X-ray absorption spectroscopy study of TiO_2-x thin films for memory applications

Carta, Daniela, Mountjoy, Gavin, Regoutz, Anna, Khiat, Ali, Serb, Alexantrou and Prodromakis, Themistoklis (2015) X-ray absorption spectroscopy study of TiO_2-x thin films for memory applications. The Journal of Physical Chemistry C, 1-40. (doi:10.1021/jp511739h).

Abstract

Metal-insulator-metal (MIM) devices based on titanium dioxide thin films exhibit resistive switching behaviour (RS) i.e. they have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage. This behaviour makes titanium dioxide thin films extremely valuable for memory applications. The physical mechanism behind RS remains a controversial subject but has been suggested that it could be interface-type, without accompanying structural changes of the oxide, or filament-type with formation of reduced titanium oxide phases in the film. In this work, X-ray absorption spectroscopy (XAS) at the Ti K-edge (4966 eV) was used to characterize the atomic-scale structure of a non-stoichiometric TiO_2-x thin film before and after annealing and for the first time after inclusion in a MIM device based on a Cr/Pt/TiO_2-x/Pt stack developed on an oxidised silicon wafer. The advantage of the XAS technique is that is element-specific. Therefore, by tuning the energy to the Ti K-edge absorption, contributions from the Pt, Cr and Si in the stack are eliminated. In order to investigate the structure of the film after electrical switching, XAS analysis at the Ti K-edge was again performed for the first time on the Cr/Pt/TiO_2-x/Pt stack in its virgin state and after switching to LRS by application of an appropriate bias. X-ray absorption near-edge structure (XANES) was employed to assess local coordination and oxidation state of the Ti and X-ray absorption fine structure (EXAFS) was used to assess bond distances, coordination numbers and Debye-Waller factors. XAS analysis revealed that the as-deposited film is amorphous with a distorted local octahedral arrangement around Ti (average Ti-O distance of 1.95 Å and coordination number of 5.2) and has a majority oxidation state of Ti⁺⁴ with a slight content of Ti⁺³. The film remains amorphous upon insertion into the stack structure and after electrical switching but crystallizes as anatase upon annealing at 600ºC. These results do not give any indication of the appearance of conducting filaments upon switching and are more compatible with homogeneous interface mechanisms.

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Contributors

Author: Themistoklis Prodromakis

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