Regensburg 2004 – wissenschaftliches Programm
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O: Oberflächenphysik
O 14: Postersitzung (Adsorption an Oberflächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Phasenübergänge, Rastersondentechniken, Struktur und Dynamik reiner Oberflächen)
O 14.20: Poster
Montag, 8. März 2004, 18:00–21:00, Bereich C
Diffusion pathways of hydrogen across Si(001) double-height steps — •M. Lawrenz, C. Schwalb, M. Dürr, and U. Höfer — Fachbereich Physik, Philipps-Universität, D-35032 Marburg
Steps can play an important role for macroscopic diffusion processes at surfaces. An interesting system to study diffusion across steps at the atomic scale is hydrogen on vicinal Si(001). At room temperature, molecular hydrogen dissociates selectively at the threefold coordinated DB-step sites. The adsorption takes place on adjacent Si atoms aligned with the dimer rows of the upper terrace. Surfaces consisting of long hydrogen chains at steps and of clean terraces can be prepared easily in this way. They provide a well-defined starting configuration for imaging hydrogen diffusion with a variable-temperature STM in real time.
Hydrogen was found to diffuse across the steps and along the dimer rows at temperatures in excess of 550 K. The hopping rates for different pathways could be determined from successive scans of the same 300 Å× 300 Å region. At 570 K, the rate for diffusion from the steps sites to the dimers of the upper terrace is 2.2×10−3 s−1 which is comparable to the rate of intra-row diffusion on the terraces. In contrast, the diffusion rate to the lower terrace is only 1×10−4 s−1. At the first dimer of the upper terraces hydrogen has a preference to diffuse back to the step sites. The overall step depletion rate could also be determined. Its value of 1.25 × 10−4 s−1 at 570 K is in good agreement with Rdiff = 1011s−1 exp(−1.7 eV/kT) as measured previously with optical second-harmonic generation [1].
[1] M. B. Raschke and U. Höfer, Phys. Rev. B 59, 2783 (1999)