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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

Top: Widening of the optical bandgap according to the α 10k criterion for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe during annealing at 353 K for 27 hours. For all three materials the bandgap increases during annealing in a  manner. Starting values, absolute and relative total changes are given in the figure legends. Bottom: The relative change of the apparent activation energy for conduction  upon annealing at 353 K for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Direct experimental data from resistance drift measurements (grey data points from47) are compared with what could be expected purely based on bandgap widening determined with infrared spectroscopy in this work (coloured crosses). The ratio  between the activation energy  and the bandgap  is determined at the beginning of annealing and kept fixed afterwards.
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f5: Top: Widening of the optical bandgap according to the α 10k criterion for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe during annealing at 353 K for 27 hours. For all three materials the bandgap increases during annealing in a manner. Starting values, absolute and relative total changes are given in the figure legends. Bottom: The relative change of the apparent activation energy for conduction upon annealing at 353 K for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Direct experimental data from resistance drift measurements (grey data points from47) are compared with what could be expected purely based on bandgap widening determined with infrared spectroscopy in this work (coloured crosses). The ratio between the activation energy and the bandgap is determined at the beginning of annealing and kept fixed afterwards.

Mentions: Next we apply the 10k criterion to the data measured during annealing. As expected from the earlier qualitative examination of the spectra, the temporal evolution of shows a monotonous increase for all three materials (Fig. 5, top). The bandgap widens in a manner with total changes ranging from 19 meV (3.4%) for AIST over to 23 meV (2.9%) for GST and 30 meV (3.4%) for GeTe. To relate these changes upon annealing to the activation energy for electrical conduction, we develop an expression for the activation energy with our results for the bandgap as input. In Boltzmann approximation the activation energy of a p-type semiconductor is given by the distance between valence band edge and Fermi level with 46. Motivated by the work of Oosthoek et al.27 we assume that the Fermi level maintains its relative position between valence and conduction band edge over the whole temperature range:


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

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

Top: Widening of the optical bandgap according to the α 10k criterion for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe during annealing at 353 K for 27 hours. For all three materials the bandgap increases during annealing in a  manner. Starting values, absolute and relative total changes are given in the figure legends. Bottom: The relative change of the apparent activation energy for conduction  upon annealing at 353 K for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Direct experimental data from resistance drift measurements (grey data points from47) are compared with what could be expected purely based on bandgap widening determined with infrared spectroscopy in this work (coloured crosses). The ratio  between the activation energy  and the bandgap  is determined at the beginning of annealing and kept fixed afterwards.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Top: Widening of the optical bandgap according to the α 10k criterion for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe during annealing at 353 K for 27 hours. For all three materials the bandgap increases during annealing in a manner. Starting values, absolute and relative total changes are given in the figure legends. Bottom: The relative change of the apparent activation energy for conduction upon annealing at 353 K for Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Direct experimental data from resistance drift measurements (grey data points from47) are compared with what could be expected purely based on bandgap widening determined with infrared spectroscopy in this work (coloured crosses). The ratio between the activation energy and the bandgap is determined at the beginning of annealing and kept fixed afterwards.
Mentions: Next we apply the 10k criterion to the data measured during annealing. As expected from the earlier qualitative examination of the spectra, the temporal evolution of shows a monotonous increase for all three materials (Fig. 5, top). The bandgap widens in a manner with total changes ranging from 19 meV (3.4%) for AIST over to 23 meV (2.9%) for GST and 30 meV (3.4%) for GeTe. To relate these changes upon annealing to the activation energy for electrical conduction, we develop an expression for the activation energy with our results for the bandgap as input. In Boltzmann approximation the activation energy of a p-type semiconductor is given by the distance between valence band edge and Fermi level with 46. Motivated by the work of Oosthoek et al.27 we assume that the Fermi level maintains its relative position between valence and conduction band edge over the whole temperature range:

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