Interaction of 50 eV electrons with D2O on GaAs(100)

Abstract

The interaction of 50 eV electrons with D2O, dosed on GaAs(100) at 100 K, has been investigated using temperature programmed desorption (TPD), x-ray photoelectron spectroscopy, and high resolution electron energy loss spectroscopy. There is good evidence that water adsorbs molecularly and does not dissociate thermally in TPD. The TPD spectra change with coverage. At the lowest exposures, the desorption is broad, peaking near 200 K, and exhibits a long tail extending to 600 K, the latter a characteristic of every exposure. As the exposure increases, this peak moves slightly to lower temperatures (196 K) and a second peak appears at 156 K; these two then merge to an unresolved peak at 176 K which moves downward to as low as 169 K. Dissociation and desorption are induced by electron irradiation at 100 K; the O(1s) binding energy shifts downward (533.2–531 eV) and broadens and the vibrational spectra take on characteristics expected for O–D formation and the desorption of molecular water. Electron-induced O–D bond breaking is limited to D2O molecules located at the GaAs interface. During heating (up to 600 K) after electron irradiation, there is (1) some D2O and D2 desorption, (2) depending on the annealing temperature, a lower O(1s) binding energy (530 eV at 400 K, 529 eV at 600 K), and (3) loss of all vibrational features attributable to D. These facts are all consistent with forming a small amount of surface oxide, possibly at defect sites. The D2O removal process is adequately described as first order with an upper bound for the cross section, σ(D2O), of 1.16×10−17 cm2. The measured isotope effect, σ(H 2O)/σ(D2O), was 2.57.