Binary alloys of Ge and Te: order, voids, and the eutectic composition

Area of Science:

• Materials Science
• Computational Chemistry
• Condensed Matter Physics

Background:

• Understanding the atomic structure of amorphous germanium-tellurium (GeTe) alloys is crucial for their application in phase-change memory devices.
• Existing models, like the "8-N rule," may not fully capture the coordination behavior of elements in these complex alloys.

Purpose of the Study:

• To characterize the liquid and amorphous structures of Ge0.15Te0.85 and GeTe alloys.
• To investigate the coordination environments and bonding characteristics of Ge and Te atoms.
• To identify and quantify structural features such as cavities in these materials.

Main Methods:

• Combined density functional theory (DFT) and molecular dynamics (MD) simulations were employed.
• Analysis focused on coordination numbers, bonding angles, and local atomic environments.

Main Results:

• Tellurium (Te) atoms exhibit threefold coordination, deviating from the "8-N rule."
• Germanium (Ge) atoms display fourfold coordination with octahedral and tetrahedral bonding angles.
• The eutectic Ge0.15Te0.85 alloy shows a high prevalence of tetrahedral Ge coordination and contains significant, previously overlooked cavities (over 25% of the volume).
• Ge0.15Te0.85 exhibits a unique structure distinct from GeTe or pure Te, with no Te segregation.

Conclusions:

• The study provides a detailed atomic-level understanding of amorphous GeTe alloys.
• The findings challenge existing bonding rules and highlight the importance of cavities in the structure of Ge0.15Te0.85.
• These insights are vital for the rational design and optimization of GeTe-based materials for technological applications.