomic percent or less. Surface hydrogen adlayers were prepared by exposing the clean Si(100) surface held at a temperature T.=690 K to a flux of atomic species generated from H2 or D2 in the presence of a hot tungsten filament (= 1950 K). TPD spectra obtained following atomic D dosing are in good agreement with previous results [7,8], indicating that the surface is dominated by the monohydride phase having a saturation coverage of 1.0 ML (1 ML=6.8xl0 14 cm-2). For the data below, the initial adlayer was prepared in the saturation regime. By comparing TPD 227 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
integrated intensities for saturated H- and D-terminated/Si(100) surfaces, the DJ/H 2 and D2jHD relative detection sensitivity ratios were determined. A pulsed valve was mounted directly in the UHV chamber with no differential pumping and operated at 10 Hz. HI was transferred from the lecture bottle to a reservoir that was held at room temperature. The reservoir contained only 0.21±0.06 HI (mole fraction), with the balanc
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