Multiple states and roles of hydrogen in p-type SnS semiconductors

Hydrogen (H) plays critical roles in the electrical properties of semiconductor materials and devices. In this work, we report multiple states and roles of H in SnS by H plasma treatment and density functional theory (DFT) calculations. The as-deposited SnS films include impurity H at 2.3 × 1019 cm-3, four orders of magnitude larger than the hole density. The DFT calculations reveal that H exists in multiple states at the equilibrium mainly at the interstitial and the Sn-substitutional sites, which have formation enthalpies lower than those for the intrinsic defects. These H states work as donors and acceptors, respectively, and strongly pin the Fermi level in the p-type region. The native p-type conduction in the actual SnS semiconductors is caused mainly by the H-on-Sn (HSn) acceptors, rather than the previously reported Sn vacancies (VSn) for pure SnS. It is also confirmed that even stronger H doping with larger H chemical potentials cannot convert SnS to an n-type conductor because it reduces SnS to Sn metal.