Adsorption and Desorption Barriers in Hydrogen Reactions on Si(100) Surfaces: Is the Barrier Puzzle Solved?

The recent progress in the study of hydrogen adsorption and desorption reactions on Si(100) surfaces has been reviewed. The so called barrier puzzle was raised 15 years ago when the hydrogen adsorption and desorption seemed to occur along the different reaction pathways. The precise measurements of sticking coefficients and energy distributions of desorbed molecules have unraveled this puzzle in such a way that detailed balance is valid at least in the regime of dynamics along the 2H, 3H and 4H inter-dimer channel pathways. However, the recent kinetics studies have revealed that the desorption barrier of hydrogen is only 1.6-1.8 eV, substantially lower than the widely accepted value of 2.5 eV. The value of 1.6-1.8 eV is only slightly lower than the isosteric heat of adsorption of 1.9±0.3 eV which was obtained under the equilibrium condition between adsorption and desorption. It is argued that in the hydrogen reactions at equilibrium on Si(100), adsorption proceeds along the 4H pathway but desorption proceeds along the 2H pathway.