Sb Te Phase-change Materials Research Articles

Revealing the crystallization dynamics of Sb–Te phase change materials by large-scale simulations

Using an efficient and accurate machine learning potential, large-scale crystallization dynamics of Sb–Te phase change materials are achieved.

Crystallization kinetics and thermodynamics of an Ag–In–Sb–Te phase change material using complementary in situ microscopic techniques

Crystallization kinetics and thermodynamics of an Ag–In–Sb–Te phase change material using complementary in situ microscopic techniques

Obtaining glasses in the extremely crystallizing Ge–Sb–Te phase change material

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the GexSbxTe100-2x join in the ternary Ge–Sb–Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe6. Results indicate a weak variation of Tg with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge2Sb2Te5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the Tg variation suggests the preferential formation of Te–Sb–Te, at variance with isochemical compounds such as Ge–Sb–Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x=8.5%. Most of our glasses could be formed around this composition.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Progress in metasurfaces based on Ge–Sb–Te phase-change materials

Recently, metasurfaces based on phase-change materials (PCMs) have attracted increasing attention due to the dramatic optical properties contrast between amorphous and crystalline states. The chalcogenide PCMs can be reversibly switched by electrical or optical pulses, offering tunability and reconfigurability for the metasurfaces. In this Perspective, the latest achievements and ongoing development in reconfigurable metasurfaces based on chalcogenide PCMs are presented, including the applications in nonlinear optics, anapole control, beam steering, perfect absorbers, and polaritons. This Perspective ends with perspectives for the growing demands of PCMs based on metasurfaces.

Chemical Design Principles for Cache-Type Sc–Sb–Te Phase-Change Memory Materials

Enhanced crystal nucleation in a Sc–Sb–Te phase-change material has enabled subnanosecond switching in phase-change memory devices, making cache-type nonvolatile memory feasible. However, the microscopic mechanisms remain to be further explored. In this work, we present a systematic ab initio study of the relevant parent compounds, namely, Sc2Te3 and Sb2Te3. Despite similar bond lengths and angles in the amorphous phases of the two compounds, Sc2Te3 displays a much more ordered amorphous network without homopolar bonds. As a result, the local structural order in amorphous Sc2Te3 is dominated by square motifs, remarkably similar to those of the metastable rocksalt-like phase. Chemical bonding analysis indicates more robust Sc–Te bonds compared with Sb–Te bonds in the amorphous phase, as well as a substantial role of electrostatic interactions in Sc2Te3 but not in Sb2Te3. The robustness of Sc–Te bonds explains the enhanced nucleation in Sc–Sb–Te compounds. Finally, we discuss an alloying strategy of Sc2Te3 ...

Properties of Ti–Sb–Te doped with SbSe alloy for application in nonvolatile phase change memory

Sb2Se3 alloy is used as a dopant for Ti–Sb–Te phase change material aiming to improve the thermal stability of Ti–Sb–Te based phase change memory cell. In this paper, the thermal and electrical properties of Sb2Se3-doped Ti0.32Sb2Te3 are respectively investigated. Compared with Ti0.32Sb2Te3 material, (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72 has better thermal stability with crystallization temperature of 203 °C and estimated 10-year data retention of 102 °C. For the (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72-based PCM cell, it inherits the advantage of fast switching speed and good endurance of TST based one with wider SET/RESET window. The effects of Sb2Se3 doping on the structure of Ti0.32Sb2Te3 are also systemically studied. In crystalline (Sb2Se3)0.28(Ti0.32Sb2Te3)0.72 film structure, Se is prone to substitute Te and bond with Sb and Te in the Sb2Te3 lattice structure which may decrease the crystal lattice constant.

Performance improvement in a Ti–Sb–Te phase change material by GaSb doping

Herein, the GaSb-doped Ti0.09Sb0.38Te0.53(TST) material is investigated and considered to be a potential candidate for phase change random access memory (PCM) application due to its overall good performance. With a high crystallization temperature of 220 °C, (GaSb)0.11(Ti0.09Sb0.38T0.53)0.89 ((GaSb)0.11TST) exhibits a data retention of 10 y at 136.8 °C, which is much better than that of Ge2Sb2Te5 as well as TST. For the (GaSb)0.11TST-based cell, an electric pulse as short as 5 ns can fulfill the SET operation, thus demonstrating an extremely rapid crystallization speed. Furthermore, the cell shows a significantly lower power consumption for SET/RESET reversible switching than that of the Ge2Sb2Te5-based cell. The programming cycles can reach 1 × 104 cycles with stability resistance of about a two orders of magnitude on/off ratio.

Aluminum-centered tetrahedron-octahedron transition in advancing Al-Sb-Te phase change properties.

