The phenomenon of resistivity change in transition metal oxide insulators under an electric-field was first demonstrated back in 1962. More than thirty years later, the concept of resistive switching was proposed resulting in a new field of research with applications towards non-volatile memory [1].
Among a wide group of materials exhibiting resistive switching is the binary metal oxide of TaOx, which is studied widely for resistance-change random access memory (RRAM). One major reason for the Ta-O system being a leading candidate for RRAM applications is because it contains only a single equilibrium oxide phase, tantalum pentoxide (Ta2O5), which allows an excellent switching characteristics such as very high endurance [2].
Classified as a Fermi-glass material, amorphous Ta2O5, is an insulator in which electrical conduction is by a variable-range hopping (VRH) mechanism. Thus, according to Mott`s theory – the electrical conductance is strongly dependent on the hopping length between one conduction-center to another [3].
We investigate the effect of oxygen concentration during sputter deposition (O/Ar flow-rate) on the structure and properties of the amorphous TaOx thin films: electrical resistivity and the electronic band structure.
Related to Mott`s equation, the temperature-dependent hopping lengths must be evaluated in different Ta/O systems in order to show a trend between the electronic properties and the amorphous structure.
Oxygen vacancies serve as conduction-center for the VRH mechanism, so the hopping length can be determined from the inter-atomic distances in the short-range order of the Ta oxide. Moreover, since the memristive behavior exists only in a nano-layered structures, transmission electron microscopy (TEM) is a favorable method for characterizing the short range structural order. Illuminating a nanoscale region of the amorphous thin layer with a parallel electron beam in order to collect an electron diffraction, enables to calculate the radial distribution function (RDF).
![(left) - bright field TEM image of TaOx amorphous thin film sputter deposited with a 30%-Oxygen-flow. (right) radial distribution function curves of Ta2O5 based on micro-beam electron diffraction and selected area electron diffraction [5]. Conductivity 𝝈 is proportional to the hopping probability as 𝜶 is the decay length of the localized-electron wave function in r.s, W is the activation energy for hopping, and (𝑻) is the hopping length [3].](https://events.eventact.com/duin/29086/11.JPG)
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Figure 1: (left) - bright field TEM image of TaOx amorphous thin film sputter deposited with a 30%-Oxygen-flow. (right) radial distribution function curves of Ta2O5 based on micro-beam electron diffraction and selected area electron diffraction [5]. Conductivity 𝝈 is proportional to the hopping probability as 𝜶 is the decay length of the localized-electron wave function in r.s, W is the activation energy for hopping, and (𝑻) is the hopping length [3].
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The chemical bonding states and electronic band structure were characterized by X-ray photoemission spectroscopy (core-level and valence band measurements), Resistivity was measured by a four-point-probe method. We will discuss the role of oxygen flow during sputter deposition on the structure and functional properties of TaOx thin films.
References
[1] D. Zhu, Yi Li, W. Shen, Z. Zhou, L. Liu , and X. Zhang, Journal of Semiconductors, Vol. 38 (2017).
[2] I. Goldfarb, F. Miao, J. Joshua Yang, W. Yi, J.P. Strachan, M.-X. Zhang, M.D. Pickett, G. Medeiros-Ribeiro, R. Stanley Williams. Appl. Phys A, Vol. 107 (2012).
[3] N.F. Mott, E.A. Davies, Electronic Processes in Non-crystalline Materials, Clarendon (1979).
[4] R. Bassiri, K. B. Borisenko, D. J. H. Cockayne, J. Hough1, I. MacLaren, and S. Rowan. Applied Physics Letters, Vol. 98 (2011).
[5] J. Shanmugam, K. B. Borisenko, Y. Chou, A. I. Kirkland, SoftwareX, Vol. 6 (2017)
