Reaction kinetics of H 2O with chlorinated Si(111)-(7 × 7) and porous silicon surfaces

Abstract

Atomic layer control of SiO 2 film growth can be achieved on silicon surfaces using the SiCl 4 +H 2O reaction applied in an ABAB … binary reaction sequence (A) SiCl ∗ + H 2O→ SiOH ∗ + HCl , (B) SiOH ∗ + SiCl 4→ SiOSiCl 3 ∗ + HCl , where the asterisks indicate the surface species. The reaction of H 2O with SiCl∗ species on the silicon surface is an important initial step for this controlled SiO 2 deposition. In this study, the reaction of H 2O on Si(111)-(7 × 7) surfaces chlorinated by SiCl 4 exposures was studied using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) techniques. Complementary transmission FTIR experiments monitored the SiCl, SiH and SiOSi vibrations of the surface species during the reaction of H 2O on chlorinated porous silicon surfaces. At temperatures T≤700 K, the oxygen uptake resulting from H 2O adsorption was small, and chlorine loss was negligible on chlorinated Si(111)-(7 × 7) and porous silicon surfaces. In contrast, both oxygen uptake and chlorine removal were measurable and thermally activated at T>700 K. The kinetics of oxygen uptake and chlorine loss were also studied on the Si(111)-(7 × 7) surface versus chlorine coverage. The oxygen uptake rates in the temperature range from 700–820 K were independent of the initial surface chlorine coverage. A simple kinetics model employing H 2O adsorption kinetics and H 2, HCl and SiCl 2 desorption kinetics was used to explain the temperature threshold at ∼ 700 K and to determine the reaction mechanism. These model calculations were consistent with chlorine loss that was rate-limited by HCl desorption that occurs at T>700 K. The independence of oxygen uptake on the initial chlorine coverage was attributed to the similarity between the HCl and H 2 desorption kinetics. In contrast to recent observations on the chlorinated SiO 2 surface, these results indicate that the first H 2O reaction in the initial AB sequence on chlorinated silicon surfaces does not involve a direct substitution reaction with an SiCl∗ surface species. Rather, the reaction is consistent with a Langmuir-Hinshelwood mechanism involving H 2O adsorption followed by HCl desorption.