Crystalline Phase-change Materials Research Articles

Metavalent or Hypervalent Bonding: Is There a Chance for Reconciliation?

A family of solids including crystalline phase change materials such as GeTe and Sb2 Te3 , topological insulators like Bi2 Se3, and halide perovskites such as CsPbI3 possesses an unconventional property portfolio that seems incompatible with ionic, metallic, or covalent bonding. Instead, evidence is found for a bonding mechanism characterized by half-filled p-bands and a competition between electron localization and delocalization. Different bonding concepts have recently been suggested based on quantum chemical bonding descriptors which either define the bonds in these solids as electron-deficient (metavalent) or electron-rich (hypervalent). This disagreement raises concerns about the accuracy of quantum-chemical bonding descriptors is showed. Here independent of the approach chosen, electron-deficient bonds govern the materials mentioned above is showed. A detailed analysis of bonding in electron-rich XeF2 and electron-deficient GeTe shows that in both cases p-electrons govern bonding, while s-electrons only play a minor role. Yet, the properties of the electron-deficient crystals are very different from molecular crystals of electron-rich XeF2 or electron-deficient B2 H6 . The unique properties of phase change materials and related solids can be attributed to an extended system of half-filled bonds, providing further arguments as to why a distinct nomenclature such as metavalent bonding is adequate and appropriate for these solids.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Terahertz Nanoimaging and Nanospectroscopy of Chalcogenide Phase-Change Materials

Chalcogenide phase-change materials (PCMs) exhibit optical phonons at terahertz (THz) frequencies, which can be used for studying basic properties of the phase transition and which lead to a strong dielectric contrast that could be exploited for THz photonics applications. Here, we demonstrate that the phonons of PCMs can be studied by frequency-tunable THz scattering-type scanning near-field optical microscopy (s-SNOM). Specifically, we perform spectroscopic THz nanoimaging of a PCM sample comprising amorphous and crystalline phases. We observe phonon signatures, yielding strong s-SNOM signals and, most important, clear spectral differences between the amorphous and crystalline PCM, which allows for distinguishing the PCM phases with high confidence on the nanoscale. We also found that the spectral signature can be enhanced, regarding both signal strength and spectral contrast, by increasing the radius of the probing tip. From a general perspective, our results establish THz s-SNOM for nanoscale structural and chemical mapping based on local phonon spectroscopy.

Chemical Bonding in Chalcogenides: The Concept of Multicenter Hyperbonding.

The precise nature of chemical-bonding interactions in amorphous, and crystalline, chalcogenides is still unclear due to the complexity arising from the delocalization of bonding, and nonbonding, electrons. Although an increasing degree of electron delocalization for elements down a column of the periodic table is widely recognized, its influence on chemical-bonding interactions, and on consequent material properties, of chalcogenides has not previously been comprehensively understood from an atomistic point of view. Here, a chemical-bonding framework is provided for understanding the behavior of chalcogenides (and, in principle, other lone-pair materials) by studying prototypical telluride nonvolatile-memory, "phase-change" materials (PCMs), and related chalcogenide compounds, via density-functional-theory molecular-dynamics (DFT-MD) simulations. Identification of the presence of previously unconsidered multicenter "hyperbonding" (lone-pair-antibonding-orbital) interactions elucidates not only the origin of various material properties, and their contrast in magnitude between amorphous and crystalline phases, but also the very similar chemical-bonding nature between crystalline PCMs and one of the bonding subgroups (with the same bond length) found in amorphous PCMs, in marked contrast to existing viewpoints. The structure-property relationship established from this new bonding-interaction perspective will help in designing improved chalcogenide materials for diverse applications, based on a fundamental chemical-bonding point of view.

