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Electronic Conduction in Ti/Poly-TiO2/Ti Structures.

Hossein-Babaei F, Alaei-Sheini N - Sci Rep (2016)

Bottom Line: Containing no interface energy barrier, Ti/poly-TiO2/Ti devices demonstrate high resistance ohmic conduction at biasing fields below 5 × 10(6) V.m(-1); higher fields drive the samples to a distinctly nonlinear and hysteretic low resistance status.The observed threshold is two orders of magnitude smaller than the typical resistance switching fields reported for the nanosized single grain memristors.This is consistent with the smaller activation energies reported for the IOV motion on the rutile facets than its interior.

View Article: PubMed Central - PubMed

Affiliation: Electronic Materials Laboratory, Electrical Engineering Department, K. N. Toosi University of Technology, Tehran 16317-14191, Iran.

ABSTRACT
Recent intensive investigations on metal/metal oxide/metal structures have targeted nanometric single grain oxides at high electric fields. Similar research on thicker polycrystalline oxide layers can bridge the results to the prior literature on varistors and may uncover novel ionic/electronic features originating from the conduction mechanisms involving grain boundaries. Here, we investigate electronic conduction in Ti/poly-TiO2-x/Ti structures with different oxygen vacancy distributions and describe the observed features based on the motion and rearrangement of the ionized oxygen vacancies (IOVs) on the grain facets rather than the grain interiors. Containing no interface energy barrier, Ti/poly-TiO2/Ti devices demonstrate high resistance ohmic conduction at biasing fields below 5 × 10(6) V.m(-1); higher fields drive the samples to a distinctly nonlinear and hysteretic low resistance status. The observed threshold is two orders of magnitude smaller than the typical resistance switching fields reported for the nanosized single grain memristors. This is consistent with the smaller activation energies reported for the IOV motion on the rutile facets than its interior. The presented model describes the observed dependence of the threshold field on the relative humidity of the surrounding air based on the lower activation energies reported for the hydroxyl-assisted IOV motion on the rutile facets.

No MeSH data available.


Related in: MedlinePlus

The schematic presentation of the model used for describing the I–V diagram of B-samples.(a) The biasing field applied is insufficient to cause IOV motion; the electronic conduction takes place via both grains and grain boundaries; the device is at its HRS. (b) At higher biasing fields, IOVs adjacent to the anode move in the field direction via grainboundaries to create conductive filaments; the field intensity is still 100 times smaller than that required for IOV motion within the grains. (c) The IOV filament is growing via grain boundaries and the resistance of the sample is still determined by the thinning oxide region with normal grain boundaries. (d) The IOV filament connecting the anode to the cathode provides an easy route for the electrons; high current levels through the filament decrease the field intensity in the oxide layer preventing more IOV migration; the device is at its LRS. (e) The external field is removed and the repelling coulombic forces have ruptured the filament and increased the device resistance by almost two orders of magnitude; the device is back in its HRS. (f) The remaining filament segment is dissolved by thermal diffusion; applying a negative bias would bring the device back to its original status (a).
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f7: The schematic presentation of the model used for describing the I–V diagram of B-samples.(a) The biasing field applied is insufficient to cause IOV motion; the electronic conduction takes place via both grains and grain boundaries; the device is at its HRS. (b) At higher biasing fields, IOVs adjacent to the anode move in the field direction via grainboundaries to create conductive filaments; the field intensity is still 100 times smaller than that required for IOV motion within the grains. (c) The IOV filament is growing via grain boundaries and the resistance of the sample is still determined by the thinning oxide region with normal grain boundaries. (d) The IOV filament connecting the anode to the cathode provides an easy route for the electrons; high current levels through the filament decrease the field intensity in the oxide layer preventing more IOV migration; the device is at its LRS. (e) The external field is removed and the repelling coulombic forces have ruptured the filament and increased the device resistance by almost two orders of magnitude; the device is back in its HRS. (f) The remaining filament segment is dissolved by thermal diffusion; applying a negative bias would bring the device back to its original status (a).

