REACTION-KINETICS OF H2O WITH CHLORINATED SI(111)-(7X7) AND POROUS SILICON SURFACES

Atomic layer control of SiO2 film growth can be achieved on silicon su rfaces using the SiCl4+H2O reaction applied in an ABAB binary reaction sequence (A) SiCl+H2O-->SiOCH*+HCl, (B) SiOH*+SiCl4-->SiOSiCl3*+HCl, where the asterisks indicate the surface species. The reaction of H2O with SiCl species on the silicon surface is an important initial ste p for this controlled SiO2 deposition. In this study, the reaction of H2O on Si(111)-(7 x 7) surfaces chlorinated by SiCl4 exposures was stu died using laser-induced thermal desorption (LITD), temperature-progra mmed 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 H2O on chlorinated porous silicon surfaces. At temperatures T less th an or equal to 700 K, the oxygen uptake resulting from H2O adsorption was small, and chlorine loss was negligible on chlorinated Si(111)-(7 x 7) and porous silicon surfaces. In contrast, both oxygen uptake and chlorine removal were measurable and thermally activated at T>700 K. T he kinetics of oxygen uptake and chlorine loss were also studied on th e Si(111)-(7 x 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 SiCl2 desorption kinetics was used to explain the temperature threshold at similar to 700 K and to d etermine the reaction mechanism. These model calculations were consist ent with chlorine loss that was rate-limited by HCl desorption that oc curs at T>700 K. The independence of oxygen uptake on the initial chlo rine coverage was attributed to the similarity between the HCl and H-2 desorption kinetics. In contrast to recent observations on the chlori nated SiO2 surface, these results indicate that the firt H2O reaction in the initial AB sequence on chlorinated silicon surfaces does not in volve a direct substitution reaction with an SiCl surface species. Ra ther, the reaction is consistent with a Langmuir-Hinshelwood mechanism involving H2O adsorption followed by HCl desorption.