Electronic And Optical Properties Of PbTiO₃with Nitrogen Dopants And Vacancy Defects

It is very important to develop the high-performance transparent conducting oxidesno matter in fabricating transparent electronics or solar cells. In many electronic devices,the n-type transparent conducting oxides are present in many devices, but their p-typecounterparts are not largely commercialized, as they exhibit much lower carriermobilities due to the large hole effective masses of most oxides. In recent years, byconducting a high-throughput computational search on thousands of binary and ternaryoxides, some researchers have predicted that the material of lead titanate is an idealp-type semiconductor. Its hole effective mass is relatively small. The energy band gap ofintrinsic lead titanate is3.4eV measured in experiments, so the conductivity of intrinsiclead titanate is weak. Further more, the light transmissivity of intrinsic lead titanate isnot very high. So we often introduce some defects (impurities or vacancies) to improvethe conductivity and light transmissivity of lead titanate. The purpose of the theorystudying is to make guidance for getting the high light transmissivity and largeconductivity of lead titanate films in enperiment.Based on the first-principles calculations, the stability properties, geometryparameters, electronic and optical properties of N-doped tetragonal symmetry leadtitanate and cubic symmetry lead titanate have been investigated. Results shows that theTi-O bonds in lead titanate shows relatively strong ionic characteristics and Pb-O bondsshows less covalence characteristics, the band structures and density of states are alsostudied, the Fermi energy gets into the valance bands, so the N-doped PbTiO3shows atypical p-type semiconductor characteristic, the hole effective masses and the relativehole numbers are calculated based on Origin8.0. Due to the charge imbalance in thedoped crystal, it is likely to form oxygen vacancy in N-doped PbTiO3, theself-compensation effects are discussed, the formation of oxygen vacancy leads to theband gap narrowing, Fermi energy gets into the top of the valance band maximum, thereare almost no acceptors near the top of the valance bands, this illustrates that the p-typeconductivity is fully compensated by oxygen vacancy. When the spin-polarized densityfunctional theory calculations are considered, we find that the N-doped PbTiO3systemis spin-polarized, but there is no spin-polarized in N-doped PbTiO3with oxygenvacancy. The p-type N-doped PbTiO3with different doping concentrations have been studied by first-principles calculations, the band structures, hole effective masses,conductivities and optical absorption properties are studied. Results show that thevalues of the band gaps and the hole effective masses are decreased, the conductivitiesincrease firstly then decrease, when the N doping concentration is2.5at%, the p-typeconductivity is the biggest and the visible-light absorption can be enhanced most. Inaddition, the Pb, Ti and O vacancy in PbTiO3are studied, respectively. The formationenthalpy of Pb vacancy is smallest, so it is easy to form, in its band structure, Fermienergy enters into the valance bands, impurities form acceptor states, the band gap is1.677eV. The effective mass of PbTiO3within Pb vacancy is small, so its hole mobilityis big, in the solar energy of2eV, there is an absorption peak, at the region of5eV～10eV, the absorption is enhanced. The Ti vacancy shows the similar changing tendencywith Pb vacancy, but the absorption line at4eV～5.6eV is reduced. The Fermi energyof PbTiO3within oxygen vacancy gets into the conduction bands, it shows n-typesemiconductor characteristic, the band gap is1.554eV, the property of donor stateslocalization is strong, the absorption intensity in low energy region become weaken incomparison with that of PbTiO3within Pb vacancy or Ti vacancy.