Soft phonon mode and ferroelectricity in GeTe

Steigmeier, E. F.; Harbeke, G.

doi:10.1016/0038-1098(70)90619-8

Cited by 182 publications

(139 citation statements)

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“…The previous Raman spectroscopy studies on bulk single crystal α-GeTe showed both E and A 1 modes at 93 and 127 cm −1 at room temperature, where the later one found to be as soft optical phonon mode. 21 In the present work, we observe the both E and A 1 modes at 88.4 and 125.7 cm −1 at 298 K with 10 µm crystal size and these results are closer to those reported earlier on single crystal of α-GeTe. However, the recent experimental study on bulk polycrystalline α-GeTe specimen reveals that the Raman modes of E and A 1 occur at 95 and 125 cm −1 along with additional bands with lesser intensity at high frequencies.…”

Section: Resultssupporting

confidence: 92%

“…However, the recent experimental study on bulk polycrystalline α-GeTe specimen reveals that the Raman modes of E and A 1 occur at 95 and 125 cm −1 along with additional bands with lesser intensity at high frequencies. 22 On the basis of ab initio results [17][18][19][20] and earlier Raman spectroscopy observations, 21,22 we confirm that the observed vibrational modes in 10µm crystal size are intrinsic part of bulk α-GeTe structure. Furthermore, the decrease in crystal size leads to downshift of phonon modes in addition to the peak broadening of the Raman spectra as shown in inset of Figure 3.…”

Section: Resultssupporting

confidence: 82%

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The role of atomic vacancies on phonon confinement in α-GeTe

Kalra

Murugavel

2015

AIP Advances

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Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys.

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Section: Resultssupporting

confidence: 92%

Section: Resultssupporting

confidence: 82%

The role of atomic vacancies on phonon confinement in α-GeTe

Kalra

Murugavel

2015

AIP Advances

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“…In other words, the material exhibits a displacive phase transition, similar to that of perovskite ferroelectrics, with the dipole moment in the low-temperature phase stemming from the relative shift of Ge and Te sublattices. 10 Raman scattering studies showed some mode softening, 11 which provided indirect support of the proposed structural change. This description of the phase transition has been recently challenged by extended X-ray absorption fine structure (EXAFS) 12 and total scattering studies.…”

mentioning

confidence: 84%

“…The authors concluded that the ferroelectricto-paraelectric transition results from an order-disorder transition, where the structure remains rhombohedrally distorted locally but the distortions change from coherent at low temperatures to stochastic at higher temperatures. [9][10][11] In the latter case, the average structure becomes cubic. This discrepancy between the Bragg diffraction and the EXAFS/total scattering results is not unknown and has also been reported for other materials.…”

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confidence: 99%

Ferroelectric switching in epitaxial GeTe films

et al. 2014

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In this paper, using a resonance-enhanced piezoresponse force microscopy approach supported by density functional theory computer simulations, we have demonstrated the ferroelectric switching in epitaxial GeTe films. It has been shown that in films with thickness on the order of several nanometers reversible reorientation of polarization occurs due to swapping of the shorter and longer Ge-Te bonds in the interior of the material. It is also hinted that for ultra thin films consisting of just several atomic layers weakly bonded to the substrate, ferroelectric switching may proceed through exchange of Ge and Te planes within individual GeTe layers.

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“…GeTe might make a promising ferroelectric were it not for large concentrations of free carriers arising from Ge vacancies and a Fermi energy lying within the valence band. [19][20][21] Again, the GeTe structure is a consequence of s-p hybridization and lone pairs on both Ge and Te. 22 SnTe also forms the rhombohedral A7 structure below ∼16 K, 23,24 while all other IV-VI compounds form the rocksalt or GeS structures at room temperature.…”

Section: Fig 1 (Color Online)mentioning

confidence: 99%

Chemical ordering rather than random alloying in SbAs

Shoemaker

Chasapis

et al. 2013

Phys. Rev. B

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The semimetallic Group V elements display a wealth of correlated electron phenomena due to a small indirect band overlap that leads to relatively small, but equal, numbers of holes and electrons at the Fermi energy with high mobility. Their electronic bonding characteristics produce a unique crystal structure, the rhombohedral A7 structure, which accommodates lone pairs on each site. Here we show via single-crystal and synchrotron x-ray diffraction that SbAs is a compound and the A7 structure can display chemical ordering of Sb and As, which were previously thought to mix randomly. Formation of this compound arises due to differences in electronegativity that are common to IV-VI compounds of average group V such as GeTe, SnS, PbS, and PbTe, and also ordered intra-period compounds such as CuAu and NiPt. High-temperature diffraction studies reveal an order-disorder transition around 550 K in SbAs, which is in stark contrast to IV-VI compounds GeTe and SnTe that become cubic at elevated temperatures but do not disorder. Transport and infrared reflectivity measurements, along with first-principles calculations, confirm that SbAs is a semimetal, albeit with a direct band separation larger than that of Sb or As. Because even subtle substitutions in the semimetals, notably Bi1−xSbx, can open semiconducting energy gaps, a further investigation of the interplay between chemical ordering and electronic structure on the A7 lattice is warranted.

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Soft phonon mode and ferroelectricity in GeTe

Cited by 182 publications

References 5 publications

The role of atomic vacancies on phonon confinement in α-GeTe

The role of atomic vacancies on phonon confinement in α-GeTe

Ferroelectric switching in epitaxial GeTe films

Chemical ordering rather than random alloying in SbAs

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