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The role of vacancies and local distortions in the design of new phase-change materials

A Corrigendum to this article was published on 01 February 2007

Abstract

Phase-change materials are of tremendous technological importance ranging from optical data storage to electronic memories. Despite this interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. ‘GeSbTe’-based phase-change materials contain vacancy concentrations around 10% in their metastable crystalline structure. By using density-functional theory, the origin of these vacancies has been clarified and we show that the most stable crystalline phases with rocksalt-like structures are characterized by large vacancy concentrations and local distortions. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge–Te and Sb–Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by new experimental data.

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Figure 1: Rocksalt structure of Ge1Sb2Te4.
Figure 2: Defect formation energies.
Figure 3: COHP bonding analysis.
Figure 4: Pair correlation functions in the case of Ge vacancies.
Figure 5: XRD diffractograms of Ge1.5Sb2Te4, Ge2Sb2Te4 and Ge2Sb1Te4.
Figure 6: Optical properties and recrystallization behaviour.

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Acknowledgements

We gratefully acknowledge support from the Deutsche Forschungsgemenischaft (Wu 243/11). We thank J. Neugebauer for fruitful discussions, Umicore (Liechtenstein) for the careful preparation of phase-change targets and M. Heidelmann for carrying out the static tester measurements. Furthermore, we thank the Rechenzentrum of the RWTH Aachen for providing computer time and technical support.

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D.L. carried out the plane-wave pseudopotential calculations (structural relaxation, formation energies); M.G. carried out the calculations for the quantum-chemical bonding analysis (COHP); D.W. carried out the experiments (XRD, ellipsometry, Kissinger analysis, recrystallization behaviour); M.W. initialized this research project and M.W., W.W. and R.D. carried out the analysis of the data.

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Correspondence to Matthias Wuttig.

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Wuttig, M., Lüsebrink, D., Wamwangi, D. et al. The role of vacancies and local distortions in the design of new phase-change materials. Nature Mater 6, 122–128 (2007). https://doi.org/10.1038/nmat1807

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