Optical absorption and small-polaron hopping in oxygen deficient and lithium-ion-intercalated amorphous titanium oxide films
2016 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 119, no 1, 015701Article in journal (Refereed) PublishedText
Optical absorption in oxygen-deficient and Li+-ion inserted titanium oxide films was studied in the framework of small-polaron hopping. Non-stoichiometric TiOy films with 1.68 <= y <= 2.00 were deposited by reactive DC magnetron sputtering and were subjected to electrochemical intercalation of Li+-ions and charge-balancing electrons to obtain LixTiOy films with 0.12 <= x <= 0.34. Dispersion analysis was applied to calculate the complex dielectric function epsilon((h) over bar omega) =epsilon(1) ((h) over bar omega) + i epsilon(2)((h) over bar omega) from numerical inversion of optical transmittance and reflectance spectra; a superposition of Tauc-Lorentz and Lorentz oscillator models was used for this purpose. Data on epsilon(2)((h) over bar omega) were employed to calculate the optical conductivity and fit this property to a small-polaron model for disordered systems with strong electron-phonon interaction and involving transitions near the Fermi level. The introduction of oxygen vacancies and/or Li+ insertion yielded band gap widening by similar to 0.20-0.35 eV, and both processes induced similar low-energy optical absorption. The small-polaron-based analysis indicated increases in the Fermi level by similar to 0.15-0.3 eV for sub-stoichiometric and/or Li+-inserted films. This suggests the existence of polaronic Ti3+ states in the lower part of the conduction band arising from transfer of electrons from oxygen vacancies and/or inserted Li+ species. The present article is a sequel to an earlier paper on oxygen-deficient and/or Li+-inserted amorphous WOy thin films and forms part of a comprehensive investigation of optical absorption in amorphous transition metal oxides with different valence states of the metallic ions.
Place, publisher, year, edition, pages
2016. Vol. 119, no 1, 015701
Condensed Matter Physics Engineering and Technology
IdentifiersURN: urn:nbn:se:uu:diva-276818DOI: 10.1063/1.4939091ISI: 000367902600048OAI: oai:DiVA.org:uu-276818DiVA: diva2:903499
FunderSwedish Research CouncilEU, European Research Council, 267234