REVIEW
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Reversible Crystalline-Crystalline Transitions in
Chalcogenide Phase-Change Materials
Bin Liu, Kaiqi Li, Jian Zhou, and Zhimei Sun*
depicted in Figure 1a,b.[11,12] Initially, crystalline PCMs are heated above their melting temperature by either a laser beam
or a short-width, high-amplitude electrical pulse. Subsequently, the melted PCMs
are rapidly quenched to the non-crystalline
(amorphous or glassy) state. This two-step
process is referred to as reset/write. During this reset/write process, a non-crystalline
mark is created against the crystalline background to represent a recorded bit. The
disparity in optical or electrical properties between the non-crystalline and crystalline states allows for easy retrieval of
the recorded information. To revert the
non-crystalline mark back to its original crystalline state, a longer, moderateamplitude laser beam or electrical pulse
is employed to heat the non-crystalline
PCMs just above their crystallization temperature. This heating process triggers
the rearrangement of atoms, transitioning the material back to an ordered state,
known as set/erase. It is necessary to
highlight that both laser beams and electrical pulses operate on
thermal effects to trigger the typical phase transition between
non-crystalline and crystalline states of PCMs, denoted as the
non-crystalline-to-crystalline phase transition (n-CCPT) in this
review.
After over 60 years of development, PCRAM has been considered the most technologically mature candidate for the next
generation of non-volatile memory.[13–16] In 2015, Intel and Micron jointly announced the first electronic PCRAM product
based on a 3D XPoint memory architecture.[17,18] Subsequently,
Intel introduced Optane solid-state disk (SSD) and persistent
memory,[19,20] while Micron unveiled the fastest SSD utilizing
the 3D XPoint Technology.[21] Positioned between NAND flash
and dynamic random access memory (DRAM) in terms of performance and cost, PCRAM is anticipated to enter the market to narrow the performance gap between storage-class memory and DRAM technologies (Figure 1c[22] ). Notably, recent advancements have demonstrated that PCMs have the potential
to emulate the analog characteristics of biological synapses and
neurons by leveraging continuous transitions in their electrical/optical properties.[23,24] Consequently, PCMs have emerged
as a promising medium for memristive and neuro-inspired electronic/optical devices (Figure 1d).[25–27] More specifically, the partial disordering of crystalline PCMs can yield multiple reset states,
meanwhile, the stepwise set process can be achieved through the
Phase-change random access memory (PCRAM) is one of the most
technologically mature candidates for next-generation non-volatile memory
and is currently at the forefront of artificial intelligence and neuromorphic
computing. Traditional PCRAM exploits the typical phase transition and
electrical/optical contrast between non-crystalline and crystalline states of
chalcogenide phase-change materials (PCMs). Currently, traditional PCRAM
faces challenges that vastly hinder further memory optimization, for example,
the high-power consumption, significant resistance drift, and the
contradictory nature between crystallization speed and thermal stability,
nearly all of them are related to the non-crystalline state of PCMs. In this
respect, a reversible crystalline-to-crystalline phase transition can solve the
above problems. This review delves into the atomic structures and switching
mechanisms of the emerging atypical crystalline-to-crystalline transitions, and
the understanding of the thermodynamic and kinetic features. Ultimately, an
outlook is provided on the future opportunities that atypical all-crystalline
phase transitions offer for the development of a novel PCRAM, along with the
key challenges that remain to be addressed.
1. Introduction
Since Ovshinsky filed the patent entitled Resistance Switches
and the Like in 1962,[1] phase-change random access memory
(PCRAM), exploiting a substantial change in electrical or optical properties of chalcogenide phase-change materials (PCMs),
has become a promising technology for data storage.[2–10] The
working principle of PCRAM is relatively straightforward, as
B. Liu, K. Li, J. Zhou, Z. Sun
School of Materials Science and Engineering
Beihang University
Beijing 100191, China
E-mail: zmsun@buaa.edu.cn
B. Liu
National Key Laboratory of Spintronics
Hangzhou International Innovation Institute
Beihang University
Hangzhou 311115, China
B. Liu
State Key Laboratory of Materials for Integrated Circuits
Shanghai Institute of Microsystem and Information Technology
Chinese Academy of Sciences
865 Changning Road, Shanghai 200050, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202407239
DOI: 10.1002/adfm.202407239
Adv. Funct. Mater. 2024, 34, 2407239
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Figure 1. PCMs and PCRAM. a,b) The working principle of PCMs for PCRAM. c) The current memory hierarchy in terms of performance and cost. PCRAM
combines high speed, non-volatility, high storage density, and manufacturing cost.[50] d) Biological neuron based on PCRAM cells.[28] e) Properties
comparison of PCMs with other three types of resistive switching material (RSM). Artificial neural networks and spiking neural networks are respectively
abbreviated as ANN and SNN.[51] Panels (a,b) are Reproduced with permission.[12] Copyright 2007, Springer Nature. Panel (c) is Reproduced with
permission.[50] Copyright 2019, Springer Nature. Panel (d) is Reproduced with permission.[28] Copyright 2016, Springer Nature. Panel (e) is Reproduced
with permission.[51] Copyright 2020, Springer Nature.
application of medium-amplitude multiple-short voltage pulses,
reflecting a gradual crystallization of non-crystalline PCMs.[28]
Furthermore, the electrical/optical properties of a given state in a
single PCRAM cell depend on its excitation history, enabling the
construction of intricate neural networks based on an extensive
array of PCRAM cells. With the demand for increased computing
power due to the rapid growth of artificial intelligence, PCRAM
technology has taken a prominent role in designing and implementing specialized electronic computing systems to bolster processing in artificial neural networks,[28–33] such as applicationspecific integrated circuits,[34] neuromorphic computing,[35] and
in-memory computing[36] devices.
The unique storage principle gives PCRAM based on nCCPT (PCRAM-n-CCPT) a unique set of features, such as
good radiation-tolerance ability,[37] high scalability,[7] and multilevel storage capability,[38] making it a technologically mature candidate for advanced non-volatile memory. Nevertheless,
the n-CCPT also brings PCRAM some inherent bottlenecks
(Figure 1e). First, the contradiction between thermal stability and
crystallization speed.[39,40] Thermal stability refers to the ability
Adv. Funct. Mater. 2024, 34, 2407239
of PCMs to resist crystallization under thermal fields. Therefore, the usual method of ensuring stability is to sacrifice speed.
Another bottleneck is the high power consumption associated
with the reset operation of PCMs,[41,42] wherein the melting of
PCMs leads to a power-intensive process (the power consumption during the reset operation of Ge2 Sb2 Te5 -based PCRAM is
≈901.8 pJ[43] ). Additionally, PCRAM-n-CCPT faces the issue of
serious resistance drift.[44] During the reset operation, a highstrain, low-order non-crystalline state was created, which undergoes spontaneous structural relaxation toward an energetically
more favorable ideal glassy state (called physical aging).[45] Physical aging would lead to a resistance increase over time, viz. resistance drift, which is well-recognized and poses challenges to
further optimizing PCRAM for high storage density and neuroinspired computing devices that need significant programming
consistencies.[46] Despite substantial efforts have been made to
understand the crystallization kinetics and structural aging of
non-crystalline PCMs,[47–49] this field is challenged by the presence of varied and sometimes conflicting physical mechanisms
proposed to explain these phenomena, which in turn vastly
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hinder the overall performance optimization of PCRAM-n-CCPT
for universal memory and neuro-inspired computing.
