Experiments with Supersonic Beams as a Source of Cold Atoms

that they can be prepared ex-situ and will remain clean enough to still reflect helium
well. The crystal chosen for this experiment, is hydrogen passivated Si(111). Lithium
fluoride was also examined as a possible crystal substrate, and the details of this
examination may be found in [55].
To use Si(111) as an atomic mirror which is prepared ex-situ, thecrystalmust
be passivated. A freshly cleaved Si(111) surface will have dangling bonds and the
surface will not be passive enough to keep contaminants from sticking to the surface.
By wet etching the surface, the dangling bonds are terminated with hydrogen, which
passivates the surface, resulting in Si(111)-(1×1)H [56, 57]. The mirrors are created
from phosphor doped 200μm thick wafers with a pre-existing oxide layer and a miscut
angle of ±0.1 ◦ acquired from Virginia Semiconductor. The 100 mm diameter waferes
were pre-cut into smaller circles [58], but the cutting process scratched the crystal
surfaces and rendered the area within 2 mm of a cut unusable. The wafers are cut to
their final size for etching and insertion into the vacuum chamber by a dicing saw,
removing the damaged regions. The wet etching process used to create this surface
is now described.
3.2.2 Crystal Preparation and Wet Etching
To produce an atomically flat passivated Si surface, the crystal must first be
cleaned as much as possible to remove contaminants, especially organic residues. The
procedure described below is based on suggestions from the group of Dr. Ken Shih at
The University of Texas at Austin, and the group of Dr. Bill Allison at the Cavendish
Lab in Cambridge [57]. All chemicals used are LP grade or better, and the ammonium
fluoride is MB grade. Additionally, any water used in the procedure is ultra pure and
has a resistivity of greater than 17 MΩ cm. Finally, all beakers, bottles, containers,
and tools used to handle the wafers or in contact with the chemicals are made of
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