Association

44 3. Experimental details
Figure 3.9.: Thermal hydrogen cracker for in situ substrate treatments, for example silicon surfaces.
Left: degas mode. Right: cracking mode with H 2 supply (T fil ≈ 2000 K).
In order to provide chemically clean and atomically sharp silicon surfaces, we treat Si wafer
pieces by an in situ thermal flashing procedure. A flashing sample holder consisting of tantalum
clamping, tungsten fixation screws, and alumina isolation spacers enables the effective
thermal cleaning of Si pieces by a direct current heating. We constructed the flashing
stage according to the textbook of Yates (1998) 132 and under practical advice from Westphal
(2011). 133 The thermal flashing procedure takes place in the main chamber under the
best UHV conditions (Fig. 3.7c), and substrate temperatures of T Si 1100 ◦ C are easily reached
for low-doped silicon (ρ Si 3kΩ cm).
Atomic hydrogen is suited for the surface passivation of silicon dangling bonds, which we
provide in situ in order to ensure chemical cleanliness and to cover a broad range of supply
parameters. For H 2 crackers using a tungsten hot surface, the efficiency of H production is
proportional to (p H2 ) 1 /2
in a regime p H2 > 10 −5 mbar, and commercial H 2 crackers work at
p H2 ≈ 10 −2 mbar in the tungsten cracking capillary. 132 Using a white glowing tungsten surface
(T W 2000 ◦ C), a dissociation efficiency of 0.3 per H 2 collision with W was reported. 134
In order to match these efficiency parameters and considering the limited space in the prechamber,
we developed a source for atomic hydrogen and mounted it to the pre-chamber. It
is fabricated from a high-power halogen lamp (Fig. 3.7a). Ultraclean (99.999%) molecular
hydrogen gas is leaked by a dose valve into the cracking bulb, in which the tungsten filament
is heated with a power of ∼60 W. While a hydrogen partial pressure in the chamber of
10 −3 mbar is found to optimally passivate the clean Si, we estimate the hydrogen pressure in
the cracking bulb to be in 10 −1 mbar regime, in agreement with the reported working pressure
of commercial crackers. Moreover, the cracking bulb architecture with a short nozzle
protects the sample surface from unwanted evaporation of the hot cracking filament.
For this work, only a persistent high chemical and structural quality of the EuO heterostructures
permits one to systematically investigate a tuning of composition, strain, or interface
structure. The presented specializations of the Oxide MBE setup render high quality growth
of EuO thin films possible on various oxide substrates (as shown in Ch. 4). Moreover, advanced
treatments of clean Si crystals are applicable in order to apply atomically thin passi-
Fabrication from high-power halogen lamps suggested by F.-J. Köhne.