  PbSe, a predicted two-dimensional (2D) topological crystalline insulator
(TCI) in the monolayer limit, possess excellent thermoelectric and infrared
optical properties. Native defects in PbSe take a crucial role for the
applications. However, little attention has been paid to the defect induced
doping characteristics. Here, we provide an experimental and theoretical
investigation of defects induced p-type characteristic on epitaxial monolayer
PbSe on Au(111). Scanning tunneling microscopy (STM) measurements demonstrate
an epitaxial PbSe monolayer with a fourfold symmetric lattice. Combined
scanning tunneling spectroscopy (STS) and density functional theory (DFT)
calculations reveal a quasi-particle bandgap of 0.8eV of PbSe. STM results
unveil that there are two types of defects on the surface, one is related the
vacancies of Pb atoms and the other is the replacement of the absent Se atoms
by Pb. Corresponding theoretical optimization confirms the structures of the
defects. More importantly, both STS measurements and DFT calculations give
evidence that the Pb vacancies move the Fermi energy inside the valence band
and produce extra holes, leading to p-type characteristics of PbSe. Our work
provides effective information for the future research of device performance
based on PbSe films.