In situ study of vacancy disordering in crystalline phase-change materials under electron beam irradiation

Unconventionally high amount of atomic vacancies up to more than 10% are known to form in Ge-Sb-Te crystals upon rapid crystallization from the amorphous phase. Upon thermal annealing, an ordering process of these atomic vacancies is observed, triggering a structural transition from the recrystallized rocksalt structure to a stable layered trigonal structure and a transition from insulator to metal. In this work, we demonstrate an opposite vacancy disordering process upon extensive electron beam irradiation, which is accompanied by the reverse transition from the stable trigonal phase to the metastable cubic phase. The combined in situ transmission electron microscopy experiments and density functional theory nudged elastic band calculations reveal three transition stages, including (I) the vacancy diffusion in the trigonal phase, (II) the change in atomic stacking, and (III) the disappearance of vacancy-rich planes. The mechanism of vacancy disordering is attributed to kinetic knock-on collision effects of the high-energy electron beam, which prevail over the heating effects.

Persistence of spin memory in a crystalline, insulating phase-change material

The description of disorder-induced electron localization by Anderson over 60 years ago began a quest for novel phenomena emerging from electronic interactions in the presence of disorder. Even today, the interplay of interactions and disorder remains incompletely understood. This holds in particular for strongly disordered materials where charge transport depends on ‘hopping’ between localized sites. Here we report an unexpected spin sensitivity of the electrical conductivity at the transition from diffusive to hopping conduction in a material that combines strong spin-orbit coupling and weak inter-electronic interactions. In thin films of the disordered crystalline phase change material SnSb2Te4, a distinct change in electrical conductance with applied magnetic field is observed at low temperatures. This magnetoconductance changes sign and becomes anisotropic at the disorder-driven crossover from strongly localized (hopping) to weakly localized (diffusive) electron motion. The positive and isotropic magnetoconductance arises from disruption of spin correlations that inhibit hopping transport. This experimental observation of a recently hypothesized ‘spin memory’ demonstrates the spin plays a previously overlooked role in the disorder-driven transition between weak and strong localization in materials with strong spin–orbit interactions.

Direct observation of partial disorder and zipperlike transition in crystalline phase change materials

Chalcogenide phase change materials, such as $\mathrm{G}{\mathrm{e}}_{1}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ (GST), are of tremendous importance in emerging data storage technology, which takes advantage of rapid and reversible switching of GST between the amorphous and crystalline phases. To date, however, the atomic arrangement of the crystalline GST structure has not been fully resolved, resulting in a controversial understanding of the polymorphic transition mechanism. Here, the atomic and chemical arrangements of stable hexagonal structures of GST are determined by state-of-the-art aberration-corrected scanning transmission electron microscopy. A partially ordered Ge/Sb atomic stacking is resolved in the hexagonal structure to balance the enthalpy and entropy, differing from completely disordered Ge/Sb intermixing arrangement in a metastable rock-salt structure. The transition mechanism between these two phases is proposed, achieved by opening the van de Waals gap like a zipper triggered by the Ge/Sb hopping near vacancy grooves rather than the interplanar random atomic migration. The present results shed light on the understanding of the atomic arrangement and polymorphic phase transitions as well as the control of disorder in GST phase change memory.

From octahedral structure motif to sub-nanosecond phase transitions in phase change materials for data storage

Phase change random access memory (PCRAM) has been successfully applied in the computer storage architecture, as storage class memory, to bridge the performance gap between DRAM and Flash-based solid-state drive due to its good scalability, 3D-integration ability, fast operation speed and compatible with CMOS technology. Focusing on phase change materials and PCRAM for decades, we have successfully developed 128 Mb embedded PCRAM chips, which can meet the requirements of most embedded systems. 3D Xpoint (3D PCRAM), invented by Intel and Micron, has been regarded as a new breakthrough in the last 25 years since the application of NAND in 1989, which represents state-of-the-art memory technology. This technology has some remarkable features, such as the confined device structure with 20 nm size, the metal crossbar electrodes to reduce the resistance variations in PCRAM arrays, and the ovonic threshold switching selector that can provide a high drive current and a low leakage current. A good understanding of phase change mechanism is of great help to design new phase change materials with fast operation speed, low power consumption and long-lifetime. In this paper, we firstly review the development of PCRAM and different understandings on phase change mechanisms in recent years, and then propose a new view on the mechanism, which is based on the octahedral structure motifs and vacancies. Octahedral structure motifs are generally found in both amorphous and crystalline phase change materials. They are considered to be the basic units during phase transition, which are severely defective in the amorphous phase. These configurations turn into more ordered ones after minor local rearrangements, the growth of which results in the crystallization of rocksalt (RS) phase with a large amount of vacancies in the cation sites. Further driven by thermodynamic driving force, these vacancies move and layer along certain directions; consequently, the metastable RS structure transforms into the stable hexagonal (HEX) structure. Based on our results, we find that reversible phase transition between amorphous phase and RS phase, without further changing into HEX phase, would greatly decrease the required power consumption. Robust octahedra and plenty of vacancies in both amorphous and RS phase, respectively avoiding large atomic rearrangement and providing necessary space, are crucial to achieve the nanosecond or even sub-nanosecond operation of PCRAM.

