Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials

Andrikopoulos, Konstantinos S.; Yannopoulos, Spyros N.; Kolobov, Alexander V.; Fons, Paul; Tominaga, Junji

doi:10.1016/j.jpcs.2007.02.027

Cited by 189 publications

(165 citation statements)

References 16 publications

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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%

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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Section: Resultsmentioning

confidence: 99%

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

et al. 2015

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“…1,2 Recently, extensive theoretical investigation on the mechanism of the phase change in GST have been made using molecular-dynamics simulations. [3][4][5] In addition, experimental studies using extended x-ray absorption fine structure ͑XAFS͒ and Raman-scattering measurements have examined dynamics of phase transition in GST, [6][7][8][9] suggesting that the structure of amorphous GST can be described as a cross section of a distorted rocksalt structure with vacancies and the amorphization of GST is due to an umbrella flip of Ge atoms from an octahedral position into a tetrahedral one. Moreover, Sun et al 4 theoretically proposed that the vacancies in the crystalline ͑cubic͒ GST are highly ordered and layered followed by the recent prediction of the formation of large voids in the amorphous GST films.…”

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confidence: 99%

Ultrafast dephasing of coherent optical phonons in atomically controlledGeTe/Sb2Te3superlattices

2009

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Femtosecond dynamics of coherent optical phonons in GeTe/ Sb 2 Te 3 superlattices ͑SLs͒, a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity obtained in as-grown SLs exhibits the coherent A 1 optical modes at 5.10 and 3.78 THz while only the single A 1 mode at 3.68 THz is observed in annealed SLs. The decay rate of the A 1 mode in annealed SLs is strongly temperature dependent while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A 1 phonons in amorphous SLs is governed by the phonon-defect ͑vacancy͒ scattering rather than the anharmonic phonon-phonon coupling. DOI: 10.1103/PhysRevB.79.174112 PACS number͑s͒: 78.47.JϪ, 63.20.kp, 63.50.Ϫx, 68.35.Rh One of the most common materials for optical recording media is Ge 2 Sb 2 Te 5 ͑GST͒ in which phase transition between crystalline and amorphous phases serve rewritable recording. 1,2 Recently, extensive theoretical investigation on the mechanism of the phase change in GST have been made using molecular-dynamics simulations. [3][4][5] In addition, experimental studies using extended x-ray absorption fine structure ͑XAFS͒ and Raman-scattering measurements have examined dynamics of phase transition in GST, [6][7][8][9] suggesting that the structure of amorphous GST can be described as a cross section of a distorted rocksalt structure with vacancies and the amorphization of GST is due to an umbrella flip of Ge atoms from an octahedral position into a tetrahedral one. Moreover, Sun et al. 4 theoretically proposed that the vacancies in the crystalline ͑cubic͒ GST are highly ordered and layered followed by the recent prediction of the formation of large voids in the amorphous GST films. 10 The experimental information on the existence of vacancies from the lattice dynamical point of view, however, has not been explored.One of the advantages of GST as the optical recording media is its high-speed switching of read-write characteristics whose time scale has been believed to be less than a nanosecond. In order to understand and control the rapid phase change in GST, a time-resolved study of phonon dynamics in GST is strongly demanded, however, the timeresolved studies are still very few.11 Moreover, a new class of semiconductor superlattices ͑GeTe/ Sb 2 Te 3 ͒ with three different states has recently been proposed which will enable us to realize reversible transition among the three states by the means of the irradiation of laser pulses. 12The coherent phonon spectroscopy ͑CPS͒ is a powerful tool to study ultrafast dynamics of structural phase transitions, occurring within picosecond and femtosecond time scales and in fact it has been applied to semimetals, 13 ferroelectric materials, 14,15 and Mott insulators. 16,17 In the CPS, the pump pulse impulsively generates coherent lattice vibration through real or virtual electronic transitions. The pulse length ͑⌬͒ used s...

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“…Raman peaks at ∼124 cm 1 is attributed to the symmetric stretching mode (A 1 ) of corner-sharing tetrahedral units GeTe 4 n Ge n (n = 1,2), and the peaks at ∼158 cm -1 is related to vibrations in the Sb-Te 3 units. 33 For better comparison, Gaussian fitted Raman spectra of the GST, C5.5-GST and C6.7-GST are depicted in Fig. 3(b)-(d), respectively.…”

confidence: 99%

Carbon doping induced Ge local structure change in as-deposited Ge2Sb2Te5 film by EXAFS and Raman spectrum

et al. 2018

AIP Advances

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The local structure change of Ge induced by carbon doping in as-deposited Ge2Sb2Te5 films were studied by extended X-ray absorption fine structure and Raman spectrum. Ge-C bonds are formed at the expense of reducing the coordination of Ge-Ge and Ge-Te bonds, and make the local structure of Ge to be a well-defined tetrahedral geometry, which increases the rigidity of amorphous network and reduces the number of ABAB rings, thus the crystallization temperature of carbon-doped Ge2Sb2Te5 (CGST) films are enhanced. The reduced proportion of the tetrahedral units GeTe4−nGen (n = 1, 2) caused by carbon doping accounts for the weaker Raman peak intensity at ∼124 cm−1 in CGST films. Meanwhile, the impact of doping carbon on the crystalline structure of CGST films were investigated by high resolution transmission electron microscope.

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Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials

Cited by 189 publications

References 16 publications

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

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

Ultrafast dephasing of coherent optical phonons in atomically controlledGeTe/Sb2Te3superlattices

Carbon doping induced Ge local structure change in as-deposited Ge2Sb2Te5 film by EXAFS and Raman spectrum

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