Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3

V Bragaglia,D Schick,J E Boschker,M Ramsteiner,R Calarco,K Holldack,R Mitzner

doi:10.1038/s41598-020-69663-y

Abstract

In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers.

Highlights

Electronic and lattice dynamics in chalcogenide-based materials are important factors in the performance of opto-electrical data-storage media and thermoelectric devices
In this work we present a dedicated study in the THz range of the S b2Te3 constituent which is paramount for the study of chalcogenide superlattices (CSLs) dielectric properties and its carrier and lattice dynamics upon laser excitation
The absorption feature at ~ 62 cm−1 is assigned to the IR-active Eu phonon mode, which shows a peculiar softening of 10 cm−1 wavenumbers upon temperature decrease

Summary

Introduction

Electronic and lattice dynamics in chalcogenide-based materials are important factors in the performance of opto-electrical data-storage media and thermoelectric devices Among these materials, Sb2Te3 is a prototype as it is, together with GeTe and BiTe, one of the constituents of chalcogenide superlattices (CSLs)[1,2]. The THz regime results ideal for the understanding of fundamental properties in such class of materials, as shown by our previous works on GeSbTe a lloys[9,10] Such measurement capability combined to a CSLs engineering could show future avenues for device improvements. Rather large concentration of p-type carriers around 1 020 cm−3 1 due to the presence of native antisite defects–Sb atoms occupying Te lattice sites–and is diamagnetic

Methods

Results

Conclusion