Phase Change Memory Applications Research Articles

Advantages of Ta-Doped Sb3Te1 Materials for Phase Change Memory Applications.

Phase change memory (PCM), a typical representative of new storage technologies, offers significant advantages in terms of capacity and endurance. However, among the research on phase change materials, thermal stability and switching speed performance have always been the direction where breakthroughs are needed. In this research, as a high-speed and good thermal stability material, Ta was proposed to be doped in Sb3Te1 alloy to improve the phase transition performance and electrical properties. The characterization shows that Ta-doped Sb3Te1 can crystallize at temperatures up to 232 °C and devices can operate at speeds of 6 ns and 8 × 104 operation cycles. The reduction of grain size and the density change rate (3.39%) show excellent performances, which are both smaller than that of Ge2Sb2Te5 (GST) and Sb3Te1. These properties conclusively demonstrate that Ta incorporation of Sb3Te1 alloy is a material with better thermal stability and faster crystallization rates for PCM applications.

GeTe/CrSb2Te superlattice-like thin film for excellent thermal stability and high phase change speed

Thermal stability and operation speed are two key challenges in phase change memory (PCM). This work reports GeTe/Cr0.16Sb2Te (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 (T10-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.

Elemental Redistribution During the Crystallization of Ge–Cu–Te Thin Films for Phase-Change Memory

Herein, a GeCu2Te2 alloy is proposed as a phase-change material for application in nonvolatile phase-change random access memory (PRAM). The crystallization kinetics and microchemical changes during phase transformation are investigated, and their correlation with the electrical behaviors of the GeCu2Te2 thin films are examined. The key findings are as follows: (ⅰ) the GeCu2Te2 alloy shows a higher crystallization temperature (∼185 °C) than the classic Ge2Sb2Te5 (GST) thin films, thus demonstrating superior thermal stability; (ⅱ) the crystallization kinetics demonstrate a decreasing in the Avrami exponent n from 4, which is related to the growth-dominated crystallization process evidenced by the micromorphology; (ⅲ) a massive redistribution of the chemical elements along the depth of the thin films during crystallization is considered to be driven by selective surface oxidation at amorphous state, and stress buildup during crystallization. In addition, the crystallization-induced stress is determined as ∼168 MPa by utilizing the wafer curvature and X-ray diffraction methods for the GeCu2Te2 thin films. Finally, the lower threshold switching voltage ∼1.72 V for amorphous GeCu2Te2 thin films is beneficial for reducing the SET operating power consumption. The authors believe that these results are valuable for the optimal phase change material design.

Influence of yttrium element on the crystallization performance of ZnSb alloy for phase change memory application

The crystallization behavior and electrical performance of Y-doped Zn15Sb85 phase change material were proposed and experimental examined. The relationship between the resistance and temperature reveals that the Y dopant is able to enhancing the thermal stability (Tc ~ 237°C, T10 ~ 169 °C) and resistivity (Ra ~ 105 Ω, Rc ~ 102 Ω) of Zn15Sb85 films, which are suitable for automobile design and surface mount technology. XRD results indicate that the Y dopant could effectively inhibit grain growth and reduce grain size. Meanwhile, XPS illustrates that Y is more likely to form bonds with Sb. Further, the resistance drift index and surface roughness become small after doping Y element, which is of great benefit to improving the reliability and electrical performance of the device. Moreover, T-shaped phase change memory cells based on Y0.36(Zn15Sb85)0.64 film were also built and analyzed. The characteristic of current − voltage (I-V) and resistance − voltage (R-V) prove that the SET and RESET operations can be achieved by electric induction, implying the excellent candidate of Y-doped Zn15Sb85 material for high thermal and high reliability application.

