Inverse and Direct Photoemission

In Deutsch

Responsible:  Dr. L. Johansson, L. Duda, M. Krieftewirth, M. Laurenzis

A major research project at Experimentelle Physik I is the study of surface electronic structures with angle-resolved inverse and (direct) ultra-violet photoemission. Both techniques are combined in a custom-built surface-science facility, originally set up at the IBM Zurich Research Laboratory. The system also contains LEED, Auger-electron spectroscopy, EELS, ion sputtering, crystal cleaver, evaporators and a film-thickness monitor. The samples can be heated or cooled in the range 30 K - 1800 K, after they have been inserted through a load-lock system into the ultra-high vacuum chambers (pressures in the 10 exp(-11) mbar range).

Inverse photoemission spectroscopy (IPES) is performed in the isochromat mode at a photon energy of 9.5 eV, allowing band mapping E(k) of the unoccupied electronic states in the range 0-20 eV above the Fermi level. Ultra-violet photoelectron spectroscopy (UPS) provides the energy band dispersions of the occupied states below the Fermi level, employing He resonance excitation with photon energies 21.2 and 40.8 eV.

Both techniques have been applied to, e.g., alkali-metal adsorption on semiconductor surfaces (Ref. 1), which serve as model systems for metallization and Schottky barrier formation, and hence is relevant for the understanding of semiconductor devices. A particular advantage of our system is the ability to study the electronic states above and below the Fermi level on the same surface, and thereby directly observe the charge transfer and onset of metallization for increasing metal coverage.

A typical example for the application of UPS and IPES pertaining to this research subject is shown in the figures below, where potassium is adsorbed on the Si(100)2x1 surface (Ref. 2). In the figure to the left UPS and IPES spectra are shown for increasing K coverages. One observes the formation of a K-induced empty state that shifts towards the Fermi level, finally rendering the K/Si(100)2x1 surface metallic. The energy dispersion of this surface-state band was also measured (Ref. 2) and the resulting band structure is shown together with theoretical results (Ref. 3) in the second figure (to the right).

Figure 1. (left, click to enlarge it)
Angle-resolved UPS and IPES spectra are shown, recorded in normal emission/incidence, for increasing K coverage on the Si(100)2x1 surface. The coverage was controlled by measuring the shift of the work function. In the IPES spectra a K-induced empty state appears and shifts towards the Fermi level, rendering the surface metallic at saturation coverage. In the ARUPS spectra, the well-know dangling-bond surface state of the clean surface is gradually split into two states with increasing K coverage. (From Ref. 2)

Figure 2. (below, click to enlarge it)
The experimentally obtained energy dispersion of the empty band is plotted here (dots). Also shown are the results of a theoretical molecular dynamics calculation of the surface bands for the so-called double-layer model (from Morikawa et al., Ref. 3).

References

1. B. Reihl, R. Dudde, L. S. O. Johansson, and K. O. Magnusson: The Electronic Structure of Alkali-Metal Layers on Semiconductor Surfaces, Applied Physics A 55, 449 (1992).
   
2. L. S. O. Johansson and B. Reihl: Empty surface states on the Si(100)2x1-K surface: evidence for overlayer metallization, Physical Review Letters 67, 2191 (1991). Download the paper in pdf-format.
   
3. Y. Morikawa, K. Kobayashi, and K. Terakura: First-principles molecular dynamics study of alkali-metal adsorption on a Si(001) surface, Surface Science 283, 377 (1993).

Download reference 2 in pdf-format. To read it you need the Adobe Acrobat Reader, which you can download by clicking on the image to the right.

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