Crystalline phases in the GeSb[sub 2]Te[sub 4] alloy system: Phase transitions and elastic properties (original) (raw)
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Elastic properties and lattice thermal conductivity of amorphous Ge2Sb2Te5 and GeTe thin films
This study reports on the elastic properties and the lattice thermal conductivity of amorphous Ge 2 Sb 2 Te 5 and GeTe thin films using surface Brillouin scattering. It is demonstrated that this method allows for the determination of the isotropic elastic constants from the measured surface acoustic phonon frequencies. We found elastic constants of C 11 ¼ 48:7/ 43:6 and C 44 ¼ 14:3/9:4 GPa for GeTe and Ge 2 Sb 2 Te 5 , respectively. These results suggest acoustic hardening in GeTe compared to Ge 2 Sb 2 Te 5 films, and this was supported by the derived, shear, and Young's moduli. The measured longitudinal and transverse velocities were used to determine the lower limit of the lattice thermal conductivity. In general, both chalcogenides alloys exhibit low lattice thermal conductivities of κ min , 0:50 W m À1 K À1. This could be beneficial for thermal management in phase-change memory devices and for thermoelectric application. Published under license by AIP Publishing. https://doi.
Thermal analysis of quaternary Ge–Se–Sb–Te chalcogenide alloys
Journal of Thermal Analysis and Calorimetry, 2014
Tellurium-based glasses are suitable for storage devices due to their rapid amorphous-to-crystalline transformation. Alloys of Ge 19-y Se 63.8 Sb 17.2 Te y (y = 0, 2, 4, 6, 8, 10 at.%) have been synthesized using melt quench technique. Glass transition and crystallization kinetics of alloys have been investigated using differential thermal analysis at different heating rates (5, 10, 15, and 20 K min -1 ). The thermal stability of synthesized alloys has been investigated. Resistance to devitrification has been analyzed on the basis of activation energy for crystallization.
Indium based chalcogenides have an adequate potential in nonlinear and optoelectronic applications. Sb 10 Se 65 Ge 25-y In y (y = 0, 3, 6, 9, 12, 15) system has been studied theoretically for physical parameters. The connectivity of the system has been discussed in terms of average coordination number and total number of constraints which also influence the mean bond energy and cohesive energy of the system. Energy band gap has been correlated to average single bond energy and electronegativity.
In situ dynamic HR-TEM and EELS study on phase transitions of Ge2Sb2Te5 chalcogenides
Ultramicroscopy, 2008
The phase transition phenomena of Ge2Sb2Te5 chalcogenides were investigated by in situ dynamic high-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS). A 300kV field emission TEM and a 1250kV high voltage TEM were employed for the in situ heating experiments from 20 to 500 degrees C for undoped and 3wt% nitrogen-doped Ge2Sb2Te5 thin films deposited by DC sputtering. Crystallization of amorphous Ge2Sb2Te5 to its cubic structure phase started at 130 degrees C and then rapid crystal growth developed from cubic to hexagonal phase in the range of 130-350 degrees C; finally, the hexagonal crystals started to melt at 500 degrees C. For nitrogen-doped Ge2Sb2Te5, its crystallization from amorphous film occurred at higher temperature of ca. 200 degrees C, and the cubic and hexagonal phases were usually formed simultaneously without significant growth of crystals at further heating to 400 degrees C. EELS measurements showed that the electronic str...
Journal of Applied Physics, 2005
Ge-Sb-Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge 2 Sb 2 Te 5 , in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship ␣h ϰ ͑h − E g opt ͒ 2 , which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu ͓Tauc et al., Phys. Status Solidi 15, 627 ͑1966͔͒, and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorphous, face-centered-cubic, and hexagonal phases are 0.7, 0.5, and 0.5 eV, respectively. The subgap absorption in the amorphous phase shows an exponential decay with an Urbach slope of 81 meV. We measured the photoconductivity of amorphous Ge 2 Sb 2 Te 5 and determined a mobility-lifetime product of 3 ϫ 10 −9 cm 2 / V. The spectral photoconductivity shows a threshold at about 0.7 eV, in agreement with our analysis of the optical band gap. The face-centered-cubic and hexagonal phases are highly conductive and do not show freeze-out; even at 5 K the density of free carriers remains at 10 19 -10 20 cm −3 , so these are degenerate semiconductors in which the Fermi level resides inside a band. In the hexagonal phase, the effect of free electrons on the Hall coefficient is significant at high temperatures. When the Hall data are fitted using the two-carrier analysis, the hole mobility is found to decrease slowly with temperature, as expected. The considerations discussed in this paper can be readily applied to study related chalcogenide materials.
