Anti-doping in Insulators and Semiconductors

Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free electrons (holes) increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to increase the band gap and thus lead to a decrease in conductivity. This behavior we dub as “anti-doping” was seen in rare-earth nickel oxides SmNiO₃, cobalt oxides SrCoO_2.5, Li-ion battery materials and even MgO with metal vacancies. Given the apparent generality of this phenomenon and that it may offer an unconventional way of controlling conductivity, we demystify the physical origin of anti-doping as well as its inverse problem – the “design principles” that would enable intelligent search of materials. We find that electron anti-doping is expected in materials having pre-existing trapped holes and is caused by annihilation of such “hole polarons” via doping.