  Three-dimensional topological semimetals host a range of interesting quantum
phenomena related to band crossing that give rise to Dirac or Weyl fermions,
and can be potentially engineered into novel quantum devices. Harvesting the
full potential of these materials will depend on our ability to position the
Fermi level near the symmetry-protected band crossings so that their exotic
spin and charge transport properties become prominent in the devices. Recent
experiments on bulk and thin films of topological half-Heuslers show that the
Fermi level is far from the symmetry-protected crossings, leading to strong
interference from bulk bands in the observation of topologically protected
surface states. Using density functional theory calculations we explore how
intrinsic defects can be used to tune the Fermi level in the two representative
half-Heusler topological semimetals PtLuSb and PtLuBi. Our results explain
recent results of Hall and angle-resolved photoemission measurements. The
calculations show that Pt vacancies are the most abundant intrinsic defects in
these materials grown under typical growth conditions, and that these defects
lead to excess hole densities that place the Fermi level significantly below
the expected position in the pristine material. Directions for tuning the Fermi
level by tuning chemical potentials are addressed.