Ultra-Stable, Endurable, and Flexible Sb2TexSe3–x Phase Change Devices for Memory Application and Wearable Electronics

Flexible memory and wearable electronics represent an emerging technology, thanks to their reliability, compatibility, and superior performance. Here, an Sb2TexSe3-x (STSe) phase change material was grown on flexible mica, which not only exhibited superior nature in thermal stability for phase change memory application but also revealed novel function performance in wearable electronics, thanks to its excellent mechanical reliability and endurance. The thermal stability of Sb2Te3 was improved obviously with the crystallization temperature elevated 60 K after Se doping, for the enhanced charge localization and stronger bonding energy, which was validated by the Vienna ab initio simulation package calculations. Based on the ultra-stability of STSe, the STSe-based phase change memory shows 65 000 reversible phase change ability. Moreover, the assembled flexible device can show real-time monitoring and recoverability response in sensing human activities in different parts of the body, which proves its effective reusability and potential as wearable electronics. Most importantly, the STSe device presents remarkable working reliability, reflected by excellent endurance over 100 s and long retention over 100 h. These results paved a novel way to utilize STSe phase change materials for flexible memory and wearable electronics with extreme thermal and mechanical stability and brilliant performance.

Evaluation of vibrational properties and local structure change during phase transition in Ge2Sb2Te5 and In3SbTe2 phase change materials

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and x-ray photoelectron spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the phase change memory application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ∼85.5 cm−1 and ∼180 cm−1 Raman shift, respectively, having vibration modes associated with B1g symmetry and transversal optical phonons with Г15 symmetry. Further annealing at a higher temperature of 400 , a ternary IST phase is obtained at ∼163 cm−1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centred cubic phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge–Te and Sb–Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4−n Ge n (n = 1, 2) corner-sharing tetrahedra with A1 mode at ∼104.5 cm−1 and ∼137 cm−1 Raman shift, respectively, and a declining Sb2Te3 peak at ∼175 cm−1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

Electrical and structural properties of binary Ga–Sb phase change memory alloys

Material properties of Ga–Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for analog phase change memory (PCM) applications. Crystallization of this alloy was shown to occur in the temperature range of 180–264 °C, with activation energy >2.5 eV depending on the composition. X-ray diffraction (XRD) studies showed phase separation upon crystallization into two phases, Ga-doped A7 antimony and cubic zinc-blende GaSb. Synchrotron in situ XRD analysis revealed that crystallization into the A7 phase is accompanied by Ga out-diffusion from the grains. X-ray absorption fine structure studies of the local structure of these alloys demonstrated a bond length decrease with a stable coordination number of 4 upon amorphous-to-crystalline phase transformation. Mushroom cell structures built with Ga–Sb alloys on ø110 nm TiN heater show a phase change material resistance switching behavior with resistance ratio >100 under electrical pulse measurements. TEM and Energy Dispersive Spectroscopy (EDS) studies of the Ga–Sb cells after ∼100 switching cycles revealed that partial SET or intermediate resistance states are attained by the variation of the grain size of the material as well as the Ga content in the A7 phase. A mechanism for a reversible composition control is proposed for analog cell performance. These results indicate that Te-free Ga–Sb binary alloys are potential candidates for analog PCM applications.

Neuromorphic Photonic Memory Devices Using Ultrafast, Non-Volatile Phase-Change Materials.

The search for ultrafast photonic memory devices is inspired by the ever-increasing number of cloud-computing, supercomputing, and artificial-intelligence applications, together with the unique advantages of signal processing in the optical domain such as high speed, large bandwidth, and low energy consumption. By embracing silicon photonics with chalcogenide phase-change materials (PCMs), non-volatile integrated photonic memory is developed with promising potential in photonic integrated circuits and nanophotonic applications. While conventional PCMs suffer from slow crystallization speed, scandium-doped antimony telluride (SST) has been recently developed for ultrafast phase-change random-access memory applications. An ultrafast non-volatile photonic memory based on an SST thin film with a 2 ns write/erase speed is demonstrated, which is the fastest write/erase speed ever reported in integrated phase-change photonic devices. SST-based photonic memories exhibit multilevel capabilities and good stability at room temperature. By mapping the memory level to the biological synapse weight, an artificial neural network based on photonic memory devices is successfully established for image classification. Additionally, a reflective nanodisplay application using SST with optoelectronic modulation capabilities is demonstrated. Both the optical and electrical changes in SST during the phase transition and the fast-switching speed demonstrate their potential for use in photonic computing, neuromorphic computing, nanophotonics, and optoelectronic applications.

