Growth of crystalline phase change materials by physical deposition methods

Boschker, Jos E.; Calarco, Raffaella

doi:10.1080/23746149.2017.1346483

Cited by 18 publications

(18 citation statements)

References 67 publications

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“…The samples on the Si(111)-7 × 7 reconstruction additionally feature rotational domains. An XRD peak indicating the formation of vacancy layers is observed attributing the epitaxial films to the metastable vacancy-ordered phase [24,28].…”

Section: Sample Preparation and Stm/sts Experimentsmentioning

confidence: 99%

“…These layers are partly stacked in a regular sequence, as deduced from XRD and TEM [24]. This more ordered phase deviates from the rocksalt structure and has recently been dubbed the metastable vacancy-ordered phase [28]. Moreover, it is known that the epitaxial films are strongly p-type doped [35] with an unintentionally varying charge carrier density p = (0.1-3) × 10 26 m −3 [24,26].…”

Section: Introductionmentioning

confidence: 99%

“…It is known from x-ray diffraction (XRD) that our epitaxial films grow in the technologically relevant metastable phase [28]. Conventionally, this phase features the cubic rocksalt structure with alternating layers of Te and of a disordered mixture of Ge, Sb, and vacancies (Vcs).…”

Section: Introductionmentioning

confidence: 99%

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Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

et al. 2017

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Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase-change material Ge 2 Sb 2 Te 5 as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless permits the detection of the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable Ge 2 Sb 2 Te 5 . First, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of 10 −4 nm −2 are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the buildup of vacancy layers.

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Section: Sample Preparation and Stm/sts Experimentsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

et al. 2017

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“…The epitaxial growth of PCMs has established a new level of material quality 8–10 and thus allows for an improved determination of their properties using techniques that normally cannot be applied to polycrystalline samples. For example, the electronic band structure of PCMs has been studied by angle-resolved photoemission spectroscopy 11,12 , which is not possible using polycrystalline samples.…”

Section: Introductionmentioning

confidence: 99%

Electrical and optical properties of epitaxial binary and ternary GeTe-Sb2Te3 alloys

Boschker

Lü

Bragaglia

et al. 2018

Sci Rep

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Phase change materials such as pseudobinary GeTe-Sb2Te3 (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge2Sb2Te5 (GST225), and Sb2Te3. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb2Te3 that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225. Fourier transform infrared spectroscopy at room temperature demonstrates the presence of electronic states within the energy gap for α-GeTe and GST225. We conclude that these electronic states are due to vacancy clusters inside these two materials. The obtained results shed new light on the fundamental properties of phase change materials such as the high dielectric constant and persistent photoconductivity and have the potential to be included in device simulations.

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“…At the same time, superlattices have also been fabricated in a single crystal form using molecular beam epitaxy (MBE) [21][22][23][24] and pulsed laser deposition (PLD) 25 and comprehensive transmission electron microscopy (TEM) experiments were carried out on such samples. [26][27][28][29][30][31] These measurements demonstrated that the (as-grown) structures were different from the original intuitive models proposed for the iPCM SET and RESET states.…”

mentioning

confidence: 99%

Origin of resistivity contrast in interfacial phase-change memory: The crucial role of Ge/Sb intermixing

Saito

Kolobov

Fons

et al. 2019

Applied Physics Letters

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Phase-change memories based on reversible amorphous-crystal transformations in pseudobinary GeTe-Sb2Te3 alloys are one of the most promising non-volatile memory technologies. The recently proposed superlattice-based memory, or interfacial phase change memory (iPCM), is characterized by significantly faster switching, lower energy consumption and better endurance. The switching mechanism in iPCM, where both the SET and RESET states are crystalline, is still contentious. Here, using the ab initio density functional theory simulations, a conceptually new switching mechanism for iPCM is derived, which is based on the change in potential landscape of the band-gap, associated with local deviations from the pseudo-binary stoichiometry across the van der Waals gaps, and the associated shift of the Fermi level. The crucial role in this process belongs to Ge/Sb intermixing on the cation planes of iPCM. These findings offer a comprehensive understanding of the switching mechanisms in iPCM and are an essential step forward to the insightful development of phase-change memory technology. Phase change random access memory (PCRAM) is one of the most promising candidates for the next generation non-volatile memory with phase change materials forming a key component of the recently commercialized 3D XPoint memory technology where sophisticated three-dimensional integration of chalcogenide-based memory and selector components enables much faster writing speeds than Flash and denser capacity than dynamic random access memory (DRAM). 1,2 The development of phase-change materials dates back over three decades, with the most studied materials being Ge-Sb-Te (GST) ternary alloys on the GeTe-Sb2Te3 pseudobinary tie-line, and which found great success in optical disc devices. 3 On the other hand, as optical disc and non-volatile memories clearly have very different requirements, the required material properties also differ. Using the same material, GST alloys, for PCRAM memories does not in itself solve the various difficulties in developing non-volatile memories. A major breakthrough in PCRAM materials was the development of interfacial phase change memory (iPCM) inspired by the Ge-atom flip-flop model. 4-8 In iPCM, the two binary end compounds of the GeTe-Sb2Te3 pseudobinary tie line, GeTe and Sb2Te3, are alternately stacked to form an atomically aligned superlattice structure with van der Waals (vdW) gaps separating covalently bonded blocks. Devices based upon iPCM showed excellent performance such as ultra-low power consumption, faster switching speeds, and longer endurance than conventional alloy-type PCRAM. 4,9 After the development of chalcogenide superlattices and their devices based on GeTe/Sb2Te3 superlattices fabricated from sputter-deposited films, 4,10-13 several specific switching models were proposed based on the idea of bi-layer switching within GeTe blocks. 14-20 In particular switching between inverted Petrov and Ferro structures (Figure 1) was proposed based on the arguments that the relative stability of these two pha...

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Growth of crystalline phase change materials by physical deposition methods

Cited by 18 publications

References 67 publications

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

Electrical and optical properties of epitaxial binary and ternary GeTe-Sb2Te3 alloys

Origin of resistivity contrast in interfacial phase-change memory: The crucial role of Ge/Sb intermixing

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