Münster 1997 – wissenschaftliches Programm

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O: Oberflächenphysik

O 29: Metalle auf Halbleitern I

O 29.3: Vortrag

Donnerstag, 20. März 1997, 11:45–12:00, S 1

Oxygen adsorption on Na/Si(111) investigated with XSW
— •C. Sánchez-Hanke¹, A. Hille¹, V. Eteläniemi², E.G. Michel³, and G. Materlik¹ — ¹HASYLAB am DESY Notkestrasse 85, 22607 Hamburg — ²Univ. of Helsinki — ³Univ. Autónoma de Madrid.

The adsorption of oxygen on alkali-metal pre-covered semiconductors induces drastic changes in the work function. Under some conditions, the work function decrease is large enough to form a Negative Electron Affinity (NEA) surface.
To understand this electronic behaviour we investigated the O/Na/Si(111) surface structure with X-ray Standing Waves (XSW). XSW is a powerful structural technique which allows one to localize adsorbate atoms with respect to bulk lattice planes.
The samples were covered with 0.5 ML Na and further exposed to oxygen in the range 0.2-1.6 L. In these conditions, a positive work function change was induced. Na 1s and O 1s photoemission signals were detected in the Bragg reflection range at 3.3 keV photons for the (111) reflection of the substrate. The Na 1s modulation supports that Na atoms are adsorbed on top of Si atoms. The O 1s photoemision signal exhibits two components. The first is related to oxygen atoms bonded to Si, while the second comes from oxygen atoms whose enviroment is modified by the presence of Na. Both species were localized with respect to the (111) plane. This information allows one to establish a structural model for an NEA-like surface.