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Relation between bandgap and resistance drift in amorphous phase change materials.

Rütten M, Kaes M, Albert A, Wuttig M, Salinga M - Sci Rep (2015)

Bottom Line: A widening of the bandgap upon annealing accompanied by a decrease of the optical dielectric constant ε∞ is observed for all three materials.Quantitative comparison with experimental data for the apparent activation energy of conduction reveals that the temporal evolution of bandgap and activation energy can be decoupled.The case of Ag4In3Sb67Te26, where the increase of activation energy is significantly smaller than the bandgap widening, demonstrates the possibility to identify new phase change materials with reduced resistance drift.

View Article: PubMed Central - PubMed

Affiliation: Institute of Physics 1A, RWTH Aachen University, Sommerfeldstrasse 14, 52074 Aachen, Germany.

ABSTRACT
Memory based on phase change materials is currently the most promising candidate for bridging the gap in access time between memory and storage in traditional memory hierarchy. However, multilevel storage is still hindered by the so-called resistance drift commonly related to structural relaxation of the amorphous phase. Here, we present the temporal evolution of infrared spectra measured on amorphous thin films of the three phase change materials Ag4In3Sb67Te26, GeTe and the most popular Ge2Sb2Te5. A widening of the bandgap upon annealing accompanied by a decrease of the optical dielectric constant ε∞ is observed for all three materials. Quantitative comparison with experimental data for the apparent activation energy of conduction reveals that the temporal evolution of bandgap and activation energy can be decoupled. The case of Ag4In3Sb67Te26, where the increase of activation energy is significantly smaller than the bandgap widening, demonstrates the possibility to identify new phase change materials with reduced resistance drift.

No MeSH data available.


Related in: MedlinePlus

Imaginary part of the dielectric function  needed in addition to the OJL model in order to match the depth of the first measured reflectance minimum.This quantity’s dependence on temperature before (open black symbols) and after annealing (filled black symbols) is shown together with its evolution during the intermediate annealing at 353 K (coloured symbols).
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f3: Imaginary part of the dielectric function needed in addition to the OJL model in order to match the depth of the first measured reflectance minimum.This quantity’s dependence on temperature before (open black symbols) and after annealing (filled black symbols) is shown together with its evolution during the intermediate annealing at 353 K (coloured symbols).

Mentions: Before turning towards the analysis of , we take a look at the fit quality on the basis of Fig. 1. While the data within the region of interband transitions starting at 0.4 eV are described very well by the model, for lower energies the modelled reflectance is too high. This is especially visible at the first two minima at the example of AIST, but can be observed for GeTe and GST as well. We conclude from this overestimated reflectance that there is a contribution of absorption present in the phase change material, which is not considered in the OJL model. To find out about the evolution of this discrepancy between data and model during annealing and cooling, we determine the amount of missing absorption in the OJL model. This is done for the first measured reflectance minimum, where the discrepancy is most obvious and the influence of the interband transitions is smallest. At this point we can increase manually until the modelled reflectance is low enough to match the measured reflectance. This gives an estimation of how much additional contribution would be required to describe the experimental data (Fig. 3). It reveals that quite small deviations in can cause pronounced differences at the reflectance minima of the measured infrared spectra. Viewing our samples as resonators for infrared light, the remarkable sensitivity of this method is a consequence of a high quality factor also manifested in the sharpness of the minima. The pronounced changes in refractive index at the phase change material’s interfaces, both with air and with the metal, effectively extend the optical path through the thin film and thus enhance the interaction of the light with the investigated material. The absolute amount of additional of AIST is roughly one order of magnitude larger than for GeTe and GST. The relative temperature dependences of additional are similarly strong for all three materials with decreases ranging between 45% and 71%. Aside from this temperature dependence we observe a reduction of additional upon annealing by 5 to 8%.


Relation between bandgap and resistance drift in amorphous phase change materials.

Rütten M, Kaes M, Albert A, Wuttig M, Salinga M - Sci Rep (2015)

Imaginary part of the dielectric function  needed in addition to the OJL model in order to match the depth of the first measured reflectance minimum.This quantity’s dependence on temperature before (open black symbols) and after annealing (filled black symbols) is shown together with its evolution during the intermediate annealing at 353 K (coloured symbols).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4664898&req=5

f3: Imaginary part of the dielectric function needed in addition to the OJL model in order to match the depth of the first measured reflectance minimum.This quantity’s dependence on temperature before (open black symbols) and after annealing (filled black symbols) is shown together with its evolution during the intermediate annealing at 353 K (coloured symbols).
Mentions: Before turning towards the analysis of , we take a look at the fit quality on the basis of Fig. 1. While the data within the region of interband transitions starting at 0.4 eV are described very well by the model, for lower energies the modelled reflectance is too high. This is especially visible at the first two minima at the example of AIST, but can be observed for GeTe and GST as well. We conclude from this overestimated reflectance that there is a contribution of absorption present in the phase change material, which is not considered in the OJL model. To find out about the evolution of this discrepancy between data and model during annealing and cooling, we determine the amount of missing absorption in the OJL model. This is done for the first measured reflectance minimum, where the discrepancy is most obvious and the influence of the interband transitions is smallest. At this point we can increase manually until the modelled reflectance is low enough to match the measured reflectance. This gives an estimation of how much additional contribution would be required to describe the experimental data (Fig. 3). It reveals that quite small deviations in can cause pronounced differences at the reflectance minima of the measured infrared spectra. Viewing our samples as resonators for infrared light, the remarkable sensitivity of this method is a consequence of a high quality factor also manifested in the sharpness of the minima. The pronounced changes in refractive index at the phase change material’s interfaces, both with air and with the metal, effectively extend the optical path through the thin film and thus enhance the interaction of the light with the investigated material. The absolute amount of additional of AIST is roughly one order of magnitude larger than for GeTe and GST. The relative temperature dependences of additional are similarly strong for all three materials with decreases ranging between 45% and 71%. Aside from this temperature dependence we observe a reduction of additional upon annealing by 5 to 8%.

Bottom Line: A widening of the bandgap upon annealing accompanied by a decrease of the optical dielectric constant ε∞ is observed for all three materials.Quantitative comparison with experimental data for the apparent activation energy of conduction reveals that the temporal evolution of bandgap and activation energy can be decoupled.The case of Ag4In3Sb67Te26, where the increase of activation energy is significantly smaller than the bandgap widening, demonstrates the possibility to identify new phase change materials with reduced resistance drift.

View Article: PubMed Central - PubMed

Affiliation: Institute of Physics 1A, RWTH Aachen University, Sommerfeldstrasse 14, 52074 Aachen, Germany.

ABSTRACT
Memory based on phase change materials is currently the most promising candidate for bridging the gap in access time between memory and storage in traditional memory hierarchy. However, multilevel storage is still hindered by the so-called resistance drift commonly related to structural relaxation of the amorphous phase. Here, we present the temporal evolution of infrared spectra measured on amorphous thin films of the three phase change materials Ag4In3Sb67Te26, GeTe and the most popular Ge2Sb2Te5. A widening of the bandgap upon annealing accompanied by a decrease of the optical dielectric constant ε∞ is observed for all three materials. Quantitative comparison with experimental data for the apparent activation energy of conduction reveals that the temporal evolution of bandgap and activation energy can be decoupled. The case of Ag4In3Sb67Te26, where the increase of activation energy is significantly smaller than the bandgap widening, demonstrates the possibility to identify new phase change materials with reduced resistance drift.

No MeSH data available.


Related in: MedlinePlus