Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

Andrikopoulos, Konstantinos S.; Yannopoulos, Spyros N.; Voyiatzis, George A.; Kolobov, Alexander V.; Ribes, M.; Tominaga, Junji

doi:10.1088/0953-8984/18/3/014

Cited by 224 publications

(205 citation statements)

References 35 publications

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“…Reactions performed at 180°C also gave GeTe particles, whose size and shape were comparable to those obtained at 160°C. These findings somehow correlate with those of previous studies, in which the amorphous-to-crystalline phase transition for GeTe thin films was found to occur at 145°C, 53,54 whereas bulk GeTe started to crystallize at roughly 180°C. 55 However, this finding has to be considered as rather coincidentally since the solution-based growth of crystalline GeTe particles kinetically differs from the reorganization of solid GeTe from the metastable amorphous state to the crystalline state.…”

Section: ■ Results and Discussionsupporting

confidence: 92%

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

et al. 2013

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GeTe octahedra were prepared by reaction of equimolar amounts of GeCl 2 ·dioxane and Te(SiEt 3 ) 2 in oleylamine, whereas a slight excess of the Te precursor yielded GeTe octahedra decorated with elemental Te nanowires, which can be removed by washing with TOP. The mechanism of the GeTe formation is strongly influenced by the solvent. The expected elimination of Et 3 SiCl (dehalosilylation) only occurred in aprotic solvents, whereas Te(SiEt 3 ) 2 was found to react with primary and secondary amines with formation of silylamines. Temperature-dependent studies on the reaction in oleylamine showed that crystalline GeTe particles are formed at temperatures higher than 140 °C. XRD, SAED, and HRTEM studies proved the formation of rhombohedral GeTe nanoparticles. These findings were confirmed by a single-crystal and powder X-ray analysis. The rhombohedral structure modification was found, and the structure was solved in the acentric space group R3m. Disciplines Ceramic Materials | Complex Fluids | Other Materials Science and Engineering | Thermodynamics CommentsReprinted with permission from Inorg. Chem., 2013, 52 (24) ABSTRACT: GeTe octahedra were prepared by reaction of equimolar amounts of GeCl 2 · dioxane and Te(SiEt 3 ) 2 in oleylamine, whereas a slight excess of the Te precursor yielded GeTe octahedra decorated with elemental Te nanowires, which can be removed by washing with TOP. The mechanism of the GeTe formation is strongly influenced by the solvent. The expected elimination of Et 3 SiCl (dehalosilylation) only occurred in aprotic solvents, whereas Te(SiEt 3 ) 2 was found to react with primary and secondary amines with formation of silylamines. Temperature-dependent studies on the reaction in oleylamine showed that crystalline GeTe particles are formed at temperatures higher than 140°C. XRD, SAED, and HRTEM studies proved the formation of rhombohedral GeTe nanoparticles. These findings were confirmed by a single-crystal and powder X-ray analysis. The rhombohedral structure modification was found, and the structure was solved in the acentric space group R3m.

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Section: ■ Results and Discussionsupporting

confidence: 92%

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

et al. 2013

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“…51,52 Based on experimental and theoretical studies, it has been suggested that a-GST is characterized by the presence of fourfold coordinated Ge atoms, 37,42,46,48,50,[53][54][55][56][57][58][59][60] in which the sum of the occurrences of GeTe 4 , Ge͑SbTe 3 ͒, and Ge͑GeTe 3 ͒ is about 66%, 59 which is similar to the local environments observed in amorphous GeTe for GeTe 4 and Ge͑GeTe 3 ͒. 61 Ge-Ge and Ge-Sb bonds are found in those motifs, which is assumed to be due to disorder effects, since they are not present in the crystalline phases. Furthermore, a-GST shows volume expansion of 6%-7% compared with the m-GST phase, 10,36,62 and it has a higher energy ͑28-40 meV/at.͒ with respect to m-GST.…”

Section: Introductionmentioning

confidence: 73%

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

Silva

Walsh

Wei

et al. 2009

Journal of Applied Physics

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Tricritical behavior of the RI-RV rotator phase transition in a mixture of alkanes with nanoparticles J. Chem. Phys. 135, 134505 (2011) Pressure-induced amorphization in mayenite (12CaO·7Al2O3) J. Chem. Phys. 135, 094506 (2011) Influence of Al2O3 crystallization on band offsets at interfaces with Si and TiNx Appl. Phys. Lett. 99, 072103 (2011) Temperature induced transition from hexagonal to circular pits in graphite oxidation by O2 Appl. Phys. Lett. 99, 044102 (2011) Phase transformation of Ho2O3 at high pressure J. Appl. Phys. 110, 013526 (2011) Additional information on J. Appl. Phys. The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge 2 Sb 2 Te 5 has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt ͓111͔ direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from Sb 2 Te 3 , lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate configuration can be obtained experimentally by applying an intense laser beam, which overcomes the thermodynamic barrier from the octahedron to tetrahedron sites. The high figure of merit of Ge 2 Sb 2 Te 5 is achieved from the optimal combination of intrinsic vacancies provided by Sb 2 Te 3 and the instability of the tetrahedron sites provided by GeTe.

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“…For all the Raman spectra, there are peaks found at 88, 120, 139, and 160 cm -1 . The broad peaks found at 88 and 160 cm -1 are contributed by GeTe vibration modes [14,34,35]. Two distinct peaks were also observed at 120 and 139 cm -1 , and their intensities decreased with increasing deposition temperature.…”

Section: Resultsmentioning

confidence: 99%

“…The samples were excited with a He-Ne laser (k = 633 nm), and the incident laser power was adjusted to 0.5 mW in order to minimize heating effects in the illuminated sample region of interest [9,14]. The Raman spectra were recorded at an interval of 0.4 cm -1 .…”

Section: Methodsmentioning

confidence: 99%

Impact of deposition conditions on the crystallization kinetics of amorphous GeTe films

et al. 2015

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The speed at which phase change memory devices can operate depends strongly on the crystallization kinetics of the amorphous phase. To better understand factors that affect the crystallization rate, we have investigated crystallization of GeTe films as a function of their deposition temperatures and deposition rates, using X-ray synchrotron radiation and Raman spectroscopy. As-deposited films were found to be fully amorphous under all conditions, even though films deposited at higher temperatures and lower rates experienced lower effective quench rates. Non-isothermal transformation curves show that the apparent crystallization temperature of GeTe films decreases with increasing deposition temperature and decreasing deposition rate. It was found that this correlates with a decrease in the activation energy for nucleation (calculated using Kissinger's analysis), while the activation energy for crystal growth remained unaffected. From Raman spectroscopy measurements, it was found that increasing the deposition temperature or decreasing the deposition rate, and therefore the effective quench rate, reduces the number of homopolar Te-Te bonds and thereby reduces the barrier to crystal nucleation.

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Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition

Cited by 224 publications

References 35 publications

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

Solution-Based Synthesis of GeTe Octahedra at Low Temperature

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

Impact of deposition conditions on the crystallization kinetics of amorphous GeTe films

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