Activation Energy of Metastable Amorphous Ge2Sb2Te5 from Room Temperature to Melt

TL;DR

This study investigates the activation energy and electrical behavior of metastable amorphous Ge2Sb2Te5 across a broad temperature range, revealing its transition from semiconductor to metallic liquid and providing insights into its conduction mechanisms.

Contribution

It provides a detailed analysis of the temperature-dependent activation energy and resistivity of amorphous GST, linking its electronic properties to phase transitions and super-cooled liquid behavior.

Findings

Activation energy peaks at ~377 meV near 465 K.

GST transitions from semiconducting to metallic phase around 930 K.

Resistivity decline follows an exponential trend consistent with super-cooled liquid model.

Abstract

Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K - 858 K) and the metastable amorphous phase can be treated as a super-cooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to ~377 meV at ~465 K and reaching zero at ~930 K, using a reference activation energy of 111 meV (3kBT/2) at melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ~ 858 K and transitions from the semiconducting-liquid phase to the metallic-liquid phase at ~ 930 K at equilibrium. The simultaneous Seebeck (S) and resistivity versus temperature measurements of amorphous-fcc mixed-phase GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with degenerate semiconductors, and the dS/dT and room temperature activation energy show a linear correlation. The single-crystal fcc is calculated to have dS/dT = 0.153 {\mu}V/K for an activation energy of zero and a Fermi level 0.16 eV below the valance band edge.