Adsorption and thermal reactions of H₂O and H₂S on Ge(100)

The adsorption and thermal decomposition of H₂O and H ₂S on Ge(100 were studied with temperatureprogrammed desorption (TPD and X-ray photoelectron spectra (XPS, using synchrotron radiation. At 105 K, H₂O molecules either adsorb molecularly or dissociate to form surface OH and H for exposures of all durations. Chemisorbed H₂O dissociates to form surface OH on annealing to 270 K, whereas H₂S 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 H₂S undergoes dissociative adsorption with a negligible barrier (2.1 kJ/mol but H₂O dissociates with a larger barrier (26.8 kJ/mol. On annealing to 550 K, surface OH mainly recombines with surface H to evolve H₂O, 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 H₂O and H₂S is thermally activated to combine and to desorb as H₂ at 620 K. For H₂S, a small proportion of surface H recombines with surface S to desorb as H₂S. Finally, surface O and S are removable from the surface with desorption of GeO at 710 K and GeS at 695 K, respectively.