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Multi-level phase-change (MLPC) memory can not only further raise the storage density of high-density phase-change memory, but also have significant applications in neuromorphic computing. However, acquisitions of MLPC materials are very challenging. MLPC is only observed in a small number of phase-change materials and can't be designed determinately because the mechanism of MLPC emergence has been understood incompletely. Here, we investigate phase-change behaviors of six Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) bilayer films with x = 5, 10, 15, 20, 35, and 50. Three-level phase-change memory is observed for five bilayer films with x < 50, but not for one with x = 50, revealing the dependence of MLPC memory on the thickness ratio of two constituent layers composing bilayer films. A parallel resistance model is proposed to describe the measured sheet resistance of bilayer films and can be used to explain the thickness-ratio dependence of three-level phase-change memory very well. Subsequently, a design rule for MLPC memory films is given out unambiguously. Based on the parallel resistance model, MLPC memories are simulated for bilayer and trilayer films. Three-level memory indeed appears in bilayer films only when the design rule is met, confirming the validity of our design rule. Four-level memory is predicted in trilayer films as the design rule is obeyed. [Display omitted] • Multi-level phase-change behaviors of six bilayer phase-change films, Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) with x = 5, 10, 15, 20, 35 and 50, were studied by the measurement of sheet resistance as a function of heating temperature. • Appearance of three-level phase-change behaviors depended on the thickness ratio of Ge 2 Sb 2 Te 5 and Sb 7 Se 3 layers. Similar phenomena were reported previously in the literatures, but not understood and explained. • A parallel resistance model was proposed to describe the sheet resistance of bilayer films. Based on the model, the thickness-ratio dependence of three-level phase-change behaviors could be explained very well. • A design rule of multi-level phase-change multi-layer films was given out for the first time. Multi-level phase changes were simulated for bilayer films based on the sheet resistance model. It was found that three-level phase changes indeed emerged only when our design rule was met totally, conforming the validity of our design rule. Furthermore, a scheme of trilayer films was proposed. Its multi-level phase changes were simulated. Four-level phase changes were predicted only if the design rule was obeyed totally. • Multi-level phase-change films could be designed determinately under the guidance of the design rule, no longer be encountered and observed accidentally. Our results pave the way for determinate design of desired multi-level phase-change films. [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]

The phase change material Sb 2 Te 3 (ST) exhibits a fast-switching speed and poor thermal stability because the four-fold rings are dominant and homo-polar bonds are absent in the amorphous state. Many studies have been performed on the improvement of phase change properties. Chemical doping methods have been widely used because additional chemical bonds are formed between the doped element and host atoms, leading to the modification of the local structure and electronic band structure. Previous reports have indicated the importance of the bond length and lattice mismatch in element doped ST. In this study, the electronegativity of the doped elements was investigated because it determines the strength of the bond via charge transfer between two atoms. A strong correlation between the electronegativity of the doped element and the phase change properties of the doped ST was observed. An ideal electronegativity range, which corresponds to some elements doped ST with excellent thermal stability, was found in this investigation. Nb atoms were selected as the optimal dopant to stabilize the amorphous phase of ST because Nb was in the ideal range. Additional homo-polar bonds and Nb-Te bonds can stabilize the amorphous phase because additional energy is required for ring reconstruction during crystallization. Consequently, the thermal stability and resistance difference of the Nb doped ST were significantly improved. In addition, the phase change speed of ST was not affected by Nb doping. This study highlights a new approach for pursuing better properties of phase change materials by chemical doping methods. • Electronegativity of doped element plays a crucial role in tailoring the Sb 2 Te 3 (ST) properties. • An ideal electronegativity range corresponds to the elemental doping of ST. • Nb was selected as the optimal dopant to stabilize the amorphous phase of ST. • The Nb-ST material is a potential candidate for PCM applications. • This work provides a new approach for pursuing better properties by doping methods. [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]

The ultimate goal of memory researchers and developers is to devise a universal memory to replace all memories in complex hierarchies of memory systems. Phase change memory is considered by the International Semiconductor Industry Association as the most likely emerging storage technology to achieve this ambitious goal due to its excellent performance. However, the relatively slow operation speed and poor cycle endurance are stumbling blocks for this purpose. Here, we propose a new doping strategy to construct stable bonding by introducing atoms with large electronegativity differences from the matrix Te atoms enhancing the capture ability and the pinning effect to Te atoms, which facilitates fast nucleation and suppresses migration of Te atoms to achieve significant performance improvement. Using ab initio molecular dynamics simulations, we explore and compare the atomic structures, chemical bonding, and dynamics properties of Hf doped Sb 2 Te 3 (HfST), La doped Sb 2 Te 3 (LaST), Rb doped Sb 2 Te 3 (RbST) and Sr doped Sb 2 Te 3 (SrST). In the XST (X refers to Hf, Rb, La and Sr atoms) systems, the increase of the more stable ring structure fluctuation and the stronger atom capture capability of rings enable more and faster formation of nuclei during crystallization. Higher Peierls-like distortions suggest better thermal stability. The motility of Te atoms is suppressed due to the pinning effect. High coordination of atoms and the concentration reduction of the lone-pair electron and void enable small density difference. Our results show that fast universal memory with good endurance can be achieved by the construction of strong bonding by doping atoms with large electronegativity differences, providing an important guidance for experiments and industries. • Constructing stable bonding accelerates the crystallization rate of the system. • Constructing stable bonding improves the cycle endurance of the Sb 2 Te 3 system. • COHP analysis corroborate the strong atom capture ability after doping. • MSD analysis of the systems confirm the pinning effect of the doped atoms. • ELF and LED demonstrate the reduction of density difference after doping. [ABSTRACT FROM AUTHOR]

