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The coupling of ferroelectric, photoelectric, semiconducting, and phase transition properties make two-dimensional (2D) In2Se3 a material platform with great application potential in the phase change memory, intelligent sensing, and in-memory computing devices. However, at present, there are unclear phase transition mechanisms and ferroelectric dynamics in 2D In2Se3, which seriously hinder the development of device applications. In this review, we mainly highlight the phase transition mechanisms and ferroelectric devices of In2Se3 beginning with the history of bulk In2Se3 and of 2D In2Se3. The phase transition relations of the four In2Se3 phases, including α-, β-, β′-, and γ-phases, under various driving forces, are summarized. The different driving forces, including temperature, laser, electric-field, vacancy, doping, and strain, are introduced and discussed. Moreover, the phase-control growth of 2D In2Se3 films and their novel ferroelectric device applications are demonstrated. Finally, a perspective on future research directions of 2D In2Se3 is provided. [ABSTRACT FROM AUTHOR]

The coupling of ferroelectric, photoelectric, semiconducting, and phase transition properties make two-dimensional (2D) In2Se3 a material platform with great application potential in the phase change memory, intelligent sensing, and in-memory computing devices. However, at present, there are unclear phase transition mechanisms and ferroelectric dynamics in 2D In2Se3, which seriously hinder the development of device applications. In this review, we mainly highlight the phase transition mechanisms and ferroelectric devices of In2Se3 beginning with the history of bulk In2Se3 and of 2D In2Se3. The phase transition relations of the four In2Se3 phases, including α-, β-, β′-, and γ-phases, under various driving forces, are summarized. The different driving forces, including temperature, laser, electric-field, vacancy, doping, and strain, are introduced and discussed. Moreover, the phase-control growth of 2D In2Se3 films and their novel ferroelectric device applications are demonstrated. Finally, a perspective on future research directions of 2D In2Se3 is provided. [ABSTRACT FROM AUTHOR]

We have prepared Sb70Se30/HfO2 (SS/HO) multilayer thin films by magnetron sputtering method and studied the effect of the HO interlayer on the phase transition behavior of the SS thin films. The results indicate that the introduction of the HO interlayer not only enhances the crystallization temperature, data retention capability, and crystallization activation energy of the SS thin film but also reduces resistance drift and suppresses its crystallization. X-ray photoelectron spectroscopy shows that Sb atoms exhibit a preference for bonding with O atoms at multilayer interfaces, thereby forming stronger Sb–O bonds, which contribute to the thermal stability of multilayer thin films. The Raman results further confirm a reduction in bond length and an increase in bond energy. Furthermore, the SS/HO thin films exhibit enhanced substrate adhesion and a more uniform surface morphology. The study of crystallization kinetics reveals that the crystallization mechanism of SS/HO thin films is predominantly governed by one-dimensional growth. Moreover, the lower threshold switching voltage (1.65 V) in SS/HO thin films contributes to a reduction in power consumption during SET operations. All these findings collectively demonstrate that SS/HO thin films are anticipated to be excellent candidate materials for phase-change memory applications. [ABSTRACT FROM AUTHOR]

The disorder‐to‐order (crystallization) process in phase‐change materials determines the speed and storage polymorphism of phase‐change memory devices. Only by clarifying the fine‐structure variation can the devices be insightfully designed, and encode and store information. As essential phase‐change parent materials, the crystallized Sb–Te binary system is generally considered to have the cationic/anionic site occupied by Sb/Te atoms. Here, direct atomic identification and simulation demonstrate that the ultrafast crystallization speed of Sb–Te materials is due to the random nature of lattice site occupation by different classes of atoms with the resulting octahedral motifs having high similarity to the amorphous state. It is further proved that after atomic ordering with disordered chemical occupation, chemical ordering takes place, which results in different storage states with different resistance values. These new insights into the complicated route from disorder to order will play an essential role in designing neuromorphic devices with varying polymorphisms. [ABSTRACT FROM AUTHOR]

