Stepped NaCl films grown epitaxially on Si-precovered vicinal Ge(100)

Low energy electron diffraction (LEED) and programmed thermal desorption spectroscopy (TDS) techniques have been used to characterize the morphology and modifications of the desorption kinetics of stepped NaCl(100) films which have been grown epitaxially on the vicinal Ge(100) surfaces with misorientations up to 5.4° in [011] direction. Even at this high misorientation the Ge surface contains about 60% steps of monoatomic height. The number DB of steps with double atomic height, however, can be significantly increased by adsorption of 0.5 ML Si at 470 K. At room temperature NaCl grows as a rough film with non-polar NaCl steps along the [010] and [001] directions, respectively. Growth of NaCl films at surface temperatures at or below 200 K, however, produces polar steps which most likely consist of pairs of alternating Na+ and Cl− terminated steps. No significant influence on the electronic structure by either type of step could be detected with ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). Adsorbed species like water and CO2 do not dissociate on the stepped NaCl surfaces, but are bound more strongly than on flat surfaces. For CO2 the non-polar NaCl steps produced a new desorption peak located at 115 K corresponding to a desorption energy of 38 kJ/mol, close to that found for adsorption at F centers (40 kJ/mol).