Recently, various atypical phase transitions such as allcrystalline transition,[52] all-noncrystalline transition,[53,54] and
non-thermal amorphization[55,56] have been observed in PCMs.
These atypical transitions significantly differ from the typical nCCPT in terms of microscopic mechanisms and macroscopic
properties. Of particular interest is the reversible crystalline-tocrystalline phase transition (CCPT), which presents an opportunity to overcome the inherent limitations of PCRAM-n-CCPT
since the poor thermal stability, slow crystallization speed, high
power consumption, and severe resistance drift issues of PCMs
are all related to their non-crystalline phase.[57–60] However, the
CCPT is not yet a fully mature PCRAM candidate. A comprehensive understanding is crucial to facilitate its development in data
memory and computing. This review delves into the key aspects
of CCPT, including atomic structures, switching mechanisms,
and optimization strategies. Through our discussion, we aim to
enhance the community’s comprehension of PCMs exhibiting
CCPT and expedite their advancement.
2. CCPT in Ternary Chalcogenide Alloys
Most alloyed chalcogenide PCMs such as GeTe,[61] Sn2 Se3 ,[62]
In2 Se3 ,[63] In3 SbTe2 ,[64] and Ag-In-Sb-Te,[65] are known to exhibit
polymorphism, with multiple crystalline phases existing at various temperatures. However, in present PCRAM storage, only
the reversible n-CCPT is utilized. For example, the canonical
PCMs Ge2 Sb2 Te5 feature one non-crystalline phase and two crystalline phases (metastable cubic and stable hexagonal).[66] During thermal annealing, the non-crystalline Ge2 Sb2 Te5 was first
transformed into the cubic structure at ≈150 °C, leading to a significant resistance decrease of over 4 orders of magnitude.[67,68]
This large contrast in resistance serves as the basis for information storage in Ge2 Sb2 Te5 -based PCRAM-n-CCPT devices. When
the cubic Ge2 Sb2 Te5 is subjected to temperatures of 320–350 °C,
it transitions into the stable hexagonal structure, further reducing its resistance.[69] The phase transitions from disordered noncrystalline to cubic and then to hexagonal phases have been
successfully demonstrated in Ge2 Sb2 Te5 -based PCRAM devices
through current-voltage measurements, heralding an essential
feature for multilevel data storage.[69] However, some studies
indicated that the stable hexagonal phase may lead to compromised storage performance in terms of power consumption,[70]
cyclability,[71] and operating speed.[72] Consequently, preventing
the formation of the hexagonal phase has been identified as a
crucial concern for PCRAM applications, leading to the exclusive utilization of the n-CCPT between non-crystalline and cubic Ge2 Sb2 Te5 for PCRAM design. Until recently, many polycrystalline phase transitions in chalcogenide PCMs were observed
and investigated, foreboding their potential application in data
storage devices.
It has been investigated that the vacancy layers (VLs) in
Ge2 Sb2 Te5 have a pronounced impact on the electrical/thermal
conductivity[73–75] and the CCPT.[76–78] By aberration-corrected
scanning transmission electron microscopy (STEM), Lotnyk
et al.[79] observed the formation and vanish of VLs in cubic
Ge2 Sb2 Te5 induced by the focused electron beam. Therefore,
the work of Lotnyk et al. might be regarded as a prelude to
Adv. Funct. Mater. 2024, 34, 2407239
the CCPT in chalcogenide Ge-Sb-Te alloys. However, the exact atomic arrangement in both cubic and hexagonal Ge2 Sb2 Te5
is still under debate,[80] making it difficult to accurately determine the reversible CCPT mechanism. According to Yamada and
Matsunaga,[81] the 4(a) sites in cubic Ge2 Sb2 Te5 are completely
occupied by Te atoms, while the 4(b) sites exhibit random occupancy by Ge, Sb, and vacancies, with probabilities determined by
the actual composition of Ge2 Sb2 Te5 (Figure 2a). In contrast, utilizing high-resolution TEM, Park et al.[82] proposed that atoms
at the 4(b) site are arranged in order, and Ge and Sb atoms display a tendency to align on particular planes (Figure 2b). Subsequently, Sun et al. asserted ordering within the cations and VLs
of the cubic structure could result in more stable configurations
compared to completely random structures.[6] Through their calculations, they demonstrated the polymorphic nature of the cubic
phase, showing a spectrum ranging from structures with random
vacancies to those featuring ordered VLs.[83]
The hexagonal Ge2 Sb2 Te5 is a high-temperature phase with a
„
space group P3m1.
Its atomic structure is comprised of 9-layer
stacking blocks along the [0001] axis, distinguishing it from its cubic counterpart. However, its exact atomic arrangement remains
a subject of ongoing debate, with three primary proposed stacking sequences along the [0001]. I) the Petrov model,[84] -(Te-SbTe-Ge-Te-Te-Ge-Te-Sb)-, was first introduced to describe the structure of Ge1 Sb4 Te7 , with layers of GeTe situated outside the layers
of Sb2 Te3 .[84] The atomic layers of Te in GeTe create vdW gaps
in this model. II) the Kooi model,[85] -(Te-Ge-Te-Sb-Te-Te-Sb-TeGe)-, was proposed to describe the crystal Gex Sb2 Te3+x (x = 1, 2,
3),[85] wherein the GeTe sublayers are chemically bonded with the
Sb2 Te3 sublayers and the outermost Te atomic layers form vdW
gaps. The Kooi model density functional theory (DFT) calculations have indicated that the Kooi sequence is the most energyfavored configuration.[6,86–88] Nonetheless, Da Silva et al. have
suggested that the stability of the configuration requires Ge and
Sb atoms to reduce strain energy, leading to the atoms mixing.[89]
III) the Matsunaga model:[90] -(Te-Sb/Ge-Te-Sb/Ge-Te-Te-Sb/GeTe-Sb/Ge)-. The controversy on the hexagonal Ge2 Sb2 Te5 structure extends beyond the theoretical domain to experimental observations. Studies have revealed that the electronic and atomic
structures of hexagonal Ge2 Sb2 Te5 exhibit high sensitivity to the
layer sequence.[91] Additionally, the experimental absorption coefficient has indicated a better fit to the Petrov sequence,[92] however, the broadening Raman peaks [93] and the non-uniform contrast of cationic layers[94,95] in HAADF-STEM images have both
indicated a preference for the Matsunaga sequence.[90]
Although the atomic model of Ge2 Sb2 Te5 remains under debate, the underlying mechanism of CCPT has been properly investigated based on the existing microstructure. Solid-state reactions, including CCPT, require both the correct thermodynamic
conditions to provide driving forces and the right kinetic factors to determine the rate and preferred path.[96] For a reaction
to occur, the initial structure should possess a higher energy to
create the necessary driving force of reaction. Using Ge2 Sb2 Te5
as an example, the metastable cubic phase has a higher energy
than the equilibrium hexagonal phase, thus promoting the stabilization reaction (phase transition) from cubic to hexagonal
phases. It is suggested that a diffusionless controlled mechanism is likely to occur during this stabilization process due to
the similar atomic sequence in the cubic and hexagonal lattice
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Figure 2. The atomic model of the cubic structure of Ge2 Sb2 Te5 . a) The cubic atomic model with randomly arranged cations and vacancies. b) The
cubic atomic model with alternatively arranged Ge, Sb, and vacancies. Panels are Reproduced with permission.[82] Copyright 2005, American Institute
of Physics.
