  Source oxidation of easily oxidizing elements such as Ca, Sr, Ba, and Ti in
an oxidizing ambient leads to their flux instability and is one of the biggest
problems in the multi-elemental oxide Molecular Beam Epitaxy technique. Here we
report a new scheme that can completely eliminate the source oxidation problem:
a self-gettering differential pump using the source itself as the pumping
medium. The pump simply comprises a long collimator mounted in front of the
source in extended port geometry. With this arrangement, the oxygen partial
pressure near the source was easily maintained well below the source oxidation
regime, resulting in a stabilized flux, comparable to that of an
ultra-high-vacuum environment. Moreover, this pump has a self-feedback
mechanism that allows a stronger pumping effectiveness for more easily
oxidizing elements, which is a desired property for eliminating the source
oxidation problem.
