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Nanostrukturen und Grenzflächen Poster: Do., 13:00–15:30 D-P320
VUV Ellipsometry using Synchrotron Radiation with Sub-monolayer Resolution
Dietrich R.T. Zahn 1 , Ovidiu D. Gordan 1 , Sascha Hermann 1 , Daniel
Lehmann 1 , Cameliu Himcinschi 1 , Christoph Cobet 2,3 , Simona Silaghi 3 , Walter
Braun 4 , Norbert Esser 3
1 Institut für Physik, TU Chemnitz, Reichenhainerstr. 70, 09107 Chemnitz, Germany –
2 Institut für Festkörperphysik, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany –
3 ISAS Institute for Analytical Sciences, Albert-Einstein-Str. 9, 12489 Berlin, Germany
– 4 BESSY GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany
Ellipsometry is a non-destructive and very sensitive surface and thin film measurement technique
which can detect the changes in the optical response produced by an overlayer on a
smooth surface. Due to the long wavelengths of the spectral range accessible with most commercial
ellipsometers it can, however, be difficult to separate the refractive index of the film
and the film thickness. Consequently only the product of these parameters can be uniquely
determined. The lower wavelength of the UV light should be even more sensitive for ultra-thin
films. Still to our knowledge there is no investigation yet of organic monolayers using UV
ellipsometry.
Therefore the aim of this work is to show that sub-monolayer sensitivity can be achieved using
in situ ellipsometry to monitor the evolution of the dielectric response of tris-(8-hydroxyquinoline)-aluminum(III)
(Alq3) layers from ultra-low coverages to bulk-like layers. Alq3 was
deposited under ultra-high vacuum conditions onto hydrogen passivated silicon and zinc oxide
substrates. The characteristic Vacuum-Ultra-Violet (VUV) absorption lines of Alq3 were detected
using synchrotron radiation as light source.
In order to separate overlayer and substrate contributions to the effective dielectric function we
use two different ways: analysis in terms of a conventional three phase model and a parameterfree
approximation valid for ultra-thin films proposed by Aspnes [1].
In such ultra-thin films the absorption lines corresponding to molecular transitions of the Alq3
are found to be spectrally blue-shifted with respect to bulk-like layers. While it still remains to
fully understand why the observed shift depends on the substrate, we proved for the first time
that the dielectric response can be reliably obtained in the VUV range for strong absorbing
materials even for sub-monolayer coverage.
[1] D. Aspnes, Optical Properties of Solids-New Developments, ed B.Seraphin, Chap 15 (1976)
Fig. 1: Imaginary part of the dielectric function;
comparison between the dielectric response for a
sub-monolayer of Alq3 on H-Si(111) and on ZnO
obtained from a model fit and calculated using the
Aspnes approximation. The dielectric function for
bulk Alq3 is also plotted. The calculated ɛ2 from
the Aspnes approximation is affected by the experimental
noise of two different measurements.