structures.[6] This mechanism could involve various processes
such as shearing martensitic movement, epitaxial growth, or vacancy ordering mechanisms.[74,97,98] By investigating the structure characteristics of the cubic Ge2 Sb2 Te5 using first-principles
calculations, Kim et al.[99] proposed a transition model for the
stabilization process (Figure 3a). As the temperature rises, the assembly of Sb and Ge atoms in the cubic phase on the (111) planes
triggers the ordering of cations and vacancies, resulting in a contraction in lattice constants and leading to the cubic-to-hexagonal
transition (Figure 3b).
The stabilization process has also been scrutinized by atomicresolution energy dispersive X-ray spectroscopy and HAADFSTEM.[80,100] VLs are found frequently between the cubic and
hexagonal phases and are considered precursors to vdW gaps
in the hexagonal phases. EDX mapping revealed weaker Sb signals in VLs but stronger signals in neighboring Ge/Sb planes
(Figure 3c). Notably, Te atoms remain nearly immobile during the transition, serving as the structural framework for
both phases. These microstructure characterizations propose an
energy-effective zipperlike transition mechanism (Figure 3d),
Figure 3. Transition model for the stabilization process in Ge2 Sb2 Te5 . a) Atomic disorder-dependent structural stability and lattice constant, and c/a
ratio-dependent total energy. b) Schematic transition model for the stabilization process in Ge2 Sb2 Te5 . c) HAADF-STEM image and EDX mapping of
the transition region. d) The zipperlike transition model for the stabilization process. Panels (a,b) are Reproduced with permission.[99] Copyright 2015,
American Institute of Physics. Panels (c,d) are Reproduced with permission.[100] Copyright 2019, American Physical Society.
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Figure 4. Phase transition from hexagonal to cubic in Ge2 Sb2 Te5 . a–f) Schematic and STEM images of an as-grown (a), a pulse-induced (b), and a
repeatedly switched (c) Ge2 Sb2 Te5 NW. Metastabilization process from hexagonal g) to transitional state h) and final cubic i) phases. j) Switching
performance of this NW device.[101] k) The energy diagram compares the disordered cubic and hexagonal phases of Ge1 Sb2 Te4 with varying degrees of
vacancy layer formation.[103] Configurations marked by red dots are less likely to be observed in experimental settings, as they possess higher energy
levels, making them less energetically favorable when compared to other configurations. Panels (a–j) are Reproduced with permission.[101] Copyright
2016, American Chemical Society. Panel (k) is Reproduced with permission.[103] Copyright 2016, American Physical Society.
whereby under thermal agitation, minor Ge/Sb atoms break
bonds and exchange sites with nearby vacancies in a zipperlike flipping process, leading to Sb-rich atomic arrangements
in the intermediate phase. Subsequently, the relaxation process
and atomic hoppings result in the creation of vdW gaps. Investigations suggest that the atomic arrangements are somewhat sequential, with Sb atoms migrating before Ge atoms. Ge
and Sb atoms respectively favor the inner and the outer layers in the forming stacking blocks.[80] The atomic is arranged
closer to the hexagonal structure as the degree of Ge/Sb migration in the intermediate state is greater. In the final, Sb
and Ge atoms aggregate in the outer and inner layers, respectively, forming the stacking block of -Te-Sbx /Gey -Te-Gex /Sby Te-Gex /Sby -Te-Sbx /Gey -Te- (x > y), resembling the Matsunaga
model.[90]
The critical feature of phase transitions that are designed for
random access memory is reversible, which ensures the encoded
data can be repeatedly erased and written. Therefore, the primary basis for Ge2 Sb2 Te5 to integrate all-crystalline PCRAM is
the reversible CCPT, viz. the stabilization reaction from cubic
to hexagonal phases and the reversible reaction from hexagonal to cubic phases. Contrary to the stabilization transition, the
Adv. Funct. Mater. 2024, 34, 2407239
transformation from hexagonal to cubic phases presents an apparent paradox, as the metastable cubic phase exhibits a higher
Gibbs free energy (G) compared to the equilibrium hexagonal
phase. The absence of a thermodynamic driving force for this
transition is perplexing. Consequently, the change from hexagonal to cubic phase is typically accomplished through a process of
melt-quenching amorphization followed by the crystallization of
the non-crystalline phase. Remarkably, Lee et al.[101] documented
the direct transition from a hexagonal to a cubic phase within a
single-crystal hexagonal Ge2 Sb2 Te5 nanowire (NW) using atomicresolution STEM (Figure 4a–j). The term “metastabilization” was
coined to distinguish this occurrence from the opposing stabilization process. Lee et al.[101] have delineated a two-step mechanism for metastabilization. The first step is characterized by the
relocation of Ge and Sb atoms from their inherent octahedral
positions within the mixed layers of Ge and Sb to the tetrahedral sites of vacancy loops (VLs), which is triggered by an applied
electrical pulse. The second step involves the reconfiguration of
these Ge and Sb atoms, leading to the transformation of an intermediate structure into a metastable cubic configuration. The potential CCPT process via an intermediate metastable local structure in asymmetrical Ge2 Sb2 Te5 NW was further confirmed by
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combining the nanoscale X-ray absorption fine structure (XAFS),
Raman, and TEM analyses.[102]
Meanwhile, a research study was carried out by Privitera
et al.[103] focusing on the impact of ion irradiation on both the
structural and electronic characteristics of a hexagonal Ge1 Sb2 Te4
film. They discovered that ion irradiation could also induce a
metastabilization transition to a rock-salt structure, accompanied
by a metal-insulator transition (Figure 4k). Through DFT simulations, Privitera et al. determined that the total energy of hexagonal Ge1 Sb2 Te4 with partial vacancy layer formation was higher
compared to corresponding cubic models. These findings provided a preliminary theoretical framework, suggesting that the
substantial energy cost linked to the disorder caused by vacancies
in the hexagonal phase was the driving force behind the metastabilized structural transformation. Recently, Jiang et al.[78] observed vacancy disordering in hexagonal Ge1 Sb2 Te4 film as a result of electron irradiation. They attributed this disordering to the
kinetic knock-on collision effects caused by high-energy electron
beams, which generated displacement forces triggering vacancy
diffusion. Jiang et al. demonstrated that the vacancy disordering
process correlated with the metastabilization process. This transition comprised three distinct stages: I) vacancy diffusion within
the trigonal phase, II) alterations in atomic stacking, and III) the
elimination of planes rich in vacancy. These stages were elucidated through in situ TEM experiments and DFT nudged elastic
band calculations. Furthermore, Jiang et al.[78] summarized and
illustrated how vacancy ordering and disordering, induced by either heating effects or displacement forces, played a pivotal role
in driving phase transitions in the Ge1 Sb2 Te4 film.
