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Cr-Triggered Local Structural Change in Cr2Ge2Te6 Phase Change Material
Shogo Hatayama1, Yi Shuang1, Paul Fons2,3
1Department of Materials Science, Graduate School of Engineering , Tohoku University , 6-6-11, Aoba-yama , Aoba-ku, Sendai 980-8579 , Japan.
ACS Applied Materials & Interfaces
|October 25, 2019
Summary
The crystalline phase of chromium germanium telluride (Cr2Ge2Te6) shows higher resistance due to filled chromium vacancies. This resistance increase is linked to chromium atom diffusion from nanoclusters, impacting electronic states.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Solid State Chemistry
Background:
- Chromium germanium telluride (Cr2Ge2Te6) is a phase change material.
- It exhibits higher resistivity in the crystalline phase than the amorphous phase.
- Cr2Ge2Te6 demonstrates ultralow operation energy for amorphization.
Purpose of the Study:
- Investigate the origin of increased resistance in crystalline Cr2Ge2Te6 compared to its amorphous state.
- Elucidate the phase change mechanism in terms of local structural and electronic state changes.
- Understand the role of chromium nanoclusters and vacancies in the material's properties.
Main Methods:
- Density of states measurements at the Fermi level.
- Annealing experiments at varying temperatures.
- Photoelectron spectroscopy (Cr 2s core level).
- Analysis of local structure and coordination numbers.
Main Results:
- Density of states at the Fermi level decreased with annealing, becoming negligible at 380 °C.
- Fermi level shifted to the band gap center, increasing resistance.
- Phase change involved a metastable crystalline phase.
- Chromium vacancies, induced by Cr nanoclusters, generated hole carriers (p-type conduction).
- Cr-Cr bonds decreased, while Cr-Te bonds increased upon annealing.
Conclusions:
- Increased resistance in crystalline Cr2Ge2Te6 is attributed to the filling of Cr vacancies.
- Chromium atoms diffuse from Cr nanoclusters to fill these vacancies.
- This process alters the local structure and electronic states, leading to higher resistivity.
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