The magnetostructural phase transition in Fe50Al50 alloy with chemically ordered paramagnetic B2 and disordered ferromagnetic (FM) A2 phase has applications in spintronics such as phase-change magnetic memory and magnonic devices. We first conducted a systematic first-principles density functional theory study of the A2 and B2 phases in the Fe50Al50 alloy. A theoretical understanding of this equiatomic alloy's electronic and spin-dynamical properties leads us to the experimental exploration of the FM A2 phase. Therefore, we deposit the 50 nm Fe50Al50 alloy thin film using sputtering and investigate the influence of the Ta buffer layer on its structural and magnetic properties. Our results reveal that the film with a buffer layer exhibits the A2 phase with appreciably higher saturation magnetization (848 emu/cc) than the film without a buffer layer (576 emu/cc). However, the surface roughness and Gilbert damping (α) slightly increase with the presence of the buffer layer from 0.41 to 0.56 nm and 4.35 × 10−3–4.94 × 10−3, respectively. The enhancement in α is due to extrinsic contributions induced by surface inhomogeneities. [ABSTRACT FROM AUTHOR]

Flexible information memory is the key component of flexible electronic devices and the core of intelligent wearable devices. In this paper, Ge/Sb multilayer phase change films of various thickness ratios were prepared using polyether ether ketone as substrate, and their flexible phase change properties and device conversion characteristics were studied. After bending for 100000 times and bending experiments with different bending radius, the film can still realize the transition from amorphous to crystalline states, and the resistance fluctuation was small. Bending, stretching and pressing of the film resulted in grain refinement and increasing of crystalline resistance. The flexible electronic devices using Ge/Sb multilayer films were prepared. The phase change memory device can realize reversible conversion between SET and RESET states with different pulse widths in flat, bent states and after bending many times. All findings show that Ge/Sb multilayer films on PEEK substrate have broad application prospects in high-performance flexible memory in the future. [ABSTRACT FROM AUTHOR]

This work presents the optimization of the crystallization behavior and reliability of Sn15Sb85 thin films by doping Sm element. The phase transition behaviors induced by thermal were investigated by in situ resistance measurement. With the addition of Sm element, Sn15Sb85 film exhibits the superior crystallization temperature (232 °C) and data conservation (172.32 °C for 10 years), larger activation energy of crystallization (4.91 eV) and crystalline resistance (∼103 Ω), which contributes to the increased thermal stability of the amorphous state and decrease in the programming energy. The Sm-doping can broaden the energy band gap from 0.55 to 1.07 eV. The amorphous Sm and Sn compositions could retard grain growth and refine grain size from 21.13 to 11.13 nm, combining with x-ray diffraction and x-ray photoelectron spectroscopy. The surface morphology of Sn15Sb85 film becomes smoother after Sm doping as determined by atomic force microscopy images, resulting in the improved interfacial reliability. Phase change memory devices based on Sm0.095(Sn15Sb85)0.905 films can successfully achieve the complete SET and RESET reversible operation process with high operating speed (200 ns) and low power consumption (1.6 × 10−10 J). The results suggest that doping the proper concentration of Sm element will be an effectual solution to adapt and optimize the crystallization properties of Sn15Sb85 phase change material. [ABSTRACT FROM AUTHOR]

In phase-change memory, heat dissipation towards the electrode is an important obstacle to energy efficiency. Low crystalline resistance requires a higher Joule heat for the RESET operation, and a high current density is required. In this study, Ge2Sb2Te5 (GST) and C super-lattice-like (SLL) films were proposed to reduce power consumption. The 0.3 nm ultrathin C nanolayers led to a relatively low phase change temperature compared with the pure GST. The periodically deposited C nanolayer can act as an ideal thermal barrier and heat source in the SLL film-based device. The grain growth of GST was confined by the carbon layers, indicating a confined heat diffusion. To investigate the heat distribution in the SLL-film-based device cell, we performed two-dimensional finite element analysis. Because of scattering effect, a higher thermal efficiency in the active region can be expected. In the device performance test, a highly reduced SET/SESET plus was observed in the device. Fabricating an ultrathin C nanolayer is an excellent solution for reducing the power consumption of the device. [ABSTRACT FROM AUTHOR]