Ab initio density functional theory study of the electronic, dynamic, and thermoelectric properties of the crystalline pseudobinary chalcogenide (GeTe)x/(Sb2Te3)(x=1,2,3)

We use ab initio density functional theory calculations to understand the electronic, dynamical, and thermoelectric behavior of layered crystalline phase-change materials. We perform calculations on the pseudobinary compounds ${(\mathrm{GeTe})}_{x}/({\mathrm{Sb}}_{2}{\mathrm{Te}}_{3})$ (GST) with $x=1$, 2, and 3. Since the stable configuration of these compounds remains somehow unsettled, we study one stacking configuration for GST124 ($x=1$), three for GST225 ($x=2$), and two for GST326 ($x=3$). A supercell approach is used to check the dynamical stability of the systems while thermoelectric properties are obtained by solving the Boltzmann transport equation. We report that the most accepted stacking configuration of GST124, GST225, and GST326 have metallic character and for the case of $x=2$ and 3, those are the ones with the lowest energy. However, we find the metallic of GST326 configuration to be dynamically unstable. In general, our values of the Seebeck coefficient and thermal conductivity for compounds with $x=1$ and 2 agree very well with the available experimental data. The small differences that we observe with respect to experimental data are attributed to the disorder that is present experimentally and that we have not taken into account. We do not find a Dirac cone in the electronic band structure of GST225, contrarily to previous reports. We attribute this due to the theoretical strain induced by the choice of the pseudopotential.

Unique Bond Breaking in Crystalline Phase Change Materials and the Quest for Metavalent Bonding.

Laser-assisted field evaporation is studied in a large number of compounds, including amorphous and crystalline phase change materials employing atom probe tomography. This study reveals significant differences in field evaporation between amorphous and crystalline phase change materials. High probabilities for multiple events with more than a single ion detected per laser pulse are only found for crystalline phase change materials. The specifics of this unusual field evaporation are unlike any other mechanism shown previously to lead to high probabilities of multiple events. On the contrary, amorphous phase change materials as well as other covalently bonded compounds and metals possess much lower probabilities for multiple events. Hence, laser-assisted field evaporation in amorphous and crystalline phase change materials reveals striking differences in bond rupture. This is indicative for pronounced differences in bonding. These findings imply that the bonding mechanism in crystalline phase change materials differs substantially from conventional bonding mechanisms such as metallic, ionic, and covalent bonding. Instead, the data reported here confirm a recently developed conjecture, namely that metavalent bonding is a novel bonding mechanism besides those mentioned previously.

Observation of carrier localization in cubic crystalline Ge2Sb2Te5 by field effect measurement

The tunable disorder of vacancies upon annealing is an important character of crystalline phase-change material Ge2Sb2Te5 (GST). A variety of resistance states caused by different degrees of disorder can lead to the development of multilevel memory devices, which could bring a revolution to the memory industry by significantly increasing the storage density and inspiring the neuromorphic computing. This work focuses on the study of disorder-induced carrier localization which could result in multiple resistance levels of crystalline GST. To analyze the effect of carrier localization on multiple resistant levels, the intrinsic field effect (the change in surface conductance with an applied transverse electric field) of crystalline GST was measured, in which GST films were annealed at different temperatures. The field effect measurement is an important complement to conventional transport measurement techniques. The field effect mobility was acquired and showed temperature activation, a hallmark of carrier localization. Based on the relationship between field effect mobility and annealing temperature, we demonstrate that the annealing shifts the mobility edge towards the valence-band edge, delocalizing more carriers. The insight of carrier transport in multilevel crystalline states is of fundamental relevance for the development of multilevel phase change data storage.

