Structural and Phase Analysis
Spectroscopic Ellipsometer
In ellipsometry, the polarization change of a reflected light beam on a substrate is analyzed to determine the dielectric properties of thin films or their layer thickness. Once the thin films have been sufficiently precise defined, further conclusions can be drawn from the dielectric properties of the material regarding composition, crystal structure, anisotropy, conductivity, doping or roughness.
For the analysis, the incident light is initially polarized linearly and directed onto the substrate. Polarization is usually adjusted at an angle of 45° to the plane of incidence, so that the electromagnetic field of the incident beam can be divided equally into a parallel and a vertical component. The behaviour of these components in the reflection on a thin film or layer system can be described by the Fresnel formulas.
An analyzer detects the amplitude ratio and phase shift of the vertical and parallel components in the reflected beam. In addition to these measured values, a model system is also required which is created from the known parameters, e. g. layer thicknesses of the thin film or the layer system. The analysis software now adapts the parameters still unknown from the model in Fresnel's formulas until the experimental data are reproduced with the smallest possible error.
Woollam M2000
The M2000 is a spectroscopic ellipsometer that offers a wide spectral range and enables very fast measurements. With 470 wavelengths, the M2000 covers a wavelength range of 245 nm - 1000 nm. The near infrared upgrade installed in 2015 extends the measuring range with an additional 190 wavelengths from 1005 nm to 1690 nm. The angle of incidence can be traversed fully automatically from 45° - 90°. A measurement typically takes only a few seconds per angle of incidence. The modular design also enables operation directly on coating systems for in-situ growth control.
STM - Scanning Tunneling Microscope
The scanning tunneling microscope (STM) works, similar to an AFM, with a fine tip that is scanned across a substrate in a grid-like manner. With the STM, the tip is not brought into contact with the surface, but is approached at a distance of 0.1 - 1 nm. Electrons can tunnel over this short distance by applying an electrical voltage between the tip and the substrate. The tunnel current is exponentially dependent on the distance and allows a vertical resolution of up to 0.01 Å. The lateral resolution can be up to 1 Å. The measurement signal does not directly reflect the topography, but rather the height information of a constant electron density. The STM therefore also provides access to the chemical properties of a substrate through local measurement of the electronic structure.
Omicron VT-STM
Omicron's Variable Temperature (VT)-STM allows measurement over a wide temperature range from 25 K - 1500 K. The STM has the optional QPlus sensor, which allows the STM to be operated as an AFM in non-contact mode. Thus, overview images can be acquired relatively quickly in order to approach the position of the actual STM measurement. The system also allows scanning tunneling spectroscopy, in which the voltage at each pixel is passed through a small area around the tunnel voltage and the change in the tunnel current is recorded. The dI/dV signal can be used to determine local state densities.
The VT-STM is mounted in a UHV system of the group of Prof. Dr. Thomas Chassé.