Group IIIA elements, Al, Ga, or In, etc., doped Sb-Te materials have proven good phase change properties, especially the superior data retention ability over popular Ge2Sb2Te5, while their phase transition mechanisms are rarely investigated. In this paper, aiming at the phase transition of Al-Sb-Te materials, we reveal a dominant rule of local structure changes around the Al atoms based on ab initio simulations and nuclear magnetic resonance evidences. By comparing the local chemical environments around Al atoms in respective amorphous and crystalline Al-Sb-Te phases, we believe that Al-centered motifs undergo reversible tetrahedron-octahedron reconfigurations in phase transition process. Such Al-centered local structure rearrangements significantly enhance thermal stability of amorphous phase compared to that of undoped Sb-Te materials, and facilitate a low-energy amorphization due to the weak links among Al-centered and Sb-centered octahedrons. Our studies may provide a useful reference to further understand the underlying physics and optimize performances of all IIIA metal doped Sb-Te phase change materials, prompting the development of NOR/NAND Flash-like phase change memory technology.

A chemical link between Ge–Sb–Te and In–Sb–Te phase-change materials

Chemical-bonding theory reveals a common electronic “fingerprint” in seemingly different phase-change materials for data storage.

Sb-rich Zn–Sb–Te phase-change materials: A candidate for the trade-off between crystallization speed and data retention

We explored the structural and physical properties of Sb-rich Zn–Sb–Te films in order to combine the good thermal stability of ZnSb with the high crystallization speed of Sb2Te. The films generally exhibit two different crystallization characteristics described as follows: amorphous → Sb2Te crystalline phase if the Zn content in the film is less than ∼10 at. % and amorphous → Sb crystalline phase if the Zn content is more than ∼10 at. %. Among the films, the Zn28.62Sb53.69Te17.69 film was found to show the highest crystallization temperature (∼255 °C), best 10 year data retention (∼165.9 °C), and shortest crystallization time of ∼58 ns at 70 mW with a stable rhombohedral Sb phase; thus, it is considered promising for phase-change memory applications.

Screened remote plasma-enhanced atomic vapor deposition of Sb–Te thin film for the improvement of trench-covering ability

A new screened remote plasma-enhanced atomic vapor deposition (SPEAVD) technique was studied for depositing Sb–Te phase-change materials inside trench structures with high aspect ratios. The theoretical model of the screening mechanism included the concepts of a plasma sheath and a sticking coefficient. Plasma’s limitation of poor step coverage was overcome by filtering its high-energy ions and electrons. Cross-sectional observation was conducted using field emission scanning electron microscopy (SEM). The trench-covering ability of SPEAVD was much better than that of direct remote PEAVD under broad chamber conditions. Surface morphology was observed by SEM and atomic force microscopy. The surface chemical state of the deposited film was measured by X-ray photoelectron spectroscopy. The crystallinity in various deposition temperatures was analyzed by X-ray diffraction measurements. The electrical characteristics of the films were measured by a four-point probe. We expect this research to provide a new deposition method that will allow fine control of step coverage and other characteristics.

Nitrogen-doped Sb-rich Si–Sb–Te phase-change material for high-performance phase-change memory

The effects of nitrogen doping on the phase-change performance of Sb-rich Si–Sb–Te materials are systemically investigated, focusing on the chemical state and the role of nitrogen upon crystallization. The tendency of N atoms to bond with Si (SiNx) in the crystalline film is analyzed by X-ray photoelectron spectroscopy. The microstructures of the materials mixed with Sb2Te crystal grains and amorphous Si/SiNx regions are elucidated via in situ transmission electron microscopy, from which a percolation behavior is demonstrated to possibly describe the random crystallization feature in the nucleation-dominated nanocomposite material. The phase-change memory cells based on N-doped Sb-rich Si–Sb–Te materials display more stable and reliable electrical performance than the nitrogen-free ones. An endurance characteristic in the magnitude of 107cycles of the phase-change memory cells is realized with moderate nitrogen addition, meaning that the nitrogen incorporation into Si–Sb–Te material is a suitable method to achieve high-performance phase-change memory for commercial applications.

Fast and scalable memory characteristics of Ge‐doped SbTe phase change materials

AbstractPhase change memory (PCM) has opportunities of various applications on the premise of its high performance operations, which are still to develop with innovations such as change of a memory material. In respects of high‐speed and high‐scalability memory characteristics, δ‐phase Ge‐doped SbTe (GeST) materials stand as highly promising candidates. An overview of the material and device characteristics of these materials is presented primarily based on our recent experimental and computational studies and with a particular regard to their Sb‐to‐Te ratio (STR) dependence.TEM images of the δ‐phase GeST microstructures of varying STR and a highly scaled PCM cell with a δ‐phase GeST of high STR. magnified image

Insight into the Growth and Control of Single-Crystal Layers of Ge–Sb–Te Phase-Change Material