The superlattice-like Sn 15 Sb 85 /Ti phase change films were fabricated by magnetron sputtering, and the effects of Ti layer on the phase change properties and crystal structure of the Sn 15 Sb 85 films were systematically researched. With the addition of Ti layers, the crystallization temperature of the films increased, indicating their thermal stability and data retention were improved. X-ray diffraction results reflect that the presence of Ti layers restrains grain growth and refines grain size. X-ray photoelectron spectroscopy analysis reveals that the incorporation of Ti layers enhances the interatomic binding energy and improves the stability. X-ray reflectivity analysis and atomic force microscopy indicate that the Sn 15 Sb 85 /Ti films have smaller volume change and smoother surface than pure Sn 15 Sb 85 films, implying the better interfacial property and reliability. The T-type phase change memory cell based on Sn 15 Sb 85 /Ti thin film has faster operation speed (100 ns) and lower power consumption (2.6 ×10−12 J) than the conventional Ge 2 Sb 2 Te 5 material. The results show that the superlattice-like Sn 15 Sb 85 /Ti film will be a potential candidate with outstanding properties in thermostability and power consumption. • Proposing the Sn 15 Sb 85 /Ti thin film with superlattice-like structure. • Performance optimization and structural analysis of film. • PCM cells based on Sn 15 Sb 85 /Ti film was prepared and tested. [ABSTRACT FROM AUTHOR]

In this study, the multilayer Zn 15 Sb 85 /SiO 2 materials were investigated for phase change memory application with the intention of improving thermal stability and switching speed. The Zn 15 Sb 85 /SiO 2 materials exhibited a high crystallization temperature (T c ∼230 °C), good data retention (T 10 ∼ 160 °C), small density change (∼3.3%), flat surface and rapid amorphization speed (∼1.34 ns). T-shaped phase change devices based on Zn 15 Sb 85 /SiO 2 materials achieved low RESET voltage (V RESET ∼ 1.3 V) in programming operation. The results showed that the multilayer Zn 15 Sb 85 /SiO 2 material was a promising phase change material with high thermal ability, low power consumption and rapid switching speed. • The films were prepared by alternate sputtering Zn 15 Sb 85 and SiO 2 materials. • The fast phase change speed and good thermal stability were achieved. • The adding of SiO 2 interlayers restrained the crystallization. • The SiO 2 interlayers made the thin film more compact. • The PCM devices based on Zn 15 Sb 85 /SiO 2 had a lower power consumption. [ABSTRACT FROM AUTHOR]

Ovonic threshold switching (OTS) selector is a key technology for high-density crossbar memory array. The basic characteristics of GeSe OTS material including structural information, film properties, microscopic properties, and electrical threshold switching behaviors are investigated by combining ab-initio molecular dynamics and experiments. Amorphous GeSe contains numerous Ge Se covalent bonds and defective structure motifs. The original resistance is extremely high compared to phase change materials, leading to lower leakage currents in device cells. The structure of GeSe remains stable at ∼350 °C and transforms to orthorhombic phase at higher temperature up to 450 °C. The severe phase separation of 500°C-annealed GeSe is observed with Se crystal grains. The high threshold voltage (>4 V) and holding voltage (>1 V) with large fluctuation (>1 V) are presented in the OTS cells with GeSe material. Due to the comprehensive investigations of GeSe material, we can improve the thermal stability and device performance of GeSe as a base material to find promising candidates for crossbar memory. Image 1 • A full investigation on GeSe (an important base material in OTS selectors). • Film properties, microstructure, and electrical behaviors are studied. • We find the shortcomings of GeSe base material and provide solutions. [ABSTRACT FROM AUTHOR]

Abstract Chalcogenide alloys in pseudobinary line between Sb 2 Te 3 and GeTe are extensively utilied in phase change memories for optical and electronic contrast between crystalline and amorphous phases. Different from the crystalline structure, the understanding of amorphous structures is still insufficient due to disorder and distortion. By employing first-principle molecular dynamics simulations and atomistic cluster alignment analysis, the short-range orders and the associated distortions of amorphous Sb 2 Te 3 , Ge 1 Sb 2 Te 4 , Ge 2 Sb 2 Te 5 , Ge 3 Sb 2 Te 6 and GeTe are investigated to explore the origin of nature difference. The results reveal that Ge Ge and Sb Sb bonds present a notable competitive mechanism with GeTe content. The GeTe content has a great influence on Ge- and Te-centered short-range orders but little influence on the structures of Sb-centered clusters, especially for the octahedral sites and unidentified structures. The tetrahedrons in Ge-centered clusters of each alloy show a close proportion, but the fraction of tetrahedrons in total clusters increases with the increasing GeTe content due to the increase in the ratio of Ge atom. As for the distortions of clusters, the distorted tetrahedrons in Ge 2 Sb 2 Te 5 are closest to the standard tetrahedron, the Peierls distortion of Ge-centered 6-fold octahedron reduces with an increase in GeTe content whereas that of Sb-centered 6-fold octahedron shows a little change. Additionally, it is found that GeTe content can inhibit the formation of nanocavity but contribute to the formation of the odd rings. Our findings deepens the understanding of amorphous structures, which can promote the design and application of phase-change materials. Highlights • A newly atomistic cluster alignment method is used to explore the local structures of amorphous Ge-Sb-Te alloys. • The fractions of defective octahedrons contribute to understand the structural difference. • The GeTe content affects the Ge- and Te-centered short-range orders notably. • The GeTe content inhibits the formation of nanocavity but promotes the formation of the odd rings. [ABSTRACT FROM AUTHOR]

