Origin of n- and p-type conductivity in undoped $\alpha$-PbO: role of defects

TL;DR

This study uses first principles calculations to reveal how defects in undoped α-PbO lead to its intrinsic n- and p-type conductivity, depending on the defect environment during deposition.

Contribution

It demonstrates the defect mechanisms responsible for n- and p-type conductivity in undoped α-PbO through detailed defect formation energy analysis.

Findings

O vacancies cause n-type doping by excess electrons.

O interstitials induce p-type doping by excess holes.

Defect concentrations can reach extremely high levels (~10^{22}cm^{-3}).

Abstract

The first principles calculations (GGA) have been applied to study the crystallographic defects in $\alpha$-PbO in order to understand an origin of $n$- and $p$-type conductivity in otherwise undoped $\alpha$-PbO. It was found that deposition in the oxygen-deficient environment to be defined in our simulations by the Pb-rich/O-poor limit stimulates a formation of the O vacancies and the Pb interstitials both to be characterized by quite low formation energies $\sim$ 1.0 eV. The O vacancy being occupied by two electrons shifts a balance of electrons and holes between these two defects to excess of electrons (four electrons against two holes) that causes the $n$-type doping. For the Pb-poor/O-rich limit, an excess of oxygen triggers a formation of the O interstitials characterized by such a low formation energy that spontaneous appearance of this defect is predicted. It is shown that the concentration of the O interstitials is able to reach the extreme magnitude equal to number of the possible defect sites ($\sim 10^{22}$cm$^{-3}$). The localized state formed by the O interstitial is occupied by two holes and because there are no other defects in reasonable concentration to balance the hole redundancy, $p$-type doping is induced.