Thermal stability and phase transition rate are two important issues that restrict the engineering applications of phase change memories. In this paper, GeSb/Sb multilayer films were prepared by multilayer stacking, and their phase transition properties were investigated. GeSb/Sb multilayer films had higher crystallization temperature, better data retention, and lower resistance drift than pure Sb films, which indicated that they had excellent thermal stability and accuracy of resistance identification. The test results of X‐ray diffraction and atomic force microscopy showed that GeSb spacer layers could inhibit grain growth of Sb film layers, thus reducing the roughness and potential fluctuation of the film surface. X‐ray reflectivity experiments showed that the GeSb/Sb multilayer films before and after crystallization had small volume changes. The devices based on [GeSb(7 nm)/Sb(3 nm)]8 multilayer films could achieve reversible resistive transition operation between SET and RESET at a pulse width of 10 ns, showing satisfactory transition rate and operational power consumption. The low resistance drift and ultra‐fast phase transition speed make GeSb/Sb multilayer films one of the promising choices for phase change memories. [ABSTRACT FROM AUTHOR]

The layered semiconductor In2Se3 has a low temperature crystalline–crystalline (α → β) phase transformation with distinct electrical properties that make it a promising candidate for phase change memory. Here, using scanning tunneling microscopy, correlative in situ micro‐Raman, and electrical measurements, it is shown that the β phase can persist in bulk crystals at room temperature in non‐oxidative environments. Of particular note, the stability of β phase crystals in ambient conditions under encapsulation of graphene and similar passivation layers, is reported for the first time. The strategy of encapsulation to ensure the persistence of β phase overlaps with efforts to passivate switching materials. It is further demonstrated that degradation from the elevated temperatures required for the phase change is slowed through examination of Raman signatures. These results demonstrate an alternative method of phase manipulation with a new stabilization of β‐In2Se3 in ambient conditions potentially extendable to other polymorphic materials, and the importance of passivation in In2Se3 memory devices. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is an exceptional universal memory innovation that offers qualities like fast speed and non-volatility. But major obstacle still exists in achieving better thermal stability and rapid switching at the same time. Therefore, it is crucial to investigate new materials with enhanced properties. To overcome the contrast of access time between storage systems and memories, storage class memory concept based on emerging nonvolatile memory technology has been introduced. Resistive switching has gained much awareness in last decade due to its application in PCM technologies. Investigation of resistive switching on novel Te (1−x) (GeSe 0.5)Sc x (x = 0, 0.05, 0.1, 0.15) quaternary alloy has been analyzed in this paper. Material has revealed electrical resistivity contrast of ∼103 orders of magnitude that offers better thermal stability than the majority of other phase transition alloys. Additionally, current-voltage (I-V) properties and voltage dependent threshold switching support an incredibly fast threshold switching operation. Switching is observed at 18–30 V and threshold voltage has been noticed to decrease (10–12 V) with increasing temperature. From XPS study, it can be observed that binding energy is shifting towards lower energies by 0.25 eV. Sc makes the GeTeSe parent material appealing for next-generation high-speed, non-volatile, and improved thermal stability memory applications. [Display omitted] • Electrical resistivity contrast of ∼103 orders of magnitude. • Better thermal stability than the majority of other phase transition alloys. • Switching is observed at 18-30 V and threshold voltage has been noticed to decrease (10-12 V) with increasing temperature. • The barrier-cluster model is used to interpret compensating effect in thermally controlled resistive switching and d.c. conduction. • Sc makes the GeTeSe appealing for next-generation high-speed, non-volatilememory applications. [ABSTRACT FROM AUTHOR]