3. CCPT in Binary Chalcogenide Alloys
The reversible CCPT in ternary Ge-Sb-Te alloys as discussed
above, has garnered significant attention in recent years. Despite
the growing interest, the practical application of CCPT in ternary
Ge-Sb-Te alloys in PCRAM devices remains limited. In contrast,
the reversible CCPT behavior in binary In2 Se3 has been studied
extensively and is seen as more mature for potential data storage applications. In2 Se3 is characterized by a variety of crystalline
phases, encompassing four prevalent ones (𝛼, 𝛽, 𝛾, and 𝛿) as well
as four less frequently encountered ones (𝛼′, 𝛽′, 𝛾′, and 𝜅). Studies on the crystal structure and CCPT of In2 Se3 compounds have
extensively been studied through various analytical techniques
such as lab-based or synchrotron X-ray diffraction, in situ TEM,
differential thermal analysis, and summaries available in works
by Li et al.[63] and Han et al.[104] The phase transitions in In2 Se3
are complex, and at times, ambiguous and conflicting. The generally accepted phase transition sequence with increasing temperature is 𝛼 → 𝛽 → 𝛾 → 𝛿. The 𝛼-phase, characterized by a layered
structure, and the 𝛾-phase, featuring a defective wurtzite structure, are both considered stable at room temperature.[104] In contrast, the 𝛽 and 𝛿 phases are deemed stable only within specific
temperature ranges. This understanding of the phase transitions
in In2 Se3 provides valuable insights into the material behavior
and potential applications in PCRAM devices.
In2 Se3 emerged as a material of interest for binary PCRAM
design in 2005, employing the large resistance contrast between
the non-crystalline and crystalline phases, in line with the typical
n-CCPT framework.[105] Notably, In2 Se3 nanowires exhibit a 105 -
Adv. Funct. Mater. 2024, 34, 2407239
fold variation in electric resistivity upon crystallization, offering a
significantly larger resistance ratio compared to the conventional
Ge2 Sb2 Te5 , which enhances data storage capabilities.[106,107] Furthermore, the higher crystalline resistivity of In2 Se3 nanowires
enhances their ability to efficiently deliver switching power.[106]
In the initial research on PCRAM utilizing In2 Se3 nanowires or
thin films, the resistive switching characteristics were examined
concerning both the duration and the magnitude of the electrical pulses applied,[105–107] the specific crystalline phase present in
the set state remained elusive due to the coexistence of stable 𝛼and 𝛾-phases at room temperature. A significant breakthrough
came when Huang et al. successfully fabricated an Au/In2 Se3 nanowire/Au PCRAM device and tracked the dynamic phase
transition process through a reset/set operation using pulsed and
DC voltage sweeps within an in situ TEM setup.[108] However,
this type of PCRAM based on the typical n-CCPT within In2 Se3
falls outside the scope of this review.
Upon annealing, the bulk 𝛼-In2 Se3 transforms into the 𝛽 phase
at 473 K. However, when the temperature is reduced back to room
temperature, the 𝛽 phase consistently returns to the 𝛼 phase.[109]
Therefore, even if the bulk single-crystal In2 Se3 is used for data
storage, the encoded data is volatile. In contrast to the bulk, polycrystalline In2 Se3 powders have shown the coexistence of 𝛼 and 𝛽
phases at room temperature, suggesting that the 𝛽 phase may be
stabilized as the grain size is reduced.[110] Additionally, it has been
noted that 𝛼-In2 Se3 can transform into the 𝛽 phase under a pressure of 0.7 GPa, and this phase change can be maintained even after the pressure is reduced back to ambient levels.[111] These two
significant studies offer valuable insights into the development of
a stable 𝛽 phase that can persist at room temperature. Soon, thin
layers of single-crystal In2 Se3 , with thicknesses ranging from
≈87 nm down to about 4 nm, were exfoliated from crystalline
powders (Figure 5[109] ). The transition from the 𝛼 to 𝛽 phase,
along with the associated changes in electrical properties due to
thermal heating, were detected. It has also been characterized
that the structural transition from the high-resistance 𝛼 phase to
the low-resistance 𝛽 phase, induced by different forms of energy
input (thermal or electrical), shares a remarkable similarity.[52]
Zhang et al. synthesized 2D In2 Se3 using chemical vapor deposition and discovered that In2 Se3 layers, varying from a single layer
to about 20 layers, could stabilize in the 𝛽 phase with a superstructure at room temperature.[112] Furthermore, recent research
has shown that preventing the oxidation of the In2 Se3 surface can
lead to the persistence of the 𝛽 phase in both bulk and multi-layer
flakes, even after cooling down to room temperature.[113] This
finding highlights the potential use of graphene encapsulation
as an innovative strategy to maintain the stability of 𝛽-In2 Se3 in
open air and at higher temperatures. These findings provide the
basis for designing PCRAM-CCPT devices using single-crystal
In2 Se3 thin layers, which exhibit a stable 𝛽 phase even at room
temperature.
A test PCRAM device (Figure 6) was fabricated by layering In2 Se3 thin layers on graphene.[52] Repeatable set and reset programming was accomplished. Analysis of STEM images,
selected area electron diffraction (SAED) patterns, and corresponding electron energy-loss spectroscopy (EELS) profiles revealed that the set state consists of pure 𝛽-In2 Se3 , while the reset state unexpectedly manifests as the 𝛾 phase, rather than the
anticipated 𝛼-In2 Se3 . In the set state, the 𝛽 phase structure is
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Figure 5. The CCPT and the associated alterations in the electrical characteristics of single-crystal In2 Se3 thin films. a) Atomic force microscopy image
for the as-exfoliated In2 Se3 thin films. Electron diffraction patterns b) and Raman spectra c) are depicted for the as-exfoliated In2 Se3 thin film before
and after the annealing. The micro-Raman spectra d) and the electrical resistance measurements e) are shown for an In2 Se3 thin film during both the
heating and cooling cycles. f) The phase transition temperature from the 𝛼 to 𝛽 phases with a function of the layer thickness. The open circles on the
plot represent the residuals from the data analysis. Panels (a–f) are Reproduced with permission.[109] Copyright 2013, American Chemical Society.
Figure 6. Phase-transition characteristics of In2 Se3 -based all-crystalline PCRAM device. a) A schematic and an optical micrograph for an In2 Se3 test
device. b) The current-voltage characteristics of the device for the periods before, during, and after the phase transition. c,d) STEM images along with
the corresponding SAED patterns for the set and reset state of In2 Se3 devices, respectively. e) The atomic structures for the set and reset states. Panels
(a–e) are Reproduced with permission.[52] Copyright 2017, Wiley.
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distinguished by an octahedral bonding arrangement around
each indium (In) atom. Contrastingly, the reset state is characterized by a tetrahedral arrangement, typical of the 𝛾 phase.
