Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3

Abstract

in this study we report on the investigation of epitaxially grown Sb 2 te 3 by employing fouriertransform 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. 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, Sb 2 Te 3 is a prototype as it is, together with GeTe and BiTe, one of the constituents of chalcogenide superlattices (CSLs) 1,2. Combining Sb 2 Te 3 with one of the above-mentioned crystals together in one lattice, creates a superlattice with intriguing properties different or greater than those of its individual components. In a recent study it has been demonstrated that a strong link exists among structural and thermoelectric properties in epitaxial Sb 2+x Te 3 , the latter having implications also in phase change SLs upon strain engineering in designed Sb 2+x Te 3 /GeTe multilayers 1. Improved thermoelectric properties by nanostructuring Sb 2 Te 3 /BiTe multilayer have already been demonstrated 3. Some more interesting properties include ultra-low power (non-melting) phase-change switching if compared to the established devices based on single active phase change material (PCM) and controlled topological states upon stack engineering 4-6. Those properties can be exploited for applications such as in photonic and electronic memories and processors, in Terahertz (THz) detection and to regenerate electricity from waste heat. However, the electro-optic properties of these chalcogenide topological insulators have barely been explored 7,8. The THz regime results ideal for the understanding of fundamental properties in such class of materials, as also shown by our previous works on GeSbTe alloys 9,10. Such measurement capability combined to a CSLs engineering could show future avenues for device improvements. To this purpose, in this wo...

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“…The photoinduced absorbance changes shown in Figure b,c clearly indicate softening of the phonon, Δω 0 , up to 2.6 meV (0.9 meV) for the undoped Sb 2 Te 3 (Ti 0.4 Sb 2 Te 3 ). Noticeably, the amount of this photoinduced Δω 0 is greater than or comparable to the thermal phonon shift (from 298 to 5 K) of ∼1.24 meV for Sb 3 Te 3 thin films . Moreover, as shown in Figure d,e, we track the temporal Δω 0 from the absorbance spectra over the entire τ range (see Tables S2-1 and S2-2 for fitting parameters).…”

“…The phonon absorption peak for the undoped Sb 2 Te 3 (Ti 0.4 Sb 2 Te 3 ) appears at ω 0 ≈ 8.6 meV (8 meV) in the equilibrium state. 24 The undoped Sb 2 Te 3 exhibits a 3-fold greater phonon shift, whereas Ti 0.4 Sb 2 Te 3 exhibits a more than 50-fold longer relaxation time. The photoinduced absorbance changes shown in Figure 3b,c clearly indicate softening of the phonon, Δω 0 , up to 2.6 meV (0.9 meV) for the undoped Sb 2 Te 3 (Ti 0.4 Sb 2 Te 3 ).…”

“…The resulting Δσ 1 primarily involves the responses of photoexcited carriers at low frequencies and IR-active phonons at ω 0 ≈ 8 meV (Figure a). The phonon absorption peak for the undoped Sb 2 Te 3 (Ti 0.4 Sb 2 Te 3 ) appears at ω 0 ≈ 8.6 meV (8 meV) in the equilibrium state …”

“…Noticeably, the amount of this photoinduced Δω 0 is greater than or comparable to the thermal phonon shift (from 298 to 5 K) of ∼1.24 meV for Sb 3 Te 3 thin films. 24 Moreover, as shown in Figure 3d,e, we track the temporal Δω 0 from the absorbance spectra over the entire τ range (see Tables S2-1 and S2-2 for fitting parameters). Within ∼1 ps of the optical pumping, the phonon softening occurs abruptly in both samples.…”

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“…For both MBT and MBST, f F ano shifts towards the lower frequency side with decreasing temperature. Such softening of the phonon mode upon cooling, has been seen in other TIs like Sb 2 Te 3 and Bi 2 Se 3 (both pristine and doped) samples [36,38,43]. The softening of the IR phonon mode, E u , upon cooling is called anomalous as it is in contrast to the usual temperature dependence (hardening upon cooling) of phonons driven via the anharmonic phonon decay process and the thermal lattice expansion [36,44,45].…”