  As progress is made on thin-film synthesis of Heusler compounds, a more
complete understanding of the surface will be required to control their
properties, especially as functional heterostructures are explored. Here, the
surface reconstructions of semiconducting half-Heusler NiTiSn(001), and
Ni1+xTiSn(001) (x=0.0-1.0) are explored as a way to optimize growth conditions
during molecular beam epitaxy. Density functional theory (DFT) calculations
were carried out to guide the interpretation of the experimental results. For
NiTiSn(001) a c(2x2) surface reconstruction was observed for Sn rich samples,
while a (1x1) unreconstructed surface was observed for Ti-rich samples. A
narrow range around 1:1:1 stoichiometry exhibited a (2x1) surface
reconstruction. Electrical transport is used to relate the observed reflection
high energy electron diffraction (RHEED) pattern during and after growth with
carrier concentration and stoichiometry. Scanning tunneling microscopy and
RHEED were used to examine surface reconstructions, the results of which are in
good agreement with density functional calculations. X-ray photoelectron
spectroscopy was used to determine surface termination and stoichiometry.
Atomic surface models are proposed, which suggest Sn-dimers form in
reconstructed Ni1+xTiSn(001) half-Heusler surfaces (x<0.25) with a transition
to Ni terminated surfaces for x > 0.25.
