Abstract:Phase change materials such as pseudobinary GeTe-Sb2Te3 (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge2Sb2Te5 (GST225), and Sb2Te3. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb2Te3 that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225… Show more
“…1 a, the plasma frequency ω p,D controls the total transmittance value and Γ D the transmittance slope increase over the considered bandwidth. At room temperature, the value of ω p,D / 2π = 7,305 cm −1 is remarkably close to the value determined from Hall effect data 19 in bulk material. Moreover, ω p,D is related to the free carrier concentration and their effective mass m * via .…”
Section: Resultssupporting
confidence: 83%
“…Moreover, recent density functional theory (DFT) calculations show that the band gap of Sb 2 Te 3 increases with increasing temperature due to the thermal expansion of Sb 2 Te 3 18 and such raise of the bandgap would result in a reduction of the free carrier concentration. Experimental verification of the theory can be found in Boschker et al 19 . Figure 1 (a) Temperature dependent THz transmittance change of Sb 2 Te 3 .…”
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...
“…1 a, the plasma frequency ω p,D controls the total transmittance value and Γ D the transmittance slope increase over the considered bandwidth. At room temperature, the value of ω p,D / 2π = 7,305 cm −1 is remarkably close to the value determined from Hall effect data 19 in bulk material. Moreover, ω p,D is related to the free carrier concentration and their effective mass m * via .…”
Section: Resultssupporting
confidence: 83%
“…Moreover, recent density functional theory (DFT) calculations show that the band gap of Sb 2 Te 3 increases with increasing temperature due to the thermal expansion of Sb 2 Te 3 18 and such raise of the bandgap would result in a reduction of the free carrier concentration. Experimental verification of the theory can be found in Boschker et al 19 . Figure 1 (a) Temperature dependent THz transmittance change of Sb 2 Te 3 .…”
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...
“…Hall measurements revealed that GeTe films are intrinsically p-type doped (p ≈ 2 × 10 20 cm −3 -a typical concentration for this material 35 ) due to the presence of about 10% Ge vacancies 48 . The mobility at room temperature 50 is 95 cm 2 V −1 s −1 .…”
“…The crystallization temperature for GeTe is ≈ 180 • C (453 K) [9,10]. GeTe has also been implemented with a superlattice configuration as GeTe-Sb 2 Te 3 that are extensively used for their application in optical as well as PCM storage devices [11]. Further, the interface between the GeTe and Sb 2 Te 3 in the superlattice configuration is found to control the phase transition, accompanied by a reduced entropy loss, which helps in making fast and efficient PCMs [12].…”
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