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An IR-transmission spectroscopical study of the influence of substrate surface defects on the morphology and the electronic structure of ultrathin Fe grown on MgO(001)

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Thin Solid Films

Volume 364, Issues 1-2
27 March 2000
Pages 177-180

PII: S0040-6090(99)00921-9
Copyright © 2000 Elsevier Science S.A. All rights reserved.

Gerhard Fahsold ^,, Andreas Priebe, Norbert Magg and Annemarie Pucci

Institut für Kirchhoff-Physik, Ruprecht-Karls-Universität Heidelberg, Albert-Ueberle-Strasse 3-5, D-69120 Heidelberg, Germany

Available online 7 April 2000.

IR broad-band properties are sensitive to electronic transport and charge localization. This allows to investigate the electronic structure of ultrathin metal films by means of IR-transmission spectroscopy. As this electronic structure is determined by the interfaces of the film, the dynamic conductivity also reflects its morphology. We performed in-situ transmission spectroscopy in the middle infrared (MIR) during the evaporation of Fe on MgO(001) at room temperature in ultrahigh vacuum. We varied the quality of the substrate surface by using MgO(001) cleaved in air and cleaved in ultrahigh vacuum. The difference in the thicknesses for the onset of continuous transport properties of the growing films is well detected by their different IR-spectroscopical behavior. For continuous films, we calculate the thickness dependent scattering of electrons and the thickness dependent effective IR-optical oscillator strength from their IR-optical data. For MgO cleaved in air compared with cleavage in ultrahigh vacuum, the continuous thin film model becomes relevant at a smaller thickness, i.e. the substrate is covered faster in this case.

Author Keywords: Infrared spectroscopy; Dynamic conductivity; Thin films; Classical size effect; Iron; MgO

1. Introduction
2. IR-dynamical conductivity of continuous thin films
3. Experiment
4. Calculation of IR spectra
5. Discussion
6. Conclusion
Acknowledgements
References

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Fig. 1. Upper part: Selection of IR-transmittance spectra from series measured at normal incidence during Fe-film growth. The MgO(100) substrates are at room temperature. Their surfaces are prepared by cleavage in air (left) and by cleavage in UHV (right). Unity means the same transmittance as the substrate. The labels indicate the film thicknesses for the selected spectra. Lower part: Development of IR-transmission at 2000 cm^-1 (open symbols) and at 4000 cm^-1 (filled symbols) during Fe-film growth. The MgO(001) substrates are cleaved in air (squares) or cleaved in UHV (circles). The thicknesses of the optical crossover (solid arrows) and upper limits for the d.c.-percolation threshold (dashed arrows, see Section 5) are indicated.

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Fig. 2. Upper part: Measured spectra (solid) as shown in Fig. 1 for air-cleaved substrates and best-fit thin film calculation (dashed). Lower part: Fit results for the parameters

(see Eq. (2)) and

² (see Eq. (3)) versus film thickness d for a representative selection of our spectral data for air-cleaved and UHV-cleaved substrates. The inset shows the mean-square error between calculated spectrum and measurement for d=1.5 nm (on UHV-cleaved MgO). Each line indicates an increase by a factor of two. An increase of this mean-square error by a factor of two is used to read out the error bars for

and

².

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Corresponding author. Fax: +49-6221-549-262; email: fahsold@urz.uni-heidelberg.de

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Thin Solid Films
Volume 364, Issues 1-2
27 March 2000
Pages 177-180

32 of 59

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