Improved thermal stability and direct hexagonal transition accompanied by metal-insulator transition in Arsenic substituted Ge2Sb2Te5

Amorphous chalcogenides, particularly Ge2Sb2Te5 (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 Ge2Sb2−xAsxTe5 (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.

Liquid and Glass Phases of an Alkylguanidinium Sulfonate Hydrogen-Bonded Organic Framework.

Glassy phases of framework materials feature unique and tunable properties that are advantageous for gas separation membranes, solid electrolytes, and phase-change memory applications. Here, we report a new guanidinium organosulfonate hydrogen-bonded organic framework (HOF) that melts and vitrifies below 100 °C. In this low-temperature regime, non-covalent interactions between guest molecules and the porous framework become a dominant contributor to the overall stability of the structure, resulting in guest-dependent melting, glass, and recrystallization transitions. Through simulations and X-ray scattering, we show that the local structures of the amorphous liquid and glass phases resemble those of the parent crystalline framework.

Tailorable fragile-to-strong kinetics features of metal oxides nanocomposite phase-change antimony films

A fragile-to-strong (F-S) kinetics feature is desirable in phase-change supercooled liquids, for decoupling the contradictory relation between good thermal stability nearby glass transition temperature and fast crystallization speed around melting temperature. We designed three metal oxides (MOs), i.e., the fragile TiO2, the moderate ZnO, and the strong GeO2, doping into a very fragile Sb matrix, which is an ideal elementary phase-change material (PCM) for overcoming the composition segregation in fatigued device, to tailor the F-S transition behavior in their supercooled liquids. Depending on the crystallization kinetics analyses of three representative films, which all exhibit the similar thermal stability with crystallization temperature of 200 ± 6 ºC, crystallization activation energy of 2.83 ± 0.09 eV, and the temperature for ten years data retention of 101 ± 7 ºC, we found no F-S behavior in the Sb62.1(TiO2)37.9 film, but weak and strong F-S behavior in Sb82.7(ZnO)17.3 and Sb73.6(GeO2)26.4 film, respectively. It shows that, the Sb based nanocomposite films with different MOs that have various fragilities can bring tailorable F-S kinetics features. Nevertheless, although the metal elementaries of Ti, Zn, Ge, have been confirmed could not bond with the Sb atoms, the Sb2O3 phase are detected more or less in Sb62.1(TiO2)37.9 and Sb82.7(ZnO)17.3 but not found in Sb73.6(GeO2)26.4 film. It is thus believed the formation of Sb2O3 would impede the F-S behavior in the supercooled liquids. The Sb73.6(GeO2)26.4 film that has large F-S transition magnitude of 2.3 and high F-S transition temperature of 536 K, is suggested to be a candidate for potential phase-change memory application. Such distinct F-S behavior in nano-size confined Sb73.6(GeO2)26.4 matrix, which consists of phase-changeable Sb and non-phase-changeable GeO2, stems from the competition between short-range ordering cluster that with abundant Peierls-like distortions in fragile Sb phase and medium-range ordering cluster that with energetical favorable networks in strong GeO2 phase.

Improved multilevel storage capacity in Ge2Sb2Te5-based phase-change memory using a high-aspect-ratio lateral structure

Further improvement of storage density is a key challenge for the application of phase-change memory (PCM) in storage-class memory. However, for PCM, storage density improvements include feature size scaling down and multi-level cell (MLC) operation, potentially causing thermal crosstalk issues and phase separation issues, respectively. To address these challenges, we propose a high-aspect-ratio (25:1) lateral nanowire (NW) PCM device with conventional chalcogenide Ge2Sb2Te5 (GST-225) to realize stable MLC operations, i.e., low intra- and inter-cell variability and low resistance drift (coefficient = 0.009). The improved MLC performance is attributed to the high aspect ratio, which enables precise control of the amorphous region because of sidewall confinement, as confirmed by transmission electron microscopy analysis. In summary, the NW devices provide guidance for the design of future high-aspect-ratio three-dimensional PCM devices with MLC capability.