Mentions: The model schematically presented in Fig. 7a–f describes all observations detailed above. At each cycle of the applied field, the IOV motion at the grain boundaries occurs once the field intensity reaches ca. 2.5 × 106 V/m. At higher field levels, the mobilized IOVs move along grain boundaries to form filaments in directions close to the applied field. The IOV population within the grains does not take part in this process as the applied field is much smaller than that required for their mobilization. The driving force for filament formation comes from the reduced field energy within the oxide layer. The electronic conduction through the formed filament reduces the sample resistance. This field reduction stops further IOV filament development, but filament dissolution occurs when the external field is below 106 V/m. The driving force for filament dissolution originates from the thermal diffusion of the IOVs assisted by the repelling coulombic forces. The hysteretic behavior of the device at high fields is attributed to the two different driving forces acting on the formation and dissolution stages of the IOV filaments. The strong frequency dependence and slowness of the observed phenomena are interpreted based on the fact that both the forming and dissolving of the IOV filaments involve the motion of oxygen ions. The profound asymmetry of the I–V characteristic in B-samples (Fig. 2d) is attributed to the uneven IOV distribution within the oxide layer. (This asymmetric I–V is distinctly different from the I–V asymmetry observed in Schottky type metal/semiconductor junctions262851). In all biasing conditions, the IOVs close to the positively biased electrode play a more constructive role in the filament formation. They are driven away from the positively biased electrode towards the interior of the oxide layer where they are most effective in filament formation. The IOVs adjacent to the cathode, on the other hand, get attracted to the electrode and are less effective in the process. The color coding used in Fig. 7 suggests larger IOV concentration in the grain boundaries than the grains, but the background literature is not clear on this point; opposite IOV concentration variations from inside to the surface of a grain has been reported for ZnO43 and SrTiO360, and those of TiO2 has not yet been experimentally clarified. The model presented here would remain untouched by the direction of those variations.


Electronic Conduction in Ti/Poly-TiO2/Ti Structures.

Hossein-Babaei F, Alaei-Sheini N - Sci Rep (2016)

The schematic presentation of the model used for describing the I–V diagram of B-samples.(a) The biasing field applied is insufficient to cause IOV motion; the electronic conduction takes place via both grains and grain boundaries; the device is at its HRS. (b) At higher biasing fields, IOVs adjacent to the anode move in the field direction via grainboundaries to create conductive filaments; the field intensity is still 100 times smaller than that required for IOV motion within the grains. (c) The IOV filament is growing via grain boundaries and the resistance of the sample is still determined by the thinning oxide region with normal grain boundaries. (d) The IOV filament connecting the anode to the cathode provides an easy route for the electrons; high current levels through the filament decrease the field intensity in the oxide layer preventing more IOV migration; the device is at its LRS. (e) The external field is removed and the repelling coulombic forces have ruptured the filament and increased the device resistance by almost two orders of magnitude; the device is back in its HRS. (f) The remaining filament segment is dissolved by thermal diffusion; applying a negative bias would bring the device back to its original status (a).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4940738&req=5