Journal of Applied Physics, 2002
The crystal structures of GeSb 2 Te 4 , Ge 2 Sb 2 Te 5 , and Ge 3 Sb 2 Te 6 were determined using electron diffraction and high-resolution transmission electron microscopy. The structure determined for the former two crystals deviates from the ones proposed in the literature. These crystal structures were developed jointly upon cooling of liquid Ge 2 Sb 2 Te 5. A stacking disorder parallel to the basal plane was observed that increases with increasing cooling rates. For the Ge x Sb 2 Te 3ϩx (xϭ1,2,3) crystals it is shown that an a,b,c stacking holds with an alternating stacking of x GeTe double layers identically present in binary GeTe and one Te-Sb-Te-Te-Sb-repeat unit also present in binary Sb 2 Te 3. A stacking disorder is a logical consequence of building crystals with these two principal units. On the other hand, it is likely that all stable crystals of the Ge-Sb-Te systems are an ordered sequence of these two units. Some of the implications of these findings of the stable and metastable crystal structures that develop from amorphous Ge 2 Sb 2 Te 5 are presented so as to understand the crucial crystallization process in Ge 2 Sb 2 Te 5 phase change material.
Inorganic chemistry, 2017
We demonstrate that pressure-induced amorphization in Ge-Sb-Te alloys across the ferroelectric-paraelectric transition can be represented as a mixture of coherently distorted rhombohedral Ge8Sb2Te11 and randomly distorted cubic Ge4Sb2Te7 and high-temperature Ge8Sb2Te11 phases. While coherent distortion in Ge8Sb2Te11 does not prevent the crystalline state from collapsing into its amorphous counterpart in a similar manner to pure GeTe, the pressure-amorphized Ge8Sb2Te11 phase begins to revert to the crystalline cubic phase at ∼9 GPa in contrast to Ge4Sb2Te7, which remains amorphous under ambient conditions when gradually decompressed from 40 GPa. Moreover, experimentally, it was observed that pressure-induced amorphization in Ge8Sb2Te11 is a temperature-dependent process. Ge8Sb2Te11 transforms into the amorphous phase at ∼27.5 and 25.2 GPa at room temperature and 408 K, respectively, and completely amorphizes at 32 GPa at 408 K, while some crystalline texture could be seen until 38 GP...
Crystallization process in Ge2Sb2Te5 amorphous films
Vacuum, 2010
The aim of this work is to investigate the isokinetic and isothermal amorphous-to-crystalline phase transformation process in Ge 2 Sb 2 Te 5 ternary alloys. The experiments were carried out using electrical impedance, X-ray diffraction and reflection measurements. The results have shown that, upon annealing, the crystallization process in amorphous Ge 2 Sb 2 Te 5 films starts with nuclei which were identified as the Ge 1 Sb 4 Te 7 crystalline phase. As temperature increases (or time of isothermal annealing) these nuclei are transformed into the fcc-Ge 2 Sb 2 Te 5 phase. In order to establish the mechanism of crystallization for this system, a stochastic lattice model was implemented to analyze nucleation and growth of the two phases involved (i.e., the metastable Ge 1 Sb 4 Te 7 nuclei followed by the stable fcc-Ge 2 Sb 2 Te 5). The results of the simulations demonstrate close agreement with experimental results. Furthermore, the crystallization process in amorphous films with the Ge 1 Sb 4 Te 7 composition shows the existence of only one phase during the whole process and can be described by the classical Johnson-Mehl-Avrami-Kolmogorov model.
New Structural Picture of the Ge_{2}Sb_{2}Te_{5} Phase-Change Alloy
Physical Review Letters, 2011
Using electron microscopy and diffraction techniques, as well as first-principles calculations, we demonstrate that as much as 35% of the total Ge atoms in the cubic phase of Ge 2 Sb 2 Te 5 locate in tetrahedral environments. The Ge-vacancy interactions play a crucial stabilizing role, leading to Ge-vacancy pairs and the sharing of vacancies that clusters tetrahedral Ge into domains. The Ge 2 Sb 2 Te 5 structure with coexisting octahedral and tetrahedral Ge produces optical and structural properties in good agreement with experimental data and explains the property contrast as well as the rapid transformation in this phase-change alloy.