Association

40 3. Experimental details
a very good electron focusing is achieved, one can observe very sharp diffraction spots of
only one order rather than elongated rods. Some inelastically scattered electrons penetrate
the bulk crystal and can be detected besides the surface diffraction pattern. If they fulfill
Bragg diffraction conditions in the bulk, they are observed as diagonal Kikuchi pattern connecting
the intense surface diffraction points. 130 If there is no pattern at all or if it consists of
concentric circles, the surface is not well ordered or even polycrystalline.
Recording RHEED intensity oscillations of the specular reflex with time allows one to quantify
the number of monolayers grown in MBE, if the Van-der-Merve growth mode is predominant,
as illustrated in Fig. 3.4. The Oxide-MBE system at PGI-6 is equipped with a
self-constructed RHEED system having a VARIAN 981 e-gun source in the main deposition
chamber.
3.2.2. Low-energy electron diffraction (LEED)
camera
LEED
window
screen
incident beam
Ewald sphere
sample
reciprocal
rods
electron gun
spots
4-grid LEED optics
Figure 3.5.: LEED working principle.
Low-energy electrons with energies from about 20 to 500 eV are ideal probes for surface
studies, because they are mainly elastically scattered by atoms. According to the de Broglie
relation, λ = √ 150.4/E0 e− ([λ] = Å, [E] = eV), typical LEED wavelengths are of the same magnitude
as interatomic distances in crystals and comparable with that used in X-ray diffraction
(see Sec. 3.4.2). The Ewald sphere intersects only the lattice rods at particular points that
fulfill the Laue diffraction condition eq. (3.4), and the resulting LEED pattern consists of an
ordered arrangement of spots. All surface diffraction patterns show a symmetry reflecting
that of the crystal class and are centrally symmetric. The scale of the diffraction pattern
depends on the electron energy and the size of the surface unit cell as
√
2π 2mE
d ∝ 2 −|G ‖ | 2 , (3.5)
where d is the real space lattice parameter, and G ‖ a reciprocal lattice vector in the surface
plane. 79,131
In the experimental work, a LEED image shall exhibit a sharp pattern and low background
intensity. Defects and crystallographic imperfections will broaden spots and increase the