Association

3. Experimental details
Die Pläne hab’ ich bereits im Kopf! Ich muss nur noch einen
passenden Apparat konstruieren.
(Daniel Düsentrieb)
The fabrication and detailed investigation of functional magnetic oxide heterostructures requires
suitable experimental methods. First, their synthesis by reactive molecular beam epitaxy
is illustrated in detail, focusing on in situ passivation techniques for silicon surfaces
and specifications of the growth of magnetic oxide thin films. In a second step, insights
into the surface crystal structure can be obtained from in situ electron diffraction techniques,
and the surface chemical properties (e. g. the silicon oxidation) are controlled by Auger electron
spectroscopy. Finally, a variety of ex situ methods covering superconducting quantum
interference device magnetometry, X-ray diffraction techniques, hard X-ray photoemission
spectroscopy, and the MCD effect in photoemission allow one to compile a detailed picture
of the magnetic properties, crystal structure, and electronic structure of the surface, buried
interfaces, and the bulk of magnetic oxide heterostructures.
3.1. Molecular beam epitaxy for thin magnetic oxide films
In order to fabricate high quality thin films or coatings for electronic and semiconductor
technologies, molecular beam epitaxy (MBE) was invented in the 1960s at Bell Telephone
Laboratories. Nowadays it is used mainly for the fabrication of transistors, diodes, solar
cells, and for fundamental thin film research. MBE is a very fine-controllable deposition
technique, which provides the opportunity to grow epitaxial thin films with a low deposition
rate (typically < 1 nm/s) such that a monolayer-controlled growth is possible.
Thin films growth modes
An essential advantage of the MBE technique is its ability to achieve layer-by-layer growth of
epitaxial thin films. This highest-quality growth mode, however, needs several key prerequisites.
First, the mean free path l of the emitted particles should be larger than the distance
from the effusion cell to the substrate surface,
l =
k B T !
√ 40 cm, (3.1)
2πpd 2
with k B : Boltzmann’s constant, T : the system temperature, p: the chamber pressure, and
d: the diameter of the particle. This limits the residual gas pressure to roughly 10 −5 mbar.
Nevertheless, UHV conditions in the 10 −10 –10 −11 mbar range are required in order to maintain
the purity of chemical composition of the film – or in other words – to keep the ratio of
contaminants 10 −5 .
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