Multilevel storage and resistance drift are two challenges in phase-change memories. Here, we report that [C(5 nm)/Sb 2 Te 3 (12 nm)] 5 phase-change heterostructure film has multilevel phase change properties and low resistance drift. The addition of carbon broke the Sb 2 Te 3 bond and formed new C-Sb and C-Te bonds. The reliability of phase-change heterostructure was demonstrated by transmission electron microscopy. The amorphous resistance drift index of C/Sb 2 Te 3 film was reduced effectively which may be caused by the structure relaxation. In addition, PCM devices based on C/Sb 2 Te 3 phase-change heterostructure films also confirmed the potential of multilevel storage. This research provided a new method to realize multilevel phase-change memories with low resistance drift. • [C(5 nm)/Sb 2 Te 3 (12 nm)] 5 (C/ST) phase-change heterostructure (PCH) films were prepared by magnetron sputtering. • C/ST film has two resistance changes and low resistance drift index. • The clear multilayer structure confirms the good reliability of superlattice structure. • The PCM devices based on C/ST PCH film exhibit three stable storage states and has low power consumption. [ABSTRACT FROM AUTHOR]

Thermal stability and operation speed are two key challenges in phase change memory (PCM). This work reports GeTe/Cr 0.16 Sb 2 Te (GT/CrST) superlattice-like (SLL) films for both high operation speed and superior thermal stability. The thermal properties, phase change behaviors, microstructural evolution and the electrical performances are investigated in detail. Results show that [GT(8 nm)/CrST(2 nm)] 7 film possesses excellent ten-year data-retention (T 10-year =156 ºC), small density change rate (2.6%), ultrafast reversible phase change (0.5 ns), and low resistance drift coefficient (∼0.07). Furthermore, the [GT(8 nm)/CrST(2 nm)] 7 based PCM cell has the reversible operation time of as low as 8 ns and low power consumption of 8.1 × 10−12 J. The two-dimensional finite element analysis confirms that the lower power consumption is ascribed to the existence of interfacial thermal resistance in SLL thin film, leading to the lower thermal conductivity. Results indicate that [GT(8 nm)/CrST(2 nm)] 7 thin film has great potential for PCM application with excellent thermal stability and phase change speed. • [GeTe(8 nm)/Cr 0.16 Sb 2 Te(2 nm)] 7 film possesses excellent ten-year data-retention. • Ultrafast reversible phase change, and low resistance drift are achieved. • The device cell has the reversible operation time of 8 ns. • The device cell has the lower power consumption of 8.1 × 10−12 J. • The lower power consumption is ascribed to interfacial thermal resistance. [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 has been widely used as thermoelectric material due to its overall good properties, such as low thermal conductivity, high electrical conductivity, and flexible atoms arrangement for structural optimization et al. However, its potential in phase change memory (PCM) application is not well evaluated due to its unstable amorphous state. In this work, stability of amorphous Bi 2 Te 3 has been improved by N doping. Crystallization temperature of 175 °C and data retention of 43.6 °C have been achieved. Smaller and defective grains have been directly observed. Abnormal volume expansion of 7.2% has been observed after crystallization. Low-power consumption and good reversible phase change ability of N doped Bi 2 Te 3 has been verified in PCM cell. [ABSTRACT FROM AUTHOR]

The thermal properties of Sb-Ge superlattice-like thin films have been experimentally studied for phase change memory. The Sb-Ge superlattice-like thin films have a high crystallization temperature (223 °C). The interdiffusion coefficient D λ was determined by measuring the intensity of the low-angle X-ray diffraction arising from the modulation as a function of annealing time. The temperature dependence in the temperature range 80–200 °C is described by D λ = 8.3 × 10 −24 exp(-0.42 eV/kT) m 2 /s . The thermal stability of Sb-Ge superlattice-like thin films can also dramatically be improved with the increase of the temperature. The density variation of Sb-Ge superlattice-like thin films changes by only around 8% between amorphous and crystalline states, which is very important for device reliability. The threshold current and threshold voltage of a cell based on Sb-Ge are 8.15 μA, 1.07 V, respectively. The lower RESET power is presented in the PCM cells of Sb-Ge superlattice-like thin films, benefiting from its high resistivity. [ABSTRACT FROM AUTHOR]

GeTe is a fundamental material for phase-change memory, one of the most promising next-generation data storage devices. Doping GeTe with W has achieved both high writing (crystallization) speed at elevated temperature and long data retention (amorphous stability) at room temperature, which overcame a key challenge for phase-change memory. Yet the effect of W on amorphous GeTe remains ambiguous at atomic and electronic scales. By means of ab initio calculations and molecular dynamics (AIMD) simulations, we shed light on this issue and reveal that W would agglomerate during the melt-quench process and the chemically bonded W cluster plays a key role in tuning the overall phase-change performances of GeTe. Furthermore, the strong W-Ge and W-Te bonds show vital impact on the local structure and crystallization of amorphous GeTe as well. The present work provides valuable clues for advancing the understanding of transition metals doping effects on amorphous phase-change materials, and hence promotes the development of novel phase-change alloys with improved information storage performance. [ABSTRACT FROM AUTHOR]

Solving the contradictory between data retention and switching speed has been the subject of numerous investigations on phase change materials. Towards this end, Ge 1.6 Te-GaSb nano-composite is proposed, which combines advantages of fast crystallization speed and high thermal stability. The characterization results elucidate that doped materials exhibit a high crystallization temperature due to the enhanced stability of the amorphous state associated with the generated larger energy barrier. Furthermore, the reversible electrical switching capability of the phase-change devices is improved in terms of an ultrafast speed of 5 ns with Sb-rich GaSb addition. A good endurance of 20 K and long data retention are achieved simultaneously, indicating that Sb-rich GaSb incorporation into Ge 1.6 Te alloy is a promising material for high-temperature performance applications. [ABSTRACT FROM AUTHOR]