Dielectric properties of amorphous phase-change materials

The dielectric function of several amorphous phase-change materials has been determined by employing a combination of impedance spectroscopy (9 kHz--3 GHz) and optical spectroscopy from the far- $(20\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, 0.6 THz) to the near- $(12\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, 360 THz) infrared, i.e., from the DC limit to the first interband transition. While phase-change materials undergo a change from covalent bonding to resonant bonding on crystallization, the amorphous and crystalline phases of ordinary chalcogenide semiconductors are both governed by virtually the same covalent bonds. Here, we study the dielectric properties of amorphous phase-change materials on the pseudobinary line between GeTe and $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$. These data provide important insights into the charge transport and the nature of bonding in amorphous phase-change materials. No frequency dependence of permittivity and conductivity is discernible in the impedance spectroscopy measurements. Consequently, there are no dielectric relaxations. The frequency-independent conductivity is in line with charge transport via extended states. The static dielectric constant significantly exceeds the optical dielectric constant. This observation is corroborated by transmittance measurements in the far infrared, which show optical phonons. From the intensity of these phonon modes, a large Born effective charge is derived. Nevertheless, it is known that crystalline phase-change materials such as GeTe possess even significantly larger Born effective charges. Crystallization is hence accompanied by a huge increase in the Born effective charge, which reveals a significant change of bonding upon crystallization. In addition, a clear stoichiometry trend in the static dielectric constant along the pseudobinary line between GeTe and $\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{3}$ has been identified.

Effects of stoichiometry on the transport properties of crystalline phase-change materials

It has recently been shown that a metal-insulator transition due to disorder occurs in the crystalline state of the GeSb2Te4 phase-change compound. The transition is triggered by the ordering of the vacancies upon thermal annealing. In this work, we investigate the localization properties of the electronic states in selected crystalline (GeTe)x-(Sb2Te3)y compounds with varying GeTe content by large-scale density functional theory simulations. In our models, we also include excess vacancies, which are needed to account for the large carrier concentrations determined experimentally. We show that the models containing a high concentration of stoichiometric vacancies possess states at the Fermi energy localized inside vacancy clusters, as occurs for GeSb2Te4. On the other hand, the GeTe-rich models display metallic behavior, which stems from two facts: a) the tail of localized states shrinks due to the low probability of having sizable vacancy clusters, b) the excess vacancies shift the Fermi energy to the region of extended states. Hence, a stoichiometry-controlled metal-insulator transition occurs. In addition, we show that the localization properties obtained by scalar-relativistic calculations with gradient-corrected functionals are unaffected by the inclusion of spin-orbit coupling or the use of hybrid functionals.

Penn gap rule in phase-change memory materials: No clear evidence for resonance bonds

Although a proposal of resonance bonds in crystalline phase-change materials based on the GeSbTe system has been provided, we do not find any clear evidence in favor of the proposal. The ellipsometric study demonstrates that a change in the high frequency dielectric constant ε∞ between the amorphous and crystalline phases is only scaled by the average bandgap (the Penn gap rule). Even for a pure antimony film, regarded as a prototype resonance bonding material, ε∞ was found to follow the Penn gap rule. Experimentally, we did not find any evidence of a significant change in the optical transition matrix element during the phase change, which is necessary to support the idea of resonance bonds.