Alternative electronic density of states model for metastable crystalline phase of Ge2Sb2Te5

Motivated by future data storage requirements, Ge2Sb2Te5 is studied for application in phase-change random access memory. The currently accepted density of states (DOS) models for the cubic crystalline phase, based on first-principles calculations, are reviewed. An alternative DOS model, which incorporates band tails and an antimony vacancy multivalent defect, is proposed. Solar cell capacitance simulation results reveal that the alternative model is successful in predicting a free hole concentration and Fermi level position consistent with previous Hall effect and thermopower measurements respectively. The conduction band tail, which has not previously been incorporated within the DOS model of the crystalline phase, is shown to contribute to this success.

Design strategy of phase change material properties for low-energy memory application

Aiming at reducing energy consumptions in data writing, the development of new memory materials is highly required. To develop new phase change materials (PCMs) with extremely low operation energy used in non-volatile memory, such as storage-class memory, we performed Bayesian optimization for the physical properties of PCMs through numerical simulations. In this numerical simulation, the electrical potential and temperature distribution were solved simultaneously. It was found that a PCM with low thermal conductivity, low melting temperature, and low ratio of contact resistance to volumetric resistance results in a low operation energy of a PCM-based memory application. Finally, we developed a design strategy for PCMs. New PCMs should be developed by lowering the operation energy E, described as E = κ(1 + C)ΔT/Δz, where κ is the thermal conductivity of the PCM, ΔT is the melting temperature, C is the ratio of contact resistance to the volumetric resistance, and Δz is the thickness of the PCM. The present results clarify the relationship between thermal and electrical properties for lowering operation energy ever hidden in previous studies. According to the design strategy, an operation energy in phase change memory application can be decreased to less than 1/100 compared with conventional Ge-Sb-Te compounds.

Structural investigations on Ge2Sb2Te5 thin films using polarised Raman studies

Ge-Sb-Te (GST) based materials are the main source of ingredient for Phase change memory (PCM) applications. This ternary compound has the capacity to shift faster between crystalline as well as amorphous state when a suitable electrical or optical pulse is applied on it. Its crystalline phase has two states viz. face centered cubic (FCC) which is actually the metastable state and hexagonal phase which is primarily more stable state as compared to the FCC state. However, only FCC phase is suitable for PCM applications, but this study emphasizes the study of stable hexagonal phase also. The need to understand the stable hexagonal phase arises to further understand the useful metastable FCC stage. In this work, local anatomical studies have been carried out using X-ray diffraction technique (XRD) along with Raman spectroscopy that includes polarization studies on Ge2Sb2Te5 thin films in amorphous, FCC and hexagonal states. This work provides information about the symmetry of vibrational states using inelastic Raman scattering studies in polarization mode having vertical and horizontal geometries showing different Raman scattering intensities corresponding to parallel (VV-vertical vertical) and perpendicular (VH-vertical horizontal) scattering configurations. It is expected that interpretations from these studies will be helpful to understand the molecular structure and orientation of various flawed structural states such as coordination and orientation defects.

Extraction of thermal and optical parameters for As–Se–Te thin films according to phase-change pathways

The technique of thermal evaporation was used to synthesize the As50Se50-xTex (x = 0, 25, and 50 at.%) thin films (∼500 nm). The scenarios of this research were focused on the phase change pathways of the fresh and annealed (at Tc) thin films. By measuring the surface resistance of the studied thin films in the thermal range between 300 and 600 K, the path of phase transitions from the amorphous to the crystalline state was determined. The average activation energy of phase transitions was computed for the entire system. The phase-transition temperature has been determined utilizing the first derivative of the sheet resistance as a function of temperature (Rs–T). The optical bandgap of the fresh and annealed (at Tc) thin films was determined utilizing Kubelka–Munk (K-M) function that was extracted after measuring the diffuse reflectivity spectra in the range of (300–2500 nm). The two molar optical constants (n, k), the molar dispersion and dielectric parameters have been computed. Values of effective plasmon energy (Ψ), Molar volume Vm, the effective number of electrons neff, the electron density parameter rs, the homopolar gap (E‾h), the heteropolar gap (C), the average gap (E‾g), the theoretical static dielectric constant (ε0th) and Phillips ionicity of the glasses under study were calculated according to Penn model. All computed parameters in this work were given to the phase change paths in the studied system to consider the possibility of using the chalcogenide system in the phase-change memory (PCM) applications.

