Experimental evidence for presence of voids in phase change memory material

Upadhyay, Manisha; Abhaya, S.; Murugavel, Sevi; Amarendra, G.

doi:10.1039/c3ra44246b

Cited by 13 publications

(14 citation statements)

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“…Because Te excess in both GeTe and TAGS-85 affects the two-peak structure, we can suggest that it reduces the rhombohdral distortion, possibly due to an increase in the concentration of Ge vacancies. An insignificant amount of Te may form a second phase as was reported for Ge x Te 100-x alloys [18]. A wide range of observed Knight shifts in a given sample allows us to use these materials for detection of 125 Te NMR signals and measure T 1 from Te in different chemical environments.…”

Section: Resultsmentioning

confidence: 81%

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

Levin

Cui

Schmidt‐Rohr

2016

Solid State Nuclear Magnetic Resonance

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(125)Te NMR spectra and spin-lattice relaxation times, T1, have been measured for several GeTe-based materials with Te excess. The spectra show inhomogeneous broadening by several thousand ppm and a systematic variation in T1 relaxation time with resonance frequency. The quadratic dependence of the spin-lattice relaxation rate, 1/T1, on the Knight shift in the Korringa relation is found to be valid over a wide range of Knight shifts. This result confirms that T1 relaxation in GeTe-based materials is mostly dominated by hyperfine interaction between nuclei and free charge carriers. In GeTe with 2.5% excess of Te, about 15% of the material exhibits a Knight shift of ≥4500ppm and a T1 of only 0.3ms, indicating a high hole concentration that could correspond to close to 50% vacancies on the Ge sublattice in this component. Our findings provide a basis for determining the charge carrier concentration and its distribution in complex thermoelectric and phase-change tellurides, which should lead to a better understanding of electronic and thermal transport properties as well as chemical bonding in these materials.

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Section: Resultsmentioning

confidence: 81%

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

Levin

Cui

Schmidt‐Rohr

2016

Solid State Nuclear Magnetic Resonance

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“…10,11 In order to understand the nature of defects between the two states of PCM based materials, an experimental effort has been made by using positron lifetime spectroscopy on Ge-Te alloys. 12 These studies elucidated with varying compositions, the associated defect are multi vacancies or vacancy clusters (nanoscale voids) rather simple mono vacancies or dangling bond defects in Ge-Te glass and crystal. Therefore, both the size of cavity and concentration of defects play an important role in the fast reversible phase transition process.…”

Section: Introductionmentioning

confidence: 99%

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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“…The low-frequency band at around 70 and 90 cm À1 (bands A 1 and A 2 ) is the bending modes of amorphous Te chain connecting the Ge atom in GeTe 4 (Ge-(Te-Te) 4/2 ) tetrahedra in the network. [22,23] The band peaking at 126 cm À1 (Band A 3 ) represents the symmetric stretching vibrational modes of GeTe 4 tetrahedral units, [24,25] whereas the peaks at 149 and 165 cm À1 (corresponding to band A 4 and A 5 ) are assigned to the vibrations of Te─Te related to short-disordered chain [25,26] which elucidates the vitreous Ge 20 Te 80 that has an essential built up of GeTe 4 tetrahedra connected by Te─Te linkages. In addition, the broad spectra peaking at 225 cm À1 (band A 6 ) at the highfrequency sphere embodies the vibrations of defective tetrahedral units, i.e., GeTe 4-n Ge n (n ¼ 0-3).…”

Section: Resultsmentioning

confidence: 99%

Temperature‐Dependent Local Structural Changes of Amorphous Thin Ge₂₀Te₈₀ Film Revealed by In Situ Resistance, X‐Ray Diffraction, and Raman Spectroscopy Studies

Sengottaiyan

Manivannan

2020

Physica Status Solidi (b)

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Although numerous chalcogenide glass systems including GeTe6 and GexSe100−x have been explored for Ovonic Threshold Switch (OTS) selectors in vertically stackable cross‐point memory applications, yet an improved thermal and structural stability at elevated temperatures remains a key challenge. Herein, a systematic temperature‐dependent experimental investigation on the thermal stability and local structural changes of as‐deposited amorphous thin Ge20Te80 film is conducted for a wide range of temperatures from 25 to 260 °C. The coherent experimental studies reveal that the amorphous phase is stable up to 180 °C, and the crystallization process is initiated above 180 °C, by simultaneous crystallization of Te and GeTe until ≈238 °C as substantiated by temperature‐dependent sheet resistance, X‐ray diffraction, and Raman spectroscopic measurements. Furthermore, the local structural changes over the crystallization process of Ge20Te80 thin film are elucidated by Raman spectroscopy. These experimental findings provide a decisive understanding of the thermal stability and structural evolution of Ge20Te80 thin films toward designing stable OTS selector materials.

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Experimental evidence for presence of voids in phase change memory material

Cited by 13 publications

References 50 publications

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

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

Temperature‐Dependent Local Structural Changes of Amorphous Thin Ge₂₀Te₈₀ Film Revealed by In Situ Resistance, X‐Ray Diffraction, and Raman Spectroscopy Studies

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Experimental evidence for presence of voids in phase change memory material

Cited by 13 publications

References 50 publications

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

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

Temperature‐Dependent Local Structural Changes of Amorphous Thin Ge20Te80 Film Revealed by In Situ Resistance, X‐Ray Diffraction, and Raman Spectroscopy Studies

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Temperature‐Dependent Local Structural Changes of Amorphous Thin Ge₂₀Te₈₀ Film Revealed by In Situ Resistance, X‐Ray Diffraction, and Raman Spectroscopy Studies