Electronic Structure of Tantalum Oxide with Oxygen Vacancy and Polyvacancy, Oxygen Interstitial, Tantalum Interstitial and Tantalum Substituting Oxygen

Abstract

The electronic properties of Ta2O5 are limited by its defects. The defects are problem if Ta2O5 is used in DRAM or as a blocking layer in a flash charge trapping memory (CTM), because it leads to a high conductivity via the traps and the threshold voltage instability. On the contrary, the high defects concentration opens the possibility of using Ta2O5 as a storage medium in the CTM [1] and resistive random-access memory (ReRAM) [2]. The aim of the study is the investigation of the electronic structure of O vacancy (VO), O polyvacancy, interstitial O and Ta atoms, O-to-Ta substitution in Ta2O5 to clarify the trap nature responsible for the charge localization and transport. The simulations carried out for the λ-Ta2O5 [3] 168-atom supercell in terms of the DFT in Quantum-ESPRESSO code. The calculated bandgap of λ-Ta2O5 is 4.2 eV. All types of native defects, except for the O interstitial, form defect states into the λ-Ta2O5 bandgap. The value of trap thermal ionization energy 0.5 eV are comparable to the experimental ones 0.7-0.8 eV [4], and optical trap energy 0.9 eV for the VO indicate that the VO in Ta2O5 can act as the charge trap, and participate in charge transport. The width of density of states defect peaks indicate the charge localization in space. The electron localization on the VO in Ta2O5 is due to the polaronic effect. The negative charge distributed between the nearest Ta atoms and it reflects the bonding character of the Ta orbitals charge density. Its allows us to conclude that the chains of closely located oxygen vacancies in TaOx can act as a conductive shunt and participate in the resistive switching. Each subsequent O vacancy forms near the already existing one, and no more than 2 removed O atoms, related to Ta atom. The O polyvacancy forms a filled defective band in Ta2O5 bandgap. The work is supported by the Russian Science Foundation grant #17-72-10103. [1] K. Hota, F.H. Alshammari, K. N. Salama et. al, Acs. Appl. Mater. Inter. 9, 21856 (2017). [2] M. Kim, S.R. Lee, S. Kim, M. Chang et. al, Adv. Funct. Mater. 25, 1527 (2015). [3] H. Lee, J. Kim, S.J. Kim et. al, Phys. Rev. Lett. 110, 235502 (2013). [4] V. Egorov, D.S. Kuzmichev, P.S. Chizhov et. al, Acs. Appl. Mater. Inter. 9, 13286 (2017). Figure 1