Rutherford Backscattering Spectrometry (RBS)
The RBS-equipment,
used for the characterization of thin films of complex oxides and switchable
mirrors produced in the Condensed Matter group at the VU, is an Ion
Beam Analysis facility situated at the Amolf Institute in Amsterdam in the
group of Prof. A. Polman.
Rutherford backscattering analysis is a
powerful tool for the characterization of thin films.
It provides information about the composition
of a sample as well as depth distributions.
By aligning the crystallographic axes of the
sample to the incoming He+-beam, the so-called channelling
analysis provides information about the epitaxial growth of the thin
films.
A van de Graaff accelerator produces 2 MeV He+
ions, which are focused on the thin film to be analysed.
A surface barrier detector under 165 degrees back
reflection measures the scattered ions coming from the sample.
By means of a multi channel analyser the signal
height, which is proportional to the energy, is converted into 512
channels ranging from 0 to 2MeV.
In each channel those events are collected
which have a specific energy of the recoiled He+ ions.
A typical spectrum is shown in which a
Palladium film is deposited on a sapphire substrate:
The
Pd peak is well separated from the substrate signal, which is due to the higher mass (m2) of Pd compared to Al or O. The He particles (m1) are
scattered with a much higher recoil energy than the He from the substrate in
this elastic scattering process. This enables a simple peak integration of the
Pd peak for an accurate calculation of the number of Pd atoms in the film.
When
the elements of the substrate (SrTiO3)
have a mass comparable to the masses of the film (PrCuO) grown on top,
an overlap may occur as can be seen in the graph above. Now the peak
integration is no longer possible. For the calculation of compositions the RUMP program provides a simulation, which
gives an overall accuracy of 3 %.
The
RUMP program can also simulate multi-layers and interface reactions such as
in-diffusion of complicate multiplayer structures like the above picture
presents
Literature:
J.H.
Rector, P. Koster, F. Peerdeman, D.G. de Groot, B. Dam.
Twinfree YBa2Cu3O7-d films on (001) NdGaO3 showing
isotropic electrical