f7: The schematic presentation of the model used for describing the I–V diagram of B-samples.(a) The biasing field applied is insufficient to cause IOV motion; the electronic conduction takes place via both grains and grain boundaries; the device is at its HRS. (b) At higher biasing fields, IOVs adjacent to the anode move in the field direction via grainboundaries to create conductive filaments; the field intensity is still 100 times smaller than that required for IOV motion within the grains. (c) The IOV filament is growing via grain boundaries and the resistance of the sample is still determined by the thinning oxide region with normal grain boundaries. (d) The IOV filament connecting the anode to the cathode provides an easy route for the electrons; high current levels through the filament decrease the field intensity in the oxide layer preventing more IOV migration; the device is at its LRS. (e) The external field is removed and the repelling coulombic forces have ruptured the filament and increased the device resistance by almost two orders of magnitude; the device is back in its HRS. (f) The remaining filament segment is dissolved by thermal diffusion; applying a negative bias would bring the device back to its original status (a).
Mentions: The model schematically presented in Fig. 7a–f describes all observations detailed above. At each cycle of the applied field, the IOV motion at the grain boundaries occurs once the field intensity reaches ca. 2.5 × 106 V/m. At higher field levels, the mobilized IOVs move along grain boundaries to form filaments in directions close to the applied field. The IOV population within the grains does not take part in this process as the applied field is much smaller than that required for their mobilization. The driving force for filament formation comes from the reduced field energy within the oxide layer. The electronic conduction through the formed filament reduces the sample resistance. This field reduction stops further IOV filament development, but filament dissolution occurs when the external field is below 106 V/m. The driving force for filament dissolution originates from the thermal diffusion of the IOVs assisted by the repelling coulombic forces. The hysteretic behavior of the device at high fields is attributed to the two different driving forces acting on the formation and dissolution stages of the IOV filaments. The strong frequency dependence and slowness of the observed phenomena are interpreted based on the fact that both the forming and dissolving of the IOV filaments involve the motion of oxygen ions. The profound asymmetry of the I–V characteristic in B-samples (Fig. 2d) is attributed to the uneven IOV distribution within the oxide layer. (This asymmetric I–V is distinctly different from the I–V asymmetry observed in Schottky type metal/semiconductor junctions262851). In all biasing conditions, the IOVs close to the positively biased electrode play a more constructive role in the filament formation. They are driven away from the positively biased electrode towards the interior of the oxide layer where they are most effective in filament formation. The IOVs adjacent to the cathode, on the other hand, get attracted to the electrode and are less effective in the process. The color coding used in Fig. 7 suggests larger IOV concentration in the grain boundaries than the grains, but the background literature is not clear on this point; opposite IOV concentration variations from inside to the surface of a grain has been reported for ZnO43 and SrTiO360, and those of TiO2 has not yet been experimentally clarified. The model presented here would remain untouched by the direction of those variations.

Bottom Line: Containing no interface energy barrier, Ti/poly-TiO2/Ti devices demonstrate high resistance ohmic conduction at biasing fields below 5 × 10(6) V.m(-1); higher fields drive the samples to a distinctly nonlinear and hysteretic low resistance status.The observed threshold is two orders of magnitude smaller than the typical resistance switching fields reported for the nanosized single grain memristors.This is consistent with the smaller activation energies reported for the IOV motion on the rutile facets than its interior.

View Article: PubMed Central - PubMed

Affiliation: Electronic Materials Laboratory, Electrical Engineering Department, K. N. Toosi University of Technology, Tehran 16317-14191, Iran.

ABSTRACT
Recent intensive investigations on metal/metal oxide/metal structures have targeted nanometric single grain oxides at high electric fields. Similar research on thicker polycrystalline oxide layers can bridge the results to the prior literature on varistors and may uncover novel ionic/electronic features originating from the conduction mechanisms involving grain boundaries. Here, we investigate electronic conduction in Ti/poly-TiO2-x/Ti structures with different oxygen vacancy distributions and describe the observed features based on the motion and rearrangement of the ionized oxygen vacancies (IOVs) on the grain facets rather than the grain interiors. Containing no interface energy barrier, Ti/poly-TiO2/Ti devices demonstrate high resistance ohmic conduction at biasing fields below 5 × 10(6) V.m(-1); higher fields drive the samples to a distinctly nonlinear and hysteretic low resistance status. The observed threshold is two orders of magnitude smaller than the typical resistance switching fields reported for the nanosized single grain memristors. This is consistent with the smaller activation energies reported for the IOV motion on the rutile facets than its interior. The presented model describes the observed dependence of the threshold field on the relative humidity of the surrounding air based on the lower activation energies reported for the hydroxyl-assisted IOV motion on the rutile facets.

No MeSH data available.


Related in: MedlinePlus