Self-passivation of vacancies in α -PbO

Abstract

We introduce a self-passivation of single lead (Pb) and oxygen (O) vacancies in the α-PbO compound through formation of a Pb-O vacancy pair. The preferential mechanism for pair formation involves initial development of the single Pb vacancy which, by weakening the covalent bonding, sets up the crystal lattice for an appearance of the O vacancy. Binding of the Pb and O vacancies occurs through the ionization interactions. Since no dangling bonds appear at the Pb-O pair site, this defect has a minor effect on the electronic properties. In such, vacancy self-passivation offers a practical way to improve the transport properties in thermally grown PbO layers.Polycrystalline Lead Oxide (PbO) is one of the few photoconductive materials with a long-more than 60 years -history of employment in imaging devices. Although PbO is extensively used as a photoconductor, very little is known about the electronic properties and charge transport in this material. It is generally believed that transport in PbO is controlled by trapping in defects and that trapping is a cause of low mobility-time product [1]. However, the nature of defects is not fully understood. Emerging applications of PbO in the direct conversion flat panel radiation medical imaging detectors revived the interest in studying defects in this material as defects can significantly affect imaging performance [2].Our recent modeling of the native point defects in α-PbO [3] has shown that thermally deposited PbO layers should contain a significant concentration of single vacancies due to their moderate formation energies. Single vacancies are amphoteric defects appearing in the different charge states. In the neutral charge state, the O vacancy (V O(0) )) holds two electrons and forms the deep donor level. The neutral Pb vacancy (V Pb(0) )) is filled with holes, and is a shallow acceptor. It was established that these vacancies prefer to appear doubly ionized, V Pb(2−) and V O(2+) , acting as compensating centres to each other. Indeed, two compensating vacancies have a lower formation energy than neutral ones, such asThe fact that vacancies prefer to appear in charge states suggests that we have to consider the ionization interactions between them and the formation of a neutral vacancy pair V Pb-O instead of two separate compensating vacancies. This should further decrease the formation energy, approximately speaking, the free energy required to insert a defect in a lattice is reduced by the energy liberated due to ionization of the donor and acceptor. Indeed, the formation of the defect complexes in favor over the single defects is often observed during material deposition [4]. In this work, we present our study of the formation mechanism of V Pb-O pair in PbO layers and its effect on the electronic properties. Analysis of the formation of VPb-O vacancy pair was performed using the density functional theory (DFT) available in the Wien2k package [5] which utilizes the fullpotential augmented plane-wave method. The PerdewBurke-Ernzerhof parameterization [6] of the gen...

Interstitial impurity-induced magnetism inα-PbO surface

“…Lead oxide or PbO has a rich history as a photoconductive material in imaging devices and additional applications of PbO may be potentially realised in the field of spintronics. In recent studies of Bershevich and Reznik on α-PbO, the low temperature polymorph of lead oxide, it is shown that magnetism can be induced by non-magnetic impurities situated at the interstitial sites between crystal layers [21,22]. α-PbO in its bulk form is thermodynamically stable at room temperature and has a layered structure, with a space group P 4/nmm − D [23].…”

Origin of n- and p-type conductivity in undoped α-PbO: role of defects

“…1 (d)). Since each electron stays on its own orbital, Hund's rules dictate the ferromagnetic ordering of these electrons [18]. The spin-polarization energy defined as the energy difference between the spin-unpolarized and spin-polarized states is found to be E pol =0.235 eV that indicates the stable triplet ground state [18].…”

“…For the Pb-poor/O-rich conditions, the interaction between I O and V Pb can potentially occur but because each localized state is occupied by two holes, such interaction is repulsive. Under equilibrium growth conditions, the formation energy of V Pb and V O would become comparable, indicating a possibility of their pairing as well [23]. This combination of defects is unique as it induces vanishing of the dangling bonds and, therefore, occurs without generation of a defect state inside the band gap.…”

“…Since each electron stays on its own orbital, Hund's rules dictate the ferromagnetic ordering of these electrons [18]. The spin-polarization energy defined as the energy difference between the spin-unpolarized and spin-polarized states is found to be E pol =0.235 eV that indicates the stable triplet ground state [18]. The Pb interstitial induces a localized state inside the band gap and spin-polarized solution separates the occupied and unoccupied states as E D (I Pb )-E V =0.99 eV and E D (I Pb )-E V =1.51 eV (see Fig.…”