Phase diagram of the Cu-Ge-Sb ternary system has been investigated experimentally and assessed thermodynamically by means of CALPHAD approach. Experimental results were obtained by using differential thermal analysis (DTA), scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), and X-ray powder diffraction (XRD) methods. Examined samples can construct two isothermal sections at 400 and 500 °C as well as three vertical sections of Cu-GeSb, Ge-CuSb, and Sb-CuGe. From equilibrated samples in Cu-rich part of the 500 °C isothermal section, one ternary compound was detected. Based on the present experimental data and available literature data, a thermodynamic description of the ternary Cu-Ge-Sb system has been developed. A reasonable agreement between experimental data and the calculated phase equilibria is obtained. [ABSTRACT FROM AUTHOR]

The synthesis of SnSe 2 thin films, for phase change memory applications, generally require selenization and higher temperature. However, there are many critical problems associated with selenization, such as use of highly toxic H 2 Se/Se gas at high temperature, slow reaction rate and non-uniform composition of thin films. To overcome these problems, a novel and relatively low temperature synthesis of SnSe 2 thin films has been reported here. Selenium rich Sn 30 Se 70 (prepared by melt quenching technique) has been thermally evaporated in presence of Ar followed by vacuum annealing at 200 °C. An attempt has been made to elucidate the structural, optical and electrical properties of these thin films. X-ray diffraction and UV–Vis studies confirmed a phase change from orthorhombic SnSe to hexagonal SnSe 2 followed by improvement in crystallinity (from ≈ 5 nm to 15 nm) and a red shift in optical band gap on annealing. Electrical studies revealed that SnSe phase have single thermal and photo activation energies in temperature range from −20 °C to 100 °C. A large contrast (four orders) in dark-conductivity [(3.19 ± 0.02) × 10 −5 Ω − 1 c m − 1 to (5.16 ± 0.02) × 10 −1 Ω − 1 c m − 1 ] between two phases of Sn 30 Se 70 , suggests its use in phase change memory devices. The SnSe 2 phase is observed to be highly stable in the presence of visible light as compared to SnSe. [ABSTRACT FROM AUTHOR]

Phase change memory (PCM) is one of the most promising candidates for the next generation non-volatile memory, but how to balance the contradiction between thermal stability and speed remains the bottleneck problem that limit its application as a universal memory. Here, we innovatively introducing the tetrahedral cluster and the pseudo-octahedron composed of fourfold abab rings at the same time into Sb 2 Te 3 amorphous matrix by elemental Cu doping. The increased Cu tetrahedral clusters bring about enhanced amorphous stability in Sb 2 Te 3 and raise its crystallization temperature from 153.3 ℃ to 203.7 ℃. Besides, the reduced number of the lone-pair electrons, the stronger bond strength and the greater Peierls-like distortions also play crucial roles. Simultaneously, crystallization process simulation results show the fourfold abab ring structure, as a crystalline precursor, accelerates the crystallization speed remarkably. Experimentally, 14 ns SET speed is achieved in Cu 0.2 (Sb 2 Te 3) 0.8 based PCM device, which is four times faster than in pure Sb 2 Te 3 device. Therefore, by a simple single-element doping process, both amorphous stability and crystallization speed are improved at the same time. • Both tetrahedra and pseudo-octahedra are introduced simultaneously by single-element doping in order to balance the contradiction of phase change memory between thermal stability and speed. • Studies on local structure of materials verifies the increase of tetrahedrons and fourfold abab rings after doping. • AIMD simulation studies and experiment tests based device demonstrate the acceleration of crystallization rate after doping. • ELF, ICOHP, ALTBC and crystallization temperature test prove the improve of the stability after doping. [ABSTRACT FROM AUTHOR]

3D cross (X)–point memory arrays have attracted attention for future memory architecture due to their cost efficiency and high density. Chalcogenide–based materials are suitable candidates for 3D X–point memory because they can be stacked to form both memory and selector with a simple process. Conventional binary chalcogenide ovonic threshold switching (OTS) selectors such as Ge X Se 1−x or GeTe suffer from thermal instability. Furthermore, ternary OTS selectors are being investigated by doping various materials to overcome thermal instability. Here, we introduce a wide range of Ge 1−x S x films to analyze and confirm the selector behavior for future applications. S–rich region (0.5 ≤ x ≤ 0.67) devices behaved as OTS selector with low leakage current (1.3 nA) and high thermal stability (amorphous up to 600 °C). To study the mechanism of devices with different compositions, the modified PF–model was used to calculate trap density, activation energy, subthreshold swing, and distance between traps. Additionally, extracted optical bandgap (1.65 – 3.45 eV) and urbach energy (166 – 103 meV) had good accordance with electrical characteristics of each device. This innovative research has potential to enable 3D X–point memory by combining conformal OTS selector and chalcogenide–based phase change memory. [Display omitted] ● Easy fabrication of binary OTS selector using earth-friendly and thermally stable materials. ● Analyses of wide ranges of Ge 1−x S x films for future applications. ● Excellent thermal stability (∼600 °C) and high electrical performances (up to off current of 1.3 nA). ● Fast switching on/off speeds (15/20 ns) along with the admirable endurance (∼ 109 cycles). [ABSTRACT FROM AUTHOR]

W doping is proposed to reduce the grain size of Sb 2 Te 3 . The grain size of W 0.08 (Sb 2 Te 3 ) 0.92 is about 5 nm, which is smaller than that of Sb 2 Te 3 (∼80 nm) under the same annealing condition. The phase change memory based on W 0.08 (Sb 2 Te 3 ) 0.92 material shows 10 ns operation speed, 6 ns Set operation speed, and 1.3 × 10 5 reversible phase change ability. Through experimental and theoretical investigation, the W atoms are suggested to be located at the Sb positions, and bind with the Te atoms. The stronger W Te bonds induce the small grain size of W 0.08 (Sb 2 Te 3 ) 0.92 , leading to high speed phase change ability. [ABSTRACT FROM AUTHOR]