Employed for a long time in optical disks, Ge2Sb2Te5 is nowadays considered the most promising material also for phase-change nonvolatile memories. In the current paper, Ge–Sb–Te phase-change material thin films with a nominal composition of Ge2Sb2Te5 were grown by molecular beam epitaxy on slightly mismatched GaSb and InAs substrates with (001) and (111) orientations. In situ quadrupole mass spectrometry and reflection high-energy electron diffraction allowed tight control of the growth parameters, revealing that Ge2Sb2Te5 grows in epitaxial fashion only within a narrow window of substrate temperatures around 200 °C. Smooth surfaces were achieved solely on (111)-oriented substrates. Rough surfaces and interfaces were observed by transmission electron microscopy for films grown on (001)-oriented substrates. Whereas films deposited on (001) substrates possess two different vertical epitaxial orientations, single-crystalline layers exclusively (111) oriented were achieved on (111) substrates, as shown by synchrotron radiation X-ray diffraction. All the results point to the superior crystalline quality and morphology of Ge2Sb2Te5 layers grown on (111) surfaces, independent of the substrate material.

Epitaxial growth and structure of Ge–Sb–Te phase change materials on GaSb

The growth rate of nominal Ge2Sb2Te5 phase change material epitaxially grown from the elements by molecular beam epitaxy rapidly decreases above ≈180 °C substrate temperature. A theoretical estimate of this behavior reveals congruent sublimation of the layer, suggesting that it may possess characteristics of a molecular crystal consisting of rather stable subunits with weaker bonds between them. In situ synchrotron x-ray diffraction data of the layers show a peculiar anisotropic broadening of the reflections, consistent with a small rhombohedral distortion of the unit cell analogous to GeTe. The three-dimensional mosaicity due to the growth on a strictly cubic substrate can be avoided by growing on the (111) plane, orienting the distortion in the growth direction to establish an exact match of the in-plane symmetries at the interface.

Fast switching behavior of nanoscaleAg6In5Sb59Te30 based nanopillar type phase change memory

Ag and In co-doped SbTe phase change material (AgInSbTe) was used to fabricate nanopillarphase change memory. The AgInSbTe nanopillar type phase change device with 200 nm of diameterwas fabricated by nanoimprint lithography and was reversibly changed between the resistances of104 and106 Ω by applying set/reset pulses using conducting atomic force microscopy. Due to the fastcrystallization nature of AgInSbTe, the set operation of the device could be donewith only 50 ns of set pulse. The fast crystallization nature of AgInSbTe is alsoresponsible for the decrease in reset voltage of devices set with a short pulse.

Analysis and improvement of interfacial adhesion of growth-dominant Ge-doped SbTe phase change materials

A semiquantitative characterization of the interfacial adhesion of phase change materials is developed, which consists of determining critical adhesion temperature (TCA) via measuring the probability of adhesion failure with temperature using patterned films. By comparison of TCA values, Ge-doped SbTe (Ge-ST) is shown to have weaker adhesion than Ge2Sb2Te5 (GST), which results from its limited ability in relaxation of crystallization-induced stress. Nitrogen or oxygen doping in Ge-ST produces significant increase in TCA, close to that of GST. This improvement is due to smaller grain size of N-/O-doped Ge-ST, which facilitates the relaxation of the stress via grain boundary diffusion or sliding.

Comparison of the crystallization of Ge–Sb–Te and Si–Sb–Te in a constant-temperature annealing process

The crystallization of Ge–Sb–Te and Si–Sb–Te phase-change materials has been characterized by in situ time-dependent resistance measurement and transmission electron microscopy . Although silicon has various properties that are similar to those of germanium , Si–Sb–Te and Ge–Sb–Te crystallize via different processes. Si–Sb–Te has a complex structure (mainly hexagonal) while Ge–Sb–Te has a simple face-centered cubic structure when annealed at 433 K for several hours.

Stoichiometry of various Ag(In)SbTe phase change materials (PCMs) determined using LA-ICP-MS

In this study major element concentrations of AgInSbTe phase change materials of different stoichiometry were quantified using UV-ns-LA-ICP-MS. Performing raster ablation sampling, precision and accuracy of the determined concentrations were improved compared to single hole drilling ablation. Using raster mode, possible fractionation effects during the ablation and temporal mass load effects in the ICP were found less variable for the respective materials. Additionally, using raster mode a more representative sampling (mass) was achieved, which results in a more bulk-equivalent description of the sample. An indication for that is given by the improved RSD of the determined concentrations to values as low as 1%. A single calibration material composed of all four elements of interest was applied to determine the major element content in AgInSbTe samples with unknown stoichiometry. A quantification approach based on a 100% normalisation (see ref. 1) was used, due to the similar behaviour of the elements during raster ablation sampling. The high purity of these materials allows using the 100% normalisation, based on the four major elements only. Furthermore, the applicability of a four-component calibration material for the quantification of three-component AgSbTe materials was investigated in terms of precision and accuracy. The stoichiometry-dependent particle size distribution of the laser induced aerosol was studied and is suspected to be the limiting factor for accuracy. It is shown that a matrix-matched calibration material, or at least ablation conditions that provide a particle size-matched aerosol between calibration- and sample-material, provides access to accurate stoichiometry of such materials.