Thermal conductivity of GeTe/Sb$_{2}$Te$_{3}$ superlattices measured by coherent phonon spectroscopy

TL;DR

This study measures the lattice thermal conductivity of GeTe/Sb2Te3 superlattices using femtosecond coherent phonon spectroscopy, revealing insights into phonon scattering mechanisms across different phases and temperatures.

Contribution

It provides a novel experimental approach to evaluate thermal conductivity in superlattices via coherent phonon spectroscopy and applies Debye theory to interpret phonon scattering effects.

Findings

Amorphous superlattices show weak temperature dependence of thermal conductivity.

Coherent optical phonon modes are observed at THz frequencies.

Phonon-defect scattering dominates in amorphous structures.

Abstract

We report on evaluation of lattice thermal conductivity of GeTe/Sb$_{2}$Te$_{3}$ superlattice (SL) by using femtosecond coherent phonon spectroscopy at various lattice temperatures. The time-resolved transient reflectivity obtained in amorphous and crystalline GeTe/Sb$_{2}$Te$_{3}$ SL films exhibits the coherent $A_{1}$ optical modes at terahertz (THz) frequencies with picoseconds dephasing time. Based on the Debye theory, we calculate the lattice thermal conductivity, including scattering by grain boundary and point defect, umklapp process, and phonon resonant scattering. The results indicate that the thermal conductivity in amorphous SL is less temperature dependent, being attributed to dominant phonon-defect scattering.