The structural characteristics of epitaxial Ge 1 Sb 2 Te 4 deposited by MOVPE are reported. X-ray diffraction, electron backscatter diffraction as well as high resolution transmission electron microscopy were carried out. The Ge 1 Sb 2 Te 4 layers crystallize in the trigonal space group R 3 ¯ m with lattice constants a = 4.27 Å and c = 41.0 Å (in hexagonal description). Seven alternating anion and cation layers forming blocks separated by (van der Waals) gaps oriented parallel to the Si ( 111 ) substrate can be used to describe the structure. Except for the formation of crystallographic twins (rotation by 60 ° around the [ 0001 ] direction), no other defects are found. The results indicate the existence of mixed cation layers and a stacking sequence (Ge 0.5 Sb 0.5 )-Te-(Ge 0.25 Sb 0.75 )-Te-Te-(Ge 0.25 Sb 0.75 )-Te. The structural relation to interfacial phase change memories based on superlattices is discussed. [ABSTRACT FROM AUTHOR]

The compound GeTe, despite its simple stoichiometry, is rather unconventional and has been investigated both from a fundamental and technological perspective: it is of high interest for several technologies such as data-storage (phase change memories) and thermoelectricity. The understanding of GeTe growth is thus a key issue for technological applications and fundamental understanding. In this work, GeTe crystallization kinetics is compared to Ge/Te reactive diffusion kinetics using in situ X-ray diffraction measurements, as well as in situ transmission electron microscopy. GeTe crystallization from an amorphous solid solution exhibiting the stoichiometry of the compound GeTe is found to occur at the same temperature as for the reactive diffusion of an amorphous Ge layer on top of a polycrystalline Te layer. Furthermore, GeTe growth is of tridimensional type in the two cases, and can be modeled by the Johnson-Mehl-Avrami-Kolmogorov model. Using this model, the activation energies of nucleation and growth were determined for both crystallization and reactive diffusion. The results suggest that GeTe exhibits nucleation/reaction kinetics unusually low compared to atomic transport kinetics, contrasting with other germanides. [Display omitted] • Crystallization is compared to reactive diffusion for the GeTe growth. • Crystallization temperatures for GeTe compound are the same in both cases. • GeTe growth is of 3D type and modeled by the JMAK model in both cases. • GeTe growth kinetics differs for crystallization and reactive diffusion. • Nucleation/reaction kinetics is indeed low compared to atomic transport. [ABSTRACT FROM AUTHOR]

The contradictory between thermal stability and operation speed has traditionally been the restriction to apply phase change memory (PCM) in the harsh condition. For providing a feasible solution, in this paper, Ru-modification is introduced to boost the data retention of Sb 2 Te 3 without sacrificing the operation speed over the conventional Ge 2 Sb 2 Te 5 compound. The both experimental and simulation results indicate that Ru locates at the grain boundary to inhibit the formation of large grains. Several performance of the device was enhanced, including: ultra-high 10-year data retention above 200 °C, lower Reset voltage about 1.9 V at 100 ns pulse width and a fast operation speed of 6 ns. Refined grains and smaller density change contributed to the reduced voltage and increased endurance. These findings demonstrated that PCM based Ru doped Sb 2 Te 3 will be a potential application in high performance memories. [Display omitted] • Ru-modification is introduced to boost data retention of Sb 2 Te 3 without sacrificing operation speed. • Ru locates at the grain boundary proved through the both experimental and simulation. • The temperature for 10-year data retention of Ru 0.08 (Sb 2 Te 3) 0.92 is up to 200 °C. • Ru 0.08 (Sb 2 Te 3) 0.92 possesses Reset voltage of 1.9 V at 100 ns pulse width and operation speed of 10 ns. [ABSTRACT FROM AUTHOR]

Sb–Te alloy is widely considered as one of the important materials for phase change memory (PCM) with fast operation speed. However, the poor amorphous phase stability limits its wide application. In this work, silicon carbide (SiC) doped Sb 2 Te (Sb 2 Te-SiC) nanomaterial was proposed to improve the thermal stability of Sb 2 Te. It was found that the crystallization temperature of Sb 2 Te was remarkably improved from 149 °C to 251 °C. Accordingly, the temperature for 10-year data retention increases from 56.5 °C to 156.4 °C. X-ray diffraction and transmission electron microscope results indicate that the grain size of Sb 2 Te-SiC film was largely reduced with SiC doping. Besides, experimental results show the Sb 2 Te-SiC nanomaterial is very uniform and no phase separation was observed even after 300 °C annealing. Furthermore, the Sb 2 Te-SiC nanomaterial based PCM cells show fast operation speed of 7 ns and good endurance ability of 2.1 × 10 4 cycles. [ABSTRACT FROM AUTHOR]

Stacked Ge 8 Sb 92 /Ga 30 Sb 70 films exhibited better thermal stability. Smaller volume change (2.21%) before and after phase change than Ge 2 Sb 2 Te 5 (GST). Crystallization process of the [Ge 8 Sb 92 (25 nm)/Ga 30 Sb 70 (25 nm)] 1 thin film was within 11 ns, which was measured by a picosecond laser pump-probe system. Reversible transition of the phase change memory cell based on the [Ge 8 Sb 92 (25 nm)/Ga 30 Sb 70 (25 nm) ] 1 can be achieved by using an electrical pulse of as short as 50 ns. [ABSTRACT FROM AUTHOR]

