Intrinsic defects and electronic conductivity of TaON: First-principles insights

Abstract

As a compound in between the tantalum oxide and nitride, the tantalum oxynitride TaON is expected to combine their advantages and act as an efficient visible-light-driven photocatalyst. In this letter, using hybrid functional calculations we show that TaON has different defect properties from the binary tantalum oxide and nitride: (i) instead of O or N vacancies or Ta interstitials, the ON antisite is the dominant defect, which determines its intrinsic n-type conductivity and the p-type doping difficulty; (ii) the ON antisite has a shallower donor level than O or N vacancies, with a delocalized distribution composed mainly of the Ta 5d orbitals, which gives rise to better electronic conductivity in the oxynitride than in the oxide and nitride. The phase stability analysis reveals that the easy oxidation of TaON is inevitable under O rich conditions, and a relatively O poor condition is required to synthesize stoichiometric TaON samples. The splitting of water into H 2 and O 2 using semiconductor photocatalysts has been considered as a clean and renewable way to utilize solar energy. One key issue for realizing this application is to search for suitable photoelectrode semiconductors which can absorb visible light. [1][2][3] Owing to their good chemical stability in aqueous solution, transition metal oxides are the subject of most previous studies.[2, 4] However, transition metal oxides usually have large band gaps (≥ 3 eV), limiting the absorption of visible light and setting an upper limit on the energy conversion efficiency at ∼2%. [2,4] In contrast, the transition metal nitrides have smaller band gaps due to N 2p orbitals being shallower than O 2p orbitals, but the easy oxidation of nitrides in aqueous solution makes them degraded quickly. Being in between the oxides and nitrides, the oxynitrides are believed to have intermediate properties, such as smaller band gaps than oxides and better stability than the nitrides.[2, 4, 5] A well known example, tantalum oxynitride TaON, which has a band gap around 2.5 eV, between those of Ta 2 O 5 (3.9 eV) and Ta 3 N 5 (2

Effects of intrinsic defects and extrinsic doping on the electronic and photocatalytic properties of Ta3N5

“…µi (i=Ta, N or O) refers to the atomic chemical potential of the type i atoms, and ni is the number of atoms of type i added to (ni<0) and/or removed from (ni>0) the perfect crystal. 29,30 However, its reliability and generality remain controversial. ∆V is the VBM correction term, which can be obtained by aligning the average electrostatic potentials between the defect contained supercell and the defect-free supercell.…”

“…15 Generally, for metal oxides and nitrides such as TiO 2 and GaN, the most common origin of the n-type conductivity is attributed to anion vacancies. 17 On the other hand, some experimental studies reported that O defects as unintentional impurities are included in Ta 3 N 5 inevitably during the synthesis even under the anaerobic condition. For example, previous theoretical calculation indicated that O N anti-site, as a dominant intrinsic defect in TaON, is responsible for its n-type conductivity.…”

Oxynitrogenography: Controlled Synthesis of Single-Phase Tantalum Oxynitride Photoabsorbers

“…For example, in our previous work we were unable to form single phase β-TaON thin films in either wet or dry ammonia. 27,36 Moreover, while a significant amount of literature is available on its crystal and electronic structure, 28,29,30,31 and photoelectrochemical performance, 8,10 to date a quantitative analysis of the carrier mobility and lifetime of β-TaON is missing.…”

First‐Principles Study on Site Preference of Aluminum Vacancy and Nitrogen Atoms in γ–Alon

“…Their theoretical study on Si 5 AlON 7 obtained the preferential distribution of O and N atoms in β‐ sialon . Chen and Wang carried out first–principles study on the intrinsic defects of TaON . Their results indicated that the O N antisite is the dominant defect in the binary tantalum oxide and nitride system.…”