Diplomarbeit Diplom-Ingenieur - Institut für Halbleiter

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conditions are essential for the crystal growth. A common method for in situ
characterisation is Reflection High Energy Electron Diffraction (RHEED).
The basic processes of crystal growth take place in three zones between the
effusion cell and the sample surface. Firstly, there is the molecular beam generation
zone. The vapour elements mix in the mixing zone and crystal growth takes place in
the crystallisation zone on the substrate surface. The growth itself is controlled by the
substrate temperature, the beam flux and the process time. A schematic drawing of a
MBE system with the tree process zones can be seen in Fig. 2.3.
Figure 2.3: Sketch of a MBE System [27]. The essential parts of the system are shown,
and the tree main process zones are indicated.
In general, three epitaxial growth modes driven by energy minimisation can be
distinguished. The energy terms are the surface, interface and strain energy. The
Frank-van der Merwe growth is a monolayer-by-monolayer growth. The substrate and
the epi-material have a fitting lattice constant, thus no strain is established during the
crystal growth. The interface energy is smaller than the surface energy, consequently
the epi-layer minimises its surface and grows in closed monolayer steps. The Volmer-
Weber mode refers to a pure island growth on the surface. The strain energy term
plays again no role. The surface energy is now the smaller term and the crystal grows
with the minimum interface in the shape of islands. The Stranski-Krastanov growth is