Real-time Ellipsometry

Basic Principles

Ellipsometry, first employed by P. Drude in the late 19th century, is a optical technique that uses polarised light to probe the dielectric properties of a sample. Due to its high sensitivity it can provide information about thin films starting in the monolayer regime. In combination with other techniques, such as X-ray scattering, a range of properties such as layer thickness, morphology, and chemical composition can be studied.

Ellipsometry scheme

Ellipsometry generally measures two of the four Stokes parameters, denoted as Ψ and Δ. These depend on the components Rp and Rs of the reflectivity via

Equation 1

From this (complex) ratio of reflectivities, the fundamental equation of ellipsometry, the optical properties of the sample and the layer thicknesses can be determined.

This requires a modelling procedure which takes into account the Fresnel reflection coefficients (including multiple reflections).

Unlike other spectroscopic techniques ellipsometry does not require intensity calibration of the measured light A particularly attractive feature of ellipsometry is its insensitivity to the intensity of the measured light.

Further details can be found in this introduction to ellipsometry provided by the J. A. Woollam Co.

Experimental Setup

As the ellipsometer can be mounted to our ultra-high vacuum system in situ studies during the growth of the film are feasible.

  • The organic thin films are prepared by organic molecular beam deposition.
  • A spectroscopic ellipsometer (Woollam M2000) aquires Ψ and Δ for wavelenghts between 245 nm and 1000 nm.
  • This in situ technique allows realtime measurements with good time resolution during growth.
  • A fit procedure, implemented in the analysis software, yields the optical constants n and k over the complete spectral range.
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Example

In a current project, we are combining organic molecular beam deposition with real-time ellipsometry. This allows us to see how the electronic transitions of the material change during growth.
Using complementary information from x-ray scattering experiments allows us to learn how functional properties (band-gap, electronic transitions) relate to the thin film structure at interfaces.

For our previous work on ellipsometry, see our list of publications.