Amorphous chalcogenides, particularly Ge 2 Sb 2 Te 5 (GST) based alloys, are well known for their non-volatile phase-change random access memory applications (PCRAM). In this work, the phase change properties of Ge 2 Sb 2−x As x Te 5 (x = 0, 0.5, 1.0, 2.0) thin films deposited by thermal evaporation are reported. The As substituted samples crystallize at higher temperatures compared to parent GST. During the phase change for x > 1.0, a direct transition from amorphous to the stable hexagonal structure has been observed. A distinct two-step transition in Sb rich samples and a single step transition for As rich samples are observed in R-T measurements with a high contrast in electrical resistivity. The transition is becoming sharper and sharper with increasing As substitution. A composition-dependent Metal-Insulator Transition (MIT) is also observed in these samples. Compared to GST, As substituted samples show an increase in crystallization temperature and activation energy for crystallization. For GST, the 10-year data retention temperature is 67 °C, and with complete As substitution, it increases to 169 °C, with a significant rise of 102 °C in the data retention temperature. High thermal stability, sharp transition and increased data retention of As substituted Ge-Sb-Te suggest that they are promising candidates for PCRAM applications. • As substituted samples show a direct transition to stable hexagonal phase. • An increase in electrical resistivity contrast by 2 orders of magnitude. • Significant increase in the 10- year data retention. • Greater amorphous phase stability for As substituted samples. • Amorphous to crystal phase change is accompanied by metal-insulator transition. [ABSTRACT FROM AUTHOR]

• Two new phase-change materials of Er 20 Sb 80 and Er 13 Te 87 compounds are screened out. • Er 20 Sb 80 achieves a ultra-fast writing speed of 700 ps. • Origin of high thermal stability proposed to the Er-alloyed Sb/Te compounds have been discussed. • Characteristics of structure of Er-alloyed Sb/Te films are analyzed from the experimental and theoretical perspective. The erbium (Er)-alloyed Sb 2 Te 3 phase-change material has recently been found to enable fast writing speed and high data retention due to the Er-stabilized precursor and the formation of coordinate bonds between Er and Te. However, the detailed characteristics of binary alloys about Er-Sb-Te system are still missing. In this work, Er-alloyed Sb/Te binary material are investigated. The Er 20 Sb 80 compound we designed allows a ultra-fast writing speed of 0.7 ns in the conventional PCRAM device, and the Er 13 Te 87 also can be switched under 50 ns. The impurity Er improves the ability of forming the amorphous structure of Sb/Te, and this stems from the strong Er-Sb and Er-Te bonds stabilize glassy states. Our study demonstrates the device performance of Er-alloyed Sb/Te binary phase-change material, and the origins of these improved performance are also explained by the microscopic observation and molecular dynamics simulation. [ABSTRACT FROM AUTHOR]

• The chemical vapor deposition method realizes the controllable preparation of uniform γ-In 2 Se 3 nanosheets. • Growing γ-In 2 Se 3 nanosheets on SiO 2 /Si substrates can avoid potential contamination and damage during the transfer process. • The prepared γ-In 2 Se 3 based photodetector has a stable broadband optical response. [Display omitted] Two-dimensional (2D) In 2 Se 3 with excellent optical and electrical properties has great application potential for flexible devices, photodetectors, and phase change memories. Here, we report the successes of growing In 2 Se 3 nanosheets on SiO 2 /Si substrates by chemical vapor deposition (CVD) and fabricating broadband photodetector based on In 2 Se 3. The Raman spectroscopy and transmission electron microscopy studies indicate that our 2D In 2 Se 3 is in γ-phase. Our 2D γ-In 2 Se 3 photodetector shows a broadband response range from near ultraviolet (350 nm) to infrared (1000 nm). The stable broadband response provides opportunities for the application of 2D γ-In 2 Se 3 -based devices and also a reference for the preparation and application of other 2D materials. [ABSTRACT FROM AUTHOR]

• XPS chemical shift of 3d Te , Sb and Ge with Sm show bonding of Sm with Te , Sb and Ge. • 3d 3/2 state in XPS shows Sm in both + 2 and + 3 oxidation states in Sm-GST thin films. • Sm incorporation enhances the thermal stability of Sm-GST thin films. • The crystallization mechanism in GST thin films improves for x = 0.4 at% Sm content. • Sm doping improves GST crystallization which may enhance the PCM device performance. The thin films of (Ge 2 Sb 2 Te 5) 100−x Sm x (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) (Sm-GST) phase change material have been investigated employing X-ray photoelectron spectroscopy (XPS) to examine the nature of chemical bonding in as-deposited thin films of Sm-GST. The composition of as-deposited thin films of Sm-GST has been also analyzed from the peak area ratios of XPS core-level spectra and the morphology of the thin film has been studied using field emission scanning electron microscopy (FESEM). The powder samples obtained from the as-deposited thin films have been utilized for the non-isothermal differential scanning calorimetry (DSC) measurements at the constant heating rate of 10 K/min. The values of glass transition temperature (T g), onset crystallization (T c), peak crystallization temperature (T p) and melting temperature (T m) obtained from DSC curves of Sm-GST thin films have been used for the evaluation of thermal stability parameters. The activation energy for crystallization (E c) and avrami exponent (n) for fcc and hexagonal phase of Sm-GST thin films have been evaluated using Henderson's method and Matusita's method. The impact of Sm doping on the thermal stability, glass-forming ability and crystallization activation energy of as-deposited thin films have been examined and its possible influence on the memory device performance has been correlated. [ABSTRACT FROM AUTHOR]

Highlights: [•] W doped Sb4Te was studied for phase change memory. [•] W0.04(Sb4Te)0.96 has been confirmed to be high speed. [•] W0.04(Sb4Te)0.96 has been confirmed to be high thermal stability. [Copyright &y& Elsevier]

Highlights: [•] Phase change behavior of Sb-rich Ga–Sb–Se was studied for phase change memory. [•] Sb-rich Ga–Sb–Se films possess crystallization temperature higher than 225°C. [•] Sb-rich Ga–Sb–Se film has retention ability in excess of 135°C for 10years. [•] Compared with Ge2Sb2Te5, Ga10Sb60Se30 has fast crystallization speed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Al-doped Sn2Se3 thin films were investigated for phase-change memory applications. [•] The thermal stability of Sn2Se3 thin films were improved by Al doping. [•] A fast transition speed was observed in Al-doped Sn2Se3 thin films by the picosecond laser pulses. [•] A lower power consumption of PCM cells could be achieved by Al doping. [ABSTRACT FROM AUTHOR]