A multi-scale analysis of the impact of pressure on melting of crystalline phase change material germanium telluride

The impact of the moderate pressure (about 100 GPa) on the melting of crystalline (c-) phase change material (PCM) germanium telluride (GeTe) is analyzed, by combining the heat transfer equation in the PCM device scale (101–102 nm and beyond), and the ab initio molecular dynamics and the nudged elastic band simulations in the atomistic scale (10−1–100 nm). The multi-scale analysis unravels that a pressure P = 1.0 GPa can increase the melting temperature of c-GeTe and the PCM device “reset” operation energy consumption by 6%–7%. It is shown that the melting temperature increase originates from the pressure-induced raise of the energy barrier of the umbrella-flip transition of the Ge atom from the octahedral symmetry site to the tetrahedral symmetry site. It is revealed that when P > 1.0 GPa, which is normal in PCM devices, the “reset” energy will be increased even by more. Based on the analysis, suggestions to alleviate pressure-induced raise of melting temperature and “reset” energy are provided.

Study on the impact of the initialization process on the phase change memory

The set processes of T-shape phase change memory (PCRAM) cell are compared to study the change caused by the initialization. After the initialization, PCRAM cell shows lower Vth and minor resistances than those of the 1st operation, which can be explained by the reduction of contact resistance caused by the formation of the crystalline phase change material interface (CPI) near the electrodes. The temperature transition regions between the programmed region and the electrodes preserve the CPI after its formation in the initial operation. Thus, we suggest the formation of CPI is the key role for the initialization.

Weak antilocalization and disorder-enhanced electron interactions in annealed films of the phase-change compound GeSb2Te4

Phase-change materials can be reversibly switched between amorphous and crystalline states and often show strong contrast in the optical and electrical properties of these two phases. They are now in widespread use for optical data storage, and their fast switching and a pronounced change of resistivity upon crystallization are also very attractive for nonvolatile electronic data storage. Nevertheless, several open questions remain regarding the electronic states and charge transport in these compounds. In this work, we study electrical transport in thin metallic films of the disordered, crystalline phase-change material GeSb${}_{2}$Te${}_{4}$. We observe weak antilocalization and disorder-enhanced Coulomb interaction effects at low temperatures, and separate the contributions of these two phenomena to the temperature dependence of the resistivity, Hall effect, and magnetoresistance. Strong spin-orbit scattering causes positive magnetoresistance at all temperatures, and a careful analysis of the low-field magnetoresistance allows us to extract the temperature-dependent electron dephasing rate and study other scattering phenomena. We find electron dephasing due to inelastic electron-phonon scattering at higher temperatures, electron-electron scattering dephasing at intermediate temperatures, and a crossover to weak temperature dependence below 1 K.

Role of vacancies in metal–insulator transitions of crystalline phase-change materials

The study of metal-insulator transitions (MITs) in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of materials. Recently, a MIT governed by disorder was observed in crystalline phase-change materials. Here we report on calculations employing density functional theory, which identify the microscopic mechanism that localizes the wavefunctions and is driving this transition. We show that, in the insulating phase, the electronic states responsible for charge transport are localized inside regions having large vacancy concentrations. The transition to the metallic state is driven by the dissolution of these vacancy clusters and the formation of ordered vacancy layers. These results provide important insights on controlling the wavefunction localization, which should help to develop conceptually new devices based on multiple resistance states.

Phase Change Materials: Challenges on the Path to a Universal Storage Device

Phase change materials, in which a material is reversibly switched between an amorphous and crystalline state with corresponding contrast in optical and electronic transport properties, are excellent nonvolatile storage media. Rewritable digital versatile disks (DVDs) and Blu-ray discs are based on such materials in which the optical contrast between the amorphous and crystalline phases enables data storage. Additionally, the large change in electronic transport properties with resistivity contrast of up to six orders of magnitude on crystallization and fast switching speed is at the heart of a new class of nonvolatile data storage devices with electronic read/write operation and potential for future miniaturization. The amorphous state is characterized by saturated covalent bonds, whereas the crystalline phase forms resonant bonds. This bonding mechanism can account for the high electronic polarizabilities that characterize crystalline phase change materials. Interestingly, the relevant electronic states also govern the charge transport in the crystalline phase, leading to unique transport properties including a high degree of electronic localization, in those phase change materials, which are characterized by a high degree of disorder.

Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.