  Tin monosulfide (SnS) is a naturally p-type semiconductor with a layered
crystal structure, but no reliable n-type SnS has been obtained by conventional
aliovalent ion substitution. In this work, carrier polarity conversion to
n-type was achieved by isovalent ion substitution for polycrystalline SnS thin
films on glass substrates. Substituting Pb2+ for Sn2+ converted the majority
carrier from hole to electron, and the free electron density ranged from 1012
to 1015 cm-3 with the largest electron mobility of 7.0 cm2/(Vs). The n-type
conduction was confirmed further by the position of the Fermi level (EF) based
on photoemission spectroscopy and electrical characteristics of pn
heterojunctions. Density functional theory calculations reveal that the Pb
substitution invokes a geometrical size effect that enlarges the interlayer
distance and subsequently reduces the formation energies of Sn and Pb
interstitials, which work as electron donors.
