Dielectric properties of amorphous phase-change materials

Chen, Chao; Jost, Peter; Völker, H; Kaminski, Marvin; Wirtssohn, Matti; Engelmann, U.; Kruger, Karl; Schlich, Franziska F.; Schlockermann, Carl; Lobo, R. P. S. M.; Wuttig, Matthias

doi:10.1103/physrevb.95.094111

Cited by 48 publications

(42 citation statements)

References 49 publications

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“…6 Indeed, the transition from the amorphous to the crystalline state in materials such as GeTe or Ge 2 Sb 2 Te 5 is accompanied by a significant increase of ε ∞ and Z*. [6][7][8] This observation is in line with a transition from ordinary covalent bonding in the amorphous state, where the atoms have on average three nearest neighbors, 9 that are held by saturated bonds, to resonant bonding in the crystalline state.…”

Section: Introductionmentioning

confidence: 76%

Formation of resonant bonding during growth of ultrathin GeTe films

et al. 2017

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A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) || Si(111): GeTe [−110] || Si[−110] epitaxial relationship rigorously. Hence, in striking contrast to conventional lattice-matched epitaxial systems, a drastic improvement in atomic order is observed above a critical film thickness. Raman spectra show a remarkable change of vibrational modes above the critical thickness that is attributed to a change in the nature of the bonds: While ordinary covalent bonding is found in ultrathin films, resonant bonding can prevail only once a critical thickness is reached. This scenario is further supported by density functional theory calculations showing that ultrathin films do not utilize resonant bonding in contrast to the bulk phase. These findings are important not only for ultrathin films of phase-change materials such as GeTe and GeSbTe, which are employed in phase-change memories, but also for thermoelectrics and topological insulators such as Bi 2 Te 3 and Sb 2 Te 3 , where resonant bonding might also have a significant role.

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Section: Introductionmentioning

confidence: 76%

Formation of resonant bonding during growth of ultrathin GeTe films

et al. 2017

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“…The simplest description of Seebeck coefficient and electrical conductivity of metals and degenerate semiconductors is provided by the Pisarenko and the Drude formula

S = \frac{8 π^{2} k_{normalB}^{2}}{3 e h^{2}} m_{DOS}^{normal*} T {false(\frac{π}{3 n} false)}^{2 / 3}

σ = e μ n = e \frac{e τ}{m_{Drude}^{normal*}} n

where

m_{DOS}^{normal*}

is the density of states effective mass,

m_{Drude}^{normal*}

is the conductivity effective mass, n is the carrier concentration, μ is the carrier mobility, τ is the carrier relaxation time, T is the temperature, k B is Boltzmann's constant, e is the elementary charge, and h is Planck's constant. The strong screening associated to the very large polarizabilities found in phase‐change materials makes electron–electron interactions negligible, hence justifying Equation . Accordingly, the power factor only depends upon the concentration of charge carriers, their relaxation time, the two different effective masses

m_{DOS}^{normal*}

and

m_{Drude}^{normal*}

as well as temperature.…”

Section: Upper Valence Band At the High‐symmetry Points Of The Brillomentioning

confidence: 99%

Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective

2018

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Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial-and-error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV-VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV-VI compounds with octahedral-like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density-of-states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight-binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.

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“…Since this value is much smaller than the grain size of the crystalline samples, ~100–200 Å [44], the scattering of electrons should be attributed to an intra-grain mechanism. In GST, the electron-correlation effects are weak due to the high static dielectric constant of 98, which stems from the alignment of directional p orbitals, forming resonant bonding [40,41,78]. Therefore, Mott physics is expected not to be the dominant mechanism responsible for the MIT.…”

Section: Discussionmentioning

confidence: 99%

A Review on Disorder-Driven Metal–Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials

Wang

Mazzarello

et al. 2017

Materials

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Metal–insulator transition (MIT) is one of the most essential topics in condensed matter physics and materials science. The accompanied drastic change in electrical resistance can be exploited in electronic devices, such as data storage and memory technology. It is generally accepted that the underlying mechanism of most MITs is an interplay of electron correlation effects (Mott type) and disorder effects (Anderson type), and to disentangle the two effects is difficult. Recent progress on the crystalline Ge1Sb2Te4 (GST) compound provides compelling evidence for a disorder-driven MIT. In this work, we discuss the presence of strong disorder in GST, and elucidate its effects on electron localization and transport properties. We also show how the degree of disorder in GST can be reduced via thermal annealing, triggering a disorder-driven metal–insulator transition. The resistance switching by disorder tuning in crystalline GST may enable novel multilevel data storage devices.

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Dielectric properties of amorphous phase-change materials

Cited by 48 publications

References 49 publications

Formation of resonant bonding during growth of ultrathin GeTe films

Formation of resonant bonding during growth of ultrathin GeTe films

Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective

A Review on Disorder-Driven Metal–Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials

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