Lateral electrical transport and field-effect characteristics of sputtered p-type chalcogenide thin films

Investigating lateral electrical transport in p-type thin film chalcogenides is important to evaluate their potential for field-effect transistors (FETs) and phase-change memory applications. For instance, p-type FETs with materials sputtered at low temperature (≤ 250 °C) could play a role in flexible electronics or back-end-of-line silicon-compatible processes. Here, we explore lateral transport in chalcogenide films (Sb2Te3, Ge2Sb2Te5, and Ge4Sb6Te7) and multilayers, with Hall measurements (in ≤ 50 nm thin films) and with p-type transistors (in ≤ 5 nm ultrathin films). The highest Hall mobilities are measured for Sb2Te3/GeTe superlattices (∼18 cm2 V−1 s−1 at room temperature), over 2–3× higher than the other films. In ultrathin p-type FETs with Ge2Sb2Te5, we achieve field-effect mobility up to ∼5.5 cm2 V−1 s−1 with on/off current ratio of ∼104, the highest for Ge2Sb2Te5 transistors to date. We also explore process optimizations (e.g., the AlOx capping layer, type of developer for lithography) and uncover their tradeoffs toward the realization of p-type transistors with acceptable mobility and on/off current ratio. Our study provides essential insights into the optimization of electronic devices based on p-type chalcogenides.

Improvement in thermal stability and crystallization mechanism of Sm doped Ge2Sb2Te5 thin films for phase change memory applications

The thin films of (Ge2Sb2Te5)100−xSmx(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 (Tg), onset crystallization (Tc), peak crystallization temperature (Tp) and melting temperature (Tm) obtained from DSC curves of Sm-GST thin films have been used for the evaluation of thermal stability parameters. The activation energy for crystallization (Ec) 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.

Surface oxidation phenomena in Ge-rich GeSbTe alloys and N doping influence for Phase-Change Memory applications

Phase-Change Memory technology is increasing in maturity and in interest for next generation of non-volatile memory applications. In order to take advantage of the properties of innovative phase-change layers such as Ge-rich GST alloys, the understanding of the phenomena behind the possible evolution of the layer in time and temperature becomes fundamental. In this work, we present in detail the protocol we have developed using pARXPS technique, to reliably measure the surficial oxidation thickness on Ge-rich GST alloys with different nitrogen doping level. A double oxidation kinetic was observed on these samples for the first time, as well as an influence by nitrogen doping, and analyzed in detail by complementarily using pARXPS and TEM-EDX techniques.

Performance Improvement of Sb Phase Change Thin Film by Y Doping

Yttrium-doped Sb phase change thin films were prepared by magnetron sputtering and the in-situ resistance temperature measurement system was used to study the process of phase change thin films. With the addition of yttrium atoms, the crystallization temperature of Sb thin film increases, indicating improvement of their thermal stability and data retention ability. Furthermore, yttrium-doped Sb thin films have higher crystalline and amorphous resistance, which is conducive to the phase transition by Joule heating under electric pulse current. The optical band gap energy increases after yttrium doping, which means lower conductivity and higher conduction activation energy. X-ray diffraction shows that the doping of yttrium atoms can inhibit the grain growth and refine the grain size. X-ray photoelectron spectroscopy was used to examine the chemical state of the element, indicating the formation of a new Y-Sb bond, which verifies the improvement of stability. According to atomic force microscopy images, the yttrium-doped Sb material exhibits a surface morphology with less roughness, implying better interfacial properties and reliability. In general, yttrium-doped Sb thin films have better development in the application of phase change memory.