• Based on the alloy strategy, Er 0.17 Sb 2 Te compound is screened out. • Er 0.17 Sb 2 Te shows high comprehensive performance in phase-change memory. • Origin of high thermal stability proposed to the Er 0.17 Sb 2 Te compound has been discussed. • Effect of replacing Sb by Er on physical properties of Er 0.17 Sb 2 Te compound is analyzed. To date, operation speed and data retention are still bottlenecks for phase-change memory. Doping impurity is a practical strategy to improve these performances of phase-change memory. Here, to avoid the precipitation of impurity, we choose erbium (Er) dopant due to the most matched structure with parental Sb 2 Te (EST), which is manifested by both experimental and simulation results that Er locates at lattice position. The device tests show the excellent comprehensive performances: operation speed as fast as 6 ns and 10-year data retention as high as 121 °C. This excellent amorphous thermal stability stems from the strong Er Te bonds, and these stable local patterns stabilize glassy states and result in high stability. Our study not only demonstrates that the performance of designed EST meets the requirement of phase-change memory, but also provides general doping rules to obtain better comprehensive performance. [ABSTRACT FROM AUTHOR]

• Superlattice-like (SLL) Ge 2 Sb 2 Te 5 /C film was fabricated to suppress the resistance drift and heat loss of the device. • Ab initio molecular dynamics simulations were used to evaluate the evolution of C nanofilms at the atomic scale. • Two-dimensional finite element analysis reveals the suppressed heat dissipation in the programming regions. • The Ge 2 Sb 2 Te 5 /C SLL thin film demonstrates potential for high-performance phase change memory applications. [Display omitted] Resistance drift and heat loss are two challenges in phase change memory. In this work, superlattice-like (SLL) Ge 2 Sb 2 Te 5 /C film was fabricated, which was designed to suppress the resistance drift and heat loss of the device. Ab initio molecular dynamics simulations were used to evaluate the evolution of C nanofilms at the atomic scale and most C atoms were found accumulated spontaneously as nanometer-scale carbon clusters. Long-range atomic migrations were restrained because of the artificial carbon clusters, which contributed to the resistance drift suppression. The fabricated carbon nanolayers were also believed to serve as an artificial thermal barrier, and an energy-inexpensive amorphization process based on the device has been achieved. With two-dimensional finite element analysis, we found that the suppressed heat dissipation in the programming regions were related to the low interfacial thermal conductance between the C and Ge 2 Sb 2 Te 5. The Ge 2 Sb 2 Te 5 /C SLL thin film demonstrates potential for high-performance phase change memory applications. [ABSTRACT FROM AUTHOR]

Abstract: Compositional dependent investigations of the bulk GeTe chalcogenides alloys added with different selenium concentrations are carried out by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron probe micro-analyzer (EPMA) and differential scanning calorimetry (DSC). The measurements reveal that GeTe crystals are predominant in alloys up to 0.20at.% of Se content indicating interstitial occupancy of Se in the Ge vacancies. Raman modes in the GeTe alloys changes to GeSe modes with the addition of Se. Amorphousness in the alloy increases with increase of Se and 0.50at.% Se alloy forms a homogeneous amorphous phase with a mixture of Ge–Se and Te–Se bonds. Structural changes are explained with the help of bond theory of solids. Crystallization temperature is found to be increasing with increase of Se, which will enable the amorphous stability. For the optimum 0.50at.% Se alloy, the melting temperature has reduced which will reduce the RESET current requirement for the phase change memory applications. [Copyright &y& Elsevier]

Abstract: Crystallization temperatures of the Sb2Te films increase remarkably from 139.4°C to 223.0°C as the N2 flow rates increasing from 0sccm to 1.5sccm. Electrical conduction activation energies for amorphous and crystalline states increase by doping nitrogen. A small amount of nitrogen atoms can locate at interstitial sites in the hexagonal structure, generating a strain field, and improving the thermal stability of amorphous state. The best 10-years lifetime at temperature up to 141°C is found in Sb2TeN1 films. Doping excessively high nitrogen in Sb2Te film will form nitride and make Te separate out. As a result, the activation energy for crystallization decreases instead, accompanying with the deterioration of thermal stability. The power consumption of PCRAM test cell based on Sb2TeN1 film is ten times lower than that of PCRAM device using Ge2Sb2Te5 films. [Copyright &y& Elsevier]

In this paper, the phase transition mechanism and device conversion performance of the Sb/GeTe superlattice-like films were studied and showed improved amorphous thermal stability and resistance drift compared to the pure Sb films. Transmission electron microscopy showed that the Sb layer plays a leading role in the crystallization process, forming a large number of Sb grains. The metallic behavior of the Sb/GeTe thin films was confirmed by density functional theory calculations. The thermal conductivity of the crystalline film was found to be higher than that of the amorphous film, and the thermal conductivity of the Sb/GeTe film was lower than that of the other monolayer films. In the Sb/GeTe film, the Sb–Sb and Ge–Te bonds were the main bond structures, and some Sb–Te bonds were formed at the layer interfaces, enhancing the stability of the film. Laser picosecond measurements showed that Sb/GeTe film had a higher crystallization rate than the GeTe film. The phase change memory devices based on the Sb/GeTe thin films were found to have lower operational power consumption and higher transition speed. These results showed that the Sb/GeTe thin film explored in this study was a promising phase change storage thin film with excellent performance. ● The coupling effect of multiple interfaces in Sb/GeTe superlattice films were investigated in detail. ● The redistribution of the atoms after crystallization was observed. ● The energy band structure of the Sb/GeTe superlattice films was obtained by density functional theory calculations. ● The interface coupling improved the phase transition behavior and the phase transition performance. [ABSTRACT FROM AUTHOR]

