Defect pairing in Fe-doped SnS van der Waals crystals: a photoemission and scanning tunneling microscopy study

We investigate the effect of low concentrations of iron on the physical properties of SnS van der Waals crystals grown from the melt. By means of scanning tunneling microscopy (STM) and photoemission spectroscopy we study Fe-induced defects and observe an electron doping effect in the band structure of the native p-type SnS semiconductor. Atomically resolved and bias dependent STM data of characteristic defects are compared to ab initio density functional theory simulations of vacancy (V_S and V_Sn), Fe substitutional (Fe_Sn), and Fe interstitial (Fe_int) defects. While native SnS is dominated by acceptor-like V_Sn vacancies, our results show that Fe preferentially occupies donor-like interstitial Fe_int sites in close proximity to V_Sn defects along the high-symmetry c-axis of SnS. The formation of such well-defined coupled (V_Sn, Fe_int) defect pairs leads to local compensation of the acceptor-like character of V_Sn, which is in line with a reduction of p-type carrier concentrations observed in our Hall transport measurements.