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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.


Top: Change in infrared reflectance spectra upon annealing for 27 hours at 353 K (left) and due to cooling from 353 K to 10 K (right) for amorphous Ag4In3Sb67Te26, exemplary for all three materials. Circles mark manually determined reflectance minima. The Al-reflectance forms an upper bound for the overall sample reflectance (light grey line). Each spectrum is fitted with the reflectance model including the dielectric function for the phase change material, as described in the methods. For clarity only the fit to the first spectrum during annealing at  and for cooling at  is shown (dashed black line). All fits are limited to the region of interband transitions (in this case 0.4–1 eV) Bottom: Change in the dielectric function  and the refractive index , which results from fitting the spectra in the top part. Despite the limited fit region, the model still gives a good description of  in the low energy range, as the two manually determined values for  (black circles) confirm. The low energy limit of  is used for a detailed investigation of  in Fig. 2. Applying the heuristic 10k criterion, bandgaps from different annealing times and temperatures can be compared easily among Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Here again the bandgap is only marked for the first spectrum of each series.
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f1: Top: Change in infrared reflectance spectra upon annealing for 27 hours at 353 K (left) and due to cooling from 353 K to 10 K (right) for amorphous Ag4In3Sb67Te26, exemplary for all three materials. Circles mark manually determined reflectance minima. The Al-reflectance forms an upper bound for the overall sample reflectance (light grey line). Each spectrum is fitted with the reflectance model including the dielectric function for the phase change material, as described in the methods. For clarity only the fit to the first spectrum during annealing at and for cooling at is shown (dashed black line). All fits are limited to the region of interband transitions (in this case 0.4–1 eV) Bottom: Change in the dielectric function and the refractive index , which results from fitting the spectra in the top part. Despite the limited fit region, the model still gives a good description of in the low energy range, as the two manually determined values for (black circles) confirm. The low energy limit of is used for a detailed investigation of in Fig. 2. Applying the heuristic 10k criterion, bandgaps from different annealing times and temperatures can be compared easily among Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Here again the bandgap is only marked for the first spectrum of each series.

Mentions: The results of measuring a 1 μm thick layer of phase change material on top of an aluminum mirror with FTIR spectroscopy is shown in the top part of Fig. 1 (see also supplementary Figs 1, 2). Before analyzing the experimental data quantitatively by fitting the spectra with a model, we give a qualitative overview of their core features. All recorded spectra, regardless of annealing or temperature variation, exhibit the same general pattern, i.e. characteristic minima in reflectance up to a certain energy. These reflectance minima can be explained by multiple reflections at the sample interfaces leading to interference. For energies far below the bandgap, absorption by the phase change material is negligible and the maximum sample reflectance is only limited by the aluminum mirror. Under this condition, a simple expression can be derived to determine values of at photon energies of minimum reflectance. These values for serve as reference points for later analysis. We obtain for the reflectance minimum at wavelength , with being the optical path length through the phase change layer. The imaginary part of the dielectric function in the index of refraction is negligible, allowing us to determine at the photon energies of minimum reflectance only based on the position of the minima and the thickness of the film . We determine values of for the first two minima, as can be seen in Fig. 1, since these energies are sufficiently far away from the bandgap. For energies near and above the bandgap, interband absorption in the phase change material increases and cannot be neglected anymore. Accordingly, the height of the reflectance maxima decreases with increasing energy up to the point where interference within the phase change layer and the related oscillations in the spectra vanish completely.


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: Change in infrared reflectance spectra upon annealing for 27 hours at 353 K (left) and due to cooling from 353 K to 10 K (right) for amorphous Ag4In3Sb67Te26, exemplary for all three materials. Circles mark manually determined reflectance minima. The Al-reflectance forms an upper bound for the overall sample reflectance (light grey line). Each spectrum is fitted with the reflectance model including the dielectric function for the phase change material, as described in the methods. For clarity only the fit to the first spectrum during annealing at  and for cooling at  is shown (dashed black line). All fits are limited to the region of interband transitions (in this case 0.4–1 eV) Bottom: Change in the dielectric function  and the refractive index , which results from fitting the spectra in the top part. Despite the limited fit region, the model still gives a good description of  in the low energy range, as the two manually determined values for  (black circles) confirm. The low energy limit of  is used for a detailed investigation of  in Fig. 2. Applying the heuristic 10k criterion, bandgaps from different annealing times and temperatures can be compared easily among Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Here again the bandgap is only marked for the first spectrum of each series.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Top: Change in infrared reflectance spectra upon annealing for 27 hours at 353 K (left) and due to cooling from 353 K to 10 K (right) for amorphous Ag4In3Sb67Te26, exemplary for all three materials. Circles mark manually determined reflectance minima. The Al-reflectance forms an upper bound for the overall sample reflectance (light grey line). Each spectrum is fitted with the reflectance model including the dielectric function for the phase change material, as described in the methods. For clarity only the fit to the first spectrum during annealing at and for cooling at is shown (dashed black line). All fits are limited to the region of interband transitions (in this case 0.4–1 eV) Bottom: Change in the dielectric function and the refractive index , which results from fitting the spectra in the top part. Despite the limited fit region, the model still gives a good description of in the low energy range, as the two manually determined values for (black circles) confirm. The low energy limit of is used for a detailed investigation of in Fig. 2. Applying the heuristic 10k criterion, bandgaps from different annealing times and temperatures can be compared easily among Ag4In3Sb67Te26, Ge2Sb2Te5 and GeTe. Here again the bandgap is only marked for the first spectrum of each series.
Mentions: The results of measuring a 1 μm thick layer of phase change material on top of an aluminum mirror with FTIR spectroscopy is shown in the top part of Fig. 1 (see also supplementary Figs 1, 2). Before analyzing the experimental data quantitatively by fitting the spectra with a model, we give a qualitative overview of their core features. All recorded spectra, regardless of annealing or temperature variation, exhibit the same general pattern, i.e. characteristic minima in reflectance up to a certain energy. These reflectance minima can be explained by multiple reflections at the sample interfaces leading to interference. For energies far below the bandgap, absorption by the phase change material is negligible and the maximum sample reflectance is only limited by the aluminum mirror. Under this condition, a simple expression can be derived to determine values of at photon energies of minimum reflectance. These values for serve as reference points for later analysis. We obtain for the reflectance minimum at wavelength , with being the optical path length through the phase change layer. The imaginary part of the dielectric function in the index of refraction is negligible, allowing us to determine at the photon energies of minimum reflectance only based on the position of the minima and the thickness of the film . We determine values of for the first two minima, as can be seen in Fig. 1, since these energies are sufficiently far away from the bandgap. For energies near and above the bandgap, interband absorption in the phase change material increases and cannot be neglected anymore. Accordingly, the height of the reflectance maxima decreases with increasing energy up to the point where interference within the phase change layer and the related oscillations in the spectra vanish completely.

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.