• The thermal stability were improved upon an increase in the Hf content. • The grain size reduced due to the presence of amorphous Hf-Ge and Hf-Sb content. • The volume change became smaller as a result of the confined grain size. • The cell based on Hf 0.14 (Ge 5 Sb 95) 0.86 thin film exhibits a relatively fast switching speed and lower power consumption. The influences of Hf addition on the phase change character of Ge 5 Sb 95 thin films were systematically studied. The heat induced crystallization was evaluated by in situ resistance measurement. By doping Hf element, both the resistance and the crystallization activation energy rise, resulting in better thermal stability and higher operating energy efficiency. Various analysis indicate that due to the existed amorphous germanium and antimony element, the hafnium addition can delay the growth of crystal grains and constrain the crystal size. A shift in the Raman mode corresponding to Sb after crystallization can be observed. By using the X-ray reflectance and atomic force microscope, it is observed that after doping Hf, the volume fluctuation and the surface roughness decrease. To evaluate the electrical characteristics, a phase change memory chip based on Hf doped Ge 5 Sb 95 thin film was also fabricated successfully. The results show that proper incorporation of Hf with Ge 5 Sb 95 is an helpful method for adjusting and optimizing the crystallization properties of the material in the applications of the phase change memory. [ABSTRACT FROM AUTHOR]

• Data retention of Re doped Sb 2 Te (RST) was remarkably increased to 152 °C@10 years. • Density change and power consumption of RST are largely reduced. • Operation speed of RST based device is as fast as 10 ns. Sb 2 Te material is an important base material for phase change memory or optical data storage, but the poor amorphous stability of Sb 2 Te severely limits its application. In this work, we proposed a superhard and infusible Rhenium (Re) doped Sb 2 Te (RST) phase change material to improve the properties of Sb 2 Te. The data retention of RST was remarkably increased to 152 °C@10 years, which is much higher than those of traditional Ge 2 Sb 2 Te 5 (87 °C@10 years) and Sb 2 Te (56 °C@10 years). The operation speed of the RST based device is as fast as 10 ns and the resistance ratio is more than 102 times. The density change of RST (4.2%) is smaller than those of Ge 2 Sb 2 Te 5 (6.5%) and Sb 2 Te (5.7%). Furthermore, the operation voltage and power consumption of RST based device are much lower than those of Ge 2 Sb 2 Te 5 and Sb 2 Te based devices. The results indicate that the RST is a promising material for high performance phase change memory or optical data storage applications. [ABSTRACT FROM AUTHOR]

The nature of crystallization in telluride based phase change memory materials is being actively discussed, and we present new experimental results on the effect of a pulsed laser treatment on the amorphous thin film crystallization of composition Ge 2 Sb 2 Te 5 (GST225). It is shown that there are two ways of crystalline phase formation under femtosecond pulses excitation. At the moderate value of laser fluence, crystalline fraction forms inside the solid amorphous phase. This process is apparently two-stage, and this leads to formation of fine-grained polycrystalline material. The second way of crystallization proceeds at higher levels of the laser fluence, closer to the beam axis. In this case, larger grains are produced closer to the beam axis, and eventually no new nuclei can be formed due to high cooling velocity leading to inhibition of nucleation. The article also presents the results on the effect of the substrate type (metallic or oxide) and film thickness on crystallization, which is due to the light penetration depth and reflection from the film-substrate interface. Image 1 • Threshold value of laser fluence for crystallization depends on the number of pulses. • Two types of Ge 2 Sb 2 Te 5 crystallization under femtosecond laser pulses are shown. • Both conductive and dielectric substrates could promote or inhibit crystallization. • Initial laser pulses provide incubation of crystalline nuclei. [ABSTRACT FROM AUTHOR]

Binary Sb 2 Te is considered as a potential recording material for phase-change memory due to its higher crystallization speed than the widely investigated ternary Ge-Sb-Te alloys. To further improve the performance of Sb 2 Te, various dopants such as scandium (Sc) have been explored. Yet, the thermal conductivity of the pristine and doped Sb 2 Te is unknown, even though the thermal conductivity (κ) is an essential parameter for phase change memory devices as thermal conductivity determines the exchange of energy with surroundings and heat transport in the SET/RESET process. In this work, by means of ab initio calculations, we have calculated the thermal conductivity of Sb 2 Te, scandium (Sc)-doped and yttrium (Y)-doped Sb 2 Te. The calculated lattice thermal conductivity values at 300 K are 3.76 Wm−1K−1, 3.22 Wm−1K−1 and 3.35 Wm−1K−1 for pristine Sb 2 Te, Sc-doped Sb 2 Te, and Y-doped Sb 2 Te, respectively, showing a reduction in lattice thermal conductivity by doping. Further analysis of the electron localization function and Bader charge distribution suggest that the increased bonding strength of the dopants with the surrounding atoms contributes to the reduction in lattice thermal conductivity of the doped Sb 2 Te. On the other hand, the electronic thermal conductivity of Sb 2 Te should also decrease due to the metal to semiconductor transition by Sc/Y doping as analyzed by the calculated density of states. Overall, the thermal conductivity of Sb 2 Te decreases to some extent by doping Sc and Y, which can lower the energy consumption and improve the efficiency of energy utilization in the data SET/RESET process and is thus desirable for phase-change memory devices. • Sc or Y dopants prefer to substitute the Sb1 atoms due to the lowest formation energies compared to other substitutional sites. • Sc/Y can lower the lattice thermal conductivity, which can be attributed to the stronger ionic bonds between the Sc/Y and pristine atoms. • The electronic thermal conductivity significantly decreases after doping Sc and Y due to the metal-to-semiconductor transition. • The reduction of thermal conductivity after Scandium/Yttrium doping is highly desirable in the phase change random access memory (PCRAM) devices. [ABSTRACT FROM AUTHOR]