Correlating ultrafast calorimetry, viscosity, and structural measurements in liquid GeTe and Ge15Te85

Hans Weber,Jiri Orava,Ivan Kaban,Julian Pries,A Lindsay Greer

doi:10.1103/physrevmaterials.2.093405

Abstract

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped ${\mathrm{Sb}}_{2}\mathrm{Te}$ and ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$, the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The differences in structures and dynamics of liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$ appear closely correlated to their distinctly different crystallization mechanisms.

Highlights

Phase-change memory (PCM), based on chalcogenides such as Ge-Sb-Te and Ag-In-Sb-Te, is a resistive-type memory with Joule-heating-induced reversible glassy-tocrystalline transitions [1]
Two conflicting requirements must be met in PCM operation: the crystallization time must be short at elevated temperatures, preferably below ∼1 ns [3,4,5], but the glass must be resistant to spontaneous crystallization at and to some extent above room temperature for data retention
The present work shows that the trends in the temperaturedependent viscosity, measured directly and inferred from calorimetry, can be correlated with the temperature evolution of the structure factor in the liquid chalcogenides

Summary

Introduction

Phase-change memory (PCM), based on chalcogenides such as Ge-Sb-Te and Ag-In-Sb-Te, is a resistive-type memory with Joule-heating-induced reversible glassy-tocrystalline transitions [1]. The phase change (PC) of interest is from the glass to the crystal via supercooled liquid Change in this direction is the SET (writing) operation in PCM and crystallization, characterized by the crystal growth rate (U ), is the rate-limiting step.

Methods

Results

Conclusion