Adsorption and Thermal Reactions of H₂O and H₂S on Ge(100)

The adsorption and thermal decomposition of H 2 O and H 2 S on Ge(100) were studied with temperature-programmed desorption (TPD) and X-ray photoelectron spectra (XPS), using synchrotron radiation. At 105 K, H 2 O molecules either adsorb molecularly or dissociate to form surface OH and H for exposures of all durations. Chemisorbed H 2 O dissociates to form surface OH on annealing to 270 K, whereas H 2 S dissociates to form surface SH and H on an initial exposure and further adsorbs molecularly on protracted exposure to the surface at 105 K. The calculation with density-functional theory (DFT) shows that H 2 S undergoes dissociative adsorption with a negligible barrier (2.1 kJ/mol) but H 2 O dissociates with a larger barrier (26.8 kJ/mol). On annealing to 550 K, surface OH mainly recombines with surface H to evolve H 2 O, but a small proportion of surface OH decomposes to form surface O and H. Most surface SH undergoes decomposition to form surface S and H at 520 K. According to a DFT calculation, surface OH has an activation energy for decomposition greater than that for recombination with surface H, whereas activation energies for decomposition and recombination of surface SH show the reverse order. Surface H resulting from the dissociation of H 2 O and H 2 S is thermally activated to combine and to desorb as H 2 at 620 K. For H 2 S, a small proportion of surface H recombines with surface S to desorb as H 2 S. Finally, surface O and S are removable from the surface with desorption of GeO at 710 K and GeS at 695 K, respectively.