The 𝛾 to 𝛽 phase transition involves a vacancy reconfiguration
process akin to that observed during the stabilization process
in Ge2 Sb2 Te5 materials.[52] The transformation from the 𝛽 to 𝛾
phase might theoretically entail the vertical migration of In atoms
from their octahedral positions to the nearby van der Waals (vdW)
spaces that exist across the selenium (Se) layers. Subsequently,
these atoms could undergo lateral diffusion or displacive shifts
toward tetrahedral sites, influenced by temperature and strain
fields. Thereafter, the vacancies created by this migration would
be harnessed for the ensuing structural reconfigurations that culminate in the reset state.
In recent years, Mori et al. have documented a multilevel reversible CCPT phenomenon in MnTe, where the 𝛽-phase transitions to the 𝛼-phase via an intermediary phase known as 𝛽′phase. This intermediate phase is marked by minor adjustments
in the positions of the tellurium (Te) atoms relative to their arrangement in the 𝛽-phase.[114–120] The transformation from 𝛽 to
𝛽′ is initiated by a puckering mechanism, causing the manganese
(Mn) and Te atomic planes to move in opposite directions along
the c-axis (refer to Figure 7). Subsequently, the transition from 𝛽′
to 𝛼 is initiated by a buckling mechanism, which results in pairs
of Mn and Te atomic planes moving alternately in opposite directions along the [210] crystallographic direction. It is noteworthy
that MnTe films not protected by a tungsten (W) capping layer
failed to maintain the out-of-plane z-axis alignment during the
polymorphic transformation. These observations imply that the
stable two-step polymorphic transformation, which maintains
the z-axis orientation, is likely affected by the confinement effect
of the W capping layer on the MnTe film. Notably, the reversible
displacive transformation between the 𝛽′ (wurtzite-type) and 𝛼
(nickeline-type) phases, facilitated by an atomic-plane shuffling
mechanism under rapid Joule/laser heating, produces significant
electrical and optical contrasts. Furthermore, this approach facilitates the creation of non-volatile memory devices that offer improved energy efficiency and quicker operational speeds when
compared to conventional PCMs that experience diffusional nCCPT transitions. Given that displacive transformations are characterized by diffusionless shearing and shuffling of atoms without the need for random atomic diffusion, their rate is notably
high.[114]
Thus far, the transition from the 2H to the 1T′ phase in MoTe2
has been achieved through the application of heat and mechanical stress. However, the phase transitions that have been reported are volatile, which significantly restricts the potential use
of MoTe2 in various applications, including memory devices, reconfigurable electronic circuits, and topological transistors. Recently, a significant breakthrough was made with the first demonstration of a reversible and non-volatile phase transition between
the 2H and 1T′ phases in a Te-deficient MoTe2 .[121] The energy
gap between these two phases narrows as the concentration of
vacancies in Te increases, and when the concentration of Te vacancies surpasses 4%, the 1T′ phase becomes more stable than
the 2H phase. In its original, unaltered state, the concentration
of Te vacancies is too low for the 1T′ phase to be stable at room
temperature, making the 2H phase the more stable configuration. Upon the application of an electric field, there is a localized
Adv. Funct. Mater. 2024, 34, 2407239
increase in the concentration of Te vacancies as these vacancies
move along the direction of the field. This increase in Te vacancy
concentration further reduces the energy difference between the
2H and 1T′ phases, which can trigger the 2H-to-1T′ phase transition. When the direction of the electric field is reversed, the Te vacancies are driven back in the opposite direction. This migration
leads to a decrease in the concentration of Te vacancies and an
increase in the energy difference between the phases, effectively
returning the material to its original 2H phase. Significantly, this
phase transition process is highly controllable, exhibiting exceptional performance characteristics such as a cycling endurance
surpassing 105 cycles, ultrafast switching times of ≈5 ns for set
operation and 10 ns for reset operation, and remarkable retention
properties exceeding 105 seconds at an elevated temperature of
85 °C.[121]
Recently, Liu et al. reported the reversible cubic-tohexagonal phase transition in yttrium-doped antimony telluride (Sb2 Te3 ).[122] Their research integrated the CCPT with
non-crystalline states to establish a novel multi-level storage
phase-change system. This system exhibits ultralow power
consumption, minimal resistance drift for the lower two states,
and competitive operating speed. The authors determined that
yttrium plays a crucial role in stabilizing the cubic phase and
facilitating the reversible cubic-to-hexagonal transition through
the sequential and directional migration of Sb atoms based
on atomic-resolution structural characterization and ab initio
calculations (Figure 8a–e[122] ). Researchers have provided further
validation of the reversible displacive phase transformations in
yttrium-doped Sb2 Te3 through the use of ab initio molecular
dynamics (AIMD) simulations (Figure 8f[123] ). Specifically, the
forward transformation, which is a displacive shift from the
cubic to the hexagonal phase, occurs in a two-step mechanism
involving shearing and contraction of the Te-Sb-Te-Sb-Te structural units. In contrast, the reverse transformation from the
hexagonal back to the cubic phase requires elevated temperatures and the process of atomic diffusion. This reverse transition
is set in motion by the diffusion of antimony (Sb) cations from
the interlayer regions of the structural units into the interstitial
spaces. The incorporation of yttrium dopants serves to enhance
the thermal stability of the cubic phase, thus promoting the
reverse transformation. These discoveries offer an enhanced
comprehension of the displacive transformations that occur in
the archetypal material Sb2 Te3 and may yield significant insights
for the advancement of PRAM-CCPT technology. The controllable all-crystalline phase transitions in binary chalcogenide
alloys, such as In2 Se3 , MnTe, MoTe2 , and Y-Sb2 Te3 , present
promising opportunities for the development of energy-efficient
and high-speed electronic/photonic phase-change storage, and
neuromorphic computing devices.[122–124]
4. CCPT in Superlattice PCMs
Simpson and his colleagues[15] developed a type of interfacial
PCRAM, based on a superlattice consisting of GeTe and Sb2 Te3
layers with thicknesses ranging between 5 and 40 Å, separated
by vdW gaps (Figure 9a,b). In the reset state, Simpson et al. observed ordered lattices and diffraction spots in the interfacial
PCRAM adjacent to the electrode (Figure 9c,d). It was noted
that interfacial PCRAM in the reset state exhibited a crystalline
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Figure 7. MnTe-based PCRAM and its CCPT mechanism. a) Voltage-dependent resistance curve for MnTe and Ge2 Sb2 Te5 devices. Cross-sectional
diagrammatic (b) and TEM c) image of the MnTe device. d) The SAED patterns are displayed for two regions: the matrix, which is outlined by the
red-dotted line (top), and the active region, enclosed by the blue-dotted line (bottom). The crystal structures of MnTe are illustrated for the e) 𝛽-phase,
f) 𝛽′-phase, and g) 𝛼-phase. The two-stage displacive transition mechanism is described for the phase changes from 𝛽 to 𝛽′ (h) and 𝛽′ to 𝛼 (i). Panels
(a–i) are Reproduced with permission.[114] Copyright 2020, Springer Nature.