In this paper, the phase transformation properties of C/Sb composite multilayer films based on flexible substrates were studied. After different times of bending, the flexible films can still achieve amorphous to crystalline phase transition, but the crystalline resistance increases to different degrees. When the bending times are less than 15000 times, the deformation makes the grain fine, the surface of the film tends to be flat, the surface electric field effect is weakened, and the resistance drift is reduced. When the bending times are higher than 15000 times, cracks begin to appear in the film, and the cracks' depth and width increase with the bending times. The distribution of C and Sb elements at the crack was analyzed by scanning electronic microscope energy spectrum. The phase change memory device of C/Sb composite multilayer film based on flexible PEEK substrate has been made, and the rapid reversible resistance transition (∼10 ns) in various complex states has been successfully achieved. This research provides a new idea for flexible information storage. • The addition of carbon increases the phase transition temperature and improves the thermal stability of pure antimony. • Complete the phase transition in the case of external bending and internal bending. • After 80,000 bending, the phase transition can still be completed, showing good bending resistance. • The device made of composite material realizes the SET-RESET process at the pulse width of 10ns. [ABSTRACT FROM AUTHOR]

[Display omitted] • [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 Superlattice-like film exhibits high comprehensive performance in phase change memory. • C3/SS3 SLL film has a high crystallization temperature (∼240 ℃) and a ten-year data retention temperature (∼170 ℃). • The PCM based on C3/SS3 SLL film has ultrafast operating speed (∼6 ns) and low power consumption (∼2.18 × 10-11 J). • The addition of carbon nanolayers has formed C-Sb and C-Se bonds, which have been verified by experiments and simulations. The C/Sb 7 Se 3 superlattice-like film was prepared by magnetron sputtering technology, and the effects of carbon nanolayers on the film and device properties were studied. The C/Sb 7 Se 3 superlattice-like films show excellent thermal stability and data retention, especially the [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 film. The analysis of transmission electron microscopy and x-ray diffraction reveal that the high-quality multilayer structure of [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 is maintained. The segmentation of phase change material by amorphous carbon layer reduces the size of crystalline region, thus accelerates the crystallization rate. Element mapping indicates that C atoms and Se atoms in the annealed [C(3 nm)/Sb 7 Se 3 (3 nm)] 5 films show obvious interlayer diffusion. The results of x-ray photoelectron spectroscopy and density functional theory manifest that atomic diffusion results in the formation of strong C-Sb and C-Se bonds in the phase transition layer and C atom prefers to occupying interstitial site in the center of the periodic structural unit of Sb 2 Se 3 phase. Carbon atoms can effectively suppress grain growth and gradually move to the boundary of the grain, refining grain size, thus improving the stability of the amorphous phase. This study indicates that carbon nanolayers play an important role in improving the thermal stability and operation speed, reducing power consumption and resistance drift in Sb 7 Se 3 films. [ABSTRACT FROM AUTHOR]

In this paper, the flexible phase change behavior of Ge 2 Sb 2 Te 5 /Mg 35 Sb 65 superlattice-like memory based on PEEK substrate is investigated in detail. The flexible memory has good thermal stability (T c ∼270.4 °C), resistance to bending (more than 2 ×104 bending times), low resistance drift (less than 25% of the Ge 2 Sb 2 Te 5 film), stable amorphous to crystalline resistance ratio (∼2 orders of magnitude) and ultra-fast information access speed (∼5 ns). Bending refines the grain of the film and widens the band gap. This study provides a potential option for flexible information storage electronics. • Ge 2 Sb 2 Te 5 /Mg 35 Sb 65 superlattice-like films on PEEK substrates possess good thermal stability and mechanical bending durability. • After several bending cycles, the FPCM can even complete the ultrafast phase transition within 5 ns. [ABSTRACT FROM AUTHOR]

Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160 °C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220 and 240 °C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to 〈 11 2 ¯ 0 〉 -oriented t-grains occurs at 280 °C. Unlike other grains, the 〈001〉 t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers. [Display omitted] [ABSTRACT FROM AUTHOR]