Adsorption, desorption, and decomposition of HCl and HBr on Ge(100): Competitive pairing and near-first-order desorption kinetics

Abstract

We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge(100), produced by dissociative chemisorption of HCl and HBr, by temperature-programmed desorption. The initial sticking probability S0 for HCl decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr S0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining (1−f) H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide (HX) near 580–590 K. The activation energies for desorption of H2, HCl, and HBr are all approximately 40 kcal/mol. For both HCl and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeCl2 and GeBr2 near 675 and 710 K, respectively. Desorption of H2, HCl, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the π bond on unoccupied Ge dimers. We introduce and solve a generalized doubly occupied dimer model incorporating competitive pairing of H+H, H+X, and X+X on Ge dimers to explain the near-first-order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of ≊3 kcal/mol. Implications of the present results for surface thermochemistry, chemical vapor deposition, and atomic layer epitaxy of Ge and Si(100)2×1 surfaces are discussed.