structure, contrary to conventional chalcogenide alloys that are
typically non-crystalline at this state. This unique observation
led Simpson et al. to suggest that the transition of interfacial
PCRAM was achieved through the flipping of Ge atoms between
covalently and resonantly bonded sites (Figure 9e), resulting in
a CCPT without melting. Compared to conventional PCRAM
based on chalcogenide alloys, interfacial PCRAM offers advan-
Adv. Funct. Mater. 2024, 34, 2407239
tages in power consumption, switching speed, and cycling endurance. The reduced energy cost of interfacial PCRAM is attributed to a 95% decrease in entropic losses compared to related
alloys.[15] Moreover, the electro-thermal confinement aspect of interfacial PCRAM provides a significant reduction (approximately
four-fold) in the effective cross-plane thermal conductivity of
the superlattice when compared to polycrystalline Ge2 Sb2 Te5 .[125]
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Figure 8. The process of metastabilization from the hexagonal to the cubic phase in yttrium-doped antimony telluride. a–d) HAADF-STEM image
captures the intermediate state during the metastabilization process. e) The enthalpy, entropy, and energy changes of Y-Sb2 Te3 are associated with
varying concentrations of cations within the vdW gap. f) The mechanism of the forward displacive transformation. Panels (a–e) are Reproduced with
permission.[122] Copyright 2021, Elsevier. Panel (f) is Reproduced with permission.[123] Copyright 2023, Elsevier.
Thermal properties play a more predominant role in heat confinement in mushroom cells than electrical properties.[126] Additionally, the proposed CCPT in interfacial PCRAM involves less
atomic diffusion, potentially explaining its superior endurance
and faster switching capabilities at the nanoscale. In conclusion,
the remarkable phase-transition behavior and low power consumption of interfacial PCRAM offer a new avenue for controlling phase transitions in PCMs and enhancing the performance
of PCRAM devices.[127]
Chong et al.[128] were the first to introduce and implement
the superlattice concept in PCRAM by utilizing alternately arranged GeTe, Sb2 Te3 superlattice layers, and vdW gaps along the
z-axis, sparking a surge in research and application of superlattice PCMs following the interfacial PCRAM. The superlattice
PCRAM device has recently garnered considerable attention due
Adv. Funct. Mater. 2024, 34, 2407239
to its promise to reduce energy while also sustaining commendable performance characteristics, including high storage density,
rapid operation speeds, extensive cyclicity, and significant electrical/optical contrast.[129–131] Nonetheless, the atomic structure and
the underlying mechanisms of phase transitions within these devices remain subjects of intense discussion and debate. This ongoing discourse has been identified as a major factor that hinders
advancements in performance optimization and the creation of
innovative PCRAM devices that capitalize on superlattice technology. In early, Petrov[84] and Kooi[85] models that proposed to
delineate the structure of Gex Sb2 Te3+x (x = 1, 2, 3) alloys were borrowed to describe the atomic configuration of the layered blocks
within the GeTe/Sb2 Te3 superlattice devices (Figure 10a). Following this, Tominaga and Kolobov et al. argued that for the GeTe/
Sb2 Te3 superlattice, separating the GeTe and Sb2 Te3 sublayers to
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Figure 9. The interfacial PCRAM device and operating mechanism. a) High-resolution TEM image of an as-deposited interfacial PCRAM cell. b) A meltamorphized dome forms above the TiN heater. c) The SAED pattern for the entire interfacial PCRAM structure. d) High-resolution TEM images and
SAED patterns are displayed for four distinct regions. e) Two potential models are proposed to explain the 1D motion of germanium (Ge) atoms. Panels
(a–e) are Reproduced with permission.[15] Copyright 2011, Springer Nature.
create more vdW gaps is essential.[132] This development resulted
in the formulation of the Inverted Petrov model, a modification
where the order of the GeTe atomic layers is reversed in comparison to the original Petrov model. Additionally, Tominaga et al.[132]
proposed the Ferro model, characterized by a bilayer of GeTe with
a ferroelectric sequence. Various studies have attempted to determine the most stable phase at different temperatures. The Kooi
phase was found to be the most energy-favored at lower temperatures based on AIMD simulations.[6] Conversely, at temperatures exceeding 500 K, the Inverted Petrov model and the Ferro
model were identified as energetically favorable.[132] Subsequent
research conducted by Yu and Robertson, which involved calculating the enthalpy variations concerning temperature using the
phonon dispersion spectrum, demonstrated that the Ferro model
had the greatest stability at temperatures exceeding 125 K.[133]
Nonetheless, the inconsistency in results obtained from different methods has been ascribed to the inadequate consideration
of Ge/Sb intermixing in simulations.[127]
The compounds (GeTe)n (Sb2 Te3 )m , which exhibit a blend of
Ge and Sb in the cationic layers, have been separately suggested by Karpinsky et al.[134] and Shelimova et al.[135] Subsequent research further confirmed the cationic intermixing in
Ge2 Sb2 Te5 alloys.[136–139] Recent microstructure characterization
studies[140–143] have demonstrated that the GeTe/Sb2 Te3 superlat-
Adv. Funct. Mater. 2024, 34, 2407239
tices deposited via molecular beam epitaxy, pulsed laser deposition, or magnetic sputtering also exhibit intermixed Ge and Sb
atoms. Utilizing extended X-ray absorption fine structure spectroscopy and TEM, Momand et al.[140] observed the intermixing of
GeTe and Sb2 Te3 at the interfaces and formed Ge-Sb-Te alloys.[141]
Notably, as revealed by X-ray diffraction and Raman spectroscopy,
the GeTe blocks were found to be incorporated inside the Sb2 Te3
layers, challenging conventional models.[141] Then, a theoretical
model was further put forward to explain the formation of these
Ge-Sb-Te alloys when GeTe is deposited onto Sb2 Te3 in a superlattice arrangement (Figure 10b[144] ). Kolobov et al.[145] have further
shown that the inversion of the Sb-Te terminating layers nearest
to the vdW gaps results in a substantial alteration in the density of
states. This provides a different explanation for the property differences between the set and reset states in the GeTe/Sb2 Te3 superlattices. The stability of Ge/Sb intermixing is attributed to its
ability to enhance configurational entropy and reduce sublattice
mismatch between GeTe and Sb2 Te3 . While traditional superlattice models are constructed based on alternating pure GeTe and
Sb2 Te3 sublattices, it is suggested that simulations or calculations
of the GeTe/Sb2 Te3 superlattice model should take into account
the Ge/Sb intermixing.[127]
Elucidating the phase transition mechanism is another critical
issue for improving the performance of the interfacial PCRAM
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Figure 10. Atomic structure of interfacial PCRAM and the schematic of transition mechanism. a) Simple superlattice stacking sequences in the case
of (GeTe)2 (Sb2 Te3 )1 . b) A step-by-step process of Ge-Sb-Te alloy is formed during the deposition of GeTe onto Sb2 Te3 . Switching model of interfacial
PCRAM based on a double c) and a single d) Ge umbrella flip. The proposed routes of the set and reset process for the switching model are proposed
by Ohyanagi et al. e) and Tominaga et al. f). Panels (a,c,d) are Reproduced with permission.[140] Copyright 2015, IEEE. Panel (b) is Reproduced with
permission.[144] Copyright 2016, American Chemical Society. Panel (e) is Reproduced with permission.[133] Copyright 2015, Springer Nature.
