Substitutional tin acceptor states in black phosphorus

TL;DR

This paper identifies substitutional tin impurities as the cause of p-type doping and defect features in black phosphorus, revealing their electronic states and impact on STM imaging.

Contribution

It demonstrates that tin impurities are responsible for p-type behavior and defect features in black phosphorus, providing insights into their electronic structure and origin.

Findings

Tin impurities cause p-type doping in black phosphorus.

Substitutional tin creates localized negative charge states.

The 2p level of tin induces observable defect features in STM images.

Abstract

Nominally-pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunnelling microscopy (STM) images of black phosphorus reveal the presence of long-ranged double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP bandgap. The localised negative charge of the tin impurities induces hydrogenic states in the bandgap and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.