Kinetics of boron deposition from BBR 3 + H 2

Abstract

The kinetics of the reaction between the two vapor-phase species BBr 3 and H 2 to form solid B and HBr in a well-mixed flow reactor at feed pressures from 0.001 to 1.0 Torr have been studied by monitoring reaction rates with a differentially pumped mass spectrometer to determine reactant and product partial pressures. A resistively heated tantalum foil was used as the deposition substrate to provide surface temperatures up to 1500°C. This technique provides rapid measurement (∼ 2s) of gas composition in a well-mixed flow reactor so that reaction rates could be examined quantitatively. We observed reaction orders that varied from + 1 2 to approximately − 1 with respect to P H 2 and orders that varied from + 1 to approximately − 2 with respect to P BBr 3 . Simple Langmuir—Hinshelwood kinetic models qualitatively explain our experimental observations, suggesting that reaction order variations can be attributed to competitive adsorption of species such as H atoms and BBr x fragments on the boron surface. Two more realistic chemical mechanisms are proposed, but no single expression fits the data for all compositions considered. An expression based on a mechanism involving Br abstraction from BBr x(ads) by H (ads) was found to give best agreement with data.