and superlattice device. One of the well-known proposals for
the superlattice device is the CCPT which was proposed as
early as the interfacial PCRAM was invented.[15] Despite numerous hypotheses positing new switching mechanisms to account for the low switching current densities, a consensus has
yet to emerge. Two primary proposals exist for the CCPT mechanism, the first involving a reversible phase transition between
Petrov and Inverted Petrov models (Figure 10c), and the second
proposing a transition between Ferro and Inverted Petrov models (Figure 10d). Initially, Takaura et al. introduced the reversible
CCPT mechanism between Petrov and Inverted Petrov,[146,147]
where the atomic layer sequence transitions from -Sb-Te-GeTe-vdW-Te-Ge-Te-Sb-Te- to -Sb-Te-vdW-Te-Ge-Ge-Te-vdW-Te/SbTe- through the flipping of Ge atoms into the vdW gaps. However, discrepancies between theoretical calculations and experimental results persist, specifically in the resistance ratio. Yu
Adv. Funct. Mater. 2024, 34, 2407239
and Robertson later suggested a two-step process for Ge flipping, with calculated energy barriers indicating challenges in
achieving vertical flipping (Figure 10e[133] ). The computed energy thresholds for the initial phase (vertical flipping) and the
subsequent phase (lateral movement) were identified as 2.59 and
0.05 electron volts (eV), respectively. However, the substantial energy barrier associated with the first step suggests that the vertical flipping of atoms is not a likely occurrence. As a result,
Song et al. proposed that the vertical flipping of Ge atoms is
more probable to occur sequentially, meaning that the Ge atoms
would flip individually rather than all at once in a simultaneous manner.[148] More recently, Nakamura et al. investigated the
switching mechanism by constructing a GeTe/Sb2 Te3 superlattice model and proposed the metastable phase as the set state
in a one-step transition process.[149,150] On the other hand, Tominaga et al.[132] put forth a reversible CCPT mechanism between
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Figure 11. The evolutionary approach to designing vdW heterostructures for interfacial PCRAM. a) The algorithmic process for refining the architecture
of vdW heterostructure superlattices. b) A proposed model for the diffusive atomic switching mechanism of Ge atoms. Panels (a,b) are Reproduced
with permission.[151] Copyright 2016, The Royal Society of Chemistry.
Ferro and Inverted Petrov models, highlighting the transitions
from -Sb-Te-Ge-Te-Ge-Te-vdW-Te-Sb-Te- to -SbTe-vdW-Te-Ge-GeTe-vdW-Te-Sb-Te- through a single Ge-layer flip (Figure 10f). Significant energy barriers were identified for the vertical flipping
and lateral motion steps, underscoring the challenge of achieving transitions in practice.
Using a genetic algorithms (GA) method, Kalikka et al.
investigated alternative structures of GeTe/Sb2 Te3 superlattice
(Figure 11a[151] ), presenting novel models distinct from the established Petrov, Inverted Petrov, Ferro, and Kooi structures.
Their proposed models feature stoichiometric Ge-Sb-Te layer
blocks interconnected through vdW interactions, encompassing
Ge3 Sb2 Te6 and Ge1 Sb2 Te4 layer blocks. They highlight the phasetransition mechanism involving Ge-atom flipping, where flipping a Ge layer in Ge3 Sb2 Te6 into the vdW interspace adjacent
to Ge1 Sb2 Te4 transforms the structure into two vdW-connected
Ge2 Sb2 Te5 layer blocks. The calculated energy barriers for this
transformation range from 1.83–1.99 eV (Figure 11b[151] ), albeit
relatively high for rapid switching. Tominaga et al. proposed that
the transition in interfacial PCRAM is driven by a polarized electrical field due to its polar switching mode.[146,149] In contrast,
Bolotov et al.[152] investigated the switching of electric conductance triggered by voltage pulses through the use of scanning
probe techniques. They noticed a current response that was delayed, occurring between 0.05 to 10 seconds after the applica-
Adv. Funct. Mater. 2024, 34, 2407239
tion of the voltage pulse, suggesting an electric field-induced
switching mechanism. Conversely, Nakamura et al. contend that
the driving force in nonpolar interfacial PCRAM switching is
the Joule heat generated by the current rather than the electrical field effect.[149] All the above research suggested that Geatom flipping has a critical role in this CCPT of GeTe/Sb2 Te3
superlattice.
Research by Hase et al. and Makino et al. utilizing coherent phonon spectroscopy investigated the transition dynamics
in GeTe/Sb2 Te3 superlattice.[153–157] The changes in phonon frequency induced by femtosecond laser were detected and were
attributed to the variation of Ge configuration. Furthermore,
the frequency change influenced by the laser pulse polarization
was further discovered, pointing to the anisotropic flipping of
Ge atoms. However, the technique remains limited in capturing the atomic-level details, merely capturing transient states.
In contrast, observations from HAADF-STEM characterization
reveal the flipping of the Ge/Sb plane into neighboring vdW
gaps within the GeTe/Sb2 Te3 superlattice induced by electron
exposure.[142,158] Notably, the kinetic energy of an electron beam
surpasses that of typical electrical or optical pulses used for
switching. While existing models examine Ge atom flipping near
vdW gaps in superlattices, they fail to account for the critical factor of Ge/Sb intermixing, significantly contributing to the behavior of the GeTe/Sb2 Te3 superlattice.[140–143]
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The proposed crystalline-crystalline transition has been discussed since the invention of the interfacial PCRAM, with some
researchers considering it as a potential mechanism.[15] However, Momand et al.[140] questioned the ability of the TEM characterization provided by Simpson et al. to differentiate whether
the reset state is partially or entirely crystalline. Contradictory
findings to the crystalline-crystalline phase transition hypothesis, such as the commonly observed melt-quench-induced nCCPT,[159] as well as partial melting or amorphization,[160,161]
strain-assisted phase transition,[162,163] and stacking fault-assisted
metal-insulator transition,[164,165] have also been documented.
Recent studies by Yoo et al. suggested a different perspective,
proposing that the reset switching in interfacial PCRAM occurs through a transition from the GeTe/Sb2 Te3 superlattice
to the cubic Ge2 Sb2 Te5 alloy followed by melt-quenching-free
amorphization.[129] Interestingly, the set operation no longer reverts the material to the initial superlattice but to the FCC structure, potentially indicating that the superlattice might not be
actively involved in subsequent cycling processes. This finding
raises questions about the critical role of interfaces in interfacial
PCRAM devices, despite the uncertainties regarding the superlattice’s exact influence on device performance. Supporting this
notion, research indicates that both the switching current density
and resistance drift coefficient decrease as the superlattice period
thickness decreases, reflecting a higher density of interfaces.[166]
In conclusion, the debate surrounding the phase change mechanism of interfacial PCRAM and superlattice phase change memory persists due to the intricate manufacturing processes and
complex structural considerations.
