  Black phosphorus (BP) is receiving significant attention because of its
direct 0.4-1.5 eV layer-dependent band gap and high mobility. Because BP
devices rely on exfoliation from bulk crystals, there is a need to understand
native impurities and defects in the source material. In particular, samples
are typically p-doped, but the source of the doping is not well understood.
Here, we use scanning tunneling microscopy and spectroscopy to compare atomic
defects of BP samples from two commercial sources. Even though the sources
produced crystals with an order of magnitude difference in impurity atoms, we
observed a similar defect density and level of p-doping. We attribute these
defects to phosphorus vacancies and provide evidence that they are the source
of the p-doping. We also compare these native defects to those induced by air
exposure and show they are distinct and likely more important for control of
electronic structure. These results indicate that impurities in BP play a minor
role compared to vacancies, which are prevalent in commercially-available
materials, and call for better control of vacancy defects.
