Anti-doping in Insulators and Semiconductors having Intermediate Bands with Trapped Carriers

TL;DR

This paper investigates a counterintuitive doping phenomenon called 'anti-doping' in insulators and semiconductors with intermediate bands, where electron doping decreases conductivity due to merging of bands, revealing new control mechanisms.

Contribution

It explains the physical origin of anti-doping and proposes design principles for discovering materials exhibiting this behavior.

Findings

Anti-doping occurs in materials with trapped holes.

Electron doping can annihilate hole polarons, reducing conductivity.

The study offers a framework for designing anti-doped materials.

Abstract

Ordinary doping by electrons (holes) generally means that the Fermi level shifts towards the conduction band (valence band) and that the conductivity of free carriers increases. Recently, however, some peculiar doping characteristics were sporadically recorded in different materials without noting the mechanism: electron doping was observed to cause a portion of the lowest unoccupied band to merge into the valance band, leading to a decrease in conductivity. This behavior we dub as 'anti-doping' was seen in rare-earth nickel oxides SmNiO3, cobalt oxides SrCoO2.5, Li-ion battery materials and even MgO with metal vacancies. We describe 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 electron doping. This may offer an unconventional way of controlling conductivity.