5. Conclusion and Outlook
This systematic review examines the historical development and
recent advancements of PCMs featuring reversible CCPT. Analysis is conducted on the properties, structures, and potential
switching mechanisms of these PCMs. One of the primary challenges in this area pertains to the ambiguity of the structure and
switching mechanisms of PCRAM-CCPT. To address them, it is
vital to meticulously compare the structures of the set/reset states
in practical devices using advanced structural characterization
techniques, such as ultrafast dark-field electron microscopy.[167]
In addition to experimental approaches, AIMD has become a
valuable theoretical tool for modeling the structural characteristics and dynamic behaviors of chalcogenide alloys over the past
few decades.[168–173] However, the computational expense associated with AIMD hinders the transition from material-level simulations to real device-level simulations, presenting a significant
obstacle that necessitates the consideration of realistic operating
conditions such as active volume fluctuations and applied electric biases.[174,175] The advent of artificial intelligence has led to
the development of machine learning-based interatomic potential models, derived from a range of representative configurations. These models combine the efficacy of empirical potentials
with the precision of DFT, enabling a deeper understanding of
the structural, chemical, and kinetic characteristics of PCMs.[176]
Several machine learning potentials, including artificial neural
networks and Gaussian approximation potentials (GAP), have
been utilized to explore the properties and behaviors of materials such as GeTe[177–180] and Ge2 Sb2 Te5 .[181–184] Through large-
Adv. Funct. Mater. 2024, 34, 2407239
scale simulations, it is now possible to elucidate the crystal structure and phase transition mechanisms of CCPT, with a particular focus on unraveling the role of interfaces within superlattice
PCRAM.
Recently, materials such as Ge-Sb-Te,[101] Y-Sb2 Te3 ,[122]
In2 Se3 ,[185] MoTe2 ,[121] and GeTe/Sb2 Te3 superlattices[15] exhibit
structural transitions between two crystalline phases potentially
affording an insulator-metal transition. However, PCMs with
characteristics of reversible CCPT have not received sufficient
attention. Compared to typical n-CCPT, CCPT possesses many
advanced phase-change natures in data storage and processing.
First, CCPT does not undergo a melt-quench process during
the reversible phase transition, which gives it the advantage of
low power consumption and slight thermal crosstalk.[15] Second,
CCPT is a diffusionless transition through shearing and shuffling of atoms and does not require random diffusion of atoms,
enabling memory with lower energy, faster operation, and a
more controlled phase transition process compared with the
conventional diffusional n-CCPT.[114] Third, CCPT has improved
cycling endurance than n-CCPT, as cycling failures, including
stuck-set (caused by elemental segregation) and stuck-reset
(caused by void formation), are essentially caused by atomic
migration.[186] Various driving forces such as hole-wind force,
electrostatic force, and crystallization-induced segregation have
been identified as responsible for the atomic migration, and
have a strong effect in regions where the PCMs are molten
or undercooled liquid. In contrast, CCPT does not involve a
molten or undercooled liquid phase, thus suppressing atomic
migration and improving cycling endurance. Fourth, the optical/electrical signal of PCRAM-CCPT is more stable than that
of non-crystalline chalcogenide alloys due to the resistance
drift issue induced by spontaneous structural relaxation, which
impedes conventional chalcogenide alloys for multilevel storage
and neuro-inspired computing.[46]
Due to the advanced phase-change properties, PCMs featuring reversible CCPT characteristics are poised to revolutionize
the development of high-performance binary PCRAM devices.
Despite the nascent stage of research on reversible CCPT and
the resulting limited performance data, the potential of CCPT
is already being realized in superlattice-like devices. Notably,
these superlattice-based PCRAM devices demonstrate a remarkable reduction in energy consumption to just 12% of that required by comparable Ge2 Sb2 Te5 -based devices.[15] Additionally,
they achieve a significant reduction in operational time by 75%.
Most impressively, the cycling performance of these devices has
been enhanced by an extraordinary three orders of magnitude,
underscoring the promising future of CCPT in this field.[15] In
addition, compared to the typical diffusional n-CCPT, the diffusionless CCPT is a more controllable phase transition process. PCMs with characteristics of CCPT thus have greater potential in achieving more quantity and precise intermediate storage states.[32] Combining the characteristics of high stability, low
power consumption, and long durability, such PCMs show great
application prospects in the field of integrating multilevel data
storage devices and neuromorphic computing chips. Unfortunately, PCMs with reversible CCPT, especially those with large
storage windows, remain very rare. More efforts should be made
to explore new materials with this characteristic, to optimize synthesis strategies, to strengthen structure characterization, and to
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reveal phase transition mechanisms, and finally to achieve effective control of reversible CCPT. Material exploration aims to identify and develop new materials with inherent CCPT characteristics, that can offer improved performance metrics over existing
PCMs. The direct strategy is refining the synthesis techniques
to produce materials with more uniform and desirable properties, which has a great effect on improving the efficiency and endurance of CCPT-type PCRAM devices. Revealing the structural
transformation mechanism of CCPT by using the most advanced
structural characterization techniques and computational simulation methods is also crucial for achieving effective control over
the reversible CCPT process and is essential for the practical implementation of high-performance PCRAM devices. Meanwhile,
research on its application in CCPT memory devices and neuromorphic chips should also be carried out as soon as possible.
Acknowledgements
Financial support was provided by the National Natural Science Foundation of China (Grant Nos. 52332005 and 62304016). B.L. acknowledges
support from the Open Research Fund of State Key Laboratory of Materials
for Integrated Circuits (No. NKLJC-K2023-07), and Fundamental Research
Funds for the Central Universities (Grant No. 62304016).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
All-crystalline phase-change memory, Crystalline-crystalline, Phase
transition, Phase-change materials, Phase-change memory
Received: April 28, 2024
Revised: June 23, 2024
Published online: July 16, 2024
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Bin Liu is an associate professor at National Key Laboratory of Spintronics, Hangzhou International
Innovation Institute of Beihang University, China. He received his Ph.D. degree of Materials Science
from Beihang University in 2021. His current research interests are the structure and properties of
phase-change materials for random access memory.
Kaiqi Li received his Ph.D. degree of Materials Science from Beihang University in 2024. His research
interests are multi-scale simulations of phase-change random access memory and the transport theory of 2D devices.
Jian Zhou is a professor at the School of Materials Science and Engineering, Beihang University, China.
He obtained his Ph.D. from the Institute of Metal Research, Chinese Academy of Sciences in 2003.
He worked at RWTH Aachen University (Germany), Royal Institute of Technology (Sweden), and Xiamen University (China), before joining Beihang University. His research interests are intermetallics,
thermoelectric materials, low dimensional magnetic materials and 2D transition metal carbides and
borides (MXenes and MBenes). He has published over 180 peer reviewed SCI papers.
Adv. Funct. Mater. 2024, 34, 2407239
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Zhimei Sun is a Cheung Kong Scholar Chair Professor at School of Materials Science and Engineering of Beihang University, China. She received her Ph.D. of Materials Science from Institute of Metal
Research (CAS) in 2002, and after which she worked at RWTH Aachen University (Germany) and Uppsala University (Sweden) from 2002 to 2007, and at Xiamen University (China) from 2007 to 2013. Her
research includes phase-change memory materials, high-performance structural materials and 2D
transition metal carbides/borides.
Adv. Funct. Mater. 2024, 34, 2407239
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